CBOR Object Signing and Encryption (COSE)
draft-ietf-cose-msg-23
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8152.
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|---|---|---|---|
| Author | Jim Schaad | ||
| Last updated | 2016-10-18 (Latest revision 2016-10-16) | ||
| Replaces | draft-schaad-cose-msg | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Göran Selander | ||
| Shepherd write-up | Show Last changed 2016-08-14 | ||
| IESG | IESG state | Became RFC 8152 (Proposed Standard) | |
| Consensus boilerplate | Yes | ||
| Telechat date |
(None)
Needs a YES. Needs 10 more YES or NO OBJECTION positions to pass. |
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| Responsible AD | Kathleen Moriarty | ||
| Send notices to | "Goeran Selander" <goran.selander@ericsson.com> | ||
| IANA | IANA review state | Version Changed - Review Needed |
draft-ietf-cose-msg-23
Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 set of values. For this document however, the size of the authentication tag is fixed at 128 bits. The set of algorithms defined in this document are in Table 9. +---------+-------+------------------------------------------+ | name | value | description | +---------+-------+------------------------------------------+ | A128GCM | 1 | AES-GCM mode w/ 128-bit key, 128-bit tag | | | | | | A192GCM | 2 | AES-GCM mode w/ 192-bit key, 128-bit tag | | | | | | A256GCM | 3 | AES-GCM mode w/ 256-bit key, 128-bit tag | +---------+-------+------------------------------------------+ Table 9: Algorithm Value for AES-GCM Keys may be obtained either from a key structure or from a recipient structure. Implementations encrypting and decrypting MUST validate that the key type, key length and algorithm are correct and appropriate for the entities involved. When using a COSE key for this algorithm, the following checks are made: o The 'kty' field MUST be present and it MUST be 'Symmetric'. o If the 'alg' field is present, it MUST match the AES-GCM algorithm being used. o If the 'key_ops' field is present, it MUST include 'encrypt' or 'wrap key' when encrypting. o If the 'key_ops' field is present, it MUST include 'decrypt' or 'unwrap key' when decrypting. 10.1.1. Security Considerations When using AES-GCM, the following restrictions MUST be enforced: o The key and nonce pair MUST be unique for every message encrypted. o The total amount of data encrypted for a single key MUST NOT exceed 2^39 - 256 bits. An explicit check is required only in environments where it is expected that it might be exceeded. Consideration was given to supporting smaller tag values; the constrained community would desire tag sizes in the 64-bit range. Schaad Expires April 21, 2017 [Page 46] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 Doing so drastically changes both the maximum messages size (generally not an issue) and the number of times that a key can be used. Given that CCM is the usual mode for constrained environments, restricted modes are not supported. 10.2. AES CCM Counter with CBC-MAC (CCM) is a generic authentication encryption block cipher mode defined in [RFC3610]. The CCM mode is combined with the AES block encryption algorithm to define a commonly used content encryption algorithm used in constrained devices. The CCM mode has two parameter choices. The first choice is M, the size of the authentication field. The choice of the value for M involves a trade-off between message growth (from the tag) and the probably that an attacker can undetectably modify a message. The second choice is L, the size of the length field. This value requires a trade-off between the maximum message size and the size of the Nonce. It is unfortunate that the specification for CCM specified L and M as a count of bytes rather than a count of bits. This leads to possible misunderstandings where AES-CCM-8 is frequently used to refer to a version of CCM mode where the size of the authentication is 64 bits and not 8 bits. These values have traditionally been specified as bit counts rather than byte counts. This document will follow the convention of using bit counts so that it is easier to compare the different algorithms presented in this document. We define a matrix of algorithms in this document over the values of L and M. Constrained devices are usually operating in situations where they use short messages and want to avoid doing recipient specific cryptographic operations. This favors smaller values of both L and M. Less constrained devices will want to be able to use larger messages and are more willing to generate new keys for every operation. This favors larger values of L and M. The following values are used for L: 16 bits (2) limits messages to 2^16 bytes (64 KiB) in length. This is sufficiently long for messages in the constrained world. The nonce length is 13 bytes allowing for 2^(13*8) possible values of the nonce without repeating. 64 bits (8) limits messages to 2^64 bytes in length. The nonce length is 7 bytes allowing for 2^56 possible values of the nonce without repeating. Schaad Expires April 21, 2017 [Page 47] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 The following values are used for M: 64 bits (8) produces a 64-bit authentication tag. This implies that there is a 1 in 2^64 chance that a modified message will authenticate. 128 bits (16) produces a 128-bit authentication tag. This implies that there is a 1 in 2^128 chance that a modified message will authenticate. Schaad Expires April 21, 2017 [Page 48] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 +--------------------+-------+----+-----+-----+---------------------+ | name | value | L | M | k | description | +--------------------+-------+----+-----+-----+---------------------+ | AES-CCM-16-64-128 | 10 | 16 | 64 | 128 | AES-CCM mode | | | | | | | 128-bit key, 64-bit | | | | | | | tag, 13-byte nonce | | | | | | | | | AES-CCM-16-64-256 | 11 | 16 | 64 | 256 | AES-CCM mode | | | | | | | 256-bit key, 64-bit | | | | | | | tag, 13-byte nonce | | | | | | | | | AES-CCM-64-64-128 | 12 | 64 | 64 | 128 | AES-CCM mode | | | | | | | 128-bit key, 64-bit | | | | | | | tag, 7-byte nonce | | | | | | | | | AES-CCM-64-64-256 | 13 | 64 | 64 | 256 | AES-CCM mode | | | | | | | 256-bit key, 64-bit | | | | | | | tag, 7-byte nonce | | | | | | | | | AES-CCM-16-128-128 | 30 | 16 | 128 | 128 | AES-CCM mode | | | | | | | 128-bit key, | | | | | | | 128-bit tag, | | | | | | | 13-byte nonce | | | | | | | | | AES-CCM-16-128-256 | 31 | 16 | 128 | 256 | AES-CCM mode | | | | | | | 256-bit key, | | | | | | | 128-bit tag, | | | | | | | 13-byte nonce | | | | | | | | | AES-CCM-64-128-128 | 32 | 64 | 128 | 128 | AES-CCM mode | | | | | | | 128-bit key, | | | | | | | 128-bit tag, 7-byte | | | | | | | nonce | | | | | | | | | AES-CCM-64-128-256 | 33 | 64 | 128 | 256 | AES-CCM mode | | | | | | | 256-bit key, | | | | | | | 128-bit tag, 7-byte | | | | | | | nonce | +--------------------+-------+----+-----+-----+---------------------+ Table 10: Algorithm Values for AES-CCM Keys may be obtained either from a key structure or from a recipient structure. Implementations encrypting and decrypting MUST validate that the key type, key length and algorithm are correct and appropriate for the entities involved. Schaad Expires April 21, 2017 [Page 49] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 When using a COSE key for this algorithm, the following checks are made: o The 'kty' field MUST be present and it MUST be 'Symmetric'. o If the 'alg' field is present, it MUST match the AES-CCM algorithm being used. o If the 'key_ops' field is present, it MUST include 'encrypt' or 'wrap key' when encrypting. o If the 'key_ops' field is present, it MUST include 'decrypt' or 'unwrap key' when decrypting. 10.2.1. Security Considerations When using AES-CCM, the following restrictions MUST be enforced: o The key and nonce pair MUST be unique for every message encrypted. Note that the value of L influences the number of unique nonces. o The total number of times the AES block cipher is used MUST NOT exceed 2^61 operations. This limitation is the sum of times the block cipher is used in computing the MAC value and in performing stream encryption operations. An explicit check is required only in environments where it is expected that it might be exceeded. [RFC3610] additionally calls out one other consideration of note. It is possible to do a pre-computation attack against the algorithm in cases where portions of the plaintext are highly predictable. This reduces the security of the key size by half. Ways to deal with this attack include adding a random portion to the nonce value and/or increasing the key size used. Using a portion of the nonce for a random value will decrease the number of messages that a single key can be used for. Increasing the key size may require more resources in the constrained device. See sections 5 and 10 of [RFC3610] for more information. 10.3. ChaCha20 and Poly1305 ChaCha20 and Poly1305 combined together is an AEAD mode that is defined in [RFC7539]. This is an algorithm defined to be a cipher that is not AES and thus would not suffer from any future weaknesses found in AES. These cryptographic functions are designed to be fast in software-only implementations. The ChaCha20/Poly1305 AEAD construction defined in [RFC7539] has no parameterization. It takes a 256-bit key and a 96-bit nonce, as well Schaad Expires April 21, 2017 [Page 50] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 as the plain text and additional data as inputs and produces the cipher text as an option. We define one algorithm identifier for this algorithm in Table 11. +-------------------+-------+---------------------------------------+ | name | value | description | +-------------------+-------+---------------------------------------+ | ChaCha20/Poly1305 | 24 | ChaCha20/Poly1305 w/ 256-bit key, | | | | 128-bit tag | +-------------------+-------+---------------------------------------+ Table 11: Algorithm Value for AES-GCM Keys may be obtained either from a key structure or from a recipient structure. Implementations encrypting and decrypting MUST validate that the key type, key length and algorithm are correct and appropriate for the entities involved. When using a COSE key for this algorithm, the following checks are made: o The 'kty' field MUST be present and it MUST be 'Symmetric'. o If the 'alg' field is present, it MUST match the ChaCha20/Poly1305 algorithm being used. o If the 'key_ops' field is present, it MUST include 'encrypt' or 'wrap key' when encrypting. o If the 'key_ops' field is present, it MUST include 'decrypt' or 'unwrap key' when decrypting. 10.3.1. Security Considerations The pair of key, nonce MUST be unique for every invocation of the algorithm. Nonce counters are considered to be an acceptable way of ensuring that they are unique. 11. Key Derivation Functions (KDF) Key Derivation Functions (KDFs) are used to take some secret value and generate a different one. The secret value comes in three flavors: o Secrets that are uniformly random: This is the type of secret that is created by a good random number generator. Schaad Expires April 21, 2017 [Page 51] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 o Secrets that are not uniformly random: This is type of secret that is created by operations like key agreement. o Secrets that are not random: This is the type of secret that people generate for things like passwords. General KDF functions work well with the first type of secret, can do reasonably well with the second type of secret, and generally do poorly with the last type of secret. None of the KDF functions in this section are designed to deal with the type of secrets that are used for passwords. Functions like PBES2 [I-D.moriarty-pkcs5-v2dot1] need to be used for that type of secret. The same KDF function can be setup to deal with the first two types of secrets in a different way. The KDF function defined in Section 11.1 is such a function. This is reflected in the set of algorithms defined for HKDF. When using KDF functions, one component that is included is context information. Context information is used to allow for different keying information to be derived from the same secret. The use of context based keying material is considered to be a good security practice. This document defines a single context structure and a single KDF function. These elements are used for all of the recipient algorithms defined in this document that require a KDF process. These algorithms are defined in Section 12.1.2, Section 12.4.1, and Section 12.5.1. 