| COSE Working Group | J. Schaad |
| Internet-Draft | August Cellars |
| Obsoletes: 8152 (if approved) | August 22, 2018 |
| Intended status: Standards Track | |
| Expires: February 23, 2019 |
CBOR Object Signing and Encryption (COSE) - Structures and Process
draft-schaad-cose-rfc8152bis-struct-00
Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need for the ability to have basic security services defined for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR.
This document along with [I-D.schaad-cose-rfc8152bis-algs] obsoletes RFC8152.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on February 23, 2019.
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There has been an increased focus on small, constrained devices that make up the Internet of Things (IoT). One of the standards that has come out of this process is "Concise Binary Object Representation (CBOR)" [RFC7049]. CBOR extended the data model of the JavaScript Object Notation (JSON) [RFC7159] 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 and be 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 that specified how to process encryption, signatures, and Message Authentication Code (MAC) operations and how to encode keys using JSON. This document defines the CBOR Object Signing and Encryption (COSE) standard, which does the same thing for the CBOR encoding format. While there is a strong attempt to keep the flavor of the original JSON Object Signing and Encryption (JOSE) documents, two considerations are taken into account:
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 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
When the words appear in lowercase, this interpretation does not apply.
There is currently no standard CBOR grammar available for use by specifications. The CBOR structures are therefore described 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 CBOR Data Definition Language (CDDL) [CDDL]. In this specification, the following primitive types are used:
Two syntaxes from CDDL appear in this document as shorthand. These are:
As well as the prose description, a version of a CBOR grammar is presented in CDDL. Since CDDL has not been published in an RFC, this grammar may not work with the final version of CDDL. 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.)
//artwork[@type='CDDL']/text()
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.
start = COSE_Messages / COSE_Key / COSE_KeySet / Internal_Types
; This is defined to make the tool quieter:
Internal_Types = Sig_structure / Enc_structure / MAC_structure /
COSE_KDF_Context
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.
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. The inclusion of strings allows for an additional range of short encoded values to be used as well. Since the word "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 COSE map that 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 defines the non-terminal 'label', as in the previous paragraph, and 'values', which permits any value to be used.
label = int / tstr values = any
In this document, we use the following terminology:
Byte is a synonym for octet.
Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use in constrained systems. It is defined in [RFC7252].
Authenticated Encryption (AE) [RFC5116] algorithms are those encryption algorithms that provide an authentication check of the contents algorithm with the encryption service.
Authenticated Encryption with Authenticated Data (AEAD) [RFC5116] algorithms provide the same content authentication service as AE algorithms, but they additionally provide for authentication of non-encrypted data as well.
The COSE object structure is designed so that there can be a large amount of common code when parsing and processing the different types of security messages. All of the message structures are built on the CBOR array type. The first three elements of the array always contain the same information:
Elements after this point are dependent on the specific message type.
COSE messages are also built using the concept of layers to separate different types of cryptographic concepts. As an example of how this works, consider the COSE_Encrypt message (Section 5.1). This message type is broken into two layers: the content layer and the recipient layer. In the content layer, the plaintext is encrypted and information about the encrypted message is placed. In the recipient layer, the content encryption key (CEK) is encrypted and information about how it is encrypted for each recipient is placed. A single layer version of the encryption message COSE_Encrypt0 (Section 5.2) is provided for cases where the CEK is pre-shared.
Identification of which type of message has been presented is done by the following methods:
| CBOR Tag | cose-type | Data Item | Semantics |
|---|---|---|---|
| 98 | cose-sign | COSE_Sign | COSE Signed Data Object |
| 18 | cose-sign1 | COSE_Sign1 | COSE Single Signer Data Object |
| 96 | cose-encrypt | COSE_Encrypt | COSE Encrypted Data Object |
| 16 | cose-encrypt0 | COSE_Encrypt0 | COSE Single Recipient Encrypted Data Object |
| 97 | cose-mac | COSE_Mac | COSE MACed Data Object |
| 17 | cose-mac0 | COSE_Mac0 | COSE Mac w/o Recipients Object |
The following CDDL fragment identifies all of the top messages defined in this document. Separate non-terminals are defined for the tagged and the untagged versions of the messages.
