ecc_sign_hash.c

/* LibTomCrypt, modular cryptographic library -- Tom St Denis
 *
 * LibTomCrypt is a library that provides various cryptographic
 * algorithms in a highly modular and flexible manner.
 *
 * The library is free for all purposes without any express
 * guarantee it works.
 */

#include "tomcrypt.h"

#ifdef LTC_MECC

/**
  @file ecc_sign_hash.c
  ECC Crypto, Tom St Denis
*/

static int _ecc_sign_hash(const unsigned char *in,  unsigned long inlen,
                                unsigned char *out, unsigned long *outlen,
                                prng_state *prng, int wprng, ecc_key *key, int sigformat)
{
   ecc_key       pubkey;
   void          *r, *s, *e, *p, *b;
   int           err, max_iterations = LTC_PK_MAX_RETRIES;
   unsigned long pbits, pbytes, i, shift_right;
   unsigned char ch, buf[MAXBLOCKSIZE];

   LTC_ARGCHK(in     != NULL);
   LTC_ARGCHK(out    != NULL);
   LTC_ARGCHK(outlen != NULL);
   LTC_ARGCHK(key    != NULL);

   /* is this a private key? */
   if (key->type != PK_PRIVATE) {
      return CRYPT_PK_NOT_PRIVATE;
   }

   /* is the IDX valid ?  */
   if (ltc_ecc_is_valid_idx(key->idx) != 1) {
      return CRYPT_PK_INVALID_TYPE;
   }

   if ((err = prng_is_valid(wprng)) != CRYPT_OK) {
      return err;
   }

   /* init the bignums */
   if ((err = mp_init_multi(&r, &s, &p, &e, &b, NULL)) != CRYPT_OK) {
      return err;
   }
   if ((err = mp_read_radix(p, (char *)key->dp->order, 16)) != CRYPT_OK)              { goto errnokey; }

   /* get the hash and load it as a bignum into 'e' */
   pbits = mp_count_bits(p);
   pbytes = (pbits+7) >> 3;
   if (pbits > inlen*8) {
      if ((err = mp_read_unsigned_bin(e, (unsigned char *)in, inlen)) != CRYPT_OK)    { goto errnokey; }
   }
   else if (pbits % 8 == 0) {
      if ((err = mp_read_unsigned_bin(e, (unsigned char *)in, pbytes)) != CRYPT_OK)   { goto errnokey; }
   }
   else {
      shift_right = 8 - pbits % 8;
      for (i=0, ch=0; i<pbytes; i++) {
        buf[i] = ch;
        ch = (in[i] << (8-shift_right));
        buf[i] = buf[i] ^ (in[i] >> shift_right);
      }
      if ((err = mp_read_unsigned_bin(e, (unsigned char *)buf, pbytes)) != CRYPT_OK)  { goto errnokey; }
   }

   /* make up a key and export the public copy */
   do {
      if ((err = ecc_make_key_ex(prng, wprng, &pubkey, key->dp)) != CRYPT_OK) {
         goto errnokey;
      }

      /* find r = x1 mod n */
      if ((err = mp_mod(pubkey.pubkey.x, p, r)) != CRYPT_OK)                          { goto error; }

      if (mp_iszero(r) == LTC_MP_YES) {
         ecc_free(&pubkey);
      } else {
         if ((err = rand_bn_upto(b, p, prng, wprng)) != CRYPT_OK)          { goto error; } /* b = blinding value */
         /* find s = (e + xr)/k */
         if ((err = mp_mulmod(pubkey.k, b, p, pubkey.k)) != CRYPT_OK)      { goto error; } /* k = kb */
         if ((err = mp_invmod(pubkey.k, p, pubkey.k)) != CRYPT_OK)         { goto error; } /* k = 1/kb */
         if ((err = mp_mulmod(key->k, r, p, s)) != CRYPT_OK)               { goto error; } /* s = xr */
         if ((err = mp_mulmod(pubkey.k, s, p, s)) != CRYPT_OK)             { goto error; } /* s = xr/kb */
         if ((err = mp_mulmod(pubkey.k, e, p, e)) != CRYPT_OK)             { goto error; } /* e = e/kb */
         if ((err = mp_add(e, s, s)) != CRYPT_OK)                          { goto error; } /* s = e/kb + xr/kb */
         if ((err = mp_mulmod(s, b, p, s)) != CRYPT_OK)                    { goto error; } /* s = b(e/kb + xr/kb) = (e + xr)/k */
         ecc_free(&pubkey);
         if (mp_iszero(s) == LTC_MP_NO) {
            break;
         }
      }
   } while (--max_iterations > 0);

   if (max_iterations == 0) {
      goto errnokey;
   }

   if (sigformat == 1) {
      /* RFC7518 format */
      if (*outlen < 2*pbytes) { err = CRYPT_MEM; goto errnokey; }
      zeromem(out, 2*pbytes);
      i = mp_unsigned_bin_size(r);
      if ((err = mp_to_unsigned_bin(r, out + (pbytes - i)))   != CRYPT_OK) { goto errnokey; }
      i = mp_unsigned_bin_size(s);
      if ((err = mp_to_unsigned_bin(s, out + (2*pbytes - i))) != CRYPT_OK) { goto errnokey; }
      *outlen = 2*pbytes;
      err = CRYPT_OK;
   }
   else {
      /* store as ASN.1 SEQUENCE { r, s -- integer } */
      err = der_encode_sequence_multi(out, outlen,
                               LTC_ASN1_INTEGER, 1UL, r,
                               LTC_ASN1_INTEGER, 1UL, s,
                               LTC_ASN1_EOL, 0UL, NULL);
   }
   goto errnokey;
error:
   ecc_free(&pubkey);
errnokey:
   mp_clear_multi(r, s, p, e, b, NULL);
   return err;
}

/**
  Sign a message digest
  @param in        The message digest to sign
  @param inlen     The length of the digest
  @param out       [out] The destination for the signature
  @param outlen    [in/out] The max size and resulting size of the signature
  @param prng      An active PRNG state
  @param wprng     The index of the PRNG you wish to use
  @param key       A private ECC key
  @return CRYPT_OK if successful
*/
int ecc_sign_hash(const unsigned char *in,  unsigned long inlen,
                        unsigned char *out, unsigned long *outlen,
                        prng_state *prng, int wprng, ecc_key *key)
{
   return _ecc_sign_hash(in, inlen, out, outlen, prng, wprng, key, 0);
}

/**
  Sign a message digest in RFC7518 format
  @param in        The message digest to sign
  @param inlen     The length of the digest
  @param out       [out] The destination for the signature
  @param outlen    [in/out] The max size and resulting size of the signature
  @param prng      An active PRNG state
  @param wprng     The index of the PRNG you wish to use
  @param key       A private ECC key
  @return CRYPT_OK if successful
*/
int ecc_sign_hash_rfc7518(const unsigned char *in,  unsigned long inlen,
                                unsigned char *out, unsigned long *outlen,
                                prng_state *prng, int wprng, ecc_key *key)
{
   return _ecc_sign_hash(in, inlen, out, outlen, prng, wprng, key, 1);
}

#endif