Concise Binary Object Representation (CBOR) is data format designed for small code size and small message size. There is a need for the ability to have the basic security services defined for this data format. This document specifies processing for signatures, message authentication codes, and encryption using CBOR. This document also specifies a representation for cryptographic keys using CBOR.

The source for this draft is being maintained in GitHub. Suggested changes should be submitted as pull requests at <https://github.com/cose-wg/cose-spec>. Instructions are on that page as well. Editorial changes can be managed in GitHub, but any substantial issues need to be discussed on the COSE mailing list.

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

1. Introduction

There has been an increased focus on the small, constrained devices that make up the Internet of Things (IOT). One of the standards that has come out of this process is the Concise Binary Object Representation (CBOR). CBOR extended the data model of the JavaScript Object Notation (JSON) by allowing for binary data among other changes. CBOR is being adopted by several of the IETF working groups dealing with the IOT world as their encoding of data structures. CBOR was designed specifically to be both small in terms of messages transport and implementation size as well having a schema free decoder. A need exists to provide message security services for IOT and using CBOR as the message encoding format makes sense.

The JOSE working group produced a set of documents [RFC7515][RFC7516][RFC7517][RFC7518] using JSON [RFC7159] that specified how to process encryption, signatures and message authentication (MAC) operations, and how to encode keys using JSON. This document does the same work for use with the CBOR [RFC7049] document format. While there is a strong attempt to keep the flavor of the original JOSE documents, two considerations are taken into account:

1.1. Design changes from JOSE

1.2. Requirements Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

1.3. CBOR Grammar

There currently is no standard CBOR grammar available for use by specifications. We therefore describe the CBOR structures in prose.

The document was developed by first working on the grammar and then developing the prose to go with it. An artifact of this is that the prose was written using the primitive type strings defined by CDDL. In this specification, the following primitive types are used:

There is a version of a CBOR grammar in the CBOR Data Definition Language (CDDL) [I-D.greevenbosch-appsawg-cbor-cddl]. Since CDDL has not be published as an RFC, this grammar may not work with the final version of CDDL when it is published. For those people who prefer using a formal language to describe the syntax of the CBOR, an informational version of the CBOR grammar is interweaved into the text as well. The CDDL grammar is informational, the prose description is normative.

The collected CDDL can be extracted from the XML version of this document via the following XPath expression below. (Depending on the XPath evaluator one is using, it may be necessary to deal with > as an entity.)

CDDL expects the initial non-terminal symbol to be the first symbol in the file. For this reason the first fragment of CDDL is presented here.

The non-terminal Internal_Types is defined for dealing with the automated validation tools used during the writing of this document. It references those non-terminals that are used for security computations, but are not emitted for transport.

1.4. CBOR Related Terminology

In JSON, maps are called objects and only have one kind of map key: a string. In COSE, we use strings, negative integers and unsigned integers as map keys. The integers are used for compactness of encoding and easy comparison. Since the work "key" is mainly used in its other meaning, as a cryptographic key, we use the term "label" for this usage as a map key.

The presence of a label in a map which is not a string or an integer is an error. Applications can either fail processing or process messages with incorrect labels, however they MUST NOT create messages with incorrect labels.

A CDDL grammar fragment is defined that defines the non-terminals 'label' as in the previous paragraph and 'values' which permits any value to be used.

1.5. Document Terminology

Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use in constrained systems. It is defined in [RFC7252].

Key management is used as a term to describe how a key at level n is obtained from level n+1 in encrypted and MACed messages. The term is also used to discuss key life cycle management, this document does not discuss key life cycle operations.

2. Basic COSE Structure

The COSE Message structure is designed so that there can be a large amount of common code when parsing and processing the different security messages. All of the message structures are built on the CBOR array type. The first three elements of the array contain the same information.

Identification of which type of message has been presented is done by the following method:

COSE Object Identification
Tag Value	cose-type	Data Item	Semantics
TBD1	cose-sign	COSE_Sign	COSE Signed Data Object
TBD7	cose-sign1	COSE_Sign1	COSE Single Signer Data Object
TBD2	cose-enveloped	COSE_Enveloped	COSE Enveloped Data Object
TBD3	cose-encrypted	COSE_Encrypted	COSE Encrypted Data Object
TBD4	cose-mac	COSE_Mac	COSE Mac-ed Data Object
TBD6	cose-mac0	COSE_Mac0	COSE Mac w/o Recipients Object
TBD5	N/A	COSE_Key, COSE_KeySet	COSE Key or COSE Key Set Object

The following CDDL fragment identifies all of the top level messages defined in this document. Separate non-terminals are defined for the tagged and the untagged versions of the messages for the convenience of applications.

COSE_Messages = COSE_Untagged_Message / COSE_Tagged_Message
      
COSE_Untagged_Message = COSE_Sign / COSE_Sign1 /
    COSE_Enveloped / COSE_Encrypted /
    COSE_Mac / COSE_Mac0

COSE_Tagged_Message = COSE_Sign_Tagged / COSE_Sign1_Tagged /
    COSE_Enveloped_Tagged / COSE_Encrypted_Tagged /
    COSE_Mac_Tagged / COSE_Mac0_Tagged

3. Header Parameters

The structure of COSE has been designed to have two buckets of information that are not considered to be part of the payload itself, but are used for holding information about content, algorithms, keys, or evaluation hints for the processing of the layer. These two buckets are available for use in all of the structures except for keys. While these buckets can be present, they may not all be usable in all instances. For example, while the protected bucket is defined as part of the recipient structure, some of the algorithms used for recipient structures do not provide for authenticated data. If this is the case, the protected bucket should be left empty.

Both buckets are implemented as CBOR maps. The map key is a 'label' (Section 1.4). The value portion is dependent on the definition for the label. Both maps use the same set of label/value pairs. The integer and string values for labels has been divided into several sections with a standard range, a private range, and a range that is dependent on the algorithm selected. The defined labels can be found in the 'COSE Header Parameters' IANA registry (Section 16.2).

Two buckets are provided for each layer:

protected:: Contains parameters about the current layer that are to be cryptographically protected. This bucket MUST be empty if it is not going to be included in a cryptographic computation. This bucket is encoded in the message as a binary object. This value is obtained by CBOR encoding the protected map and wrapping it in a bstr object. Senders SHOULD encode an empty protected map as a zero length binary object (it is both shorter and the version used in the authentication structures). Recipients MUST accept both a zero length binary value and a zero length map encoded in the binary value. The wrapping allows for the encoding of the protected map to be transported with a greater chance that it will not be altered in transit. (Badly behaved intermediates could decode and re-encode, but this will result in a failure to verify unless the re-encoded byte string is identical to the decoded byte string.) This finesses the problem of all parties needing to be able to do a common canonical encoding.
unprotected:: Contains parameters about the current layer that are not cryptographically protected.

Only parameters that deal with the current layer are to be placed at that layer. As an example of this, the parameter 'content type' describes the content of the message being carried in the message. As such, this parameter is placed only in the content layer and is not placed in the recipient or signature layers. In principle, one should be able to process any given layer without reference to any other layer. (With the exception of the COSE_Sign structure, the only data that needs to cross layers is the cryptographic key.)

The buckets are present in all of the security objects defined in this document. The fields in order are the 'protected' bucket (as a CBOR 'bstr' type) and then the 'unprotected' bucket (as a CBOR 'map' type). The presence of both buckets is required. The parameters that go into the buckets come from the IANA "COSE Header Parameters" (Section 16.2). Some common parameters are defined in the next section, but a number of parameters are defined throughout this document.

Labels in each of the maps MUST be unique. When processing messages, if a label appears multiple times the message MUST be rejected as malformed. Applications SHOULD perform the same checks that the labels appearing in the protected and unprotected headers are unique as well. If the message is not rejected as malformed, attributes MUST be obtained from the protected bucket before they are obtained from the unprotected bucket.

The following CDDL fragment represents the two header buckets. A group Headers is defined in CDDL which represents the two buckets in which attributes are placed. This group is used to provide these two fields consistently in all locations. A type is also defined which represents the map of header values. It uses forward references to a group definition of headers for generic and algorithms.

Headers = (
    protected : bstr,                  ; Contains a header_map
    unprotected : header_map
)

header_map = {
    Generic_Headers,
    ; Algorithm_Headers,
    * label => values
}

3.1. Common COSE Headers Parameters

This section defines a set of common header parameters. A summary of these parameters can be found in Table 2. This table should be consulted to determine the value of label as well as the type of the value.

The set of header parameters defined in this section are:

alg

This parameter is used to indicate the algorithm used for the security processing. This parameter MUST be present at each level of a signed, encrypted or authenticated message except the COSE_Sign structure. When the algorithm supports authenticating associated data, this parameter MUST be in the protected header bucket. The value is taken from the 'COSE Algorithm Registry' (see Section 16.4).

crit

The parameter is used to indicate which protected header labels an application that is processing a message is required to understand. Parameters defined in this document do not need to be included as they should be understood by all implementations. When present, this parameter MUST be placed in the protected header bucket. The array MUST have at least one value in it.
Not all labels need to be included in the 'crit' parameter. The rules for deciding which header labels are placed in the array are:

Integer labels in the range of 0 to 8 SHOULD be omitted.
Integer labels in the range -1 to -255 can be omitted as they are algorithm dependent. If an application can correctly process an algorithm, it can be assumed that it will correctly process all of the common parameters associated with that algorithm. (The algorithm range is -1 to -65536, the higher end is for more optional algorithm specific items.)
Labels for parameters required for an application MAY be omitted. Applications should have a statement if the label can or cannot be omitted.

The header parameter values indicated by 'crit' can be processed by either the security library code or by an application using a security library, the only requirement is that the parameter is processed. If the 'crit' value list includes a value for which the parameter is not in the protected bucket, this is a fatal error in processing the message.

content type

This parameter is used to indicate the content type of the data in the payload or ciphertext fields. Integers are from the 'CoAP Content-Formats' IANA registry table. Strings are from the IANA 'Media Types' registry. Applications SHOULD provide this parameter if the content structure is potentially ambiguous.

kid

This parameter identifies one piece of data that can be used as input to find the needed cryptographic key. The value of this parameter can be matched against the 'kid' member in a COSE_Key structure. Other methods of key distribution can define an equivalent field to be matched. Applications MUST NOT assume that 'kid' values are unique. There may be more than one key with the same 'kid' value, it may be required that all of the keys need to be checked to find the correct one. The internal structure of 'kid' values is not defined and cannot be relied on by applications. Key identifier values are hints about which key to use. They are not directly a security critical field. For this reason, they can be placed in the unprotected headers bucket.

Initialization Vector

This parameter holds the Initialization Vector (IV) value. For some symmetric encryption algorithms this may be referred to as a nonce. As the IV is authenticated by encryption process, it can be placed in the unprotected header bucket.

Partial Initialization Vector

This parameter holds a part of the IV value. When using the COSE_Encrypted structure, frequently a portion of the IV is part of the context associated with the key value. This field is used to carry a value that causes the IV to be changed for each message. As the IV is authenticated by the encryption process, this value can be placed in the unprotected header bucket. The 'Initialization Vector' and 'Partial Initialization Vector' parameters MUST NOT be present in the same security layer.
The message IV is generated by the following steps:

Left pad the partial IV with zeros to the length of IV.
XOR the padded partial IV with the context IV.

counter signature

This parameter holds a counter signature value. Counter signatures provide a method of having a second party sign some data. The counter signature can occur as an unprotected attribute in any of the following structures: COSE_Sign, COSE_Sign1, COSE_Signature, COSE_Enveloped, COSE_recipient, COSE_Encrypted, COSE_Mac and COSE_Mac0. These structures all have the same beginning elements so that a consistent calculation of the counter signature can be computed. Details on computing counter signatures are found in Section 4.5.

operation time

This parameter provides the time the content cryptographic operation is performed. For signatures and recipient structures, this would be the time that the signature or recipient key object was created. For content structures, this would be the time that the content structure was created. The unsigned integer value is the number of seconds, excluding leap seconds, after midnight UTC, January 1, 1970. The field is primarily intended to be to be used for counter signatures, however it can additionally be used for replay detection as well.

Common Header Parameters
name	label	value type	value registry	description
alg	1	int / tstr	COSE Algorithm Registry	Cryptographic algorithm to use
crit	2	[+ label]	COSE Header Label Registry	Critical headers to be understood
content type	3	tstr / int	CoAP Content- Formats or Media Types registry	Content type of the payload
kid	4	bstr		key identifier
IV	5	bstr		Full Initialization Vector
Partial IV	6	bstr		Partial Initialization Vector
counter signature	7	COSE_Signature / [+ COSE_Signature ]		CBOR encoded signature structure
operation time	8	uint		Time the COSE structure was created

The CDDL fragment that represents the set of headers defined in this section is given below. Each of the headers is tagged as optional because they do not need to be in every map, headers required in specific maps are discussed above.

Generic_Headers = (
    ? 1 => int / tstr,  ; algorithm identifier
    ? 2 => [+label],    ; criticality
    ? 3 => tstr / int,  ; content type
    ? 4 => bstr,        ; key identifier
    ? 5 => bstr,        ; IV
    ? 6 => bstr,        ; Partial IV
    ? 7 => COSE_Signature / [+COSE_Signature], ; Counter signature
    ? 8 => uint         ; Operation time
)

4. Signing Objects

COSE supports two different signature structures. COSE_Sign allows for one or more signers to be applied to a single content. COSE_Sign1 is restricted to a single signer. The structures cannot be converted between each other, the signature computation includes a parameter identifying which structure is being used.

4.1. Signing with One or More Signers

The signature structure allows for one or more signatures to be applied to a message payload. There are provisions for parameters about the content and parameters about the signature to be carried along with the signature itself. These parameters may be authenticated by the signature, or just present. An example of a parameter about the content is the content type. Examples of parameters about the signature would be the algorithm and key used to create the signature, when the signature was created, and counter signatures.

When more than one signature is present, the successful validation of one signature associated with a given signer is usually treated as a successful signature by that signer. However, there are some application environments where other rules are needed. An application that employs a rule other than one valid signature for each signer must specify those rules. Also, where simple matching of the signer identifier is not sufficient to determine whether the signatures were generated by the same signer, the application specification must describe how to determine which signatures were generated by the same signer. Support of different communities of recipients is the primary reason that signers choose to include more than one signature. For example, the COSE_Sign structure might include signatures generated with the RSA signature algorithm and with the Elliptic Curve Digital Signature Algorithm (ECDSA) signature algorithm. This allows recipients to verify the signature associated with one algorithm or the other. (The original source of this text is [RFC5652].) More detailed information on multiple signature evaluation can be found in [RFC5752].

The signature structure can be encoded either with or without a tag depending on the context it will be used in. The signature structure is identified by the CBOR tag TBD1. The CDDL fragment that represents this is.

COSE_Sign_Tagged = #6.991(COSE_Sign) ; Replace 991 with TBD1

A COSE Signed Message is divided into two parts. The CBOR object that carries the body and information about the body is called the COSE_Sign structure. The CBOR object that carries the signature and information about the signature is called the COSE_Signature structure. Examples of COSE Signed Messages can be found in Appendix C.1.

The COSE_Sign structure is a CBOR array. The fields of the array in order are:

protected: as described in Section 3.
unprotected: as described in Section 3.
payload: contains the serialized content to be signed. If the payload is not present in the message, the application is required to supply the payload separately. The payload is wrapped in a bstr to ensure that it is transported without changes. If the payload is transported separately (i.e. detached content), then a nil CBOR object is placed in this location and it is the responsibility of the application to ensure that it will be transported without changes.

Note: When a signature with message recovery algorithm is used (Section 8), the maximum number of bytes that can be recovered is the length of the payload. The size of the payload is reduced by the number of bytes that will be recovered. If all of the bytes of the payload are consumed, then the payload is encoded as a zero length binary string rather than as being absent.
signatures: is an array of signatures. Each signature is represented as a COSE_Signature structure.

The CDDL fragment which represents the above text for COSE_Sign follows.

COSE_Sign = [
    Headers,
    payload : bstr / nil,
    signatures : [+ COSE_Signature]
]

The COSE_Signature structure is a CBOR array. The fields of the array in order are:

protected: as described in Section 3.
unprotected: as described in Section 3.
signature: contains the computed signature value. The type of the field is a bstr.

The CDDL fragment which represents the above text for COSE_Signature follows.

COSE_Signature =  [
    Headers,      
    signature : bstr
]

4.2. Signing with One Signer

The signature structure can be encoded either with or without a tag depending on the context it will be used in. The signature structure is identified by the CBOR tag TBD7. The CDDL fragment that represents this is:

COSE_Sign1_Tagged = #6.997(COSE_Sign1) ; Replace 997 with TBD7

The CBOR object that carries the body, the signature and the information about the body and signature is called the COSE_Sign1 structure. Examples of COSE Single signature messages can be found in Appendix C.2.

The COSE_Sign1 structure is a CBOR array. The fields of the array in order are:

protected: as described in Section 3.
unprotected: as described in Section 3.
payload: as described in Section 4.1.
signature: contains the computed signature value. The type of the field is a bstr.

