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openssl/crypto/ec/ecdsa_ossl.c
Matt Caswell a3e9d5aa98 Add blinding to an ECDSA signature 
Keegan Ryan (NCC Group) has demonstrated a side channel attack on an
ECDSA signature operation. During signing the signer calculates:

s:= k^-1 * (m + r * priv_key) mod order

The addition operation above provides a sufficient signal for a
flush+reload attack to derive the private key given sufficient signature
operations.

As a mitigation (based on a suggestion from Keegan) we add blinding to
the operation so that:

s := k^-1 * blind^-1 (blind * m + blind * r * priv_key) mod order

Since this attack is a localhost side channel only no CVE is assigned.

Reviewed-by: Rich Salz <rsalz@openssl.org>
2018-06-13 16:19:22 +01:00

521 lines
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/*
* Copyright 2002-2018 The OpenSSL Project Authors. All Rights Reserved.
*
* Licensed under the OpenSSL license (the "License"). You may not use
* this file except in compliance with the License. You can obtain a copy
* in the file LICENSE in the source distribution or at
* https://www.openssl.org/source/license.html
*/
#include <string.h>
#include <openssl/err.h>
#include <openssl/obj_mac.h>
#include <openssl/bn.h>
#include <openssl/rand.h>
#include <openssl/ec.h>
#include "ec_lcl.h"
int ossl_ecdsa_sign(int type, const unsigned char *dgst, int dlen,
unsigned char *sig, unsigned int *siglen,
const BIGNUM *kinv, const BIGNUM *r, EC_KEY *eckey)
{
ECDSA_SIG *s;
s = ECDSA_do_sign_ex(dgst, dlen, kinv, r, eckey);
if (s == NULL) {
*siglen = 0;
return 0;
}
*siglen = i2d_ECDSA_SIG(s, &sig);
ECDSA_SIG_free(s);
return 1;
}
static int ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in,
BIGNUM **kinvp, BIGNUM **rp,
const unsigned char *dgst, int dlen)
{
BN_CTX *ctx = NULL;
BIGNUM *k = NULL, *r = NULL, *X = NULL;
const BIGNUM *order;
EC_POINT *tmp_point = NULL;
const EC_GROUP *group;
int ret = 0;
int order_bits;
if (eckey == NULL || (group = EC_KEY_get0_group(eckey)) == NULL) {
ECerr(EC_F_ECDSA_SIGN_SETUP, ERR_R_PASSED_NULL_PARAMETER);
return 0;
}
if (!EC_KEY_can_sign(eckey)) {
ECerr(EC_F_ECDSA_SIGN_SETUP, EC_R_CURVE_DOES_NOT_SUPPORT_SIGNING);
return 0;
}
if (ctx_in == NULL) {
if ((ctx = BN_CTX_new()) == NULL) {
ECerr(EC_F_ECDSA_SIGN_SETUP, ERR_R_MALLOC_FAILURE);
return 0;
}
} else
ctx = ctx_in;
k = BN_new(); /* this value is later returned in *kinvp */
r = BN_new(); /* this value is later returned in *rp */
X = BN_new();
if (k == NULL || r == NULL || X == NULL) {
ECerr(EC_F_ECDSA_SIGN_SETUP, ERR_R_MALLOC_FAILURE);
goto err;
}
if ((tmp_point = EC_POINT_new(group)) == NULL) {
ECerr(EC_F_ECDSA_SIGN_SETUP, ERR_R_EC_LIB);
goto err;
}
order = EC_GROUP_get0_order(group);
if (order == NULL) {
ECerr(EC_F_ECDSA_SIGN_SETUP, ERR_R_EC_LIB);
goto err;
}
/* Preallocate space */
order_bits = BN_num_bits(order);
if (!BN_set_bit(k, order_bits)
|| !BN_set_bit(r, order_bits)
|| !BN_set_bit(X, order_bits))
goto err;
do {
/* get random k */
do
if (dgst != NULL) {
if (!BN_generate_dsa_nonce
