crypto_edx25519.c

Go to the documentation of this file.

/*
     This file is part of GNUnet.
     Copyright (C) 2022 GNUnet e.V.
 
     GNUnet is free software: you can redistribute it and/or modify it
     under the terms of the GNU Affero General Public License as published
     by the Free Software Foundation, either version 3 of the License,
     or (at your option) any later version.
 
     GNUnet is distributed in the hope that it will be useful, but
     WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
     Affero General Public License for more details.
 
     You should have received a copy of the GNU Affero General Public License
     along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
     SPDX-License-Identifier: AGPL3.0-or-later
 */
 
#include "platform.h"
#include <gcrypt.h>
#include <sodium.h>
#include "gnunet_util_lib.h"
 
#define CURVE "Ed25519"
 
void
GNUNET_CRYPTO_edx25519_key_clear (struct GNUNET_CRYPTO_Edx25519PrivateKey *pk)
{
  memset (pk, 0, sizeof(struct GNUNET_CRYPTO_Edx25519PrivateKey));
}
 
 
void
GNUNET_CRYPTO_edx25519_key_create_from_seed (
  const void *seed,
  size_t seedsize,
  struct GNUNET_CRYPTO_Edx25519PrivateKey *pk)
{
 
  GNUNET_static_assert (sizeof(*pk) == sizeof(struct GNUNET_HashCode));
  GNUNET_CRYPTO_hash (seed,
                      seedsize,
                      (struct GNUNET_HashCode *) pk);
 
  /* Clamp the first half of the key. The second half is used in the signature
   * process. */
  pk->a[0] &= 248;
  pk->a[31] &= 127;
  pk->a[31] |= 64;
}
 
 
void
GNUNET_CRYPTO_edx25519_key_create (
  struct GNUNET_CRYPTO_Edx25519PrivateKey *pk)
{
  char seed[256 / 8];
  GNUNET_CRYPTO_random_block (GNUNET_CRYPTO_QUALITY_NONCE,
                              seed,
                              sizeof (seed));
  GNUNET_CRYPTO_edx25519_key_create_from_seed (seed,
                                               sizeof(seed),
                                               pk);
}
 
 
void
GNUNET_CRYPTO_edx25519_key_get_public (
  const struct GNUNET_CRYPTO_Edx25519PrivateKey *priv,
  struct GNUNET_CRYPTO_Edx25519PublicKey *pub)
{
  crypto_scalarmult_ed25519_base_noclamp (pub->q_y,
                                          priv->a);
}
 
 
enum GNUNET_GenericReturnValue
GNUNET_CRYPTO_edx25519_sign_ (
  const struct GNUNET_CRYPTO_Edx25519PrivateKey *priv,
  const struct GNUNET_CRYPTO_EccSignaturePurpose *purpose,
  struct GNUNET_CRYPTO_Edx25519Signature *sig)
{
 
  crypto_hash_sha512_state hs;
  unsigned char r[64];
  unsigned char hram[64];
  unsigned char P[32];
  unsigned char r_mod[64];
  unsigned char R[32];
  unsigned char tmp[32];
 
  crypto_hash_sha512_init (&hs);
 
  crypto_scalarmult_ed25519_base_noclamp (P,
                                          priv->a);
 
  crypto_hash_sha512_update (&hs,
                             priv->b,
                             sizeof(priv->b));
  crypto_hash_sha512_update (&hs,
                             (uint8_t*) purpose,
                             ntohl (purpose->size));
  crypto_hash_sha512_final (&hs,
                            r);
 
  memcpy (sig->s, P, 32);
 
  crypto_core_ed25519_scalar_reduce (r_mod, r);
 
  crypto_scalarmult_ed25519_base_noclamp (R, r_mod);
  memcpy (sig->r, R, sizeof (R));
 
  crypto_hash_sha512_init (&hs);
  crypto_hash_sha512_update (&hs, (uint8_t*) sig, 64);
  crypto_hash_sha512_update (&hs, (uint8_t*) purpose,
                             ntohl (purpose->size));
  crypto_hash_sha512_final (&hs, hram);
 
  unsigned char hram_mod[64];
  crypto_core_ed25519_scalar_reduce (hram_mod, hram);
 
  crypto_core_ed25519_scalar_mul (tmp, hram_mod, priv->a);
  crypto_core_ed25519_scalar_add (sig->s, tmp, r_mod);
 
  sodium_memzero (r, sizeof (r));
  sodium_memzero (r_mod, sizeof (r_mod));
 