The CDDL fragment which represents the above text for COSE_Sign1 follows.

COSE_Sign1 = [
    Headers,
    payload : bstr / nil,
    signature : bstr
]

4.3. Externally Supplied Data

One of the features that we supply in the COSE document is the ability for applications to provide additional data to be authenticated as part of the security, but that is not carried as part of the COSE object. The primary reason for supporting this can be seen by looking at the CoAP message structure [RFC7252] where the facility exists for options to be carried before the payload. An example of data that can be placed in this location would be CoAP options for transaction ids and nonces to check for replay protection. If the data is in the options section, then it is available for routers to help in performing the replay detection and prevention. However, it may also be desired to protect these values so that if they are be modified in transit it can be detected. This is the purpose of the externally supplied data field.

This document describes the process for using a byte array of externally supplied authenticated data, however the method of constructing the byte array is a function of the application. Applications that use this feature need to define how the externally supplied authenticated data is to be constructed. Such a construction needs to take into account the following issues:

If multiple items are included, care needs to be taken that data cannot bleed between the items. This is usually addressed by making fields fixed width and/or encoding the length of the field. Using options from CoAP [RFC7252] as an example, these fields use a TLV structure so they can be concatenated without any problems.
If multiple items are included, a defined order for the items needs to be defined. Using options from CoAP as an example, an application could state that the fields are to be ordered by the option number.

4.4. Signing and Verification Process

In order to create a signature, a consistent byte stream is needed. This algorithm takes in the body information (COSE_Sign), the signer information (COSE_Signature), and the application data (External). A CBOR array is used to construct the byte stream. The fields of the array in order are:

A text string identifying the context of the signature. The context string is:
"Signature"
for signatures using the COSE_Signature structure.
"Signature1"
for signatures using the COSE_Sign1 structure.
"CounterSignature"
for signatures used as counter signature attributes.
The protected attributes from the body structure encoded in a bstr type. If there are no protected attributes, a bstr of length zero is used.
The protected attributes from the signer structure encoded in a bstr type. If there are no protected attributes, a bstr of length zero is used. This field is omitted for the COSE_Sign1 signature structure.
The protected attributes from the application encoded in a bstr type. If this field is not supplied, it defaults to a zero length binary string. (See Section 4.3 for application guidance on constructing this field.)
The payload to be signed encoded in a bstr type. The payload is placed here independent of how it is transported.

The CDDL fragment which describes the above text is.

Sig_structure = [
    context: "Signature" / "Signature1" / "CounterSignature",
    body_protected: bstr,
    ? sign_protected: bstr,
    external_aad: bstr,
    payload: bstr
]

How to compute a signature:

Create a Sig_structure and populate it with the appropriate fields.
Create the value ToBeSigned by encoding the Sig_structure to a byte string using the encoding described in Section 14.
Call the signature creation algorithm passing in K (the key to sign with), alg (the algorithm to sign with) and ToBeSigned (the value to sign).
Place the resulting signature value in the 'signature' field of the array.

How to verify a signature:

Create a Sig_structure object and populate it with the appropriate fields.
Create the value ToBeSigned by encoding the Sig_structure to a byte string using the encoding described in Section 14..
Call the signature verification algorithm passing in K (the key to verify with), alg (the algorithm used sign with), ToBeSigned (the value to sign), and sig (the signature to be verified).

In addition to performing the signature verification, one must also perform the appropriate checks to ensure that the key is correctly paired with the signing identity and that the appropriate authorization is done.

4.5. Computing Counter Signatures

Counter signatures provide a method of having a different signature occur on some piece of content. This is normally used to provide a signature on a signature allowing for a proof that a signature existed at a given time (i.e. a Timestamp). In this document we allow for counter signatures to exist in a greater number of environments. As an example, it is possible to place a counter signature in the unprotected attributes of a COSE_Enveloped object. This would allow for an intermediary to either verify that the encrypted byte stream has not been modified, without being able to decrypt it. Or for the intermediary to assert that an encrypted byte stream either existed at a given time or passed through it in terms of routing (i.e. a proxy signature).

An example of a counter signature on a signature can be found in Appendix C.1.3. An example of a counter signature in an encryption object can be found in Appendix C.3.3.

The creation and validation of counter signatures over the different items relies on the fact that the structure of the objects have the same structure. The elements are a set of protected attributes, a set of unprotected attributes and a body in that order. This means that the Sig_structure can be used in a uniform manner to get the byte stream for processing a signature. If the counter signature is going to be computed over a COSE_Enveloped structure, the body_protected and payload items can be mapped into the Sig_structure in the same manner as from the COSE_Sign structure.

It should be noted that only a signature algorithm with appendix (see Section 8) can be used for counter signatures. This is because the body should be able to be processed without having to evaluate the counter signature, and this is not possible for signature schemes with message recovery.

5. Encryption Objects

COSE supports two different encryption structures. COSE_Encrypted is used when a recipient structure is not needed because the key to be used is known implicitly. COSE_Enveloped is used the rest of time time. This includes cases where there are multiple recipients, a recipient algorithm other than direct is to be used, or the key to be used is not known.

5.1. Enveloped COSE Structure

The enveloped structure allows for one or more recipients of a message. There are provisions for parameters about the content and parameters about the recipient information to be carried in the message. The protected parameters associated with the content are authenticated by the content encryption algorithm. The protected parameters associated with the recipient are authenticated by the recipient algorithm (when the algorithm supports it). Examples of parameters about the content are the type of the content, and the content encryption algorithm. Examples of parameters about the recipient are the recipient's key identifier, the recipient encryption algorithm.

The same techniques and structures are used for encrypting both the plain text and the keys used to protect the text. This is different from the approach used by both [RFC5652] and [RFC7516] where different structures are used for the content layer and for the recipient layer. Two structures are defined: COSE_Enveloped to hold the encrypted content, and COSE_recipient to hold the encrypted keys for recipients. Examples of encrypted messages can be found in Appendix C.3.

The COSE Enveloped structure can be encoded either with or without a tag depending on the context it will be used in. The COSE Enveloped structure is identified by the CBOR tag TBD2. The CDDL fragment that represents this is.

COSE_Enveloped_Tagged = #6.992(COSE_Enveloped) ; Replace 992 with TBD2

The COSE_Enveloped structure is a CBOR array. The fields of the array in order are:

protected: as described in Section 3.
unprotected: as described in Section 3.
ciphertext: contains the cipher text encoded as a bstr. If the ciphertext is to be transported independently of the control information about the encryption process (i.e. detached content) then the field is encoded as a null object.
recipients: contains an array of recipient information structures. The type for the recipient information structure is a COSE_recipient.

The CDDL fragment that corresponds to the above text is:

COSE_Enveloped = [
    Headers,
    ciphertext: bstr / nil,
    recipients: [+COSE_recipient]
]

The COSE_recipient structure is a CBOR array. The fields of the array in order are:

protected: as described in Section 3.
unprotected: as described in Section 3.
ciphertext: contains the encrypted key encoded as a bstr. If there is not an encrypted key, then this field is encoded as a nil value.
recipients: contains an array of recipient information structures. The type for the recipient information structure is a COSE_recipient. (And example of this can be found in Appendix B.) If there are no recipient information structures, this element is absent.

The CDDL fragment that corresponds to the above text for COSE_recipient is:

COSE_recipient = [    
    Headers,
    ciphertext: bstr / nil,
    ? recipients: [+COSE_recipient]
]

5.1.1. Recipient Algorithm Classes

A typical encrypted message consists of an encrypted content and an encrypted CEK for one or more recipients. The CEK is encrypted for each recipient, using a key specific to that recipient. The details of this encryption depends on which class the recipient algorithm falls into. Specific details on each of the classes can be found in Section 12. A short summary of the five recipient algorithm classes is:

direct:: The CEK is the same as the identified previously distributed symmetric key or derived from a previously distributed secret. No CEK is transported in the message.
symmetric key-encryption keys:: The CEK is encrypted using a previously distributed symmetric KEK.
key agreement:: The recipient's public key and a sender's private key are used to generate a pairwise secret, a KDF is applied to derive a key, and then the CEK is either the derived key or encrypted by the derived key.
key transport:: The CEK is encrypted with the recipient's public key. No key transport algorithms are defined in this document.
passwords:: The CEK is encrypted in a KEK that is derived from a password. No password algorithms are defined in this document.

5.2. Encrypted COSE structure

The encrypted structure does not have the ability to specify recipients of the message. The structure assumes that the recipient of the object will already know the identity of the key to be used in order to decrypt the message. If a key needs to be identified to the recipient, the enveloped structure ought to be used.

The structure defined to hold an encrypted message is COSE_Encrypted. Examples of encrypted messages can be found in Appendix C.3.

The COSE_Encrypted structure can be encoded either with or without a tag depending on the context it will be used in. The COSE_Encrypted structure is identified by the CBOR tag TBD3. The CDDL fragment that represents this is.

COSE_Encrypted_Tagged = #6.993(COSE_Encrypted) ; Replace 993 with TBD3

The COSE_Enveloped structure is a CBOR array. The fields of the array in order are:

protected: as described in Section 3.
unprotected: as described in Section 3.
ciphertext: as described in Section 5.1.

The CDDL fragment for COSE_Encrypted that corresponds to the above text is:

COSE_Encrypted = [
    Headers,
    ciphertext: bstr / nil,
]

5.3. Encryption Algorithm for AEAD algorithms

The encryption algorithm for AEAD algorithms is fairly simple. The first step is to create a consistent byte stream for the authenticated data structure. For this purpose we use a CBOR array, the fields of the array in order are:

A text string identifying the context of the authenticated data structure. The context string is:
"Encrypted"
for the content encryption of an encrypted data structure.
"Enveloped"
for the first level of an enveloped data structure (i.e. for content encryption).
"Env_Recipient"
for a recipient encoding to be placed in an enveloped data structure.
"Mac_Recipient"
for a recipient encoding to be placed in a MAC message structure.
"Rec_Recipient"
for a recipient encoding to be placed in a recipient structure.
The protected attributes from the body structure encoded in a bstr type. If there are no protected attributes, a bstr of length zero is used.
The protected attributes from the application encoded in a bstr type. If this field is not supplied, it defaults to a zero length bstr. (See Section 4.3 for application guidance on constructing this field.)

The CDDL fragment which describes the above text is:

Enc_structure = [
    context : "Enveloped" / "Encrypted" / "Env_Recipient" /
        "Mac_Recipient" / "Rec_Recipient",
    protected: bstr,
    external_aad: bstr
]

How to encrypt a message:

Create a Enc_structure and populate it with the appropriate fields.
Encode the Enc_structure to a byte stream (AAD) using the encoding described in Section 14.
Determine the encryption key. This step is dependent on the class of recipient algorithm being used. For:
No Recipients:
The key to be used is determined by the algorithm and key at the current level.
Direct and Direct Key Agreement:
The key is determined by the key and algorithm in the recipient structure. The encryption algorithm and size of the key to be used are inputs into the KDF used for the recipient. (For direct, the KDF can be thought of as the identity operation.)
Other:
The key is randomly generated.
Call the encryption algorithm with K (the encryption key to use), P (the plain text) and AAD. Place the returned cipher text into the 'ciphertext' field of the structure.
For recipients of the message, recursively perform the encryption algorithm for that recipient using the encryption key as the plain text.

How to decrypt a message:

Create a Enc_structure and populate it with the appropriate fields.
Encode the Enc_structure to a byte stream (AAD) using the encoding described in Section 14.
Determine the decryption key. This step is dependent on the class of recipient algorithm being used. For:
No Recipients:
The key to be used is determined by the algorithm and key at the current level.
Direct and Direct Key Agreement:
The key is determined by the key and algorithm in the recipient structure. The encryption algorithm and size of the key to be used are inputs into the KDF used for the recipient. (For direct, the KDF can be thought of as the identity operation.)
Other:
The key is determined by decoding and decrypting the recipient structure.
Call the decryption algorithm with K (the decryption key to use), C (the cipher text) and AAD.

5.4. Encryption algorithm for AE algorithms

How to encrypt a message:

Verify that the 'protected' field is absent.
Verify that there was no external additional authenticated data supplied for this operation.
Determine the encryption key. This step is dependent on the class of recipient algorithm being used. For:
No Recipients:
The key to be used is determined by the algorithm and key at the current level.
Direct and Direct Key Agreement:
The key is determined by the key and algorithm in the recipient structure. The encryption algorithm and size of the key to be used are inputs into the KDF used for the recipient. (For direct, the KDF can be thought of as the identity operation.)
Other:
The key is randomly generated.
Call the encryption algorithm with K (the encryption key to use) and the P (the plain text). Place the returned cipher text into the 'ciphertext' field of the structure.
For recipients of the message, recursively perform the encryption algorithm for that recipient using the encryption key as the plain text.

How to decrypt a message:

Verify that the 'protected' field is absent.
Verify that there was no external additional authenticated data supplied for this operation.
Determine the decryption key. This step is dependent on the class of recipient algorithm being used. For:
No Recipients:
The key to be used is determined by the algorithm and key at the current level.
Direct and Direct Key Agreement:
The key is determined by the key and algorithm in the recipient structure. The encryption algorithm and size of the key to be used are inputs into the KDF used for the recipient. (For direct, the KDF can be thought of as the identity operation.)
Other:
The key is determined by decoding and decrypting the recipient structure.
Call the decryption algorithm with K (the decryption key to use), C (the cipher text) and AAD.

6. MAC Objects

COSE supports two different MAC structures. COSE_MAC0 is used when a recipient structure is not needed because the key to be used is implicitly known. COSE_MAC is used for all other cases. These include a requirement for multiple recipients, the key being unknown, a recipient algorithm of other than direct.

6.1. MAC Message with Recipients

In this section we describe the structure and methods to be used when doing MAC authentication in COSE. This document allows for the use of all of the same classes of recipient algorithms as are allowed for encryption.

When using MAC operations, there are two modes in which it can be used. The first is just a check that the content has not been changed since the MAC was computed. Any class of recipient algorithm can be used for this purpose. The second mode is to both check that the content has not been changed since the MAC was computed, and to use the recipient algorithm to verify who sent it. The classes of recipient algorithms that support this are those that use a pre-shared secret or do static-static key agreement (without the key wrap step). In both of these cases, the entity that created and sent the message MAC can be validated. (This knowledge of sender assumes that there are only two parties involved and you did not send the message yourself.)

The MAC message uses two structures, the COSE_Mac structure defined in this section for carrying the body and the COSE_recipient structure (Section 5.1) to hold the key used for the MAC computation. Examples of MAC messages can be found in Appendix C.5.

The MAC structure can be encoded either with or without a tag depending on the context it will be used in. The MAC structure is identified by the CBOR tag TBD4. The CDDL fragment that represents this is:

COSE_Mac_Tagged = #6.994(COSE_Mac)         ; Replace 994 with TBD4

The COSE_Mac structure is a CBOR array. The fields of the array in order are:

protected: as described in Section 3.
unprotected: as described in Section 3.
payload: contains the serialized content to be MACed. If the payload is not present in the message, the application is required to supply the payload separately. The payload is wrapped in a bstr to ensure that it is transported without changes. If the payload is transported separately (i.e. detached content), then a null CBOR object is placed in this location and it is the responsibility of the application to ensure that it will be transported without changes.
tag: contains the MAC value.
recipients: as described in Section 5.1.

The CDDL fragment which represents the above text for COSE_Mac follows.

COSE_Mac = [
   Headers,
   payload: bstr / nil,
   tag: bstr,
   recipients: [+COSE_recipient]
]

6.2. MAC Messages with Implicit Key

In this section we describe the structure and methods to be used when doing MAC authentication for those cases where the recipient is implicitly known.

The MAC message uses the COSE_Mac0 structure defined in this section for carrying the body. Examples of MAC messages with an implicit key can be found in Appendix C.6.

The MAC structure can be encoded either with or without a tag depending on the context it will be used in. The MAC structure is identified by the CBOR tag TBD6. The CDDL fragment that represents this is:

COSE_Mac0_Tagged = #6.996(COSE_Mac0)    ; Replace 996 with TBD6

The COSE_Mac0 structure is a CBOR array. The fields of the array in order are:

protected: as described in Section 3.
unprotected: as described in Section 3.
payload: as described in Section 6.1.
tag: contains the MAC value.

The CDDL fragment which corresponds to the above text is:

COSE_Mac0 = [
   Headers,
   payload: bstr / nil,
   tag: bstr,
]

6.3. How to compute and verify a MAC

In order to get a consistent encoding of the data to be authenticated, the MAC_structure is used to have a canonical form. The MAC_structure is a CBOR array. The fields of the MAC_structure in order are:

A text string that identifies the structure that is being encoded. This string is "MAC" for the COSE_Mac structure. This string is "MAC0" for the COSE_Mac0 structure.
The protected attributes from the COSE_MAC structure. If there are no protected attributes, a zero length bstr is used.
If the application has supplied external authenticated data, encode it as a binary value and place in the MAC_structure. If there is no external authenticated data, then use a zero length 'bstr'. (See Section 4.3 for application guidance on constructing this field.)
The payload to be MAC-ed encoded in a bstr type. The payload is placed here independent of how it is transported.

