4. Output Feedback (OFB) mode:

This mode is very similar to CFB mode, with the only difference: each bit in the ciphertext is independent of the previous bit or bits. This avoids error propagation. If any error occurs in transmission, it does not affect the bits that follow.

• It involves feeding the successive output blocks from the underlying block cipher back to it. These feedback blocks provide string of bits to feed the encryption algorithm which act as the key-stream generates as in case of OFB mode.
• The key-stream generated is XORed with the plaintext blocks. The OFB mode requires an IV as the initial random n-bit input block. The IV need not be secret.
• The operation is depicted in the following illustration.