text: /* 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.
   */
  
  /* Implements ECC over Z/pZ for curve y^2 = x^3 - 3x + b
   *
   * All curves taken from NIST recommendation paper of July 1999
   * Available at http://csrc.nist.gov/cryptval/dss.htm
   */
  #include "tomcrypt.h"
  
  /**
    @file ecc_shared_secret.c
    ECC Crypto, Tom St Denis
  */
  
  #ifdef LTC_MECC
  
  /**
    Create an ECC shared secret between two keys
    @param private_key      The private ECC key
    @param public_key       The public key
    @param out              [out] Destination of the shared secret (Conforms to EC-DH from ANSI X9.63)
    @param outlen           [in/out] The max size and resulting size of the shared secret
    @return CRYPT_OK if successful
  */
  int ecc_shared_secret(ecc_key *private_key, ecc_key *public_key,
                        unsigned char *out, unsigned long *outlen)
  {
     unsigned long  x;
     ecc_point     *result;
     void          *prime;
     int            err;
  
     LTC_ARGCHK(private_key != NULL);
     LTC_ARGCHK(public_key  != NULL);
     LTC_ARGCHK(out         != NULL);
     LTC_ARGCHK(outlen      != NULL);
  
     /* type valid? */
     if (private_key->type != PK_PRIVATE) {
        return CRYPT_PK_NOT_PRIVATE;
     }
  
     if (ltc_ecc_is_valid_idx(private_key->idx) == 0 || ltc_ecc_is_valid_idx(public_key->idx) == 0) {
        return CRYPT_INVALID_ARG;
     }
  
     if (XSTRCMP(private_key->dp->name, public_key->dp->name) != 0) {
        return CRYPT_PK_TYPE_MISMATCH;
     }
  
     /* make new point */
     result = ltc_ecc_new_point();
     if (result == NULL) {
        return CRYPT_MEM;
     }
  
     if ((err = mp_init(&prime)) != CRYPT_OK) {
        ltc_ecc_del_point(result);
        return err;
     }
  
     if ((err = mp_read_radix(prime, (char *)private_key->dp->prime, 16)) != CRYPT_OK)                               { goto done; }
     if ((err = ltc_mp.ecc_ptmul(private_key->k, &public_key->pubkey, result, prime, 1)) != CRYPT_OK)                { goto done; }
  
     x = (unsigned long)mp_unsigned_bin_size(prime);
     if (*outlen < x) {
        *outlen = x;
        err = CRYPT_BUFFER_OVERFLOW;
        goto done;
     }
     zeromem(out, x);
     if ((err = mp_to_unsigned_bin(result->x, out + (x - mp_unsigned_bin_size(result->x))))   != CRYPT_OK)           { goto done; }
  
     err     = CRYPT_OK;
     *outlen = x;
  done:
     mp_clear(prime);
     ltc_ecc_del_point(result);
     return err;
  }
  
  #endif
  /* ref:         $Format:%D$ */
  /* git commit:  $Format:%H$ */
  /* commit time: $Format:%ai$ */
  
  