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

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

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

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

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

1. Introduction

In the process of writing RFCXXXX [I-D.ietf-cose-msg] several items were removed from that ocument to be addressed at a later date. This document was created to address those items.

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 [RFC2119].

1.2. CBOR Grammar

There currently is no standard CBOR grammar available for use by specifications. We therefore describe the CBOR structures in prose. There is a version of a CBOR grammar in the CBOR Data Definition Language (CDDL) [I-D.greevenbosch-appsawg-cbor-cddl]. An informational version of the CBOR grammar that reflects what is in the prose can be found in Appendix A. CDDL has not been fixed, so this grammar may will only work with the version of CDDL at the time of publishing.

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

NOTE: For the purposes of review, we are currently interlacing the CDLL grammar into the text of document. This is being done for simplicity of comparision of the grammar againist the prose. The grammar will be removed to an appendix during WGLC.

1.3. Document Terminology

2. Signature Algorithms

There are two basic signature algorithm structures that can be used. The first is the common signature with appendix. In this structure, the message content is processed and a signature is produced, the signature is called the appendix. This is the message structure used by our common algorithms such as ECDSA and RSASSA-PSS. (In fact the SSA in RSASSA-PSS stands for Signature Scheme with Appendix.) The basic structure becomes:

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

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

2.1. RSASSA-PSS

The RSASSA-PSS signature algorithm is parametized with a hash function (h), a mask generation function (mgf) and a salt length (sLen). For this specification, the mask generation function is fixed to be MGF1 as defined in [RFC3447]. It has been recommended that the same hash function be used for hashing the data as well as in the mask generation function, for this specification we following this recommendation. The salt length is the same length as the hash function output.

Implementations need to check that the key type is 'RSA' when creating or verifying a signature.

RSASSA-PSS Algorithm Values
name	value	hash	salt length	description
PS256	-26	SHA-256	32	RSASSA-PSS w/ SHA-256
PS384	-27	SHA-384	48	RSASSA-PSS w/ SHA-384
PS512	-28	SHA-512	64	RSASSA-PSS w/ SHA-512

2.1.1. Security Considerations

In addition to needing to worry about keys that are too small to provide the required security, there are issues with keys that are too large. Denial of service attacks have been mounted with overly large keys. This has the potential to consume resources with potentially bad keys. There are two reasonable ways to address this attack. First, a key should not be used for a cryptographic operation until it has been matched back to an authorized user. This approach means that no cryptography would be done except for authorized users. Second, applications can impose maximum as well as minimum length requirements on keys. This limits the resources consumed even if the matching is not performed until the cryptography has been done.

There is a theoretical hash substitution attack that can be mounted against RSASSA-PSS. However, the requirement that the same hash function be used consistently for all operations is an effective mitigation against it. Unlike ECDSA, hash functions are not truncated so that the full hash value is always signed. The internal padding structure of RSASSA-PSS means that one needs to have multiple collisions between the two hash functions in order to be successful in producing a forgery based on changing the hash function. This is highly unlikely.

2.2. Edwards-curve Digital Signature Algorithms (EdDSA)

Reference for these is [I-D.irtf-cfrg-eddsa].

The algorithms defined in this document can be found in Table 2.

EdDSA Algorithm Values
name	value	description
Ed25519	*	EdDSA for Curve 25591
Ed488	*	EdDSA for Curve 448

3. Message Authentication (MAC) Algorithms

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

MACs are designed in the same basic structure as signature with appendix algorithms. The message content is processed and an authentication code is produced, the authentication code is frequently called a tag. The basic structure becomes:

        
tag = MAC_Create(message content, key)

valid = MAC_Verify(message content, key, tag)

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

4. Content Encryption Algorithms

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

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

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

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

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

4.1. ChaCha20 and Poly1305

ChaCha20 and Poly1305 combined together is a new AEAD mode that is defined in [RFC7539]. This is a new algorithm defined to be a cipher which 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 an a 96-bit nonce as well as the plain text and additional data as inputs and produces the cipher text as an option. We define one algorithm identifier for this algorithm in Table 3.

