File 0001-Add-blinding-to-an-ECDSA-signature.patch of Package compat-openssl098.29129
From 949ff36623eafc3523a9f91784992965018ffb05 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/ecdsa/ecdsatest.c | 9 ++++-
crypto/ecdsa/ecs_ossl.c | 82 ++++++++++++++++++++++++++++++++--------
3 files changed, 79 insertions(+), 16 deletions(-)
Index: openssl-0.9.8j/crypto/ecdsa/ecdsatest.c
===================================================================
--- openssl-0.9.8j.orig/crypto/ecdsa/ecdsatest.c 2018-08-13 12:22:52.525216069 +0200
+++ openssl-0.9.8j/crypto/ecdsa/ecdsatest.c 2018-08-13 12:24:05.001676253 +0200
@@ -137,7 +137,7 @@ int restore_rand(void)
return 1;
}
-static int fbytes_counter = 0;
+static int fbytes_counter = 0, use_fake = 0;
static const char *numbers[8] = {
"651056770906015076056810763456358567190100156695615665659",
"6140507067065001063065065565667405560006161556565665656654",
@@ -157,6 +157,11 @@ int fbytes(unsigned char *buf, int num)
int ret;
BIGNUM *tmp = NULL;
+ if (use_fake == 0)
+ return old_rand->bytes(buf, num);
+
+ use_fake = 0;
+
if (fbytes_counter >= 8)
return 0;
tmp = BN_new();
@@ -200,11 +205,13 @@ int x9_62_test_internal(BIO *out, int ni
/* create the key */
if ((key = EC_KEY_new_by_curve_name(nid)) == NULL)
goto x962_int_err;
+ use_fake = 1;
if (!EC_KEY_generate_key(key))
goto x962_int_err;
BIO_printf(out, ".");
(void)BIO_flush(out);
/* create the signature */
+ use_fake = 1;
signature = ECDSA_do_sign(digest, 20, key);
if (signature == NULL)
goto x962_int_err;
Index: openssl-0.9.8j/crypto/ecdsa/ecs_ossl.c
===================================================================
--- openssl-0.9.8j.orig/crypto/ecdsa/ecs_ossl.c 2018-08-13 12:23:01.033270090 +0200
+++ openssl-0.9.8j/crypto/ecdsa/ecs_ossl.c 2018-08-13 12:40:21.767908367 +0200
@@ -225,6 +225,7 @@ static ECDSA_SIG *ecdsa_do_sign(const un
{
int ok = 0;
BIGNUM *kinv=NULL, *s, *m=NULL,*tmp=NULL,*order=NULL;
+ BIGNUM *blind = NULL, *blindm = NULL;
const BIGNUM *ckinv;
BN_CTX *ctx = NULL;
const EC_GROUP *group;
@@ -250,9 +251,19 @@ static ECDSA_SIG *ecdsa_do_sign(const un
}
s = ret->s;
- if ((ctx = BN_CTX_new()) == NULL || (order = BN_new()) == NULL ||
- (tmp = BN_new()) == NULL || (m = BN_new()) == NULL)
- {
+ ctx = BN_CTX_new();
+ if (ctx == NULL) {
+ ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_MALLOC_FAILURE);
+ goto err;
+ }
+
+ BN_CTX_start(ctx);
+ order = BN_CTX_get(ctx);
+ tmp = BN_CTX_get(ctx);
+ m = BN_CTX_get(ctx);
+ blind = BN_CTX_get(ctx);
+ blindm = BN_CTX_get(ctx);
+ if (blindm == NULL) {
ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_MALLOC_FAILURE);
goto err;
}
@@ -303,18 +314,59 @@ static ECDSA_SIG *ecdsa_do_sign(const un
}
}
- 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 := blind^-1 * k^-1 * (blind * m + blind * r * priv_key) mod order
+ */
+
+ /* Generate a blinding value */
+ do {
+ if (!BN_rand(blind, BN_num_bits(order) - 1, -1, 0))
+ 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)) {
+ ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);
+ goto err;
+ }
+ if (!BN_mod_mul(tmp, tmp, ret->r, order, ctx)) {
+ ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);
+ goto err;
+ }
+
+ /* blindm := blind * m mod order */
+ if (!BN_mod_mul(blindm, blind, m, order, ctx)) {
+ ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);
+ goto err;
+ }
+
+ /* s : = (blind * priv_key * r) + (blind * m) mod order */
+ if (!BN_mod_add_quick(s, tmp, blindm, order)) {
+ ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);
+ goto err;
+ }
+
+ /* s := s * k^-1 mod order */
+ if (!BN_mod_mul(s, s, ckinv, order, ctx)) {
ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);
goto err;
}
- if (!BN_mod_add_quick(s, tmp, m, order))
- {
+ /* s:= s * blind^-1 mod order */
+ if (BN_mod_inverse(blind, blind, order, ctx) == NULL) {
ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);
goto err;
}
- if (!BN_mod_mul(s, s, ckinv, order, ctx))
- {
+ /* s := s * k^-1 mod order */
+ if (!BN_mod_mul(s, s, blind, order, ctx)) {
ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);
goto err;
}
@@ -341,15 +393,11 @@ err:
ECDSA_SIG_free(ret);
ret = NULL;
}
- if (ctx)
+ if (ctx != NULL) {
+ BN_CTX_end(ctx);
BN_CTX_free(ctx);
- if (m)
- BN_clear_free(m);
- if (tmp)
- BN_clear_free(tmp);
- if (order)
- BN_free(order);
- if (kinv)
+ }
+ if (kinv != NULL)
BN_clear_free(kinv);
return ret;
}
