CBOR Algoritms for Object Signing and Encryption (COSE)
draft-schaad-cose-rfc8152bis-algs-00

Abstract

Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need for the ability to have basic security services defined for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. COSE additionally describes how to represent cryptographic keys using CBOR.

In this specification the conventions for the use of a number of cryptographic algorithms with COSE. The details of the structure of COSE are defined in [I-D.schaad-cose-rfc8152bis-struct].

This document along with [I-D.schaad-cose-rfc8152bis-struct] obsoletes RFC8152.

Status of This Memo

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

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

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

This Internet-Draft will expire on February 23, 2019.

Copyright Notice

Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.

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

Table of Contents

• 1. Introduction
• 1.1. Requirements Terminology
• 1.2. Document Terminology
• 2. Signature Algorithms
• 2.1. ECDSA
• 2.1.1. Security Considerations
• 2.2. Edwards-Curve Digital Signature Algorithms (EdDSAs)
• 2.2.1. Security Considerations
• 3. Message Authentication Code (MAC) Algorithms
• 3.1. Hash-Based Message Authentication Codes (HMACs)
• 3.1.1. Security Considerations
• 3.2. AES Message Authentication Code (AES-CBC-MAC)
• 3.2.1. Security Considerations
• 4. Content Encryption Algorithms
• 4.1. AES GCM
• 4.1.1. Security Considerations
• 4.2. AES CCM
• 4.2.1. Security Considerations
• 4.3. ChaCha20 and Poly1305
• 4.3.1. Security Considerations
• 5. Key Derivation Functions (KDFs)
• 5.1. HMAC-Based Extract-and-Expand Key Derivation Function (HKDF)
• 5.2. Context Information Structure
• 6. Content Key Distribution Methods
• 6.1. Direct Key
• 6.1.1. Security Considerations
• 6.2. Direct Key with KDF
• 6.2.1. Security Considerations
• 6.3. AES Key Wrap
• 6.3.1. Security Considerations for AES-KW
• 6.4. Direct ECDH
• 6.4.1. Security Considerations
• 6.5. ECDH with Key Wrap
• 7. Key Object Parameters
• 7.1. Elliptic Curve Keys
• 7.1.1. Double Coordinate Curves
• 7.2. Octet Key Pair
• 7.3. Symmetric Keys
• 8. IANA Considerations
• 8.1. COSE Algorithms Registry
• 8.2. COSE Key Type Parameters Registry
• 8.3. COSE Key Types Registry
• 8.4. COSE Elliptic Curves Registry
• 8.5. Expert Review Instructions
• 9. Security Considerations
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Appendix A. Examples
• A.1. Examples of Signed Messages
• A.1.1. Single Signature
• A.1.2. Multiple Signers
• A.1.3. Counter Signature
• A.1.4. Signature with Criticality
• A.2. Single Signer Examples
• A.2.1. Single ECDSA Signature
• A.3. Examples of Enveloped Messages
• A.3.1. Direct ECDH
• A.3.2. Direct Plus Key Derivation
• A.3.3. Counter Signature on Encrypted Content
• A.3.4. Encrypted Content with External Data
• A.4. Examples of Encrypted Messages
• A.4.1. Simple Encrypted Message
• A.4.2. Encrypted Message with a Partial IV
• A.5. Examples of MACed Messages
• A.5.1. Shared Secret Direct MAC
• A.5.2. ECDH Direct MAC
• A.5.3. Wrapped MAC
• A.5.4. Multi-Recipient MACed Message
• A.6. Examples of MAC0 Messages
• A.6.1. Shared Secret Direct MAC
• A.7. COSE Keys
• A.7.1. Public Keys
• A.7.2. Private Keys
• Acknowledgments
• Author's Address

1. Introduction

There has been an increased focus on small, constrained devices that make up the Internet of Things (IoT). One of the standards that has come out of this process is "Concise Binary Object Representation (CBOR)" [RFC7049]. CBOR extended the data model of the JavaScript Object Notation (JSON) [RFC7159] by allowing for binary data, among other changes. CBOR is being adopted by several of the IETF working groups dealing with the IoT world as their encoding of data structures. CBOR was designed specifically to be both small in terms of messages transport and implementation size and be a schema-free decoder. A need exists to provide message security services for IoT, and using CBOR as the message-encoding format makes sense.

