[Cryptography] Perfect Integrity?

[Peter Gutmann]

Peter Fairbrother <peter at tsto.co.uk> writes:

>A one-bit W-C MAC will give an attacker no advantage in guessing the bit -
>but he will still have a 50% chance of guessing right.
>
>For information-theoretic security the MAC has to be as long as the message.
>I think.

Depends on the circumstances.  Lets say the MAC is being used as part of an
alarm circuit, where a keepalive is sent across the circuit every 50ms, with a
1-bit MAC attached.  The attacker would have to guess the bit, then 50ms later
guess the next bit, then 50ms later guess the next one, etc.  Get a single bit
wrong and you trigger the alarm.

Peter.