(k, order, EC_KEY_get0_private_key(eckey), dgst, dlen,
ctx)) {
ECerr(EC_F_ECDSA_SIGN_SETUP,
EC_R_RANDOM_NUMBER_GENERATION_FAILED);
goto err;
}
} else {
if (!BN_priv_rand_range(k, order)) {
ECerr(EC_F_ECDSA_SIGN_SETUP,
EC_R_RANDOM_NUMBER_GENERATION_FAILED);
goto err;
}
}
while (BN_is_zero(k));
/* compute r the x-coordinate of generator * k */
if (!EC_POINT_mul(group, tmp_point, k, NULL, NULL, ctx)) {
ECerr(EC_F_ECDSA_SIGN_SETUP, 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, tmp_point, X, NULL, ctx)) {
ECerr(EC_F_ECDSA_SIGN_SETUP, 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,
tmp_point, X, NULL,
ctx)) {
ECerr(EC_F_ECDSA_SIGN_SETUP, ERR_R_EC_LIB);
goto err;
}
}
#endif
if (!BN_nnmod(r, X, order, ctx)) {
ECerr(EC_F_ECDSA_SIGN_SETUP, ERR_R_BN_LIB);
goto err;
}
}
while (BN_is_zero(r));
/* Check if optimized inverse is implemented */
if (EC_GROUP_do_inverse_ord(group, k, k, ctx) == 0) {
/* compute the inverse of k */
if (group->mont_data != NULL) {
/*
* We want inverse in constant time, therefore we utilize the fact
* order must be prime and use Fermats Little Theorem instead.
*/
if (!BN_set_word(X, 2)) {
ECerr(EC_F_ECDSA_SIGN_SETUP, ERR_R_BN_LIB);
goto err;
}
if (!BN_mod_sub(X, order, X, order, ctx)) {
ECerr(EC_F_ECDSA_SIGN_SETUP, ERR_R_BN_LIB);
goto err;
}
BN_set_flags(X, BN_FLG_CONSTTIME);
if (!BN_mod_exp_mont_consttime(k, k, X, order, ctx,
group->mont_data)) {
ECerr(EC_F_ECDSA_SIGN_SETUP, ERR_R_BN_LIB);
goto err;
}
} else {
if (!BN_mod_inverse(k, k, order, ctx)) {
ECerr(EC_F_ECDSA_SIGN_SETUP, ERR_R_BN_LIB);
goto err;
}
}
}
/* clear old values if necessary */
BN_clear_free(*rp);
BN_clear_free(*kinvp);
/* save the pre-computed values */
*rp = r;
*kinvp = k;
ret = 1;
err:
if (!ret) {
BN_clear_free(k);
BN_clear_free(r);
}
if (ctx != ctx_in)
BN_CTX_free(ctx);
EC_POINT_free(tmp_point);
BN_clear_free(X);
return ret;
}
int ossl_ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in, BIGNUM **kinvp,
BIGNUM **rp)
{
return ecdsa_sign_setup(eckey, ctx_in, kinvp, rp, NULL, 0);
}
ECDSA_SIG *ossl_ecdsa_sign_sig(const unsigned char *dgst, int dgst_len,
const BIGNUM *in_kinv, const BIGNUM *in_r,
EC_KEY *eckey)
{
int ok = 0, i;
BIGNUM *kinv = NULL, *s, *m = NULL, *tmp = NULL, *blind = NULL;
BIGNUM *blindm = NULL;
const BIGNUM *order, *ckinv;
BN_CTX *ctx = NULL;
const EC_GROUP *group;
ECDSA_SIG *ret;
const BIGNUM *priv_key;
group = EC_KEY_get0_group(eckey);
priv_key = EC_KEY_get0_private_key(eckey);
if (group == NULL || priv_key == NULL) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_PASSED_NULL_PARAMETER);
return NULL;
}
if (!EC_KEY_can_sign(eckey)) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, EC_R_CURVE_DOES_NOT_SUPPORT_SIGNING);
return NULL;
}
ret = ECDSA_SIG_new();
if (ret == NULL) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_MALLOC_FAILURE);
return NULL;
}
ret->r = BN_new();
ret->s = BN_new();
if (ret->r == NULL || ret->s == NULL) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_MALLOC_FAILURE);
goto err;
}
s = ret->s;