The core COSE specification consists of two documents. [I-D.schaad-cose-rfc8152bis-struct] contains the serialization structures and the procedures for using the different cryptographic algorithms. This document provides for an initial set of algorithms that are then use with those structures.

1.1. Requirements Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

1.2. Document Terminology

In this document, we use the following terminology:

Byte is a synonym for octet.

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

Authenticated Encryption (AE) [RFC5116] algorithms are those encryption algorithms that provide an authentication check of the plain text contents as part of the encryption service.

Authenticated Encryption with Authenticated Data (AEAD) [RFC5116] algorithms provide the same content authentication service as AE algorithms, but they additionally provide for authentication of non-encrypted data as well.

2. Signature Algorithms

The document defines signature algorithm identifiers for two signature algorithms.

2.1. ECDSA

ECDSA [DSS] defines a signature algorithm using ECC. Implementations SHOULD use a deterministic version of ECDSA such as the one defined in [RFC6979]. The use of a deterministic signature algorithm allows for systems to avoid relying on random number generators in order to avoid generating the same value of 'k' (the per-message random value). Biased generation of the value 'k' can be attacked, and collisions of this value leads to leaked keys. It additionally allows for doing deterministic tests for the signature algorithm. The use of deterministic ECDSA does not lessen the need to have good random number generation when creating the private key.

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

The algorithms defined in this document can be found in Table 1.

This document defines ECDSA to work only with the curves P-256, P-384, and P-521. This document requires that the curves be encoded using the 'EC2' (2 coordinate elliptic curve) key type. Implementations need to check that the key type and curve are correct when creating and verifying a signature. Other documents can define it to work with other curves and points in the future.

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

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

Using the function defined in [RFC8017], the signature is:

Signature = I2OSP(R, n) | I2OSP(S, n)
where n = ceiling(key_length / 8)

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

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

2.1.1. Security Considerations

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

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

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

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

2.2. Edwards-Curve Digital Signature Algorithms (EdDSAs)

[RFC8032] describes the elliptic curve signature scheme Edwards-curve Digital Signature Algorithm (EdDSA). In that document, the signature algorithm is instantiated using parameters for edwards25519 and edwards448 curves. The document additionally describes two variants of the EdDSA algorithm: Pure EdDSA, where no hash function is applied to the content before signing, and HashEdDSA, where a hash function is applied to the content before signing and the result of that hash function is signed. For EdDSA, the content to be signed (either the message or the pre-hash value) is processed twice inside of the signature algorithm. For use with COSE, only the pure EdDSA version is used. This is because it is not expected that extremely large contents are going to be needed and, based on the arrangement of the message structure, the entire message is going to need to be held in memory in order to create or verify a signature. This means that there does not appear to be a need to be able to do block updates of the hash, followed by eliminating the message from memory. Applications can provide the same features by defining the content of the message as a hash value and transporting the COSE object (with the hash value) and the content as separate items.

The algorithms defined in this document can be found in Table 2. A single signature algorithm is defined, which can be used for multiple curves.

[RFC8032] describes the method of encoding the signature value.

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

• The 'kty' field MUST be present, and it MUST be 'OKP' (Octet Key Pair).
• The 'crv' field MUST be present, and it MUST be a curve defined for this signature algorithm.
• If the 'alg' field is present, it MUST match 'EdDSA'.
• If the 'key_ops' field is present, it MUST include 'sign' when creating an EdDSA signature.
• If the 'key_ops' field is present, it MUST include 'verify' when verifying an EdDSA signature.

2.2.1. Security Considerations

How public values are computed is not the same when looking at EdDSA and Elliptic Curve Diffie-Hellman (ECDH); for this reason, they should not be used with the other algorithm.

If batch signature verification is performed, a well-seeded cryptographic random number generator is REQUIRED. Signing and non-batch signature verification are deterministic operations and do not need random numbers of any kind.

3. Message Authentication Code (MAC) Algorithms

This section defines the usages for two MAC algorithms.

3.1. Hash-Based Message Authentication Codes (HMACs)

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

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

The algorithms defined in this document can be found in Table 3.

