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Versions: (draft-housley-suit-cose-hash-sig) 00 01

Network Working Group                                         R. Housley
Internet-Draft                                            Vigil Security
Intended status: Standards Track                          March 06, 2019
Expires: September 7, 2019


 Use of the Hash-based Signature Algorithm with CBOR Object Signing and
                           Encryption (COSE)
                      draft-ietf-cose-hash-sig-01

Abstract

   This document specifies the conventions for using the HSS/LMS hash-
   based signature algorithm with the CBOR Object Signing and Encryption
   (COSE) syntax.  The HSS/LMS algorithm is one form of hash-based
   digital signature; it is described in [HASHSIG].

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 7, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Algorithm Security Considerations . . . . . . . . . . . .   2
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  LMS Digital Signature Algorithm Overview  . . . . . . . . . .   3
     2.1.  Hierarchical Signature System (HSS) . . . . . . . . . . .   4
     2.2.  Leighton-Micali Signature (LMS) . . . . . . . . . . . . .   5
     2.3.  Leighton-Micali One-time Signature Algorithm (LM-OTS) . .   6
   3.  Hash-based Signature Algorithm Identifiers  . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     4.1.  Implementation Security Considerations  . . . . . . . . .   7
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  COSE Algorithms Registry Entry  . . . . . . . . . . . . .   9
     6.2.  COSE Key Types Registry Entry . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   This document specifies the conventions for using the HSS/LMS hash-
   based signature algorithm with the CBOR Object Signing and Encryption
   (COSE) [RFC8152] syntax.  The Leighton-Micali Signature (LMS) system
   provides a one-time digital signature that is a variant of Merkle
   Tree Signatures (MTS).  The Hierarchical Signature System (HSS) is
   built on top of the LMS system to efficiently scale for a larger
   numbers of signatures.  The HSS/LMS algorithm is one form of hash-
   based digital signature, and it is described in [HASHSIG].  The HSS/
   LMS signature algorithm can only be used for a fixed number of
   signing operations.  The number of signing operations depends upon
   the size of the tree.  The HSS/LMS signature algorithm uses small
   public keys, and it has low computational cost; however, the
   signatures are quite large.  The HSS/LMS private key can be very
   small when the signer is willing to perform additional computation at
   signing time; alternatively, the private key can consume additional
   memory and provide a faster signing time.

1.1.  Algorithm Security Considerations

   At Black Hat USA 2013, some researchers gave a presentation on the
   current sate of public key cryptography.  They said: "Current
   cryptosystems depend on discrete logarithm and factoring which has
   seen some major new developments in the past 6 months" [BH2013].




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   They encouraged preparation for a day when RSA and DSA cannot be
   depended upon.

   A post-quantum cryptosystem [PQC] is a system that is secure against
   quantum computers that have more than a trivial number of quantum
   bits.  It is open to conjecture when it will be feasible to build
   such a machine.  RSA, DSA, and ECDSA are not post-quantum secure.

   The HSS/LMS signature algorithm does not depend on discrete logarithm
   or factoring, as a result these algorithms are considered to be post-
   quantum secure.

   Today, the RSA digital signature algorithm is often used to sign
   software updates.  In preparation for a day when RSA, DSA, and ECDSA
   cannot be depended upon, a digital signature algorithm is needed that
   will remain secure even if there are significant cryptoanalytic
   advances or a large-scale quantum computer is invented.  The HSS/LMS
   hash-based digital signature algorithm specified in [HASHSIG] is one
   such algorithm.  The use of hash-based signatures to protect software
   update distribution will allow the deployment of software that
   implements new cryptosystems even if such advances break current
   digital signature mechanisms.

1.2.  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.

2.  LMS Digital Signature Algorithm Overview

   This specification makes use of the hash-based signature algorithm
   specified in [HASHSIG], which is the Leighton and Micali adaptation
   [LM] of the original Lamport-Diffie-Winternitz-Merkle one-time
   signature system [M1979][M1987][M1989a][M1989b].

