CBOR Object Signing and Encryption (COSE): Hash Algorithms

Introduction The CBOR Object Signing and Encryption (COSE) syntax does not define any direct methods for the use of hash algorithms. It also does not define a structure syntax that is used to encode a digested object structure along the lines of the DigestedData ASN.1 structure in . This omission was intentional as a structure consisting of just a digest identifier, the content, and a digest value does not by itself provide any strong security service. Additionally, an application is going to be better off defining this type of structure so that it can include add any additional data that needs to be hashed, as well as methods of obtaining the data. While the above is true, there are some cases where having some standard hash algorithms defined for COSE with a common identifier makes a great deal of sense. Two of the cases where these are going to be used are:

Indirect signing of content, and
Object identification.

Indirect signing of content is a paradigm where the content is not directly signed, but instead a hash of the content is computed and that hash value, along with the hash algorithm, is included in the content that will be signed. Doing indirect signing allows for the a signature to be validated without first downloading all of the content associated with the signature. This capability can be of even grater importance in a constrained environment as not all of the content signed may be needed by the device. The use of hashes to identify objects is something that has been very common. One of the primary things that has been identified by a hash function for secure message is a certificate. Two examples of this can be found in and the newly defined COSE equivalents in .

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 when, and only when, they appear in all capitals, as shown here.

Open Issues RFC Editor: Please remove this section before publishing.

No Open Issues

Hash Algorithm Usage As noted in the previous section, hash functions can be used for a variety of purposes. Some of these purposes require that a hash function be cryptographically strong, these include direct and indirect signatures. That is, using the hash as part of the signature or using the hash as part of the body to be signed. Other uses of hash functions do not require the same level of strength. This document contains some hash functions that are not designed to be used for cryptographic operations. An application that is using a hash function needs to carefully evaluate exactly what hash properties are needed and which hash functions are going to provide them. Applications should also make sure that the ability to change hash functions is part of the base design as cryptographic advances are sure to reduce strength of a hash function. A hash function is a map from a large bit string to a smaller bit string. There are going to be collisions by a hash function, the trick is to make sure that it is difficult to find two values that are going to map to the same output value. The standard "Collision Attack" is one where an attacker can find two different messages that have the same hash value. If a collision attack exists, then the function SHOULD NOT be used for a cryptographic purpose. The only reason why such a hash function is used is when there is absolutely no other choice (e.g. a HSM that cannot be replaced), and only after looking at the possible security issues. Cryptographic purposes would include creation of signatures or the use of hashes for indirect signatures. These functions may still be usable for non-cryptographic purposes. An example of a non-cryptographic use of a hash is for filtering from a collection of values to find possible candidates that can later be checked to see if they are the correct one. A simple example of this is the classic thumbprint of a certificate. If the thumbprint is used to verify that it is the correct certificate, then that usage is subject to a collision attack as above. If however, the thumbprint is used to sort through a collection of certificates to find those that might be used for the purpose of verifying a signature, a simple filter capability is sufficient. In this case, one still needs to validate that the public key validates the signature (and the certificate is trusted), and all certificates that don't contain a key that validates the signature can be discarded as false positives. To distinguish between these two cases, a new value in the recommended column of the COSE Algorithms registry is to be added. "Filter Only" indicates that the only purpose of a hash function should be to filter results and not those which require collision resistance.

Example CBOR hash structure did not provide a default structure for holding a hash value not only because no separate hash algorithms were defined, but because how the structure is setup is frequently application specific. There are four fields that are often included as part of a hash structure:

The hash algorithm identifier.
The hash value.
A pointer to the value that was hashed, this could be a pointer to a file, an object that can be obtained from the network, or a pointer to someplace in the message, or something very application specific.
Additional data, this can be something as simple as a random value to make finding hash collisions slightly harder as the value handed to the application cannot have been selected to have a collision, or as complicated as a set of processing instructions that are to be used with the object that is pointed to.

An example of a structure which permits all of the above fields to exist would look like the following. There is no definition here of what goes into the 'any' value and how it would be included in the computed hash value. COSE_Hash_V = ( 1 : int / tstr, # Algorithm identifier 2 : bstr, # Hash value 3 : tstr ?, # Location of object hashed 4 : any ? # object containing other details and things - prefixed to the object to be hashed ) An alternate structure that could be used for situations where one is searching a group of objects for a match. In this case, the location would not be needed and adding extra data to the hash would be counterproductive. This results in a structure that looks like this: COSE_Hash_Find = [ hashAlg : int / tstr, hashValue : bstr ]

