tls-cipher, cipher and auth

anatoli

Could someone please clarify the details of the 3 subj options?

The doubts are:

1. Is there any relation between --tls-cipher and --cipher options? AFAIU, --tls-cipher should specify the ciphers to use for peers' authentication and symmetric key exchange, i.e. asymmetric public-key encryption. Then the data (symmetric) encryption is done with a cipher specified with --cipher option and every packet is authenticated with a digest specified by --auth option.

If this is correct, then why there are values for --tls-cipher like TLS-DHE-RSA-WITH-AES-256-GCM-SHA384? TLS-DHE-RSA would be the TLS protocol, Diffie–Hellman key exchange method and the RSA public-key cryptosystem.

But what does the WITH-AES-256-GCM-SHA384 part mean? AES is the symmetric encryption algo, GCM is its mode of operation, SHA384 should be the HMAC algo (unclear how it is related to GCM, as GCM has its own message authentication).

Does this value specify both asymmetric and symmetric encryption algorithms? In this case what does cipher option specify? If I comment it out, the logs show cipher BF-CBC, which is the default value for --cipher option.

Also, there is no indication in the logs of --tls-cipher used, why?

2. --auth option should be the HMAC digest algo for symmetric encryption authentication. The --show-digests option shows a list of available digests, among them there are SHA512 and RSA-SHA512 entries. The second digest I suppose is encrypt-then-MAC, but specifying one on one peer and another on another produces no errors when they connect to each other, what I interpret as RSA-SHA512 value for --auth option being the same as SHA512. Is this interpretation correct? What are the obstacles for encrypt-then-MAC functionality? Or does SHA512 already use this form of HMAC? Then what does RSA-SHA512 mean?

3. According to the different values available for --tls-cipher, --cipher and --auth, openvpn has support for AES-GCM mode of operation and SHA512 hashing. Then why there is no AES-256-GCM symmetric cipher-mode (which is considered the best method for symmetric encryption) for --cipher option or TLS-DHE-RSA-WITH-AES-256-GCM-SHA512 for --tls-cipher (assuming that SHA512 part means something additional to GCM authentication)?

maikcat

interesting questions... 🙂

i am NOT a crypto expert but after some digging i found out that:

Then why there is no AES-256-GCM symmetric cipher-mode

This is supported only with TLS 1.2 ,AFAIK openvpn does not support TLS 1.2 (yet?)
also GCM support only 128bit block size,correct?

can you also read this...

https://community.openvpn.net/openvpn/ticket/304

Michael.

IncreasedSecurity

maikcat wroteinteresting questions... 🙂

i am NOT a crypto expert but after some digging i found out that:

Then why there is no AES-256-GCM symmetric cipher-mode
This is supported only with TLS 1.2 ,AFAIK openvpn does not support TLS 1.2 (yet?)
also GCM support only 128bit block size,correct?

can you also read this...

https://community.openvpn.net/openvpn/ticket/304

Michael.

Also, see https://community.openvpn.net/openvpn/ticket/301 - "Support AEAD cipher modes"