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INTERNET-DRAFT                                                R. Housley
Intended Status: Proposed Standard                        Vigil Security
Expires: 2 January 2019                                      2 July 2018


             Use of the Hash-based Signature Algorithm with
               CBOR Object Signing and Encryption (COSE)
                 <draft-housley-suit-cose-hash-sig-04>


Abstract

   This document specifies the conventions for using the Leighton-Micali
   Signature (LMS) algorithm for digital signatures with the CBOR Object
   Signing and Encryption (COSE) syntax.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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Housley                                                         [Page 1]


INTERNET-DRAFT                                                 July 2018


Copyright and License Notice

   Copyright (c) 2018 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  LMS Digital Signature Algorithm Overview . . . . . . . . . . .  3
     3.1.  Hierarchical Signature System (HSS)  . . . . . . . . . . .  4
     3.2.  Leighton-Micali Signature (LMS)  . . . . . . . . . . . . .  5
     3.3.  Leighton-Micali One-time Signature Algorithm (LM-OTS)  . .  6
   4.  Hash-based Signature Algorithm Identifiers . . . . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
     5.1.  Implementation Security Considerations . . . . . . . . . .  7
     5.2.  Algorithm Security Considerations  . . . . . . . . . . . .  8
   6.  Operational Considerations . . . . . . . . . . . . . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  COSE Algorithms Registry Entry . . . . . . . . . . . . . .  9
     7.2.  COSE Key Types Registry Entry  . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11















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1.  Introduction

   Today, RSA is often used to digitally 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 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.

   This document specifies the conventions for using the Leighton-Micali
   Signature (LMS) algorithm [HASHSIG] for digital signatures with the
   CBOR Object Signing and Encryption (COSE) [RFC8152] syntax.  The LMS
   algorithm is one form of hash-based digital signature; it can only be
   used for a fixed number of signatures.  The LMS algorithm uses small
   private and public keys, and it has low computational cost; however,
   the signatures are quite large.

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.

3.  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 3.1;

      o  Leighton-Micali Signature (LMS) -- see Section 3.2; and

      o  Leighton-Micali One-time Signature Algorithm (LM-OTS) -- see
           Section 3.3.

   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



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   to permit the registration of additional one-way hash functions in
   the future.

3.1.  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 3.2.
   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 3.2.

   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[1] ||
      signed_public_key[2] ||
         ...
      signed_public_key[Nspk-1] ||
      signed_public_key[Nspk] ||
      lms_signature  /* signature of message */

   where, as defined in Section 7 of [HASHSIG], a signed_public_key is
   the lms_signature over the public key followed by the public key
   itself.










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3.2.  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 hash-based signature algorithm supports five values for 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.

   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 3.3, 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]








Housley                                                         [Page 5]


INTERNET-DRAFT                                                 July 2018


3.3.  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]







Housley                                                         [Page 6]


INTERNET-DRAFT                                                 July 2018


4.  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.

5.  Security Considerations

5.1.  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.

   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.




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   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.

5.2.  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].
   They encouraged preparation for a day when RSA and DSA cannot be
   depended upon.

   A post-quantum cryptosystem 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 LM-OTP one-time signature, LMS, and HSS do not depend on discrete
   logarithm or factoring, as a result these algorithms are considered
   to be post-quantum secure.

   Today, RSA is often used to digitally sign software updates.  This
   means that the distribution of software updates could be compromised
   if a significant advance is made in factoring or a quantum computer
   is invented.  The use of hash-based signatures to protect software
   update distribution will allow the deployment of software that
   implements new cryptosystems.

6.  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



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   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.

7.  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.

7.1.  COSE Algorithms Registry Entry

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

      Name:  HASHSIG-HSS-LMS

      Value:  TBD (Value to be assigned by IANA)

      Description:  Hash-based digital signatures using HSS/LMS

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

      Recommended:  Yes

7.2.  COSE Key Types Registry Entry

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

      Name:  HASHSIG-HSS-LMS

      Value:  TBD (Value to be assigned by IANA)

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

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







Housley                                                         [Page 9]


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8.  References

8.1.  Normative References

   [HASHSIG]  McGrew, D., M. Curcio, and S. Fluhrer, "Hash-Based
              Signatures", Work in progress.  <draft-mcgrew-hash-
              sigs-11>

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

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

8.2.  Informative References

   [BH2013]   Ptacek, T., T. Ritter, J. Samuel, 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, T. 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.




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   [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, <http://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>.

Acknowledgements

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

Author's Address

   Russ Housley
   Vigil Security, LLC
   918 Spring Knoll Drive
   Herndon, VA 20170
   USA

   EMail: housley@vigilsec.com


















Housley                                                        [Page 11]


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