## Use of the HSS/LMS Hash-Based Signature Algorithm with CBOR Object Signing and Encryption (COSE)

draft-ietf-cose-hash-sig-09

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 This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on September 7, 2019. Copyright Notice Copyright (c) 2019 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. Housley Expires September 7, 2019 [Page 1]

Internet-Draft HashSig with COSE March 2019 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]. Housley Expires September 7, 2019 [Page 2]

Internet-Draft HashSig with COSE March 2019 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. Housley Expires September 7, 2019 [Page 3]

Internet-Draft HashSig with COSE March 2019 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. Housley Expires September 7, 2019 [Page 4]

Internet-Draft HashSig with COSE March 2019 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] Housley Expires September 7, 2019 [Page 5]

Internet-Draft HashSig with COSE March 2019 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] Housley Expires September 7, 2019 [Page 6]

Internet-Draft HashSig with COSE March 2019 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. Housley Expires September 7, 2019 [Page 7]

Internet-Draft HashSig with COSE March 2019 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. Housley Expires September 7, 2019 [Page 8]

Internet-Draft HashSig with COSE March 2019 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>. Housley Expires September 7, 2019 [Page 9]

Internet-Draft HashSig with COSE March 2019 [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>. Housley Expires September 7, 2019 [Page 10]

Internet-Draft HashSig with COSE March 2019 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 Housley Expires September 7, 2019 [Page 11]