From: Steffan Karger Date: Fri, 28 Oct 2016 13:57:01 +0200 Message-ID: <1477655821-6711-1-git-send-email-steffan.karger@...1435...> In-Reply-To: <8018809a-aa16-32df-4c1a-724926994732@...1435...> References: <8018809a-aa16-32df-4c1a-724926994732@...1435...> MIME-Version: 1.0 Subject: [Openvpn-devel] [PATCH v2 (master)] Limit --reneg-bytes to 64MB when using small block ciphers List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit To: openvpn-devel@lists.sourceforge.net Following the earlier warning about small block ciphers, now limit the --reneg-bytes value when using a cipher that susceptible to SWEET32-like attacks. The 64 MB value has been selected with the researchers who published the SWEET32 paper. Note that this will not change a user-set --reneg-bytes value, to allow a user to align a gun with his feet^w^w^w^w^w^w override this behaviour if really needed. v2: obey user-set --reneg-bytes 0 to revert to old behaviour, use more firm language in warning message, and add URL to man page. Signed-off-by: Steffan Karger --- doc/openvpn.8 | 1 + src/openvpn/crypto.c | 5 +++-- src/openvpn/options.c | 1 + src/openvpn/ssl.c | 29 ++++++++++++++++++++++++++++- 4 files changed, 33 insertions(+), 3 deletions(-) diff --git a/doc/openvpn.8 b/doc/openvpn.8 index 2e58f33..0ac8184 100644 --- a/doc/openvpn.8 +++ b/doc/openvpn.8 @@ -4101,6 +4101,7 @@ an abbreviation for Blowfish in Cipher Block Chaining mode. Using BF-CBC is no longer recommended, because of it's 64-bit block size. This small block size allows attacks based on collisions, as demonstrated by SWEET32. +See https://community.openvpn.net/openvpn/wiki/SWEET32 for details. To see other ciphers that are available with OpenVPN, use the .B \-\-show\-ciphers diff --git a/src/openvpn/crypto.c b/src/openvpn/crypto.c index 45689f2..f600070 100644 --- a/src/openvpn/crypto.c +++ b/src/openvpn/crypto.c @@ -836,8 +836,9 @@ init_key_ctx (struct key_ctx *ctx, struct key *key, cipher_kt_iv_size(kt->cipher)); if (cipher_kt_block_size(kt->cipher) < 128/8) { - msg (M_WARN, "WARNING: this cipher's block size is less than 128 bit " - "(%d bit). Consider using a --cipher with a larger block size.", + msg (M_WARN, "WARNING: INSECURE cipher with block size less than 128" + " bit (%d bit). This allows attacks like SWEET32. Mitigate by " + "using a --cipher with a larger block size (e.g. AES-256-CBC).", cipher_kt_block_size(kt->cipher)*8); } } diff --git a/src/openvpn/options.c b/src/openvpn/options.c index 281ef0b..996c26e 100644 --- a/src/openvpn/options.c +++ b/src/openvpn/options.c @@ -852,6 +852,7 @@ init_options (struct options *o, const bool init_gc) #endif o->key_method = 2; o->tls_timeout = 2; + o->renegotiate_bytes = -1; o->renegotiate_seconds = 3600; o->handshake_window = 60; o->transition_window = 3600; diff --git a/src/openvpn/ssl.c b/src/openvpn/ssl.c index dfdc200..33122a3 100644 --- a/src/openvpn/ssl.c +++ b/src/openvpn/ssl.c @@ -283,6 +283,27 @@ tls_get_cipher_name_pair (const char * cipher_name, size_t len) { return NULL; } +/** + * Limit the reneg_bytes value when using a small-block (<128 bytes) cipher. + * + * @param cipher The current cipher (may be NULL). + * @param reneg_bytes Pointer to the current reneg_bytes, updated if needed. + * May *not* be NULL. + */ +static void +tls_limit_reneg_bytes (const cipher_kt_t *cipher, int *reneg_bytes) +{ + if (cipher && (cipher_kt_block_size(cipher) < 128/8)) + { + if (*reneg_bytes == -1) /* Not user-specified */ + { + msg (M_WARN, "WARNING: cipher with small block size in use, " + "reducing reneg-bytes to 64MB to mitigate SWEET32 attacks."); + *reneg_bytes = 64 * 1024 * 1024; + } + } +} + /* * Max number of bytes we will add * for data structures common to both @@ -1742,6 +1763,8 @@ tls_session_update_crypto_params(struct tls_session *session, msg (D_TLS_ERRORS, "TLS Error: server generate_key_expansion failed"); goto cleanup; } + tls_limit_reneg_bytes (session->opt->key_type.cipher, + &session->opt->renegotiate_bytes); ret = true; cleanup: CLEAR (*ks->key_src); @@ -2126,6 +2149,8 @@ key_method_2_write (struct buffer *buf, struct tls_session *session) } CLEAR (*ks->key_src); + tls_limit_reneg_bytes (session->opt->key_type.cipher, + &session->opt->renegotiate_bytes); } return true; @@ -2354,6 +2379,8 @@ key_method_2_read (struct buffer *buf, struct tls_multi *multi, struct tls_sessi } CLEAR (*ks->key_src); + tls_limit_reneg_bytes (session->opt->key_type.cipher, + &session->opt->renegotiate_bytes); } gc_free (&gc); @@ -2410,7 +2437,7 @@ tls_process (struct tls_multi *multi, if (ks->state >= S_ACTIVE && ((session->opt->renegotiate_seconds && now >= ks->established + session->opt->renegotiate_seconds) - || (session->opt->renegotiate_bytes + || (session->opt->renegotiate_bytes > 0 && ks->n_bytes >= session->opt->renegotiate_bytes) || (session->opt->renegotiate_packets && ks->n_packets >= session->opt->renegotiate_packets) -- 2.7.4