In Cryptography, a block cipher is a keyed permutation from N-bit blocks to N-bit blocks.

Modes of operation

A block cipher is a simple primitive that takes a key, and can encrypt and decrypt fixed-size blocks of data. This means you need to use a block cipher in a way that makes it suitable to encrypt different sizes of data.

Electronic Code Book (ECB)

Electronic Code Book (ECB) mode is the block cipher in its raw form.

• Use the key with the block cipher to get your N-bit to N-bit permutation.
• Go over the plaintext in blocks, encrypt each block using the raw block cipher.
• There is no IV. With the same key, the same N-bit block always encrypts to the same ciphertext.

Counter mode (CTR)

• Instead of putting the plaintext through the block cipher, counter mode instead encrypts an incrementing counter to generate a keystream.
• The plaintext is then XOR’ed with the keystream.