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

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

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

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

3.1.1. Security Considerations

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

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

AES-CBC-MAC is defined in [MAC]. (Note that this is not the same algorithm as AES Cipher-Based Message Authentication Code (AES-CMAC) [RFC4493].)

AES-CBC-MAC is parameterized by the key length, the authentication tag length, and the IV used. For all of these algorithms, the IV is fixed to all zeros. We provide an array of algorithms for various key lengths and tag lengths. The algorithms defined in this document are found in Table 4.

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

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

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

3.2.1. Security Considerations

A number of attacks exist against Cipher Block Chaining Message Authentication Code (CBC-MAC) that need to be considered.

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

4. Content Encryption Algorithms

This docuement defines the identifier and usages for three content encryption algorithms.

4.1. AES GCM

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

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

The set of algorithms defined in this document are in Table 5.

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

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

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

4.1.1. Security Considerations

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

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

Consideration was given to supporting smaller tag values; the constrained community would desire tag sizes in the 64-bit range. Doing 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 Counter with CBC-MAC (CCM) is the usual mode for constrained environments, restricted modes are not supported.

4.2. AES CCM

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 probability 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):

This 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):

This limits messages to 2^64 bytes in length. The nonce length is 7 bytes allowing for 2^56 possible values of the nonce without repeating.

The following values are used for M:

64 bits (8):

This 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):

This produces a 128-bit authentication tag. This implies that there is a 1 in 2^128 chance that a modified message will authenticate.

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

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

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

4.2.1. Security Considerations

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

• The key and nonce pair MUST be unique for every message encrypted. Note that the value of L influences the number of unique nonces.
• 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.

4.3. 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 as the plaintext and additional data as inputs and produces the ciphertext as an option. We define one algorithm identifier for this algorithm in Table 7.

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

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

• The 'kty' field MUST be present, and it MUST be 'Symmetric'.
• If the 'alg' field is present, it MUST match the ChaCha20/Poly1305 algorithm being used.
• If the 'key_ops' field is present, it MUST include 'encrypt' or 'wrap key' when encrypting.
• If the 'key_ops' field is present, it MUST include 'decrypt' or 'unwrap key' when decrypting.

4.3.1. Security Considerations

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

5. Key Derivation Functions (KDFs)

This document defines a single context structure and a single KDF. These elements are used for all of the recipient algorithms defined in this document that require a KDF process. These algorithms are defined in Sections 6.2, 6.4, and 6.5.

5.1. 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 Diffie-Hellman (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 9 is used. While [RFC5869] suggests that the length of the salt be the same as the length of the underlying hash value, any amount of salt will improve the security as different key values will be generated. This parameter is protected by being included in the key computation and does not need to be separately authenticated. The salt value does not need to be unique for every message sent.
• length -- the number of bytes of output that need to be generated.
• context information -- Information that describes the context in which the resulting value will be used. Making this information specific to the context 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 5.2 is used by the KDFs in this document.
• PRF -- The underlying pseudorandom 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 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 8.

5.2. 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:

• Fields can be defined by the application. This is commonly used to assign fixed names to parties, but it can be used for other items such as nonces.
• Fields can be defined by usage of the output. Examples of this are the algorithm and key size that are being generated.
• Fields can be defined by parameters from the message. We define a set of parameters in Table 10 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.

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 Algorithms" 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. 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. First, 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 be carried as a value in the unprotected headers.

We define an algorithm parameter 'PartyU nonce' that can be used to carry this value in the message; however, the nonce value could be determined by the application and the value determined from elsewhere.

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

The following CDDL fragment corresponds to the text above.

PartyInfo = (
    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
]

6. Content Key Distribution Methods

This document defines the identifiers and usage for a number of content key distribution methods.

6.1. 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 11.

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

6.1.1. Security Considerations

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

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

6.2. Direct Key with KDF

These recipient algorithms take a common shared secret between the two parties and applies the HKDF function (Section 5.1), using the context structure defined in Section 5.2 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 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 12.

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

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

6.2.1. 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 that requires the same amount of work to get the key.

6.3. AES Key Wrap

The AES Key Wrap 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:

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

6.3.1. 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.