11.1. HMAC-based Extract-and-Expand Key Derivation Function (HKDF) The HKDF key derivation algorithm is defined in [RFC5869]. The HKDF algorithm takes these inputs: secret - a shared value that is secret. Secrets may be either previously shared or derived from operations like a DH key agreement. salt - an optional value that is used to change the generation process. The salt value can be either public or private. If the salt is public and carried in the message, then the 'salt' algorithm header parameter defined in Table 13 is used. While [RFC5869] suggests that the length of the salt be the same as the length of the underlying hash value, any amount of salt will improve the security as different key values will be generated. This parameter is protected by being included in the key Schaad Expires April 21, 2017 [Page 52] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 computation and does not need to be separately authenticated. The salt value does not need to be unique for every message sent. length - the number of bytes of output that need to be generated. context information - Information that describes the context in which the resulting value will be used. Making this information specific to the context in which the material is going to be used ensures that the resulting material will always be tied to that usage. The context structure defined in Section 11.2 is used by the KDF functions in this document. PRF - The underlying pseudo-random function to be used in the HKDF algorithm. The PRF is encoded into the HKDF algorithm selection. HKDF is defined to use HMAC as the underlying PRF. However, it is possible to use other functions in the same construct to provide a different KDF function that is more appropriate in the constrained world. Specifically, one can use AES-CBC-MAC as the PRF for the expand step, but not for the extract step. When using a good random shared secret of the correct length, the extract step can be skipped. For the AES algorithm versions, the extract step is always skipped. The extract step cannot be skipped if the secret is not uniformly random, for example, if it is the result of an ECDH key agreement step. (This implies that the AES HKDF version cannot be used with ECDH.) If the extract step is skipped, the 'salt' value is not used as part of the HKDF functionality. The algorithms defined in this document are found in Table 12. +---------------+-----------------+---------------------------------+ | name | PRF | description | +---------------+-----------------+---------------------------------+ | HKDF SHA-256 | HMAC with | HKDF using HMAC SHA-256 as the | | | SHA-256 | PRF | | | | | | HKDF SHA-512 | HMAC with | HKDF using HMAC SHA-512 as the | | | SHA-512 | PRF | | | | | | HKDF AES- | AES-CBC-MAC-128 | HKDF using AES-MAC as the PRF | | MAC-128 | | w/ 128-bit key | | | | | | HKDF AES- | AES-CBC-MAC-256 | HKDF using AES-MAC as the PRF | | MAC-256 | | w/ 256-bit key | +---------------+-----------------+---------------------------------+ Table 12: HKDF algorithms Schaad Expires April 21, 2017 [Page 53] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 +------+-------+------+-------------------------------+-------------+ | name | label | type | algorithm | description | +------+-------+------+-------------------------------+-------------+ | salt | -20 | bstr | direct+HKDF-SHA-256, direct | Random salt | | | | | +HKDF-SHA-512, direct+HKDF- | | | | | | AES-128, direct+HKDF-AES-256, | | | | | | ECDH-ES+HKDF-256, ECDH- | | | | | | ES+HKDF-512, ECDH- | | | | | | SS+HKDF-256, ECDH- | | | | | | SS+HKDF-512, ECDH-ES+A128KW, | | | | | | ECDH-ES+A192KW, ECDH- | | | | | | ES+A256KW, ECDH-SS+A128KW, | | | | | | ECDH-SS+A192KW, ECDH- | | | | | | SS+A256KW | | +------+-------+------+-------------------------------+-------------+ Table 13: HKDF Algorithm Parameters 11.2. Context Information Structure The context information structure is used to ensure that the derived keying material is "bound" to the context of the transaction. The context information structure used here is based on that defined in [SP800-56A]. By using CBOR for the encoding of the context information structure, we automatically get the same type and length separation of fields that is obtained by the use of ASN.1. This means that there is no need to encode the lengths for the base elements as it is done by the encoding used in JOSE (Section 4.6.2 of [RFC7518]). The context information structure refers to PartyU and PartyV as the two parties that are doing the key derivation. Unless the application protocol defines differently, we assign PartyU to the entity that is creating the message and PartyV to the entity that is receiving the message. By doing this association, different keys will be derived for each direction as the context information is different in each direction. The context structure is built from information that is known to both entities. This information can be obtained from a variety of sources: o Fields can be defined by the application. This is commonly used to assign fixed names to parties, but can be used for other items such as nonces. o Fields can be defined by usage of the output. Examples of this are the algorithm and key size that are being generated. Schaad Expires April 21, 2017 [Page 54] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 o Fields can be defined by parameters from the message. We define a set of parameters in Table 14 that can be used to carry the values associated with the context structure. Examples of this are identities and nonce values. These parameters are designed to be placed in the unprotected bucket of the recipient structure. (They do not need to be in the protected bucket since they already are included in the cryptographic computation by virtue of being included in the context structure.) +----------+-------+------+---------------------------+-------------+ | name | label | type | algorithm | description | +----------+-------+------+---------------------------+-------------+ | PartyU | -21 | bstr | direct+HKDF-SHA-256, | Party U | | identity | | | direct+HKDF-SHA-512, | identity | | | | | direct+HKDF-AES-128, | Information | | | | | direct+HKDF-AES-256, | | | | | | ECDH-ES+HKDF-256, ECDH- | | | | | | ES+HKDF-512, ECDH- | | | | | | SS+HKDF-256, ECDH- | | | | | | SS+HKDF-512, ECDH- | | | | | | ES+A128KW, ECDH- | | | | | | ES+A192KW, ECDH- | | | | | | ES+A256KW, ECDH- | | | | | | SS+A128KW, ECDH- | | | | | | SS+A192KW, ECDH-SS+A256KW | | | | | | | | | PartyU | -22 | bstr | direct+HKDF-SHA-256, | Party U | | nonce | | / | direct+HKDF-SHA-512, | provided | | | | int | direct+HKDF-AES-128, | nonce | | | | | direct+HKDF-AES-256, | | | | | | ECDH-ES+HKDF-256, ECDH- | | | | | | ES+HKDF-512, ECDH- | | | | | | SS+HKDF-256, ECDH- | | | | | | SS+HKDF-512, ECDH- | | | | | | ES+A128KW, ECDH- | | | | | | ES+A192KW, ECDH- | | | | | | ES+A256KW, ECDH- | | | | | | SS+A128KW, ECDH- | | | | | | SS+A192KW, ECDH-SS+A256KW | | | | | | | | | PartyU | -23 | bstr | direct+HKDF-SHA-256, | Party U | | other | | | direct+HKDF-SHA-512, | other | | | | | direct+HKDF-AES-128, | provided | | | | | direct+HKDF-AES-256, | information | | | | | ECDH-ES+HKDF-256, ECDH- | | | | | | ES+HKDF-512, ECDH- | | | | | | SS+HKDF-256, ECDH- | | | | | | SS+HKDF-512, ECDH- | | Schaad Expires April 21, 2017 [Page 55] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 | | | | ES+A128KW, ECDH- | | | | | | ES+A192KW, ECDH- | | | | | | ES+A256KW, ECDH- | | | | | | SS+A128KW, ECDH- | | | | | | SS+A192KW, ECDH-SS+A256KW | | | | | | | | | PartyV | -24 | bstr | direct+HKDF-SHA-256, | Party V | | identity | | | direct+HKDF-SHA-512, | identity | | | | | direct+HKDF-AES-128, | Information | | | | | direct+HKDF-AES-256, | | | | | | ECDH-ES+HKDF-256, ECDH- | | | | | | ES+HKDF-512, ECDH- | | | | | | SS+HKDF-256, ECDH- | | | | | | SS+HKDF-512, ECDH- | | | | | | ES+A128KW, ECDH- | | | | | | ES+A192KW, ECDH- | | | | | | ES+A256KW, ECDH- | | | | | | SS+A128KW, ECDH- | | | | | | SS+A192KW, ECDH-SS+A256KW | | | | | | | | | PartyV | -25 | bstr | direct+HKDF-SHA-256, | Party V | | nonce | | / | direct+HKDF-SHA-512, | provided | | | | int | direct+HKDF-AES-128, | nonce | | | | | direct+HKDF-AES-256, | | | | | | ECDH-ES+HKDF-256, ECDH- | | | | | | ES+HKDF-512, ECDH- | | | | | | SS+HKDF-256, ECDH- | | | | | | SS+HKDF-512, ECDH- | | | | | | ES+A128KW, ECDH- | | | | | | ES+A192KW, ECDH- | | | | | | ES+A256KW, ECDH- | | | | | | SS+A128KW, ECDH- | | | | | | SS+A192KW, ECDH-SS+A256KW | | | | | | | | | PartyV | -26 | bstr | direct+HKDF-SHA-256, | Party V | | other | | | direct+HKDF-SHA-512, | other | | | | | direct+HKDF-AES-128, | provided | | | | | direct+HKDF-AES-256, | information | | | | | ECDH-ES+HKDF-256, ECDH- | | | | | | ES+HKDF-512, ECDH- | | | | | | SS+HKDF-256, ECDH- | | | | | | SS+HKDF-512, ECDH- | | | | | | ES+A128KW, ECDH- | | | | | | ES+A192KW, ECDH- | | | | | | ES+A256KW, ECDH- | | | | | | SS+A128KW, ECDH- | | | | | | SS+A192KW, ECDH-SS+A256KW | | +----------+-------+------+---------------------------+-------------+ Schaad Expires April 21, 2017 [Page 56] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 Table 14: Context Algorithm Parameters We define a CBOR object to hold the context information. This object is referred to as COSE_KDF_Context. The object is based on a CBOR array type. The fields in the array are: AlgorithmID This field indicates the algorithm for which the key material will be used. This normally is either a Key Wrap algorithm identifier or a Content Encryption algorithm identifier. The values are from the "COSE Algorithm Value" registry. This field is required to be present. The field exists in the context information so that if the same environment is used for different algorithms, then completely different keys will be generated for each of those algorithms. (This practice means if algorithm A is broken and thus is easier to find, the key derived for algorithm B will not be the same as the key derived for algorithm A.) PartyUInfo This field holds information about party U. The PartyUInfo is encoded as a CBOR array. The elements of PartyUInfo are encoded in the order presented, however if the element does not exist no element is placed in the array. The elements of the PartyUInfo array are: identity This contains the identity information for party U. The identities can be assigned in one of two manners. Firstly, a protocol can assign identities based on roles. For example, the roles of "client" and "server" may be assigned to different entities in the protocol. Each entity would then use the correct label for the data they send or receive. The second way for a protocol to assign identities is to use a name based on a naming system (i.e., DNS, X.509 names). We define an algorithm parameter 'PartyU identity' that can be used to carry identity information in the message. However, identity information is often known as part of the protocol and can thus be inferred rather than made explicit. If identity information is carried in the message, applications SHOULD have a way of validating the supplied identity information. The identity information does not need to be specified and is set to nil in that case. nonce This contains a nonce value. The nonce can either be implicit from the protocol or carried as a value in the unprotected headers. We define an algorithm parameter 'PartyU nonce' that can be used to carry this value in the message However, the nonce value could be determined by the application and the value determined from elsewhere. Schaad Expires April 21, 2017 [Page 57] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 This option does not need to be specified and is set to nil in that case other This contains other information that is defined by the protocol. This option does not need to be specified and is set to nil in that case PartyVInfo This field holds information about party V. The content of the structure are the same as for the PartyUInfo but for party V. SuppPubInfo This field contains public information that is mutually known to both parties. keyDataLength This is set to the number of bits of the desired output value. (This practice means if algorithm A can use two different key lengths, the key derived for longer key size will not contain the key for shorter key size as a prefix.) protected This field contains the protected parameter field. If there are no elements in the protected field, then use a zero length bstr. other This field is for free form data defined by the application. An example is that an application could define two different strings to be placed here to generate different keys for a data stream vs a control stream. This field is optional and will only be present if the application defines a structure for this information. Applications that define this SHOULD use CBOR to encode the data so that types and lengths are correctly included. SuppPrivInfo This field contains private information that is mutually known private information. An example of this information would be a pre-existing shared secret. (This could, for example, be used in combination with an ECDH key agreement to provide a secondary proof of identity.) The field is optional and will only be present if the application defines a structure for this information. Applications that define this SHOULD use CBOR to encode the data so that types and lengths are correctly included. The following CDDL fragment corresponds to the text above. Schaad Expires April 21, 2017 [Page 58] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 PartyInfo = ( identity : bstr / nil, nonce : bstr / int / nil, other : bstr / nil, ) COSE_KDF_Context = [ AlgorithmID : int / tstr, PartyUInfo : [ PartyInfo ], PartyVInfo : [ PartyInfo ], SuppPubInfo : [ keyDataLength : uint, protected : empty_or_serialized_map, ? other : bstr ], ? SuppPrivInfo : bstr ] 12. Content Key Distribution Methods 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 are defined in [RFC7516]. Other specifications use different terms for the recipient algorithm classes or do not support some of the recipient algorithm classes. 