[Openvpn-devel,v2] Allow running a default configuration with TLS libraries without BF-CBC

Message ID 20210125124330.30046-1-arne@rfc2549.org
State New
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  • [Openvpn-devel,v2] Allow running a default configuration with TLS libraries without BF-CBC
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Commit Message

Arne Schwabe Jan. 25, 2021, 12:43 p.m.
Modern TLS libraries might drop Blowfish by default or distributions
might disable Blowfish in OpenSSL/mbed TLS. We still signal OCC
options with BF-CBC compatible strings. To avoid requiring BF-CBC
for this, special this one usage of BF-CBC enough to avoid a hard
requirement on Blowfish in the default configuration.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>

Patch v2: add more clarifying comment, do not warn about OCC only insecure
          ciphers, code improvements

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
---
 src/openvpn/init.c    | 32 ++++++++++++++++++++++--------
 src/openvpn/options.c | 46 +++++++++++++++++++++++++++++++++++++------
 2 files changed, 64 insertions(+), 14 deletions(-)

Comments

Antonio Quartulli Jan. 29, 2021, 8:51 a.m. | #1
Hi,

On 25/01/2021 13:43, Arne Schwabe wrote:
> Modern TLS libraries might drop Blowfish by default or distributions
> might disable Blowfish in OpenSSL/mbed TLS. We still signal OCC
> options with BF-CBC compatible strings. To avoid requiring BF-CBC
> for this, special this one usage of BF-CBC enough to avoid a hard
> requirement on Blowfish in the default configuration.
> 
> Signed-off-by: Arne Schwabe <arne@rfc2549.org>
> 
> Patch v2: add more clarifying comment, do not warn about OCC only insecure
>           ciphers, code improvements
> 
> Signed-off-by: Arne Schwabe <arne@rfc2549.org>

Acked-by: Antonio Quartulli <antonio@openvpn.net>
Gert Doering Feb. 14, 2021, 2:50 p.m. | #2
Hi,

On Mon, Jan 25, 2021 at 01:43:30PM +0100, Arne Schwabe wrote:
> Modern TLS libraries might drop Blowfish by default or distributions
> might disable Blowfish in OpenSSL/mbed TLS. We still signal OCC
> options with BF-CBC compatible strings. To avoid requiring BF-CBC
> for this, special this one usage of BF-CBC enough to avoid a hard
> requirement on Blowfish in the default configuration.

I was about to merge this, based on Antonio's ACK, but this part of
the code confuses me:

> diff --git a/src/openvpn/options.c b/src/openvpn/options.c
> index b81137cf..d52057cc 100644
> --- a/src/openvpn/options.c
> +++ b/src/openvpn/options.c
> @@ -3836,18 +3856,32 @@ options_string(const struct options *o,
>                 + (TLS_SERVER == true)
>                 <= 1);
>  
> -        init_key_type(&kt, o->ciphername, o->authname, o->keysize, true,
> -                      false);
> +        /* Skip resolving BF-CBC to allow SSL libraries without BF-CBC
> +         * to work here in the default configuration */
> +        const char *ciphername = o->ciphername;
> +        int keysize;
> +
> +        if (strcmp(o->ciphername, "BF-CBC") == 0) {
> +            init_key_type(&kt, "none", o->authname, o->keysize, true,
> +                          false);
> +            ciphername = cipher_kt_name(kt.cipher);
> +            keysize = 128;
> +        }
> +        else
> +        {
> +            init_key_type(&kt, o->ciphername, o->authname, o->keysize, true,
> +                          false);
> +            keysize = kt.cipher_length * 8;
> +        }
>          /* Only announce the cipher to our peer if we are willing to
>           * support it */
> -        const char *ciphername = cipher_kt_name(kt.cipher);

So the old code always sends "cipher_kt_name(kt.cipher)".

The new code adds a special case handling for "BF-CBC", calling
init_key_type(none), but then does the "cipher_kt_name()" only for
the "BF-CBC/none" case, no more for the "all other ciphers".

This looks like the wrong way around - shouldn't it do the

> +            ciphername = cipher_kt_name(kt.cipher);

for the "not BF-CBC" case, and "ciphername = o->cipher" for "only BF-CBC"?


Call me confused...

