OFB

OFB stands for Output Feedback, a mode of operation for block ciphers in cryptography. It converts a block cipher into a stream-like generator that produces a keystream independent of the plaintext, which is then XORed with the message to produce ciphertext.

In OFB, a block cipher E with key k and an initialization vector IV is used to generate the keystream. Start with a state S0 = IV and iteratively compute Si = E_k(Si-1) for i = 1, 2, … The keystream blocks Ki are taken as Si, and ciphertext blocks Ci are obtained by Ci = Pi XOR Ki. Decryption uses the same keystream: Pi = Ci XOR Ki. Because the keystream depends only on the IV and the key, the same process is used for encryption and decryption.

OFB has several notable properties. It behaves like a synchronous stream cipher: a bit error in a ciphertext block generally affects only the corresponding plaintext bit, with no error propagation to later blocks. The keystream does not depend on ciphertext, so keystream blocks can be generated in advance if the IV is known. A major security caveat is that reusing the same key and IV produces identical keystream, which compromises confidentiality; therefore IV uniqueness for each encryption with the same key is essential. OFB is not self-synchronizing, and loss or misalignment of data can prevent correct decryption without reestablishing the proper state.

OFB may also be an acronym for terms in other domains, but in cryptography it most often refers to Output Feedback as described above.