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      1 /* LibTomCrypt, modular cryptographic library -- Tom St Denis
      2  *
      3  * LibTomCrypt is a library that provides various cryptographic
      4  * algorithms in a highly modular and flexible manner.
      5  *
      6  * The library is free for all purposes without any express
      7  * guarantee it works.
      8  *
      9  * Tom St Denis, tomstdenis (at) gmail.com, http://libtomcrypt.com
     10  */
     11 
     12 /* Implements ECC over Z/pZ for curve y^2 = x^3 - 3x + b
     13  *
     14  * All curves taken from NIST recommendation paper of July 1999
     15  * Available at http://csrc.nist.gov/cryptval/dss.htm
     16  */
     17 #include "tomcrypt.h"
     18 
     19 /**
     20   @file ecc_shared_secret.c
     21   ECC Crypto, Tom St Denis
     22 */
     23 
     24 #ifdef MECC
     25 
     26 /**
     27   Create an ECC shared secret between two keys
     28   @param private_key      The private ECC key
     29   @param public_key       The public key
     30   @param out              [out] Destination of the shared secret (Conforms to EC-DH from ANSI X9.63)
     31   @param outlen           [in/out] The max size and resulting size of the shared secret
     32   @return CRYPT_OK if successful
     33 */
     34 int ecc_shared_secret(ecc_key *private_key, ecc_key *public_key,
     35                       unsigned char *out, unsigned long *outlen)
     36 {
     37    unsigned long  x;
     38    ecc_point     *result;
     39    void          *prime;
     40    int            err;
     41 
     42    LTC_ARGCHK(private_key != NULL);
     43    LTC_ARGCHK(public_key  != NULL);
     44    LTC_ARGCHK(out         != NULL);
     45    LTC_ARGCHK(outlen      != NULL);
     46 
     47    /* type valid? */
     48    if (private_key->type != PK_PRIVATE) {
     49       return CRYPT_PK_NOT_PRIVATE;
     50    }
     51 
     52    if (ltc_ecc_is_valid_idx(private_key->idx) == 0 || ltc_ecc_is_valid_idx(public_key->idx) == 0) {
     53       return CRYPT_INVALID_ARG;
     54    }
     55 
     56    if (XSTRCMP(private_key->dp->name, public_key->dp->name) != 0) {
     57       return CRYPT_PK_TYPE_MISMATCH;
     58    }
     59 
     60    /* make new point */
     61    result = ltc_ecc_new_point();
     62    if (result == NULL) {
     63       return CRYPT_MEM;
     64    }
     65 
     66    if ((err = mp_init(&prime)) != CRYPT_OK) {
     67       ltc_ecc_del_point(result);
     68       return err;
     69    }
     70 
     71    if ((err = mp_read_radix(prime, (char *)private_key->dp->prime, 16)) != CRYPT_OK)                               { goto done; }
     72    if ((err = ltc_mp.ecc_ptmul(private_key->k, &public_key->pubkey, result, prime, 1)) != CRYPT_OK)                { goto done; }
     73 
     74    x = (unsigned long)mp_unsigned_bin_size(prime);
     75    if (*outlen < x) {
     76       *outlen = x;
     77       err = CRYPT_BUFFER_OVERFLOW;
     78       goto done;
     79    }
     80    zeromem(out, x);
     81    if ((err = mp_to_unsigned_bin(result->x, out + (x - mp_unsigned_bin_size(result->x))))   != CRYPT_OK)           { goto done; }
     82 
     83    err     = CRYPT_OK;
     84    *outlen = x;
     85 done:
     86    mp_clear(prime);
     87    ltc_ecc_del_point(result);
     88    return err;
     89 }
     90 
     91 #endif
     92 /* $Source: /cvs/libtom/libtomcrypt/src/pk/ecc/ecc_shared_secret.c,v $ */
     93 /* $Revision: 1.8 $ */
     94 /* $Date: 2006/12/04 02:19:48 $ */
     95 
     96