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INTERNET-DRAFT                                                R. Housley
Intended Status: Proposed Standard                        Vigil Security
Expires: 11 December 2018                                   11 June 2018


      Use of the Hash-based Merkle Tree Signature (MTS) Algorithm
               in the Cryptographic Message Syntax (CMS)
                  <draft-housley-cms-mts-hash-sig-09>


Abstract

   This document specifies the conventions for using the Merkle Tree
   Signatures (MTS) digital signature algorithm with the Cryptographic
   Message Syntax (CMS).  The MTS algorithm is one form of hash-based
   digital signature.

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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   Internet-Drafts are draft documents valid for a maximum of six months
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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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  ASN.1  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  MTS Digital Signature Algorithm Overview . . . . . . . . . . .  3
     2.1.  Hierarchical Signature System (HSS)  . . . . . . . . . . .  3
     2.2.  Leighton-Micali Signature (LMS)  . . . . . . . . . . . . .  4
     2.3.  Leighton-Micali One-time Signature Algorithm (LM-OTS)  . .  5
   3.  Algorithm Identifiers and Parameters . . . . . . . . . . . . .  6
   4.  Signed-data Conventions  . . . . . . . . . . . . . . . . . . .  6
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
     5.1.  Implementation Security Considerations . . . . . . . . . .  7
     5.2.  Algorithm Security Considerations  . . . . . . . . . . . .  7
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   7.  Normative References . . . . . . . . . . . . . . . . . . . . .  8
   8.  Informative References . . . . . . . . . . . . . . . . . . . .  8
   Appendix: ASN.1 Module . . . . . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11

















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1.  Introduction

   This document specifies the conventions for using the Merkle Tree
   Signatures (MTS) digital signature algorithm with the Cryptographic
   Message Syntax (CMS) [CMS] signed-data content type.  The MTS
   algorithm is one form of hash-based digital signature that can only
   be used for a fixed number of signatures.  The MTS algorithm is
   described in [HASHSIG].  The MTS algorithm uses small private and
   public keys, and it has low computational cost; however, the
   signatures are quite large.

1.1.  ASN.1

   CMS values are generated using ASN.1 [ASN1-B], using the Basic
   Encoding Rules (BER) and the Distinguished Encoding Rules (DER)
   [ASN1-E].

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.  MTS Digital Signature Algorithm Overview

   Merkle Tree Signatures (MTS) are a method for signing a large but
   fixed number of messages.  An MTS system depends on a one-time
   signature method and a collision-resistant hash function.

   This specification makes use of the MTS 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].  It makes use of the LM-OTS one-time
   signature scheme and the SHA-256 one-way hash function [SHS].

2.1.  Hierarchical Signature System (HSS)

   The MTS system specified in [HASHSIG] uses a hierarchy of trees.  The
   Hierarchical N-time Signature System (HSS) allows subordinate trees
   to be generated when needed by the signer.  Otherwise, generation of
   the entire tree might take weeks or longer.

   An HSS signature as specified in 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



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   at the root of the tree, and the signature over that 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_on_message

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

      u32str(Nspk) ||
      lms_signature_on_public_key[0] || public_key[1] ||
      lms_signature_on_public_key[1] || public_key[2] ||
         ...
      lms_signature_on_public_key[Nspk-2] || public_key[Nspk-1] ||
      lms_signature_on_public_key[Nspk-1] || public_key[Nspk] ||
      lms_signature_on_message

2.2.  Leighton-Micali Signature (LMS)

   Each tree in the system 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] specification 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 the amount
   of data associated with each node in the tree.  The [HASHSIG]
   specification supports only the SHA-256 hash function [SHS], with
   m=32.

   Five tree sizes are specified in [HASHSIG]:

      LMS_SHA256_M32_H5;
      LMS_SHA256_M32_H10;
      LMS_SHA256_M32_H15;
      LMS_SHA256_M32_H20; and
      LMS_SHA256_M32_H25.