12.1. Direct Encryption The direct encryption class algorithms share a secret between the sender and the recipient that is used either directly or after manipulation as the CEK. When direct encryption mode is used, it MUST be the only mode used on the message. The COSE_Encrypt structure for the recipient is organized as follows: o The 'protected' field MUST be a zero length item unless it is used in the computation of the content key. o The 'alg' parameter MUST be present. o A parameter identifying the shared secret SHOULD be present. o The 'ciphertext' field MUST be a zero length item. o The 'recipients' field MUST be absent. Schaad Expires April 21, 2017 [Page 59] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 12.1.1. Direct Key This recipient algorithm is the simplest; the identified key is directly used as the key for the next layer down in the message. There are no algorithm parameters defined for this algorithm. The algorithm identifier value is assigned in Table 15. When this algorithm is used, the protected field MUST be zero length. The key type MUST be 'Symmetric'. +--------+-------+-------------------+ | name | value | description | +--------+-------+-------------------+ | direct | -6 | Direct use of CEK | +--------+-------+-------------------+ Table 15: Direct Key 12.1.1.1. Security Considerations This recipient algorithm has several potential problems that need to be considered: o These keys need to have some method to be regularly updated over time. All of the content encryption algorithms specified in this document have limits on how many times a key can be used without significant loss of security. o These keys need to be dedicated to a single algorithm. There have been a number of attacks developed over time when a single key is used for multiple different algorithms. One example of this is the use of a single key both for CBC encryption mode and CBC-MAC authentication mode. o Breaking one message means all messages are broken. If an adversary succeeds in determining the key for a single message, then the key for all messages is also determined. 12.1.2. Direct Key with KDF These recipient algorithms take a common shared secret between the two parties and applies the HKDF function (Section 11.1), using the context structure defined in Section 11.2 to transform the shared secret into the CEK. The 'protected' field can be of non-zero length. Either the 'salt' parameter of HKDF or the partyU 'nonce' parameter of the context structure MUST be present. The salt/nonce parameter can be generated either randomly or deterministically. The Schaad Expires April 21, 2017 [Page 60] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 requirement is that it be a unique value for the shared secret in question. If the salt/nonce value is generated randomly, then it is suggested that the length of the random value be the same length as the hash function underlying HKDF. While there is no way to guarantee that it will be unique, there is a high probability that it will be unique. If the salt/nonce value is generated deterministically, it can be guaranteed to be unique and thus there is no length requirement. A new IV must be used for each message if the same key is used. The IV can be modified in a predictable manner, a random manner or an unpredictable manner (i.e., encrypting a counter). The IV used for a key can also be generated from the same HKDF functionality as the key is generated. If HKDF is used for generating the IV, the algorithm identifier is set to "IV- GENERATION". When these algorithms are used, the key type MUST be 'symmetric'. The set of algorithms defined in this document can be found in Table 16. +---------------------+-------+-------------+-----------------------+ | name | value | KDF | description | +---------------------+-------+-------------+-----------------------+ | direct+HKDF-SHA-256 | -10 | HKDF | Shared secret w/ HKDF | | | | SHA-256 | and SHA-256 | | | | | | | direct+HKDF-SHA-512 | -11 | HKDF | Shared secret w/ HKDF | | | | SHA-512 | and SHA-512 | | | | | | | direct+HKDF-AES-128 | -12 | HKDF AES- | Shared secret w/ AES- | | | | MAC-128 | MAC 128-bit key | | | | | | | direct+HKDF-AES-256 | -13 | HKDF AES- | Shared secret w/ AES- | | | | MAC-256 | MAC 256-bit key | +---------------------+-------+-------------+-----------------------+ Table 16: Direct Key with KDF When using a COSE key for this algorithm, the following checks are made: o The 'kty' field MUST be present and it MUST be 'Symmetric'. Schaad Expires April 21, 2017 [Page 61] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 o If the 'alg' field is present, it MUST match the algorithm being used. o If the 'key_ops' field is present, it MUST include 'deriveKey' or 'deriveBits'. 12.1.2.1. Security Considerations The shared secret needs to have some method to be regularly updated over time. The shared secret forms the basis of trust. Although not used directly, it should still be subject to scheduled rotation. While these methods do not provide for perfect forward secrecy, as the same shared secret is used for all of the keys generated, if the key for any single message is discovered only the message (or series of messages) using that derived key are compromised. A new key derivation step will generate a new key which requires the same amount of work to get the key. 12.2. Key Wrapping In key wrapping mode, the CEK is randomly generated and that key is then encrypted by a shared secret between the sender and the recipient. All of the currently defined key wrapping algorithms for COSE are AE algorithms. Key wrapping mode is considered to be superior to direct encryption if the system has any capability for doing random key generation. This is because the shared key is used to wrap random data rather than data that has some degree of organization and may in fact be repeating the same content. The use of Key Wrapping loses the weak data origination that is provided by the direct encryption algorithms. The COSE_Encrypt structure for the recipient is organized as follows: o The 'protected' field MUST be absent if the key wrap algorithm is an AE algorithm. o The 'recipients' field is normally absent, but can be used. Applications MUST deal with a recipient field being present, not being able to decrypt that recipient is an acceptable way of dealing with it. Failing to process the message is not an acceptable way of dealing with it. o The plain text to be encrypted is the key from next layer down (usually the content layer). Schaad Expires April 21, 2017 [Page 62] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 o At a minimum, the 'unprotected' field MUST contain the 'alg' parameter and SHOULD contain a parameter identifying the shared secret. 12.2.1. AES Key Wrapping The AES Key Wrapping algorithm is defined in [RFC3394]. This algorithm uses an AES key to wrap a value that is a multiple of 64 bits. As such, it can be used to wrap a key for any of the content encryption algorithms defined in this document. The algorithm requires a single fixed parameter, the initial value. This is fixed to the value specified in Section 2.2.3.1 of [RFC3394]. There are no public parameters that vary on a per invocation basis. The protected header field MUST be empty. Keys may be obtained either from a key structure or from a recipient structure. Implementations encrypting and decrypting MUST validate that the key type, key length and algorithm are correct and appropriate for the entities involved. When using a COSE key for this algorithm, the following checks are made: o The 'kty' field MUST be present and it MUST be 'Symmetric'. o If the 'alg' field is present, it MUST match the AES Key Wrap algorithm being used. o If the 'key_ops' field is present, it MUST include 'encrypt' or 'wrap key' when encrypting. o If the 'key_ops' field is present, it MUST include 'decrypt' or 'unwrap key' when decrypting. +--------+-------+----------+-----------------------------+ | name | value | key size | description | +--------+-------+----------+-----------------------------+ | A128KW | -3 | 128 | AES Key Wrap w/ 128-bit key | | | | | | | A192KW | -4 | 192 | AES Key Wrap w/ 192-bit key | | | | | | | A256KW | -5 | 256 | AES Key Wrap w/ 256-bit key | +--------+-------+----------+-----------------------------+ Table 17: AES Key Wrap Algorithm Values Schaad Expires April 21, 2017 [Page 63] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 12.2.1.1. Security Considerations for AES-KW The shared secret needs to have some method to be regularly updated over time. The shared secret is the basis of trust. 12.3. Key Transport 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: o The 'protected' field MUST be absent. o The plain text to be encrypted is the key from next layer down (usually the content layer). o At a minimum, the 'unprotected' field MUST contain the 'alg' parameter and SHOULD contain a parameter identifying the asymmetric key. 12.4. Direct Key Agreement The 'direct key agreement' class of recipient algorithms uses a key agreement method to create a shared secret. A KDF is then applied to the shared secret to derive a key to be used in protecting the data. This key is normally used as a CEK or MAC key, but could be used for other purposes if more than two layers are in use (see Appendix B). The most commonly used key agreement algorithm is Diffie-Hellman, but other variants exist. Since COSE is designed for a store and forward environment rather than an on-line environment, many of the DH variants cannot be used as the receiver of the message cannot provide any dynamic key material. One side-effect of this is that perfect forward secrecy (see [RFC4949]) is not achievable. A static key will always be used for the receiver of the COSE object. Two variants of DH that are supported are: Ephemeral-Static DH: where the sender of the message creates a one-time DH key and uses a static key for the recipient. The use of the ephemeral sender key means that no additional random input is needed as this is randomly generated for each message. Schaad Expires April 21, 2017 [Page 64] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 Static-Static DH: where a static key is used for both the sender and the recipient. The use of static keys allows for recipient to get a weak version of data origination for the message. When static-static key agreement is used, then some piece of unique data for the KDF is required to ensure that a different key is created for each message. When direct key agreement mode is used, there MUST be only one recipient in the message. This method creates the key directly and that makes it difficult to mix with additional recipients. If multiple recipients are needed, then the version with key wrap needs to be used. The COSE_Encrypt structure for the recipient is organized as follows: o At a minimum, headers MUST contain the 'alg' parameter and SHOULD contain a parameter identifying the recipient's asymmetric key. o The headers SHOULD identify the sender's key for the static-static versions and MUST contain the sender's ephemeral key for the ephemeral-static versions. 12.4.1. ECDH The mathematics for Elliptic Curve Diffie-Hellman can be found in [RFC6090]. In this document, the algorithm is extended to be used with the two curves defined in [RFC7748]. ECDH is parameterized by the following: o Curve Type/Curve: The curve selected controls not only the size of the shared secret, but the mathematics for computing the shared secret. The curve selected also controls how a point in the curve is represented and what happens for the identity points on the curve. In this specification, we allow for a number of different curves to be used. A set of curves are defined in Table 22. The math used to obtain the computed secret is based on the curve selected and not on the ECDH algorithm. For this reason, a new algorithm does not need to be defined for each of the curves. o Computed Secret to Shared Secret: Once the computed secret is known, the resulting value needs to be converted to a byte string to run the KDF function. The X coordinate is used for all of the curves defined in this document. For curves X25519 and X448, the resulting value is used directly as it is a byte string of a known length. For the P-256, P-384 and P-521 curves, the X coordinate is run through the I2OSP function defined in [I-D.moriarty-pkcs1], using the same computation for n as is defined in Section 8.1. Schaad Expires April 21, 2017 [Page 65] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 o Ephemeral-static or static-static: The key agreement process may be done using either a static or an ephemeral key for the sender's side. When using ephemeral keys, the sender MUST generate a new ephemeral key for every key agreement operation. The ephemeral key is placed in the 'ephemeral key' parameter and MUST be present for all algorithm identifiers that use ephemeral keys. When using static keys, the sender MUST either generate a new random value or otherwise create a unique value. For the KDF functions used, this means either in the 'salt' parameter for HKDF (Table 13) or in the 'PartyU nonce' parameter for the context structure (Table 14) MUST be present. (Both may be present if desired.) The value in the parameter MUST be unique for the pair of keys being used. It is acceptable to use a global counter that is incremented for every static-static operation and use the resulting value. When using static keys, the static key should be identified to the recipient. The static key can be identified either by providing the key ('static key') or by providing a key identifier for the static key ('static key id'). Both of these parameters are defined in Table 19. o Key derivation algorithm: The result of an ECDH key agreement process does not provide a uniformly random secret. As such, it needs to be run through a KDF in order to produce a usable key. Processing the secret through a KDF also allows for the introduction of context material: how the key is going to be used, and one-time material for static-static key agreement. All of the algorithms defined in this document use one of the HKDF algorithms defined in Section 11.1 with the context structure defined in Section 11.2. o Key Wrap algorithm: No key wrap algorithm is used. This is represented in Table 18 as 'none'. The key size for the context structure is the content layer encryption algorithm size. The set of direct ECDH algorithms defined in this document are found in Table 18. Schaad Expires April 21, 2017 [Page 66] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 +-----------+-------+---------+------------+--------+---------------+ | name | value | KDF | Ephemeral- | Key | description | | | | | Static | Wrap | | +-----------+-------+---------+------------+--------+---------------+ | ECDH-ES + | -25 | HKDF - | yes | none | ECDH ES w/ | | HKDF-256 | | SHA-256 | | | HKDF - | | | | | | | generate key | | | | | | | directly | | | | | | | | | ECDH-ES + | -26 | HKDF - | yes | none | ECDH ES w/ | | HKDF-512 | | SHA-512 | | | HKDF - | | | | | | | generate key | | | | | | | directly | | | | | | | | | ECDH-SS + | -27 | HKDF - | no | none | ECDH SS w/ | | HKDF-256 | | SHA-256 | | | HKDF - | | | | | | | generate key | | | | | | | directly | | | | | | | | | ECDH-SS + | -28 | HKDF - | no | none | ECDH SS w/ | | HKDF-512 | | SHA-512 | | | HKDF - | | | | | | | generate key | | | | | | | directly | +-----------+-------+---------+------------+--------+---------------+ Table 18: ECDH Algorithm Values Schaad Expires April 21, 2017 [Page 67] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 +-----------+-------+----------+---------------------+--------------+ | name | label | type | algorithm | description | +-----------+-------+----------+---------------------+--------------+ | ephemeral | -1 | COSE_Key | ECDH-ES+HKDF-256, | Ephemeral | | key | | | ECDH-ES+HKDF-512, | Public key | | | | | ECDH-ES+A128KW, | for the | | | | | ECDH-ES+A192KW, | sender | | | | | ECDH-ES+A256KW | | | | | | | | | static | -2 | COSE_Key | ECDH-SS+HKDF-256, | Static | | key | | | ECDH-SS+HKDF-512, | Public key | | | | | ECDH-SS+A128KW, | for the | | | | | ECDH-SS+A192KW, | sender | | | | | ECDH-SS+A256KW | | | | | | | | | static | -3 | bstr | ECDH-SS+HKDF-256, | Static | | key id | | | ECDH-SS+HKDF-512, | Public key | | | | | ECDH-SS+A128KW, | identifier | | | | | ECDH-SS+A192KW, | for the | | | | | ECDH-SS+A256KW | sender | +-----------+-------+----------+---------------------+--------------+ Table 19: ECDH Algorithm Parameters This document defines these algorithms to be used with the curves P-256, P-384, P-521, X25519, and X448. Implementations MUST verify that the key type and curve are correct. Different curves are restricted to different key types. Implementations MUST verify that the curve and algorithm are appropriate for the entities involved. When using a COSE key for this algorithm, the following checks are made: o The 'kty' field MUST be present and it MUST be 'EC2' or 'OKP'. o If the 'alg' field is present, it MUST match the Key Agreement algorithm being used. o If the 'key_ops' field is present, it MUST include 'derive key' or 'derive bits' for the private key. o If the 'key_ops' field is present, it MUST be empty for the public key. Schaad Expires April 21, 2017 [Page 68] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 12.4.2. Security Considerations Some method of checking that points provided from external entities are valid. For the 'EC2' key format, this can be done by checking that the x and y values form a point on the curve. For the 'OKP' format, there is no simple way to do point validation. Consideration was given to requiring that the public keys of both entities be provided as part of the key derivation process. (As recommended in section 6.1 of [RFC7748].) This was not done as COSE is used in a store and forward format rather than in on line key exchange. In order for this to be a problem, either the receiver public key has to be chosen maliciously or the sender has to be malicious. In either case, all security evaporates anyway. A proof of possession of the private key associated with the public key is recommended when a key is moved from untrusted to trusted. (Either by the end user or by the entity that is responsible for making trust statements on keys.) 12.5. Key Agreement with Key Wrap 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 function for this would be: encryptedKey = KeyWrap(KDF(DH-Shared, context), CEK) The COSE_Encrypt structure for the recipient is organized as follows: o The 'protected' field is fed into the KDF context structure. o The plain text to be encrypted is the key from next layer down (usually the content layer). o The 'alg' parameter MUST be present in the layer. o A parameter identifying the recipient's key SHOULD be present. A parameter identifying the sender's key SHOULD be present. 12.5.1. ECDH These algorithms are defined in Table 20. ECDH with Key Agreement is parameterized by the same parameters as for ECDH Section 12.4.1 with the following modifications: Schaad Expires April 21, 2017 [Page 69] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 o Key Wrap Algorithm: Any of the key wrap algorithms defined in Section 12.2.1 are supported. The size of the key used for the key wrap algorithm is fed into the KDF function. The set of identifiers are found in Table 20. +-----------+-------+---------+------------+--------+---------------+ | name | value | KDF | Ephemeral- | Key | description | | | | | Static | Wrap | | +-----------+-------+---------+------------+--------+---------------+ | ECDH-ES + | -29 | HKDF - | yes | A128KW | ECDH ES w/ | | A128KW | | SHA-256 | | | Concat KDF | | | | | | | and AES Key | | | | | | | wrap w/ 128 | | | | | | | bit key | | | | | | | | | ECDH-ES + | -30 | HKDF - | yes | A192KW | ECDH ES w/ | | A192KW | | SHA-256 | | | Concat KDF | | | | | | | and AES Key | | | | | | | wrap w/ 192 | | | | | | | bit key | | | | | | | | | ECDH-ES + | -31 | HKDF - | yes | A256KW | ECDH ES w/ | | A256KW | | SHA-256 | | | Concat KDF | | | | | | | and AES Key | | | | | | | wrap w/ 256 | | | | | | | bit key | | | | | | | | | ECDH-SS + | -32 | HKDF - | no | A128KW | ECDH SS w/ | | A128KW | | SHA-256 | | | Concat KDF | | | | | | | and AES Key | | | | | | | wrap w/ 128 | | | | | | | bit key | | | | | | | | | ECDH-SS + | -33 | HKDF - | no | A192KW | ECDH SS w/ | | A192KW | | SHA-256 | | | Concat KDF | | | | | | | and AES Key | | | | | | | wrap w/ 192 | | | | | | | bit key | | | | | | | | | ECDH-SS + | -34 | HKDF - | no | A256KW | ECDH SS w/ | | A256KW | | SHA-256 | | | Concat KDF | | | | | | | and AES Key | | | | | | | wrap w/ 256 | | | | | | | bit key | +-----------+-------+---------+------------+--------+---------------+ Table 20: ECDH Algorithm Values with Key Wrap Schaad Expires April 21, 2017 [Page 70] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 When using a COSE key for this algorithm, the following checks are made: o The 'kty' field MUST be present and it MUST be 'EC2' or 'OKP'. o If the 'alg' field is present, it MUST match the Key Agreement algorithm being used. o If the 'key_ops' field is present, it MUST include 'derive key' or 'derive bits' for the private key. o If the 'key_ops' field is present, it MUST be empty for the public key. 13. Key Object Parameters The COSE_Key object defines a way to hold a single key object. It is still required that the members of individual key types be defined. This section of the document is where we define an initial set of members for specific key types. For each of the key types, we define both public and private members. The public members are what is transmitted to others for their usage. Private members allow for the archival of keys by individuals. However, there are some circumstances in which private keys may be distributed to entities in a protocol. Examples include: entities that have poor random number generation, centralized key creation for multi-cast type operations, and protocols in which a shared secret is used as a bearer token for authorization purposes. Key types are identified by the 'kty' member of the COSE_Key object. In this document, we define four values for the member: +-----------+-------+--------------------------------------------+ | name | value | description | +-----------+-------+--------------------------------------------+ | OKP | 1 | Octet Key Pair | | | | | | EC2 | 2 | Elliptic Curve Keys w/ X,Y Coordinate pair | | | | | | Symmetric | 4 | Symmetric Keys | | | | | | Reserved | 0 | This value is reserved | +-----------+-------+--------------------------------------------+ Table 21: Key Type Values Schaad Expires April 21, 2017 [Page 71] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 13.1. Elliptic Curve Keys Two different key structures could be defined for Elliptic Curve keys. One version uses both an x and a y coordinate, potentially with point compression ('EC2'). This is the traditional EC point representation that is used in [RFC5480]. The other version uses only the x coordinate as the y coordinate is either to be recomputed or not needed for the key agreement operation ('OKP'). Applications MUST check that the curve and the key type are consistent and reject a key if they are not. +---------+----------+-------+------------------------------------+ | name | key type | value | description | +---------+----------+-------+------------------------------------+ | P-256 | EC2 | 1 | NIST P-256 also known as secp256r1 | | | | | | | P-384 | EC2 | 2 | NIST P-384 also known as secp384r1 | | | | | | | P-521 | EC2 | 3 | NIST P-521 also known as secp521r1 | | | | | | | X25519 | OKP | 4 | X25519 for use w/ ECDH only | | | | | | | X448 | OKP | 5 | X448 for use w/ ECDH only | | | | | | | Ed25519 | OKP | 6 | Ed25519 for use w/ EdDSA only | | | | | | | Ed448 | OKP | 7 | Ed448 for use w/ EdDSA only | +---------+----------+-------+------------------------------------+ Table 22: EC Curves 13.1.1. Double Coordinate Curves The traditional way of sending EC curves has been to send either both the x and y coordinates, or the x coordinate and a sign bit for the y coordinate. The latter encoding has not been recommended in the IETF due to potential IPR issues. However, for operations in constrained environments, the ability to shrink a message by not sending the y coordinate is potentially useful. For EC keys with both coordinates, the 'kty' member is set to 2 (EC2). The key parameters defined in this section are summarized in Table 23. The members that are defined for this key type are: crv contains an identifier of the curve to be used with the key. The curves defined in this document for this key type can be found Schaad Expires April 21, 2017 [Page 72] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 in Table 22. Other curves may be registered in the future and private curves can be used as well. x contains the x coordinate for the EC point. The integer is converted to an octet string as defined in [SEC1]. Leading zero octets MUST be preserved. y contains either the sign bit or the value of y coordinate for the EC point. When encoding the value y, the integer is converted to an octet string (as defined in [SEC1]) and encoded as a CBOR bstr. Leading zero octets MUST be preserved. The compressed point encoding is also supported. Compute the sign bit as laid out in the Elliptic-Curve-Point-to-Octet-String Conversion function of [SEC1]. If the sign bit is zero, then encode y as a CBOR false value, otherwise encode y as a CBOR true value. The encoding of the infinity point is not supported. d contains the private key. For public keys, it is REQUIRED that 'crv', 'x' and 'y' be present in the structure. For private keys, it is REQUIRED that 'crv' and 'd' be present in the structure. For private keys, it is RECOMMENDED that 'x' and 'y' also be present, but they can be recomputed from the required elements and omitting them saves on space. +------+-------+-------+---------+----------------------------------+ | name | key | label | type | description | | | type | | | | +------+-------+-------+---------+----------------------------------+ | crv | 2 | -1 | int / | EC Curve identifier - Taken from | | | | | tstr | the COSE Curves registry | | | | | | | | x | 2 | -2 | bstr | X Coordinate | | | | | | | | y | 2 | -3 | bstr / | Y Coordinate | | | | | bool | | | | | | | | | d | 2 | -4 | bstr | Private key | +------+-------+-------+---------+----------------------------------+ Table 23: EC Key Parameters 13.2. Octet Key Pair A new key type is defined for Octet Key Pairs (OKP). Do not assume that keys using this type are elliptic curves. This key type could be used for other curve types (for example, mathematics based on hyper-elliptic surfaces). Schaad Expires April 21, 2017 [Page 73] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 The key parameters defined in this section are summarized in Table 24. The members that are defined for this key type are: crv contains an identifier of the curve to be used with the key. The curves defined in this document for this key type can be found in Table 22. Other curves may be registered in the future and private curves can be used as well. x contains the x coordinate for the EC point. The octet string represents a little-endian encoding of x. d contains the private key. For public keys, it is REQUIRED that 'crv' and 'x' be present in the structure. For private keys, it is REQUIRED that 'crv' and 'd' be present in the structure. For private keys, it is RECOMMENDED that 'x' also be present, but it can be recomputed from the required elements and omitting it saves on space. +------+------+-------+-------+-------------------------------------+ | name | key | label | type | description | | | type | | | | +------+------+-------+-------+-------------------------------------+ | crv | 1 | -1 | int / | EC Curve identifier - Taken from | | | | | tstr | the COSE Key Common Parameters | | | | | | registry | | | | | | | | x | 1 | -2 | bstr | X Coordinate | | | | | | | | d | 1 | -4 | bstr | Private key | +------+------+-------+-------+-------------------------------------+ Table 24: Octet Key Pair Parameters 13.3. Symmetric Keys Occasionally it is required that a symmetric key be transported between entities. This key structure allows for that to happen. For symmetric keys, the 'kty' member is set to 3 (Symmetric). The member that is defined for this key type is: k contains the value of the key. This key structure does not have a form that contains only public members. As it is