From a3e9d5aa980f238805970f420adf5e903d35bf09 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/ec/ecdsa_ossl.c | 70 +++++++++++++++++++++++++++++++++++++----- 2 files changed, 67 insertions(+), 7 deletions(-) diff --git a/CHANGES b/CHANGES index 7a478521c1..fe3e13aa0d 100644 --- a/CHANGES +++ b/CHANGES @@ -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. diff --git a/crypto/ec/ecdsa_ossl.c b/crypto/ec/ecdsa_ossl.c index 7842851778..640593bc2d 100644 --- a/crypto/ec/ecdsa_ossl.c +++ b/crypto/ec/ecdsa_ossl.c @@ -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; } -- 2.34.1