Sunday, October 17, 2021

OFB Mode | Output Feedback Mode | Mode of Block Cipher | Application of OFB Mode



Cryptographic algorithm works on main two techniques: block and stream ciphers.

In a stream cipher, the plaintext is encrypted one bit at a time. In a block cipher, the plaintext is broken into blocks of a fixed length and the bits in each block are encrypted together. One of the main issues with block ciphers is that they only allow you to encrypt messages the fixed size as their block length.

If plaintext, which has a block size 64 bits easily encrypt. But encrypt a 65-bit message, you need a way to define how the second block should be encrypted.

The solution to this is called block cipher modes of operation. Need of block cipher mode is basic building block for providing data security. In block cipher rather than encrypting one bit at a time, block of bits is encrypted at a time.

There are 5 modes of operation for block cipher that may be used in a wide variety of applications like symmetric key cryptographic algorithm. These modes define how data encrypted and decrypted.

Output Feedback Mode (OFB)

The output feedback (OFB) mode is similar in structure to that of CFB. In CFB, the ciphertext unit is fed back to the shift register. In case of output feedback mode (OFB), difference is that output of encryption process O1 instead of generating text C1 is directly placed in next stage of shift register without XOR operation.

In OFB mode, if there is a small error in individual bits, it remains an error in individual bits which do not corrupt the whole encrypted message (to avoid bit error transmission) which is the biggest advantage of OFB mode over all other mode. Other difference of OFB mode, operates on full blocks of plaintext and ciphertext, not on an s-bit subset.


Figure: Encryption Process of OFB Mode

Decryption process of OFB reverse of encryption process as shown in figure.

Figure: Decryption Process of OFB Mode

Application of OFB Mode

Use of OFB, Transmission over noisy channel. i.e., satellite communication.

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