A block cipher is a symmetric-key encryption scheme that encrypts () and decrypts () data in fixed blocks. The key must have the same length as the block size.
- Encryption:
- Decryption:
A block cipher should be a pseudorandom permutation as it uses a (keyed) bijective mapping from a bitstream to another bitstream in a way that is not practical to undo without knowing the key. As long as the recipient of the ciphertexts knows the key, he can map the ciphertext back to the original plaintext.
Given that an ideal block cipher is close to a random permutation, knowing pairs do not reveal information about any other such pairs. Essentially, an adversary has to do a brute-force search over the key space.