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/* 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.
*
* Tom St Denis, tomstdenis@gmail.com, http://libtomcrypt.com
*/
#include "tomcrypt.h"
/**
@file dsa_verify_key.c
DSA implementation, verify a key, Tom St Denis
*/
#ifdef MDSA
/**
Verify a DSA key for validity
@param key The key to verify
@param stat [out] Result of test, 1==valid, 0==invalid
@return CRYPT_OK if successful
*/
int dsa_verify_key(dsa_key *key, int *stat)
{
void *tmp, *tmp2;
int res, err;
LTC_ARGCHK(key != NULL);
LTC_ARGCHK(stat != NULL);
/* default to an invalid key */
*stat = 0;
/* first make sure key->q and key->p are prime */
if ((err = mp_prime_is_prime(key->q, 8, &res)) != CRYPT_OK) {
return err;
}
if (res == 0) {
return CRYPT_OK;
}
if ((err = mp_prime_is_prime(key->p, 8, &res)) != CRYPT_OK) {
return err;
}
if (res == 0) {
return CRYPT_OK;
}
/* now make sure that g is not -1, 0 or 1 and <p */
if (mp_cmp_d(key->g, 0) == LTC_MP_EQ || mp_cmp_d(key->g, 1) == LTC_MP_EQ) {
return CRYPT_OK;
}
if ((err = mp_init_multi(&tmp, &tmp2, NULL)) != CRYPT_OK) { return err; }
if ((err = mp_sub_d(key->p, 1, tmp)) != CRYPT_OK) { goto error; }
if (mp_cmp(tmp, key->g) == LTC_MP_EQ || mp_cmp(key->g, key->p) != LTC_MP_LT) {
err = CRYPT_OK;
goto error;
}
/* 1 < y < p-1 */
if (!(mp_cmp_d(key->y, 1) == LTC_MP_GT && mp_cmp(key->y, tmp) == LTC_MP_LT)) {
err = CRYPT_OK;
goto error;
}
/* now we have to make sure that g^q = 1, and that p-1/q gives 0 remainder */
if ((err = mp_div(tmp, key->q, tmp, tmp2)) != CRYPT_OK) { goto error; }
if (mp_iszero(tmp2) != LTC_MP_YES) {
err = CRYPT_OK;
goto error;
}
if ((err = mp_exptmod(key->g, key->q, key->p, tmp)) != CRYPT_OK) { goto error; }
if (mp_cmp_d(tmp, 1) != LTC_MP_EQ) {
err = CRYPT_OK;
goto error;
}
/* now we have to make sure that y^q = 1, this makes sure y \in g^x mod p */
if ((err = mp_exptmod(key->y, key->q, key->p, tmp)) != CRYPT_OK) { goto error; }
if (mp_cmp_d(tmp, 1) != LTC_MP_EQ) {
err = CRYPT_OK;
goto error;
}
/* at this point we are out of tests ;-( */
err = CRYPT_OK;
*stat = 1;
error:
mp_clear_multi(tmp, tmp2, NULL);
return err;
}
#endif
/* $Source: /cvs/libtom/libtomcrypt/src/pk/dsa/dsa_verify_key.c,v $ */
/* $Revision: 1.6 $ */
/* $Date: 2006/12/04 03:18:43 $ */
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