6.4. Direct ECDH

The mathematics for ECDH 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:

• 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 18.
  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.
• 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. 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 [RFC8017], using the same computation for n as is defined in Section 2.1.
• 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 create a unique value. For the KDFs used, this means either the 'salt' parameter for HKDF (Table 9) or the 'PartyU nonce' parameter for the context structure (Table 10) MUST be present (both can 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 15.
• 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 5.1 with the context structure defined in Section 5.2.
• Key Wrap Algorithm: No key wrap algorithm is used. This is represented in Table 14 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 14.

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:

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

6.4.1. Security Considerations

There is a 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 online 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).

6.5. ECDH with Key Wrap

These algorithms are defined in Table 16.

ECDH with Key Agreement is parameterized by the same parameters as for ECDH; see Section 6.4, with the following modifications:

• Key Wrap Algorithm: Any of the key wrap algorithms defined in Section 6.3 are supported. The size of the key used for the key wrap algorithm is fed into the KDF. The set of identifiers are found in Table 16.

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

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

7. 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:

7.1. Elliptic Curve Keys

Two different key structures are defined for elliptic curve keys. One version uses both an x-coordinate 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.

7.1.1. Double Coordinate Curves

The traditional way of sending ECs has been to send either both the x-coordinate and y-coordinate 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 19. The members that are defined for this key type are:

crv:

This 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 18. Other curves may be registered in the future, and private curves can be used as well.

x:

This 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:

This contains either the sign bit or the value of the 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:

This 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.

7.2. 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).

The key parameters defined in this section are summarized in Table 20. The members that are defined for this key type are:

crv:

This 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 18. Other curves may be registered in the future and private curves can be used as well.

x:

This contains the x-coordinate for the EC point. The octet string represents a little-endian encoding of x.

d:

This 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.

7.3. 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 4 ('Symmetric'). The member that is defined for this key type is:

k:

This 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 taken that it is never transmitted accidentally or insecurely. For symmetric keys, it is REQUIRED that 'k' be present in the structure.

8. IANA Considerations

8.1. COSE Algorithms Registry

IANA has created a new registry titled "COSE Algorithms". The registry has been created to use the "Expert Review Required" registration procedure. Guidelines for the experts are provided in Section 8.5. It should be noted that, in addition to the expert review, some portions of the registry require a specification, potentially a Standards Track RFC, be supplied as well.

The columns of the registry are:

Name:

A value that can be used to identify an algorithm in documents for easier comprehension. The name SHOULD be unique. However, the 'Value' field is what is used to identify the algorithm, not the 'name' field.

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 Action". Integer values from -65536 to 65535 and strings of length 2 are designated as "Specification Required". Integer values 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.

Reference:

A document where the algorithm is defined (if publicly available).

Recommended:

Does the IETF have a consensus recommendation to use the algorithm? The legal values are 'Yes', 'No', and 'Deprecated'.

The initial contents of the registry can be found in Tables 1, 2, 3, 4, 5, 6, 7, 11, 12, 13, 14, and 16. All of the entries in the "References" column of this registry point to this document. All of the entries in the "Recommended" column 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.

8.2. COSE Key Type Parameters Registry

IANA has created a new registry titled "COSE Key Type Parameters". The registry has been created to use the "Expert Review Required" registration procedure. Expert review guidelines are provided in Section 8.5.

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" registry and is placed in the 'kty' field of a COSE Key structure.

Name:

This is a descriptive name that enables easier reference to the item. It is not used in the encoding.

Label:

The label is to be unique for every value of key type. The range of values is from -65536 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.

Reference:

This contains a pointer to the public specification for the field if one exists.

This registry has been initially populated by the values in Tables 19, 20, and 21. All of the entries in the "References" column of this registry point to this document.

8.3. COSE Key Types Registry

IANA has created a new registry titled "COSE Key Types". The registry has been created to use the "Expert Review Required" registration procedure. Expert review guidelines are provided in Section 8.5.

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.

References:

This contains a pointer to the public specification for the curve if one exists.

This registry has been initially populated by the values in Table 17. The specification column for all of these entries will be this document.

8.4. COSE Elliptic Curves Registry

IANA has created a new registry titled "COSE Elliptic Curves". The registry has been created to use the "Expert Review Required" registration procedure. Guidelines for the experts are provided in Section 8.5. It should be noted that, in addition to the expert review, some portions of the registry require a specification, potentially a Standards Track RFC, be supplied as well.

The columns of the table are:

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 Action". 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.

