cipher feedback mode

cipher feedback mode (CFB) is another mode of operation for a block cipher. a weakness of CBC is a full block is required before the ciphertext can be sent, which CFB attempts to improve.

1. start with a random initial vector $C_0$ as in CBC.
2. output $C_i:=M_i\oplus E_K(C_{i-1})$

note that once $C_{i-1}$ is computed, the block cipher step for $C_i$ can be computed immediately — no waiting for $M_i$ required. this means that as bits of $M_i$ arrive, we can encrypt them immediately without waiting for the block to fill.

an ancillary advantage is that we don’t have to pad the last block.