From 41d23d435221411b4d70c08b6c5424d0afcf4c19 Mon Sep 17 00:00:00 2001 From: Matt Caswell Date: Tue, 19 Jun 2018 15:07:02 +0100 Subject: [PATCH] Add blinding to a DSA signature This extends the recently added ECDSA signature blinding to blind DSA too. This is based on side channel attacks demonstrated by Keegan Ryan (NCC Group) for ECDSA which are likely to be able to be applied to DSA. Normally, as in ECDSA, during signing the signer calculates: s:= k^-1 * (m + r * priv_key) mod order In ECDSA, 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. This commit also tweaks the previous ECDSA blinding so that blinding is only removed at the last possible step. Reviewed-by: Rich Salz Reviewed-by: Richard Levitte (Merged from https://github.com/openssl/openssl/pull/6524) --- CHANGES | 4 +-- crypto/dsa/dsa_ossl.c | 73 +++++++++++++++++++++++++++++------------ crypto/ecdsa/ecs_ossl.c | 14 ++++---- 3 files changed, 61 insertions(+), 30 deletions(-) diff --git a/CHANGES b/CHANGES index a3861abac1..f9562dd712 100644 --- a/CHANGES +++ b/CHANGES @@ -9,8 +9,8 @@ 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). + *) Add blinding to ECDSA and DSA signatures 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 diff --git a/crypto/dsa/dsa_ossl.c b/crypto/dsa/dsa_ossl.c index aa10dd12f6..2dcfedeeee 100644 --- a/crypto/dsa/dsa_ossl.c +++ b/crypto/dsa/dsa_ossl.c @@ -133,17 +133,13 @@ const DSA_METHOD *DSA_OpenSSL(void) static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa) { BIGNUM *kinv = NULL, *r = NULL, *s = NULL; - BIGNUM m; - BIGNUM xr; + BIGNUM *m, *blind, *blindm, *tmp; BN_CTX *ctx = NULL; int reason = ERR_R_BN_LIB; DSA_SIG *ret = NULL; int noredo = 0; - BN_init(&m); - BN_init(&xr); - - if (!dsa->p || !dsa->q || !dsa->g) { + if (dsa->p == NULL || dsa->q == NULL || dsa->g == NULL) { reason = DSA_R_MISSING_PARAMETERS; goto err; } @@ -154,6 +150,13 @@ static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa) ctx = BN_CTX_new(); if (ctx == NULL) goto err; + m = BN_CTX_get(ctx); + blind = BN_CTX_get(ctx); + blindm = BN_CTX_get(ctx); + tmp = BN_CTX_get(ctx); + if (tmp == NULL) + goto err; + redo: if ((dsa->kinv == NULL) || (dsa->r == NULL)) { if (!DSA_sign_setup(dsa, ctx, &kinv, &r)) @@ -173,20 +176,52 @@ static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa) * 4.2 */ dlen = BN_num_bytes(dsa->q); - if (BN_bin2bn(dgst, dlen, &m) == NULL) + if (BN_bin2bn(dgst, dlen, m) == NULL) goto err; - /* Compute s = inv(k) (m + xr) mod q */ - if (!BN_mod_mul(&xr, dsa->priv_key, r, dsa->q, ctx)) - goto err; /* s = xr */ - if (!BN_add(s, &xr, &m)) - goto err; /* s = m + xr */ - if (BN_cmp(s, dsa->q) > 0) - if (!BN_sub(s, s, dsa->q)) + /* + * The normal signature calculation is: + * + * s := k^-1 * (m + r * priv_key) mod q + * + * We will blind this to protect against side channel attacks + * + * s := blind^-1 * k^-1 * (blind * m + blind * r * priv_key) mod q + */ + + /* Generate a blinding value */ + do { + if (!BN_rand(blind, BN_num_bits(dsa->q) - 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 q */ + if (!BN_mod_mul(tmp, blind, dsa->priv_key, dsa->q, ctx)) + goto err; + if (!BN_mod_mul(tmp, tmp, r, dsa->q, ctx)) + goto err; + + /* blindm := blind * m mod q */ + if (!BN_mod_mul(blindm, blind, m, dsa->q, ctx)) + goto err; + + /* s : = (blind * priv_key * r) + (blind * m) mod q */ + if (!BN_mod_add_quick(s, tmp, blindm, dsa->q)) + goto err; + + /* s := s * k^-1 mod q */ if (!BN_mod_mul(s, s, kinv, dsa->q, ctx)) goto err; + /* s:= s * blind^-1 mod q */ + if (BN_mod_inverse(blind, blind, dsa->q, ctx) == NULL) + goto err; + if (!BN_mod_mul(s, s, blind, dsa->q, ctx)) + goto err; + /* * Redo if r or s is zero as required by FIPS 186-3: this is very * unlikely. @@ -210,13 +245,9 @@ static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa) BN_free(r); BN_free(s); } - if (ctx != NULL) - BN_CTX_free(ctx); - BN_clear_free(&m); - BN_clear_free(&xr); - if (kinv != NULL) /* dsa->kinv is NULL now if we used it */ - BN_clear_free(kinv); - return (ret); + BN_CTX_free(ctx); + BN_clear_free(kinv); + return ret; } static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, diff --git a/crypto/ecdsa/ecs_ossl.c b/crypto/ecdsa/ecs_ossl.c index 1d37551803..6115df7407 100644 --- a/crypto/ecdsa/ecs_ossl.c +++ b/crypto/ecdsa/ecs_ossl.c @@ -334,7 +334,7 @@ static ECDSA_SIG *ecdsa_do_sign(const unsigned char *dgst, int dgst_len, * * We will blind this to protect against side channel attacks * - * s := k^-1 * blind^-1 * (blind * m + blind * r * priv_key) mod order + * s := blind^-1 * k^-1 * (blind * m + blind * r * priv_key) mod order */ /* Generate a blinding value */ @@ -368,18 +368,18 @@ static ECDSA_SIG *ecdsa_do_sign(const unsigned char *dgst, int dgst_len, goto err; } - /* s:= s * blind^-1 mod order */ - if (BN_mod_inverse(blind, blind, order, ctx) == NULL) { + /* 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_mul(s, s, blind, order, ctx)) { + + /* 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; } - - /* s := s * k^-1 mod order */ - if (!BN_mod_mul(s, s, ckinv, order, ctx)) { + if (!BN_mod_mul(s, s, blind, order, ctx)) { ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB); goto err; } -- 2.25.1