CipherModes

Cipher modes, also known as modes of operation, are specific algorithms that define how a block cipher is used to encrypt messages of arbitrary length. Block ciphers encrypt fixed-size blocks of data, and cipher modes extend this capability to handle longer messages by repeatedly applying the block cipher. Each mode has different properties concerning security, efficiency, and error propagation.

Electronic Codebook (ECB) mode is the simplest mode. Each block of plaintext is encrypted independently with the same key. This simplicity makes it easy to implement but also insecure, as identical plaintext blocks will result in identical ciphertext blocks, revealing patterns.

Cipher Block Chaining (CBC) mode addresses ECB's weakness by introducing a dependency between consecutive blocks. Each plaintext block is XORed with the previous ciphertext block before encryption. This ensures that identical plaintext blocks produce different ciphertext blocks. CBC requires an initialization vector (IV) for the first block, which must be unpredictable and unique for each encryption.

Cipher Feedback (CFB) mode is a self-synchronizing stream cipher. It encrypts ciphertext in blocks, but it is similar to CBC in that it uses the previous ciphertext block to generate the keystream. This mode allows for encrypted characters to be transmitted as soon as they are generated, without waiting for a full block.

Output Feedback (OFB) mode is also a stream cipher. It generates a pseudorandom stream of bits that are then XORed with the plaintext to produce the ciphertext. Unlike CFB, OFB generates the keystream independently of the plaintext and ciphertext, making it resistant to bit errors. However, if the same keystream is used twice, it compromises security.

Counter (CTR) mode is another stream cipher that encrypts by XORing the plaintext with an output of a cryptographically strong pseudorandom number generator. The generator is driven by a succession of values, typically an initial counter value and a nonce. CTR mode is highly efficient and allows for parallel processing of blocks.