Header Keys

name	label	value	registry	description
alg	1	int / tstr	COSE Algorithm Registry	Integers are taken from table Appendix E
crit	2	[+ label]	COSE Header Label Registry	integer values are from this table.
cty	3	tstr / int		Value is either a mime-content type or an integer from the mime-content type table
jku	*	tstr		URL to COSE key object
jwk	*	COSE_Key		contains a COSE key not a JWK key
kid	4	bstr		key identifier
x5c	*	bstr*		X.509 Certificate Chain
x5t	*	bstr		SHA-1 thumbprint of key
x5t#S256	*	bstr		SHA-256 thumbprint of key
x5u	*	tstr		URL for X.509 certificate
zip	*	int / tstr		Integers are taken from the table Appendix E

OPEN ISSUES:

1. Which of the following items do we want to have standardized in this document: jku, jwk, x5c, x5t, x5t#S256, x5u, zip
2. I am currently torn on the question "Should epk and iv/nonce be algorithm specific or generic headers?" They are really specific to an algorithm and can potentially be defined in different ways for different algorithms. As an example, it would make sense to defined nonce for CCM and GCM modes that can have the leading zero bytes stripped, while for other algorithms this might be undesirable.
3. We might want to define some additional items. What are they? A possible example would be a sequence number as this might be common. On the other hand, this is the type of things that is frequently used as the nonce in some places and thus should not be used in the same way. Other items might be challenge/response fields for freshness as these are likely to be common.

4. Signing Structure

The signature structure allows for one or more signatures to be applied to a message payload. There are provisions for attributes about the content and attributes about the signature to be carried along with the signature itself. These attributes may be authenticated by the signature, or just present. Examples of attributes about the content would be the type of content, when the content was created, and who created the content. Examples of attributes 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 CDDL grammar for a signature message is:

COSE_Sign = {
    msg_type => msg_type_signed,
    Headers,
    ? payload => bstr,
    signatures => [+ COSE_signature]
}

The fields is the structure have the following semantics:

msg_type

identifies this as providing the signed security service. The value MUST be msg_type_signed (1).

protected

contains attributes about the payload which are to be protected by the signature. An example of such an attribute would be the content type ('cty') attribute. The content is a CBOR map of attributes which is encoded to a byte stream. This field MUST NOT contain attributes about the signature, even if those attributes are common across multiple signatures. The labels in this map are typically taken from Table 2.

unprotected

contains attributes about the payload which are not protected by the signature. An example of such an attribute would be the content type ('cty') attribute. This field MUST NOT contain attributes about a signature, even if the attributes are common across multiple signatures. The labels in this map are typically taken from Table 2.

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 it is the responsibility of the application to ensure that it will be transported without changes.

signatures

is an array of signature items. Each of these items uses the COSE_signature structure for its representation.

We use the values in Table 1 as the labels in the COSE_Sign map. While other labels can be present in the map, it is not generally a recommended practice. The other labels can be either of integer or string type, use of other types SHOULD be treated as an error.

The CDDL grammar structure for a signature is:

COSE_signature =  {
    Headers,      
    signature => bstr
}

The fields in the structure have the following semantics:

protected

contains additional information to be authenticated by the signature. The field holds data about the signature operation. The field MUST NOT hold attributes about the payload being signed. The content is a CBOR map of attributes which is encoded to a byte stream. At least one of protected and unprotected MUST be present.

unprotected

contains attributes about the signature which are not protected by the signature. This field MUST NOT contain attributes about the payload being signed. At least one of protected and unprotected MUST be present.

signature

contains the computed signature value.

The COSE structure used to create the byte stream to be signed uses the following CDDL grammar structure:

Sig_structure = [
    body_protected: bstr,
    sign_protected: bstr,
    payload: bstr
]

How to compute a signature:

1. Create a Sig_structure object and populate it with the appropriate fields. For body_protected and sign_protected, if the fields are not present in their corresponding maps, an bstr of length zero is used.
2. Create the value ToBeSigned by encoding the Sig_structure to a byte string.
3. 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).
4. Place the resulting signature value in the 'signature' field of the map.

How to verify a signature:

1. Create a Sig_structure object and populate it with the appropriate fields. For body_protected and sign_protected, if the fields are not present in their corresponding maps, an bstr of length zero is used.
2. Create the value ToBeSigned by encoding the Sig_structure to a byte string.
3. Call the signature verification algorithm passing in K (the key to verify with), alg (the algorithm to 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 appropriated authorization is done.