The CDDL fragment that corresponds to the above text is:

MAC_structure = [
     context: "MAC" / "MAC0",
     protected: bstr,
     external_aad: bstr,
     payload: bstr
]

The steps to compute a MAC are:

Create a MAC_structure and populate it with the appropriate fields.
Encode the MAC_structure to a byte stream using the encoding described in Section 14.
Call the MAC creation algorithm passing in K (the key to use), alg (the algorithm to MAC with) and ToBeMaced (the value to compute the MAC on).
Place the resulting MAC in the 'tag' field of the COSE_Mac structure.
Encrypt and encode the MAC key for each recipient of the message.

How to verify a MAC are:

Create a MAC_structure object and populate it with the appropriate fields.
Encode the MAC_structure to a byte stream using the encoding described in Section 14.
Obtain the cryptographic key from one of the recipients of the message.
Call the MAC creation algorithm passing in K (the key to use), alg (the algorithm to MAC with) and ToBeMaced (the value to compute the MAC on).
Compare the MAC value to the 'tag' field of the COSE_Mac structure.

7. Key Structure

A COSE Key structure is built on a CBOR map object. The set of common parameters that can appear in a COSE Key can be found in the IANA registry 'COSE Key Common Parameter Registry' (Section 16.5). Additional parameters defined for specific key types can be found in the IANA registry 'COSE Key Type Parameters' (Section 16.6).

A COSE Key Set uses a CBOR array object as its underlying type. The values of the array elements are COSE Keys. A Key Set MUST have at least one element in the array.

The element "kty" is a required element in a COSE_Key map.

The CDDL grammar describing COSE_Key and COSE_KeySet is:

COSE_Key = {
    key_kty => tstr / int,
    ? key_ops => [+ (tstr / int) ],
    ? key_alg => tstr / int,
    ? key_kid => bstr,
    * label => values
}

COSE_KeySet = [+COSE_Key]

7.1. COSE Key Common Parameters

This document defines a set of common parameters for a COSE Key object. Table 3 provides a summary of the parameters defined in this section. There are also parameters that are defined for specific key types. Key type specific parameters can be found in Section 13.

Key Map Labels
name	label	CBOR type	registry	description
kty	1	tstr / int	COSE General Values	Identification of the key type
key_ops	4	[+ (tstr/int)]		Restrict set of permissible operations
alg	3	tstr / int	COSE Algorithm Values	Key usage restriction to this algorithm
kid	2	bstr		Key Identification value - match to kid in message
Base IV	5	bstr		Base IV to be xor-ed with Partial IVs

kty:: This parameter is used to identify the family of keys for this structure, and thus the set of key type specific parameters to be found. The set of values defined in this document can be found in Table 19. This parameter MUST be present in a key object. Implementations MUST verify that the key type is appropriate for the algorithm being processed. The key type MUST be included as part of the trust decision process.
alg:: This parameter is used to restrict the algorithms that are to be used with this key. If this parameter is present in the key structure, the application MUST verify that this algorithm matches the algorithm for which the key is being used. If the algorithms do not match, then this key object MUST NOT be used to perform the cryptographic operation. Note that the same key can be in a different key structure with a different or no algorithm specified, however this is considered to be a poor security practice.
kid:: This parameter is used to give an identifier for a key. The identifier is not structured and can be anything from a user provided string to a value computed on the public portion of the key. This field is intended for matching against a 'kid' parameter in a message in order to filter down the set of keys that need to be checked.
key_ops:: This parameter is defined to restrict the set of operations that a key is to be used for. The value of the field is an array of values from Table 4.
Base IV:: This parameter is defined to carry the base portion of an IV. It is designed to be used with the partial IV header parameter defined in Section 3.1. This field provides the ability to associate a partial IV with a key that is then modified on a per message basis with the parital IV.

Care needs to be taken that this is only used as part of a key distribution algorithm that will ensure that it will be given only to parties that will use it correctly. This is due to the fact that many of the content encryption algorithms defined for COSE require that IVs be unique for every message. Use of this field will easily allow for this rule to be broken if not used carefully. This field MUST be ignored unless an application specifically calls for its use.

Key Operation Values
name	value	description
sign	1	The key is used to create signatures. Requires private key fields.
verify	2	The key is used for verification of signatures.
encrypt	3	The key is used for key transport encryption.
decrypt	4	The key is used for key transport decryption. Requires private key fields.
wrap key	5	The key is used for key wrapping.
unwrap key	6	The key is used for key unwrapping. Requires private key fields.
derive key	7	The key is used for deriving keys. Requires private key fields.
derive bits	8	The key is used for deriving bits. Requires private key fields.

The following provides a CDDL fragment which duplicates the assignment labels from Table 3.

;key_labels
key_kty=1
key_kid=2
key_alg=3
key_ops=4

8. Signature Algorithms

There are two signature algorithm schemes. The first is signature with appendix. In this scheme, the message content is processed and a signature is produced, the signature is called the appendix. This is the scheme used by algorithms such as ECDSA and RSASSA-PSS. (In fact the SSA in RSASSA-PSS stands for Signature Scheme with Appendix.)

The signature functions for this scheme are:

signature = Sign(message content, key)

valid = Verification(message content, key, signature)

The second scheme is signature with message recovery. (An example of such an algorithm is [PVSig].) In this scheme, the message content is processed, but part of it is included in the signature. Moving bytes of the message content into the signature allows for smaller signatures, the signature size is still potentially large, but the message content has shrunk. This has implications for systems implementing these algorithms and for applications that use them. The first is that the message content is not fully available until after a signature has been validated. Until that point the part of the message contained inside of the signature is unrecoverable. The second is that the security analysis of the strength of the signature is very much based on the structure of the message content. Messages which are highly predictable require additional randomness to be supplied as part of the signature process. In the worst case, it becomes the same as doing a signature with appendix. Finally, in the event that multiple signatures are applied to a message, all of the signature algorithms are going to be required to consume the same number of bytes of message content. This means that mixing of the different schemes in a single message is not supported, and if a recovery signature scheme is used then the same amount of content needs to be consumed by all of the signatures.

The signature functions for this scheme are:

signature, message sent = Sign(message content, key)

valid, message content = Verification(message sent, key, signature)

At this time, only signatures with appendixes are defined for use with COSE, however considerable interest has been expressed in using a signature with message recovery algorithm due to the effective size reduction that is possible. Implementations will need to keep this in mind for later possible integration.

8.1. ECDSA

ECDSA [DSS] defines a signature algorithm using ECC.

The ECDSA signature algorithm is parameterized with a hash function (h). In the event that the length of the hash function output is greater than the group of the key, the left-most bytes of the hash output are used.

The algorithms defined in this document can be found in Table 5.

ECDSA Algorithm Values
name	value	hash	description
ES256	-7	SHA-256	ECDSA w/ SHA-256
ES384	-35	SHA-384	ECDSA w/ SHA-384
ES512	-36	SHA-512	ECDSA w/ SHA-512

This document defines ECDSA to work only with the curves P-256, P-384 and P-521. This document requires that the curves be encoded using the 'EC2' key type. Implementations need to check that the key type and curve are correct when creating and verifying a signature. Other documents can defined it to work with other curves and points in the future.

In order to promote interoperability, it is suggested that SHA-256 be used only with curve P-256, SHA-384 be used only with curve P-384 and SHA-512 be used with curve P-521. This is aligned with the recommendation in Section 4 of [RFC5480].

The signature algorithm results in a pair of integers (R, S). These integers will the same length as length of the key used for the signature process. The signature is encoded by converting the integers into byte strings of the same length as the key size. The length is rounded up to the nearest byte and is left padded with zero bits to get to the correct length. The two integers are then concatenated together to form a byte string that is the resulting signature.

Using the function defined in [RFC3447] the signature is:
Signature = I2OSP(R, n) | I2OSP(S, n)
where n = ceiling(key_length / 8)

When using a COSE key for this algorithm, the following checks are made:

The 'kty' field MUST be present and it MUST be 'EC2'.
If the 'alg' field present, it MUST match the ECDSA signature algorithm being used.
If the 'key_ops' field is present, it MUST include 'sign' when creating an ECDSA signature.
If the 'key_ops' field is present, it MUST include 'verify' when verifying an ECDSA signature.

8.1.1. Security Considerations

The security strength of the signature is no greater than the minimum of the security strength associated with the bit length of the key and the security strength of the hash function.

System which have poor random number generation can leak their keys by signing two different messages with the same value 'k' (the per-message random value). [RFC6979] provides a method to deal with this problem by making 'k' be deterministic based on the message content rather than randomly generated. Applications that specify ECDSA should evaluate the ability to get good random number generation and require this when it is not possible.

Note: Use of this technique a good idea even when good random number generation exists. Doing so both reduces the possibility of having the same value of 'k' in two signature operations and allows for reproducible signature values which helps testing.

There are two substitution attacks that can theoretically be mounted against the ECDSA signature algorithm.

Changing the curve used to validate the signature: If one changes the curve used to validate the signature, then potentially one could have a two messages with the same signature each computed under a different curve. The only requirement on the new curve is that its order be the same as the old one and it be acceptable to the client. An example would be to change from using the curve secp256r1 (aka P-256) to using secp256k1. (Both are 256 bit curves.) We current do not have any way to deal with this version of the attack except to restrict the overall set of curves that can be used.
Change the hash function used to validate the signature: If one has either two different hash functions of the same length, or one can truncate a hash function down, then one could potentially find collisions between the hash functions rather than within a single hash function. (For example, truncating SHA-512 to 256 bits might collide with a SHA-256 bit hash value.) This attack can be mitigated by including the signature algorithm identifier in the data to be signed.

9. Message Authentication (MAC) Algorithms

Message Authentication Codes (MACs) provide data authentication and integrity protection. They provide either no or very limited data origination. (One cannot, for example, be used to prove the identity of the sender to a third party.)

MACs use the same scheme as signature with appendix algorithms. The message content is processed and an authentication code is produced. The authentication code is frequently called a tag.

The MAC functions are:

tag = MAC_Create(message content, key)

valid = MAC_Verify(message content, key, tag)

MAC algorithms can be based on either a block cipher algorithm (i.e. AES-MAC) or a hash algorithm (i.e. HMAC). This document defines a MAC algorithm for each of these two constructions.

9.1. Hash-based Message Authentication Codes (HMAC)

The Hash-base Message Authentication Code algorithm (HMAC) [RFC2104][RFC4231] was designed to deal with length extension attacks. The algorithm was also designed to allow for new hash algorithms to be directly plugged in without changes to the hash function. The HMAC design process has been vindicated as, while the security of hash algorithms such as MD5 has decreased over time, the security of HMAC combined with MD5 has not yet been shown to be compromised [RFC6151].

The HMAC algorithm is parameterized by an inner and outer padding, a hash function (h) and an authentication tag value length. For this specification, the inner and outer padding are fixed to the values set in [RFC2104]. The length of the authentication tag corresponds to the difficulty of producing a forgery. For use in constrained environments, we define a set of HMAC algorithms that are truncated. There are currently no known issues with truncation, however the security strength of the message tag is correspondingly reduced in strength. When truncating, the left-most tag length bits are kept and transmitted.

The algorithm defined in this document can be found in Table 6.

HMAC Algorithm Values
name	value	Hash	Tag Length	description
HMAC 256/64	4	SHA-256	64	HMAC w/ SHA-256 truncated to 64 bits
HMAC 256/256	5	SHA-256	256	HMAC w/ SHA-256
HMAC 384/384	6	SHA-384	384	HMAC w/ SHA-384
HMAC 512/512	7	SHA-512	512	HMAC w/ SHA-512

Some recipient algorithms carry the key while others derive a key from secret data. For those algorithms that carry the key (i.e. AES-KeyWrap), the size of the HMAC key SHOULD be the same size as the underlying hash function. For those algorithms that derive the key (i.e. ECDH), the derived key MUST be the same size as the underlying hash function.

When using a COSE key for this algorithm, the following checks are made:

The 'kty' field MUST be present and it MUST be 'Symmetric'.
If the 'alg' field present, it MUST match the HMAC algorithm being used.
If the 'key_ops' field is present, it MUST include 'sign' when creating an HMAC authentication tag.
If the 'key_ops' field is present, it MUST include 'verify' when verifying an HMAC authentication tag.

Implementations creating and validating MAC values MUST validate that the key type, key length, and algorithm are correct and appropriate for the entities involved.

9.1.1. Security Considerations

HMAC has proved to be resistant to attack even when used with weakening hash algorithms. The current best method appears to be a brute force attack on the key. This means that key size is going to be directly related to the security of an HMAC operation.

9.2. AES Message Authentication Code (AES-CBC-MAC)

AES-CBC-MAC is defined in [MAC]. (Note this is not the same algorithm as AES-CMAC [RFC4493]).

AES-CBC-MAC is parameterized by the key length, the authentication tag length and the IV used. For all of these algorithms, the IV is fixed to all zeros. We provide an array of algorithms for various key lengths and tag lengths. The algorithms defined in this document are found in Table 7.

AES-MAC Algorithm Values
name	value	key length	tag length	description
AES-MAC 128/64	14	128	64	AES-MAC 128 bit key, 64-bit tag
AES-MAC 256/64	15	256	64	AES-MAC 256 bit key, 64-bit tag
AES-MAC 128/128	25	128	128	AES-MAC 128 bit key, 128-bit tag
AES-MAC 256/128	26	256	128	AES-MAC 256 bit key, 128-bit tag

Keys may be obtained either from a key structure or from a recipient structure. Implementations creating and validating MAC values MUST validate that the key type, key length and algorithm are correct and appropriate for the entities involved.

When using a COSE key for this algorithm, the following checks are made:

The 'kty' field MUST be present and it MUST be 'Symmetric'.
If the 'alg' field present, it MUST match the AES-MAC algorithm being used.
If the 'key_ops' field is present, it MUST include 'sign' when creating an AES-MAC authentication tag.
If the 'key_ops' field is present, it MUST include 'verify' when verifying an AES-MAC authentication tag.

9.2.1. Security Considerations

A number of attacks exist against CBC-MAC that need to be considered.

A single key must only be used for messages of a fixed and known length. If this is not the case, an attacker will be able to generate a message with a valid tag given two message, tag pairs. This can be addressed by using different keys for different length messages. The current structure mitigates this problem as a specific encoding structure which includes lengths is build and signed. (CMAC mode also addresses this issue.)
If the same key is used for both encryption and authentication operations, using CBC modes an attacker can produce messages with a valid authentication code.
If the IV can be modified, then messages can be forged. This is addressed by fixing the IV to all zeros.

10. Content Encryption Algorithms

Content Encryption Algorithms provide data confidentiality for potentially large blocks of data using a symmetric key. They provide integrity on the data that was encrypted, however they provide either no or very limited data origination. (One cannot, for example, be used to prove the identity of the sender to a third party.) The ability to provide data origination is linked to how the symmetric key is obtained.

COSE restricts the set of legal content encryption algorithms to those that support authentication both of the content and additional data. The encryption process will generate some type of authentication value, but that value may be either explicit or implicit in terms of the algorithm definition. For simplicity sake, the authentication code will normally be defined as being appended to the cipher text stream. The encryption functions are:

ciphertext = Encrypt(message content, key, additional data)

valid, message content = Decrypt(cipher text, key, additional data)

Most AEAD algorithms are logically defined as returning the message content only if the decryption is valid. Many but not all implementations will follow this convention. The message content MUST NOT be used if the decryption does not validate.

10.1. AES GCM

The GCM mode is a generic authenticated encryption block cipher mode defined in [AES-GCM]. The GCM mode is combined with the AES block encryption algorithm to define an AEAD cipher.

The GCM mode is parameterized with by the size of the authentication tag and the size of the nonce. This document fixes the size of the nonce at 96-bits. The size of the authentication tag is limited to a small set of values. For this document however, the size of the authentication tag is fixed at 128 bits.

The set of algorithms defined in this document are in Table 8.

Algorithm Value for AES-GCM
name	value	description
A128GCM	1	AES-GCM mode w/ 128-bit key, 128-bit tag
A192GCM	2	AES-GCM mode w/ 192-bit key, 128-bit tag
A256GCM	3	AES-GCM mode w/ 256-bit key, 128-bit tag

Keys may be obtained either from a key structure or from a recipient structure. Implementations encrypting and decrypting MUST validate that the key type, key length and algorithm are correct and appropriate for the entities involved.

When using a COSE key for this algorithm, the following checks are made:

The 'kty' field MUST be present and it MUST be 'Symmetric'.
If the 'alg' field present, it MUST match the AES-GCM algorithm being used.
If the 'key_ops' field is present, it MUST include 'encrypt' or 'key wrap' when encrypting.
If the 'key_ops' field is present, it MUST include 'decrypt' or 'key unwrap' when decrypting.

