A block cipher is an encryption function for fix-sized blocks.
Given n-bit sized block 'p', encryption function Ek(p) will encrypt that block using key 'k'.
It can be compared to a lookup table where for each pair (k,p) encrypted block 'c' of size equal to size of block 'p' can be produced.
A n-bit block cipher can be compared to lookup table of size 2n, with each element consisting of n bits.
Such cipher's encryption can be reversed.
For more details, see Literature: , .
See also: Block Cipher Modes.