Algorithm Value for AES-GCM
name	value	description
ChaCha20/Poly1305	11	ChaCha20/Poly1305 w/ 256-bit key

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

4.1.1. 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.

5. Key Derivation Functions (KDF)

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

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

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

Many functions are going to handle the first two type of secrets differently. The KDF function defined in [HKDF] can use different underlying constructions if the secret is uniformly random than if the secret is not uniformly random. This is reflected in the set of algorithms defined for HKDF.

When using KDF functions, one component that is generally 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.

6. Recipient Algorithm Classes

Recipient algorithms can be defined into a number of different classes. COSE has the ability to support many classes of recipient algorithms. In this section, a number of classes are listed and then a set of algorithms are specified for each of the classes. The names of the recipient algorithm classes used here are the same as are defined in [RFC7517]. Other specifications use different terms for the recipient algorithm classes or do not support some of our recipient algorithm classes.

6.1. Direct Encryption

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

The COSE_encrypt structure for the recipient is organized as follows:

The 'protected' field MUST be a zero length item if it is not used in the computation of the content key.
The 'alg' parameter MUST be present.
A parameter identifying the shared secret SHOULD be present.
The 'ciphertext' field MUST be a zero length item.
The 'recipients' field MUST be absent.

6.1.1. Direct Key with KDF

These recipient algorithms take a common shared secret between the two parties and applies the HKDF function using the context structure defined in [CONTEXT] to transform the shared secret into the necessary key. Either the 'salt' parameter of HKDF or the partyU 'nonce' parameter of the context structure MUST be present. This parameter can be generated either randomly or deterministically, the requirement is that it be a unique value for the key pair in question.

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

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

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

The set of algorithms defined in this document can be found in Table 4.

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

6.1.1.1. Security Considerations

The shared secret need to have some method to be regularly updated over time. The shared secret is forming the basis of trust, although not used directly it should still be subject to scheduled rotation.

6.2. Key Wrapping

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

The COSE_encrypt structure for the recipient is organized as follows:

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

6.3. Key Encryption

Key Encryption mode is also called key transport mode in some standards. Key Encryption mode differs from Key Wrap mode in that it uses an asymmetric encryption algorithm rather than a symmetric encryption algorithm to protect the key. This document defines one Key Encryption mode algorithm.

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

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

6.3.1. RSAES-OAEP

RSAES-OAEP is an asymmetric key encryption algorithm. The defintion of RSAEA-OAEP can be find in Section 7.1 of [RFC3447]. The algorithm is parameterized using a masking generation function (mgf), a hash function (h) and encoding parameters (P). For the algorithm identifiers defined in this section: Table 5 summarizes the rest of the values.

mgf is always set to MFG1 from [RFC3447] and uses the same hash function as h.
P is always set to the empty octet string.

RSAES-OAEP Algorithm Values
name	value	hash	description
RSAES-OAEP w/SHA-256	-25	SHA-256	RSAES OAEP w/ SHA-256
RSAES-OAEP w/SHA-512	-26	SHA-512	RSAES OAEP w/ SHA-512

The key type MUST be 'RSA'.

6.3.1.1. Security Considerations for RSAES-OAEP

A key size of 2048 bits or larger MUST be used with these algorithms. This key size corresponds roughly to the same strength as provided by a 128-bit symmetric encryption algorithm.

It is highly recommended that checks on the key length be done before starting a decryption operation. One potential denial of service operation is to provide encrypted objects using either abnormally long or oddly sized RSA modulus values. Implementations SHOULD be able to encrypt and decrypt with modulus between 2048 and 16K bits in length. Applications can impose additional restrictions on the length of the modulus.

6.4. 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 [THREE-LAYER] ).

The most commonly used key agreement algorithm used 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 key material. One side-effect of this is that perfect forward security is not achievable, a static key will always be used for the receiver of the COSE message.

Two variants of DH that are easily supported are:

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

In this specification, both variants are specified. This has been done to provide the weak data origination option for use with MAC operations.

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:

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

6.4.1. ECDH

The basic mathematics for Elliptic Curve Diffie-Hellman can be found in [RFC6090]. Two new curves have been defined in [I-D.irtf-cfrg-curves].

