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CBOR Object Signing and Encryption (COSE): Hash Algorithms

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9054.
Author Jim Schaad
Last updated 2023-02-07 (Latest revision 2020-09-14)
Replaces draft-schaad-cose-hash-algs
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Informational
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Ivaylo Petrov
Shepherd write-up Show Last changed 2020-04-14
IESG IESG state Became RFC 9054 (Informational)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Murray Kucherawy
Send notices to Ivaylo Petrov <>
IANA IANA review state Version Changed - Review Needed
IANA action state RFC-Ed-Ack
Network Working Group                                          J. Schaad
Internet-Draft                                            August Cellars
Intended status: Informational                        September 14, 2020
Expires: March 18, 2021

       CBOR Object Signing and Encryption (COSE): Hash Algorithms


   The CBOR Object Signing and Encryption (COSE) syntax
   [I-D.ietf-cose-rfc8152bis-struct] does not define any direct methods
   for using hash algorithms.  There are, however, circumstances where
   hash algorithms are used, such as indirect signatures where the hash
   of one or more contents are signed, and X.509 certificate or other
   object identification by the use of a fingerprint.  This document
   defines a set of hash algorithms that are identified by COSE
   Algorithm Identifiers.

Contributing to this document

   This note is to be removed before publishing as an RFC.

   The source for this draft is being maintained in GitHub.  Suggested
   changes should be submitted as pull requests at
   cose-wg/X509 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 March 18, 2021.

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Copyright Notice

   Copyright (c) 2020 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 (
   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   3
   2.  Hash Algorithm Usage  . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Example CBOR hash structure . . . . . . . . . . . . . . .   4
   3.  Hash Algorithm Identifiers  . . . . . . . . . . . . . . . . .   5
     3.1.  SHA-1 Hash Algorithm  . . . . . . . . . . . . . . . . . .   6
     3.2.  SHA-2 Hash Algorithms . . . . . . . . . . . . . . . . . .   6
     3.3.  SHAKE Algorithms  . . . . . . . . . . . . . . . . . . . .   8
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  COSE Algorithm Registry . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .  10
   7.  Informative References  . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

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 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:

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   *  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 an identifier for the hash algorithm, is
   included in the content that will be signed.  Doing indirect signing
   allows for a signature to be validated without first downloading all
   of the content associated with the signature.  Rather the signature
   can be validated on all of the hash values and pointers to the
   associated contents, then those associated parts can be downloaded,
   the hash value of that part computed, and then compared to the hash
   value in the signed content.  This capability can be of even greater
   importance in a constrained environment as not all of the content
   signed may be needed by the device.  An example of how this is used
   can be found in [I-D.ietf-suit-manifest].

   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 in a secure message is a certificate.  Two examples of this
   can be found in [ESS] and the COSE equivalents in

1.1.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

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 may 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 the strength of a hash function [BCP201].

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   A hash function is a map from one, normally large, bit string to a
   second, usually smaller, bit string.  As the number of possible input
   values is far greater than the number of possible output values, it
   is inevitable that there are going to be collisions.  The trick is to
   make sure that it is difficult to find two values that are going to
   map to the same output value.  A "Collision Attack" is one where an
   attacker can find two different messages that have the same hash
   value.  A hash function that is susceptible to practical collision
   attacks, SHOULD NOT be used for a cryptographic purpose.  The
   discovery of theoretical collision attacks against a given hash
   function SHOULD trigger protocol maintainers and users to do a review
   of the continued suitability of the algorithm if alternatives are
   available and migration is viable.  The only reason why such a hash
   function is used is when there is absolutely no other choice (e.g. a
   Hardware Security Module (HSM) that cannot be replaced), and only
   after looking at the possible security issues.  Cryptographic
   purposes would include the 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 a set of possible candidates; the
   candidates can then be checked to see if they can successfully be
   used.  A simple example of this is the classic fingerprint of a
   certificate.  If the fingerprint is used to verify that it is the
   correct certificate, then that usage is a cryptographic one and is
   subject to the warning above about collision attack.  If, however,
   the fingerprint 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 confirm 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 it is not intended for applications
   which require a cryptographically strong algorithm.

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:

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   *  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

   *  Additional data; this can be something as simple as a random value
      (i.e. salt) to make finding hash collisions slightly harder (as
      the payload handed to the application could have been selected to
      have a collision), or as complicated as a set of processing
      instructions that are used with the object that is pointed to.
      The additional data can be dealt with in a number of ways,
      prepending or appending to the content, but it is strongly
      suggested that it either be a fixed known size, or the lengths of
      the pieces being hashed be included.  (Encoding as a CBOR array
      accomplishes this requirement.)

   An example of a structure which permits all of the above fields to
   exist would look like the following.

   COSE_Hash_V = (
       1 : int / tstr, # Algorithm identifier
       2 : bstr, # Hash value
       ? 3 : tstr, # Location of object that was hashed
       ? 4 : any   # object containing other details and things

   Below is an alternative structure that could be used in situations
   where one is searching a group of objects for a matching hash value.
   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

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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 codepoint for the use of
   this hash algorithm.  Some of these situations are with historic HSMs
   where only SHA-1 is implemented; other situations are 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 it should no
   longer be used, the algorithm will be registered with the
   recommendation of "Filter Only".  This provides guidance about when
   the algorithm is safe for use, while discouraging usage where it is
   not safe.

