Hi,
On 19/02/2021 17:52, 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 <a...@rfc2549.org>
>
> Patch v2: add more clarifying comment, do not warn about OCC only insecure
> ciphers, code improvements
>
> Patch V3: Put ciphername resolution via ciper_kt_name in the right branch
>
> Patch V4: Fix cornercase of BF-CBC in data-ciphers not itialising cipher.
>
> Patch v5: I accidently resend v3 as v4. So v5 is just a resend of the real v4
> ---
> src/openvpn/crypto_backend.h | 2 ++
> src/openvpn/init.c | 37 ++++++++++++++++++++++------
> src/openvpn/options.c | 47 +++++++++++++++++++++++++++++++-----
> 3 files changed, 72 insertions(+), 14 deletions(-)
>
> diff --git a/src/openvpn/crypto_backend.h b/src/openvpn/crypto_backend.h
> index 384ffc80..93f7e475 100644
> --- a/src/openvpn/crypto_backend.h
> +++ b/src/openvpn/crypto_backend.h
> @@ -241,6 +241,8 @@ const cipher_kt_t *cipher_kt_get(const char *ciphername);
> * The returned name is normalised to the OpenVPN config name in case the
> * name differs from the name used by the crypto library.
> *
> + * Returns [null-cipher] in case the cipher_kt is NULL.
> + *
> * @param cipher_kt Static cipher parameters
> *
> * @return a statically allocated string describing the cipher.
> diff --git a/src/openvpn/init.c b/src/openvpn/init.c
> index 46c933b1..cfd71482 100644
> --- a/src/openvpn/init.c
> +++ b/src/openvpn/init.c
> @@ -2769,14 +2769,35 @@ 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 or explicitly
> + * allowed in the in ncp_ciphers list.
> + * In all other cases do not 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).
> + * - BF-CBC is in data-ciphers and we negotiate to use BF-CBC:
> + * If the negotiated cipher and options->ciphername are the
> + * same we do not reinit the cipher
> + *
> + * 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->ncp_enabled && tls_item_in_cipher_list("BF-CBC",
> options->ncp_ciphers))
> + || 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 059386b3..c02ad051 100644
> --- a/src/openvpn/options.c
> +++ b/src/openvpn/options.c
> @@ -3609,9 +3609,29 @@ 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);
Please add space around the / operators.
> + }
> +
> + 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,
> @@ -3781,18 +3801,33 @@ 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);
> + keysize = 128;
> + }
> + else
> + {
> + init_key_type(&kt, o->ciphername, o->authname, o->keysize, true,
> + false);
> + ciphername = cipher_kt_name(kt.cipher);
> + 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");
>
Other than the whitepsace complaint, this patch makes sense to me (as it
did the previous version, but thanks Gert for spotting the BF-CBC problem).
It's ACK for me, but Gert's test rig will give its final verdict.
Acked-by: Antonio Quartulli <anto...@openvpn.net>
--
Antonio Quartulli
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