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. The curves are defined in Table 9.
Since the only the math is changed by changing the curve, the curve is not fixed for any of the algorithm identifiers we define, instead it is defined by the points used.
Ephemeral-static or static-static: The key agreement process may be done using either a static or an ephemeral key at the senders side. When using ephemeral keys, the sender MUST generate a new ephemeral key for every key agreement operation. The ephemeral key is placed in in the 'ephemeral key' parameter and MUST be present for all algorithm identifiers which use ephemeral keys. When using static keys, the sender MUST either generate a new random value placed in either in the KDF parameters or the context structure. For the KDF functions used, this means either in the 'salt' parameter for HKDF [HKDF_Alg_Params]) or in in the 'PartyU nonce' parameter for the context struture ([KDF_Context_Alg_Params]) MUST be present. (Both may be present if desired.) The value in the parameter MUST be unique for the key pair being used. It is acceptable to use a global counter which is incremented for every static-static operation and use the resulting value. When using static keys, the static key needs to 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 7
Key derivation algorithm: The result of an ECDH key agreement process does not provide a uniformly random secret, as such it needs to be run through a KDF in order to produce a usable key. Processing the secret through a KDF also allows for the introduction of both context material, how the key is going to be used, and one time material in the even to of a static-static key agreement.
Key Wrap algorithm: The key wrap algorithm can be 'none' if the result of the KDF is going to be used as the key directly. This option, along with static-static, should be used if knowledge about the sender is desired. If 'none' is used then the content layer encryption algorithm size is value fed to the context structure. Support is also provided for any of the key wrap algorithms defined in section [KEY_WRAP_ALGS]. If one of these options is used, the input key size to the key wrap algorithm is the value fed into the context structure as the key size.

The set of algorithms direct ECDH defined in this document are found in Table 6.

ECDH Algorithm Values
name	value	KDF	Ephemeral-Static	Key Wrap	description
ECDH-ES + HKDF-256	50	HKDF - SHA-256	yes	none	ECDH ES w/ HKDF - generate key directly
ECDH-ES + HKDF-512	51	HKDF - SHA-256	yes	none	ECDH ES w/ HKDF - generate key directly
ECDH-SS + HKDF-256	52	HKDF - SHA-256	no	none	ECDH ES w/ HKDF - generate key directly
ECDH-SS + HKDF-512	53	HKDF - SHA-256	no	none	ECDH ES w/ HKDF - generate key directly
ECDH-ES+A128KW	54	HKDF - SHA-256	yes	A128KW	ECDH ES w/ Concat KDF and AES Key wrap w/ 128 bit key
ECDH-ES+A192KW	55	HKDF - SHA-256	yes	A192KW	ECDH ES w/ Concat KDF and AES Key wrap w/ 192 bit key
ECDH-ES+A256KW	56	HKDF - SHA-256	yes	A256KW	ECDH ES w/ Concat KDF and AES Key wrap w/ 256 bit key
ECDH-SS+A128KW	57	HKDF - SHA-256	no	A128KW	ECDH SS w/ Concat KDF and AES Key wrap w/ 128 bit key
ECDH-SS+A192KW	58	HKDF - SHA-256	no	A192KW	ECDH SS w/ Concat KDF and AES Key wrap w/ 192 bit key
ECDH-SS+A256KW	59	HKDF - SHA-256	no	A256KW	ECDH SS w/ Concat KDF and AES Key wrap w/ 256 bit key

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

This document defines these algorithms to be used with the curves P-256, P-384, P-521, 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.

6.5. Key Agreement with KDF

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

The COSE_encrypt structure for the recipient is organized as follows:

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

6.5.1. ECDH

These algorithms are defined in Table 6.

6.6. Password

[CREF2]JLS: Do we want/need to support this? JOSE did it mainly to support the encryption of private keys.