Reference:

This contains a pointer to the public specification for the curve if one exists.

Recommended:

Does the IETF have a consensus recommendation to use the algorithm? The legal values are 'Yes', 'No', and 'Deprecated'.

This registry has been initially populated by the values in Table 18. All of the entries in the "References" column of this registry point to this document. All of the entries in the "Recommended" column are set to "Yes".

8.5. Expert Review Instructions

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

Expert reviewers should take into consideration the following points:

• Point squatting should be discouraged. Reviewers are encouraged to get sufficient information for registration requests to ensure that the usage is not going to duplicate one that is already registered, and that the point is likely to be used in deployments. The zones tagged as private use are intended for testing purposes and closed environments; code points in other ranges should not be assigned for testing.
• Specifications are required for the standards track range of point assignment. Specifications should exist for specification required ranges, but early assignment before a specification is available is considered to be permissible. Specifications are needed for the first-come, first-serve range if they are expected to be used outside of closed environments in an interoperable way. When specifications are not provided, the description provided needs to have sufficient information to identify what the point is being used for.
• Experts should take into account the expected usage of fields when approving point assignment. The fact that there is a range for standards track documents does not mean that a standards track document cannot have points assigned outside of that range. 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.
• 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 requesting 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.

9. 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.

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

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

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

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

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

There are a large number of algorithms presented in this document that use nonce values. For all of the nonces defined in this document, there is some type of restriction on the nonce being a unique value either for a key or for some other conditions. In all of these cases, there is no known requirement on the nonce being both unique and unpredictable; under these circumstances, it's reasonable to use a counter for creation of the nonce. In cases where one wants the pattern of the nonce to be unpredictable as well as unique, one can use a key created for that purpose and encrypt the counter to produce the nonce value.

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

10. References

10.1. Normative References

10.2. Informative References

Appendix A. 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 [CDDL]) rather than as a binary dump.

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

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

gem install cbor-diag

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

diag2cbor.rb < inputfile > outputfile

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

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

A.1. Examples of Signed Messages

A.1.1. Single Signature

This example uses the following:

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

Size of binary file is 103 bytes

98(
  [
    / protected / h'', 
    / unprotected / {}, 
    / payload / 'This is the content.', 
    / signatures / [
      [
        / protected / h'a10126' / {
            \ alg \ 1:-7 \ ECDSA 256 \
          } / , 
        / unprotected / {
          / kid / 4:'11'
        }, 
        / signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
98f53afd2fa0f30a'
      ]
    ]
  ]
)

A.1.2. Multiple Signers

This example uses the following:

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

Size of binary file is 277 bytes

98(
  [
    / protected / h'', 
    / unprotected / {}, 
    / payload / 'This is the content.', 
    / signatures / [
      [
        / protected / h'a10126' / {
            \ alg \ 1:-7 \ ECDSA 256 \
          } / , 
        / unprotected / {
          / kid / 4:'11'
        }, 
        / signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
98f53afd2fa0f30a'
      ], 
      [
        / protected / h'a1013823' / {
            \ alg \ 1:-36
          } / , 
        / unprotected / {
          / kid / 4:'bilbo.baggins@hobbiton.example'
        }, 
        / signature / h'00a2d28a7c2bdb1587877420f65adf7d0b9a06635dd1
de64bb62974c863f0b160dd2163734034e6ac003b01e8705524c5c4ca479a952f024
7ee8cb0b4fb7397ba08d009e0c8bf482270cc5771aa143966e5a469a09f613488030
c5b07ec6d722e3835adb5b2d8c44e95ffb13877dd2582866883535de3bb03d01753f
83ab87bb4f7a0297'
      ]
    ]
  ]
)

A.1.3. Counter Signature

This example uses the following:

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

Size of binary file is 180 bytes

98(
  [
    / protected / h'', 
    / unprotected / {
      / countersign / 7:[
        / protected / h'a10126' / {
            \ alg \ 1:-7 \ ECDSA 256 \
          } / , 
        / unprotected / {
          / kid / 4:'11'
        }, 
        / signature / h'5ac05e289d5d0e1b0a7f048a5d2b643813ded50bc9e4
9220f4f7278f85f19d4a77d655c9d3b51e805a74b099e1e085aacd97fc29d72f887e
8802bb6650cceb2c'
      ]
    }, 
    / payload / 'This is the content.', 
    / signatures / [
      [
        / protected / h'a10126' / {
            \ alg \ 1:-7 \ ECDSA 256 \
          } / , 
        / unprotected / {
          / kid / 4:'11'
        }, 
        / signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
98f53afd2fa0f30a'
      ]
    ]
  ]
)

