Which of the following encryption modes can make protocols without integrity protection even
more susceptible to replay attacks, since each block gets decrypted in exactly the same way?
A.
Cipher feedback mode
B.
Cipher block chaining mode
C.
Output feedback mode
D.
Electronic codebook mode
Explanation:
The electronic codebook (ECB) mode is the simplest encryption mode. In this mode,
the message is divided into blocks and each block is
encrypted separately. The disadvantage of this method is that identical plaintext blocks are
encrypted into identical ciphertext blocks; thus, it
does not hide data patterns well. In some senses, it doesn’t provide serious message confidentiality,
and it is not recommended for use in
cryptographic protocols at all. ECB mode can also make protocols without integrity protection even
more susceptible to replay attacks, since
each block gets decrypted in exactly the same way.
Answer option A is incorrect. The cipher feedback (CFB) mode, a close relative of CBC, makes a block
cipher into a self-synchronizing stream
cipher.
Answer option C is incorrect. 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 B is incorrect. In the cipher-block chaining (CBC) mode, each block of plaintext is
XORed with the previous ciphertext block
before being encrypted.