5. Encryption object

In this section we describe the structure and methods to be used when doing an encryption in COSE. In COSE, we use the same techniques and structures 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 plain text and for the different key management techniques.

One of the byproducts of using the same technique for encrypting and encoding both the content and the keys using the various key management techniques, is a requirement that all of the key management techniques use an Authenticated Encryption (AE) algorithm. (For the purpose of this document we use a slightly loose definition of AE algorithms.) When encrypting the plain text, it is normal to use an Authenticated Encryption with Additional Data (AEAD) algorithm. For key management, either AE or AEAD algorithms can be used. See Appendix A for more details about the different types of algorithms. [CREF7]Ilari: I don't follow/understand this text

The CDDL grammar structure for encryption is:

COSE_encrypt = {
    msg_type=>msg_type_encrypted,
    COSE_encrypt_fields
}

COSE_encrypt_fields = (
    Headers,
    ? ciphertext => bstr,
    ? recipients => [+{COSE_encrypt_fields}]
)

Description of the fields:

msg_type

identifies this as providing the encrypted security service. The value MUST be msg_type_encrypted (2).

protected

contains the information about the plain text or encryption process that is to be integrity protected. The field is encoded in CBOR as a 'bstr'. The contents of the protected field is a CBOR map of the protected data names and values. The map is CBOR encoded before placing it into the bstr. Only values associated with the current cipher text are to be placed in this location even if the value would apply to multiple recipient structures.

unprotected

contains information about the plain text that is not integrity protected. Only values associated with the current cipher text are to be placed in this location even if the value would apply to multiple recipient structures.

cipherText

contains the encrypted plain text. If the cipherText is to be transported independently of the control information about the encryption process (i.e. detached content) then the field is omitted.

recipients

contains the recipient information. It is required that at least one recipient MUST be present for the content encryption layer.

5.1. Key Management Methods

There are a number of different key management methods that can be used in the COSE encryption system. In this section we will discuss each of the key management methods and what fields need to be specified to deal with each of them.

The names of the key management methods used here are the same as are defined in [RFC7517]. Other specifications use different terms for the key management methods or do not support some of the key management methods.

At the moment we do not have any key management methods that allow for the use of protected headers. This may be changed in the future if, for example, the AES-GCM Key wrap method defined in [RFC7518] were extended to allow for authenticated data. In that event the use of the 'protected' field, which is current forbidden below, would be permitted.

5.1.1. Direct Encryption

In direct encryption mode, a shared secret between the sender and the recipient is used as the CEK. When direct encryption mode is used, it MUST be the only mode used on the message. It is a massive security leak to have both direct encryption and a different key management mode on the same message.

For JOSE, direct encryption key management is the only key management method allowed for doing MAC-ed messages. In COSE, all of the key management methods can be used for MAC-ed messages.

The COSE_encrypt structure for the recipient is organized as follows:

• The 'protected', 'ciphertext' and 'recipients' fields MUST be absent.
• At a minimum, the 'unprotected' field MUST contain the 'alg' parameter and SHOULD contain a parameter identifying the shared secret.

5.1.2. 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 JOSE (and thus 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 COSE_encrypt structure for the recipient is organized as follows:

• The 'protected', and 'recipients' fields 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 shared secret.

5.1.3. 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. The only current Key Encryption mode algorithm supported is RSAES-OAEP.

The COSE_encrypt 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.

5.1.4. Direct Key Agreement

Direct Key Agreement derives the CEK from the shared secret computed by the key agreement operation.

When direct key agreement mode is used, it SHOULD be the only mode used on the message. This method creates the CEK directly and that makes it difficult to mix with additional recipients.

The COSE_encrypt structure for the recipient is organized as follows:

• The 'protected' field MUST be absent.
• At a minimum, the 'unprotected' field MUST contain the 'alg' parameter and SHOULD contain a parameter identifying the asymmetric key.
• The 'unprotected' field MUST contain the 'epk' parameter.

5.1.5. Key Agreement with Key Wrapping

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_encrypt 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, a parameter identifying the recipient asymmetric key, and a parameter with the sender's asymmetric public key.