   The hash-based signature algorithm has three major components:

      o  Hierarchical Signature System (HSS) -- see Section 2.1;

      o  Leighton-Micali Signature (LMS) -- see Section 2.2; and

      o  Leighton-Micali One-time Signature Algorithm (LM-OTS) -- see
            Section 2.3.





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   As implied by the name, the hash-based signature algorithm depends on
   a collision-resistant hash function.  The the hash-based signature
   algorithm specified in [HASHSIG] currently makes use of the SHA-256
   one-way hash function [SHS], but it also establishes an IANA registry
   to permit the registration of additional one-way hash functions in
   the future.

2.1.  Hierarchical Signature System (HSS)

   The hash-based signature algorithm specified in [HASHSIG] uses a
   hierarchy of trees.  The Hierarchical Signature System (HSS) allows
   subordinate trees to be generated when needed by the signer.  By
   using trees-of-trees, a very large number of nodes can be
   accommodated, where each node enables a single digital signature.
   Without the HSS, the generation of such a large tree might take weeks
   or longer.

   An HSS signature as specified in [HASHSIG] carries the number of
   signed public keys (Nspk), followed by that number of signed public
   keys, followed by the LMS signature as described in Section 2.2.
   Each signed public key is represented by the hash value at the root
   of the tree, and it also contains information about the tree
   structure.  The signature over the public key is an LMS signature as
   described in Section 2.2.

   The elements of the HSS signature value for a stand-alone tree can be
   summarized as:

      u32str(0) ||
      lms_signature  /* signature of message */

   The elements of the HSS signature value for a tree with Nspk levels
   can be summarized as:

      u32str(Nspk) ||
      signed_public_key[0] ||
      signed_public_key[1] ||
         ...
      signed_public_key[Nspk-2] ||
      signed_public_key[Nspk-1] ||
      lms_signature  /* signature of message */

   where, as defined in Section 3.3 of [HASHSIG], a signed_public_key is
   the lms_signature over the public key followed by the public key
   itself.  Note that Nspk is the number of levels in the hierarchy of
   trees minus 1.





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2.2.  Leighton-Micali Signature (LMS)

   Each tree in the hash-based signature algorithm specified in
   [HASHSIG] uses the Leighton-Micali Signature (LMS) system.  LMS
   systems have two parameters.  The first parameter is the height of
   the tree, h, which is the number of levels in the tree minus one.
   The [HASHSIG] includes support for five values of this parameter:
   h=5; h=10; h=15; h=20; and h=25.  Note that there are 2^h leaves in
   the tree.  The second parameter is the number of bytes output by the
   hash function, m, which is the amount of data associated with each
   node in the tree.  This specification supports only SHA-256, with
   m=32.  An IANA registry is defined so that other hash functions could
   be used in the future.

   Currently, the hash-based signature algorithm supports five tree
   sizes:

      LMS_SHA256_M32_H5;
      LMS_SHA256_M32_H10;
      LMS_SHA256_M32_H15;
      LMS_SHA256_M32_H20; and
      LMS_SHA256_M32_H25.

   The [HASHSIG] specification establishes an IANA registry to permit
   the registration of additional tree sizes in the future.

   An LMS signature consists of four elements: the number of the leaf
   associated with the LM-OTS signature, an LM-OTS signature as
   described in Section 2.3, a typecode indicating the particular LMS
   algorithm, and an array of values that is associated with the path
   through the tree from the leaf associated with the LM-OTS signature
   to the root.  The array of values contains the siblings of the nodes
   on the path from the leaf to the root but does not contain the nodes
   on the path itself.  The array for a tree with height h will have h
   values.  The first value is the sibling of the leaf, the next value
   is the sibling of the parent of the leaf, and so on up the path to
   the root.