COSE_Messages = COSE_Untagged_Message / COSE_Tagged_Message
COSE_Untagged_Message = COSE_Sign / COSE_Sign1 /
COSE_Encrypt / COSE_Encrypt0 /
COSE_Mac / COSE_Mac0
COSE_Tagged_Message = COSE_Sign_Tagged / COSE_Sign1_Tagged /
COSE_Encrypt_Tagged / COSE_Encrypt0_Tagged /
COSE_Mac_Tagged / COSE_Mac0_Tagged
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 are 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 is 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 have been divided into several sections including 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 15.2).
Two buckets are provided for each layer:
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" registry (Section 15.2). 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 verify that the same label does not occur in both the protected and unprotected headers. 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 that 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 that represents the map of common headers.
Headers = (
protected : empty_or_serialized_map,
unprotected : header_map
)
header_map = {
Generic_Headers,
* label => values
}
empty_or_serialized_map = bstr .cbor header_map / bstr .size 0
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 and the type of the value.
The set of header parameters defined in this section are:
The header parameter values indicated by 'crit' can be processed by either the security library code or 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.
| Name | Label | Value Type | Value Registry | Description |
|---|---|---|---|---|
| alg | 1 | int / tstr | COSE Algorithms registry | Cryptographic algorithm to use |
| crit | 2 | [+ label] | COSE Header Parameters registry | Critical headers to be understood |
| content type | 3 | tstr / uint | CoAP Content-Formats or Media Types registries | 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 |
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
)
COSE supports two different signature structures. COSE_Sign allows for one or more signatures to be applied to the same content. COSE_Sign1 is restricted to a single signer. The structures cannot be converted between each other; as the signature computation includes a parameter identifying which structure is being used, the converted structure will fail signature validation.
The COSE_Sign structure allows for one or more signatures to be applied to a message payload. Parameters relating to the content and parameters relating to the signature are 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 and counter signatures.
RFC 5652 indicates that:
For example, the COSE_Sign structure might include signatures generated with the Edwards-curve Digital Signature Algorithm (EdDSA) [RFC8032] and with the Elliptic Curve Digital Signature Algorithm (ECDSA) [DSS]. This allows recipients to verify the signature associated with one algorithm or the other. More-detailed information on multiple signature evaluations can be found in [RFC5752].
The signature structure can be encoded as either tagged or untagged depending on the context it will be used in. A tagged COSE_Sign structure is identified by the CBOR tag 98. The CDDL fragment that represents this is:
COSE_Sign_Tagged = #6.98(COSE_Sign)
A COSE Signed Message is defined in 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:
The CDDL fragment that 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:
The CDDL fragment that represents the above text for COSE_Signature follows.
COSE_Signature = [
Headers,
signature : bstr
]
The COSE_Sign1 signature structure is used when only one signature is going to be placed on a message. The parameters dealing with the content and the signature are placed in the same pair of buckets rather than having the separation of COSE_Sign.
The structure can be encoded as either tagged or untagged depending on the context it will be used in. A tagged COSE_Sign1 structure is identified by the CBOR tag 18. The CDDL fragment that represents this is:
COSE_Sign1_Tagged = #6.18(COSE_Sign1)
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_Sign1 messages can be found in Appendix C.2.
The COSE_Sign1 structure is a CBOR array. The fields of the array in order are:
The CDDL fragment that represents the above text for COSE_Sign1 follows.
COSE_Sign1 = [
Headers,
payload : bstr / nil,
signature : bstr
]
One of the features offered in the COSE document is the ability for applications to provide additional data to be authenticated, 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. Examples of data that can be placed in this location would be the CoAP code or CoAP options. If the data is in the header section, then it is available for proxies to help in performing its operations. For example, the Accept Option can be used by a proxy to determine if an appropriate value is in the proxy's cache. But the sender can prevent a proxy from changing the set of values that it will accept by including that value in the resulting authentication tag. However, it may also be desired to protect these values so that if they are modified in transit, it can be detected.
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:
In order to create a signature, a well-defined byte stream is needed. The Sig_struture is used to create the canonical form. This signing and verification process takes in the body information (COSE_Sign or COSE_Sign1), the signer information (COSE_Signature), and the application data (external source). A Sig_structure is a CBOR array. The fields of the Sig_struture in order are:
The CDDL fragment that describes the above text is:
Sig_structure = [
context : "Signature" / "Signature1" / "CounterSignature",
body_protected : empty_or_serialized_map,
? sign_protected : empty_or_serialized_map,
external_aad : bstr,
payload : bstr
]
How to compute a signature:
The steps for verifying a signature are:
In addition to performing the signature verification, the application may also perform the appropriate checks to ensure that the key is correctly paired with the signing identity and that the signing identity is authorized before performing actions.