ctx = BN_CTX_secure_new();
if (ctx == NULL) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_MALLOC_FAILURE);
goto err;
}
BN_CTX_start(ctx);
tmp = BN_CTX_get(ctx);
m = BN_CTX_get(ctx);
blind = BN_CTX_get(ctx);
blindm = BN_CTX_get(ctx);
if (blindm == NULL) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_MALLOC_FAILURE);
goto err;
}
order = EC_GROUP_get0_order(group);
if (order == NULL) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_EC_LIB);
goto err;
}
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)) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, 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))) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_BN_LIB);
goto err;
}
do {
if (in_kinv == NULL || in_r == NULL) {
if (!ecdsa_sign_setup(eckey, ctx, &kinv, &ret->r, dgst, dgst_len)) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_ECDSA_LIB);
goto err;
}
ckinv = kinv;
} else {
ckinv = in_kinv;
if (BN_copy(ret->r, in_r) == NULL) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_MALLOC_FAILURE);
goto err;
}
}
/*
* The normal signature calculation is:
*
* s := k^-1 * (m + r * priv_key) mod order
*
* We will blind this to protect against side channel attacks
*
* s := k^-1 * blind^-1 * (blind * m + blind * r * priv_key) mod order
*/
/* Generate a blinding value */
do {
if (!BN_priv_rand(blind, BN_num_bits(order) - 1,
BN_RAND_TOP_ANY, BN_RAND_BOTTOM_ANY))
goto err;
} while (BN_is_zero(blind));
BN_set_flags(blind, BN_FLG_CONSTTIME);
BN_set_flags(blindm, BN_FLG_CONSTTIME);
BN_set_flags(tmp, BN_FLG_CONSTTIME);
/* tmp := blind * priv_key * r mod order */
if (!BN_mod_mul(tmp, blind, priv_key, order, ctx)) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_BN_LIB);
goto err;
}
if (!BN_mod_mul(tmp, tmp, ret->r, order, ctx)) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_BN_LIB);
goto err;
}
/* blindm := blind * m mod order */
if (!BN_mod_mul(blindm, blind, m, order, ctx)) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_BN_LIB);
goto err;
}
/* s : = (blind * priv_key * r) + (blind * m) mod order */
if (!BN_mod_add_quick(s, tmp, blindm, order)) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_BN_LIB);
goto err;
}
/* s:= s * blind^-1 mod order */
if (BN_mod_inverse(blind, blind, order, ctx) == NULL) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_BN_LIB);
goto err;
}
if (!BN_mod_mul(s, s, blind, order, ctx)) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_BN_LIB);
goto err;
}
/* s := s * k^-1 mod order */
if (!BN_mod_mul(s, s, ckinv, order, ctx)) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_BN_LIB);
goto err;
}
if (BN_is_zero(s)) {
/*
* if kinv and r have been supplied by the caller, don't
* generate new kinv and r values
*/
if (in_kinv != NULL && in_r != NULL) {
ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, EC_R_NEED_NEW_SETUP_VALUES);
goto err;
}
} else
/* s != 0 => we have a valid signature */
break;
}
while (1);
ok = 1;
err:
if (!ok) {
ECDSA_SIG_free(ret);
ret = NULL;
}
BN_CTX_end(ctx);
BN_CTX_free(ctx);
BN_clear_free(kinv);