   An LMS signature consists of four elements: a typecode indicating the
   particular LMS algorithm, the number of the leaf associated with the
   LM-OTS signature, an LM-OTS signature as described in Section 2.3,
   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



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   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]

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

   Merkle Tree Signatures (MTS) depend on a one-time signature method.
   [HASHSIG] specifies the use of the LM-OTS.  An LM-OTS has five
   parameters.

      n -  The number of bytes associated with the hash function.
           [HASHSIG] 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.

      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].

   Four LM-OTS variants are defined in [HASHSIG]:

      LMOTS_SHA256_N32_W1;
      LMOTS_SHA256_N32_W2;
      LMOTS_SHA256_N32_W4; and
      LMOTS_SHA256_N32_W8.

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

   The LM-OTS signature value can be summarized as:

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



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3.  Algorithm Identifiers and Parameters

   The algorithm identifier for an MTS signature is id-alg-mts-hashsig:

      id-alg-mts-hashsig  OBJECT IDENTIFIER ::= { iso(1) member-body(2)
            us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) alg(3) 17 }

   When the id-alg-mts-hashsig algorithm identifier is used for a
   signature, the AlgorithmIdentifier parameters field MUST be absent
   (that is, the parameters are not present; the parameters are not set
   to NULL).

   The signature values is a large OCTET STRING.  The signature format
   is designed for easy parsing.  Each format includes a counter and
   type codes that indirectly providing all of the information that is
   needed to parse the value during signature validation.  The first 4
   octets of the signature value contains the number of signed public
   keys (Nspk) in the HSS.  The first 4 octets of each LMS signature
   value contains type code, which tells how to parse the remaining
   parts of the signature value.  The first 4 octets of each LM-OTS
   signature value contains type code, which tells how to parse the
   remaining parts of the signature value.

4.  Signed-data Conventions

   As specified in [CMS], the digital signature is produced from the
   message digest and the signer's private key.  If signed attributes
   are absent, then the message digest is the hash of the content.  If
   signed attributes are present, then the hash of the content is placed
   in the message-digest attribute, the set of signed attributes is DER
   encoded, and the message digest is the hash of the encoded
   attributes.  In summary:

      IF (signed attributes are absent)
      THEN md = Hash(content)
      ELSE message-digest attribute = Hash(content);
           md = Hash(DER(SignedAttributes))

      Sign(md)

   When using [HASHSIG], the fields in the SignerInfo are used as
   follows:

      digestAlgorithms SHOULD contain the one-way hash function used to
         compute the message digest on the eContent value.  Since the
         hash-based signature algorithms all depend on SHA-256, it is
         strongly RECOMMENDED that SHA-256 also be used to compute the
         message digest on the content.



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         Further, the same one-way hash function SHOULD be used to
         compute the message digest on both the eContent and the
         signedAttributes value if signedAttributes are present.  Again,
         since the hash-based signature algorithms all depend on
         SHA-256, it is strongly RECOMMENDED that SHA-256 be used.

      signatureAlgorithm MUST contain id-alg-mts-hashsig.  The algorithm
         parameters field MUST be absent.

      signature contains the single HSS signature value resulting from
         the signing operation as specified in [HASHSIG].

5.  Security Considerations

5.1.  Implementation Security Considerations

   Implementations must 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 an 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.

   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.  RFC 4086 [RANDOM] offers important
   guidance in this area.

   When computing signatures, the same hash function SHOULD be used for
   all operations.  In this specification, only SHA-256 is used.  Using
   only SHA-256 reduces the number of possible failure points in the
   signature process.

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



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   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 MTS signatures to protect software update
   distribution, perhaps using the format described in [FWPROT], will
   allow the deployment of software that implements new cryptosystems.

6.  IANA Considerations

   This document has no actions for IANA.

7.  Acknowledgements

   Many thanks to Panos Kampanakis, Jim Schaad, and Sean Turner for
   their careful review and comments.

