Network Working Group                                          J. Schaad
Internet-Draft                                            August Cellars
UpdatesRFC8152 (if approved)                               June 10, 2019
Intended status: Informational
Expires: December 12, 2019

       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
   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
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   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 December 12, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
     1.2.  Open Issues . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Hash Algorithm Usage  . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Example CBOR hash structure . . . . . . . . . . . . . . .   4
   3.  Hash Algorithm Identifiers  . . . . . . . . . . . . . . . . .   5
     3.1.  SHA-1 Hash Algorithm  . . . . . . . . . . . . . . . . . .   5
     3.2.  SHA-2 Hash Algorithms . . . . . . . . . . . . . . . . . .   6
     3.3.  SHAKE Algorithms  . . . . . . . . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  COSE Algorithm Registry . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   9
   7.  Informative References  . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

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, and

   *  Object identification.

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   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

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.

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

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   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

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:

   *  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

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   *  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 ]

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".

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

                       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 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 [I-D.ietf-cose-x509], 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

   *  *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-

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

                      Table 2: SHA-2 Hash Algorithms

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.

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

                      Table 3: SHAKE Hash Functions

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:

   *  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

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      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.

6.  Normative References

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

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

              Schaad, J., "CBOR CBOR Object Signing and Encryption
              (COSE): Structures and Process", draft-ietf-cose-
              rfc8152bis-struct-02 (work in progress), March 11, 2019,

   [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, <>.

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):

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              Headers for carrying and referencing X.509 certificates",
              draft-ietf-cose-x509-01 (work in progress), March 11,

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

   [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,

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

Author's Address

   Jim Schaad
   August Cellars


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