5.2. Encryption Algorithm for AEAD algorithms

The encryption algorithm for AEAD algorithms is fairly simple.
In order to get a consistent encoding of the data to be authenticated, the Enc_structure is used to have canonical form of the AAD.

Enc_structure = [
    protected: bstr,
    external_aad: bstr
]

1. Copy the protected header field from the message to be sent.
2. If the application has supplied external additional authenticated data to be included in the computation, then it is placed in the 'external_aad' field. If no data was supplied, then a zero length binary value is used.
3. Encode the Enc_structure using a CBOR Canonical encoding Section 8 to get the AAD value.
4. Determine the encryption key. This step is dependent on the key management method 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.

5. Call the encryption algorithm with K (the encryption key to use), P (the plain text) and AAD (the additional authenticated data). Place the returned cipher text into the 'ciphertext' field of the structure.
6. For recipients of the message, recursively perform the encryption algorithm for that recipient using the encryption key as the plain text.

5.3. Encryption algorithm for AE algorithms

1. Verify that the 'protected' field is absent.
2. Verify that there was no external additional authenticated data supplied for this operation.
3. Determine the encryption key. This step is dependent on the key management method 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.

4. 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.
5. For recipients of the message, recursively perform the encryption algorithm for that recipient using the encryption key as the plain text.

6. MAC objects

In this section we describe the structure and methods to be used when doing MAC authentication in COSE. JOSE used a variant of the signature structure for doing MAC operations and it is restricted to using a single pre-shared secret to do the authentication. This document allows for the use of all of the same methods of key management 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 of the key management methods 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 key management to verify who sent it. The key management modes that support this are ones that either use a pre-shared secret, or do static-static key agreement. In both of these cases the entity MAC-ing the message can be validated by a key binding. (The binding of identity assumes that there are only two parties involved and you did not send the message yourself.)

COSE_mac = {
   msg_type=>msg_type_mac, 
   Headers,
   ? payload => bstr,
   tag => bstr,
   recipients => [+{COSE_encrypt_fields}]
}

Field descriptions:

msg_type

identifies this as providing the encrypted security service. The value MUST be msg_type_mac (3).

protected

contains attributes about the payload which are to be protected by the MAC. An example of such an attribute would be the content type ('cty') attribute. The content is a CBOR map of attributes which is encoded to a byte stream. This field MUST NOT contain attributes about the recipient, even if those attributes are common across multiple recipients. At least one of protected and unprotected MUST be present.

unprotected

contains attributes about the payload which are not protected by the MAC. An example of such an attribute would be the content type ('cty') attribute. This field MUST NOT contain attributes about a recipient, even if the attributes are common across multiple recipients. At least one of protected and unprotected MUST be present.

payload

contains the serialized content to be MAC-ed.
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 it is the responsibility of the application to ensure that it will be transported without changes.

tag

contains the MAC value.

recipients

contains the recipient information. See the description under COSE_Encryption for more info.

 MAC_structure = [
     protected: bstr,
     external_aad: bstr,
     payload: bstr
]

How to compute a MAC:

1. Create a MAC_structure and copy the protected and payload elements from the COSE_mac structure.
2. 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'.
3. Encode the MAC_structure using a canonical CBOR encoder. The resulting bytes is the value to compute the MAC on.
4. Compute the MAC and place the result in the 'tag' field of the COSE_mac structure.
5. Encrypt and encode the MAC key for each recipient of the message.

7. Key Structure

There are only a few changes between JOSE and COSE for how keys are formatted. As with JOSE, COSE uses a map to contain the elements of a key. Those values, which in JOSE, are base64url encoded because they are binary values, are encoded as bstr values in COSE.

For COSE we use the same set of fields that were defined in [RFC7517].

COSE_Key = {
    kty => tstr / int,
    ? key_ops => [+ tstr / int ],
    ? alg => tstr / int,
    ? kid => bstr,
    * keys => values
}

COSE_KeySet = [+COSE_Key]

The element “kty” is a required element in a COSE_Key map.
All other elements are optional and not all of the elements listed in [RFC7517] or [RFC7518] have been listed here even though they can all appear in a COSE_Key map.

The “key_ops” element is preferred over the “use” element as the information provided that way is more finely detailed about the operations allowed. It is strongly suggested that this element be present for all keys.