   The COSE capabilities for this algorithm is an empty array.

   |Name |Value | Description | Capabilities | Reference | Recommended |
   |SHA-1| -14  | SHA-1 Hash  | []           | [This     | Filter Only |
   |     |      |             |              | Document] |             |

                       Table 1: SHA-1 Hash Algorithm

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 have 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.

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   *  *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.  Use of this hash function needs analysis
      of the potential problems with having a collision occur, or must
      be limited to where the function of the hash is non-cryptographic.

      The latter is the case for [I-D.ietf-cose-x509].  The hash value
      is used to select possible certificates and, if there are multiple
      choices remaining 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

   *  *SHA-384* and *SHA-512* hash functions are efficient for 64-bit

   *  *SHA-512/256* provides a hash function that runs more efficiently
      on 64-bit hardware, but offers the same security levels as SHA-

   The COSE capabilities array for these algorithms is empty.

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   | Name      |Value|Description| Capabilities |Reference|Recommended |
   |SHA-256/64 | -15 | SHA-2     | []           | [This   |Filter Only |
   |           |     | 256-bit   |              |Document]|            |
   |           |     | Hash      |              |         |            |
   |           |     | truncated |              |         |            |
   |           |     |to 64-bits |              |         |            |
   | SHA-256   | -16 | SHA-2     | []           | [This   | Yes        |
   |           |     | 256-bit   |              |Document]|            |
   |           |     | Hash      |              |         |            |
   | SHA-384   | -43 | SHA-2     | []           | [This   | Yes        |
   |           |     | 384-bit   |              |Document]|            |
   |           |     | Hash      |              |         |            |
   | SHA-512   | -44 | SHA-2     | []           | [This   | Yes        |
   |           |     | 512-bit   |              |Document]|            |
   |           |     | Hash      |              |         |            |
   |SHA-512/256| -17 | SHA-2     | []           | [This   | Yes        |
   |           |     | 512-bit   |              |Document]|            |
   |           |     | Hash      |              |         |            |
   |           |     | truncated |              |         |            |
   |           |     |to 256-bits|              |         |            |

                       Table 2: SHA-2 Hash Algorithms

3.3.  SHAKE Algorithms

   The family of SHA-3 hash algorithms [FIPS-202] 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.  This is the reason for including these
   algorithms in this document.

   The SHA-3 hash algorithms have a significantly different structure
   than the SHA-2 hash algorithms.

   Unlike the SHA-2 hash functions, no algorithm identifier is created
   for shorter lengths.  The length of the hash value stored is 256-bits
   for SHAKE-128 and 512-bits for SHAKE-256.

   The COSE capabilities array for these algorithms is empty.

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   | Name   |Value| Description | Capabilities |Reference| Recommended |
   |SHAKE128| -18 | SHAKE-128   | []           | [This   | Yes         |
   |        |     |256-bit Hash |              |Document]|             |
   |        |     | Value       |              |         |             |
   |SHAKE256| -45 | SHAKE-256   | []           | [This   | Yes         |
   |        |     |512-bit Hash |              |Document]|             |
   |        |     | Value       |              |         |             |

                       Table 3: SHAKE Hash Functions

4.  IANA Considerations

   The IANA actions in [I-D.ietf-cose-rfc8152bis-struct] and
   [I-D.ietf-cose-rfc8152bis-algs] need to be executed before the
   actions in this document.  Where early allocation of codepoints has
   been made, these should be preserved.

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 'Filter Only' and thus a longer algorithm identifier is
   appropriate even though it is a shorter hash value.

   IANA is requested 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.  IANA is requested to
   add this document to the reference section for this table due to this

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5.  Security Considerations

   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, such as 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 and, 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 [BCP201].  As with any cryptographic function, hash
   functions are under constant attack and the cryptographic strength of
   hash algorithms will be reduced over time.

6.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-struct-12, August 24, 2020,

              National Institute of Standards and Technology, "Secure
              Hash Standard", FIPS PUB 180-4, August 2015.

   [FIPS-202] National Institute of Standards and Technology, "SHA-3
              Standard: Permutation-Based Hash and Extendable-Output
              Functions", FIPS PUB 202, August 2015.

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   [RFC3174]  Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,

7.  Informative References

   [CMS]      Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,

   [ESS]      Hoffman, P., Ed., "Enhanced Security Services for S/MIME",
              RFC 2634, DOI 10.17487/RFC2634, June 1999,

              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Header parameters for carrying and referencing X.509
              certificates", Work in Progress, Internet-Draft, draft-
              ietf-cose-x509-06, March 9, 2020,

   [RFC6194]  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, March 2011,

              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-algs-11, July 1, 2020,

              Moran, B., Tschofenig, H., Birkholz, H., and K. Zandberg,
              "A Concise Binary Object Representation (CBOR)-based
              Serialization Format for the Software Updates for Internet
              of Things (SUIT) Manifest", Work in Progress, Internet-
              Draft, draft-ietf-suit-manifest-09, July 13, 2020,

   [BCP201]   Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, November 2015.


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              Stevens, M., Bursztein, E., Karpman, P., Albertini, A.,
              and Y. Markov, "The first collision for full SHA-1",
              February 2017,

   [COSE]     Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,

Author's Address

   Jim Schaad
   August Cellars


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