Cipher Feedback (CFB) Mode
The CFB mode uses an initial chaining vector (ICV) in its processing. CFB mode performs cipher feedback encryption. CFB mode operates on segments instead of blocks. The segment length (called s) is between one bit and the block size (called b) for the underlying algorithm (DES or AES), inclusive. ICSF only allows segment sizes which are a multiple of eight bits (complete bytes). Each encryption step takes an input block, enciphers it with the key provided to generate an output block, takes the most significant s bits of the output block, and then exclusive ORs that with the plaintext segment. The first input block is the ICV and each subsequent input block is formed by concatenating the (b-s) least significant bits of the previous input block and the ciphertext (s bits) from the previous step to form a full block. The input text can be of any length. The output text will have the same length as the input text.