Hello,
I haven't found a definitive statement on whether OpenVPN has Perfect Forward Secrecy by design. If i understand it right, there are 2 different things that are sometimes confused when it comes to forward secrecy:
1. forward security, as explained here:
https://forums.openvpn.net/topic9408.html#p18625
This protects against an attacker that manages to break a single session key (he can only decrypt e.g. 1h of data).
2. forward secrecy as in using a key exchange protocol that doesn't allow reconstructing the session key by an attacker that has access to the server private key.
Point 1 does not offer any protection if the attacker at some time in the future gets the private key of the server, because he can the decrypt the session key exchange and by this, gets the session keys.
The Question is: does OpenVPN use a forward secrecy key exchange protocol such as DH by design, or
does this depend on whether the selected cipher suite uses DH ? Do I need to provide DH parameters? Or to put it another way: will I have PFS if e.g. "AES256-SHA" is used instead of "ECDH-RSA-AES256-SHA" and no DH params are specified in the config?
Please correct me if I am totally off the track here.
Another Question: is there a specific reason why OpenVPN still uses/supports Blowfish instead of the newer (and perhaps securer) Twofish? And what mode of operation should I choose, CBC, CFB or OFB? The manpage only says CBC is recommended.
Thanks!