10.1.1. Security Considerations

When using AES-GCM, the following restrictions MUST be enforced:

The key and nonce pair MUST be unique for every message encrypted.
The total amount of data encrypted for a single key MUST NOT exceed 2^39 - 256 bits. An explicit check is required only in environments where it is expected that it might be exceeded.

Consideration was given to supporting smaller tag values, the constrained community would desire tag sizes in the 64-bit range. Doing show drastically changes both the maximum messages size (generally not an issue) and the number of times that a key can be used. Given that CCM is the usual mode for constrained environments restricted modes are not supported.

10.2. AES CCM

Counter with CBC-MAC (CCM) is a generic authentication encryption block cipher mode defined in [RFC3610]. The CCM mode is combined with the AES block encryption algorithm to define a commonly used content encryption algorithm used in constrained devices.

The CCM mode has two parameter choices. The first choice is M, the size of the authentication field. The choice of the value for M involves a trade-off between message expansion and the probably that an attacker can undetectably modify a message. The second choice is L, the size of the length field. This value requires a trade-off between the maximum message size and the size of the Nonce.

It is unfortunate that the specification for CCM specified L and M as a count of bytes rather than a count of bits. This leads to possible misunderstandings where AES-CCM-8 is frequently used to refer to a version of CCM mode where the size of the authentication is 64 bits and not 8 bits. These values have traditionally been specified as bit counts rather than byte counts. This document will follow the tradition of using bit counts so that it is easier to compare the different algorithms presented in this document.

We define a matrix of algorithms in this document over the values of L and M. Constrained devices are usually operating in situations where they use short messages and want to avoid doing recipient specific cryptographic operations. This favors smaller values of both L and M. Less constrained devices do will want to be able to user larger messages and are more willing to generate new keys for every operation. This favors larger values of L and M.

The following values are used for L:

16 bits (2): limits messages to 2^16 bytes (64 KiB) in length. This sufficiently long for messages in the constrained world. The nonce length is 13 bytes allowing for 2^(13*8) possible values of the nonce without repeating.
64 bits (8): limits messages to 2^64 bytes in length. The nonce length is 7 bytes allowing for 2^56 possible values of the nonce without repeating.

The following values are used for M:

64 bits (8): produces a 64-bit authentication tag. This implies that there is a 1 in 2^64 chance that a modified message will authenticate.
128 bits (16): produces a 128-bit authentication tag. This implies that there is a 1 in 2^128 chance that a modified message will authenticate.

Algorithm Values for AES-CCM
name	value	L	M	k	description
AES-CCM-16-64-128	10	16	64	128	AES-CCM mode 128-bit key, 64-bit tag, 13-byte nonce
AES-CCM-16-64-256	11	16	64	256	AES-CCM mode 256-bit key, 64-bit tag, 13-byte nonce
AES-CCM-64-64-128	12	64	64	128	AES-CCM mode 128-bit key, 64-bit tag, 7-byte nonce
AES-CCM-64-64-256	13	64	64	256	AES-CCM mode 256-bit key, 64-bit tag, 7-byte nonce
AES-CCM-16-128-128	30	16	128	128	AES-CCM mode 128-bit key, 128-bit tag, 13-byte nonce
AES-CCM-16-128-256	31	16	128	256	AES-CCM mode 256-bit key, 128-bit tag, 13-byte nonce
AES-CCM-64-128-128	32	64	128	128	AES-CCM mode 128-bit key, 128-bit tag, 7-byte nonce
AES-CCM-64-128-256	33	64	128	256	AES-CCM mode 256-bit key, 128-bit tag, 7-byte nonce

When using a COSE key for this algorithm, the following checks are made:

The 'kty' field MUST be present and it MUST be 'Symmetric'.
If the 'alg' field present, it MUST match the AES-CCM algorithm being used.
If the 'key_ops' field is present, it MUST include 'encrypt' or 'key wrap' when encrypting.
If the 'key_ops' field is present, it MUST include 'decrypt' or 'key unwrap' when decrypting.

10.2.1. Security Considerations

When using AES-CCM, the following restrictions MUST be enforced:

The key and nonce pair MUST be unique for every message encrypted.
The total number of times the AES block cipher is used MUST NOT exceed 2^61 operations. This limitation is the sum of times the block cipher is used in computing the MAC value and in performing stream encryption operations. An explicit check is required only in environments where it is expected that it might be exceeded.

[RFC3610] additionally calls out one other consideration of note. It is possible to do a pre-computation attack against the algorithm in cases where the portions encryption content is highly predictable. This reduces the security of the key size by half. Ways to deal with this attack include adding a random portion to the nonce value and/or increasing the key size used. Using a portion of the nonce for a random value will decrease the number of messages that a single key can be used for. Increasing the key size may require more resources in the constrained device. See sections 5 and 10 of [RFC3610] for more information.

10.3. ChaCha20 and Poly1305

ChaCha20 and Poly1305 combined together is a new AEAD mode that is defined in [RFC7539]. This is a new algorithm defined to be a cipher that is not AES and thus would not suffer from any future weaknesses found in AES. These cryptographic functions are designed to be fast in software-only implementations.

The ChaCha20/Poly1305 AEAD construction defined in [RFC7539] has no parameterization. It takes a 256-bit key and a 96-bit nonce as well as the plain text and additional data as inputs and produces the cipher text as an option. We define one algorithm identifier for this algorithm in Table 10.

Algorithm Value for AES-GCM
name	value	description
ChaCha20/Poly1305	24	ChaCha20/Poly1305 w/ 256-bit key, 128-bit tag

When using a COSE key for this algorithm, the following checks are made:

The 'kty' field MUST be present and it MUST be 'Symmetric'.
If the 'alg' field present, it MUST match the ChaCha algorithm being used.
If the 'key_ops' field is present, it MUST include 'encrypt' or 'key wrap' when encrypting.
If the 'key_ops' field is present, it MUST include 'decrypt' or 'key unwrap' when decrypting.

10.3.1. Security Considerations

The pair of key, nonce MUST be unique for every invocation of the algorithm. Nonce counters are considered to be an acceptable way of ensuring that they are unique.

11. Key Derivation Functions (KDF)

Key Derivation Functions (KDFs) are used to take some secret value and generate a different one. The secret value comes in three flavors:

Secrets that are uniformly random: This is the type of secret which is created by a good random number generator.
Secrets that are not uniformly random: This is type of secret which is created by operations like key agreement.
Secrets that are not random: This is the type of secret that people generate for things like passwords.

General KDF functions work well with the first type of secret, can do reasonable well with the second type of secret and generally do poorly with the last type of secret. None of the KDF functions in this section are designed to deal with the type of secrets that are used for passwords. Functions like PBSE2 [RFC2898] need to be used for that type of secret.

The same KDF function can be setup to deal with the first two types of secrets different. The KDF function defined in Section 11.1 is such a function. This is reflected in the set of algorithms defined for HKDF.

When using KDF functions, one component that is included is context information. Context information is used to allow for different keying information to be derived from the same secret. The use of context based keying material is considered to be a good security practice. This document defines a single context structure and a single KDF function.

11.1. HMAC-based Extract-and-Expand Key Derivation Function (HKDF)

The HKDF key derivation algorithm is defined in [RFC5869].

The HKDF algorithm takes these inputs:

secret - a shared value that is secret. Secrets may be either previously shared or derived from operations like a DH key agreement.
salt - an optional value that is used to change the generation process. The salt value can be either public or private. If the salt is public and carried in the message, then the 'salt' algorithm header parameter defined in Table 12 is used. While [RFC5869] suggests that the length of the salt be the same as the length of the underlying hash value, any amount of salt will improve the security as different key values will be generated. This parameter is protected by being included in the key computation and does not need to be separately authenticated. The salt value does not need to be unique for every message sent.
length - the number of bytes of output that need to be generated.
context information - Information that describes the context in which the resulting value will be used. Making this information specific to the context that the material is going to be used ensures that the resulting material will always be tied to the context. The context structure used is encoded into the algorithm identifier.
PRF - The underlying pseudo-random function to be used in the HKDF algorithm. The PRF is encoded into the HKDF algorithm selection. u

HKDF is defined to use HMAC as the underlying PRF. However, it is possible to use other functions in the same construct to provide a different KDF function that is more appropriate in the constrained world. Specifically, one can use AES-CBC-MAC as the PRF for the expand step, but not for the extract step. When using a good random shared secret of the correct length, the extract step can be skipped. For the AES algorithm versions, the extract step is always skipped.

The extract step cannot be skipped if the secret is not uniformly random, for example if it is the result of an ECDH key agreement step. (This implies that the AES HKDF version cannot be used with ECDH.) If the extract step is skipped, the 'salt' value is not used as part of the HKDF functionality.

The algorithms defined in this document are found in Table 11.

HKDF algorithms
name	PRF	context
HKDF SHA-256	HMAC with SHA-256	HKDF using HMAC SHA-256 as the PRF
HKDF SHA-512	HMAC with SHA-512	HKDF using HMAC SHA-512 as the PRF
HKDF AES-MAC-128	AES-CBC-MAC-128	HKDF using AES-MAC as the PRF w/ 128-bit key
HKDF AES-MAC-256	AES-CBC-MAC-256	HKDF using AES-MAC as the PRF w/ 256-bit key

HKDF Algorithm Parameters
name	label	type	description
salt	-20	bstr	Random salt

11.2. Context Information Structure

The context information structure is used to ensure that the derived keying material is "bound" to the context of the transaction. The context information structure used here is based on that defined in [SP800-56A]. By using CBOR for the encoding of the context information structure, we automatically get the same type and length separation of fields that is obtained by the use of ASN.1. This means that there is no need to encode the lengths for the base elements as it is done by the encoding used in JOSE (Section 4.6.2 of [RFC7518]).

The context information structure refers to PartyU and PartyV as the two parties which are doing the key derivation. Unless the application protocol defines differently, we assign PartyU to the entity that is creating the message and PartyV to the entity that is receiving the message. By doing this association, different keys will be derived for each direction as the context information is different in each direction.

The context structure is built from information that is known to both entities. This information can be obtained from a variety of sources:

Fields can be define by the application. This is commonly used to assign fixed names to parties, but can be used for other items such as nonces.
Fields can be defined by usage of the output. Examples of this are the algorithm and key size that are being generated.
Fields can be defined by parameters from the message. We define a set of parameters in Table 13 which can be used to carry the values associated with the context structure. Examples of this are identities and nonce values. These parameters are designed to be placed in the unprotected bucket of the recipient structure. (They do not need to be in the protected bucket since they already are included in the cryptographic computation by virtue of being included in the context structure.)

Context Algorithm Parameters
name	label	type	description
PartyU identity	-21	bstr	Party U identity Information
PartyU nonce	-22	bstr / int	Party U provided nonce
PartyU other	-23	bstr	Party U other provided information
PartyV identity	-24	bstr	Party V identity Information
PartyV nonce	-25	bstr / int	Party V provided nonce
PartyV other	-26	bstr	Party V other provided information

We define a CBOR object to hold the context information. This object is referred to as CBOR_KDF_Context. The object is based on a CBOR array type. The fields in the array are:

AlgorithmID

This field indicates the algorithm for which the key material will be used. This field is required to be present. The field exists in the context information so that if the same environment is used for different algorithms, then completely different keys will be generated each of those algorithms. (This practice means if algorithm A is broken and thus can is easier to find, the key derived for algorithm B will not be the same as the key for algorithm B.)

PartyUInfo

This field holds information about party U. The PartyUInfo is encoded as a CBOR array. The elements of PartyUInfo are encoded in the order presented, however if the element does not exist no element is placed in the array. The elements of the PartyUInfo array are:

identity: This contains the identity information for party U. The identities can be assigned in one of two manners. Firstly, a protocol can assign identities based on roles. For example, the roles of "client" and "server" may be assigned to different entities in the protocol. Each entity would then use the correct label for the data they send or receive. The second way is for a protocol to assign identities is to use a name based on a naming system (i.e. DNS, X.509 names).
We define an algorithm parameter 'PartyU identity' that can be used to carry identity information in the message. However, identity information is often known as part of the protocol and can thus be inferred rather than made explicit. If identity information is carried in the message, applications SHOULD have a way of validating the supplied identity information. The identity information does not need to be specified and can be left as absent.
nonce: This contains a nonce value. The nonce can either be implicit from the protocol or carried as a value in the unprotected headers.
We define an algorithm parameter 'PartyU nonce' that can be used to carry this value in the message However, the nonce value could be determined by the application and the value determined from elsewhere.
This item is optional and can be absent.
other: This contains other information that is defined by the protocol.
This item is optional and can be absent.

PartyVInfo

This field holds information about party V. The PartyVInfo is encoded as a CBOR array. For store and forward environments, the party V information may be minimal or even absent. The elements of PartyVInfo are encoded in the order presented, however if the element does not exist no element is placed in the array. The elements of the PartyVInfo array are:

identity: See description of PartyUInfo identity.
nonce: See description of PartyUInfo nonce.
other: See description of PartyUInfo other.

SuppPubInfo

This field contains public information that is mutually known to both parties.

keyDataLength: This is set to the number of bits of the desired output value. (This practice means if algorithm A can use two different key lengths, the key derived for longer key size will not contain the key for shorter key size as a prefix.)
protected: This field contains the protected parameter field. If there are no elements in the protected field, then use a zero length bstr.
other: The field other is for free form data defined by the application. An example is that an application could defined two different strings to be placed here to generate different keys for a data stream vs a control stream. This field is optional and will only be present if the application defines a structure for this information. Applications that define this SHOULD use CBOR to encode the data so that types and lengths are correctly include.

SuppPrivInfo

This field contains private information that is mutually known information. An example of this information would be a pre-existing shared secret. (This could for example, be used in combination with an ECDH key agreement to provide a secondary proof of identity.) The field is optional and will only be present if the application defines a structure for this information. Applications that define this SHOULD use CBOR to encode the data so that types and lengths are correctly included.

The following CDDL fragment corresponds to the text above.

PartyInfo = (
    ? nonce : bstr / int,
    ? identity : bstr,
    ? other : bstr,
)

COSE_KDF_Context = [
    AlgorithmID : int / tstr,
    PartyUInfo : [ PartyInfo ],
    PartyVInfo : [ PartyInfo ],
    SuppPubInfo : [
        keyDataLength : uint,
        protected : bstr,
        ? other : bstr
    ],
    ? SuppPrivInfo : bstr
]

12. Recipient Algorithm Classes

Recipient algorithms can be defined into a number of different classes. COSE has the ability to support many classes of recipient algorithms. In this section, a number of classes are listed and then a set of algorithms are specified for each of the classes. The names of the recipient algorithm classes used here are the same as are defined in [RFC7516]. Other specifications use different terms for the recipient algorithm classes or do not support some of the recipient algorithm classes.

12.1. Direct Encryption

The direct encryption class algorithms share a secret between the sender and the recipient that is used either directly or after manipulation as the content key. When direct encryption mode is used, it MUST be the only mode used on the message.

The COSE_Enveloped structure for the recipient is organized as follows:

The 'protected' field MUST be a zero length item unless it is used in the computation of the content key.
The 'alg' parameter MUST be present.
A parameter identifying the shared secret SHOULD be present.
The 'ciphertext' field MUST be a zero length item.
The 'recipients' field MUST be absent.

12.1.1. Direct Key

This recipient algorithm is the simplest, the identified key is directly used as the key for the next layer down in the message. There are no algorithm parameters defined for this algorithm. The algorithm identifier value is assigned in Table 14.

When this algorithm is used, the protected field MUST be zero length. The key type MUST be 'Symmetric'.

Direct Key
name	value	description
direct	-6	Direct use of CEK

12.1.1.1. Security Considerations

This recipient algorithm has several potential problems that need to be considered:

These keys need to have some method to be regularly updated over time. All of the content encryption algorithms specified in this document have limits on how many times a key can be used without significant loss of security.
These keys need to be dedicated to a single algorithm. There have been a number of attacks developed over time when a single key is used for multiple different algorithms. One example of this is the use of a single key both for CBC encryption mode and CBC-MAC authentication mode.
Breaking one message means all messages are broken. If an adversary succeeds in determining the key for a single message, then the key for all messages is also determined.

12.1.2. Direct Key with KDF

These recipient algorithms take a common shared secret between the two parties and applies the HKDF function (Section 11.1) using the context structure defined in Section 11.2 to transform the shared secret into the necessary key. The 'protected' field can be of non-zero length. Either the 'salt' parameter of HKDF or the partyU 'nonce' parameter of the context structure MUST be present. The salt/nonce parameter can be generated either randomly or deterministically. The requirement is that it be a unique value for the key/IV pair in question.

If the salt/nonce value is generated randomly, then it is suggested that the length of the random value be the same length as the hash function underlying HKDF. While there is no way to guarantee that it will be unique, there is a high probability that it will be unique. If the salt/nonce value is generated deterministically, it can be guaranteed to be unique and thus there is no length requirement.

A new IV must be used if the same key is used in more than one message. The IV can be modified in a predictable manner, a random manner or an unpredictable manner. One unpredictable manner that can be used is to use the HKDF function to generate the IV. If HKDF is used for generating the IV, the algorithm identifier is set to "IV-GENERATION".

