2

Basically, every hour OpenVPN renegotiates the TLS keys. What is the advantage of this, and what are the risks to reducing that to, for example, once per day?

4

Shorter key lifetime = less time for an attacker to a) capture known-plaintext packets and b) somehow crack the key. Also, a leaked key will only help an attacker gain access to your communication for 1h instead of a day.

So, damage control, essentially.

  • Also to prevent birthday attacks. e.g. sweet32.info – Josef says Reinstate Monica Nov 29 '16 at 14:40
  • I noticed reneg-sec because we had configured reneg-bytes to mitigate sweet32. What's a scenario where the attacker could get the TLS keys for one hour but not the next? What practical plain texts attacks are there in this scenario? – Johan de Vries Nov 29 '16 at 15:28
  • @JohandeVries chosen plaintext attacks like en.wikipedia.org/wiki/CRIME rely on an attackers scenario to observe ciphertext data amount when injecting packets of known content. You need some time to play through potential secret content. Chosen plaintext is expecially relevant to VPNs, because an attacker can choose plaintext solely by e.g. offering a website that the user has open in the background. – Marcus Müller Nov 29 '16 at 17:08
  • Will this allow access to the VPN data from other clients? Or does it depend on the attacker being behind the VPN client? – Johan de Vries Nov 30 '16 at 6:58

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