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/* LibTomCrypt, modular cryptographic library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */
#include "tomcrypt_private.h"
/**
@file dsa_verify_hash.c
DSA implementation, verify a signature, Tom St Denis
*/
#ifdef LTC_MDSA
/**
Verify a DSA signature
@param r DSA "r" parameter
@param s DSA "s" parameter
@param hash The hash that was signed
@param hashlen The length of the hash that was signed
@param stat [out] The result of the signature verification, 1==valid, 0==invalid
@param key The corresponding public DSA key
@return CRYPT_OK if successful (even if the signature is invalid)
*/
int dsa_verify_hash_raw( void *r, void *s,
const unsigned char *hash, unsigned long hashlen,
int *stat, const dsa_key *key)
{
void *w, *v, *u1, *u2;
int err;
LTC_ARGCHK(r != NULL);
LTC_ARGCHK(s != NULL);
LTC_ARGCHK(stat != NULL);
LTC_ARGCHK(key != NULL);
/* default to invalid signature */
*stat = 0;
/* init our variables */
if ((err = mp_init_multi(&w, &v, &u1, &u2, NULL)) != CRYPT_OK) {
return err;
}
/* neither r or s can be null or >q*/
if (mp_cmp_d(r, 0) != LTC_MP_GT || mp_cmp_d(s, 0) != LTC_MP_GT || mp_cmp(r, key->q) != LTC_MP_LT || mp_cmp(s, key->q) != LTC_MP_LT) {
err = CRYPT_INVALID_PACKET;
goto error;
}
/* FIPS 186-4 4.7: use leftmost min(bitlen(q), bitlen(hash)) bits of 'hash' */
hashlen = MIN(hashlen, (unsigned long)(key->qord));
/* w = 1/s mod q */
if ((err = mp_invmod(s, key->q, w)) != CRYPT_OK) { goto error; }
/* u1 = m * w mod q */
if ((err = mp_read_unsigned_bin(u1, (unsigned char *)hash, hashlen)) != CRYPT_OK) { goto error; }
if ((err = mp_mulmod(u1, w, key->q, u1)) != CRYPT_OK) { goto error; }
/* u2 = r*w mod q */
if ((err = mp_mulmod(r, w, key->q, u2)) != CRYPT_OK) { goto error; }
/* v = g^u1 * y^u2 mod p mod q */
if ((err = mp_exptmod(key->g, u1, key->p, u1)) != CRYPT_OK) { goto error; }
if ((err = mp_exptmod(key->y, u2, key->p, u2)) != CRYPT_OK) { goto error; }
if ((err = mp_mulmod(u1, u2, key->p, v)) != CRYPT_OK) { goto error; }
if ((err = mp_mod(v, key->q, v)) != CRYPT_OK) { goto error; }
/* if r = v then we're set */
if (mp_cmp(r, v) == LTC_MP_EQ) {
*stat = 1;
}
err = CRYPT_OK;
error:
mp_clear_multi(w, v, u1, u2, NULL);
return err;
}
/**
Verify a DSA signature
@param sig The signature
@param siglen The length of the signature (octets)
@param hash The hash that was signed
@param hashlen The length of the hash that was signed
@param stat [out] The result of the signature verification, 1==valid, 0==invalid
@param key The corresponding public DSA key
@return CRYPT_OK if successful (even if the signature is invalid)
*/
int dsa_verify_hash(const unsigned char *sig, unsigned long siglen,
const unsigned char *hash, unsigned long hashlen,
int *stat, const dsa_key *key)
{
int err;
void *r, *s;
ltc_asn1_list sig_seq[2];
unsigned long reallen = 0;
LTC_ARGCHK(stat != NULL);
*stat = 0; /* must be set before the first return */
if ((err = mp_init_multi(&r, &s, NULL)) != CRYPT_OK) {
return err;
}
LTC_SET_ASN1(sig_seq, 0, LTC_ASN1_INTEGER, r, 1UL);
LTC_SET_ASN1(sig_seq, 1, LTC_ASN1_INTEGER, s, 1UL);
err = der_decode_sequence_strict(sig, siglen, sig_seq, 2);
if (err != CRYPT_OK) {
goto LBL_ERR;
}
err = der_length_sequence(sig_seq, 2, &reallen);
if (err != CRYPT_OK || reallen != siglen) {
goto LBL_ERR;
}
/* do the op */
err = dsa_verify_hash_raw(r, s, hash, hashlen, stat, key);
LBL_ERR:
mp_clear_multi(r, s, NULL);
return err;
}
#endif
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