What is a block cipher?
A block cipher is a type of symmetric-key encryption algorithm that transforms a fixed-length block of plaintext (unencrypted text) data into a block of ciphertext (encrypted text) data of the same length. This transformation takes place under the action of a user-provided secret key. Decryption is performed by applying the reverse transformation to the ciphertext block using the same secret key. The fixed length is called the block size, and for many block ciphers, the block size is 64 bits. In the coming years the block size will increase to 128 bits as processors become more sophisticated. For those with a mathematical background (see Appendix A): Since different plaintext blocks are mapped to different ciphertext blocks (to allow unique decryption), a block cipher effectively provides a permutation (one to one reversible correspondence) of the set of all possible messages. The permutation effected during any particular encryption is of course secret, since it is a function of the s
