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Internet Engineering Task Force (IETF) R. Housley Request for Comments: 8708 Vigil Security Category: Standards Track February 2020 ISSN: 2070-1721

Use of the HSS/LMS Hash-Based Signature Algorithm in the Cryptographic

                        Message Syntax (CMS)

Abstract

 This document specifies the conventions for using the Hierarchical
 Signature System (HSS) / Leighton-Micali Signature (LMS) hash-based
 signature algorithm with the Cryptographic Message Syntax (CMS).  In
 addition, the algorithm identifier and public key syntax are
 provided.  The HSS/LMS algorithm is one form of hash-based digital
 signature; it is described in RFC 8554.

Status of This Memo

 This is an Internet Standards Track document.

 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.

 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8708.

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
 (https://trustee.ietf.org/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
   1.1.  ASN.1
   1.2.  Terminology
   1.3.  Motivation
 2.  HSS/LMS Hash-Based Signature Algorithm Overview
   2.1.  Hierarchical Signature System (HSS)
   2.2.  Leighton-Micali Signature (LMS)
   2.3.  Leighton-Micali One-Time Signature (LM-OTS) Algorithm
 3.  Algorithm Identifiers and Parameters
 4.  HSS/LMS Public Key Identifier
 5.  Signed-Data Conventions
 6.  Security Considerations
 7.  IANA Considerations
 8.  References
   8.1.  Normative References
   8.2.  Informative References
 Appendix A.  ASN.1 Module
 Acknowledgements
 Author's Address

1. Introduction

 This document specifies the conventions for using the Hierarchical
 Signature System (HSS) / Leighton-Micali Signature (LMS) hash-based
 signature algorithm with the Cryptographic Message Syntax (CMS) [CMS]
 signed-data content type.  The LMS system provides a one-time digital
 signature that is a variant of Merkle Tree Signatures (MTS).  The 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 with a given private key, and 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.  The HSS/LMS signatures [HASHSIG] are currently defined
 to exclusively use SHA-256 [SHS].

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.

1.3. Motivation

 Recent advances in cryptanalysis [BH2013] and progress in the
 development of quantum computers [NAS2019] pose a threat to widely
 deployed digital signature algorithms.  As a result, there is a need
 to prepare for a day when cryptosystems such as RSA and DSA that
 depend on discrete logarithms and factoring cannot be depended upon.

 If large-scale quantum computers are ever built, these computers will
 be able to break many of the public key cryptosystems currently in
 use.  A post-quantum cryptosystem [PQC] is a system that is secure
 against quantum computers that have more than a trivial number of
 quantum bits (qubits).  It is open to conjecture when it will be
 feasible to build such computers; however, RSA, DSA, Elliptic Curve
 Digital Signature Algorithm (ECDSA), and Edwards-curve Digital
 Signature Algorithm (EdDSA) are all vulnerable if large-scale quantum
 computers are ever developed.

 Since the HSS/LMS signature algorithm does not depend on the
 difficulty of discrete logarithms or factoring, the HSS/LMS signature
 algorithm is considered to be post-quantum secure.  One use of post-
 quantum-secure signatures is the protection of software updates,
 perhaps using the format described in [FWPROT], to enable deployment
 of software that implements new cryptosystems.

2. HSS/LMS Hash-Based 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 hash-based 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].

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

2.1. Hierarchical Signature System (HSS)

 The MTS system specified in [HASHSIG] uses a hierarchy of trees.  The
 N-time Hierarchical 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 [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.  The
 public key for the topmost LMS tree is the public key of the HSS
 system.  The LMS private key in the parent tree signs the LMS public
 key in the child tree, and the LMS private key in the bottom-most
 tree signs the actual message.  The signature over the public key and
 the signature over the actual message are LMS signatures as described
 in Section 2.2.

 The elements of the HSS signature value for a standalone tree (a top
 tree with no children) can be summarized as:

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

 where, u32str() and || are used as defined in [HASHSIG].

 The elements of the HSS signature value for a tree with Nspk signed
 public keys 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], the signed_public_key
 structure contains 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.

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, m,
 is the number of bytes output by the hash function, and it is 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.  As a result, the [HASHSIG] specification supports five tree
 sizes; they are identified as:

LMS_SHA256_M32_H5

LMS_SHA256_M32_H10

LMS_SHA256_M32_H15

LMS_SHA256_M32_H20

LMS_SHA256_M32_H25

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

 As specified in [HASHSIG], the LMS public key consists of four
 elements: the lms_algorithm_type from the list above, the otstype to
 identify the Leighton-Micali One-Time Signature (LM-OTS) type as
 discussed in Section 2.3, the private key identifier (I) as described
 in Section 5.3 of [HASHSIG], and the m-byte string associated with
 the root node of the tree (T[1]).

