Which
of the following encryption modes has the property to allow many error correcting codes to function
normally even when applied before encryption?
A.
OFB mode
B.
CFB mode
C.
CBC mode
D.
PCBC mode
Explanation:
The output feedback (OFB) mode makes a block cipher into a synchronous stream
cipher. It generates keystream blocks, which are then
XORed with the plaintext blocks to get the ciphertext. With other stream ciphers, flipping a bit in the
ciphertext produces a flipped bit in the
plaintext at the same location. This property allows many error correcting codes to function
normally even when applied before encryption.
Answer option C is incorrect. In the cipher-block chaining (CBC) mode, each block of plaintext is
XORed with the previous ciphertext block
before being encrypted.
Answer option D is incorrect. The propagating cipher-block chaining or plaintext cipher-block
chaining (PCBC) mode is designed to cause small
changes in the ciphertext to propagate indefinitely when decrypting, as well as when encrypting.
Answer option B is incorrect. The cipher feedback (CFB) mode, a close relative of CBC, makes a block
cipher into a self-synchronizing stream
cipher.