Internet-Draft COSE Hashes June 2019
Schaad Expires 12 December 2019 [Page]
RFC8152 (if approved)
Intended Status:
J. Schaad
August Cellars

CBOR Object Signing and Encryption (COSE): Hash Algorithms


The CBOR Object Signing and Encryption (COSE) syntax RFC 8152 does not define any direct methods for using hash algorithms. There are however circumstances where hash algorithms are used: Indirect signatures, where the hash of one or more external contents are signed, or thumbprints, for identification of X.509 certificates or other objects. This document defines a set of hash algorithms that are identified by COSE Algorithm Identifiers.

Contributing to this document

The source for this draft is being maintained in GitHub. Suggested changes should be submitted as pull requests at Editorial changes can be managed in GitHub, but any substantial issues need to be discussed on the COSE mailing list.

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

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 12 December 2019.

Table of Contents

1. 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 [CMS]. 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 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 [ESS] and the newly defined COSE equivalents in [I-D.ietf-cose-x509].

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. Open Issues

RFC Editor: Please remove this section before publishing.

  • No Open Issues

2. 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.

2.1.  Example CBOR hash structure

[COSE] 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:

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

3. Hash Algorithm Identifiers

3.1. SHA-1 Hash Algorithm

The SHA-1 hash algorithm [RFC3174] 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 ([SHA-1-collision]). The IETF formally started discouraging the use of SHA-1 with the publishing of [RFC6194].

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".

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

3.2. SHA-2 Hash Algorithms

The family of SHA-2 hash algorithms [FIPS-180-4] 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.

Table 2: 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

3.3. SHAKE Algorithms

The family SHA-3 hash algorithms [FIPS-180-4] 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.

Table 3: SHAKE Hash Functions
Name Value Description Reference Recommended
SHAKE128 TBD10 128-bit SHAKE [This Document] Yes
SHAKE256 TBD11 256-bit SHAKE [This Document] Yes

4. IANA Considerations

4.1. COSE Algorithm Registry

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

  • The SHA-1 hash function found in Table 1.
  • The set of SHA-2 hash functions found in Table 2.
  • The set of SHAKE hash functions found in Table 3.

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.

5. Security Considerations

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

6. Normative References

Schaad, J., "CBOR Object Signing and Encryption (COSE)", RFC 8152, DOI 10.17487/RFC8152, , <>.
National Institute of Standards and Technology, "Secure Hash Standard", FIPS PUB 180-4, .
Schaad, J., "CBOR CBOR Object Signing and Encryption (COSE): Structures and Process", Internet-Draft draft-ietf-cose-rfc8152bis-struct-02, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.

7. Informative References

Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, , <>.
Hoffman, P., Ed., "Enhanced Security Services for S/MIME", RFC 2634, DOI 10.17487/RFC2634, , <>.
Schaad, J., "CBOR Object Signing and Encryption (COSE): Headers for carrying and referencing X.509 certificates", Internet-Draft draft-ietf-cose-x509-01, , <>.
Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, DOI 10.17487/RFC3174, , <>.
Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security Considerations for the SHA-0 and SHA-1 Message-Digest Algorithms", RFC 6194, DOI 10.17487/RFC6194, , <>.
Stevens, M., Bursztein, E., Karpman, P., Albertini, A., and Y. Markov, "The first collision for full SHA-1", , <>.

Author's Address

Jim Schaad
August Cellars