1

After the vulnerability was released for IPSec IKEv1 PSKs, our organization decided to increase the size and complexity of the PSK.

However, upon releasing the new PSK we had a few issues:

  1. Chromebooks can not accept symbols for PSK entry.
  2. Android can not accept PSK longer than 9 characters.

So we had to restrict the PSK to [a-zA-Z0-9]{9} which is log2(629) which turns into 53.5878 bits of entropy max.

I am pushing to switch to OpenVPN, but there is pushback to the idea.

How feasible would a brute force attack on 53.5878 bits be with the PSK for IKEv1 (using the vulnerability)?

Would periodically changing the PSK (like once a week) help?

2

A nearly 54-bit keyspace is not even close to enough. Consider that DES, which has an effective key size of 56 bits, is four times larger than this, yet there are numerous services that routinely crack arbitrary DES keys. Even if you changed the PSK once per week, the keyspace is just too low for it to be an effective security measure. You absolutely need to remedy this situation either by allowing for a larger PSK, or switching to an alternative protocol which is suitable for your needs. The absolute minimum key size should be 80 bits, with an ideal key being at least 128 bits.

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