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Add blinding to an ECDSA signature

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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>
This commit is contained in:
Matt Caswell 2018-05-25 12:10:13 +01:00
parent d3273ef6c5
commit a3e9d5aa98
2 changed files with 67 additions and 7 deletions
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4
CHANGES
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@ -8,6 +8,10 @@
release branch.
Changes between 1.1.0h and 1.1.1 [xx XXX xxxx]
*) Add blinding to an ECDSA signature to protect against side channel attacks
discovered by Keegan Ryan (NCC Group).
[Matt Caswell]
*) Enforce checking in the pkeyutl command line app to ensure that the input
length does not exceed the maximum supported digest length when performing
a sign, verify or verifyrecover operation.

70
crypto/ec/ecdsa_ossl.c
View file

@ -196,7 +196,8 @@ ECDSA_SIG *ossl_ecdsa_sign_sig(const unsigned char *dgst, int dgst_len,
EC_KEY *eckey)
{
int ok = 0, i;
BIGNUM *kinv = NULL, *s, *m = NULL, *tmp = NULL;
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;
@ -229,8 +230,18 @@ ECDSA_SIG *ossl_ecdsa_sign_sig(const unsigned char *dgst, int dgst_len,
}
s = ret->s;
if ((ctx = BN_CTX_new()) == NULL ||
(tmp = BN_new()) == NULL || (m = BN_new()) == NULL) {
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;
}
@ -270,18 +281,64 @@ ECDSA_SIG *ossl_ecdsa_sign_sig(const unsigned char *dgst, int dgst_len,
}
}
if (!BN_mod_mul(tmp, priv_key, ret->r, order, ctx)) {
/*
* 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_add_quick(s, tmp, m, order)) {
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
@ -303,9 +360,8 @@ ECDSA_SIG *ossl_ecdsa_sign_sig(const unsigned char *dgst, int dgst_len,
ECDSA_SIG_free(ret);
ret = NULL;
}
BN_CTX_end(ctx);
BN_CTX_free(ctx);
BN_clear_free(m);
BN_clear_free(tmp);
BN_clear_free(kinv);
return ret;
}