As a side note, it seems to fail two of my t_client test cases, 
namely "talking to a 2.3 server with --cipher BF-CBC" and "talking to 
a 2.4 server with --ncp-disable", so maybe that code needs some more 
discussion.  I have not investigated more into these failures, first 
want to understand what the code tries to do.

gert

Patch

diff --git a/src/openvpn/init.c b/src/openvpn/init.c
index c3493c42..df0c7ebc 100644
--- a/src/openvpn/init.c
+++ b/src/openvpn/init.c
@@ -2770,14 +2770,30 @@  do_init_crypto_tls_c1(struct context *c)
 #endif /* if P2MP */
         }
 
-        /* Do not warn if we only have BF-CBC in options->ciphername
-         * because it is still the default cipher */
-        bool warn = !streq(options->ciphername, "BF-CBC")
-             || options->enable_ncp_fallback;
-        /* Get cipher & hash algorithms */
-        init_key_type(&c->c1.ks.key_type, options->ciphername, options->authname,
-                      options->keysize, true, warn);
-
+       /*
+        * BF-CBC is allowed to be used only when explicitly configured
+        * as NCP-fallback or when NCP has been disabled.
+        * In all other cases don't attempt to initialize BF-CBC as it
+        * may not even be supported by the underlying SSL library.
+        *
+        * Therefore, the key structure has to be initialized when:
+        * - any non-BF-CBC cipher was selected; or
+        * - BF-CBC is selected and NCP is disabled (explicit request to
+        *   use the BF-CBC cipher); or
+        * - BF-CBC is selected, NCP is enabled and fallback is enabled
+        *   (BF-CBC will be the fallback).
+        *
+        * Note that BF-CBC will still be part of the OCC string to retain
+        * backwards compatibility with older clients.
+        */
+        if (!streq(options->ciphername, "BF-CBC") || !options->ncp_enabled
+            || options->enable_ncp_fallback)
+        {
+            /* Do not warn if the if the cipher is used only in OCC */
+            bool warn = !options->ncp_enabled || options->enable_ncp_fallback;
+            init_key_type(&c->c1.ks.key_type, options->ciphername, options->authname,
+                          options->keysize, true, warn);
+        }
         /* Initialize PRNG with config-specified digest */
         prng_init(options->prng_hash, options->prng_nonce_secret_len);
 
diff --git a/src/openvpn/options.c b/src/openvpn/options.c
index b81137cf..d52057cc 100644
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -3664,9 +3664,29 @@  calc_options_string_link_mtu(const struct options *o, const struct frame *frame)
     {
         struct frame fake_frame = *frame;
         struct key_type fake_kt;
-        init_key_type(&fake_kt, o->ciphername, o->authname, o->keysize, true,
-                      false);
+
         frame_remove_from_extra_frame(&fake_frame, crypto_max_overhead());
+
+
+        /* o->ciphername might be BF-CBC even though the underlying SSL library
+         * does not support it. For this reason we workaround this corner case
+         * by pretending to have no encryption enabled and by manually adding
+         * the required packet overhead to the MTU computation.
+         */
+        const char* ciphername = o->ciphername;
+
+        if (strcmp(o->ciphername, "BF-CBC") == 0)
+        {
+            /* none has no overhead, so use this to later add only --auth
+             * overhead */
+
+            /* overhead of BF-CBC: 64 bit block size, 64 bit IV size */
+            frame_add_to_extra_frame(&fake_frame, 64/8 + 64/8);
+        }
+
+        init_key_type(&fake_kt, ciphername, o->authname, o->keysize, true,
+                      false);
+
         crypto_adjust_frame_parameters(&fake_frame, &fake_kt, o->replay,
                                        cipher_kt_mode_ofb_cfb(fake_kt.cipher));
         frame_finalize(&fake_frame, o->ce.link_mtu_defined, o->ce.link_mtu,
@@ -3836,18 +3856,32 @@  options_string(const struct options *o,
                + (TLS_SERVER == true)
                <= 1);
 
-        init_key_type(&kt, o->ciphername, o->authname, o->keysize, true,
-                      false);
+        /* Skip resolving BF-CBC to allow SSL libraries without BF-CBC
+         * to work here in the default configuration */
+        const char *ciphername = o->ciphername;
+        int keysize;
+
+        if (strcmp(o->ciphername, "BF-CBC") == 0) {
+            init_key_type(&kt, "none", o->authname, o->keysize, true,
+                          false);
+            ciphername = cipher_kt_name(kt.cipher);
+            keysize = 128;
+        }
+        else
+        {
+            init_key_type(&kt, o->ciphername, o->authname, o->keysize, true,
+                          false);
+            keysize = kt.cipher_length * 8;
+        }
         /* Only announce the cipher to our peer if we are willing to
          * support it */
-        const char *ciphername = cipher_kt_name(kt.cipher);
         if (p2p_nopull || !o->ncp_enabled
             || tls_item_in_cipher_list(ciphername, o->ncp_ciphers))
         {
             buf_printf(&out, ",cipher %s", ciphername);
         }
         buf_printf(&out, ",auth %s", md_kt_name(kt.digest));
-        buf_printf(&out, ",keysize %d", kt.cipher_length * 8);
+        buf_printf(&out, ",keysize %d", keysize);
         if (o->shared_secret_file)
         {
             buf_printf(&out, ",secret");