From 949ff36623eafc3523a9f91784992965018ffb05 Mon Sep 17 00:00:00 2001 From: Matt Caswell 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 --- CHANGES | 4 ++ crypto/ecdsa/ecdsatest.c | 9 ++++- crypto/ecdsa/ecs_ossl.c | 82 ++++++++++++++++++++++++++++++++-------- 3 files changed, 79 insertions(+), 16 deletions(-) diff --git a/CHANGES b/CHANGES index f17fbbf837..a3861abac1 100644 --- a/CHANGES +++ b/CHANGES @@ -9,6 +9,10 @@ Changes between 1.0.2o and 1.0.2p [xx XXX xxxx] + *) Add blinding to an ECDSA signature to protect against side channel attacks + discovered by Keegan Ryan (NCC Group). + [Matt Caswell] + *) When unlocking a pass phrase protected PEM file or PKCS#8 container, we now allow empty (zero character) pass phrases. [Richard Levitte] diff --git a/crypto/ecdsa/ecdsatest.c b/crypto/ecdsa/ecdsatest.c index 0f301f86d9..a130fc9117 100644 --- a/crypto/ecdsa/ecdsatest.c +++ b/crypto/ecdsa/ecdsatest.c @@ -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", @@ -158,6 +158,11 @@ 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(); @@ -199,11 +204,13 @@ int x9_62_test_internal(BIO *out, int nid, const char *r_in, const char *s_in) /* 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; diff --git a/crypto/ecdsa/ecs_ossl.c b/crypto/ecdsa/ecs_ossl.c index 16d4f59b9b..1d37551803 100644 --- a/crypto/ecdsa/ecs_ossl.c +++ b/crypto/ecdsa/ecs_ossl.c @@ -252,6 +252,7 @@ static ECDSA_SIG *ecdsa_do_sign(const unsigned char *dgst, int dgst_len, { int ok = 0, i; 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; @@ -269,14 +270,25 @@ static ECDSA_SIG *ecdsa_do_sign(const unsigned char *dgst, int dgst_len, } ret = ECDSA_SIG_new(); - if (!ret) { + if (ret == NULL) { ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_MALLOC_FAILURE); return NULL; } 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; } @@ -315,26 +327,70 @@ static ECDSA_SIG *ecdsa_do_sign(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, -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 * 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_add_quick(s, tmp, m, order)) { + if (!BN_mod_mul(s, s, blind, order, ctx)) { 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_is_zero(s)) { /* * if kinv and r have been supplied by the caller don't to * generate new kinv and r values */ if (in_kinv != NULL && in_r != NULL) { - ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, - ECDSA_R_NEED_NEW_SETUP_VALUES); + ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ECDSA_R_NEED_NEW_SETUP_VALUES); goto err; } } else @@ -349,15 +405,11 @@ static ECDSA_SIG *ecdsa_do_sign(const unsigned char *dgst, int dgst_len, 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; } -- 2.34.1