A friend who's learning cryptography writes with a few questions about block ciphers:
(1) Let's say we're using AES-128 -- 128 bit keys, 128 bit blocks.
For a given 128 bit block of plaintext "P" - if I was to iterate through all 2**128 key permutations and encrypt the same plaintext P with each key, would the outputs all be unique, or would there be collisions?
For a given 128 bit key "K", if I was to use K to encrypt every possible (2**128) plaintext, would the outputs all be unique, or would there be collisions?
These are all reasonable questions with simple answers. But I'm not going to give them. Why bother with two simple answers when we can give one really complicated intuition?
read more.........http://blog.cryptographyengineering.com/2013/04/wonkery-mailbag-ideal-ciphers.html