  return GNUNET_OK;
}
 
 
enum GNUNET_GenericReturnValue
GNUNET_CRYPTO_edx25519_verify_ (
  uint32_t purpose,
  const struct GNUNET_CRYPTO_EccSignaturePurpose *validate,
  const struct GNUNET_CRYPTO_Edx25519Signature *sig,
  const struct GNUNET_CRYPTO_Edx25519PublicKey *pub)
{
  const unsigned char *m = (const void *) validate;
  size_t mlen = ntohl (validate->size);
  const unsigned char *s = (const void *) sig;
 
  int res;
 
  if (purpose != ntohl (validate->purpose))
    return GNUNET_SYSERR; /* purpose mismatch */
 
  res = crypto_sign_verify_detached (s, m, mlen, pub->q_y);
  return (res == 0) ? GNUNET_OK : GNUNET_SYSERR;
}
 
 
static void
derive_h (
  const struct GNUNET_CRYPTO_Edx25519PublicKey *pub,
  const void *seed,
  size_t seedsize,
  struct GNUNET_HashCode *phc)
{
  static const char *const salt = "edx25519-derivation";
 
  GNUNET_CRYPTO_kdf (/* output*/
                     phc, sizeof(*phc),
                     /* salt */
                     seed, seedsize,
                     /* ikm */
                     pub, sizeof(*pub),
                     /* ctx chunks*/
                     salt, strlen (salt),
                     NULL, 0);
 
}
 
 
void
GNUNET_CRYPTO_edx25519_private_key_derive (
  const struct GNUNET_CRYPTO_Edx25519PrivateKey *priv,
  const void *seed,
  size_t seedsize,
  struct GNUNET_CRYPTO_Edx25519PrivateKey *result)
{
  struct GNUNET_CRYPTO_Edx25519PublicKey pub;
  struct GNUNET_HashCode hc;
  uint8_t a[32];
  uint8_t eight[32] = { 8 };
  uint8_t eight_inv[32];
  uint8_t h[64] = { 0 };
 
  GNUNET_CRYPTO_edx25519_key_get_public (priv, &pub);
 
  /* Get h mod n */
  derive_h (&pub,
            seed,
            seedsize,
            &hc);
 
  memcpy (h, &hc, 64);
  crypto_core_ed25519_scalar_reduce (h,
                                     h);
#ifdef CHECK_RARE_CASES
  {
    char zero[32] = { 0 };
    char one[32] = { 1 };
 
    GNUNET_assert (0 != memcmp (zero, h, 32));
    GNUNET_assert (0 != memcmp (one, h, 32));
  }
#endif
 
  GNUNET_assert (0 == crypto_core_ed25519_scalar_invert (eight_inv,
                                                         eight));
 
  crypto_core_ed25519_scalar_mul (a, priv->a, eight_inv);
  crypto_core_ed25519_scalar_mul (a, a, h);
  crypto_core_ed25519_scalar_mul (a, a, eight);
 
#ifdef CHECK_RARE_CASES
  /* The likelihood for a' == 0 or a' == 1 is neglegible */
  {
    char zero[32] = { 0 };
    char one[32] = { 1 };
 
    GNUNET_assert (0 != memcmp (zero, a, 32));
    GNUNET_assert (0 != memcmp (one, a, 32));
  }
#endif
 
  /* We hash the derived "h" parameter with the other half of the expanded
   * private key (that is: priv->b). This ensures that for signature
   * generation, the "R" is derived from the same derivation path as "h" and is
   * not reused. */
  {
    struct GNUNET_HashCode hcb;
    struct GNUNET_HashContext *hctx;
 
    hctx = GNUNET_CRYPTO_hash_context_start ();
    GNUNET_CRYPTO_hash_context_read (hctx, priv->b, sizeof(priv->b));
    GNUNET_CRYPTO_hash_context_read (hctx, (unsigned char*) &hc, sizeof (hc));
    GNUNET_CRYPTO_hash_context_finish (hctx, &hcb);
 
    /* Truncate result, effectively doing SHA512/256 */
    for (size_t i = 0; i < 32; i++)
      result->b[i] = ((unsigned char *) &hcb)[i];
  }
 
  for (size_t i = 0; i < 32; i++)
    result->a[i] = a[i];
 
  sodium_memzero (a, sizeof(a));
}
 
 
void
GNUNET_CRYPTO_edx25519_public_key_derive (
  const struct GNUNET_CRYPTO_Edx25519PublicKey *pub,
  const void *seed,
  size_t seedsize,
  struct GNUNET_CRYPTO_Edx25519PublicKey *result)
{
  struct GNUNET_HashCode hc;
  uint8_t h[64] = { 0 };
 
  derive_h (pub,
            seed,
            seedsize,
            &hc);
  memcpy (h,
          &hc,
          64);
  crypto_core_ed25519_scalar_reduce (h,
                                     h);
  GNUNET_assert (0 == crypto_scalarmult_ed25519_noclamp (result->q_y,
                                                         h,
                                                         pub->q_y));
}