6.6.1. PBES2

name	value	description
PBES2-HS256+A128KW	*	PBES2 w/ HMAC SHA-256 and AES Key wrap w/ 128 bit key
PBES2-HS384+A192KW	*	PBES2 w/ HMAC SHA-384 and AES Key wrap w/ 192 bit key
PBES2-HS512+A256KW	*	PBES2 w/ HMAC SHA-512 and AES Key wrap w/ 256 bit key

7. Keys

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

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

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

Key Type Values
name	value	description
EC1	1	Elliptic Curve Keys w/ X Coordinate only
RSA	3	RSA Keys

7.1. Elliptic Curve Keys

Two different key structures are being defined for Elliptic Curve keys. One version uses both an x and a y coordinate, potentially with point compression. This is the traditional EC point representation that is used in [RFC5480]. The other version uses only the x coordinate as the y coordinate is either to be recomputed or not needed for the key agreement operation. An example of this is Curve25519 [I-D.irtf-cfrg-curves]. [CREF3]Ilari: Check to see what the curves are renamed to during final publishing. It appears to be X25519 now.

EC Curves
name	key type	value	description
Curve25519	EC1	1	Curve 25519
Curve448	EC1	2	Curve 448

7.1.1. Single Coordinate Curves

One class of Elliptic Curve mathematics allows for a point to be completely defined using the curve and the x coordinate of the point on the curve. The two curves that are initially setup to use is point format are Curve 25519 and Curve 448 which are defined in [I-D.irtf-cfrg-curves].

For EC keys with only the x coordinates, the 'kty' member is set to 1 (EC1). The key parameters defined in this section are summarized in Table 10. The members that are defined for this key type are:

crv: contains an identifier of the curve to be used with the key. [CREF4]JLS: Do we create a registry for curves? Is is the same registry for both EC1 and EC2? The curves defined in this document for this key type can be found in Table 9. 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.

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

7.2. RSA Keys

This document defines a key structure for both the public and private halves of RSA keys. Together, an RSA public key and an RSA private key form an RSA key pair. [CREF5]JLS: Looking at the CBOR specification, the bstr that we are looking in our table below should most likely be specified as big numbers rather than as binary strings. This means that we would use the tag 6.2 instead. From my reading of the specification, there is no difference in the encoded size of the resulting output. The specification of bignum does explicitly allow for integers encoded with leading zeros.

The document also provides support for the so-called "multi-prime" RSA where the modulus may have more than two prime factors. The benefit of multi-prime RSA is lower computational cost for the decryption and signature primitives. For a discussion on how multi-prime affects the security of RSA crypto-systems, the reader is referred to [MultiPrimeRSA].

This document follows the naming convention of [RFC3447] for the naming of the fields of an RSA public or private key. The table Table 11 provides a summary of the label values and the types associated with each of those labels. The requirements for fields for RSA keys are as follows:

For all keys, 'kty' MUST be present and MUST have a value of 3.
For public keys, the fields 'n' and 'e' MUST be present. All other fields defined in Table 11 MUST be absent.
For private keys with two primes, the fields 'other', 'r_i', 'd_i' and 't_i' MUST be absent, all other fields MUST be present.
For private keys with more than two primes, all fields MUST be present. For the third to nth primes, each of the primes is represented as a map containing the fields 'r_i', 'd_i' and 't_i'. The field 'other' is an array of those maps.

RSA Key Parameters
name	key type	value	type	description
n	3	-1	bstr	Modulus Parameter
e	3	-2	int	Exponent Parameter
d	3	-3	bstr	Private Exponent Parameter
p	3	-4	bstr	First Prime Factor
q	3	-5	bstr	Second Prime Factor
dP	3	-6	bstr	First Factor CRT Exponent
dQ	3	-7	bstr	Second Factor CRT Exponent
qInv	3	-8	bstr	First CRT Coefficient
other	3	-9	array	Other Primes Info
r_i	3	-10	bstr	i-th factor, Prime Factor
d_i	3	-11	bstr	i-th factor, Factor CRT Exponent
t_i	3	-12	bstr	i-th factor, Factor CRT Coefficient

8. IANA Considerations

8.1. COSE Header Parameter Registry

There are currently no registration requests here

8.2. COSE Header Algorithm Label Table

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

The columns of the registry are:

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

The initial contents of the registry can be found in: Table 7. The specification column for all rows in that table should be this document.

