cipher AES-256-CBC

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OpenVpn Newbie
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Joined: Mon Oct 25, 2010 12:28 pm

cipher AES-256-CBC

Post by rossi2000 » Mon Oct 25, 2010 12:39 pm

Good Evening all.

ive succesfully installed openvpn on ubuntu and have a few clients connecing, all is good.

i decided to enable cipher AES-256-CBC encryption in the server and client config files.
everyhting works fine but.
i need to know if using the 256 aes encryption uses more processing/bandwidth than the default 128bit blowfish encryption? its just i noticed that ping times seem to fluctaute slightly.

also i noticed that clients disconnect/reconnect every hour (re-negotiate tls session), and on the openvpn site it says it is set to default of 1 hour, but gives no details on how to change this. is there a way to increase this?

any info appreciated!!


OpenVPN Super User
Posts: 219
Joined: Mon Nov 23, 2009 8:24 pm

Re: cipher AES-256-CBC

Post by mwandelaar » Mon Oct 25, 2010 10:37 pm

Ofcourse is AES-256-CBC more compute-intensive than the "default" crypto-scheme, BF-128-CBC.

On the other hand, on a normal CPU you won't notify the difference. And on linux-systems you can speed-up the AES cycles by loading the AES kernel module. Even better: openssl (the crypto-suite used by openvpn) supports the VIA Padlock engine. By using a via processor with this extension, you could accelerate all the crypto-calculations using the hardware.

You allso mentioned a slight variable RTT for ICMP-packets. I don't think this has anything to do with the crypto, but more on the local network, unless the client or server has a significant cpu-load. So can you explain more on your setup and the values of ping-times?

Hint: be aware: only using "tls-cipher AES-256-CBC" isn't enough to totally encrypt your traffic 256 bits. This TLS cipher encrypts the control-channel. The datachannel is still BF-160. If you want EAS-256 for all the traffic, both server and client need the directive "keysize 256" in there configfile.

The "disconnect-reconnect" is not really disconnecting. The server and client are re-negotiating the symmetric key for the data-channel. The directive "reneg-sec <nr of seconds>" changes the default. Depending of the content you're protecting, i think 1 hour should still be fine.

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