OFB (Output Feedback)

OFB, or Output Feedback mode, is a mode of operation for block ciphers. It effectively turns a block cipher into a synchronous stream cipher. OFB mode enables the encryption of plaintext messages of any length, and it's particularly useful in scenarios where the same encryption key is used multiple times.

OFB mode starts with an IV, which must be unique for each encryption operation with the same key. The IV is typically the size of the block cipher's block size.

  • The IV is encrypted with the block cipher.
  • The output of this encryption (not the plaintext) is then XORed with the plaintext to produce the ciphertext.
  • For the next block, this encrypted output (not the ciphertext) is encrypted again with the block cipher, and the output of this is XORed with the next segment of plaintext to produce the next segment of ciphertext.
  • This process continues for the entire length of the plaintext.

The decryption process in OFB mode is essentially the same as the encryption process. The same sequence of blocks that was XORed with the plaintext during encryption is XORed with the ciphertext during decryption to recover the plaintext.

Like CFB, OFB turns a block cipher into a stream cipher. It is suitable for encrypting data of any size. In OFB mode, errors in a ciphertext do not propagate; a bit error in the ciphertext results in a corresponding bit error in the plaintext.

OFB relies on synchronization between the sender and receiver. If they become desynchronized (e.g., due to lost or inserted bits), decryption will fail until they are resynchronized. OFB can process data in segments smaller than the block size, so it does not require padding the plaintext to a multiple of the block size.

OFB mode is often used in scenarios where error propagation is undesirable and the transmission channel might introduce errors, such as in satellite or wireless communication.

An example:

# Pseudo-code example using a block cipher in OFB mode
cipher = BlockCipher(key, mode=OFB, iv=initialization_vector)

# Encrypt
ciphertext = cipher.encrypt(plaintext)

# Decrypt
decrypted_text = cipher.decrypt(ciphertext)