RFC 8708: Use of the HSS/LMS Hash-Based Signature Algorithm in the Cryptographic Message Syntax (CMS)

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.¶

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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].¶

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.¶

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

   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 */

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

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

   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:¶

         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 }

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:¶

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