Author: Herman Schoenfeld 
Version: 1.1
Date: 2020-07-20
Copyright: (c) Sphere 10 Software Pty Ltd. All Rights Reserved.

W-OTS-Sharp.pdf

Original e-print

Abstract

A very simple modification to the standard W-OTS scheme is presented called W-OTS# that achieves a security enhancement similar to W-OTS+[1] but without the overhead of hashing a randomization vector in every round of the chaining function. The idea proffered by W-OTS# is to simply thwart Birthday-attacks[2] altogether by signing an HMAC of the message-digest (keyed with cryptographically random salt) rather than the message-digest itself. The signer thwarts a birthday attack by virtue of requiring that the attacker guess the salt bits in addition to the message-digest bits during the collision scanning process. By choosing a salt length matching the message-digest length, the security of W-OTS# reduces to that of the cryptographic hash function. This essentially doubles the security level of W-OTS and facilitates the use of shorter hash functions which provide shorter and faster signatures for same security. For example, WOTS# 128-bit signatures have commensurate security to standard W-OTS 256-bit signatures yet are roughly half the size and twice as fast. It is proposed that Blake2b-128 and Winternitz parameter w=4 (i.e. base-16 digits) be adopted as the default parameter set for the W-OTS# scheme.

1. Birthday Attack

---

A birthday attack involves an attacker forging a signature for a "malicious" message M by reusing a signature for an "agreed" message m . In this class of attack, the attacker has knowledge of a message m that the victim is willing and intending to sign in the future. The attacker creates variations of m as {m_1..m_k} any of which will also be deemed "valid" and signed by the victim. Whilst the victim considers each message m_i "identical", their hash digests are unique. This can be achieved by simply varying one or more nonce values or whitespace within m to create this set. The attacker simultaneously generates variations of a "malicious" message M as the set {M1..M_l} and stops until a collision H(m_i) = H(M_j) is found (where H is the cryptographic hash function used in the scheme).

When a collision-pair (m_i, M_j) is found, the attacker asks the victim to sign valid m_i giving s = Sign(m_i, key) = SignDigest(H(m_i), key). The attacker then proceeds to forge a
signature for invalid M_i by simply re-using s , as follows:

1: S = Sign(M_j, key)
2:   = SignDigest(H(M_j), key)
3:   = SignDigest(H(m_i), key)
4:   = s

Unbeknownst to the victim, by signing m_i , they have also signed M_j.

2. W-OTS & W-OTS+

---

The Winternitz scheme is a well-documented[4][5] scheme whose description is beyond the scope of this document. However, of relevance is the relationship between the W-OTS "security parameter" n (the bit-length of H ) and it's "security level" which is generally n/2 . This follows from the fact that if a brute-force attack on H requires 2^n hash rounds then a birthday attack requires 2^(n/2)[2] hash rounds. By eliminating the birthday attack, and assuming no such other class of attacks exist for H , the security level of the scheme is restored back to that of a brute-force attack on H which is n.

W-OTS+ achieves a similar security enhancement through obfuscation of pre-images in the hashing chains, however they are performed during the chaining function which adds an overhead (significant in some implementations). W-OTS# is similar to W-OTS+ in this regard except it only obfuscates the message-digest once via an HMAC (keyed with the salt) and uses the standard W-OTS chaining function, which is faster than W-OTS+. Despite the concatenation of the salt to the signature, the overall signature size decreases by virtue of selecting a shorter hash function H .

3. W-OTS#

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The W-OTS# construction is identical to a standard W-OTS construction for Winternitz parameter w and cryptographic hash function H. The security parameter n is inferred from the the bitlength of H. In W-OTS, a message-digest md is computed as md=H(message) . During signing, digits of base 2^w are read from md and signed in a Winternitz chain. In W-OTS#, the message-digest md is replaced with the "sig-mac" smac defined as:

3.1 Signature Message Authentication Code (SMAC)

1: smac = SMAC(m, salt)
2:      = HMAC(H(m), salt)
3:      = H(Salt || H(Salt || H(m)))

The salt is concatenated to the signature and used to compute smac during verification.