The same fields defined in [RFC7517] are used here with the following changes in rules:

• Any item which is base64 encoded in JWK, is bstr encoded for COSE.
• Any item which is integer encoded in JWK, is int encoded for COSE.
• Any item which is string (but not base64) encoded in JWK, is tstr encoded for COSE.
• Exceptions to this are the following fields:

  kid

  is always bstr encoded rather than tstr encoded. This change in encoded is due to the fact that frequently, values such as a hash of the public key is used for a kid value. Since the field is defined as not having a specific structure, making it binary rather than textual makes sense.

8. CBOR Encoder Restrictions

There as 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.
• All parsers used SHOULD fail on both parsing and generation if the same label is used twice as a key for the same map.

9. IANA Considerations

9.1. CBOR Tag assignment

It is requested that IANA assign a new tag from the “Concise Binary Object Representation (CBOR) Tags” registry. It is requested that the tag be assigned in the 0 to 23 value range.

Tag Value: TBD1

Data Item: CBOR map

Semantics: COSE security message.

9.2. COSE Object Labels Registry

It is requested that IANA create a new registry entitled “COSE Object Labels Registry”. [CREF8]JLS: Finish the registration process.

This table is initially populated by the table in Table 1.

9.3. COSE Header Label Table

It is requested that IANA create a new registry entitled “COSE Header Labels”.

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 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 server. 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.

NOTE: Need to review the range assignments. It does not necessarily make sense as specification required uses 1 byte positive integers and 2 byte strings.

9.4. COSE Header Algorithm Label Table

It is requested that IANA create a new registry entitled “COSE Header Algorithm Labels”.

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 Appendix D. The specification column for all rows in that table should be this document.

9.5. COSE Algorithm Registry

It is requested that IANA create a new registry entitled “COSE Algorithm Registry”.

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 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 server. 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.

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 Appendix E. The specification column for all rows in that table should be this document.

9.6. COSE Key Map Registry

It is requested that IANA create a new registry entitled “COSE Key Map Registry”.

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. Algorithm 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 server. Integer values in the range -1 to -65536 are used for key parameters specific to a single algorithm delegated to the “COSE Key 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 Appendix G. The specification column for all of these entries will be this document.

9.7. COSE Key Parameter Registry

It is requested that IANA create a new registry “COSE Key Parameters”.

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. Keys are expected to be re-used 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 Appendix H. The specification column for all of these entries will be this document.

9.8. Media Type Registration

9.8.1. COSE Security Message

This section registers the "application/cose" and "application/cose+cbor" media types in the "Media Types" registry. [CREF9]JLS: Should we register both or just the cose+cbor one? 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: 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+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

9.8.2. COSE Key media type

This section registers the "application/jwk+json" and "application/jwk-set+json" media typesin 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

10. Security Considerations

There are security considerations:

1. Protect private keys
2. MAC messages with more than one recipient means one cannot figure out who sent the message
3. Use of direct key with other recipient structures hands the key to other recipients.
4. Use of direct ECDH direct encryption is easy for people to leak information on if there are other recipients in the message.
5. Considerations about protected vs unprotected header fields.

11. References

11.1. Normative References

[I-D.greevenbosch-appsawg-cbor-cddl]	Vigano, C., Birkholz, H. and R. Sun, "CBOR data definition language: a notational convention to express CBOR data structures.", Internet-Draft draft-greevenbosch-appsawg-cbor-cddl-05, March 2015.
[RFC7049]	Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, October 2013.

11.2. Informative References

[AES-GCM]	Dworkin, M., "NIST Special Publication 800-38D: Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC.", June 2015.
[I-D.mcgrew-aead-aes-cbc-hmac-sha2]	McGrew, D., Foley, J. and K. Paterson, "Authenticated Encryption with AES-CBC and HMAC-SHA", Internet-Draft draft-mcgrew-aead-aes-cbc-hmac-sha2-05, July 2014.
[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3394]	Schaad, J. and R. Housley, "Advanced Encryption Standard (AES) Key Wrap Algorithm", RFC 3394, September 2002.
[RFC3447]	Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003.
[RFC3610]	Whiting, D., Housley, R. and N. Ferguson, "Counter with CBC-MAC (CCM)", RFC 3610, September 2003.
[RFC5652]	Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, September 2009.
[RFC5752]	Turner, S. and J. Schaad, "Multiple Signatures in Cryptographic Message Syntax (CMS)", RFC 5752, 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, September 2010.
[RFC7159]	Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, March 2014.
[RFC7515]	Jones, M., Bradley, J. and N. Sakimura, "JSON Web Signature (JWS)", RFC 7515, May 2015.
[RFC7516]	Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)", RFC 7516, May 2015.
[RFC7517]	Jones, M., "JSON Web Key (JWK)", RFC 7517, May 2015.
[RFC7518]	Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, May 2015.