 The LMS public key can be summarized as:

    u32str(lms_algorithm_type) || u32str(otstype) || I || T[1]

 As specified in [HASHSIG], an LMS signature consists of four
 elements: the number of the leaf (q) associated with the LM-OTS
 signature value, an LM-OTS signature value 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 value 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]

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

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

 n:   The length in bytes of the hash function output.  [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 value.

 ls:  The number of bits that are left-shifted in the final step of
      the checksum function, which is defined in Section 4.4 of
      [HASHSIG].

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

 The [HASHSIG] specification supports four LM-OTS variants:

LMOTS_SHA256_N32_W1

LMOTS_SHA256_N32_W2

LMOTS_SHA256_N32_W4

LMOTS_SHA256_N32_W8

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

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

 The LM-OTS signature value can be summarized as the identifier of the
 LM-OTS variant, the random value, and a sequence of hash values (y[0]
 through y[p-1]) that correspond to the elements of the public key, as
 described in Section 4.5 of [HASHSIG]:

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

3. Algorithm Identifiers and Parameters

 The algorithm identifier for an HSS/LMS hash-based signature is:

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

 When this object identifier is used for an HSS/LMS signature, the
 AlgorithmIdentifier parameters field MUST be absent (that is, the
 parameters are not present, and the parameters are not set to NULL).

 The signature value is a large OCTET STRING, as described in
 Section 2 of this document.  The signature format is designed for
 easy parsing.  The HSS, LMS, and LM-OTS components of the signature
 value each include a counter and a typecode that indirectly provide
 all of the information that is needed to parse the value during
 signature validation.

 The signature value identifies the hash function used in the HSS/LMS
 tree.  [HASHSIG] uses only the SHA-256 hash function [SHS], but it
 establishes an IANA registry [IANA-LMS] to permit the registration of
 additional hash functions in the future.

4. HSS/LMS Public Key Identifier

 The AlgorithmIdentifier for an HSS/LMS public key uses the id-alg-
 hss-lms-hashsig object identifier, and the parameters field MUST be
 absent.

 When this AlgorithmIdentifier appears in the SubjectPublicKeyInfo
 field of an X.509 certificate [RFC5280], the certificate key usage
 extension MAY contain digitalSignature, nonRepudiation, keyCertSign,
 and cRLSign; however, it MUST NOT contain other values.

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

    HSS-LMS-HashSig-PublicKey ::= OCTET STRING

 Note that the id-alg-hss-lms-hashsig algorithm identifier is also
 referred to as id-alg-mts-hashsig.  This synonym is based on the
 terminology used in an early draft version of the document that
 became [HASHSIG].

 The public key value is an OCTET STRING.  Like the signature format,
 it is designed for easy parsing.  The value is the number of levels
 in the public key, L, followed by the LMS public key.

 The HSS/LMS public key value can be described as:

       u32str(L) || lms_public_key

 Note that the public key for the topmost LMS tree is the public key
 of the HSS system.  When L=1, the HSS system is a single tree.

5. Signed-Data Conventions

 As specified in [CMS], the digital signature is produced from the
 message digest and the signer's private key.  The signature is
 computed over different values depending on whether signed attributes
 are absent or present.

 When signed attributes are absent, the HSS/LMS signature is computed
 over the content.  When signed attributes are present, a hash is
 computed over the content using the same hash function that is used
 in the HSS/LMS tree, then a message-digest attribute is constructed
 with the hash of the content, and then the HSS/LMS signature is
 computed over the DER-encoded set of signed attributes (which MUST
 include a content-type attribute and a message-digest attribute).  In
 summary:

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

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

digestAlgorithm MUST contain the one-way hash function used in the

HSS/LMS tree. In [HASHSIG], SHA-256 is the only supported hash

    function, but other hash functions might be registered in the
    future.  For convenience, the AlgorithmIdentifier for SHA-256 from
    [PKIXASN1] is repeated here:

          mda-sha256 DIGEST-ALGORITHM ::= {
              IDENTIFIER id-sha256
              PARAMS TYPE NULL ARE preferredAbsent }

          id-sha256 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2)
              country(16) us(840) organization(1) gov(101) csor(3)
              nistAlgorithms(4) hashalgs(2) 1 }

signatureAlgorithm MUST contain id-alg-hss-lms-hashsig, and the

algorithm parameters field MUST be absent.

signature contains the single HSS signature value resulting from

the signing operation as specified in [HASHSIG].

6. 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 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 in a virtual machine environment, failed writes,
 virtual machine snapshotting or cloning, and other operational
 concerns must be considered to ensure confidentiality and integrity.

 When generating an LMS key pair, an implementation MUST generate each
 key pair independently of all other key pairs in the HSS tree.