Hash Algorithm Identifiers

SHA-1 Hash Algorithm The SHA-1 hash algorithm was designed by the United States National Security Agency and published in 1995. Since that time a large amount of cryptographic analysis has been applied to this algorithm and a successful collision attack has been created (). The IETF formally started discouraging the use of SHA-1 with the publishing of . Despite the above, there are still times where SHA-1 needs to be used and therefore it makes sense to assign a point for the use of this hash algorithm. Some of these situations are with historic HSMs where only SHA-1 is implemented or where the SHA-1 value is used for the purpose of filtering and thus the collision resistance property is not needed. Because of the known issues for SHA-1 and the fact that is should no longer be used, the algorithm will be registered with the recommendation of "Filter Only". SHA-1 Hash Algorithm

Name	Value	Description	Reference	Recommended
SHA-1	TBD6	SHA-1 Hash	[This Document]	Filter Only

SHA-2 Hash Algorithms The family of SHA-2 hash algorithms was designed by the United States National Security Agency and published in 2001. Since that time some additional algorithms have been added to the original set to deal with length extension attacks and some performance issues. While the SHA-3 hash algorithms has been published since that time, the SHA-2 algorithms are still broadly used. There are a number of different parameters for the SHA-2 hash functions. The set of hash functions which have been chosen for inclusion in this document are based on those different parameters and some of the trade-offs involved.

SHA-256/64 provides a truncated hash. The length of the truncation is designed to allow for smaller transmission size. The trade-off is that the odds that a collision will occur increase proportionally. Locations that use this hash function need either to analysis the potential problems with having a collision occur, or where the only function of the hash is to narrow the possible choices. The latter is the case for , the hash value is used to select possible certificates and, if there are multiple choices then, each choice can be tested by using the public key.
SHA-256 is probably the most common hash function used currently. SHA-256 is an efficient hash algorithm for 32-bit hardware.
SHA-384 and SHA-512 hash functions are efficient for 64-bit hardware.
SHA-512/256 provides a hash function that runs more efficiently on 64-bit hardware, but offers the same security levels as SHA-256.

SHA-2 Hash Algorithms

Name	Value	Description	Reference	Recommended
SHA-256/64	TBD1	SHA-2 256-bit Hash truncated to 64-bits	[This Document]	Filter Only
SHA-256	TBD2	SHA-2 256-bit Hash	[This Document]	Yes
SHA-384	TBD3	SHA-2 384-bit Hash	[This Document]	Yes
SHA-512	TBD4	SHA-2 512-bit Hash	[This Document]	Yes
SHA-512/256	TBD5	SHA-2 512-bit Hash truncated to 256-bits	[This Document]	Yes

SHAKE Algorithms The family SHA-3 hash algorithms was the result of a competition run by NIST. The pair of algorithms known as SHAKE-128 and SHAKE-256 are the instances of SHA-3 that are currently being standardized in the IETF. The SHA-3 hash algorithms have a significantly different structure than the SHA-2 hash algorithms. One of the benefits of this differences is that when computing a shorter SHAKE hash value, the value is not a prefix of the result of computing the longer hash. Unlike the SHA-2 hash functions, no algorithm identifier is created for shorter lengths. Applications can specify what the minimum length for a hash function for the protocol. A validator can infer the actual length from the hash value in these cases. SHAKE Hash Functions

Name	Value	Description	Reference	Recommended
SHAKE128	TBD10	128-bit SHAKE	[This Document]	Yes
SHAKE256	TBD11	256-bit SHAKE	[This Document]	Yes

IANA Considerations

COSE Algorithm Registry IANA is requested to register the following algorithms in the "COSE Algorithms" registry.

The SHA-1 hash function found in .
The set of SHA-2 hash functions found in .
The set of SHAKE hash functions found in .

Many of the hash values produced are relatively long and as such the use of a two byte algorithm identifier seems reasonable. SHA-1 is tagged as deprecated and thus a longer algorithm identifier is appropriate even though it is a shorter hash value. In addition, IANA is to add the value of 'Filter Only' to the set of legal values for the 'Recommended' column. This value is only to be used for hash functions and indicates that it is not to be used for purposes which require collision resistance.

Security Considerations The security considerations have already been called out as part of the previous text. The following issues need to be dealt with:

Protocols need to perform a careful analysis of the properties of a hash function that are needed and how they map onto the possible attacks. In particular, one needs to distinguish between those uses that need the cryptographic properties, i.e. collision resistance, and properties that correspond to possible object identification. The different attacks correspond to who or what is being protected, is it the originator that is the attacker or a third party? This is the difference between collision resistance and second pre-image resistance. As a general rule, longer hash values are "better" than short ones, but trade-offs of transmission size, timeliness, and security all need to be included as part of this analysis. In many cases the value being hashed is a public value, as such pre-image resistance is not part of this analysis.
Algorithm agility needs to be considered a requirement for any use of hash functions. As with any cryptographic function, hash functions are under constant attack and the strength of hash algorithms will be reduced over time.