Appendix A. AEAD and AE algorithms

The set of encryption algorithms that can be used with this specification is restricted to authenticated encryption (AE) and authenticated encryption with additional data (AEAD) algorithms. This means that there is a strong check that the data decrypted by the recipient is the same as what was encrypted by the sender. Encryption modes such as counter have no check on this at all. The CBC encryption mode had a weak check that the data is correct, given a random key and random data, the CBC padding check will pass one out of 256 times. There have been several times that a normal encryption mode has been combined with an integrity check to provide a content encryption mode that does provide the necessary authentication. AES-GCM [AES-GCM], AES-CCM [RFC3610], AES-CBC-HMAC [I-D.mcgrew-aead-aes-cbc-hmac-sha2] are examples of these composite modes.

2PKCS v1.5 RSA key transport does not qualify as an AE algorithm. There are only three bytes in the encoding that can be checked as having decrypted correctly, the rest of the content can only be probabilistically checked as having decrypted correctly. For this reason, PKCS v1.5 RSA key transport MUST NOT be used with this specification. RSA-OAEP was designed to have the necessary checks that that content correctly decrypted and does qualify as an AE algorithm.

When dealing with authenticated encryption algorithms, there is always some type of value that needs to be checked to see if the authentication level has passed. This authentication value may be:

• A separately generated tag computed by both the encrypter and decrypter and then compared by the decryptor. This tag value may be either placed at the end of the cipher text (the decision we made) or kept separately (the decision made by the JOSE working group). This is the approach followed by AES-GCM [AES-GCM] and AES-CCM [RFC3610].
• A fixed value which is part of the encoded plain text. This is the approach followed by the AES key wrap algorithm [RFC3394].
• A computed value is included as part of the encoded plain text. The computed value is then checked by the decryptor using the same computation path. This is the approach followed by RSAES-OAEP [RFC3447].

Appendix B. Three Levels of Recipient Information

All of the currently defined Key Management methods only use two levels of the COSE_Encrypt structure. The first level is the message content and the second level is the content key encryption. However, if one uses a key management technique such as RSA-KEM (see Appendix A of RSA-KEM [RFC5990], then it make sense to have three levels of the COSE_Encrypt 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 3: 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.

{
  1: 2,
  2: h'a10101',
  3: {
    -1: h'02d1f7e6f26c43d4868d87ce'
  },
  4: h'64f84d913ba60a76070a9a48f26e97e863e285295a44320878caceb076
3a334806857c67',
  9: [
    {
      3: {
        1: -3
      },
      4: h'5a15dbf5b282ecb31a6074ee3815c252405dd7583e078188',
      9: [
        {
          3: {
            1: "ECDH-ES",
            5: "meriadoc.brandybuck@buckland.example",
            4: {
              1: 1,
              -1: 4,
              -2: h'b2add44368ea6d641f9ca9af308b4079aeb519f11e9b8
a55a600b21233e86e68',
              -3: h'1a2cf118b9ee6895c8f415b686d4ca1cef362d4a7630a
31ef6019c0c56d33de0'
            }
          }
        }
      ]
    }
  ]
}

Appendix C. Examples

The examples can be found at https://github.com/cose-wg/Examples. I am currently still in the process of getting the examples up there along with some control information for people to be able to check and reproduce the examples.

C.1. Direct MAC

This example has some features that are in questions but not yet incorporated in the document.

To make it easier to read, this uses CBOR's diagnostic notation rather than a binary dump.

This example is uses HMAC with SHA-256 as the digest algorithm. The key management is uses two static ECDH keys along with HKDF to directly derive the key used in the HMAC operation.

