Output Feedback Mode

What

The output feedback mode is a mode of operation that uses the cipher outputs and feeds it back into the next successive encryptions. It uses these cipher output blocks to xor with the input blocks to obtain the ciphertext blocks

Because each of the output ciphertext blocks are independent from each other, therefore, if there was a problem in transmission of the ciphertext block, then only the corresponding plaintext will be garbled. Everything else is intact. Thus, this is important for transmissions that can only happen once, like on the radio

How

OFB Encryption

$I_1 = IV$

$I_i = O_{i-1}$

$O_i = E(I_i)$

$C_i = P_i \oplus O_i$

OFB Decryption

$I_1 = IV$

$I_i = O_{i-1}$

$O_i = E(I_i)$

$C_i = P_i \oplus O_i$