INTERNET-DRAFT R. Housley Intended Status: Proposed Standard Vigil Security Expires: 12 December 2018 12 June 2018 Use of the Hash-based Signature Algorithm with CBOR Object Signing and Encryption (COSE) <draft-housley-suit-cose-hash-sig-01> 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 Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Housley [Page 1]

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

INTERNET-DRAFT June 2018 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, and this specification makes use of the SHA-256 one-way hash function [SHS]. Housley [Page 3]

INTERNET-DRAFT June 2018 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 the signature over that 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_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 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. The hash-based signature algorithm supports five tree sizes: Housley [Page 4]

INTERNET-DRAFT June 2018 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 3.3, 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 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] 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]. Housley [Page 5]

INTERNET-DRAFT June 2018 The values of p and ls are dependent on the choices of the parameters n and w, as described in Appendix A of [HASHSIG]. 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. Signing involves the generation of C, which is an n-byte random value. The LM-OTS signature value can be summarized as: u32str(type) || C || y[0] || ... || y[p-1] 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. The first four bytes of the signature value contains the number of signed public keys (Nspk) in the HSS. The first four bytes of each LMS signature value contains type code, which tells how to parse the remaining parts of the LMS signature value. The first four bytes of each LM- OTS signature value contains type code, which tells how to parse the remaining parts of the LM-OTS signature value. When using a COSE key for this algorithm, the following checks are made: o The 'kty' field MUST be present, and it MUST be 'HSIG'. o If the 'alg' field is present, and it MUST be 'HSIG'. 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. Housley [Page 6]

INTERNET-DRAFT June 2018 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. 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. 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 Housley [Page 7]

INTERNET-DRAFT June 2018 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 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 Value: TBD (Value to be assigned by IANA) Description: Hash-based digital signatures Reference: This document (Number to be assigned by RFC Editor) Recommended: Yes Housley [Page 8]

INTERNET-DRAFT June 2018 7.2. COSE Key Types Registry Entry The new entry in the "COSE Key Types" registry has the following columns: Name: HASHSIG Value: TBD (Value to be assigned by IANA) Description: Hash-based digital signature public key Reference: This document (Number to be assigned by RFC Editor) 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> Housley [Page 9]

INTERNET-DRAFT June 2018 [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. [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>. Author's Address Russ Housley Vigil Security, LLC 918 Spring Knoll Drive Herndon, VA 20170 USA EMail: housley@vigilsec.com Housley [Page 10]