| From 878e2c5b13010329c203f309ed0c8f2113f85648 Mon Sep 17 00:00:00 2001 |
| From: Matt Caswell <matt@openssl.org> |
| Date: Mon, 18 Jan 2016 11:31:58 +0000 |
| Subject: [PATCH] Prevent small subgroup attacks on DH/DHE |
| |
| Historically OpenSSL only ever generated DH parameters based on "safe" |
| primes. More recently (in version 1.0.2) support was provided for |
| generating X9.42 style parameter files such as those required for RFC |
| 5114 support. The primes used in such files may not be "safe". Where an |
| application is using DH configured with parameters based on primes that |
| are not "safe" then an attacker could use this fact to find a peer's |
| private DH exponent. This attack requires that the attacker complete |
| multiple handshakes in which the peer uses the same DH exponent. |
| |
| A simple mitigation is to ensure that y^q (mod p) == 1 |
| |
| CVE-2016-0701 (fix part 1 of 2) |
| |
| Issue reported by Antonio Sanso. |
| |
| Reviewed-by: Viktor Dukhovni <viktor@openssl.org> |
| |
| Upstream-Status: Backport |
| |
| https://github.com/openssl/openssl/commit/878e2c5b13010329c203f309ed0c8f2113f85648 |
| |
| CVE: CVE-2016-0701 |
| Signed-of-by: Armin Kuster <akuster@mvisa.com> |
| |
| --- |
| crypto/dh/dh.h | 1 + |
| crypto/dh/dh_check.c | 35 +++++++++++++++++++++++++---------- |
| 2 files changed, 26 insertions(+), 10 deletions(-) |
| |
| diff --git a/crypto/dh/dh.h b/crypto/dh/dh.h |
| index b177673..5498a9d 100644 |
| --- a/crypto/dh/dh.h |
| +++ b/crypto/dh/dh.h |
| @@ -174,6 +174,7 @@ struct dh_st { |
| /* DH_check_pub_key error codes */ |
| # define DH_CHECK_PUBKEY_TOO_SMALL 0x01 |
| # define DH_CHECK_PUBKEY_TOO_LARGE 0x02 |
| +# define DH_CHECK_PUBKEY_INVALID 0x03 |
| |
| /* |
| * primes p where (p-1)/2 is prime too are called "safe"; we define this for |
| diff --git a/crypto/dh/dh_check.c b/crypto/dh/dh_check.c |
| index 347467c..5adedc0 100644 |
| --- a/crypto/dh/dh_check.c |
| +++ b/crypto/dh/dh_check.c |
| @@ -151,23 +151,38 @@ int DH_check(const DH *dh, int *ret) |
| int DH_check_pub_key(const DH *dh, const BIGNUM *pub_key, int *ret) |
| { |
| int ok = 0; |
| - BIGNUM *q = NULL; |
| + BIGNUM *tmp = NULL; |
| + BN_CTX *ctx = NULL; |
| |
| *ret = 0; |
| - q = BN_new(); |
| - if (q == NULL) |
| + ctx = BN_CTX_new(); |
| + if (ctx == NULL) |
| goto err; |
| - BN_set_word(q, 1); |
| - if (BN_cmp(pub_key, q) <= 0) |
| + BN_CTX_start(ctx); |
| + tmp = BN_CTX_get(ctx); |
| + if (tmp == NULL) |
| + goto err; |
| + BN_set_word(tmp, 1); |
| + if (BN_cmp(pub_key, tmp) <= 0) |
| *ret |= DH_CHECK_PUBKEY_TOO_SMALL; |
| - BN_copy(q, dh->p); |
| - BN_sub_word(q, 1); |
| - if (BN_cmp(pub_key, q) >= 0) |
| + BN_copy(tmp, dh->p); |
| + BN_sub_word(tmp, 1); |
| + if (BN_cmp(pub_key, tmp) >= 0) |
| *ret |= DH_CHECK_PUBKEY_TOO_LARGE; |
| |
| + if (dh->q != NULL) { |
| + /* Check pub_key^q == 1 mod p */ |
| + if (!BN_mod_exp(tmp, pub_key, dh->q, dh->p, ctx)) |
| + goto err; |
| + if (!BN_is_one(tmp)) |
| + *ret |= DH_CHECK_PUBKEY_INVALID; |
| + } |
| + |
| ok = 1; |
| err: |
| - if (q != NULL) |
| - BN_free(q); |
| + if (ctx != NULL) { |
| + BN_CTX_end(ctx); |
| + BN_CTX_free(ctx); |
| + } |
| return (ok); |
| } |
| -- |
| 2.3.5 |
| |