When these algorithms are used, the key type MUST be 'symmetric'.

The set of algorithms defined in this document can be found in Table 15.

Direct Key
name	value	KDF	description
direct+HKDF-SHA-256	-10	HKDF SHA-256	Shared secret w/ HKDF and SHA-256
direct+HKDF-SHA-512	-11	HKDF SHA-512	Shared secret w/ HKDF and SHA-512
direct+HKDF-AES-128	-12	HKDF AES-MAC-128	Shared secret w/ AES-MAC 128-bit key
direct+HKDF-AES-256	-13	HKDF AES-MAC-256	Shared secret w/ AES-MAC 256-bit key

When using a COSE key for this algorithm, the following checks are made:

The 'kty' field MUST be present and it MUST be 'Symmetric'.
If the 'alg' field present, it MUST match the KDF algorithm being used.
If the 'key_ops' field is present, it MUST include 'deriveKey or 'deriveBits'.

12.1.2.1. Security Considerations

The shared secret needs to have some method to be regularly updated over time. The shared secret forms the basis of trust. Although not used directly, it should still be subject to scheduled rotation.

12.2. Key Wrapping

In key wrapping mode, the CEK is randomly generated and that key is then encrypted by a shared secret between the sender and the recipient. All of the currently defined key wrapping algorithms for COSE are AE algorithms. Key wrapping mode is considered to be superior to direct encryption if the system has any capability for doing random key generation. This is because the shared key is used to wrap random data rather than data has some degree of organization and may in fact be repeating the same content. The use of Key Wrapping loses the weak data origination that is provided by the direct encryption algorithms.

The COSE_Enveloped structure for the recipient is organized as follows:

The 'protected' field MUST be absent if the key wrap algorithm is an AE algorithm.
The 'recipients' field is normally absent, but can be used. Applications MUST deal with a recipient field present, not being able to decrypt that recipient is an acceptable way of dealing with it. Failing to process the message is not an acceptable way of dealing with it.
The plain text to be encrypted is the key from next layer down (usually the content layer).
At a minimum, the 'unprotected' field MUST contain the 'alg' parameter and SHOULD contain a parameter identifying the shared secret.

12.2.1. AES Key Wrapping

The AES Key Wrapping algorithm is defined in [RFC3394]. This algorithm uses an AES key to wrap a value that is a multiple of 64 bits. As such, it can be used to wrap a key for any of the content encryption algorithms defined in this document. The algorithm requires a single fixed parameter, the initial value. This is fixed to the value specified in Section 2.2.3.1 of [RFC3394]. There are no public parameters that vary on a per invocation basis. The protected header field MUST be empty.

Keys may be obtained either from a key structure or from a recipient structure. If the key obtained from a key structure, the key type MUST be 'Symmetric'. Implementations encrypting and decrypting MUST validate that the key type, key length and algorithm are correct and appropriate for the entities involved.

When using a COSE key for this algorithm, the following checks are made:

The 'kty' field MUST be present and it MUST be 'Symmetric'.
If the 'alg' field present, it MUST match the AES Key Wrap algorithm being used.
If the 'key_ops' field is present, it MUST include 'encrypt' or 'key wrap' when encrypting.
If the 'key_ops' field is present, it MUST include 'decrypt' or 'key unwrap' when decrypting.

AES Key Wrap Algorithm Values
name	value	key size	description
A128KW	-3	128	AES Key Wrap w/ 128-bit key
A192KW	-4	192	AES Key Wrap w/ 192-bit key
A256KW	-5	256	AES Key Wrap w/ 256-bit key

12.2.1.1. Security Considerations for AES-KW

The shared secret need to have some method to be regularly updated over time. The shared secret is the basis of trust.

12.3. Key Encryption

Key Encryption mode is also called key transport mode in some standards. Key Encryption mode differs from Key Wrap mode in that it uses an asymmetric encryption algorithm rather than a symmetric encryption algorithm to protect the key. This document does not define any Key Encryption mode algorithms.

When using a key encryption algorithm, the COSE_Enveloped structure for the recipient is organized as follows:

The 'protected' field MUST be absent.
The plain text to be encrypted is the key from next layer down (usually the content layer).
At a minimum, the 'unprotected' field MUST contain the 'alg' parameter and SHOULD contain a parameter identifying the asymmetric key.

12.4. Direct Key Agreement

The 'direct key agreement' class of recipient algorithms uses a key agreement method to create a shared secret. A KDF is then applied to the shared secret to derive a key to be used in protecting the data. This key is normally used as a CEK or MAC key, but could be used for other purposes if more than two layers are in use (see Appendix B).

The most commonly used key agreement algorithm is Diffie-Hellman, but other variants exist. Since COSE is designed for a store and forward environment rather than an on-line environment, many of the DH variants cannot be used as the receiver of the message cannot provide any dynamic key material. One side-effect of this is that perfect forward secrecy (see [RFC4949]) is not achievable. A static key will always be used for the receiver of the COSE message.

Two variants of DH that are supported are:

Ephemeral-Static DH: where the sender of the message creates a one time DH key and uses a static key for the recipient. The use of the ephemeral sender key means that no additional random input is needed as this is randomly generated for each message.
Static-Static DH: where a static key is used for both the sender and the recipient. The use of static keys allows for recipient to get a weak version of data origination for the message. When static-static key agreement is used, then some piece of unique data for the KDF is required to ensure that a different key is created for each message.

When direct key agreement mode is used, there MUST be only one recipient in the message. This method creates the key directly and that makes it difficult to mix with additional recipients. If multiple recipients are needed, then the version with key wrap needs to be used.

The COSE_Enveloped structure for the recipient is organized as follows:

At a minimum, headers MUST contain the 'alg' parameter and SHOULD contain a parameter identifying the recipient's asymmetric key.
The headers SHOULD identify the senders key for the static-static versions and MUST contain the senders ephemeral key for the ephemeral-static versions.

12.4.1. ECDH

The mathematics for Elliptic Curve Diffie-Hellman can be found in [RFC6090].

ECDH is parameterized by the following:

Curve Type/Curve: The curve selected controls not only the size of the shared secret, but the mathematics for computing the shared secret. The curve selected also controls how a point in the curve is represented and what happens for the identity points on the curve. In this specification, we allow for a number of different curves to be used. A set of curves are defined in Table 20.
Since the only the math is changed by changing the curve, the curve is not fixed for any of the algorithm identifiers we define. Instead, it is defined by the points used.
Ephemeral-static or static-static: The key agreement process may be done using either a static or an ephemeral key for the sender's side. When using ephemeral keys, the sender MUST generate a new ephemeral key for every key agreement operation. The ephemeral key is placed in the 'ephemeral key' parameter and MUST be present for all algorithm identifiers that use ephemeral keys. When using static keys, the sender MUST either generate a new random value or otherwise create a unique value to be placed in either in the KDF parameters or the context structure. For the KDF functions used, this means either in the 'salt' parameter for HKDF (Table 12) or in the 'PartyU nonce' parameter for the context structure (Table 13) MUST be present. (Both may be present if desired.) The value in the parameter MUST be unique for the pair of keys being used. It is acceptable to use a global counter that is incremented for every static-static operation and use the resulting value. When using static keys, the static key should be identified to the recipient. The static key can be identified either by providing the key ('static key') or by providing a key identifier for the static key ('static key id'). Both of these parameters are defined in Table 18
Key derivation algorithm: The result of an ECDH key agreement process does not provide a uniformly random secret. As such, it needs to be run through a KDF in order to produce a usable key. Processing the secret through a KDF also allows for the introduction of both context material, how the key is going to be used, and one time material in the event of a static-static key agreement.
Key Wrap algorithm: No key wrap algorithm is used. This is represented in Table 17 as 'none'. The key size for the context structure is the content layer encryption algorithm size.

The set of direct ECDH algorithms defined in this document are found in Table 17.

ECDH Algorithm Values
name	value	KDF	Ephemeral- Static	Key Wrap	description
ECDH-ES + HKDF-256	-25	HKDF - SHA-256	yes	none	ECDH ES w/ HKDF - generate key directly
ECDH-ES + HKDF-512	-26	HKDF - SHA-512	yes	none	ECDH ES w/ HKDF - generate key directly
ECDH-SS + HKDF-256	-27	HKDF - SHA-256	no	none	ECDH SS w/ HKDF - generate key directly
ECDH-SS + HKDF-512	-28	HKDF - SHA-512	no	none	ECDH SS w/ HKDF - generate key directly
ECDH-ES + A128KW	-29	HKDF - SHA-256	yes	A128KW	ECDH ES w/ Concat KDF and AES Key wrap w/ 128 bit key
ECDH-ES + A192KW	-30	HKDF - SHA-256	yes	A192KW	ECDH ES w/ Concat KDF and AES Key wrap w/ 192 bit key
ECDH-ES + A256KW	-31	HKDF - SHA-256	yes	A256KW	ECDH ES w/ Concat KDF and AES Key wrap w/ 256 bit key
ECDH-SS + A128KW	-32	HKDF - SHA-256	no	A128KW	ECDH SS w/ Concat KDF and AES Key wrap w/ 128 bit key
ECDH-SS + A192KW	-33	HKDF - SHA-256	no	A192KW	ECDH SS w/ Concat KDF and AES Key wrap w/ 192 bit key
ECDH-SS + A256KW	-34	HKDF - SHA-256	no	A256KW	ECDH SS w/ Concat KDF and AES Key wrap w/ 256 bit key

ECDH Algorithm Parameters
name	label	type	algorithm	description
ephemeral key	-1	COSE_Key	ECDH-ES	Ephemeral Public key for the sender
static key	-2	COSE_Key	ECDH-ES	Static Public key for the sender
static key id	-3	bstr	ECDH-SS	Static Public key identifier for the sender

This document defines these algorithms to be used with the curves P-256, P-384, P-521. Implementations MUST verify that the key type and curve are correct. Different curves are restricted to different key types. Implementations MUST verify that the curve and algorithm are appropriate for the entities involved.

When using a COSE key for this algorithm, the following checks are made:

The 'kty' field MUST be present and it MUST be 'EC2'.
If the 'alg' field present, it MUST match the Key Agreement algorithm being used.
If the 'key_ops' field is present, it MUST include 'derive key' or 'derive bits' for the private key.
If the 'key_ops' field is present, it MUST be empty for the public key.

12.5. Key Agreement with KDF

Key Agreement with Key Wrapping uses a randomly generated CEK. The CEK is then encrypted using a Key Wrapping algorithm and a key derived from the shared secret computed by the key agreement algorithm.

The COSE_Enveloped structure for the recipient is organized as follows:

The 'protected' field is fed into the KDF context structure.
The plain text to be encrypted is the key from next layer down (usually the content layer).
The 'alg' parameter MUST be present in the layer.
A parameter identifying the recipient's key SHOULD be present. A parameter identifying the sender's key SHOULD be present.

12.5.1. ECDH

These algorithms are defined in Table 17.

ECDH with Key Agreement is parameterized by the same parameters as for ECDH Section 12.4.1 with the following modifications:

Key Wrap Algorithm: Any of the key wrap algorithms defined in Section 12.2.1 are supported. The size of the key used for the key wrap algorithm is fed into the KDF function. The set of identifiers are found in Table 17.

When using a COSE key for this algorithm, the following checks are made:

The 'kty' field MUST be present and it MUST be 'EC2'.
If the 'alg' field present, it MUST match the Key Agreement algorithm being used.
If the 'key_ops' field is present, it MUST include 'derive key' or 'derive bits' for the private key.
If the 'key_ops' field is present, it MUST be empty for the public key.

13. Keys

The COSE_Key object defines a way to hold a single key object. It is still required that the members of individual key types be defined. This section of the document is where we define an initial set of members for specific key types.

For each of the key types, we define both public and private members. The public members are what is transmitted to others for their usage. We define private members mainly for the purpose of archival of keys by individuals. However, there are some circumstances in which private keys may be distributed to entities in a protocol. Examples include: entities that have poor random number generation, centralized key creation for multi-cast type operations, and protocols in which a shared secret is used as a bearer token for authorization purposes.

Key types are identified by the 'kty' member of the COSE_Key object. In this document, we define four values for the member:

Key Type Values
name	value	description
EC2	2	Elliptic Curve Keys w/ X,Y Coordinate pair
Symmetric	4	Symmetric Keys
Reserved	0	This value is reserved

13.1. Elliptic Curve Keys

Two different key structures could be defined for Elliptic Curve keys. One version uses both an x and a y coordinate, potentially with point compression. This is the traditional EC point representation that is used in [RFC5480]. The other version uses only the x coordinate as the y coordinate is either to be recomputed or not needed for the key agreement operation. Currently no algorithms are defined using this key structure.

EC Curves
name	key type	value	description
P-256	EC2	1	NIST P-256 also known as secp256r1
P-384	EC2	2	NIST P-384 also known as secp384r1
P-521	EC2	3	NIST P-521 also known as secp521r1

13.1.1. Double Coordinate Curves

The traditional way of sending EC curves has been to send either both the x and y coordinates, or the x coordinate and a sign bit for the y coordinate. The latter encoding has not been recommended in the IETF due to potential IPR issues. However, for operations in constrained environments, the ability to shrink a message by not sending the y coordinate is potentially useful.

For EC keys with both coordinates, the 'kty' member is set to 2 (EC2). The key parameters defined in this section are summarized in Table 21. The members that are defined for this key type are:

crv: contains an identifier of the curve to be used with the key. The curves defined in this document for this key type can be found in Table 20. Other curves may be registered in the future and private curves can be used as well.
x: contains the x coordinate for the EC point. The integer is converted to an octet string as defined in [SEC1]. Leading zero octets MUST be preserved.
y: contains either the sign bit or the value of y coordinate for the EC point. When encoding the value y, the integer is converted to an octet string (as defined in [SEC1]) and encoded as a CBOR bstr. Leading zero octets MUST be preserved. The compressed point encoding is also supported. Compute the sign bit as laid out in the Elliptic-Curve-Point-to-Octet-String Conversion function of [SEC1]. If the sign bit is zero, then encode y as a CBOR false value, otherwise encode y as a CBOR true value. The encoding of the infinity point is not supported.
d: contains the private key.

For public keys, it is REQUIRED that 'crv', 'x' and 'y' be present in the structure. For private keys, it is REQUIRED that 'crv' and 'd' be present in the structure. For private keys, it is RECOMMENDED that 'x' and 'y' also be present, but they can be recomputed from the required elements and omitting them saves on space.

EC Key Parameters
name	key type	value	type	description
crv	2	-1	int / tstr	EC Curve identifier - Taken from the COSE Curve Registry
x	2	-2	bstr	X Coordinate
y	2	-3	bstr / bool	Y Coordinate
d	2	-4	bstr	Private key

13.2. Symmetric Keys

Occasionally it is required that a symmetric key be transported between entities. This key structure allows for that to happen.

For symmetric keys, the 'kty' member is set to 3 (Symmetric). The member that is defined for this key type is:

k: contains the value of the key.

This key structure contains only private key information, care must be taken that it is never transmitted accidentally. For public keys, there are no required fields. For private keys, it is REQUIRED that 'k' be present in the structure.

Symmetric Key Parameters
name	key type	value	type	description
k	4	-1	bstr	Key Value

14. CBOR Encoder Restrictions

There has been an attempt to limit the number of places where the document needs to impose restrictions on how the CBOR Encoder needs to work. We have managed to narrow it down to the following restrictions:

The restriction applies to the encoding the Sig_structure, the Enc_structure, and the MAC_structure.
The rules for Canonical CBOR (Section 3.9 of RFC 7049) MUST be used in these locations. The main rule that needs to be enforced is that all lengths in these structures MUST be encoded such that they are encoded using definite lengths and the minimum length encoding is used.
Applications MUST NOT generate messages with the same label used twice as a key in a single map. Applications MUST NOT parse and process messages with the same label used twice as a key in a single map. Applications can enforce the parse and process requirement by using parsers that will fail the parse step or by using parsers that will pass all keys to the application and the application can perform the check for duplicate keys.

15. Application Profiling Considerations

This document is designed to provide a set of security services, but not to provide implementation requirements for specific usage. The interoperability requirements are provided for how each of the individual services are used and how the algorithms are to be used for interoperability. The requirements about which algorithms and which services are needed is deferred to each application.

Applications are therefore intended to profile the usage of this document. This section provides a set of guidelines and topics that applications need to consider when using this document.