Counter signatures provide a method of associating a different signature generated by different signers with 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_Encrypt 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 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 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_Encrypt 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.
COSE supports two different encryption structures. COSE_Encrypt0 is used when a recipient structure is not needed because the key to be used is known implicitly. COSE_Encrypt is used the rest of the time. This includes cases where there are multiple recipients or a recipient algorithm other than direct is used.
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 and the recipient's encryption algorithm.
The same techniques and structures are used for encrypting both the plaintext and the keys. This is different from the approach used by both "Cryptographic Message Syntax (CMS)" [RFC5652] and "JSON Web Encryption (JWE)" [RFC7516] where different structures are used for the content layer and for the recipient layer. Two structures are defined: COSE_Encrypt 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_Encrypt structure can be encoded as either tagged or untagged depending on the context it will be used in. A tagged COSE_Encrypt structure is identified by the CBOR tag 96. The CDDL fragment that represents this is:
COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)
The COSE_Encrypt structure is a CBOR array. The fields of the array in order are:
The CDDL fragment that corresponds to the above text is:
COSE_Encrypt = [
Headers,
ciphertext : bstr / nil,
recipients : [+COSE_recipient]
]
The COSE_recipient structure is a CBOR array. The fields of the array in order are:
The CDDL fragment that corresponds to the above text for COSE_recipient is:
COSE_recipient = [
Headers,
ciphertext : bstr / nil,
? recipients : [+COSE_recipient]
]
An 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 depend 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 content key distribution methods is:
The COSE_Encrypt0 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.
Examples of encrypted messages can be found in Appendix C.3.
The COSE_Encrypt0 structure can be encoded as either tagged or untagged depending on the context it will be used in. A tagged COSE_Encrypt0 structure is identified by the CBOR tag 16. The CDDL fragment that represents this is:
COSE_Encrypt0_Tagged = #6.16(COSE_Encrypt0)
The COSE_Encrypt0 structure is a CBOR array. The fields of the array in order are:
The CDDL fragment for COSE_Encrypt0 that corresponds to the above text is:
COSE_Encrypt0 = [
Headers,
ciphertext : bstr / nil,
]
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 an Enc_structure. The Enc_structure is a CBOR array. The fields of the Enc_structure in order are:
The CDDL fragment that describes the above text is:
Enc_structure = [
context : "Encrypt" / "Encrypt0" / "Enc_Recipient" /
"Mac_Recipient" / "Rec_Recipient",
protected : empty_or_serialized_map,
external_aad : bstr
]
How to encrypt a message:
How to decrypt a message:
How to encrypt a message:
How to decrypt a message:
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, and a recipient algorithm of other than direct.
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 they 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 (SS) 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 the sender assumes that there are only two parties involved and that you did not send the message to yourself.) The origination property can be obtained with both of the MAC message structures.
The multiple recipient MACed 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 MACed messages can be found in Appendix C.5.
The MAC structure can be encoded as either tagged or untagged depending on the context it will be used in. A tagged COSE_Mac structure is identified by the CBOR tag 97. The CDDL fragment that represents this is:
COSE_Mac_Tagged = #6.97(COSE_Mac)
The COSE_Mac structure is a CBOR array. The fields of the array in order are:
The CDDL fragment that represents the above text for COSE_Mac follows.
COSE_Mac = [ Headers, payload : bstr / nil, tag : bstr, recipients :[+COSE_recipient] ]
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 MACed message uses the COSE_Mac0 structure defined in this section for carrying the body. Examples of MACed messages with an implicit key can be found in Appendix C.6.
The MAC structure can be encoded as either tagged or untagged depending on the context it will be used in. A tagged COSE_Mac0 structure is identified by the CBOR tag 17. The CDDL fragment that represents this is:
COSE_Mac0_Tagged = #6.17(COSE_Mac0)
The COSE_Mac0 structure is a CBOR array. The fields of the array in order are:
The CDDL fragment that corresponds to the above text is:
COSE_Mac0 = [ Headers, payload : bstr / nil, tag : bstr, ]
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:
The CDDL fragment that corresponds to the above text is:
MAC_structure = [
context : "MAC" / "MAC0",
protected : empty_or_serialized_map,
external_aad : bstr,
payload : bstr
]
The steps to compute a MAC are:
The steps to verify a MAC are:
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 "COSE Key Common Parameters" registry (Section 15.5). Additional parameters defined for specific key types can be found in the IANA "COSE Key Type Parameters" registry (Section 15.6).
