Introduction
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.
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Decryption
process of OFB reverse of encryption process as shown in figure.
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Application of OFB Mode
Use of
OFB, Transmission over noisy channel. i.e., satellite communication.
To learn more about OFB Mode, Click here
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