3.2 Salt

The Salt is generated by the signer using cryptographic random number generator. The length of the Salt is n bits which is the minimum value required to nullify a birthday attack (proven
below). The salt is defined as:

1: Salt = {0,1}^n   (i.e. n cryptographically random bits)

3.2.2 Proof

1. A birthday-collision is expected after 1.25 * SQRT(U)[2] hashing rounds where U is maximum hashing rounds ever required (non-repeating).
2. In W-OTS, U=2^n where n is the security parameter (bits-length of H) and thus (1) becomes 1.25 * 2^(n/2) .
3. In W-OTS#, adding a d -bit salt hardens a birthday-collision to A = 1.25 * 2^((n+d)/2)rounds. This follows from the fact that an attacker must scan for collision (HMAC(H(m_i),Salt), HMAC(H(M_j), Salt)) which involves d more bits (whereas in W-OTS they just scan for (H(m_i), H(M_j)) ).
4. A brute-force attack on H requires B = 2^n hashing rounds[2].
5. We need to choose d such A = B, since we only need to harden a birthday attack to match that of a brute-force attack. Hardening beyond is redundant since the security level of the scheme is only as strong as the weakest attack vector.
6. Evaluating (5) gives d = 2 ln(0.8)/ln(0.2) + n = 0.2773 + n which is approximately n.
7. Thus choosing d=n is sufficient to thwart birthday-attack. QED.

4. Reference Implementation

---

This section contains snippets for the full reference implementation[^6] . The reference implementation is part of the PQC library within the Hydrogen Framework[^7] .

public class WOTSSharp : WOTS {

	public WOTSSharp()
		: this(WOTSSharp.Configuration.Default) {
	}

	public WOTSSharp(int w, bool usePublicKeyHashOptimization = false)
		: this(w, Configuration.Default.HashFunction, usePublicKeyHashOptimization) {
	}

	public WOTSSharp(int w, CHF hashFunction, bool usePublicKeyHashOptimization = false)
		: this(new Configuration(w, hashFunction, usePublicKeyHashOptimization)) {
	}

	public WOTSSharp(Configuration config)
		: base(config) {
	}

	public override byte[,] SignDigest(byte[,] privateKey, ReadOnlySpan<byte> digest)
		=> SignDigest(privateKey, digest, Tools.Crypto.GenerateCryptographicallyRandomBytes(digest.Length));

	public byte[,] SignDigest(byte[,] privateKey, ReadOnlySpan<byte> digest, ReadOnlySpan<byte> seed) {
		Guard.Argument(seed.Length == digest.Length, nameof(seed), "Must be same size as digest");
		var wotsSig = base.SignDigest(privateKey, HMAC(digest, seed));
		Debug.Assert(wotsSig.Length == Config.SignatureSize.Length * Config.SignatureSize.Width);
		seed.CopyTo(wotsSig.GetRow(Config.SignatureSize.Length - 1)); // concat seed to sig
		return wotsSig;
	}

	public override bool VerifyDigest(byte[,] signature, byte[,] publicKey, ReadOnlySpan<byte> digest) {
		Debug.Assert(signature.Length == Config.SignatureSize.Length * Config.SignatureSize.Width);
		var seed = signature.GetRow(Config.SignatureSize.Length - 1);
		return base.VerifyDigest(signature, publicKey, HMAC(digest, seed));
	}

	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	private byte[] SMAC(ReadOnlySpan<byte> message, ReadOnlySpan<byte> seed)
		=> HMAC(ComputeMessageDigest(message), seed);

	private byte[] HMAC(ReadOnlySpan<byte> digest, ReadOnlySpan<byte> seed) {
		using (Hashers.BorrowHasher(Config.HashFunction, out var hasher)) {
			hasher.Transform(seed);
			hasher.Transform(digest);
			var innerHash = hasher.GetResult();
			hasher.Transform(seed);
			hasher.Transform(innerHash);
			return hasher.GetResult();
		}
	}

	public new class Configuration : WOTS.Configuration {
		public new static readonly Configuration Default;

		static Configuration() {
			Default = new Configuration(4, CHF.Blake2b_128, true);
		}

		public Configuration()
	}
}

5. References

---

1. Hülsing, A. "W-OTS+ -Shorter Signatures for Hash-Based Signature Schemes". 2013. Url: https://eprint.iacr.org/2017/965.pdf. Accessed:
   2020-07-22.
2. Wikipedia. "Birthday Attack". Url: https://en.wikipedia.org/wiki/Birthday_attack. Accessed: 2020-07-22
3. Wikipedia. "Birthday Problem". Url: https://en.wikipedia.org/wiki/Birthday_problem. Accessed: 2020-07-22
4. Ralph Merkle. "Secrecy, authentication and public key systems / A certified digital signature". Ph.D. dissertation, Dept. of Electrical
   Engineering, Stanford University, 1979. Url: http://www.merkle.com/papers/Certified1979.pdf
5. Sphere 10 Software. "Winternitz One-Time Signature Scheme (W-OTS)". URL: https://sphere10.com/articles/cryptography/pqc/wots.
6. Sphere 10 Software. PQC Library. Url: https://github.com/Sphere10/Hydrogen/tree/master/src/Hydrogen/Crypto/PQC. Accessed 2023-05-09.
7. Sphere 10 Software. Hydrogen Framework. Url: https://github.com/Sphere10/Hydrogen. Accessed 2023-05-09.