8.3. COSE Algorithm Registry

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

The columns of the registry are:
value: The value to be used to identify this algorithm. Algorithm values MUST be unique. The value can be a positive integer, a negative integer or a string. Integer values between 0 and 255 and strings of length 1 are designated as Standards Track Document required. Integer values from 256 to 65535 and strings of length 2 are designated as Specification Required. Integer values of greater than 65535 and strings of length greater than 2 are designated as first come first server. Integer values in the range -1 to -65536 are delegated to the “COSE Header Algorithm Label” registry. Integer values beyond -65536 are marked as private use.
description: A short description of the algorithm.
specification: A document where the algorithm is defined (if publicly available).

The initial contents of the registry can be found in the following: Table 3, Table 1, Table 4, Table 5, Table 6. The specification column for all rows in that table should be this document.

8.4. COSE Key Common Parameter Registry

There are currently no registration tasks inthis section.

8.5. COSE Key Type Parameter Registry

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

The columns of the table are:

key type: This field contains a descriptive string of a key type. This should be a value that is in the COSE General Values table and is placed in the 'kty' field of a COSE Key structure.
name: This is a descriptive name that enables easier reference to the item. It is not used in the encoding.
label: The label is to be unique for every value of key type. The range of values is from -256 to -1. 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 10, and Table 11. The specification column for all of these entries will be this document.

8.6. COSE Elliptic Curve Registry

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

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 the integer values from 256 to 65535 and -65536 to -257 are designated as Specification Required. Integer values over 65535 are designated as first come first serve. Integer values less than -65536 are marked as private use.
key type: This designates the key type(s) that can be used with this curve.
description: This field contains a brief description of the curve.
specification: This contains a pointer to the public specification for the curve if one exists.

This registry will be initially populated by the values in Table 8. The specification column for all of these entries will be this document.

9. Security Considerations

There are security considerations:

Protect private keys
MAC messages with more than one recipient means one cannot figure out who sent the message
Use of direct key with other recipient structures hands the key to other recipients.
Use of direct ECDH direct encryption is easy for people to leak information on if there are other recipients in the message.
Considerations about protected vs unprotected header fields.
Need to verify that: 1) the kty field of the key matches the key and algorithm being used. 2) that the kty field needs to be included in the trust decision as well as the other key fields. 3) that the algorithm be included in the trust decision.

10. References

10.1. Normative References

[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC7049]	Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, October 2013.