A COSE Key Set uses a CBOR array object as its underlying type. The values of the array elements are COSE Keys. A COSE Key Set MUST have at least one element in the array. Examples of COSE Key Sets can be found in Appendix C.7.
Each element in a COSE Key Set MUST be processed independently. If one element in a COSE Key Set is either malformed or uses a key that is not understood by an application, that key is ignored and the other keys are processed normally.
The element "kty" is a required element in a COSE_Key map.
The CDDL grammar describing COSE_Key and COSE_KeySet is:
COSE_Key = {
1 => tstr / int, ; kty
? 2 => bstr, ; kid
? 3 => tstr / int, ; alg
? 4 => [+ (tstr / int) ], ; key_ops
? 5 => bstr, ; Base IV
* label => values
}
COSE_KeySet = [+COSE_Key]
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 [I-D.schaad-cose-rfc8152bis-algs].
| Name | Label | CBOR Type | Value Registry | Description |
|---|---|---|---|---|
| kty | 1 | tstr / int | COSE Key Common Parameters | Identification of the key type |
| kid | 2 | bstr | Key identification value -- match to kid in message | |
| alg | 3 | tstr / int | COSE Algorithms | Key usage restriction to this algorithm |
| key_ops | 4 | [+ (tstr/int)] | Restrict set of permissible operations | |
| Base IV | 5 | bstr | Base IV to be xor-ed with Partial IVs |
| 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 wrap encryption. |
| unwrap key | 6 | The key is used for key wrap decryption. 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 not to be used as a key. Requires private key fields. |
| MAC create | 9 | The key is used for creating MACs. |
| MAC verify | 10 | The key is used for validating MACs. |
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 the RSA Probabilistic Signature Scheme (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 that 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 the 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)
Signature algorithms are used with the COSE_Signature and COSE_Sign1 structures. 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.
Message Authentication Codes (MACs) provide data authentication and integrity protection. They provide either no or very limited data origination. A MAC, for example, can 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., a Hash-based Message Authentication Code (HMAC)). This document defines a MAC algorithm using each of these constructions.
MAC algorithms are used in the COSE_Mac and COSE_Mac0 structures.
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 CEK 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's sake, the authentication code will normally be defined as being appended to the ciphertext 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.
These algorithms are used in COSE_Encrypt and COSE_Encrypt0.
KDFs are used to take some secret value and generate a different one. The secret value comes in three flavors:
General KDFs work well with the first type of secret, can do reasonably well with the second type of secret, and generally do poorly with the last type of secret. None of the KDFs in this section are designed to deal with the type of secrets that are used for passwords. Functions like PBES2 [RFC8018] need to be used for that type of secret.
The same KDF can be set up to deal with the first two types of secrets in a different way. The KDF defined in !!! HDKF !!! is such a function. This is reflected in the set of algorithms defined for the HMAC-based Extract-and-Expand Key Derivation Function (HKDF).
When using KDFs, 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.
Content key distribution methods (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 those defined in [RFC7516]. Other specifications use different terms for the recipient algorithm classes or do not support some of the recipient algorithm classes.
The direct encryption class algorithms share a secret between the sender and the recipient that is used either directly or after manipulation as the CEK. When direct encryption mode is used, it MUST be the only mode used on the message.
The COSE_Recipient structure for the recipient is organized as follows:
In key wrap 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 wrap algorithms for COSE are AE algorithms. Key wrap 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 that has some degree of organization and may in fact be repeating the same content. The use of key wrap loses the weak data origination that is provided by the direct encryption algorithms.
The COSE_Encrypt structure for the recipient is organized as follows:
Key transport mode is also called key encryption mode in some standards. Key transport 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 transport mode algorithms.
When using a key transport algorithm, the COSE_Encrypt structure for the recipient is organized as follows:
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 online 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 object.