return ret;
}
/*-
* returns
* 1: correct signature
* 0: incorrect signature
* -1: error
*/
int ossl_ecdsa_verify(int type, const unsigned char *dgst, int dgst_len,
const unsigned char *sigbuf, int sig_len, EC_KEY *eckey)
{
ECDSA_SIG *s;
const unsigned char *p = sigbuf;
unsigned char *der = NULL;
int derlen = -1;
int ret = -1;
s = ECDSA_SIG_new();
if (s == NULL)
return ret;
if (d2i_ECDSA_SIG(&s, &p, sig_len) == NULL)
goto err;
/* Ensure signature uses DER and doesn't have trailing garbage */
derlen = i2d_ECDSA_SIG(s, &der);
if (derlen != sig_len || memcmp(sigbuf, der, derlen) != 0)
goto err;
ret = ECDSA_do_verify(dgst, dgst_len, s, eckey);
err:
OPENSSL_clear_free(der, derlen);
ECDSA_SIG_free(s);
return ret;
}
int ossl_ecdsa_verify_sig(const unsigned char *dgst, int dgst_len,
const ECDSA_SIG *sig, EC_KEY *eckey)
{
int ret = -1, i;
BN_CTX *ctx;
const BIGNUM *order;
BIGNUM *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) {
ECerr(EC_F_OSSL_ECDSA_VERIFY_SIG, EC_R_MISSING_PARAMETERS);
return -1;
}
if (!EC_KEY_can_sign(eckey)) {
ECerr(EC_F_OSSL_ECDSA_VERIFY_SIG, EC_R_CURVE_DOES_NOT_SUPPORT_SIGNING);
return -1;
}
ctx = BN_CTX_new();
if (ctx == NULL) {
ECerr(EC_F_OSSL_ECDSA_VERIFY_SIG, ERR_R_MALLOC_FAILURE);
return -1;
}
BN_CTX_start(ctx);
u1 = BN_CTX_get(ctx);
u2 = BN_CTX_get(ctx);
m = BN_CTX_get(ctx);
X = BN_CTX_get(ctx);
if (X == NULL) {
ECerr(EC_F_OSSL_ECDSA_VERIFY_SIG, ERR_R_BN_LIB);
goto err;
}
order = EC_GROUP_get0_order(group);
if (order == NULL) {
ECerr(EC_F_OSSL_ECDSA_VERIFY_SIG, 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) {
ECerr(EC_F_OSSL_ECDSA_VERIFY_SIG, EC_R_BAD_SIGNATURE);
ret = 0; /* signature is invalid */
goto err;
}
/* calculate tmp1 = inv(S) mod order */
/* Check if optimized inverse is implemented */
if (EC_GROUP_do_inverse_ord(group, u2, sig->s, ctx) == 0) {
if (!BN_mod_inverse(u2, sig->s, order, ctx)) {
ECerr(EC_F_OSSL_ECDSA_VERIFY_SIG, 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)) {
ECerr(EC_F_OSSL_ECDSA_VERIFY_SIG, 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))) {
ECerr(EC_F_OSSL_ECDSA_VERIFY_SIG, ERR_R_BN_LIB);
goto err;
}
/* u1 = m * tmp mod order */
if (!BN_mod_mul(u1, m, u2, order, ctx)) {
ECerr(EC_F_OSSL_ECDSA_VERIFY_SIG, ERR_R_BN_LIB);
goto err;
}
/* u2 = r * w mod q */
if (!BN_mod_mul(u2, sig->r, u2, order, ctx)) {
ECerr(EC_F_OSSL_ECDSA_VERIFY_SIG, ERR_R_BN_LIB);
goto err;
}
if ((point = EC_POINT_new(group)) == NULL) {
ECerr(EC_F_OSSL_ECDSA_VERIFY_SIG, ERR_R_MALLOC_FAILURE);
goto err;
}
if (!EC_POINT_mul(group, point, u1, pub_key, u2, ctx)) {
ECerr(EC_F_OSSL_ECDSA_VERIFY_SIG, 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)) {
ECerr(EC_F_OSSL_ECDSA_VERIFY_SIG, 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)) {
ECerr(EC_F_OSSL_ECDSA_VERIFY_SIG, ERR_R_EC_LIB);
goto err;
}
}
#endif
if (!BN_nnmod(u1, X, order, ctx)) {
ECerr(EC_F_OSSL_ECDSA_VERIFY_SIG, 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);
EC_POINT_free(point);
return ret;
}
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