Association expire, explain it at the UI

I agree in principle, I would rephrase a little the message:

This timeout is necessary to ensure proper security of your identities. If an attacker sniffing the network gets a hold of the SHA1 hash and is somehow able to bruteforce it, he can perform a man in the middle attack and access the target site. Since brute force attacks take a long time, this timeout ensures this attack is impracticable.

Also, "Read more" could be "More information about this issue."

Thank you for making the text more readable.

Thanks. :) I therefore feel this is RTBC.