expected that this key structure is going to be transmitted, care must be taking that it is never transmitted Schaad Expires April 21, 2017 [Page 74] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 accidentally or insecurely. For symmetric keys, it is REQUIRED that 'k' be present in the structure. +------+----------+-------+------+-------------+ | name | key type | label | type | description | +------+----------+-------+------+-------------+ | k | 4 | -1 | bstr | Key Value | +------+----------+-------+------+-------------+ Table 25: Symmetric Key Parameters 14. CBOR Encoder Restrictions There has been an attempt to limit the number of places where the document needs to impose restrictions on how the CBOR Encoder needs to work. We have managed to narrow it down to the following restrictions: o The restriction applies to the encoding the Sig_structure, the Enc_structure, and the MAC_structure. o 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. o Applications MUST NOT generate messages with the same label used twice as a key in a single map. Applications MUST NOT parse and process messages with the same label used twice as a key in a single map. Applications can enforce the parse and process requirement by using parsers that will fail the parse step or by using parsers that will pass all keys to the application and the application can perform the check for duplicate keys. 15. Application Profiling Considerations This document is designed to provide a set of security services, but not to provide implementation requirements for specific usage. The interoperability requirements are provided for how each of the individual services are used and how the algorithms are to be used for interoperability. The requirements about which algorithms and which services are needed are deferred to each application. An example of a profile can be found in [I-D.selander-ace-object-security] 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. Schaad Expires April 21, 2017 [Page 75] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 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. o Applications need to determine the set of messages defined in this document that they will be using. The set of messages corresponds fairly directly to the set of security services that are needed and to the security levels needed. o Applications may define new header parameters for a specific purpose. Applications will often times select specific header parameters to use or not to use. For example, an application would normally state a preference for using either the IV or the partial IV parameter. If the partial IV parameter is specified, then the application would also need to define how the fixed portion of the IV would be determined. o When applications use externally defined authenticated data, they need to define how that data is encoded. This document assumes that the data will be provided as a byte stream. More information can be found in Section 4.3. o Applications need to determine the set of security algorithms that are to be used. When selecting the algorithms to be used as the mandatory to implement set, consideration should be given to choosing different types of algorithms when two are chosen for a specific purpose. An example of this would be choosing HMAC- SHA512 and AES-CMAC as different MAC algorithms; the construction is vastly different between these two algorithms. This means that a weakening of one algorithm would be unlikely to lead to a weakening of the other algorithms. Of course, these algorithms do not provide the same level of security and thus may not be comparable for the desired security functionality. o Applications may need to provide some type of negotiation or discovery method if multiple algorithms or message structures are permitted. The method can be as simple as requiring preconfiguration of the set of algorithms to providing a discovery method built into the protocol. S/MIME provided a number of different ways to approach the problem that applications could follow: * Advertising in the message (S/MIME capabilities) [RFC5751]. * Advertising in the certificate (capabilities extension) [RFC4262]. Schaad Expires April 21, 2017 [Page 76] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 * Minimum requirements for the S/MIME, which have been updated over time [RFC2633][RFC5751]. 16. IANA Considerations 16.1. CBOR Tag assignment It is requested that IANA assign the following tags from the "CBOR Tags" registry. It is requested that the tags for COSE_Sign1, COSE_Encrypt0, and COSE_Mac0 be assigned in the 1 to 23 value range (one byte long when encoded). It is requested that the tags for COSE_Sign, COSE_Encrypt and COSE_MAC be assigned in the 24 to 255 value range (two bytes long when encoded). The tags to be assigned are in Table 1. 16.2. COSE Header Parameters Registry It is requested that IANA create a new registry entitled "COSE Header Parameters". The registry should be created as Expert Review Required. Guidelines for the experts is provided Section 16.11. It should be noted that in additional to the expert review, some portions of the registry require a specification, potentially on standards track, be supplied as well. The columns of the registry are: name The name is present to make it easier to refer to and discuss the registration entry. The value is not used in the protocol. Names are to be unique in the table. label This is the value used for the label. The label can be either an integer or a string. Registration in the table is based on the value of the label requested. Integer values between 1 and 255 and strings of length 1 are designated as Standards Track Document required. Integer values from 256 to 65535 and strings of length 2 are designated as Specification Required. Integer values of greater than 65535 and strings of length greater than 2 are designated as expert review. Integer values in the range -1 to -65536 are delegated to the "COSE Header Algorithm Parameters" registry. Integer values less than -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. Schaad Expires April 21, 2017 [Page 77] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 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 and Table 27. The specification column for all rows in that table should be this document. Additionally, the label of 0 is to be marked as 'Reserved'. 16.3. COSE Header Algorithm Parameters Registry It is requested that IANA create a new registry entitled "COSE Header Algorithm Parameters". The registry is to be created as Expert Review Required. Expert review guidelines are provided in Section 16.11. 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 Values" 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. 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 13, Table 14, and Table 19. The specification column for all rows in that table should be this document. 16.4. COSE Algorithms Registry It is requested that IANA create a new registry entitled "COSE Algorithms Registry". The registry is to be created as Expert Review Required. Guidelines for the experts is provided Section 16.11. It Schaad Expires April 21, 2017 [Page 78] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 should be noted that in additional to the expert review, some portions of the registry require a specification, potentially on standards track, be supplied as well. The columns of the registry are: value: The value to be used to identify this algorithm. Algorithm values MUST be unique. The value can be a positive integer, a negative integer or a string. Integer values between -256 and 255 and strings of length 1 are designated as Standards Track Document required. Integer values from -65536 to 65535 and strings of length 2 are designated as Specification Required. Integer values of greater than 65535 and strings of length greater than 2 are designated as expert review. Integer values less than -65536 are marked as private use. description: A short description of the algorithm. specification: A document where the algorithm is defined (if publicly available). recommended: Does the IETF have a concensus recommendation to use the algorithm. The legal values are 'yes', 'no' and 'deprecated'. The initial contents of the registry can be found in Table 10, Table 9, Table 11, Table 5, Table 7, Table 8, Table 15, Table 16, Table 17, Table 6, Table 20 and Table 18. The specification column for all rows in the table should be this document. The recommneded column for all rows in the table are set to 'yes'. Additionally, the label of 0 is to be marked as 'Reserved'. 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. 16.5. COSE Key Common Parameters Registry It is requested that IANA create a new registry entitled "COSE Key Common Parameters" registry. The registry is to be created as Expert Review Required. Guidelines for the experts is provided Section 16.11. It should be noted that in additional to the expert review, some portions of the registry require a specification, potentially on standards track, be supplied as well. Schaad Expires April 21, 2017 [Page 79] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 The columns of the registry are: name This is a descriptive name that enables easier reference to the item. It is not used in the encoding. label The value to be used to identify this algorithm. Key map labels MUST be unique. The label can be a positive integer, a negative integer or a string. Integer values between 0 and 255 and strings of length 1 are designated as Standards Track Document required. Integer values from 256 to 65535 and strings of length 2 are designated as Specification Required. Integer values of greater than 65535 and strings of length greater than 2 are designated as expert review. Integer values in the range -1 to -65536 are used for key parameters specific to a single algorithm delegated to the "COSE Key Type Parameter Labels" registry. Integer values less than -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 Table 3. The specification column for all of these entries will be this document. 16.6. COSE Key Type Parameters Registry It is requested that IANA create a new registry "COSE Key Type Parameters". The registry is to be created as Expert Review Required. Expert review guidelines are provided in Section 16.11. 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 Key Common Parameters 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. Schaad Expires April 21, 2017 [Page 80] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 label The label is to be unique for every value of key type. The range of values is from -256 to -1. Labels are expected to be reused for different keys. CBOR type This field contains the CBOR type for the field. description This field contains a brief description for the field. specification This contains a pointer to the public specification for the field if one exists. This registry will be initially populated by the values in Table 23, Table 24, and Table 25. The specification column for all of these entries will be this document. 16.7. COSE Key Type Registry It is requested that IANA create a new registry "COSE Key Type Registry". The registry is to be created as Expert Review Required. Expert review guidelines are provided in Section 16.11. The columns of this table are: name This is a descriptive name that enables easier reference to the item. The name MUST be unique. It is not used in the encoding. value This is the value used to identify the curve. These values MUST be unique. The value can be a positive integer, a negative integer or a string. description This field contains a brief description of the curve. specification This contains a pointer to the public specification for the curve if one exists. This registry will be initially populated by the values in Table 21. The specification column for all of these entries will be this document. 16.8. COSE Elliptic Curve Parameters Registry It is requested that IANA create a new registry "COSE Elliptic Curve Parameters". The registry is to be created as Expert Review Required. Guidelines for the experts is provided Section 16.11. It should be noted that in additional to the expert review, some portions of the registry require a specification, potentially on standards track, be supplied as well. Schaad Expires April 21, 2017 [Page 81] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 The columns of the table are: name This is a descriptive name that enables easier reference to the item. It is not used in the encoding. value This is the value used to identify the curve. These values MUST be unique. The integer values from -256 to 255 are designated as Standards Track Document Required. The integer values from 256 to 65535 and -65536 to -257 are designated as Specification Required. Integer values over 65535 are designated as expert review. Integer values less than -65536 are marked as private use. key type This designates the key type(s) that can be used with this curve. description This field contains a brief description of the curve. specification This contains a pointer to the public specification for the curve if one exists. recommended: Does the IETF have a concensus recommendation to use the algorithm. The legal values are 'yes', 'no' and 'deprecated'. This registry will be initially populated by the values in Table 22. The specification column for all of these entries will be this document. The recommended column for all of the inital entries will be 'yes'. 