Default mode of operation in TLS cipher suites

Question

I've been trying to evaluate the security of a list of cipher suites and I've come across cipher suites, not explicitly featuring a mode of operation.

Form this list (openvpn --show-tls), one of such cipher suites would be:

DH-RSA-AES256-SHA256

As the cipher suites are provided by the underlying SSL library (OpenSSL 1.0.2g), I assume there must be a default mode, if not explicitly defined.

What mode would be used in this case?

Answer 1

If you look at the official list, you'll see that the official cipher suite name always has the mode explicitly stated; each AES-based transform is explicitly called out to be CBC, GCM or CCM.

Going through the OpenSSL source code, we find that the ciphersuite you cited has the value 0x00, 0x37, that is, TLS_DH_RSA_WITH_AES_256_CBC_SHA. Hence, CBC is the mode being implicit. I suspect this is implicit because OpenSSL predates TLS 1.2; in TLS 1.1 and prior, CBC was the only block cipher mode allowed (and hence there was no need to call it out); once the started supporting TLS 1.2, they didn't feel like they should go back and modify the name strings they used for existing ciphersuites.