   The four elements of the LMS signature value can be summarized as:

      u32str(q) ||
      ots_signature ||
      u32str(type) ||
      path[0] || path[1] || ... || path[h-1]







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2.3.  Leighton-Micali One-time Signature Algorithm (LM-OTS)

   The hash-based signature algorithm depends on a one-time signature
   method.  This specification makes use of the Leighton-Micali One-time
   Signature Algorithm (LM-OTS) [HASHSIG].  An LM-OTS has five
   parameters:

      n -  The number of bytes output by the hash function.  This
           specification supports only SHA-256 {{SHS}}, with n=32.

      H -  A preimage-resistant hash function that accepts byte strings
           of any length, and returns an n-byte string.  This
           specification supports only SHA-256 [SHS].

      w -  The width in bits of the Winternitz coefficients.  [HASHSIG]
           supports four values for this parameter: w=1; w=2; w=4; and
           w=8.

      p -  The number of n-byte string elements that make up the LM-OTS
           signature.

      ls - The number of left-shift bits used in the checksum function,
           which is defined in Section 4.5 of [HASHSIG].

   The values of p and ls are dependent on the choices of the parameters
   n and w, as described in Appendix A of [HASHSIG].

   Currently, the hash-based signature algorithm supports four LM-OTS
   variants:

      LMOTS_SHA256_N32_W1;
      LMOTS_SHA256_N32_W2;
      LMOTS_SHA256_N32_W4; and
      LMOTS_SHA256_N32_W8.

   The [HASHSIG] specification establishes an IANA registry to permit
   the registration of additional variants in the future.

   Signing involves the generation of C, which is an n-byte random
   value.

   The LM-OTS signature value can be summarized as:

      u32str(otstype) || C || y[0] || ... || y[p-1]







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3.  Hash-based Signature Algorithm Identifiers

   The CBOR Object Signing and Encryption (COSE) [RFC8152] supports two
   signature algorithm schemes.  This specification makes use of the
   signature with appendix scheme for hash-based signatures.

   The signature value is a large byte string.  The byte string is
   designed for easy parsing, and it includes a counter and type codes
   that indirectly provide all of the information that is needed to
   parse the byte string during signature validation.

   When using a COSE key for this algorithm, the following checks are
   made:

      o  The 'kty' field MUST be present, and it MUST be 'HSS-LMS'.

      o  If the 'alg' field is present, and it MUST be 'HSS-LMS'.

      o  If the 'key_ops' field is present, it MUST include 'sign' when
           creating a hash-based signature.

      o  If the 'key_ops' field is present, it MUST include 'verify'
           when verifying a hash-based signature.

      o  If the 'kid' field is present, it MAY be used to identify the
           top of the HSS tree.  In [HASHSIG], this identifier is called
           'I', and it is the 16-byte identifier of the LMS public key
           for the tree.

4.  Security Considerations

4.1.  Implementation Security Considerations

   Implementations must protect the private keys.  Use of a hardware
   security module (HSM) is one way to protect the private keys.
   Compromise of the private keys may result in the ability to forge
   signatures.  Along with the private key, the implementation must keep
   track of which leaf nodes in the tree have been used.  Loss of
   integrity of this tracking data can cause a one-time key to be used
   more than once.  As a result, when a private key and the tracking
   data are stored on non-volatile media or stored in a virtual machine
   environment, care must be taken to preserve confidentiality and
   integrity.

   When a LMS key pair is generating a LMS key pair, an implementation
   must must generate the key pair and the corresponding identifier
   independently of all other key pairs in the HSS tree.




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   An implementation must ensure that a LM-OTS private key is used to
   generate a signature only one time, and ensure that it cannot be used
   for any other purpose.

   The generation of private keys relies on random numbers.  The use of
   inadequate pseudo-random number generators (PRNGs) to generate these
   values can result in little or no security.  An attacker may find it
   much easier to reproduce the PRNG environment that produced the keys,
   searching the resulting small set of possibilities, rather than brute
   force searching the whole key space.  The generation of quality
   random numbers is difficult.  [RFC4086] offers important guidance in
   this area.

