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Fix CVE-2018-0495:
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-0495
https://www.nccgroup.trust/us/our-research/technical-advisory-return-of-the-hidden-number-problem/
Patch copied from upstream source repository:
https://github.com/openssl/openssl/commit/0c27d793745c7837b13646302b6890a556b7017a
From 0c27d793745c7837b13646302b6890a556b7017a Mon Sep 17 00:00:00 2001
From: Matt Caswell <matt@openssl.org>
Date: Fri, 25 May 2018 12:10:13 +0100
Subject: [PATCH] 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>
---
CHANGES | 4 +++
crypto/ec/ecdsa_ossl.c | 70 +++++++++++++++++++++++++++++++++++++-----
2 files changed, 67 insertions(+), 7 deletions(-)
diff --git a/crypto/ec/ecdsa_ossl.c b/crypto/ec/ecdsa_ossl.c
index 72e2f0f28b..449be0e92a 100644
--- a/crypto/ec/ecdsa_ossl.c
+++ b/crypto/ec/ecdsa_ossl.c
@@ -210,7 +210,8 @@ 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;
@@ -243,8 +244,18 @@ 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;
}
@@ -284,18 +295,64 @@ 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_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 to
@@ -317,9 +374,8 @@ 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;
}
--
2.17.1
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