Two variants of DH that are supported are:
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_Encrypt structure for the recipient is organized as follows:
Key Agreement with Key Wrap uses a randomly generated CEK. The CEK is then encrypted using a key wrap algorithm and a key derived from the shared secret computed by the key agreement algorithm. The function for this would be:
encryptedKey = KeyWrap(KDF(DH-Shared, context), CEK)
The COSE_Encrypt structure for the recipient is organized as follows:
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:
This document is designed to provide a set of security services, but not 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 are deferred to each application.
An example of a profile can be found in [OSCOAP] where two profiles are being developed. One is for carrying content by itself, and the other is for carrying content in combination with CoAP headers.
It is intended that a profile of this document be created that defines the interoperability requirements for that specific application. This section provides a set of guidelines and topics that need to be considered when profiling this document.
The registeries and registrations listed below were created during processing of RFC 8152 [RFC8152]. The only known action at this time is to update the references.
IANA has assigned the following tags from the "CBOR Tags" registry. The tags for COSE_Sign1, COSE_Encrypt0, and COSE_Mac0 were assigned in the 1 to 23 value range (one byte long when encoded). The tags for COSE_Sign, COSE_Encrypt, and COSE_Mac were assigned in the 24 to 255 value range (two bytes long when encoded).
The tags assigned are in Table 1.
IANA has created a registry titled "COSE Header Parameters". The registry has been created to use the "Expert Review Required" registration procedure [RFC8126]. Guidelines for the experts are provided in Section 15.11. It should be noted that, in addition to the expert review, some portions of the registry require a specification, potentially a Standards Track RFC, be supplied as well.
The columns of the registry are:
The initial contents of the registry is ...
IANA has created a registry titled "COSE Header Algorithm Parameters". The registry uses the "Expert Review Required" registration procedure. Expert review guidelines are provided in Section 15.11.
The columns of the registry are:
IANA has created a registry titled "COSE Algorithms". The registry has been created to use the "Expert Review Required" registration procedure. Guidelines for the experts are provided in Section 15.11. It should be noted that, in addition to the expert review, some portions of the registry require a specification, potentially a Standards Track RFC, be supplied as well.
NOTE: The assignment of algorithm identifiers in this document was done so that positive numbers were used for the first layer objects (COSE_Sign, COSE_Sign1, COSE_Encrypt, COSE_Encrypt0, COSE_Mac, and COSE_Mac0). Negative numbers were used for second layer objects (COSE_Signature and COSE_recipient). Expert reviewers should consider this practice, but are not expected to be restricted by this precedent.
IANA has created a registry titled "COSE Key Common Parameters". The registry has been created to use the "Expert Review Required" registration procedure. Guidelines for the experts are provided in Section 15.11. It should be noted that, in addition to the expert review, some portions of the registry require a specification, potentially a Standards Track RFC, be supplied as well.
The columns of the registry are:
IANA has created a registry titled "COSE Key Type Parameters". The registry has been created to use the "Expert Review Required" registration procedure. Expert review guidelines are provided in Section 15.11.
The columns of the table are:
IANA has created a new registry titled "COSE Key Types". The registry has been created to use the "Expert Review Required" registration procedure. Expert review guidelines are provided in Section 15.11.
The columns of this table are:
IANA has created a registry titled "COSE Elliptic Curves". The registry has been created to use the "Expert Review Required" registration procedure. Guidelines for the experts are provided in Section 15.11. It should be noted that, in addition to the expert review, some portions of the registry require a specification, potentially a Standards Track RFC, be supplied as well.
The columns of the table are:
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 message.
This section registers the 'application/cose-key' and 'application/cose-key-set' 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.
The template for registering 'application/cose-key' is:
The template for registering 'application/cose-key-set' is:
IANA has added the following entries to the "CoAP Content-Formats" registry.
| Media Type | Encoding | ID | Reference |
|---|---|---|---|
| application/cose; cose-type="cose-sign" | 98 | [RFC8152] | |
| application/cose; cose-type="cose-sign1" | 18 | [RFC8152] | |
| application/cose; cose-type="cose-encrypt" | 96 | [RFC8152] | |
| application/cose; cose-type="cose-encrypt0" | 16 | [RFC8152] | |
| application/cose; cose-type="cose-mac" | 97 | [RFC8152] | |
| application/cose; cose-type="cose-mac0" | 17 | [RFC8152] | |
| application/cose-key | 101 | [RFC8152] | |
| application/cose-key-set | 102 | [RFC8152] |
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:
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 material for any individuals. There are some cases in this document that need to be highlighted on this issue.
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 this 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 the opportunity 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 encouraged 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 'OKP' key is expected.
