Cryptography Week 3: Pseudorandom Functions and Block Ciphers

F is a pseudorandom function if Fk, for a uniform key k which is an element {0,1}^n, is indistinguishable from a uniform function Func_n.

Let F be a length preserving, keyed function:

F is a keyed permutation iff: F_k is a bijection for every k F_k^-1 is efficiently computable

F is a psuedorandom permutation if F_k, for a uniform key which is an element {0,1}^n, is indistinguishable from a uniform permutation.

Block Cipher: Practical Construction of pseudorandom permutation

No asypmptotic, and hard to distinguish f_k from a uniform f element perm even for attackers running in time = 2^n

AES is a standardized block cipher block length: 128 bit key length: 128,192, and 256 bit (2^x security so even a key length of 128 is fine)