8.  Normative References

   [ASN1-B]  ITU-T, "Information technology -- Abstract Syntax Notation
              One (ASN.1): Specification of basic notation", ITU-T
              Recommendation X.680, 2015.

   [ASN1-E]     ITU-T, "Information technology -- ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ITU-T Recommendation X.690, 2015.

   [CMS]      Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <http://www.rfc-editor.org/info/rfc5652>.

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




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   [RFC2219]  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>.

   [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.

9.  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>

   [CMSASN1]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for
              Cryptographic Message Syntax (CMS) and S/MIME", RFC 5911,
              DOI 10.17487/RFC5911, June 2010, <http://www.rfc-
              editor.org/info/rfc5911>.

   [CMSASN1U] Schaad, J. and S. Turner, "Additional New ASN.1 Modules
              for the Cryptographic Message Syntax (CMS) and the Public
              Key Infrastructure Using X.509 (PKIX)", RFC 6268, DOI
              10.17487/RFC6268, July 2011, <http://www.rfc-
              editor.org/info/rfc6268>.

   [FWPROT]   Housley, R., "Using Cryptographic Message Syntax (CMS) to
              Protect Firmware Packages", RFC 4108, DOI
              10.17487/RFC4108, August 2005, <http://www.rfc-
              editor.org/info/rfc4108>.

   [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.



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   [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.

   [PKIXASN1] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
              Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
              DOI 10.17487/RFC5912, June 2010, <http://www.rfc-
              editor.org/info/rfc5912>.

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

   [RANDOM]   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>.































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Appendix: ASN.1 Module

   MTS-HashSig-2013
     { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
       id-smime(16) id-mod(0) id-mod-mts-hashsig-2013(64) }

   DEFINITIONS IMPLICIT TAGS ::= BEGIN

   EXPORTS ALL;

   IMPORTS
     PUBLIC-KEY, SIGNATURE-ALGORITHM, SMIME-CAPS
       FROM AlgorithmInformation-2009  -- RFC 5911 [CMSASN1]
         { iso(1) identified-organization(3) dod(6) internet(1)
           security(5) mechanisms(5) pkix(7) id-mod(0)
           id-mod-algorithmInformation-02(58) }
     mda-sha256
       FROM PKIX1-PSS-OAEP-Algorithms-2009  -- RFC 5912 [PKIXASN1]
         { iso(1) identified-organization(3) dod(6)
           internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
           id-mod-pkix1-rsa-pkalgs-02(54) } ;

   --
   -- Object Identifiers
   --

   id-alg-mts-hashsig  OBJECT IDENTIFIER ::= { iso(1) member-body(2)
         us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) alg(3) 17 }

   --
   -- Signature Algorithm and Public Key
   --

   sa-MTS-HashSig SIGNATURE-ALGORITHM ::= {
        IDENTIFIER id-alg-mts-hashsig
        PARAMS ARE absent
        HASHES { mda-sha256 }
        PUBLIC-KEYS { pk-MTS-HashSig }
        SMIME-CAPS { IDENTIFIED BY id-alg-mts-hashsig } }

   pk-MTS-HashSig PUBLIC-KEY ::= {
       IDENTIFIER id-alg-mts-hashsig
       KEY MTS-HashSig-PublicKey
       PARAMS ARE absent
       CERT-KEY-USAGE
           { digitalSignature, nonRepudiation, keyCertSign, cRLSign } }

   MTS-HashSig-PublicKey ::= OCTET STRING



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   --
   -- Expand the signature algorithm set used by CMS [CMSASN1U]
   --

   SignatureAlgorithmSet SIGNATURE-ALGORITHM ::=
        { sa-MTS-HashSig, ... }

   --
   -- Expand the S/MIME capabilities set used by CMS [CMSASN1]
   --

   SMimeCaps SMIME-CAPS ::= { sa-MTS-HashSig.&smimeCaps, ... }

   END

Author's Address

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

   EMail: housley@vigilsec.com



























Housley                                                        [Page 12]


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