   The generation of hash-based signatures also depends on random
   numbers.  While the consequences of an inadequate pseudo-random
   number generator (PRNGs) to generate these values is much less severe
   than the generation of private keys, the guidance in [RFC4086]
   remains important.

5.  Operational Considerations

   The public key for the hash-based signature is the key at the root of
   Hierarchical Signature System (HSS).  In the absence of a public key
   infrastructure [RFC5280], this public key is a trust anchor, and the
   number of signatures that can be generated is bounded by the size of
   the overall HSS set of trees.  When all of the LM-OTS signatures have
   been used to produce a signature, then the establishment of a new
   trust anchor is required.

   To ensure that none of tree nodes are used to generate more than one
   signature, the signer maintains state across different invocations of
   the signing algorithm.  Section 12.2 of [HASHSIG] offers some
   practical implementation approaches around this statefulness.  In
   some of these approaches, nodes are sacrificed to ensure that none
   are used more than once.  As a result, the total number of signatures
   that can be generated might be less than the overall HSS set of
   trees.

6.  IANA Considerations

   IANA is requested to add entries for hash-based signatures in the
   "COSE Algorithms" registry and hash-based public keys in the "COSE
   Key Types" registry.








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6.1.  COSE Algorithms Registry Entry

   The new entry in the "COSE Algorithms" registry has the following
   columns:

      Name:  HSS-LMS

      Value:  TBD (Value to be assigned by IANA)

      Description:  HSS/LMS hash-based digital signature

      Reference:  This document (Number to be assigned by RFC Editor)

      Recommended:  Yes

6.2.  COSE Key Types Registry Entry

   The new entry in the "COSE Key Types" registry has the following
   columns:

      Name:  HSS-LMS

      Value:  TBD (Value to be assigned by IANA)

      Description:  Public key for HSS/LMS hash-based digital signature

      Reference:  This document (Number to be assigned by RFC Editor)

7.  References

7.1.  Normative References

   [HASHSIG]  McGrew, D., Curcio, M., and S. Fluhrer, "Hash-Based
              Signatures", draft-mcgrew-hash-sigs-15 (work in progress),
              January 2019.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <https://www.rfc-editor.org/info/rfc8152>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.



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   [SHS]      National Institute of Standards and Technology (NIST),
              "Secure Hash Standard", FIPS Publication 180-3, 2008.

7.2.  Informative References

   [BH2013]   Ptacek, T., Ritter, T., Samuel, J., and A. Stamos, "The
              Factoring Dead: Preparing for the Cryptopocalypse", August
              2013, <https://media.blackhat.com/us-13/
              us-13-Stamos-The-Factoring-Dead.pdf>.

   [LM]       Leighton, F. and S. Micali, "Large provably fast and
              secure digital signature schemes from secure hash
              functions", U.S. Patent 5,432,852, July 1995.

   [M1979]    Merkle, R., "Secrecy, Authentication, and Public Key
              Systems", Stanford University Information Systems
              Laboratory Technical Report 1979-1, 1979.

   [M1987]    Merkle, R., "A Digital Signature Based on a Conventional
              Encryption Function", Lecture Notes in Computer
              Science crypto87, 1988.

   [M1989a]   Merkle, R., "A Certified Digital Signature", Lecture Notes
              in Computer Science crypto89, 1990.

   [M1989b]   Merkle, R., "One Way Hash Functions and DES", Lecture
              Notes in Computer Science crypto89, 1990.

   [PQC]      Bernstein, D., "Introduction to post-quantum
              cryptography", 2009,
              <http://www.pqcrypto.org/www.springer.com/cda/content/
              document/cda_downloaddocument/9783540887010-c1.pdf>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.








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Appendix A.  Acknowledgements

   Many hanks to Jim Schaad and Tony Putman for their valuable review
   and insights.

Author's Address

   Russ Housley
   Vigil Security, LLC
   516 Dranesville Road
   Herndon, VA  20170
   US

   Email: housley@vigilsec.com





































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