crypto/slhdsa/thash.c

/* Copyright (c) 2024, Google LLC
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
 * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
 * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
 * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */

#include <openssl/base.h>

#include <assert.h>
#include <string.h>

#include <openssl/sha.h>

#include "../internal.h"
#include "./params.h"
#include "./thash.h"


// Internal thash function used by F, H, and T_l (Section 11.2, pages 44-46)
static void slhdsa_thash(uint8_t output[SLHDSA_SHA2_128S_N],
                         const uint8_t *input, size_t input_blocks,
                         const uint8_t pk_seed[SLHDSA_SHA2_128S_N],
                         uint8_t addr[32]) {
  SHA256_CTX sha256;
  SHA256_Init(&sha256);

  // Process pubseed with padding to full block.
  static const uint8_t kZeros[64 - SLHDSA_SHA2_128S_N] = {0};
  SHA256_Update(&sha256, pk_seed, SLHDSA_SHA2_128S_N);
  SHA256_Update(&sha256, kZeros, sizeof(kZeros));
  SHA256_Update(&sha256, addr, SLHDSA_SHA2_128S_SHA256_ADDR_BYTES);
  SHA256_Update(&sha256, input, input_blocks * SLHDSA_SHA2_128S_N);

  uint8_t hash[32];
  SHA256_Final(hash, &sha256);
  OPENSSL_memcpy(output, hash, SLHDSA_SHA2_128S_N);
}

// Implements PRF_msg function (Section 4.1, page 11 and Section 11.2, pages
// 44-46)
void slhdsa_thash_prfmsg(uint8_t output[SLHDSA_SHA2_128S_N],
                         const uint8_t sk_prf[SLHDSA_SHA2_128S_N],
                         const uint8_t entropy[SLHDSA_SHA2_128S_N],
                         const uint8_t header[SLHDSA_M_PRIME_HEADER_LEN],
                         const uint8_t *ctx, size_t ctx_len, const uint8_t *msg,
                         size_t msg_len) {
  // Compute HMAC-SHA256(sk_prf, entropy || header || ctx || msg). We inline
  // HMAC to avoid an allocation.
  uint8_t hmac_key[SHA256_CBLOCK];
  static_assert(SLHDSA_SHA2_128S_N <= SHA256_CBLOCK,
                "HMAC key is larger than block size");
  OPENSSL_memcpy(hmac_key, sk_prf, SLHDSA_SHA2_128S_N);
  for (size_t i = 0; i < SLHDSA_SHA2_128S_N; i++) {
    hmac_key[i] ^= 0x36;
  }
  OPENSSL_memset(hmac_key + SLHDSA_SHA2_128S_N, 0x36,
                 sizeof(hmac_key) - SLHDSA_SHA2_128S_N);

  SHA256_CTX sha_ctx;
  SHA256_Init(&sha_ctx);
  SHA256_Update(&sha_ctx, hmac_key, sizeof(hmac_key));
  SHA256_Update(&sha_ctx, entropy, SLHDSA_SHA2_128S_N);
  if (header) {
    SHA256_Update(&sha_ctx, header, SLHDSA_M_PRIME_HEADER_LEN);
  }
  SHA256_Update(&sha_ctx, ctx, ctx_len);
  SHA256_Update(&sha_ctx, msg, msg_len);
  uint8_t hash[SHA256_DIGEST_LENGTH];
  SHA256_Final(hash, &sha_ctx);

  for (size_t i = 0; i < SLHDSA_SHA2_128S_N; i++) {
    hmac_key[i] ^= 0x36 ^ 0x5c;
  }
  OPENSSL_memset(hmac_key + SLHDSA_SHA2_128S_N, 0x5c,
                 sizeof(hmac_key) - SLHDSA_SHA2_128S_N);

  SHA256_Init(&sha_ctx);
  SHA256_Update(&sha_ctx, hmac_key, sizeof(hmac_key));
  SHA256_Update(&sha_ctx, hash, sizeof(hash));
  SHA256_Final(hash, &sha_ctx);

  // Truncate to SLHDSA_SHA2_128S_N bytes
  OPENSSL_memcpy(output, hash, SLHDSA_SHA2_128S_N);
}

