Commit message: Reserve option to use BN_mod_exp_mont_consttime in ECDSA.

Submitted by Shay Gueron, Intel Corp.
RT: 3149

Reviewed-by: Rich Salz <rsalz@openssl.org>
(cherry picked from commit f54be179aa4cbbd944728771d7d59ed588158a12)
CVE: CVE-2016-7056
CWE: CWE-320
Vulnerability description: A timing attack flaw was found in OpenSSL 1.0.1u and before that could allow a malicious user with local access to recover ECDSA P-256 private keys.

Project: openssl

static int ecdsa_do_verify(const unsigned char *dgst, int dgst_len,
		const ECDSA_SIG *sig, EC_KEY *eckey)
{
	int ret = -1, i;
	BN_CTX   *ctx;
	BIGNUM   *order, *u1, *u2, *m, *X;
	EC_POINT *point = NULL;
	const EC_GROUP *group;
	const EC_POINT *pub_key;

	/* check input values */
	if (eckey == NULL || (group = EC_KEY_get0_group(eckey)) == NULL ||
	    (pub_key = EC_KEY_get0_public_key(eckey)) == NULL || sig == NULL)
	{
		ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ECDSA_R_MISSING_PARAMETERS);
		return -1;
	}

	ctx = BN_CTX_new();
	if (!ctx)
	{
		ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_MALLOC_FAILURE);
		return -1;
	}
	BN_CTX_start(ctx);
	order = BN_CTX_get(ctx);	
	u1    = BN_CTX_get(ctx);
	u2    = BN_CTX_get(ctx);
	m     = BN_CTX_get(ctx);
	X     = BN_CTX_get(ctx);
	if (!X)
	{
		ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_BN_LIB);
		goto err;
	}
	
	if (!EC_GROUP_get_order(group, order, ctx))
	{
		ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_EC_LIB);
		goto err;
	}

	if (BN_is_zero(sig->r)          || BN_is_negative(sig->r) || 
	    BN_ucmp(sig->r, order) >= 0 || BN_is_zero(sig->s)  ||
	    BN_is_negative(sig->s)      || BN_ucmp(sig->s, order) >= 0)
	{
		ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ECDSA_R_BAD_SIGNATURE);
		ret = 0;	/* signature is invalid */
		goto err;
	}
	/* calculate tmp1 = inv(S) mod order */
	if (!BN_mod_inverse(u2, sig->s, order, ctx))
	{
		ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_BN_LIB);
		goto err;
	}
	/* digest -> m */
	i = BN_num_bits(order);
	/* Need to truncate digest if it is too long: first truncate whole
	 * bytes.
	 */
	if (8 * dgst_len > i)
		dgst_len = (i + 7)/8;
	if (!BN_bin2bn(dgst, dgst_len, m))
	{
		ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_BN_LIB);
		goto err;
	}
	/* If still too long truncate remaining bits with a shift */
	if ((8 * dgst_len > i) && !BN_rshift(m, m, 8 - (i & 0x7)))
	{
		ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_BN_LIB);
		goto err;
	}
	/* u1 = m * tmp mod order */
	if (!BN_mod_mul(u1, m, u2, order, ctx))
	{
		ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_BN_LIB);
		goto err;
	}
	/* u2 = r * w mod q */
	if (!BN_mod_mul(u2, sig->r, u2, order, ctx))
	{
		ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_BN_LIB);
		goto err;
	}

	if ((point = EC_POINT_new(group)) == NULL)
	{
		ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_MALLOC_FAILURE);
		goto err;
	}
	if (!EC_POINT_mul(group, point, u1, pub_key, u2, ctx))
	{
		ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_EC_LIB);
		goto err;
	}
	if (EC_METHOD_get_field_type(EC_GROUP_method_of(group)) == NID_X9_62_prime_field)
	{
		if (!EC_POINT_get_affine_coordinates_GFp(group,
			point, X, NULL, ctx))
		{
			ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_EC_LIB);
			goto err;
		}
	}
#ifndef OPENSSL_NO_EC2M
	else /* NID_X9_62_characteristic_two_field */
	{
		if (!EC_POINT_get_affine_coordinates_GF2m(group,
			point, X, NULL, ctx))
		{
			ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_EC_LIB);
			goto err;
		}
	}
#endif	
	if (!BN_nnmod(u1, X, order, ctx))
	{
		ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_BN_LIB);
		goto err;
	}
	/*  if the signature is correct u1 is equal to sig->r */
	ret = (BN_ucmp(u1, sig->r) == 0);
err:
	BN_CTX_end(ctx);
	BN_CTX_free(ctx);
	if (point)
		EC_POINT_free(point);
	return ret;
}