Before using a key for transmission, or before acting on information received, 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:
There are a large number of algorithms presented in this document that use nonce values. For all of the nonces defined in this document, there is some type of restriction on the nonce being a unique value either for a key or for some other conditions. In all of these cases, there is no known requirement on the nonce being both unique and unpredictable; under these circumstances, it's reasonable to use a counter for creation of the nonce. In cases where one wants the pattern of the nonce to be unpredictable as well as unique, one can use a key created for that purpose and encrypt the counter to produce the nonce value.
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 the length for all of the content encryption algorithms that are defined in this document. This means that it is up to the 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 '.)
A portion of the working group has expressed a strong desire to relax the rule that the algorithm identifier be required to appear in each level of a COSE object. 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 that contains no attributes about itself.
In this section, three sets of recommendations are laid out. The first set of recommendations apply to having an implicit algorithm identified for a single layer of a COSE object. The second set of recommendations apply to having multiple implicit algorithms identified for multiple layers of a COSE object. The third set of recommendations apply to having implicit algorithms for multiple COSE object constructs.
The key words from RFC 2119 are 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 object. This normally applies to the smallest of the COSE objects, specifically COSE_Sign1, COSE_Mac0, and COSE_Encrypt0, but could apply to the other structures as well.
The following items should be taken into account:
The second case is having multiple implicit algorithm identifiers specified for a multiple layer COSE object. An example of how this would work is the encryption context that an application specifies, which contains a content encryption algorithm, a key wrap algorithm, a key identifier, and a shared secret. The sender omits sending the algorithm identifier for both the content layer and the recipient layer leaving only the key identifier. The receiver then uses the key identifier to get the implicit algorithm identifiers.
The following additional items need to be taken into consideration:
The third case is having multiple implicit algorithm identifiers, but targeted at potentially unrelated layers or different COSE objects. There are a number of different scenarios where this might be applicable. Some of these scenarios are:
For these cases, the following additional items need to be considered:
There is a group of people who want to have a counter signature parameter that is directly tied to the value being signed, and thus the authenticated and unauthenticated buckets can be removed from the message being sent. The focus on this is an even smaller size, as all of the information on the process of creating the counter signature is implicit rather than being explicitly carried in the message. This includes not only the algorithm identifier as presented above, but also items such as the key identification, which is always external to the signature structure. This means that the entities that are doing the validation of the counter signature are required to infer which key is to be used from context rather than being explicit. One way of doing this would be to presume that all data coming from a specific port (or to a specific URL) is to be validated by a specific key. (Note that this does not require that the key identifier be part of the value signed as it does not serve a cryptographic purpose. If the key validates the counter signature, then it should be presumed that the entity associated with that key produced the signature.)
When computing the signature for the bare counter signature header, the same Sig_structure defined in Section 4.4 is used. The sign_protected field is omitted, as there is no protected header field in this counter signature header. The value of "CounterSignature0" is placed in the context field of the Sig_stucture.
| Name | Label | Value Type | Value | Description |
|---|---|---|---|---|
| CounterSignature0 | 9 | bstr | Counter signature with implied signer and headers |
All of the currently defined recipient algorithm classes only use two layers of the COSE_Encrypt structure. The first layer is the message content, and the second layer is the content key encryption. However, if one uses a recipient algorithm such as the RSA Key Encapsulation Mechanism (RSA-KEM) (see Appendix A of RSA-KEM [RFC5990]), then it makes sense to have three layers of the COSE_Encrypt structure.
These layers would be:
This is an example of what a triple layer message would look like. The message has the following layers:
In effect, this example is a decomposed version of using the ECDH‑ES+A128KW algorithm.