10.2. Informative References

[AES-GCM]	Dworkin, M., "NIST Special Publication 800-38D: Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC.", Nov 2007.
[DSS]	U.S. National Institute of Standards and Technology, "Digital Signature Standard (DSS)", July 2013.
[I-D.greevenbosch-appsawg-cbor-cddl]	Vigano, C., Birkholz, H. and R. Sun, "CBOR data definition language: a notational convention to express CBOR data structures.", Internet-Draft draft-greevenbosch-appsawg-cbor-cddl-05, March 2015.
[I-D.ietf-cose-msg]	Schaad, J. and B. Campbell, "CBOR Encoded Message Syntax", Internet-Draft draft-ietf-cose-msg-05, September 2015.
[I-D.irtf-cfrg-curves]	Langley, A. and R. Salz, "Elliptic Curves for Security", Internet-Draft draft-irtf-cfrg-curves-02, March 2015.
[I-D.irtf-cfrg-eddsa]	Josefsson, S. and I. Liusvaara, "Edwards-curve Digital Signature Algorithm (EdDSA)", Internet-Draft draft-irtf-cfrg-eddsa-00, October 2015.
[MAC]	NiST, N., "FIPS PUB 113: Computer Data Authentication", May 1985.
[MultiPrimeRSA]	Hinek, M. and D. Cheriton, "On the Security of Multi-prime RSA", June 2006.
[PVSig]	Brown, D. and D. Johnson, "Formal Security Proofs for a Signature Scheme with Partial Message Recover", February 2000.
[RFC2104]	Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997.
[RFC2633]	Ramsdell, B., "S/MIME Version 3 Message Specification", RFC 2633, June 1999.
[RFC2898]	Kaliski, B., "PKCS #5: Password-Based Cryptography Specification Version 2.0", RFC 2898, DOI 10.17487/RFC2898, September 2000.
[RFC3394]	Schaad, J. and R. Housley, "Advanced Encryption Standard (AES) Key Wrap Algorithm", RFC 3394, September 2002.
[RFC3447]	Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003.
[RFC3610]	Whiting, D., Housley, R. and N. Ferguson, "Counter with CBC-MAC (CCM)", RFC 3610, September 2003.
[RFC4231]	Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512", RFC 4231, December 2005.
[RFC4262]	Santesson, S., "X.509 Certificate Extension for Secure/Multipurpose Internet Mail Extensions (S/MIME) Capabilities", RFC 4262, December 2005.
[RFC5480]	Turner, S., Brown, D., Yiu, K., Housley, R. and T. Polk, "Elliptic Curve Cryptography Subject Public Key Information", RFC 5480, March 2009.
[RFC5652]	Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, September 2009.
[RFC5751]	Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification", RFC 5751, January 2010.
[RFC5752]	Turner, S. and J. Schaad, "Multiple Signatures in Cryptographic Message Syntax (CMS)", RFC 5752, January 2010.
[RFC5869]	Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 5869, May 2010.
[RFC5990]	Randall, J., Kaliski, B., Brainard, J. and S. Turner, "Use of the RSA-KEM Key Transport Algorithm in the Cryptographic Message Syntax (CMS)", RFC 5990, September 2010.
[RFC6090]	McGrew, D., Igoe, K. and M. Salter, "Fundamental Elliptic Curve Cryptography Algorithms", RFC 6090, February 2011.
[RFC6151]	Turner, S. and L. Chen, "Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms", RFC 6151, March 2011.
[RFC6979]	Pornin, T., "Deterministic Usage of the Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August 2013.
[RFC7159]	Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, March 2014.
[RFC7252]	Shelby, Z., Hartke, K. and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014.
[RFC7515]	Jones, M., Bradley, J. and N. Sakimura, "JSON Web Signature (JWS)", RFC 7515, May 2015.
[RFC7516]	Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)", RFC 7516, May 2015.
[RFC7517]	Jones, M., "JSON Web Key (JWK)", RFC 7517, May 2015.
[RFC7518]	Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, May 2015.
[RFC7539]	Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015.
[SEC1]	Standards for Efficient Cryptography Group, "SEC 1: Elliptic Curve Cryptography", May 2009.
[SP800-56A]	Barker, E., Chen, L., Roginsky, A. and M. Smid, "NIST Special Publication 800-56A: Recommendation for Pair-Wise Key Establishment Schemes Using Discrete Logarithm Cryptography", May 2013.

Appendix A. CDDL Grammar

For people who prefer using a formal language to describe the syntax of the CBOR, in this section a CDDL grammar is given that corresponds to [I-D.greevenbosch-appsawg-cbor-cddl]. This grammar is informational, in the event of differences between this grammar and the prose, the prose is considered to be authorative.

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


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

Appendix B. Examples

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

Examples may have some features that are in questions but not yet incorporated in the document.

To make it easier to read, the examples are presented using the CBOR's diagnostic notation rather than a binary dump. A ruby based tool exists to convert between a number of formats. This tool can be installed with the command line:

        gem install cbor-diag

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

          
         diag2cbor < inputfile > outputfile

The examples can be extracted from the XML version of this docuent via an XPath expression as all of the artwork is tagged with the attribute type='CBORdiag'.

B.1. Examples of MAC messages

B.1.1. Shared Secret Direct MAC

This example users the following:

MAC: AES-CMAC, 256-bit key, trucated to 64 bits
Recipient class: direct shared secret
File name: Mac-04

Size of binary file is 73 bytes

996( [
  h'a1016f4145532d434d41432d3235362f3634',
  {
  },
  h'546869732069732074686520636f6e74656e742e',
  h'd9afa663dd740848',
  [
    [
      h'',
      {
        1: -6,
        4: h'6f75722d736563726574'
      },
      h''
    ]
  ]
])

B.1.2. ECDH Direct MAC