{
  1: 3,
  2: h'a10104',
  4: h'546869732069732074686520636f6e74656e742e',
  10: h'2ba937ca03d76c3dbad30cfcbaeef586f9c0f9ba616ad67e9205d3857
6ad9930',
  9: [
    {
      3: {
        1: "ECDH-SS",
        5: "meriadoc.brandybuck@buckland.example",
        "spk": {
          "kid": "peregrin.took@tuckborough.example"
        },
        "apu": h'4d8553e7e74f3c6a3a9dd3ef286a8195cbf8a23d19558ccf
ec7d34b824f42d92bd06bd2c7f0271f0214e141fb779ae2856abf585a58368b01
7e7f2a9e5ce4db5'
      }
    }
  ]
}

C.2. Wrapped MAC

This example has some features that are in questions but not yet incorporated in the document.

To make it easier to read, this uses CBOR's diagnostic notation rather than a binary dump.

This example uses AES-128-MAC truncated to 64-bits as the digest algorithm. It uses AES-256 Key wrap for the key management algorithm wrapping the 128-bit key used for the digest algorithm.

{
  1: 3,
  2: h'a1016e4145532d3132382d4d41432d3634',
  4: h'546869732069732074686520636f6e74656e742e',
  10: h'6d1fa77b2dd9146a',
  9: [
    {
      3: {
        1: -5,
        5: "018c0ae5-4d9b-471b-bfd6-eef314bc7037"
      },
      4: h'711ab0dc2fc4585dce27effa6781c8093eba906f227b6eb0'
    }
  ]
}

C.3. Multi-recipient MAC message

This example has some features that are in questions but not yet incorporated in the document.

To make it easier to read, this uses CBOR's diagnostic notation rather than a binary dump.

This example uses HMAC with SHA-256 for the digest algorithm. There are three different key management techniques applied:

• An ephemeral static ECDH key agreement operation using AES-128 key wrap on the digest key.
• Key transport using RSA-OAEP with SHA-256 for the hash and the mfg function operations.
• AES 256-bit Key wrap using a pre-shared secret.

{
  1: 3,
  2: h'a10104',
  4: h'546869732069732074686520636f6e74656e742e',
  10: h'7aaa6e74546873061f0a7de21ff0c0658d401a68da738dd8937486519
83ce1d0',
  9: [
    {
      3: {
        1: "ECDH-ES+A128KW",
        5: h'62696c626f2e62616767696e7340686f626269746f6e2e657861
6d706c65',
        4: {
          1: 1,
          -1: 5,
          -2: h'43b12669acac3fd27898ffba0bcd2e6c366d53bc4db71f909
a759304acfb5e18cdc7ba0b13ff8c7636271a6924b1ac63c02688075b55ef2d61
3574e7dc242f79c3',
          -3: h'812dd694f4ef32b11014d74010a954689c6b6e8785b333d1a
b44f22b9d1091ae8fc8ae40b687e5cfbe7ee6f8b47918a07bb04e9f5b1a51a334
a16bc09777434113'
        }
      },
      4: h'1b120c848c7f2f8943e402cbdbdb58efb281753af4169c70d0126c
0d16436277160821790ef4fe3f'
    },
    {
      3: {
        1: -2,
        5: h'62696c626f2e62616767696e7340686f626269746f6e2e657861
6d706c65'
      },
      4: h'46c4f88069b650909a891e84013614cd58a3668f88fa18f3852940
a20b35098591d3aacf91c125a2595cda7bee75a490579f0e2f20fd6bc956623bf
de3029c318f82c426dac3463b261c981ab18b72fe9409412e5c7f2d8f2b5abaf7
80df6a282db033b3a863fa957408b81741878f466dcc437006ca21407181a016c
a608ca8208bd3c5a1ddc828531e30b89a67ec6bb97b0c3c3c92036c0cb84aa0f0
ce8c3e4a215d173bfa668f116ca9f1177505afb7629a9b0b5e096e81d37900e06
f561a32b6bc993fc6d0cb5d4bb81b74e6ffb0958dac7227c2eb8856303d989f93
b4a051830706a4c44e8314ec846022eab727e16ada628f12ee7978855550249cc
b58'
    },
    {
      3: {
        1: -5,
        5: "018c0ae5-4d9b-471b-bfd6-eef314bc7037"
      },
      4: h'0b2c7cfce04e98276342d6476a7723c090dfdd15f9a518e7736549
e998370695e6d6a83b4ae507bb'
    }
  ]
}

C.4. Direct ECDH

This example has some features that are in questions but not yet incorporated in the document.

To make it easier to read, this uses CBOR's diagnostic notation rather than a binary dump.

