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;
* (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@xxxxxxxxxxx> 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