16.9. Media Type Registrations 16.9.1. COSE Security Message This section registers the "application/cose" media type in the "Media Types" registry. These media types are used to indicate that the content is a COSE message. Type name: application Subtype name: cose Required parameters: N/A Optional parameters: cose-type Encoding considerations: binary Schaad Expires April 21, 2017 [Page 82] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 Security considerations: See the Security Considerations section of RFC TBD. Interoperability considerations: N/A Published specification: RFC TBD Applications that use this media type: IoT applications sending security content over HTTP(S) transports. Fragment identifier considerations: N/A Additional information: * Magic number(s): N/A * File extension(s): cbor * Macintosh file type code(s): N/A Person & email address to contact for further information: iesg@ietf.org Intended usage: COMMON Restrictions on usage: N/A Author: Jim Schaad, ietf@augustcellars.com Change Controller: IESG Provisional registration? No 16.9.2. COSE Key media type 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: Type name: application Subtype name: cose-key Required parameters: N/A Schaad Expires April 21, 2017 [Page 83] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 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: Distribution of COSE based keys for IoT applications. 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 The template for registering "application/cose-key-set" is: Type name: application Subtype name: cose-key-set Required parameters: N/A Optional parameters: N/A Encoding considerations: binary Schaad Expires April 21, 2017 [Page 84] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 Security considerations: See the Security Considerations section of RFC TBD. Interoperability considerations: N/A Published specification: RFC TBD Applications that use this media type: Distribution of COSE based keys for IoT applications. Fragment identifier considerations: N/A Additional information: * Magic number(s): N/A * File extension(s): cbor * Macintosh file type code(s): N/A Person & email address to contact for further information: iesg@ietf.org Intended usage: COMMON Restrictions on usage: N/A Author: Jim Schaad, ietf@augustcellars.com Change Controller: IESG Provisional registration? No 16.10. CoAP Content-Format Registrations IANA is requested to add the following entries to the "CoAP Content- Format" registry. ID assignment in the 24-255 range is requested. Schaad Expires April 21, 2017 [Page 85] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 +---------------------------------+----------+-------+--------------+ | Media Type | Encoding | ID | Reference | +---------------------------------+----------+-------+--------------+ | application/cose; cose-type | | TBD10 | [This | | ="cose-sign" | | | Document] | | | | | | | application/cose; cose-type | | TBD11 | [This | | ="cose-sign1" | | | Document] | | | | | | | application/cose; cose-type | | TBD12 | [This | | ="cose-encrypt" | | | Document] | | | | | | | application/cose; cose-type | | TBD13 | [This | | ="cose-encrypt0" | | | Document] | | | | | | | application/cose; cose-type | | TBD14 | [This | | ="cose-mac" | | | Document] | | | | | | | application/cose; cose-type | | TBD15 | [This | | ="cose-mac0" | | | Document] | | | | | | | application/cose-key | | TBD16 | [This | | | | | Document] | | | | | | | application/cose-key-set | | TBD17 | [This | | | | | Document | +---------------------------------+----------+-------+--------------+ Table 26 16.11. Expert Review Instructions All of the IANA registries established in this document are defined as expert review. This section gives some general guidelines for what the experts should be looking for, but they are being designated as experts for a reason so they should be given substantial latitude. Expert reviewers should take into consideration the following points: o Point squatting should be discouraged. Reviewers are encouraged to get sufficient information for registration requests to ensure that the usage is not going to duplicate one that is already registered and that the point is likely to be used in deployments. The zones tagged as private use are intended for testing purposes and closed environments, code points in other ranges should not be assigned for testing. Schaad Expires April 21, 2017 [Page 86] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 o Specifications are required for the standards track range of point assignment. Specifications should exist for specification required ranges, but early assignment before a specification is available is considered to be permissible. Specifications are needed for the first-come, first-serve range if they are expected to be used outside of closed environments in an interoperable way. When specifications are not provided, the description provided needs to have sufficient information to identify what the point is being used for. o Experts should take into account the expected usage of fields when approving point assignment. The fact that there is a range for standards track documents does not mean that a standards track document cannot have points assigned outside of that range. The length of the encoded value should be weighed against how many code points of that length are left, the size of device it will be used on, and the number of code points left that encode to that size. o When algorithms are registered, vanity registrations should be discouraged. One way to do this is to require registrations to provide additional documentation on security analysis of the algorithm. Another thing that should be considered is to request for an opinion on the algorithm from the Crypto Forum Research Group (CFRG). Algorithms that do not meet the security requirements of the community and the messages structures should not be registered. 17. Implementation Status This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist. According to [RFC7942], "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. Schaad Expires April 21, 2017 [Page 87] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 It is up to the individual working groups to use this information as they see fit". 17.1. Author's Versions There are three different implementations that have been created by the author of the document both to create the examples that are included in the document and to validate the structures and methodology used in the design of COSE. Implementation Location: https://github.com/cose-wg Primary Maintainer: Jim Schaad Languages: There are three different languages that are currently supported: Java, C# and C. Cryptography: The Java and C# libraries use Bouncy Castle to provide the required cryptography. The C version uses OPENSSL Version 1.0 for the cryptography. Coverage: The libraries currently do not have full support for counter signatures of either variety. They do have support to allow for implicit algorithm support as they allow for the application to set attributes that are not to be sent in the message. Testing: All of the examples in the example library are generated by the C# library and then validated using the Java and C libraries. All three libraries have tests to allow for the creating of the same messages that are in the example library followed by validating them. These are not compared against the example library. The Java and C# libraries have unit testing included. Not all of the MUST statements in the document have been implemented as part of the libraries. One such statement is the requirement that unique labels be present. Licensing: Revised BSD License 17.2. COSE Testing Library Implementation Location: https://github.com/cose-wg/Examples Primary Maintainer: Jim Schaad Description: A set of tests for the COSE library is provided as part of the implementation effort. Both success and fail tests Schaad Expires April 21, 2017 [Page 88] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 have been provided. All of the examples in this document are part of this example set. Coverage: An attempt has been made to have test cases for every message type and algorithm in the document. Currently examples dealing with counter signatures, EdDSA, and ECDH with Curve24459 and Goldilocks are missing. Licensing: Public Domain 18. Security Considerations There are a number of security considerations that need to be taken into account by implementers of this specification. The security considerations that are specific to an individual algorithm are placed next to the description of the algorithm. While some considerations have been highlighted here, additional considerations may be found in the documents listed in the references. Implementations need to protect the private key material for any individuals. There are some cases in this document that need to be highlighted on this issue. o Using the same key for two different algorithms can leak information about the key. It is therefore recommended that keys be restricted to a single algorithm. o Use of 'direct' as a recipient algorithm combined with a second recipient algorithm, exposes the direct key to the second recipient. o Several of the algorithms in this document have limits on the number of times that a key can be used without leaking information about the key. The use of ECDH and direct plus KDF (with no key wrap) will not directly lead to the private key being leaked; the one way function of the KDF will prevent that. There is however, a different issue that needs to be addressed. Having two recipients requires that the CEK be shared between two recipients. The second recipient therefore has a CEK that was derived from material that can be used for the weak proof of origin. The second recipient could create a message using the same CEK and send it to the first recipient, the first recipient would, for either static-static ECDH or direct plus KDF, make an assumption that the CEK could be used for proof of origin even though it is from the wrong entity. If the key wrap step is added, then no proof of origin is implied and this is not an issue. Schaad Expires April 21, 2017 [Page 89] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 Although it has been mentioned before, the use of a single key for multiple algorithms has been demonstrated in some cases to leak information about a key, provide for attackers to forge integrity tags, or gain information about encrypted content. Binding a key to a single algorithm prevents these problems. Key creators and key consumers are strongly 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: o What are the permissions associated with the key owner? o Is the cryptographic algorithm acceptable in the current context? o Have the restrictions associated with the key, such as algorithm or freshness, been checked and are correct? o Is the request something that is reasonable, given the current state of the application? o Have any security considerations that are part of the message been enforced (as specified by the application or 'crit' parameter)? 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 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 Schaad Expires April 21, 2017 [Page 90] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 distinguish between two different strings (for example, 'YES' and 'NO') based on length for all of the content encryption algorithms that are defined in this document. This means that it is up to applications to document how content padding is to be done in order to prevent or discourage such analysis. (For example, the strings could be defined as 'YES' and 'NO '.) 19. References 19.1. Normative References [AES-GCM] Dworkin, M., "NIST Special Publication 800-38D: Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC.", Nov 2007. [COAP.Formats] IANA, , "CoAP Content-Formats". [DSS] U.S. National Institute of Standards and Technology, "Digital Signature Standard (DSS)", July 2013. [I-D.irtf-cfrg-eddsa] Josefsson, S. and I. Liusvaara, "Edwards-curve Digital Signature Algorithm (EdDSA)", draft-irtf-cfrg-eddsa-08 (work in progress), August 2016. [MAC] NiST, N., "FIPS PUB 113: Computer Data Authentication", May 1985. [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997, <http://www.rfc-editor.org/info/rfc2104>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC3394] Schaad, J. and R. Housley, "Advanced Encryption Standard (AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394, September 2002, <http://www.rfc-editor.org/info/rfc3394>. [RFC3610] Whiting, D., Housley, R., and N. Ferguson, "Counter with CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September 2003, <http://www.rfc-editor.org/info/rfc3610>. Schaad Expires April 21, 2017 [Page 91] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/RFC5869, May 2010, <http://www.rfc-editor.org/info/rfc5869>. [RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic Curve Cryptography Algorithms", RFC 6090, DOI 10.17487/RFC6090, February 2011, <http://www.rfc-editor.org/info/rfc6090>. [RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August 2013, <http://www.rfc-editor.org/info/rfc6979>. [RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, October 2013, <http://www.rfc-editor.org/info/rfc7049>. [RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015, <http://www.rfc-editor.org/info/rfc7539>. [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, January 2016, <http://www.rfc-editor.org/info/rfc7748>. [SEC1] Standards for Efficient Cryptography Group, "SEC 1: Elliptic Curve Cryptography", May 2009. 