Size of binary file is 183 bytes
96(
[
/ protected / h'a10101' / {
\ alg \ 1:1 \ AES-GCM 128 \
} / ,
/ unprotected / {
/ iv / 5:h'02d1f7e6f26c43d4868d87ce'
},
/ ciphertext / h'64f84d913ba60a76070a9a48f26e97e863e2852948658f0
811139868826e89218a75715b',
/ recipients / [
[
/ protected / h'',
/ unprotected / {
/ alg / 1:-3 / A128KW /
},
/ ciphertext / h'dbd43c4e9d719c27c6275c67d628d493f090593db82
18f11',
/ 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''
]
]
]
]
]
)
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 [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.rb < 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()
This example uses the following:
Size of binary file is 103 bytes
98(
[
/ protected / h'',
/ unprotected / {},
/ payload / 'This is the content.',
/ signatures / [
[
/ protected / h'a10126' / {
\ alg \ 1:-7 \ ECDSA 256 \
} / ,
/ unprotected / {
/ kid / 4:'11'
},
/ signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
98f53afd2fa0f30a'
]
]
]
)
This example uses the following:
Size of binary file is 277 bytes
98(
[
/ protected / h'',
/ unprotected / {},
/ payload / 'This is the content.',
/ signatures / [
[
/ protected / h'a10126' / {
\ alg \ 1:-7 \ ECDSA 256 \
} / ,
/ unprotected / {
/ kid / 4:'11'
},
/ signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
98f53afd2fa0f30a'
],
[
/ protected / h'a1013823' / {
\ alg \ 1:-36
} / ,
/ unprotected / {
/ kid / 4:'bilbo.baggins@hobbiton.example'
},
/ signature / h'00a2d28a7c2bdb1587877420f65adf7d0b9a06635dd1
de64bb62974c863f0b160dd2163734034e6ac003b01e8705524c5c4ca479a952f024
7ee8cb0b4fb7397ba08d009e0c8bf482270cc5771aa143966e5a469a09f613488030
c5b07ec6d722e3835adb5b2d8c44e95ffb13877dd2582866883535de3bb03d01753f
83ab87bb4f7a0297'
]
]
]
)
This example uses the following:
Size of binary file is 180 bytes
98(
[
/ protected / h'',
/ unprotected / {
/ countersign / 7:[
/ protected / h'a10126' / {
\ alg \ 1:-7 \ ECDSA 256 \
} / ,
/ unprotected / {
/ kid / 4:'11'
},
/ signature / h'5ac05e289d5d0e1b0a7f048a5d2b643813ded50bc9e4
9220f4f7278f85f19d4a77d655c9d3b51e805a74b099e1e085aacd97fc29d72f887e
8802bb6650cceb2c'
]
},
/ payload / 'This is the content.',
/ signatures / [
[
/ protected / h'a10126' / {
\ alg \ 1:-7 \ ECDSA 256 \
} / ,
/ unprotected / {
/ kid / 4:'11'
},
/ signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
98f53afd2fa0f30a'
]
]
]
)
This example uses the following:
Size of binary file is 125 bytes
98(
[
/ protected / h'a2687265736572766564f40281687265736572766564' /
{
"reserved":false,
\ crit \ 2:[
"reserved"
]
} / ,
/ unprotected / {},
/ payload / 'This is the content.',
/ signatures / [
[
/ protected / h'a10126' / {
\ alg \ 1:-7 \ ECDSA 256 \
} / ,
/ unprotected / {
/ kid / 4:'11'
},
/ signature / h'3fc54702aa56e1b2cb20284294c9106a63f91bac658d
69351210a031d8fc7c5ff3e4be39445b1a3e83e1510d1aca2f2e8a7c081c7645042b
18aba9d1fad1bd9c'
]
]
]
)
This example uses the following:
Size of binary file is 98 bytes
18(
[
/ protected / h'a10126' / {
\ alg \ 1:-7 \ ECDSA 256 \
} / ,
/ unprotected / {
/ kid / 4:'11'
},
/ payload / 'This is the content.',
/ signature / h'8eb33e4ca31d1c465ab05aac34cc6b23d58fef5c083106c4
d25a91aef0b0117e2af9a291aa32e14ab834dc56ed2a223444547e01f11d3b0916e5
a4c345cacb36'
]
)
This example uses the following:
Size of binary file is 151 bytes
96(
[
/ protected / h'a10101' / {
\ alg \ 1:1 \ AES-GCM 128 \
} / ,
/ unprotected / {
/ iv / 5:h'c9cf4df2fe6c632bf7886413'
},
/ ciphertext / h'7adbe2709ca818fb415f1e5df66f4e1a51053ba6d65a1a0
c52a357da7a644b8070a151b0',
/ 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''
]
]
]
)
This example uses the following:
Size of binary file is 91 bytes
96(
[
/ protected / h'a1010a' / {
\ alg \ 1:10 \ AES-CCM-16-64-128 \
} / ,
/ unprotected / {