Applications need to determine the set of messages defined in this document that they will be using. The set of messages corresponds fairly directly to the set of security services that are needed and to the security levels needed.
Applications may define new header parameters for a specific purpose. Applications will often times select specific header parameters to use or not to use. For example, an application would normally state a preference for using either the IV or the partial IV parameter. If the partial IV parameter is specified, then the application would also need to define how the fixed portion of the IV would be determined.
When applications use externally defined authenticated data, they need to define how that data is encoded. This document assumes that the data will be provided as a byte stream. More information can be found in Section 4.3.
Applications need to determine the set of security algorithms that are to be used. When selecting the algorithms to be used as the mandatory to implement set, consideration should be given to choosing different types of algorithms when two are chosen for a specific purpose. An example of this would be choosing HMAC-SHA512 and AES-CMAC as different MAC algorithms, the construction is vastly different between these two algorithms. This means that a weakening of one algorithm would be unlikely to lead to a weakening of the other algorithms. Of course, these algorithms do not provide the same level of security and thus may not be comparable for the desired security functionality.
Applications may need to provide some type of negotiation or discovery method if multiple algorithms or message structures are permitted. The method can be as simple as requiring preconfiguration of the set of algorithms to providing a discovery method built into the protocol. S/MIME provided a number of different ways to approach the problem that applications could follow:
- Advertising in the message (S/MIME capabilities) [RFC5751].
- Advertising in the certificate (capabilities extension) [RFC4262].
- Minimum requirements for the S/MIME, which have been updated over time [RFC2633][RFC5751].

16. IANA Considerations

16.1. CBOR Tag assignment

It is requested that IANA assign the following tags from the "Concise Binary Object Representation (CBOR) Tags" registry. It is requested that the tags for COSE_Sign1, COSE_Encrypted and COSE_Mac0 be assigned in the 1 to 23 value range (i.e. one byte long when encoded). It is requested that the rest of the tags be assigned in the 24 to 255 value range (i.e. two bytes long when encoded).

The tags to be assigned are in table Table 1.

16.2. COSE Header Parameter Registry

It is requested that IANA create a new registry entitled "COSE Header Parameters". The registry is to be created as Expert Review Required. Expert review guidelines are provided in Section 16.10

The columns of the registry are:

name: The name is present to make it easier to refer to and discuss the registration entry. The value is not used in the protocol. Names are to be unique in the table.
label: This is the value used for the label. The label can be either an integer or a string. Registration in the table is based on the value of the label requested. Integer values between 1 and 255 and strings of length 1 are designated as Standards Track Document required. Integer values from 256 to 65535 and strings of length 2 are designated as Specification Required. Integer values of greater than 65535 and strings of length greater than 2 are designated as first come, first served. Integer values in the range -1 to -65536 are delegated to the "COSE Header Algorithm Label" registry. Integer values beyond -65536 are marked as private use.
value: This contains the CBOR type for the value portion of the label.
value registry: This contains a pointer to the registry used to contain values where the set is limited.
description: This contains a brief description of the header field.
specification: This contains a pointer to the specification defining the header field (where public).

The initial contents of the registry can be found in Table 2. The specification column for all rows in that table should be this document.

Additionally, the label of 0 is to be marked as 'Reserved'.

16.3. COSE Header Algorithm Label Table

It is requested that IANA create a new registry entitled "COSE Header Algorithm Labels". The registry is to be created as Expert Review Required. Expert review guidelines are provided in Section 16.10

The columns of the registry are:

name: The name is present to make it easier to refer to and discuss the registration entry. The value is not used in the protocol.
algorithm: The algorithm(s) that this registry entry is used for. This value is taken from the "COSE Algorithm Value" registry. Multiple algorithms can be specified in this entry. For the table, the algorithm, label pair MUST be unique.
label: This is the value used for the label. The label is an integer in the range of -1 to -65536.
value: This contains the CBOR type for the value portion of the label.
value registry: This contains a pointer to the registry used to contain values where the set is limited.
description: This contains a brief description of the header field.
specification: This contains a pointer to the specification defining the header field (where public).

The initial contents of the registry can be found in Table 12, Table 13, and Table 18. The specification column for all rows in that table should be this document.

16.4. COSE Algorithm Registry

It is requested that IANA create a new registry entitled "COSE Algorithm Registry". The registry is to be created as Expert Review Required. Expert review guidelines are provided in Section 16.10

The columns of the registry are:
value: The value to be used to identify this algorithm. Algorithm values MUST be unique. The value can be a positive integer, a negative integer or a string. Integer values between -256 and 255 and strings of length 1 are designated as Standards Track Document required. Integer values from -65536 to 65535 and strings of length 2 are designated as Specification Required. Integer values of greater than 65535 and strings of length greater than 2 are designated as first come, first served. Integer values beyond -65536 are marked as private use.
description: A short description of the algorithm.
specification: A document where the algorithm is defined (if publicly available).

The initial contents of the registry can be found in Table 9, Table 8, Table 10, Table 5, Table 6, Table 7, Table 14, Table 15, Table 16, and Table 17. The specification column for all rows in that table should be this document.

NOTE: The assignment of algorithm identifiers in this document was done so that positive numbers were used for the first level objects (COSE_Sign, COSE_Sign1, COSE_Enveloped, COSE_Encrypted, COSE_Mac and COSE_Mac0). Negative numbers were used for second level objects (COSE_Signature and COSE_recipient). Expert reviewers should consider this practice, but are not expected to be restricted by this precedent.

16.5. COSE Key Common Parameter Registry

It is requested that IANA create a new registry entitled "COSE Key Common Parameter" Registry. The registry is to be created as Expert Review Required. Expert review guidelines are provided in Section 16.10

The columns of the registry are:

name: This is a descriptive name that enables easier reference to the item. It is not used in the encoding.
label: The value to be used to identify this algorithm. Key map labels MUST be unique. The label can be a positive integer, a negative integer or a string. Integer values between 0 and 255 and strings of length 1 are designated as Standards Track Document required. Integer values from 256 to 65535 and strings of length 2 are designated as Specification Required. Integer values of greater than 65535 and strings of length greater than 2 are designated as first come, first served. Integer values in the range -1 to -65536 are used for key parameters specific to a single algorithm delegated to the "COSE Key Type Parameter Label" registry. Integer values beyond -65536 are marked as private use.
CBOR Type: This field contains the CBOR type for the field
registry: This field denotes the registry that values come from, if one exists.
description: This field contains a brief description for the field
specification: This contains a pointer to the public specification for the field if one exists

This registry will be initially populated by the values in Section 7.1. The specification column for all of these entries will be this document.

16.6. COSE Key Type Parameter Registry

It is requested that IANA create a new registry "COSE Key Type Parameters". The registry is to be created as Expert Review Required. Expert review guidelines are provided in Section 16.10

The columns of the table are:

key type: This field contains a descriptive string of a key type. This should be a value that is in the COSE General Values table and is placed in the 'kty' field of a COSE Key structure.
name: This is a descriptive name that enables easier reference to the item. It is not used in the encoding.
label: The label is to be unique for every value of key type. The range of values is from -256 to -1. Labels are expected to be reused for different keys.
CBOR type: This field contains the CBOR type for the field
description: This field contains a brief description for the field
specification: This contains a pointer to the public specification for the field if one exists

This registry will be initially populated by the values in Table 21 and Table 22. The specification column for all of these entries will be this document.

16.7. COSE Elliptic Curve Registry

It is requested that IANA create a new registry "COSE Elliptic Curve Parameters". The registry is to be created as Expert Review Required. Expert review guidelines are provided in Section 16.10

The columns of the table are:

name: This is a descriptive name that enables easier reference to the item. It is not used in the encoding.
value: This is the value used to identify the curve. These values MUST be unique. The integer values from -256 to 255 are designated as Standards Track Document Required. The integer values from 256 to 65535 and -65536 to -257 are designated as Specification Required. Integer values over 65535 are designated as first come, first served. Integer values less than -65536 are marked as private use.
key type: This designates the key type(s) that can be used with this curve.
description: This field contains a brief description of the curve.
specification: This contains a pointer to the public specification for the curve if one exists.

This registry will be initially populated by the values in Table 19. The specification column for all of these entries will be this document.

16.8. Media Type Registrations

16.8.1. COSE Security Message

This section registers the "application/cose" media type in the "Media Types" registry. These media types are used to indicate that the content is a COSE_MSG.

Type name: application
Subtype name: cose
Required parameters: N/A
Optional parameters: cose-type
Encoding considerations: binary
Security considerations: See the Security Considerations section of RFC TBD.
Interoperability considerations: N/A
Published specification: RFC TBD
Applications that use this media type: To be identified
Fragment identifier considerations: N/A
Additional information:
- Magic number(s): N/A
- File extension(s): cbor
- Macintosh file type code(s): N/A
Person & email address to contact for further information: iesg@ietf.org
Intended usage: COMMON
Restrictions on usage: N/A
Author: Jim Schaad, ietf@augustcellars.com
Change Controller: IESG
Provisional registration? No

16.8.2. COSE Key media type

This section registers the "application/cose-key+cbor" and "application/cose-key-set+cbor" media types in the "Media Types" registry. These media types are used to indicate, respectively, that content is a COSE_Key or COSE_KeySet object.

Type name: application
Subtype name: cose-key+cbor
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See the Security Considerations section of RFC TBD.
Interoperability considerations: N/A
Published specification: RFC TBD
Applications that use this media type: To be identified
Fragment identifier considerations: N/A
Additional information:
- Magic number(s): N/A
- File extension(s): cbor
- Macintosh file type code(s): N/A
Person & email address to contact for further information: iesg@ietf.org
Intended usage: COMMON
Restrictions on usage: N/A
Author: Jim Schaad, ietf@augustcellars.com
Change Controller: IESG
Provisional registration? No

Type name: application
Subtype name: cose-key-set+cbor
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See the Security Considerations section of RFC TBD.
Interoperability considerations: N/A
Published specification: RFC TBD
Applications that use this media type: To be identified
Fragment identifier considerations: N/A
Additional information:
- Magic number(s): N/A
- File extension(s): cbor
- Macintosh file type code(s): N/A
Person & email address to contact for further information: iesg@ietf.org
Intended usage: COMMON
Restrictions on usage: N/A
Author: Jim Schaad, ietf@augustcellars.com
Change Controller: IESG
Provisional registration? No

16.9. CoAP Content Format Registrations

This section registers a set of content formats for CoAP. ID assignment in the 24-255 range is requested.

Media Type	ID	Reference
application/cose; cose-type="cose-sign"	TBD10	[This Document]
application/cose; cose-type="cose-sign1"	TBD11	[This Document]
application/cose; cose-type="cose-enveloped"	TBD12	[This Document]
application/cose; cose-type="cose-encrypted"	TBD13	[This Document]
application/cose; cose-type="cose-mac"	TBD14	[This Document]
application/cose; cose-type="cose-mac0"	TBD15	[This Document]
application/cose-key	TBD16	[This Document]
application/cose-key-set	TBD17	[This Document

16.10. Expert Review Instructions

All of the IANA registries established in this document are defined as expert review. This section gives some general guidelines for what the experts should be looking for, but they are being designated as experts for a reason so they should be given substantial latitude.

Expert reviewers should take into consideration the following points:

Point squatting should be discouraged. Reviewers are encouraged to get sufficient information for registration requests to ensure that the usage is not going to duplicate one that is already registered and that the point is likely to be used in deployments. The zones tagged as private use are intended for testing purposes and closed environments, code points in other ranges should not be assigned for testing.
Specifications are required for the standards track range of point assignment. Specifications should exist for specification required ranges, but early assignment before a specification is available is considered to be permissible. Specifications are needed for the first-come, first-serve range if they are expected to be used outside of closed environments in an inoperable way. When specifications are not provided, the description provided needs to have sufficient information to identify what point is being used for.
Experts should take into account the expected usage of fields when approving point assignment. The fact that there is a range for standards track documents does not mean that a standards track document cannot have points assigned outside of that range. Some of the ranges are restricted in range, items which are not expected to be common or are not expected to be used in restricted environments should be assigned to values which will encode to longer byte strings.
When algorithms are registered, vanity registrations should be discouraged. One way to do this is to require applications to provide additional documentation on security analysis of algorithms. Another thing that should be considered is to request for an opinion on the algorithm from the Cryptographic Forum Research Group. Algorithms which do not meet the security requirements of the community and the messages structures should not be registered.

17. Security Considerations

There are a number of security considerations that need to be taken into account by implementers of this specification. The security considerations that are specific to an individual algorithm are placed next to the description of the algorithm. While some considerations have been highlighted here, additional considerations may be found in the documents listed in the references.

Implementations need to protect the private key for any individuals. There are some cases in this document that need to be highlighted on this issue.

Using the same key for two different algorithms can leak information about the key. It is therefore recommended that keys be restricted to a single algorithm.
Use of 'direct' as a recipient algorithm combined with a second recipient algorithm, either directly in a separate message, exposes the direct key to the second recipient.
Several of the algorithms in this document have limits on the number of times that a key can be used without leaking information about the key.

The use of ECDH and direct plus KDF (with no key wrap) will not directly lead to the private key being leaked, the one way function of the KDF will prevent that. There is however a different issue that needs to be addressed. Having two recipients, requires that the CEK be shared between two recipients. The second recipient therefore has a CEK that was derived from material that can be used for the weak proof of origin. The second recipient could create a message using the same CEK and send it to the first recipient, the first recipient would, for either static-static ECDH or direct plus KDF, make an assumption that the CEK could be used for proof of origin even though it is from the wrong entity. If the key wrap step is added, then no proof of origin is implied and thus is not an issue.

Although it has been mentioned before, the use of a single key for multiple algorithms has been demonstrated in some cases to leak information about a key, provide for attackers to forge integrity tags, or gain information about encrypted content. Binding a key to a single algorithm prevents these problems. Key creators and key consumers are strongly encourged not only to create new keys for each different algorithm, but to include that selection of algorithm in any distribution of key material and strictly enforce the matching of algorithms in the key structure to algorithms in the message structure. In addition to checking that algorithms are correct, the key form needs to be checked as well. Do not use an 'EC2' key where an 'oct' key is expected.

Before using a key for transmission, or before acting on information recieved, a trust decision on a key needs to be made. Is the data or action something that the entity associated with the key has a right to see or a right to request. A number of factors are associated with this trust decision. Some of the ones that are highlighted here are:

What are the permissions associated with the key owner?
Is the cryptographic algorithm acceptable in the current context?
Have the restrictions associated with the key, such as algorithm or freshness, been checked and are correct?
Is the request something that is reasonable given the current state of the application?
Have any security considerations that are part of the message been enforced? (As specified by the application or crit parameter.)

One area that has been starting to get exposure is doing traffic analysis of encrypted messages based on the length of the message. This specification does not provide for a uniform method of providing padding as part of the message structure. An observer can distinguish between two different strings (for example 'YES' and 'NO') based on length for all of the content encryption algorithms that are defined in this document. This means that it is up to applications to document how content padding is to be done in order to prevent or discourage such analysis. (For example the strings could be defined as 'YES' and 'NO '.)

18. Acknowledgments

This document is a product of the COSE working group of the IETF.

19. References

19.1. Normative References

[AES-GCM]	Dworkin, M., "NIST Special Publication 800-38D: Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC.", Nov 2007.
[DSS]	U.S. National Institute of Standards and Technology, "Digital Signature Standard (DSS)", July 2013.
[MAC]	NiST, N., "FIPS PUB 113: Computer Data Authentication", May 1985.
[RFC2104]	Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997.
[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3394]	Schaad, J. and R. Housley, "Advanced Encryption Standard (AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394, September 2002.
[RFC3610]	Whiting, D., Housley, R. and N. Ferguson, "Counter with CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September 2003.
[RFC5869]	Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/RFC5869, May 2010.
[RFC6090]	McGrew, D., Igoe, K. and M. Salter, "Fundamental Elliptic Curve Cryptography Algorithms", RFC 6090, DOI 10.17487/RFC6090, February 2011.
[RFC7049]	Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, October 2013.
[RFC7539]	Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015.
[SEC1]	Standards for Efficient Cryptography Group, "SEC 1: Elliptic Curve Cryptography", May 2009.

19.2. Informative References

[I-D.greevenbosch-appsawg-cbor-cddl]	Vigano, C. and H. Birkholz, "CBOR data definition language (CDDL): a notational convention to express CBOR data structures", Internet-Draft draft-greevenbosch-appsawg-cbor-cddl-07, October 2015.
[PVSig]	Brown, D. and D. Johnson, "Formal Security Proofs for a Signature Scheme with Partial Message Recover", February 2000.
[RFC2633]	Ramsdell, B., "S/MIME Version 3 Message Specification", RFC 2633, DOI 10.17487/RFC2633, June 1999.
[RFC2898]	Kaliski, B., "PKCS #5: Password-Based Cryptography Specification Version 2.0", RFC 2898, DOI 10.17487/RFC2898, September 2000.
[RFC3447]	Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February 2003.
[RFC4231]	Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512", RFC 4231, DOI 10.17487/RFC4231, December 2005.
[RFC4262]	Santesson, S., "X.509 Certificate Extension for Secure/Multipurpose Internet Mail Extensions (S/MIME) Capabilities", RFC 4262, DOI 10.17487/RFC4262, December 2005.
[RFC4493]	Song, JH., Poovendran, R., Lee, J. and T. Iwata, "The AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June 2006.
[RFC4949]	Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007.
[RFC5480]	Turner, S., Brown, D., Yiu, K., Housley, R. and T. Polk, "Elliptic Curve Cryptography Subject Public Key Information", RFC 5480, DOI 10.17487/RFC5480, March 2009.
[RFC5652]	Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, September 2009.
[RFC5751]	Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification", RFC 5751, DOI 10.17487/RFC5751, January 2010.
[RFC5752]	Turner, S. and J. Schaad, "Multiple Signatures in Cryptographic Message Syntax (CMS)", RFC 5752, DOI 10.17487/RFC5752, January 2010.
[RFC5990]	Randall, J., Kaliski, B., Brainard, J. and S. Turner, "Use of the RSA-KEM Key Transport Algorithm in the Cryptographic Message Syntax (CMS)", RFC 5990, DOI 10.17487/RFC5990, September 2010.
[RFC6151]	Turner, S. and L. Chen, "Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms", RFC 6151, DOI 10.17487/RFC6151, March 2011.
[RFC6979]	Pornin, T., "Deterministic Usage of the Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August 2013.
[RFC7159]	Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 2014.
[RFC7252]	Shelby, Z., Hartke, K. and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014.
[RFC7515]	Jones, M., Bradley, J. and N. Sakimura, "JSON Web Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May 2015.
[RFC7516]	Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)", RFC 7516, DOI 10.17487/RFC7516, May 2015.
[RFC7517]	Jones, M., "JSON Web Key (JWK)", RFC 7517, DOI 10.17487/RFC7517, May 2015.
[RFC7518]	Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, DOI 10.17487/RFC7518, May 2015.
[SP800-56A]	Barker, E., Chen, L., Roginsky, A. and M. Smid, "NIST Special Publication 800-56A: Recommendation for Pair-Wise Key Establishment Schemes Using Discrete Logarithm Cryptography", May 2013.