 An implementation MUST ensure that an 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 pseudorandom 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, and [RFC4086] offers important guidance
 in this area.

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

 When computing signatures, the same hash function SHOULD be used to
 compute the message digest of the content and the signed attributes,
 if they are present.

7. IANA Considerations

 In the "SMI Security for S/MIME Module Identifier
 (1.2.840.113549.1.9.16.0)" registry, IANA has updated the reference
 for value 64 to point to this document.

 In the "SMI Security for S/MIME Algorithms (1.2.840.113549.1.9.16.3)"
 registry, IANA has updated the description for value 17 to "id-alg-
 hss-lms-hashsig" and updated the reference to point to this document.

 IANA has also added the following note to the "SMI Security for
 S/MIME Algorithms (1.2.840.113549.1.9.16.3)" registry:

    Value 17, "id-alg-hss-lms-hashsig", is also referred to as "id-
    alg-mts-hashsig".

8. References

8.1. Normative References

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

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

 [HASHSIG]  McGrew, D., Curcio, M., and S. Fluhrer, "Leighton-Micali
            Hash-Based Signatures", RFC 8554, DOI 10.17487/RFC8554,
            April 2019, <https://www.rfc-editor.org/info/rfc8554>.

 [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>.

 [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>.

 [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),
            "Secure Hash Standard (SHS)", FIPS PUB 180-4,
            DOI 10.6028/NIST.FIPS.180-4, August 2015,
            <https://doi.org/10.6028/NIST.FIPS.180-4>.

8.2. 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>.

 [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,
            <https://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,
            <https://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,
            <https://www.rfc-editor.org/info/rfc4108>.

 [IANA-LMS] IANA, "Leighton-Micali Signatures (LMS)",
            <https://www.iana.org/assignments/leighton-micali-
            signatures/>.

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

 [M1979]    Merkle, R., "Secrecy, Authentication, and Public Key
            Systems", Technical Report No. 1979-1, Information Systems
            Laboratory, Stanford University, 1979.

 [M1987]    Merkle, R., "A Digital Signature Based on a Conventional
            Encryption Function", Advances in Cryptology -- CRYPTO '87
            Proceedings, Lecture Notes in Computer Science Vol. 293,
            DOI 10.1007/3-540-48184-2_32, 1988,
            <https://doi.org/10.1007/3-540-48184-2_32>.

 [M1989a]   Merkle, R., "A Certified Digital Signature", Advances in
            Cryptology -- CRYPTO '89 Proceedings, Lecture Notes in
            Computer Science Vol. 435, DOI 10.1007/0-387-34805-0_21,
            1990, <https://doi.org/10.1007/0-387-34805-0_21>.

 [M1989b]   Merkle, R., "One Way Hash Functions and DES", Advances in
            Cryptology -- CRYPTO '89 Proceedings, Lecture Notes in
            Computer Science Vol. 435, DOI 10.1007/0-387-34805-0_40,
            1990, <https://doi.org/10.1007/0-387-34805-0_40>.

 [NAS2019]  National Academies of Sciences, Engineering, and Medicine,
            "Quantum Computing: Progress and Prospects", The National
            Academies Press, DOI 10.17226/25196, 2019,
            <https://doi.org/10.17226/25196>.

 [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,
            <https://www.rfc-editor.org/info/rfc5912>.

 [PQC]      Bernstein, D., "Introduction to post-quantum
            cryptography", DOI 10.1007/978-3-540-88702-7_1, 2009,
            <http://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>.

Appendix A. ASN.1 Module

 <CODE STARTS>

 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) } ;

-
- Object Identifiers
-

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

 id-alg-mts-hashsig OBJECT IDENTIFIER ::= id-alg-hss-lms-hashsig

-
- Signature Algorithm and Public Key
-

 sa-HSS-LMS-HashSig SIGNATURE-ALGORITHM ::= {
     IDENTIFIER id-alg-hss-lms-hashsig
     PARAMS ARE absent
     PUBLIC-KEYS { pk-HSS-LMS-HashSig }
     SMIME-CAPS { IDENTIFIED BY id-alg-hss-lms-hashsig } }

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

 HSS-LMS-HashSig-PublicKey ::= OCTET STRING

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

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

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

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

END

 <CODE ENDS>

Acknowledgements

 Many thanks to Joe Clarke, Roman Danyliw, Scott Fluhrer, Jonathan
 Hammell, Ben Kaduk, Panos Kampanakis, Barry Leiba, John Mattsson, Jim
 Schaad, Sean Turner, Daniel Van Geest, and Dale Worley for their
 careful review and comments.

Author's Address

 Russ Housley
 Vigil Security, LLC
 516 Dranesville Road
 Herndon, VA 20170
 United States of America

 Email: housley@vigilsec.com