// Implements H_msg function (Section 4.1, page 11 and Section 11.2, pages
// 44-46)
void slhdsa_thash_hmsg(uint8_t output[SLHDSA_SHA2_128S_DIGEST_SIZE],
                       const uint8_t r[SLHDSA_SHA2_128S_N],
                       const uint8_t pk_seed[SLHDSA_SHA2_128S_N],
                       const uint8_t pk_root[SLHDSA_SHA2_128S_N],
                       const uint8_t header[SLHDSA_M_PRIME_HEADER_LEN],
                       const uint8_t *ctx, size_t ctx_len, const uint8_t *msg,
                       size_t msg_len) {
  // MGF1-SHA-256(R || PK.seed || SHA-256(R || PK.seed || PK.root || header ||
  // ctx || M), m) input_buffer stores R || PK_SEED || SHA256(..) || 4-byte
  // index
  uint8_t input_buffer[2 * SLHDSA_SHA2_128S_N + 32 + 4] = {0};
  OPENSSL_memcpy(input_buffer, r, SLHDSA_SHA2_128S_N);
  OPENSSL_memcpy(input_buffer + SLHDSA_SHA2_128S_N, pk_seed,
                 SLHDSA_SHA2_128S_N);

  // Inner hash
  SHA256_CTX sha_ctx;
  SHA256_Init(&sha_ctx);
  SHA256_Update(&sha_ctx, r, SLHDSA_SHA2_128S_N);
  SHA256_Update(&sha_ctx, pk_seed, SLHDSA_SHA2_128S_N);
  SHA256_Update(&sha_ctx, pk_root, SLHDSA_SHA2_128S_N);
  if (header) {
    SHA256_Update(&sha_ctx, header, SLHDSA_M_PRIME_HEADER_LEN);
  }
  SHA256_Update(&sha_ctx, ctx, ctx_len);
  SHA256_Update(&sha_ctx, msg, msg_len);
  // Write directly into the input buffer
  SHA256_Final(input_buffer + 2 * SLHDSA_SHA2_128S_N, &sha_ctx);

  // MGF1-SHA-256
  uint8_t hash[32];
  static_assert(SLHDSA_SHA2_128S_DIGEST_SIZE < sizeof(hash),
                "More MGF1 iterations required");
  SHA256(input_buffer, sizeof(input_buffer), hash);
  OPENSSL_memcpy(output, hash, SLHDSA_SHA2_128S_DIGEST_SIZE);
}

// Implements PRF function (Section 4.1, page 11 and Section 11.2, pages 44-46)
void slhdsa_thash_prf(uint8_t output[SLHDSA_SHA2_128S_N],
                      const uint8_t pk_seed[SLHDSA_SHA2_128S_N],
                      const uint8_t sk_seed[SLHDSA_SHA2_128S_N],
                      uint8_t addr[32]) {
  slhdsa_thash(output, sk_seed, 1, pk_seed, addr);
}

// Implements T_l function for WOTS+ public key compression (Section 4.1, page
// 11 and Section 11.2, pages 44-46)
void slhdsa_thash_tl(uint8_t output[SLHDSA_SHA2_128S_N],
                     const uint8_t input[SLHDSA_SHA2_128S_WOTS_BYTES],
                     const uint8_t pk_seed[SLHDSA_SHA2_128S_N],
                     uint8_t addr[32]) {
  slhdsa_thash(output, input, SLHDSA_SHA2_128S_WOTS_LEN, pk_seed, addr);
}

// Implements H function (Section 4.1, page 11 and Section 11.2, pages 44-46)
void slhdsa_thash_h(uint8_t output[SLHDSA_SHA2_128S_N],
                    const uint8_t input[2 * SLHDSA_SHA2_128S_N],
                    const uint8_t pk_seed[SLHDSA_SHA2_128S_N],
                    uint8_t addr[32]) {
  slhdsa_thash(output, input, 2, pk_seed, addr);
}

// Implements F function (Section 4.1, page 11 and Section 11.2, pages 44-46)
void slhdsa_thash_f(uint8_t output[SLHDSA_SHA2_128S_N],
                    const uint8_t input[SLHDSA_SHA2_128S_N],
                    const uint8_t pk_seed[SLHDSA_SHA2_128S_N],
                    uint8_t addr[32]) {
  slhdsa_thash(output, input, 1, pk_seed, addr);
}

// Implements T_k function for FORS public key compression (Section 4.1, page 11
// and Section 11.2, pages 44-46)
void slhdsa_thash_tk(
    uint8_t output[SLHDSA_SHA2_128S_N],
    const uint8_t input[SLHDSA_SHA2_128S_FORS_TREES * SLHDSA_SHA2_128S_N],
    const uint8_t pk_seed[SLHDSA_SHA2_128S_N], uint8_t addr[32]) {
  slhdsa_thash(output, input, SLHDSA_SHA2_128S_FORS_TREES, pk_seed, addr);
}