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- From d42c477cc794163a3757956bbffca5cea000923c Mon Sep 17 00:00:00 2001
- From: Jouni Malinen <jouni@codeaurora.org>
- Date: Tue, 26 Feb 2019 11:43:03 +0200
- Subject: [PATCH 01/14] 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 <jouni@codeaurora.org>
- ---
- 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 9c2ba58..ac53cc8 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))
- --
- 2.7.4
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