From: Steffan Karger Date: Fri, 28 Oct 2016 12:10:07 +0000 (+0200) Subject: Limit --reneg-bytes to 64MB when using small block ciphers X-Git-Tag: contains X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=a91ddc99a524014ec79560d873721e8fa81a5631;p=thirdparty%2Fopenvpn.git Limit --reneg-bytes to 64MB when using small block ciphers 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. Furthermore, in contrast with the patch for master, this will not limit --reneg-bytes on the client side. This allows server administrators to revert to the old behaviour, or increase --reneg-bytes to something they believe is workable, without having to change client configs. (The master branch provides cipher negotiation as a real solution, so we can be stricter there.) v2: obey user-set --reneg-bytes 0 to revert to old behaviour, use more firm language in warning message, add URL to man page, and only limit at the server side. Signed-off-by: Steffan Karger Acked-by: David Sommerseth Message-Id: <1477656607-7440-1-git-send-email-steffan.karger@fox-it.com> URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg12799.html Signed-off-by: David Sommerseth --- diff --git a/doc/openvpn.8 b/doc/openvpn.8 index 70573dacb..7be30ec0b 100644 --- a/doc/openvpn.8 +++ b/doc/openvpn.8 @@ -3913,6 +3913,7 @@ 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 552e33323..ca4af27eb 100644 --- a/src/openvpn/crypto.c +++ b/src/openvpn/crypto.c @@ -496,8 +496,9 @@ 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 1ef02993c..19cd815d1 100644 --- a/src/openvpn/options.c +++ b/src/openvpn/options.c @@ -849,6 +849,7 @@ init_options (struct options *o, const bool init_gc) #ifdef ENABLE_SSL 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 33fd9dd6b..bfad2911f 100644 --- a/src/openvpn/ssl.c +++ b/src/openvpn/ssl.c @@ -271,6 +271,27 @@ 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 @@ -1956,6 +1977,8 @@ 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; @@ -2222,7 +2245,7 @@ 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)