Encoded in CBOR - 216 bytes, content is 14 bytes long

{
  1: 2,
  2: h'a10101',
  3: {
    -1: h'c9cf4df2fe6c632bf7886413'
  },
  4: h'45fce2814311024d3a479e7d3eed063850f3f0b9f3f948677e3ae9869b
cf9ff4e1763812',
  9: [
    {
      3: {
        1: "ECDH-ES",
        5: "meriadoc.brandybuck@buckland.example",
        4: {
          1: 1,
          -1: 4,
          -2: h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf05
4e1c7b4d91d6280',
          -3: h'f01400b089867804b8e9fc96c3932161f1934f4223069170d
924b7e03bf822bb'
        }
      }
    }
  ]
}

C.5. Single Signature

This example has some features that are in questions but not yet cooperated in the document.

To make it easier to read, this uses CBOR's diagnostic notation rather than a binary dump.

{
  1: 1,
  4: h'546869732069732074686520636f6e74656e742e',
  5: [
    {
      2: h'a20165505333383405781e62696c626f2e62616767696e7340686f
626269746f6e2e6578616d706c65',
      6: h'7c4656acc11ffe98e2ea4babff6d177b2e5a088da2034e0096a6f8
8cd50a36ed971a83f42244d40d97043d080f43dff7a7c0eb5bc322a3d3d18826c
e755d82293ecc22a3919857bc60c456017ca87b7d662971687aac7315f68c9f52
d95c24f72418b7eb07cd432875ab42658bde269534da10d8572c2d5340be660cf
cf7f6cceaf5cd02ddfdac1cf18930199c0e8721c2d71c646516cc3da79f6d555e
89825803ef1ecf7fb411cea24d15610f56d33af0d1a7e68a2e07e9790326f01f2
9ad7210e317c637f7cbd16f8cd32a9959dfd419de697dfa3145aa7cd1019072f9
5c3d041bd8d47de09abbce16117733378e1593fe2439454cd907f88aabc664d26
298'
    }
  ]
}

C.6. Multiple Signers

This example has some features that are in questions but not yet cooperated in the document.

To make it easier to read, this uses CBOR's diagnostic notation rather than a binary dump.

Encoded in CBOR - 491 bytes, content is 14 bytes long

{
  1: 1,
  4: h'546869732069732074686520636f6e74656e742e',
  5: [
    {
      2: h'a10129',
      3: {
        5: "bilbo.baggins@hobbiton.example"
      },
      6: h'93f83aaf872d9fc8c7cad30437dc8fed33a673ec5d4d8e004f16d9
ed236f6b2ce4cc15ea48aef8c5c0c4eab8ed539a4eae27ac9d5f5e6ef7b9cfb02
133f3f4ad7062db989fcff6ecc67c13624418224416116ff0a67d1a133d27bb79
1a1893a03c683d84def6742059c63670c2738efc2dde8eac364b4d714c60db424
ffbfb098c579c6538daa2a9fc5be2d829a82c148c9913a537f98ceb469b78fae2
02531353fe740ba5ef3eb01377b01e7b27f0ffad35cdd3ef8d2c010a60331b2ac
2924183022425a1a3a4495adb03d67a92fb420bd408cb0c81d5bac694d1568f11
2ec8bbab834ca02b098c3b8f4975d75114bb001408252875d14bc1516f843b9fa
bb0'
    },
    {
      3: {
        1: -9,
        5: "bilbo.baggins@hobbiton.example"
      },
      6: h'5bacf5cf8e84051f01030a688c9c5dfa867173a6038e6655374ce3
e07de671d16dbe8cee3e965f2492d7850debf63b64a93e8fe7062ea536f3f165a
e34d5852f5001f1a1caffc52b05aec184da5a4148e305d87d0ef68642701b05c6
dde19669a1ffb39158a8fa4d51e95a2557b86da7a64a719a1422568dec9ec699d
af93a58eab268'
    }
  ]
}

Appendix D. COSE Header Algorithm Label Table

name	algorithm	label	CBOR type	description
apu	ECDH	-1	bstr
apv	ECDH	-2	bstr
epk	ECDH	-3	COSE_Key	contains a COSE key not a JWK key
iv	A128GCMKW, A192GCMKW, A256GCMKW	-1	bstr
iv	A128GCM, A192GCM, A256GCM	-1	bstr
p2c	PBE	-1	int
p2s	PBE	-2	bstr