Appendix A. Making Mandatory Algorithm Header Optional

There has been a minority of the working group who have expressed a strong desire to relax the rule that the algorithm identifier be required to appear in each level of a COSE mesage. There are two basic reasons that have been advanced to support this position. First, the resulting message will be smaller if the algorithm identifier is omitted from the most common messages in a CoAP environment. Second, there is a potential bug that will arise if full checking is not done correctly between the different places that an algorithm identifier could be placed. (The message itself, an application statement, the key structure that the sender possesses and the key structure the recipient possesses.)

This appendix lays out how such a change can be made and the details that an application needs to specify in order to use this option. Two different sets of details are specified: Those needed to omit an algorithm identifier and those needed to use a variant on the counter signature attribute which contains no attributes about itself.

A.1. Algorithm Identification

In this section are laid out three sets of recommendations. The first set of recommendations apply to having an implicit algorithm identified for a single layer of a COSE message. The second set of recommendations apply to having multiple implicit algorithm identified for multiple layers of a COSE message. The third set of recommendations apply to having implicit algorithms for multiple COSE message constructs.

RFC 2119 language is deliberately not used here, this specification can provide recommendations, but it cannot enforce them.

This set of recommendations applies to the case where an application is distributing a fixed algorithm along with the key information for use in a single COSE message object. This normally applies to the smallest of the COSE messages, specifically COSE_Sign1, COSE_Mac0 and COSE_Encrypted, but could apply to the other structures as well.

The following items should be taken into account:

Applications need to list the set of COSE structures that implicit algorithms are to be used in. Applications need to require that the receipt of an explicit algorithm identifier in one of these structures will lead to the message being rejected. This requirement is stated so that there will never be a case where there is any ambiguity about the question of which algorithm should be used, the implicit or the explicit one. This applies even if the transported algorithm is a protected attribute. This applies even if the transported algorithm is the same as the implicit algorithm.
Applications need to define the set of information that is to be considered to be part of a context when omitting algorithm identifiers. At a minimum this would be the key identifier, the key, the algorithm and the COSE structures it can be used for. Applications should restrict the use of a single key to a single algorithm. As noted for some of the algorithms in this document, the use of the same key in different related algorithms can lead to leakage of information about the key, leakage about the data or the ability to perform forgeries.
In many cases applications which make the algorithm identifier will also want to make the context identifier implicit for the same reason. That is omitting the context identifier will decrease the message size (potentially significantly depending on the length of the identifier). Applications that do this will need to describe the circumstances where the context identifier is to be omitted and how the context identifier is to be inferred in these cases. (Exhaustive search would normally not be considered to be acceptable.) An example of how this can be done is to tie the context to a transaction identifier. Both would be sent on the original message, but only the transaction identifier would need to be sent after that point as the context is tied into the transaction identifier. Another way would be to associate a context with a network address. All messages coming from a single network address can be assumed to be associated with a specific context. (In this case the address would probably be distributed as part of the context.)
Applications cannot rely on key identifiers being unique unless they take significant efforts to ensure that they are computed in such a way as to create this guarantee. Even when an application does this, the uniqueness might be violated if the application is run in different contexts (i.e. with a different security coordinator) or if the system the application runs on combines security contexts from different applications together into a single store.
Applications should continue the practice of protecting the algorithm identifier. Since this is not done by placing it in the protected attributes field, applications should define an application specific external data structure which includes this value. This external data field can be used as such for content encryption, MAC and signature algorithms. It can be used in the SuppPrivInfo field for those algorithms which use a KDF function to derive a key value. Applications may also want to protect other information that is part of the context structure as well. It should be noted that those fields, such as the key or a base IV are already protected by virtue of being used in the cryptogrpahic computation and do not need to be included in the external data field.

The second case is having multiple implicit algorithm identifiers specified for a multiple layer COSE message. An example of how this would work is that the encryption context that an application specifies contains a content encryption algorithm, a key wrap algorithm, a key identifier, and a shared secret. The sender would then omit sending the algorithm identifier at both the content layer and the recipient layer leaving only the key identifier in situations where it could not be implied.

The following additional items need to be taken into consideration:

Applications that want to support this will need to define a structure that allows for, and clearly identifies, both the COSE structure to be used with a given key and the structure and algorithm to be used for the secondary layer. The key for the secondary layer is computed normally in the recipient layer.

The third case is having multiple implicit algorithm identifiers, but targeted at potentially unrelated layers or different COSE messages. There are a number of different scenarios where this might be applicable. Some of these scenarios are:

Two contexts are distributed as a pair. Each of the contexts is for use with a COSE_Encrypt message. Each context will consist of distinct secret keys and IVs and potentially even different algorithms. One context is for sending messages from party A to party B, the second context is for sending messages from party B to party A. This means that there is no chance for a reflection attack to occur as each party uses different secret keys to send its messages, a message that is reflected back to it would fail to decrypt.
Two contexts are distributed as a pair. The first context is used for encryption of the message, the second context is used to place a counter signature on the message. The intention is that the second context can be distributed to other entities independently of the first context. This allows these entities to validate that the message came from an individual without being able to decrypt the message and see the content.
Two contexts are distributed as a pair. The first context contains a key for dealing with MAC messages, the second context contains a key for dealing with encrypted messages. This allows for a unified distribution of keys to participants for different types of messages which have different keys, but where the keys may be used in coordinated manner.

For these cases, the following items need to be considered:

Applications need to ...

A.2. Counter Signature Without Headers

TBD

No parameter for counter sig
Define to be signed structure
id how key is decided
external data struture includes alg id
bind key in distribution
single alg of key structure
uniques of kid not real
very specialized for small size
kid can be either implied OR show as kid of what is counter signed

Appendix B. Three Levels of Recipient Information

All of the currently defined recipient algorithms classes only use two levels of the COSE_Enveloped structure. The first level is the message content and the second level is the content key encryption. However, if one uses a recipient algorithm such as RSA-KEM (see Appendix A of RSA-KEM [RFC5990], then it make sense to have three levels of the COSE_Enveloped structure.

These levels would be:

Level 0: The content encryption level. This level contains the payload of the message.
Level 1: The encryption of the CEK by a KEK.
Level 2: The encryption of a long random secret using an RSA key and a key derivation function to convert that secret into the KEK.

This is an example of what a triple layer message would look like. The message has the following layers:

Level 0: Has a content encrypted with AES-GCM using a 128-bit key.
Level 1: Uses the AES Key wrap algorithm with a 128-bit key.
Level 2: Uses ECDH Ephemeral-Static direct to generate the level 1 key.

In effect this example is a decomposed version of using the ECDH-ES+A128KW algorithm.

Size of binary file is 184 bytes

992(
  [
    / protected / h'a10101' / {
        \ alg \ 1:1 \ AES-GCM 128 \
      } / , 
    / unprotected / {
      / iv / 5:h'02d1f7e6f26c43d4868d87ce'
    }, 
    / ciphertext / h'64f84d913ba60a76070a9a48f26e97e863e28529bf9be9d
e3bea1788f681200d875242f6', 
    / recipients / [
      [
        / protected / h'', 
        / unprotected / {
          / alg / 1:-3 / A128KW /
        }, 
        / ciphertext / h'f4b117264ab6d4d1476e0204bb15db58c5834461e83
5e884', 
        / recipients / [
          [
            / protected / h'a1013818' / {
                \ alg \ 1:-25 \ ECDH-ES + HKDF-256 \
              } / , 
            / unprotected / {
              / ephemeral / -1:{
                / kty / 1:2, 
                / crv / -1:1, 
                / x / -2:h'b2add44368ea6d641f9ca9af308b4079aeb519f11
e9b8a55a600b21233e86e68', 
                / y / -3:false
              }, 
              / kid / 4:'meriadoc.brandybuck@buckland.example'
            }, 
            / ciphertext / h''
          ]
        ]
      ]
    ]
  ]
)

Appendix C. Examples

This appendix includes a set of examples that show the different features and message types that have been defined in this document. To make the examples easier to read, they are presented using the extended CBOR diagnostic notation (defined in [I-D.greevenbosch-appsawg-cbor-cddl]) rather than as a binary dump.

A GITHUB project has been created at https://github.com/cose-wg/Examples that contains not only the examples presented in this document, but a more complete set of testing examples as well. Each example is found in a JSON file that contains the inputs used to create the example, some of the intermediate values that can be used in debugging the example and the output of the example presented in both a hex and a CBOR diagnostic notation format. Some of the examples at the site are designed failure testing cases, these are clearly marked as such in the JSON file. If errors in the examples in this document are found, the examples on github will be updated and a note to that effect will be placed in the JSON file.

As noted, the examples are presented using the CBOR's diagnostic notation. A ruby based tool exists that can convert between the diagnostic notation and binary. This tool can be installed with the command line:

gem install cbor-diag

The diagnostic notation can be converted into binary files using the following command line:

diag2cbor < inputfile > outputfile

The examples can be extracted from the XML version of this document via an XPath expression as all of the artwork is tagged with the attribute type='CBORdiag'. (Depending on the XPath evaluator one is using, it may be necessary to deal with > as an entity.)

//artwork[@type='CDDL']/text()

C.1. Examples of Signed Message

C.1.1. Single Signature

This example uses the following:

Signature Algorithm: ECDSA w/ SHA-256, Curve P-256-1

Size of binary file is 104 bytes

991(
  [
    / protected / h'', 
    / unprotected / {}, 
    / payload / 'This is the content.', 
    / signatures / [
      [
        / protected / h'a10126' / {
            \ alg \ 1:-7 \ ECDSA 256 \
          } / , 
        / unprotected / {
          / kid / 4:'11'
        }, 
        / signature / h'eae868ecc176883766c5dc5ba5b8dca25dab3c2e56a5
51ce5705b793914348e14eea4aee6e0c9f09db4ef3ddeca8f3506cd1a98a8fb64327
be470355c9657ce0'
      ]
    ]
  ]
)

C.1.2. Multiple Signers

This example uses the following:

Signature Algorithm: ECDSA w/ SHA-256, Curve P-256-1
Signature Algorithm: ECDSA w/ SHA-512, Curve P-521

Size of binary file is 278 bytes

991(
  [
    / protected / h'', 
    / unprotected / {}, 
    / payload / 'This is the content.', 
    / signatures / [
      [
        / protected / h'a10126' / {
            \ alg \ 1:-7 \ ECDSA 256 \
          } / , 
        / unprotected / {
          / kid / 4:'11'
        }, 
        / signature / h'0dc1c5e62719d8f3cce1468b7c881eee6a8088b46bf8
36ae956dd38fe93199199951a6a5e02a24aed5edde3509748366b1c539aaef7dea34
f2cd618fe19fe55d'
      ], 
      [
        / protected / h'a1013823' / {
            \ alg \ 1:-36
          } / , 
        / unprotected / {
          / kid / 4:'bilbo.baggins@hobbiton.example'
        }, 
        / signature / h'012ce5b1dfe8b5aa6eaa09a54c58a84ad0900e4fdf27
59ec22d1c861cccd75c7e1c4025a2da35e512fc2874d6ac8fd862d09ad07ed2deac2
97b897561e04a8d42476017c11a4a34e26c570c9eff22c1dc84d56cdf6e03ed34bc9
e934c5fdf676c7948d79e97dfe161730217c57748aadb364a0207cee811e9dde65ae
37942e8a8348cc91'
      ]
    ]
  ]
)

C.1.3. Counter Signature

This example uses the following:

Signature Algorithm: ECDSA w/ SHA-256, Curve P-256-1
The same parameters are used for both the signature and the counter signature.

Size of binary file is 181 bytes

991(
  [
    / protected / h'', 
    / unprotected / {
      / countersign / 7:[
        / protected / h'a10126' / {
            \ alg \ 1:-7 \ ECDSA 256 \
          } / , 
        / unprotected / {
          / kid / 4:'11'
        }, 
        / signature / h'c9d3402485aa585cee3efc69b14496c0b00714584b26
0f8e05764b7dbc70ae2be52a463555fc78e8da59bf8b3af281e739741dbac0b6f56a
4b03ef23cb93b1e1'
      ]
    }, 
    / payload / 'This is the content.', 
    / signatures / [
      [
        / protected / h'a10126' / {
            \ alg \ 1:-7 \ ECDSA 256 \
          } / , 
        / unprotected / {
          / kid / 4:'11'
        }, 
        / signature / h'eae868ecc176883766c5dc5ba5b8dca25dab3c2e56a5
51ce5705b793914348e14eea4aee6e0c9f09db4ef3ddeca8f3506cd1a98a8fb64327
be470355c9657ce0'
      ]
    ]
  ]
)

C.1.4. Signature w/ Operation Time and Criticality

This example uses the following:

Signature Algorithm: ECDSA w/ SHA-256, Curve P-256-1
There is an operation time of 2014-02-14T12:00Z
There is a criticality marker on the "reserved" header parameter

Size of binary file is 132 bytes

991(
  [
    / protected / h'a2687265736572766564f40281687265736572766564' / 
{
        "reserved":false, 
        \ crit \ 2:[
          "reserved"
        ]
      } / , 
    / unprotected / {}, 
    / payload / 'This is the content.', 
    / signatures / [
      [
        / protected / h'a20126081a56bffbc0' / {
            \ alg \ 1:-7 \ ECDSA 256 \, 
            8:1455422400
          } / , 
        / unprotected / {
          / kid / 4:'11'
        }, 
        / signature / h'eae868ecc176883766c5dc5ba5b8dca25dab3c2e56a5
51ce5705b793914348e150d023101a60dddbf0c11f6cdaf5708e12925c67dbb5d1db
d16b2474483e367b'
      ]
    ]
  ]
)

C.2. Single Signer Examples

C.2.1. Single ECDSA signature

This example uses the following:

Signature Algorithm: ECDSA w/ SHA-256, Curve P-256-1

Size of binary file is 100 bytes

997(
  [
    / protected / h'a10126' / {
        \ alg \ 1:-7 \ ECDSA 256 \
      } / , 
    / unprotected / {
      / kid / 4:'11'
    }, 
    / payload / 'This is the content.', 
    h'eae868ecc176883766c5dc5ba5b8dca25dab3c2e56a551ce5705b793914348
e19f43d6c6ba654472da301b645b293c9ba939295b97c4bdb847782bff384c5794'
  ]
)

C.3. Examples of Enveloped Messages

C.3.1. Direct ECDH

This example uses the following:

CEK: AES-GCM w/ 128-bit key
Recipient class: ECDH Ephemeral-Static, Curve P-256

Size of binary file is 152 bytes

992(
  [
    / protected / h'a10101' / {
        \ alg \ 1:1 \ AES-GCM 128 \
      } / , 
    / unprotected / {
      / iv / 5:h'c9cf4df2fe6c632bf7886413'
    }, 
    / ciphertext / h'40970cd7ab5fbd10f505bf7a86e6fc0a99a31224b3b5895
c9fc7892ba138233e0e65af84', 
    / recipients / [
      [
        / protected / h'a1013818' / {
            \ alg \ 1:-25 \ ECDH-ES + HKDF-256 \
          } / , 
        / unprotected / {
          / ephemeral / -1:{
            / kty / 1:2, 
            / crv / -1:1, 
            / x / -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbf
bf054e1c7b4d91d6280', 
            / y / -3:true
          }, 
          / kid / 4:'meriadoc.brandybuck@buckland.example'
        }, 
        / ciphertext / h''
      ]
    ]
  ]
)

C.3.2. Direct plus Key Derivation

This example uses the following:

CEK: AES-CCM w/128-bit key, truncate the tag to 64 bits
Recipient class: Use HKDF on a shared secret with the following implicit fields as part of the context.
- salt: "aabbccddeeffgghh"
- APU identity: "lighting-client"
- APV identity: "lighting-server"
- Supplementary Public Other: "Encryption Example 02"

Size of binary file is 92 bytes

992(
  [
    / protected / h'a1010a' / {
        \ alg \ 1:10 \ AES-CCM-16-64-128 \
      } / , 
    / unprotected / {
      / iv / 5:h'89f52f65a1c580933b5261a76c'
    }, 
    / ciphertext / h'89bedc91e9909346a8fe87834445679ee12b2c953cbb685
25aa7675f', 
    / recipients / [
      [
        / protected / h'a10129' / {
            \ alg \ 1:-10
          } / , 
        / unprotected / {
          / salt / -20:'aabbccddeeffgghh', 
          / kid / 4:'our-secret'
        }, 
        / ciphertext / h''
      ]
    ]
  ]
)

C.3.3. Counter Signature on Encrypted Content

This example uses the following:

CEK: AES-GCM w/ 128-bit key
Recipient class: ECDH Ephemeral-Static, Curve P-256

Size of binary file is 327 bytes

992(
  [
    / protected / h'a10101' / {
        \ alg \ 1:1 \ AES-GCM 128 \
      } / , 
    / unprotected / {
      / iv / 5:h'c9cf4df2fe6c632bf7886413', 
      / countersign / 7:[
        / protected / h'a1013823' / {
            \ alg \ 1:-36
          } / , 
        / unprotected / {
          / kid / 4:'bilbo.baggins@hobbiton.example'
        }, 
        / signature / h'00aa98cbfd382610a375d046a275f30266e8d0faacb9
069fde06e37825ae7825419c474f416ded0c8e3e7b55bff68f2a704135bdf99186f6
6659461c8cf929cc7fb3013ac242342ddd8443c6292a1f8c78c5985aa7d86f34c0f1
ba0b3dee5f4b59737b230da980886137da6f2ca79cc5c40ee89b771c71cdb1ee966e
cfc7d4b2cdc1410a'
      ]
    }, 
    / ciphertext / h'40970cd7ab5fbd10f505bf7a86e6fc0a99a31224b3b5895
c9fc7892ba138233e0e65af84', 
    / recipients / [
      [
        / protected / h'a1013818' / {
            \ alg \ 1:-25 \ ECDH-ES + HKDF-256 \
          } / , 
        / unprotected / {
          / ephemeral / -1:{
            / kty / 1:2, 
            / crv / -1:1, 
            / x / -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbf
bf054e1c7b4d91d6280', 
            / y / -3:true
          }, 
          / kid / 4:'meriadoc.brandybuck@buckland.example'
        }, 
        / ciphertext / h''
      ]
    ]
  ]
)

C.3.4. Encrypted Content with External Data

This example uses the following:

CEK: AES-GCM w/ 128-bit key
Recipient class: ECDH static-Static, Curve P-256 with AES Key Wrap
Externally Supplied AAD: h'0011bbcc22dd44ee55ff660077'

Size of binary file is 174 bytes

992(
  [
    / protected / h'a10101' / {
        \ alg \ 1:1 \ AES-GCM 128 \
      } / , 
    / unprotected / {
      / iv / 5:h'02d1f7e6f26c43d4868d87ce'
    }, 
    / ciphertext / h'64f84d913ba60a76070a9a48f26e97e863e2852951f6f24
9e6c3616233a911748a80be95', 
    / recipients / [
      [
        / protected / h'a101381f' / {
            \ alg \ 1:-32 \ ECHD-SS+A128KW \
          } / , 
        / unprotected / {
          / static kid / -3:'peregrin.took@tuckborough.example', 
          / kid / 4:'meriadoc.brandybuck@buckland.example', 
          / U nonce / -22:h'0101'
        }, 
        / ciphertext / h'59463342fd2193f30daeb1ebb2dc7310b56cee0939d
d6692'
      ]
    ]
  ]
)

C.4. Examples of Encrypted Messages

C.4.1. Simple Encrypted Message

This example uses the following:

CEK: AES-CCM w/ 128-bit key and a 64-bit tag

Size of binary file is 54 bytes

993(
  [
    / protected / h'a1010a' / {
        \ alg \ 1:10 \ AES-CCM-16-64-128 \
      } / , 
    / unprotected / {
      / iv / 5:h'89f52f65a1c580933b5261a78c'
    }, 
    / ciphertext / h'5974e1b99a3a4cc09a659aa2e9e7fff161d38ce74693c90
dcda22121'
  ]
)

C.4.2. Encrypted Message w/ a Partial IV

This example uses the following:

CEK: AES-CCM w/ 128-bit key and a 64-bit tag
Prefix for IV is 89F52F65A1C580933B52

Size of binary file is 43 bytes

993(
  [
    / protected / h'a1010a' / {
        \ alg \ 1:10 \ AES-CCM-16-64-128 \
      } / , 
    / unprotected / {
      / partial iv / 6:h'61a7'
    }, 
    / ciphertext / h'252a8911d465c125b6764739700f0141ed09192d2e16ce9
e579fea11'
  ]
)

C.5. Examples of MAC messages

C.5.1. Shared Secret Direct MAC

This example users the following:

MAC: AES-CMAC, 256-bit key, truncated to 64 bits
Recipient class: direct shared secret

Size of binary file is 58 bytes

994(
  [
    / protected / h'a1010f' / {
        \ alg \ 1:15 \ AES-CBC-MAC-256//64 \
      } / , 
    / unprotected / {}, 
    / payload / 'This is the content.', 
    / tag / h'9e1226ba1f81b848', 
    / recipients / [
      [
        / protected / h'', 
        / unprotected / {
          / alg / 1:-6 / direct /, 
          / kid / 4:'our-secret'
        }, 
        / ciphertext / h''
      ]
    ]
  ]
)

C.5.2. ECDH Direct MAC

This example uses the following:

MAC: HMAC w/SHA-256, 256-bit key
Recipient class: ECDH key agreement, two static keys, HKDF w/ context structure

Size of binary file is 215 bytes

994(
  [
    / protected / h'a10105' / {
        \ alg \ 1:5 \ HMAC 256//256 \
      } / , 
    / unprotected / {}, 
    / payload / 'This is the content.', 
    / tag / h'42cf68ae1253948c500dff27da3904342625a23e914f7aa545dcf6
629519f18e', 
    / recipients / [
      [
        / protected / h'a101381a' / {
            \ alg \ 1:-27 \ ECDH-SS + HKDF-256 \
          } / , 
        / unprotected / {
          / static kid / -3:'peregrin.took@tuckborough.example', 
          / kid / 4:'meriadoc.brandybuck@buckland.example', 
          / U nonce / -22:h'4d8553e7e74f3c6a3a9dd3ef286a8195cbf8a23d
19558ccfec7d34b824f42d92bd06bd2c7f0271f0214e141fb779ae2856abf585a583
68b017e7f2a9e5ce4db5'
        }, 
        / ciphertext / h''
      ]
    ]
  ]
)

C.5.3. Wrapped MAC

This example uses the following:

MAC: AES-MAC, 128-bit key, truncated to 64 bits
Recipient class: AES keywrap w/ a pre-shared 256-bit key

Size of binary file is 110 bytes

994(
  [
    / protected / h'a1010e' / {
        \ alg \ 1:14 \ AES-CBC-MAC-128//64 \
      } / , 
    / unprotected / {}, 
    / payload / 'This is the content.', 
    / tag / h'36f5afaf0bab5d43', 
    / recipients / [
      [
        / protected / h'', 
        / unprotected / {
          / alg / 1:-5 / A256KW /, 
          / kid / 4:'018c0ae5-4d9b-471b-bfd6-eef314bc7037'
        }, 
        / ciphertext / h'711ab0dc2fc4585dce27effa6781c8093eba906f227
b6eb0'
      ]
    ]
  ]
)

C.5.4. Multi-recipient MAC message

This example uses the following:

MAC: HMAC w/ SHA-256, 128-bit key
Recipient class: Uses three different methods
1. ECDH Ephemeral-Static, Curve P-521, AES-Key Wrap w/ 128-bit key
2. AES-Key Wrap w/ 256-bit key

Size of binary file is 310 bytes

994(
  [
    / protected / h'a10105' / {
        \ alg \ 1:5 \ HMAC 256//256 \
      } / , 
    / unprotected / {}, 
    / payload / 'This is the content.', 
    / tag / h'bf48235e809b5c42e995f2b7d5fa13620e7ed834e337f6aa43df16
1e49e9323e', 
    / recipients / [
      [
        / protected / h'a101381c' / {
            \ alg \ 1:-29 \ ECHD-ES+A128KW \
          } / , 
        / unprotected / {
          / ephemeral / -1:{
            / kty / 1:2, 
            / crv / -1:3, 
            / x / -2:h'0043b12669acac3fd27898ffba0bcd2e6c366d53bc4db
71f909a759304acfb5e18cdc7ba0b13ff8c7636271a6924b1ac63c02688075b55ef2
d613574e7dc242f79c3', 
            / y / -3:true
          }, 
          / kid / 4:'bilbo.baggins@hobbiton.example'
        }, 
        / ciphertext / h'c07072310285bbd3f0675774418138e14388ed47a4a
81219d42a8bfbe3a5559c19de83435d21c6bc'
      ], 
      [
        / protected / h'', 
        / unprotected / {
          / alg / 1:-5 / A256KW /, 
          / kid / 4:'018c0ae5-4d9b-471b-bfd6-eef314bc7037'
        }, 
        / ciphertext / h'0b2c7cfce04e98276342d6476a7723c090dfdd15f9a
518e7736549e998370695e6d6a83b4ae507bb'
      ]
    ]
  ]
)

C.6. Examples of MAC0 messages

C.6.1. Shared Secret Direct MAC

This example users the following:

MAC: AES-CMAC, 256-bit key, truncated to 64 bits
Recipient class: direct shared secret

Size of binary file is 39 bytes

996(
  [
    / protected / h'a1010f' / {
        \ alg \ 1:15 \ AES-CBC-MAC-256//64 \
      } / , 
    / unprotected / {}, 
    / payload / 'This is the content.', 
    / tag / h'726043745027214f'
  ]
)

Note that this example uses the same inputs as Appendix C.5.1.

C.7. COSE Keys

C.7.1. Public Keys

This is an example of a COSE Key set. This example includes the public keys for all of the previous examples.

In order the keys are:

An EC key with a kid of "meriadoc.brandybuck@buckland.example"
An EC key with a kid of "peregrin.took@tuckborough.example"
An EC key with a kid of "bilbo.baggins@hobbiton.example"
An EC key with a kid of "11"

Size of binary file is 481 bytes

[
  {
    -1:1, 
    -2:h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de439c0
8551d', 
    -3:h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd008
4d19c', 
    1:2, 
    2:'meriadoc.brandybuck@buckland.example'
  }, 
  {
    -1:1, 
    -2:h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a
09eff', 
    -3:h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbf
c117e', 
    1:2, 
    2:'11'
  }, 
  {
    -1:3, 
    -2:h'0072992cb3ac08ecf3e5c63dedec0d51a8c1f79ef2f82f94f3c737bf5de
7986671eac625fe8257bbd0394644caaa3aaf8f27a4585fbbcad0f2457620085e5c8
f42ad', 
    -3:h'01dca6947bce88bc5790485ac97427342bc35f887d86d65a089377e247e
60baa55e4e8501e2ada5724ac51d6909008033ebc10ac999b9d7f5cc2519f3fe1ea1
d9475', 
    1:2, 
    2:'bilbo.baggins@hobbiton.example'
  }, 
  {
    -1:1, 
    -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91
d6280', 
    -3:h'f01400b089867804b8e9fc96c3932161f1934f4223069170d924b7e03bf
822bb', 
    1:2, 
    2:'peregrin.took@tuckborough.example'
  }
]

C.7.2. Private Keys

This is an example of a COSE Key set. This example includes the private keys for all of the previous examples.

In order the keys are:

An EC key with a kid of "meriadoc.brandybuck@buckland.example"
A shared-secret key with a kid of "our-secret"
An EC key with a kid of "peregrin.took@tuckborough.example"
A shared-secret key with a kid of "018c0ae5-4d9b-471b-bfd6-eef314bc7037"
An EC key with a kid of "bilbo.baggins@hobbiton.example"
An EC key with a kid of "11"

Size of binary file is 816 bytes

[
  {
    1:2, 
    2:'meriadoc.brandybuck@buckland.example', 
    -1:1, 
    -2:h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de439c0
8551d', 
    -3:h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd008
4d19c', 
    -4:h'aff907c99f9ad3aae6c4cdf21122bce2bd68b5283e6907154ad911840fa
208cf'
  }, 
  {
    1:2, 
    2:'11', 
    -1:1, 
    -2:h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a
09eff', 
    -3:h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbf
c117e', 
    -4:h'57c92077664146e876760c9520d054aa93c3afb04e306705db609030850
7b4d3'
  }, 
  {
    1:2, 
    2:'bilbo.baggins@hobbiton.example', 
    -1:3, 
    -2:h'0072992cb3ac08ecf3e5c63dedec0d51a8c1f79ef2f82f94f3c737bf5de
7986671eac625fe8257bbd0394644caaa3aaf8f27a4585fbbcad0f2457620085e5c8
f42ad', 
    -3:h'01dca6947bce88bc5790485ac97427342bc35f887d86d65a089377e247e
60baa55e4e8501e2ada5724ac51d6909008033ebc10ac999b9d7f5cc2519f3fe1ea1
d9475', 
    -4:h'00085138ddabf5ca975f5860f91a08e91d6d5f9a76ad4018766a476680b
55cd339e8ab6c72b5facdb2a2a50ac25bd086647dd3e2e6e99e84ca2c3609fdf177f
eb26d'
  }, 
  {
    1:4, 
    2:'our-secret', 
    -1:h'849b57219dae48de646d07dbb533566e976686457c1491be3a76dcea6c4
27188'
  }, 
  {
    1:2, 
    -1:1, 
    2:'peregrin.took@tuckborough.example', 
    -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91
d6280', 
    -3:h'f01400b089867804b8e9fc96c3932161f1934f4223069170d924b7e03bf
822bb', 
    -4:h'02d1f7e6f26c43d4868d87ceb2353161740aacf1f7163647984b522a848
df1c3'
  }, 
  {
    1:4, 
    2:'our-secret2', 
    -1:h'849b5786457c1491be3a76dcea6c4271'
  }, 
  {
    1:4, 
    2:'018c0ae5-4d9b-471b-bfd6-eef314bc7037', 
    -1:h'849b57219dae48de646d07dbb533566e976686457c1491be3a76dcea6c4
27188'
  }
]

Appendix D. Document Updates

D.1. Version -09 to -10

Add more examples
Revise Design changes
Add context string for recursive recipient structures
Change and assign some algorithm numbers

D.2. Version -08 to -09

Integrate CDDL syntax into the text
Define Expert review guidelines
Expand application profiling guidelines
Expand text around Partial IV
Creation time becomes Operation time
Add tagging for all structures so that they cannot be moved
Add optional parameter to cose media type
Add single signature and mac structures

D.3. Version -07 to -08

Redefine sequence number into a the Partial IV.

D.4. Version -06 to -07

Editorial Changes
Make new IANA registries be Expert Review

D.5. Version -05 to -06

Remove new CFRG Elliptical Curve key agreement algorithms.
Remove RSA algorithms
Define a creation time and sequence number for discussions.
Remove message type field from all structures.
Define CBOR tagging for all structures with IANA registrations.

D.6. Version -04 to -05

Removed the jku, x5c, x5t, x5t#S256, x5u, and jwk headers.
Add enveloped data vs encrypted data structures.
Add counter signature parameter.

D.7. Version -03 to -04

Change top level from map to array.
Eliminate the term "key management" from the document.
Point to content registries for the 'content type' attribute
Push protected field into the KDF functions for recipients.
Remove password based recipient information.
Create EC Curve Registry.

D.8. Version -02 to -03

Make a pass over all of the algorithm text.
Alter the CDDL so that Keys and KeySets are top level items and the key examples validate.
Add sample key structures.
Expand text on dealing with Externally Supplied Data.
Update the examples to match some of the renumbering of fields.

D.9. Version -02 to -03

Add a set of straw man proposals for algorithms. It is possible/expected that this text will be moved to a new document.
Add a set of straw man proposals for key structures. It is possible/expected that this text will be moved to a new document.
Provide guidance on use of externally supplied authenticated data.
Add external authenticated data to signing structure.

D.10. Version -01 to -2

Add first pass of algorithm information
Add direct key derivation example.

D.11. Version -00 to -01

Add note on where the document is being maintained and contributing notes.
Put in proposal on MTI algorithms.
Changed to use labels rather than keys when talking about what indexes a map.
Moved nonce/IV to be a common header item.
Expand section to discuss the common set of labels used in COSE_Key maps.
Start marking element 0 in registries as reserved.
Update examples.

Author's Address

Jim Schaad August Cellars EMail: ietf@augustcellars.com