19.2. Informative References [I-D.greevenbosch-appsawg-cbor-cddl] Vigano, C. and H. Birkholz, "CBOR data definition language (CDDL): a notational convention to express CBOR data structures", draft-greevenbosch-appsawg-cbor-cddl-09 (work in progress), September 2016. [I-D.moriarty-pkcs1] Moriarty, K., Kaliski, B., Jonsson, J., and A. Rusch, "PKCS #1 Version 2.2: RSA Cryptography Specifications", draft-moriarty-pkcs1-03 (work in progress), September 2016. Schaad Expires April 21, 2017 [Page 92] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 [I-D.moriarty-pkcs5-v2dot1] Moriarty, K., Kaliski, B., and A. Rusch, "PKCS #5: Password-Based Cryptography Specification Version 2.1", draft-moriarty-pkcs5-v2dot1-04 (work in progress), September 2016. [I-D.selander-ace-object-security] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, "Object Security of CoAP (OSCOAP)", draft-selander-ace- object-security-05 (work in progress), July 2016. [PVSig] Brown, D. and D. Johnson, "Formal Security Proofs for a Signature Scheme with Partial Message Recover", February 2000. [RFC2633] Ramsdell, B., Ed., "S/MIME Version 3 Message Specification", RFC 2633, DOI 10.17487/RFC2633, June 1999, <http://www.rfc-editor.org/info/rfc2633>. [RFC4231] Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA- 224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512", RFC 4231, DOI 10.17487/RFC4231, December 2005, <http://www.rfc-editor.org/info/rfc4231>. [RFC4262] Santesson, S., "X.509 Certificate Extension for Secure/ Multipurpose Internet Mail Extensions (S/MIME) Capabilities", RFC 4262, DOI 10.17487/RFC4262, December 2005, <http://www.rfc-editor.org/info/rfc4262>. [RFC4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June 2006, <http://www.rfc-editor.org/info/rfc4493>. [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, <http://www.rfc-editor.org/info/rfc4949>. [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008, <http://www.rfc-editor.org/info/rfc5116>. [RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk, "Elliptic Curve Cryptography Subject Public Key Information", RFC 5480, DOI 10.17487/RFC5480, March 2009, <http://www.rfc-editor.org/info/rfc5480>. Schaad Expires April 21, 2017 [Page 93] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, September 2009, <http://www.rfc-editor.org/info/rfc5652>. [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification", RFC 5751, DOI 10.17487/RFC5751, January 2010, <http://www.rfc-editor.org/info/rfc5751>. [RFC5752] Turner, S. and J. Schaad, "Multiple Signatures in Cryptographic Message Syntax (CMS)", RFC 5752, DOI 10.17487/RFC5752, January 2010, <http://www.rfc-editor.org/info/rfc5752>. [RFC5990] Randall, J., Kaliski, B., Brainard, J., and S. Turner, "Use of the RSA-KEM Key Transport Algorithm in the Cryptographic Message Syntax (CMS)", RFC 5990, DOI 10.17487/RFC5990, September 2010, <http://www.rfc-editor.org/info/rfc5990>. [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms", RFC 6151, DOI 10.17487/RFC6151, March 2011, <http://www.rfc-editor.org/info/rfc6151>. [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, January 2013, <http://www.rfc-editor.org/info/rfc6838>. [RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 2014, <http://www.rfc-editor.org/info/rfc7159>. [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014, <http://www.rfc-editor.org/info/rfc7252>. [RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May 2015, <http://www.rfc-editor.org/info/rfc7515>. [RFC7516] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)", RFC 7516, DOI 10.17487/RFC7516, May 2015, <http://www.rfc-editor.org/info/rfc7516>. Schaad Expires April 21, 2017 [Page 94] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 [RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517, DOI 10.17487/RFC7517, May 2015, <http://www.rfc-editor.org/info/rfc7517>. [RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, DOI 10.17487/RFC7518, May 2015, <http://www.rfc-editor.org/info/rfc7518>. [RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", BCP 205, RFC 7942, DOI 10.17487/RFC7942, July 2016, <http://www.rfc-editor.org/info/rfc7942>. [SP800-56A] Barker, E., Chen, L., Roginsky, A., and M. Smid, "NIST Special Publication 800-56A: Recommendation for Pair-Wise Key Establishment Schemes Using Discrete Logarithm Cryptography", May 2013. [W3C.WebCrypto] Watson, M., "Web Cryptography API", July 2016. Appendix A. Making Mandatory Algorithm Header Optional There has been a portion of the working group who have expressed a strong desire to relax the rule that the algorithm identifier be required to appear in each level of a COSE 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. A.1. Algorithm Identification In this section are laid out three sets of recommendations. The first set of recommendations apply to having an implicit algorithm identified for a single layer of a COSE object. The second set of recommendations apply to having multiple implicit algorithms Schaad Expires April 21, 2017 [Page 95] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 identified for multiple layers of a COSE object. The third set of recommendations apply to having implicit algorithms for multiple COSE object constructs. RFC 2119 language is deliberately not used here. This specification can provide recommendations, but it cannot enforce them. This set of recommendations applies to the case where an application is distributing a fixed algorithm along with the key information for use in a single COSE 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: o Applications need to list the set of COSE structures that implicit algorithms are to be used in. Applications need to require that the receipt of an explicit algorithm identifier in one of these structures will lead to the message being rejected. This requirement is stated so that there will never be a case where there is any ambiguity about the question of which algorithm should be used, the implicit or the explicit one. This applies even if the transported algorithm identifier is a protected attribute. This applies even if the transported algorithm is the same as the implicit algorithm. o Applications need to define the set of information that is to be considered to be part of a context when omitting algorithm identifiers. At a minimum, this would be the key identifier (if needed), the key, the algorithm, and the COSE structure it is used with. Applications should restrict the use of a single key to a single algorithm. As noted for some of the algorithms in this document, the use of the same key in different related algorithms can lead to leakage of information about the key, leakage about the data or the ability to perform forgeries. o In many cases, applications that make the algorithm identifier implicit will also want to make the context identifier implicit for the same reason. That is, omitting the context identifier will decrease the message size (potentially significantly depending on the length of the identifier). Applications that do this will need to describe the circumstances where the context identifier is to be omitted and how the context identifier is to be inferred in these cases. (Exhaustive search over all of the keys would normally not be considered to be acceptable.) An example of how this can be done is to tie the context to a transaction identifier. Both would be sent on the original message, but only the transaction identifier would need to be sent Schaad Expires April 21, 2017 [Page 96] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 after that point as the context is tied into the transaction identifier. Another way would be to associate a context with a network address. All messages coming from a single network address can be assumed to be associated with a specific context. (In this case the address would normally be distributed as part of the context.) o Applications cannot rely on key identifiers being unique unless they take significant efforts to ensure that they are computed in such a way as to create this guarantee. Even when an application does this, the uniqueness might be violated if the application is run in different contexts (i.e., with a different context provider) or if the system combines the security contexts from different applications together into a single store. o Applications should continue the practice of protecting the algorithm identifier. Since this is not done by placing it in the protected attributes field, applications should define an application specific external data structure that includes this value. This external data field can be used as such for content encryption, MAC, and signature algorithms. It can be used in the SuppPrivInfo field for those algorithms which use a KDF function to derive a key value. Applications may also want to protect other information that is part of the context structure as well. It should be noted that those fields, such as the key or a base IV, are protected by virtue of being used in the cryptographic computation and do not need to be included in the external data field. The second case is having multiple implicit algorithm identifiers specified for a multiple layer COSE object. An example of how this would work is the encryption context that an application specifies contains a content encryption algorithm, a key wrap algorithm, a key identifier, and a shared secret. The sender 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: o Applications that want to support this will need to define a structure that allows for, and clearly identifies, both the COSE structure to be used with a given key and the structure and algorithm to be used for the secondary layer. The key for the secondary layer is computed per normal from the recipient layer. The third case is having multiple implicit algorithm identifiers, but targeted at potentially unrelated layers or different COSE objects. Schaad Expires April 21, 2017 [Page 97] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 There are a number of different scenarios where this might be applicable. Some of these scenarios are: o Two contexts are distributed as a pair. Each of the contexts is for use with a COSE_Encrypt message. Each context will consist of distinct secret keys and IVs and potentially even different algorithms. One context is for sending messages from party A to party B, the second context is for sending messages from party B to party A. This means that there is no chance for a reflection attack to occur as each party uses different secret keys to send its messages, a message that is reflected back to it would fail to decrypt. o Two contexts are distributed as a pair. The first context is used for encryption of the message; the second context is used to place a counter signature on the message. The intention is that the second context can be distributed to other entities independently of the first context. This allows these entities to validate that the message came from an individual without being able to decrypt the message and see the content. o Two contexts are distributed as a pair. The first context contains a key for dealing with MACed messages, the second context contains a key for dealing with encrypted messages. This allows for a unified distribution of keys to participants for different types of messages that have different keys, but where the keys may be used in coordinated manner. For these cases, the following additional items need to be considered: o Applications need to ensure that the multiple contexts stay associated. If one of the contexts is invalidated for any reason, all of the contexts associated with it should also be invalidated. A.2. Counter Signature Without Headers 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 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 Schaad Expires April 21, 2017 [Page 98] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 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 in this counter signature header. The value of "CounterSignature0" is placed in the context field of the Sig_stucture. +-------------------+-------+-------+-------+-----------------------+ | name | label | value | value | description | | | | type | | | +-------------------+-------+-------+-------+-----------------------+ | CounterSignature0 | 9 | bstr | | Counter signature | | | | | | with implied signer | | | | | | and headers | +-------------------+-------+-------+-------+-----------------------+ Table 27 Appendix B. Two Layers of Recipient Information All of the currently defined recipient algorithms 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 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: o Layer 0: The content encryption layer. This layer contains the payload of the message. o Layer 1: The encryption of the CEK by a KEK. o Layer 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: Schaad Expires April 21, 2017 [Page 99] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 o Layer 0: Has a content encrypted with AES-GCM using a 128-bit key. o Layer 1: Uses the AES Key wrap algorithm with a 128-bit key. o Layer 2: Uses ECDH Ephemeral-Static direct to generate the layer 1 key. In effect, this example is a decomposed version of using the ECDH- ES+A128KW algorithm. Size of binary file is 184 bytes Schaad Expires April 21, 2017 [Page 100] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 992( [ / 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'' ] ] ] ] ] ) Appendix C. Examples This appendix includes a set of examples that show the different features and message types that have been defined in this document. To make the examples easier to read, they are presented using the extended CBOR diagnostic notation (defined in [I-D.greevenbosch-appsawg-cbor-cddl]) rather than as a binary dump. Schaad Expires April 21, 2017 [Page 101] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 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() C.1. Examples of Signed Message C.1.1. Single Signature This example uses the following: o Signature Algorithm: ECDSA w/ SHA-256, Curve P-256 Size of binary file is 104 bytes Schaad Expires April 21, 2017 [Page 102] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 991( [ / protected / h'', / unprotected / {}, / payload / 'This is the content.', / signatures / [ [ / protected / h'a10126' / { \ alg \ 1:-7 \ ECDSA 256 \ } / , / unprotected / { / kid / 4:'11' }, / signature / h'eae868ecc176883766c5dc5ba5b8dca25dab3c2e56a5 51ce5705b793914348e14eea4aee6e0c9f09db4ef3ddeca8f3506cd1a98a8fb64327 be470355c9657ce0' ] ] ] ) C.1.2. Multiple Signers This example uses the following: o Signature Algorithm: ECDSA w/ SHA-256, Curve P-256 o Signature Algorithm: ECDSA w/ SHA-512, Curve P-521 Size of binary file is 278 bytes Schaad Expires April 21, 2017 [Page 103] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 991( [ / protected / h'', / unprotected / {}, / payload / 'This is the content.', / signatures / [ [ / protected / h'a10126' / { \ alg \ 1:-7 \ ECDSA 256 \ } / , / unprotected / { / kid / 4:'11' }, / signature / h'0dc1c5e62719d8f3cce1468b7c881eee6a8088b46bf8 36ae956dd38fe93199199951a6a5e02a24aed5edde3509748366b1c539aaef7dea34 f2cd618fe19fe55d' ], [ / protected / h'a1013823' / { \ alg \ 1:-36 } / , / unprotected / { / kid / 4:'bilbo.baggins@hobbiton.example' }, / signature / h'012ce5b1dfe8b5aa6eaa09a54c58a84ad0900e4fdf27 59ec22d1c861cccd75c7e1c4025a2da35e512fc2874d6ac8fd862d09ad07ed2deac2 97b897561e04a8d42476017c11a4a34e26c570c9eff22c1dc84d56cdf6e03ed34bc9 e934c5fdf676c7948d79e97dfe161730217c57748aadb364a0207cee811e9dde65ae 37942e8a8348cc91' ] ] ] ) C.1.3. Counter Signature This example uses the following: o Signature Algorithm: ECDSA w/ SHA-256, Curve P-256 o The same parameters are used for both the signature and the counter signature. Size of binary file is 181 bytes Schaad Expires April 21, 2017 [Page 104] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 991( [ / protected / h'', / unprotected / { / countersign / 7:[ / protected / h'a10126' / { \ alg \ 1:-7 \ ECDSA 256 \ } / , / unprotected / { / kid / 4:'11' }, / signature / h'c9d3402485aa585cee3efc69b14496c0b00714584b26 0f8e05764b7dbc70ae2b23b89812f5895b805f07a792f7ce77ef6d63875dc37d6a78 ef4d175da45c9a51' ] }, / payload / 'This is the content.', / signatures / [ [ / protected / h'a10126' / { \ alg \ 1:-7 \ ECDSA 256 \ } / , / unprotected / { / kid / 4:'11' }, / signature / h'eae868ecc176883766c5dc5ba5b8dca25dab3c2e56a5 51ce5705b793914348e14eea4aee6e0c9f09db4ef3ddeca8f3506cd1a98a8fb64327 be470355c9657ce0' ] ] ] ) C.1.4. Signature w/ Criticality This example uses the following: o Signature Algorithm: ECDSA w/ SHA-256, Curve P-256 o There is a criticality marker on the "reserved" header parameter Size of binary file is 126 bytes Schaad Expires April 21, 2017 [Page 105] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 991( [ / 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'eae868ecc176883766c5dc5ba5b8dca25dab3c2e56a5 51ce5705b793914348e1ff259ead2c38d8a7d8a9c87c2ce534d762dab059773115a6 176fa780e85b6b25' ] ] ] ) C.2. Single Signer Examples C.2.1. Single ECDSA signature This example uses the following: o Signature Algorithm: ECDSA w/ SHA-256, Curve P-256 Size of binary file is 100 bytes Schaad Expires April 21, 2017 [Page 106] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 997( [ / protected / h'a10126' / { \ alg \ 1:-7 \ ECDSA 256 \ } / , / unprotected / { / kid / 4:'11' }, / payload / 'This is the content.', / signature / h'eae868ecc176883766c5dc5ba5b8dca25dab3c2e56a551ce 5705b793914348e19f43d6c6ba654472da301b645b293c9ba939295b97c4bdb84778 2bff384c5794' ] ) C.3. Examples of Enveloped Messages C.3.1. Direct ECDH This example uses the following: o CEK: AES-GCM w/ 128-bit key o Recipient class: ECDH Ephemeral-Static, Curve P-256 Size of binary file is 152 bytes Schaad Expires April 21, 2017 [Page 107] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 992( [ / 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'' ] ] ] ) C.3.2. Direct plus Key Derivation This example uses the following: o CEK: AES-CCM w/128-bit key, truncate the tag to 64 bits o Recipient class: Use HKDF on a shared secret with the following implicit fields as part of the context. * salt: "aabbccddeeffgghh" * APU identity: "lighting-client" * APV identity: "lighting-server" * Supplementary Public Other: "Encryption Example 02" Schaad Expires April 21, 2017 [Page 108] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 Size of binary file is 92 bytes 992( [ / protected / h'a1010a' / { \ alg \ 1:10 \ AES-CCM-16-64-128 \ } / , / unprotected / { / iv / 5:h'89f52f65a1c580933b5261a76c' }, / ciphertext / h'753548a19b1307084ca7b2056924ed95f2e3b17006dfe93 1b687b847', / recipients / [ [ / protected / h'a10129' / { \ alg \ 1:-10 } / , / unprotected / { / salt / -20:'aabbccddeeffgghh', / kid / 4:'our-secret' }, / ciphertext / h'' ] ] ] ) C.3.3. Counter Signature on Encrypted Content This example uses the following: o CEK: AES-GCM w/ 128-bit key o Recipient class: ECDH Ephemeral-Static, Curve P-256 Size of binary file is 327 bytes Schaad Expires April 21, 2017 [Page 109] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 992( [ / protected / h'a10101' / { \ alg \ 1:1 \ AES-GCM 128 \ } / , / unprotected / { / iv / 5:h'c9cf4df2fe6c632bf7886413', / countersign / 7:[ / protected / h'a1013823' / { \ alg \ 1:-36 } / , / unprotected / { / kid / 4:'bilbo.baggins@hobbiton.example' }, / signature / h'00aa98cbfd382610a375d046a275f30266e8d0faacb9 069fde06e37825ae7825419c474f416ded0c8e3e7b55bff68f2a704135bdf99186f6 6659461c8cf929cc7fb300f5e2b33c3b433655042ff719804ff73b0be3e988ecebc0 c70ef6616996809c6eb59a918dbe0a5edb0d15137ece0aba2a0b0f68ad2631cb62f2 ea4d7099804218b0' ] }, / 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'' ] ] ] ) Schaad Expires April 21, 2017 [Page 110] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 C.3.4. Encrypted Content with External Data This example uses the following: o CEK: AES-GCM w/ 128-bit key o Recipient class: ECDH static-Static, Curve P-256 with AES Key Wrap o Externally Supplied AAD: h'0011bbcc22dd44ee55ff660077' Size of binary file is 174 bytes 992( [ / protected / h'a10101' / { \ alg \ 1:1 \ AES-GCM 128 \ } / , / unprotected / { / iv / 5:h'02d1f7e6f26c43d4868d87ce' }, / ciphertext / h'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' ] ] ] ) C.4. Examples of Encrypted Messages C.4.1. Simple Encrypted Message This example uses the following: o CEK: AES-CCM w/ 128-bit key and a 64-bit tag Size of binary file is 54 bytes Schaad Expires April 21, 2017 [Page 111] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 993( [ / protected / h'a1010a' / { \ alg \ 1:10 \ AES-CCM-16-64-128 \ } / , / unprotected / { / iv / 5:h'89f52f65a1c580933b5261a78c' }, / ciphertext / h'5974e1b99a3a4cc09a659aa2e9e7fff161d38ce7edd5617 388e77baf' ] ) C.4.2. Encrypted Message w/ a Partial IV This example uses the following: o CEK: AES-CCM w/ 128-bit key and a 64-bit tag o Prefix for IV is 89F52F65A1C580933B52 Size of binary file is 43 bytes 993( [ / protected / h'a1010a' / { \ alg \ 1:10 \ AES-CCM-16-64-128 \ } / , / unprotected / { / partial iv / 6:h'61a7' }, / ciphertext / h'252a8911d465c125b6764739700f0141ed09192da5c69e5 33abf852b' ] ) C.5. Examples of MACed messages C.5.1. Shared Secret Direct MAC This example uses the following: o MAC: AES-CMAC, 256-bit key, truncated to 64 bits o Recipient class: direct shared secret Size of binary file is 58 bytes Schaad Expires April 21, 2017 [Page 112] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 994( [ / protected / h'a1010f' / { \ alg \ 1:15 \ AES-CBC-MAC-256//64 \ } / , / unprotected / {}, / payload / 'This is the content.', / tag / h'9e1226ba1f81b848', / recipients / [ [ / protected / h'', / unprotected / { / alg / 1:-6 / direct /, / kid / 4:'our-secret' }, / ciphertext / h'' ] ] ] ) C.5.2. ECDH Direct MAC This example uses the following: o MAC: HMAC w/SHA-256, 256-bit key o Recipient class: ECDH key agreement, two static keys, HKDF w/ context structure Size of binary file is 215 bytes Schaad Expires April 21, 2017 [Page 113] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 994( [ / 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'' ] ] ] ) C.5.3. Wrapped MAC This example uses the following: o MAC: AES-MAC, 128-bit key, truncated to 64 bits o Recipient class: AES keywrap w/ a pre-shared 256-bit key Size of binary file is 110 bytes Schaad Expires April 21, 2017 [Page 114] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 994( [ / protected / h'a1010e' / { \ alg \ 1:14 \ AES-CBC-MAC-128//64 \ } / , / unprotected / {}, / payload / 'This is the content.', / tag / h'36f5afaf0bab5d43', / recipients / [ [ / protected / h'', / unprotected / { / alg / 1:-5 / A256KW /, / kid / 4:'018c0ae5-4d9b-471b-bfd6-eef314bc7037' }, / ciphertext / h'711ab0dc2fc4585dce27effa6781c8093eba906f227 b6eb0' ] ] ] ) C.5.4. Multi-recipient MACed message This example uses the following: o MAC: HMAC w/ SHA-256, 128-bit key o Recipient class: Uses three different methods 1. ECDH Ephemeral-Static, Curve P-521, AES-Key Wrap w/ 128-bit key 2. AES-Key Wrap w/ 256-bit key Size of binary file is 310 bytes Schaad Expires April 21, 2017 [Page 115] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 994( [ / protected / h'a10105' / { \ alg \ 1:5 \ HMAC 256//256 \ } / , / unprotected / {}, / payload / 'This is the content.', / tag / h'bf48235e809b5c42e995f2b7d5fa13620e7ed834e337f6aa43df16 1e49e9323e', / recipients / [ [ / protected / h'a101381c' / { \ alg \ 1:-29 \ ECHD-ES+A128KW \ } / , / unprotected / { / ephemeral / -1:{ / kty / 1:2, / crv / -1:3, / x / -2:h'0043b12669acac3fd27898ffba0bcd2e6c366d53bc4db 71f909a759304acfb5e18cdc7ba0b13ff8c7636271a6924b1ac63c02688075b55ef2 d613574e7dc242f79c3', / y / -3:true }, / kid / 4:'bilbo.baggins@hobbiton.example' }, / ciphertext / h'339bc4f79984cdc6b3e6ce5f315a4c7d2b0ac466fce a69e8c07dfbca5bb1f661bc5f8e0df9e3eff5' ], [ / protected / h'', / unprotected / { / alg / 1:-5 / A256KW /, / kid / 4:'018c0ae5-4d9b-471b-bfd6-eef314bc7037' }, / ciphertext / h'0b2c7cfce04e98276342d6476a7723c090dfdd15f9a 518e7736549e998370695e6d6a83b4ae507bb' ] ] ] ) C.6. Examples of MAC0 messages C.6.1. Shared Secret Direct MAC This example uses the following: o MAC: AES-CMAC, 256-bit key, truncated to 64 bits Schaad Expires April 21, 2017 [Page 116] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 o Recipient class: direct shared secret Size of binary file is 39 bytes 996( [ / protected / h'a1010f' / { \ alg \ 1:15 \ AES-CBC-MAC-256//64 \ } / , / unprotected / {}, / payload / 'This is the content.', / tag / h'726043745027214f' ] ) Note that this example uses the same inputs as Appendix C.5.1. C.7. COSE Keys C.7.1. Public Keys This is an example of a COSE Key set. This example includes the public keys for all of the previous examples. In order the keys are: o An EC key with a kid of "meriadoc.brandybuck@buckland.example" o An EC key with a kid of "peregrin.took@tuckborough.example" o An EC key with a kid of "bilbo.baggins@hobbiton.example" o An EC key with a kid of "11" Size of binary file is 481 bytes Schaad Expires April 21, 2017 [Page 117] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 [ { -1:1, -2:h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de439c0 8551d', -3:h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd008 4d19c', 1:2, 2:'meriadoc.brandybuck@buckland.example' }, { -1:1, -2:h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a 09eff', -3:h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbf c117e', 1:2, 2:'11' }, { -1:3, -2:h'0072992cb3ac08ecf3e5c63dedec0d51a8c1f79ef2f82f94f3c737bf5de 7986671eac625fe8257bbd0394644caaa3aaf8f27a4585fbbcad0f2457620085e5c8 f42ad', -3:h'01dca6947bce88bc5790485ac97427342bc35f887d86d65a089377e247e 60baa55e4e8501e2ada5724ac51d6909008033ebc10ac999b9d7f5cc2519f3fe1ea1 d9475', 1:2, 2:'bilbo.baggins@hobbiton.example' }, { -1:1, -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91 d6280', -3:h'f01400b089867804b8e9fc96c3932161f1934f4223069170d924b7e03bf 822bb', 1:2, 2:'peregrin.took@tuckborough.example' } ] C.7.2. Private Keys This is an example of a COSE Key set. This example includes the private keys for all of the previous examples. In order the keys are: Schaad Expires April 21, 2017 [Page 118] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 o An EC key with a kid of "meriadoc.brandybuck@buckland.example" o A shared-secret key with a kid of "our-secret" o An EC key with a kid of "peregrin.took@tuckborough.example" o A shared-secret key with a kid of "018c0ae5-4d9b-471b- bfd6-eef314bc7037" o An EC key with a kid of "bilbo.baggins@hobbiton.example" o An EC key with a kid of "11" Size of binary file is 816 bytes [ { 1:2, 2:'meriadoc.brandybuck@buckland.example', -1:1, -2:h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de439c0 8551d', -3:h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd008 4d19c', -4:h'aff907c99f9ad3aae6c4cdf21122bce2bd68b5283e6907154ad911840fa 208cf' }, { 1:2, 2:'11', -1:1, -2:h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a 09eff', -3:h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbf c117e', -4:h'57c92077664146e876760c9520d054aa93c3afb04e306705db609030850 7b4d3' }, { 1:2, 2:'bilbo.baggins@hobbiton.example', -1:3, -2:h'0072992cb3ac08ecf3e5c63dedec0d51a8c1f79ef2f82f94f3c737bf5de 7986671eac625fe8257bbd0394644caaa3aaf8f27a4585fbbcad0f2457620085e5c8 f42ad', -3:h'01dca6947bce88bc5790485ac97427342bc35f887d86d65a089377e247e 60baa55e4e8501e2ada5724ac51d6909008033ebc10ac999b9d7f5cc2519f3fe1ea1 d9475', Schaad Expires April 21, 2017 [Page 119] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 -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' } ] Acknowledgments 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 draft 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. Schaad Expires April 21, 2017 [Page 120] Internet-Draft CBOR Object Signing and Encryption (COSE) October 2016 Jones, Ilari Liusvaara, Francesca Palombini, Goran Selander, and Ludwig Seitz. Author's Address Jim Schaad August Cellars Email: ietf@augustcellars.com Schaad Expires April 21, 2017 [Page 121]