Appendix E. COSE Algorithm Name Values

This table contains all of the defined algorithms for COSE.

name	value	description
HS256	4	HMAC w/ SHA-256
HS384	5	HMAC w/ SHA-384
HS512	6	HMAC w/ SHA-512
RS256	*	RSASSA-v1.5 w/ SHA-256
RS384	*	RSASSA-v1.5 w/ SHA-384
RSA512	*	RSASSA-v1.5 w/ SHA-256
ES256	-7	ECDSA w/ SHA-256
ES384	-8	ECDSA w/ SHA-384
ES512	-9	ECDSA w/ SHA-512
PS256	-10	RSASSA-PSS w/ SHA-256
PS384	*	RSASSA-PSS w/ SHA-384
PS512	-11	RSASSA-PSS w/ SHA-512
RSA1_5	*	RSAES v1.5 Key Encryption
RSA-OAEP	-2	RSAES OAEP w/ SHA-256
A128KW	-3	AES Key Wrap w/ 128-bit key
A192KW	-4	AES Key Wrap w/ 192-bit key
A256KW	-5	AES Key Wrap w/ 256-bit key
dir	-6	Direct use of CEK
ECDH-ES	*	ECDH ES w/ Concat KDF as CEK
ECDH-ES+A128KW	*	ECDH ES w/ Concat KDF and AES Key wrap w/ 128 bit key
ECDH-ES+A192KW	*	ECDH ES w/ Concat KDF and AES Key wrap w/ 192 bit key
ECDH-ES+A256KW	*	ECDH ES w/ Concat KDF and AES Key wrap w/ 256 bit key
A128GCMKW	*	AES GCM Key Wrap w/ 128 bit key
A192GCMKW	*	AES GCM Key Wrap w/ 192 bit key
A256GCMKW	*	AES GCM Key Wrap w/ 256 bit key
PBES2-HS256+A128KW	*	PBES2 w/ HMAC SHA-256 and AES Key wrap w/ 128 bit key
PBES2-HS384+A192KW	*	PBES2 w/ HMAC SHA-384 and AES Key wrap w/ 192 bit key
PBES2-HS512+A256KW	*	PBES2 w/ HMAC SHA-512 and AES Key wrap w/ 256 bit key
A128GCM	1	AES-GCM mode w/ 128-bit key
A192GCM	2	AES-GCM mode w/ 192-bit key
A256GCM	3	AES-GCM mode w/ 256-bit key

Appendix F. COSE General Values

name	number	description
EC	1	Elliptic Curve key Type
RSA	2	RSA Key type
oct	3	Octet Key type
P256	4	EC Curve P256 (NIST)
P521	5	EC Curve P521 (NIST)

Appendix G. COSE Key Map Labels

This table contains a list of all of the parameters defined for keys that were defined by the JOSE document set. In the table is the data value type to be used for CBOR as well as the integer value that can be used as a replacement for the name in order to further decrease the size of the sent item.

name	label	CBOR type	registry	description
kty	1	tstr / int	COSE General Values	Identification of the key type
use	*	tstr		deprecated - don't use
key_ops	*	[* tstr]
alg	3	tstr / int	COSE Algorithm Values	Key usage restriction to this algorithm
kid	2	bstr		Key Identification value - match to kid in message
x5u	*	tstr
x5c	*	bstr*
x5t	*	bstr
x5t#S256	*	bstr

;key_keys
kty=1
key_kid=2
key_alg=3

Appendix H. COSE Key Parameter Labels

This table contains a list of all of the parameters that were defined by the JOSE document set for a specific key type. In the table is the data value type to be used for CBOR as well as the integer value that can be used as a replacement for the name in order to further decrease the size of the sent item. Parameters dealing with keys

key type	name	label	CBOR type	registry	description
EC	crv	-1	int / tstr	Pull from general value registry
EC	x	-2	bstr
EC	y	-3	bstr
EC	d	-4	bstr
RSA	e	-1	bstr
RSA	n	-2	bstr
RSA	d	-3	bstr
RSA	p	-4	bstr
RSA	q	-5	bstr
RSA	dp	-6	bstr
RSA	dq	-7	bstr
RSA	qi	-8	bstr
RSA	oth	-9	bstr
RSA	r	-10	bstr
RSA	t	-11	bstr
oct	k	-1	bstr

Author's Address