A.1.4. Signature with Criticality

This example uses the following:

• Signature Algorithm: ECDSA w/ SHA-256, Curve P-256
• There is a criticality marker on the "reserved" header parameter

Size of binary file is 125 bytes

98(
  [
    / protected / h'a2687265736572766564f40281687265736572766564' / 
{
        "reserved":false, 
        \ crit \ 2:[
          "reserved"
        ]
      } / , 
    / unprotected / {}, 
    / payload / 'This is the content.', 
    / signatures / [
      [
        / protected / h'a10126' / {
            \ alg \ 1:-7 \ ECDSA 256 \
          } / , 
        / unprotected / {
          / kid / 4:'11'
        }, 
        / signature / h'3fc54702aa56e1b2cb20284294c9106a63f91bac658d
69351210a031d8fc7c5ff3e4be39445b1a3e83e1510d1aca2f2e8a7c081c7645042b
18aba9d1fad1bd9c'
      ]
    ]
  ]
)

A.2. Single Signer Examples

A.2.1. Single ECDSA Signature

This example uses the following:

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

Size of binary file is 98 bytes

18(
  [
    / protected / h'a10126' / {
        \ alg \ 1:-7 \ ECDSA 256 \
      } / , 
    / unprotected / {
      / kid / 4:'11'
    }, 
    / payload / 'This is the content.', 
    / signature / h'8eb33e4ca31d1c465ab05aac34cc6b23d58fef5c083106c4
d25a91aef0b0117e2af9a291aa32e14ab834dc56ed2a223444547e01f11d3b0916e5
a4c345cacb36'
  ]
)

A.3. Examples of Enveloped Messages

A.3.1. Direct ECDH

This example uses the following:

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

Size of binary file is 151 bytes

96(
  [
    / protected / h'a10101' / {
        \ alg \ 1:1 \ AES-GCM 128 \
      } / , 
    / unprotected / {
      / iv / 5:h'c9cf4df2fe6c632bf7886413'
    }, 
    / ciphertext / h'7adbe2709ca818fb415f1e5df66f4e1a51053ba6d65a1a0
c52a357da7a644b8070a151b0', 
    / recipients / [
      [
        / protected / h'a1013818' / {
            \ alg \ 1:-25 \ ECDH-ES + HKDF-256 \
          } / , 
        / unprotected / {
          / ephemeral / -1:{
            / kty / 1:2, 
            / crv / -1:1, 
            / x / -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbf
bf054e1c7b4d91d6280', 
            / y / -3:true
          }, 
          / kid / 4:'meriadoc.brandybuck@buckland.example'
        }, 
        / ciphertext / h''
      ]
    ]
  ]
)

A.3.2. Direct Plus Key Derivation

This example uses the following:

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

Size of binary file is 91 bytes

96(
  [
    / protected / h'a1010a' / {
        \ alg \ 1:10 \ AES-CCM-16-64-128 \
      } / , 
    / unprotected / {
      / iv / 5:h'89f52f65a1c580933b5261a76c'
    }, 
    / ciphertext / h'753548a19b1307084ca7b2056924ed95f2e3b17006dfe93
1b687b847', 
    / recipients / [
      [
        / protected / h'a10129' / {
            \ alg \ 1:-10
          } / , 
        / unprotected / {
          / salt / -20:'aabbccddeeffgghh', 
          / kid / 4:'our-secret'
        }, 
        / ciphertext / h''
      ]
    ]
  ]
)

A.3.3. Counter Signature on Encrypted Content

This example uses the following:

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

Size of binary file is 326 bytes

96(
  [
    / protected / h'a10101' / {
        \ alg \ 1:1 \ AES-GCM 128 \
      } / , 
    / unprotected / {
      / iv / 5:h'c9cf4df2fe6c632bf7886413', 
      / countersign / 7:[
        / protected / h'a1013823' / {
            \ alg \ 1:-36
          } / , 
        / unprotected / {
          / kid / 4:'bilbo.baggins@hobbiton.example'
        }, 
        / signature / h'00929663c8789bb28177ae28467e66377da12302d7f9
594d2999afa5dfa531294f8896f2b6cdf1740014f4c7f1a358e3a6cf57f4ed6fb02f
cf8f7aa989f5dfd07f0700a3a7d8f3c604ba70fa9411bd10c2591b483e1d2c31de00
3183e434d8fba18f17a4c7e3dfa003ac1cf3d30d44d2533c4989d3ac38c38b71481c
c3430c9d65e7ddff'
      ]
    }, 
    / ciphertext / h'7adbe2709ca818fb415f1e5df66f4e1a51053ba6d65a1a0
c52a357da7a644b8070a151b0', 
    / recipients / [
      [
        / protected / h'a1013818' / {
            \ alg \ 1:-25 \ ECDH-ES + HKDF-256 \
          } / , 
        / unprotected / {
          / ephemeral / -1:{
            / kty / 1:2, 
            / crv / -1:1, 
            / x / -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbf
bf054e1c7b4d91d6280', 
            / y / -3:true
          }, 
          / kid / 4:'meriadoc.brandybuck@buckland.example'
        }, 
        / ciphertext / h''
      ]
    ]
  ]
)

A.3.4. Encrypted Content with External Data

This example uses the following:

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

Size of binary file is 173 bytes

96(
  [
    / protected / h'a10101' / {
        \ alg \ 1:1 \ AES-GCM 128 \
      } / , 
    / unprotected / {
      / iv / 5:h'02d1f7e6f26c43d4868d87ce'
    }, 
    / ciphertext / h'64f84d913ba60a76070a9a48f26e97e863e28529d8f5335
e5f0165eee976b4a5f6c6f09d', 
    / recipients / [
      [
        / protected / h'a101381f' / {
            \ alg \ 1:-32 \ ECHD-SS+A128KW \
          } / , 
        / unprotected / {
          / static kid / -3:'peregrin.took@tuckborough.example', 
          / kid / 4:'meriadoc.brandybuck@buckland.example', 
          / U nonce / -22:h'0101'
        }, 
        / ciphertext / h'41e0d76f579dbd0d936a662d54d8582037de2e366fd
e1c62'
      ]
    ]
  ]
)

A.4. Examples of Encrypted Messages

A.4.1. Simple Encrypted Message

This example uses the following:

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

Size of binary file is 52 bytes

16(
  [
    / protected / h'a1010a' / {
        \ alg \ 1:10 \ AES-CCM-16-64-128 \
      } / , 
    / unprotected / {
      / iv / 5:h'89f52f65a1c580933b5261a78c'
    }, 
    / ciphertext / h'5974e1b99a3a4cc09a659aa2e9e7fff161d38ce71cb45ce
460ffb569'
  ]
)

A.4.2. Encrypted Message with a Partial IV

This example uses the following:

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

Size of binary file is 41 bytes

16(
  [
    / protected / h'a1010a' / {
        \ alg \ 1:10 \ AES-CCM-16-64-128 \
      } / , 
    / unprotected / {
      / partial iv / 6:h'61a7'
    }, 
    / ciphertext / h'252a8911d465c125b6764739700f0141ed09192de139e05
3bd09abca'
  ]
)

A.5. Examples of MACed Messages

A.5.1. Shared Secret Direct MAC

This example uses the following:

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

Size of binary file is 57 bytes

97(
  [
    / protected / h'a1010f' / {
        \ alg \ 1:15 \ AES-CBC-MAC-256//64 \
      } / , 
    / unprotected / {}, 
    / payload / 'This is the content.', 
    / tag / h'9e1226ba1f81b848', 
    / recipients / [
      [
        / protected / h'', 
        / unprotected / {
          / alg / 1:-6 / direct /, 
          / kid / 4:'our-secret'
        }, 
        / ciphertext / h''
      ]
    ]
  ]
)

A.5.2. ECDH Direct MAC

This example uses the following:

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

Size of binary file is 214 bytes

97(
  [
    / protected / h'a10105' / {
        \ alg \ 1:5 \ HMAC 256//256 \
      } / , 
    / unprotected / {}, 
    / payload / 'This is the content.', 
    / tag / h'81a03448acd3d305376eaa11fb3fe416a955be2cbe7ec96f012c99
4bc3f16a41', 
    / recipients / [
      [
        / protected / h'a101381a' / {
            \ alg \ 1:-27 \ ECDH-SS + HKDF-256 \
          } / , 
        / unprotected / {
          / static kid / -3:'peregrin.took@tuckborough.example', 
          / kid / 4:'meriadoc.brandybuck@buckland.example', 
          / U nonce / -22:h'4d8553e7e74f3c6a3a9dd3ef286a8195cbf8a23d
19558ccfec7d34b824f42d92bd06bd2c7f0271f0214e141fb779ae2856abf585a583
68b017e7f2a9e5ce4db5'
        }, 
        / ciphertext / h''
      ]
    ]
  ]
)

A.5.3. Wrapped MAC

This example uses the following:

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

Size of binary file is 109 bytes

97(
  [
    / protected / h'a1010e' / {
        \ alg \ 1:14 \ AES-CBC-MAC-128//64 \
      } / , 
    / unprotected / {}, 
    / payload / 'This is the content.', 
    / tag / h'36f5afaf0bab5d43', 
    / recipients / [
      [
        / protected / h'', 
        / unprotected / {
          / alg / 1:-5 / A256KW /, 
          / kid / 4:'018c0ae5-4d9b-471b-bfd6-eef314bc7037'
        }, 
        / ciphertext / h'711ab0dc2fc4585dce27effa6781c8093eba906f227
b6eb0'
      ]
    ]
  ]
)

A.5.4. Multi-Recipient MACed Message

This example uses the following:

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

Size of binary file is 309 bytes

97(
  [
    / protected / h'a10105' / {
        \ alg \ 1:5 \ HMAC 256//256 \
      } / , 
    / unprotected / {}, 
    / payload / 'This is the content.', 
    / tag / h'bf48235e809b5c42e995f2b7d5fa13620e7ed834e337f6aa43df16
1e49e9323e', 
    / recipients / [
      [
        / protected / h'a101381c' / {
            \ alg \ 1:-29 \ ECHD-ES+A128KW \
          } / , 
        / unprotected / {
          / ephemeral / -1:{
            / kty / 1:2, 
            / crv / -1:3, 
            / x / -2:h'0043b12669acac3fd27898ffba0bcd2e6c366d53bc4db
71f909a759304acfb5e18cdc7ba0b13ff8c7636271a6924b1ac63c02688075b55ef2
d613574e7dc242f79c3', 
            / y / -3:true
          }, 
          / kid / 4:'bilbo.baggins@hobbiton.example'
        }, 
        / ciphertext / h'339bc4f79984cdc6b3e6ce5f315a4c7d2b0ac466fce
a69e8c07dfbca5bb1f661bc5f8e0df9e3eff5'
      ], 
      [
        / protected / h'', 
        / unprotected / {
          / alg / 1:-5 / A256KW /, 
          / kid / 4:'018c0ae5-4d9b-471b-bfd6-eef314bc7037'
        }, 
        / ciphertext / h'0b2c7cfce04e98276342d6476a7723c090dfdd15f9a
518e7736549e998370695e6d6a83b4ae507bb'
      ]
    ]
  ]
)

A.6. Examples of MAC0 Messages

A.6.1. Shared Secret Direct MAC

This example uses the following:

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

Size of binary file is 37 bytes

17(
  [
    / protected / h'a1010f' / {
        \ alg \ 1:15 \ AES-CBC-MAC-256//64 \
      } / , 
    / unprotected / {}, 
    / payload / 'This is the content.', 
    / tag / h'726043745027214f'
  ]
)

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

A.7. COSE Keys

A.7.1. Public Keys

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

In order the keys are:

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

Size of binary file is 481 bytes

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

A.7.2. Private Keys

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

In order the keys are:

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

Size of binary file is 816 bytes

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

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 specification was based to some degree on the outputs of the JOSE and S​/​MIME working groups.

The following individuals provided input into the final form of the document: Carsten Bormann, John Bradley, Brain Campbell, Michael B. Jones, Ilari Liusvaara, Francesca Palombini, Ludwig Seitz, and Goran Selander.

Author's Address