Which mode is a block cipher implementation as a self-synchronizing stream…

Answer: Cipher Feedback Mode (CFB)

Why this is correct:

Cipher Feedback Mode converts a block cipher into a self-synchronizing stream cipher. The keystream used to encrypt plaintext is derived from the block cipher output combined with previous ciphertext, so the receiver can resynchronize automatically after a limited number of lost or inserted bits/blocks.

• How CFB works: the block cipher output is XORed with plaintext to produce ciphertext; previous ciphertext segments are fed back into the cipher input to generate subsequent keystream segments.

• Self-synchronization property: because the keystream depends on previous ciphertext, the receiver regains correct alignment automatically after a limited number of received bits/blocks.

• Why the other modes are not correct: Output Feedback (OFB) is a synchronous stream mode (keystream independent of ciphertext) and does not self-synchronize; Cipher Block Chaining (CBC) and Electronic Codebook (ECB) are block modes and are not stream ciphers.