checking for enable-weak-ssl-ciphers at runtime?

[Daniel Lenski]

Aha, thanks Matt.
The code I resorted to using does SSL_CTX_new(), SSL_new(),
SSL_CTX_set_cipher_list(), and then iterates
through SSL_get1_supported_ciphers() looking for anything with 3DES/RC4.
So it seems I can simplify that down to just SSL_CTX_new() and
SSL_CTX_set_cipher_list().

I know that supporting obsolete, insecure crypto isn't a high priority for
OpenSSL (though unfortunately a necessary one for OpenConnect), but it does
seem that having a simple general-purpose API for checking "is this
cipher(suite) known and can we actually use it?" would be a desirable
feature.

This particular case was a bit frustrating to me because with OpenSSL
1.0.x, there is a much simpler way to check if particular ciphers can be
instantiated with the SSL_METHOD API exposed…

/* XX: Based on my reading of the SSL_METHOD API
 * (
https://github.com/openssl/openssl/blob/5a5530a29abcf5d7ab7194d73b3807d568b06cbd/ssl/ssl_local.h
)
 * and the constants in the ssl3.h header
 * (
https://github.com/openssl/openssl/blob/master/include/openssl/ssl3.h#L45-L50
),
 * as well as the way that the 3DES/RC4 ciphers do or don't get compiled in
 * (https://github.com/openssl/openssl/blob/master/ssl/s3_lib.c#L166-L182)
 *
 * This is the most reasonable way to figure out whether the library can
 * actually instantiate a cipher. The other would be to iterate through
 * the ciphers (using m->num_ciphers() and m->get_cipher(num)) and
 * compare them by name.
 */
int can_enable_insecure_crypto()
{
const SSL_METHOD *m = SSLv23_client_method();
unsigned char ch_SSL3_CK_RSA_DES_192_CBC3_SHA[2] = { 0x00, 0x0a };
unsigned char ch_SSL3_CK_RSA_RC4_128_SHA[2] = { 0x00, 0x05 };

if (!m)
return -ENOMEM; /* XX: static, should never happen */
if (m->get_cipher_by_char(ch_SSL3_CK_RSA_DES_192_CBC3_SHA) &&
   m->get_cipher_by_char(ch_SSL3_CK_RSA_RC4_128_SHA))
return 0;
return -ENOENT;
}

On Sun, May 24, 2020 at 2:49 PM Matt Caswell <matt at openssl.org> wrote:

>
>
> On 23/05/2020 21:08, Daniel Lenski wrote:
> > When OpenConnect is explicitly requested to connect to an ancient
> > server, what I am currently trying to do is
> > SSL_CTX_set_cipher_list(ctx, "DEFAULT:+3DES:+RC4"). However, this
> > fails silently on subsequent connection if 3DES/RC4 support isn't
> > available.
>
> As long as at least one cipher is successfully set then this command
> will succeed. By setting "DEFAULT" you're getting all the ciphersuites
> in the default list and hence the command succeeds. If you want to test
> if you have any 3DES ciphersuites available then you can try this:
>
> SSL_CTX_set_cipher_list(ctx, "3DES")
>
> This will succeed if at least one 3DES cipersuite is available, and fail
> otherwise. Or you could do:
>
> SSL_CTX_set_cipher_list(ctx, "3DES:RC4")
>
> Which will succeed if there is at least one ciphersuite based on 3DES or
> RC4 available, and fail otherwise.
>
>
> > It was suggested that I should try EVP_get_ciphername().
>
>
> The ciphers available via the EVP API are only indirectly related to the
> ciphersuites available in libssl. If there are no 3DES based ciphers
> available via EVP then there won't be any libssl 3DES based
> ciphersuites. But the reverse is not true, i.e. 3DES may not be
> available in libssl, but it is via EVP. So this is not a great test for
> your purposes.
>
> Matt
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mta.openssl.org/pipermail/openssl-users/attachments/20200525/bbfb1763/attachment-0001.html>