/ iv / 5:h'89f52f65a1c580933b5261a76c'
},
/ ciphertext / h'753548a19b1307084ca7b2056924ed95f2e3b17006dfe93
1b687b847',
/ recipients / [
[
/ protected / h'a10129' / {
\ alg \ 1:-10
} / ,
/ unprotected / {
/ salt / -20:'aabbccddeeffgghh',
/ kid / 4:'our-secret'
},
/ ciphertext / h''
]
]
]
)
This example uses the following:
Size of binary file is 326 bytes
96(
[
/ 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'00929663c8789bb28177ae28467e66377da12302d7f9
594d2999afa5dfa531294f8896f2b6cdf1740014f4c7f1a358e3a6cf57f4ed6fb02f
cf8f7aa989f5dfd07f0700a3a7d8f3c604ba70fa9411bd10c2591b483e1d2c31de00
3183e434d8fba18f17a4c7e3dfa003ac1cf3d30d44d2533c4989d3ac38c38b71481c
c3430c9d65e7ddff'
]
},
/ ciphertext / h'7adbe2709ca818fb415f1e5df66f4e1a51053ba6d65a1a0
c52a357da7a644b8070a151b0',
/ 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''
]
]
]
)
This example uses the following:
Size of binary file is 173 bytes
96(
[
/ protected / h'a10101' / {
\ alg \ 1:1 \ AES-GCM 128 \
} / ,
/ unprotected / {
/ iv / 5:h'02d1f7e6f26c43d4868d87ce'
},
/ ciphertext / h'64f84d913ba60a76070a9a48f26e97e863e28529d8f5335
e5f0165eee976b4a5f6c6f09d',
/ 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'41e0d76f579dbd0d936a662d54d8582037de2e366fd
e1c62'
]
]
]
)
This example uses the following:
Size of binary file is 52 bytes
16(
[
/ protected / h'a1010a' / {
\ alg \ 1:10 \ AES-CCM-16-64-128 \
} / ,
/ unprotected / {
/ iv / 5:h'89f52f65a1c580933b5261a78c'
},
/ ciphertext / h'5974e1b99a3a4cc09a659aa2e9e7fff161d38ce71cb45ce
460ffb569'
]
)
This example uses the following:
Size of binary file is 41 bytes
16(
[
/ protected / h'a1010a' / {
\ alg \ 1:10 \ AES-CCM-16-64-128 \
} / ,
/ unprotected / {
/ partial iv / 6:h'61a7'
},
/ ciphertext / h'252a8911d465c125b6764739700f0141ed09192de139e05
3bd09abca'
]
)
This example uses the following:
Size of binary file is 57 bytes
97(
[
/ 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''
]
]
]
)
This example uses the following:
Size of binary file is 214 bytes
97(
[
/ protected / h'a10105' / {
\ alg \ 1:5 \ HMAC 256//256 \
} / ,
/ unprotected / {},
/ payload / 'This is the content.',
/ tag / h'81a03448acd3d305376eaa11fb3fe416a955be2cbe7ec96f012c99
4bc3f16a41',
/ 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''
]
]
]
)
This example uses the following:
Size of binary file is 109 bytes
97(
[
/ 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'
]
]
]
)
This example uses the following:
Size of binary file is 309 bytes
97(
[
/ 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'339bc4f79984cdc6b3e6ce5f315a4c7d2b0ac466fce
a69e8c07dfbca5bb1f661bc5f8e0df9e3eff5'
],
[
/ protected / h'',
/ unprotected / {
/ alg / 1:-5 / A256KW /,
/ kid / 4:'018c0ae5-4d9b-471b-bfd6-eef314bc7037'
},
/ ciphertext / h'0b2c7cfce04e98276342d6476a7723c090dfdd15f9a
518e7736549e998370695e6d6a83b4ae507bb'
]
]
]
)
This example uses the following:
Size of binary file is 37 bytes
17(
[
/ 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.
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:
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'
}
]
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:
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'
}
]
This document is a product of the COSE working group of the IETF.
The following individuals are to blame for getting me started on this project in the first place: Richard Barnes, Matt Miller, and Martin Thomson.
The initial version of the specification was based to some degree on the outputs of the JOSE and S/MIME working groups.
The following individuals provided input into the final form of the document: Carsten Bormann, John Bradley, Brain Campbell, Michael B. Jones, Ilari Liusvaara, Francesca Palombini, Ludwig Seitz, and Goran Selander.