From d42c477cc794163a3757956bbffca5cea000923c Mon Sep 17 00:00:00 2001 From: Jouni Malinen Date: Tue, 26 Feb 2019 11:43:03 +0200 Subject: [PATCH] OpenSSL: Use constant time operations for private bignums This helps in reducing measurable timing differences in operations involving private information. BoringSSL has removed BN_FLG_CONSTTIME and expects specific constant time functions to be called instead, so a bit different approach is needed depending on which library is used. The main operation that needs protection against side channel attacks is BN_mod_exp() that depends on private keys (the public key validation step in crypto_dh_derive_secret() is an exception that can use the faster version since it does not depend on private keys). crypto_bignum_div() is currently used only in SAE FFC case with not safe-prime groups and only with values that do not depend on private keys, so it is not critical to protect it. crypto_bignum_inverse() is currently used only in SAE FFC PWE derivation. The additional protection here is targeting only OpenSSL. BoringSSL may need conversion to using BN_mod_inverse_blinded(). This is related to CVE-2019-9494 and CVE-2019-9495. Signed-off-by: Jouni Malinen --- src/crypto/crypto_openssl.c | 20 +++++++++++++++----- 1 file changed, 15 insertions(+), 5 deletions(-) diff --git a/src/crypto/crypto_openssl.c b/src/crypto/crypto_openssl.c index 9c2ba58d5..ac53cc81a 100644 --- a/src/crypto/crypto_openssl.c +++ b/src/crypto/crypto_openssl.c @@ -607,7 +607,8 @@ int crypto_mod_exp(const u8 *base, size_t base_len, bn_result == NULL) goto error; - if (BN_mod_exp(bn_result, bn_base, bn_exp, bn_modulus, ctx) != 1) + if (BN_mod_exp_mont_consttime(bn_result, bn_base, bn_exp, bn_modulus, + ctx, NULL) != 1) goto error; *result_len = BN_bn2bin(bn_result, result); @@ -1360,8 +1361,9 @@ int crypto_bignum_exptmod(const struct crypto_bignum *a, bnctx = BN_CTX_new(); if (bnctx == NULL) return -1; - res = BN_mod_exp((BIGNUM *) d, (const BIGNUM *) a, (const BIGNUM *) b, - (const BIGNUM *) c, bnctx); + res = BN_mod_exp_mont_consttime((BIGNUM *) d, (const BIGNUM *) a, + (const BIGNUM *) b, (const BIGNUM *) c, + bnctx, NULL); BN_CTX_free(bnctx); return res ? 0 : -1; @@ -1380,6 +1382,11 @@ int crypto_bignum_inverse(const struct crypto_bignum *a, bnctx = BN_CTX_new(); if (bnctx == NULL) return -1; +#ifdef OPENSSL_IS_BORINGSSL + /* TODO: use BN_mod_inverse_blinded() ? */ +#else /* OPENSSL_IS_BORINGSSL */ + BN_set_flags((BIGNUM *) a, BN_FLG_CONSTTIME); +#endif /* OPENSSL_IS_BORINGSSL */ res = BN_mod_inverse((BIGNUM *) c, (const BIGNUM *) a, (const BIGNUM *) b, bnctx); BN_CTX_free(bnctx); @@ -1413,6 +1420,9 @@ int crypto_bignum_div(const struct crypto_bignum *a, bnctx = BN_CTX_new(); if (bnctx == NULL) return -1; +#ifndef OPENSSL_IS_BORINGSSL + BN_set_flags((BIGNUM *) a, BN_FLG_CONSTTIME); +#endif /* OPENSSL_IS_BORINGSSL */ res = BN_div((BIGNUM *) c, NULL, (const BIGNUM *) a, (const BIGNUM *) b, bnctx); BN_CTX_free(bnctx); @@ -1504,8 +1514,8 @@ int crypto_bignum_legendre(const struct crypto_bignum *a, /* exp = (p-1) / 2 */ !BN_sub(exp, (const BIGNUM *) p, BN_value_one()) || !BN_rshift1(exp, exp) || - !BN_mod_exp(tmp, (const BIGNUM *) a, exp, (const BIGNUM *) p, - bnctx)) + !BN_mod_exp_mont_consttime(tmp, (const BIGNUM *) a, exp, + (const BIGNUM *) p, bnctx, NULL)) goto fail; if (BN_is_word(tmp, 1)) -- 2.21.0