Yes, HMAC is (one of) the right solution(s) here. On the other hand, do you trust the rest of the program if the author gets this wrong? The attack does work,

Yes, HMAC is (one of) the right solution(s) here. On the other hand, do you trust the rest of the program if the author gets this wrong? The attack does work, see e.g. http://www.vnsecurity.net/t/length-extension-attack/ http://www.vnsecurity.net/t/length-extension-attack/. This example suggests that any padding should not matter; and even if it does, being able to mount this attack on a substantial part of visitors is bad. (Sorry for being vague here - I don't have the time to look at this properly now.)