Which Encryption Modes Are Vulnerable to Replay Attacks?

D.

The correct answer is D. Electronic codebook mode.

Electronic codebook (ECB) mode is a basic block cipher mode of operation that encrypts each block of plaintext separately using the same key, resulting in identical ciphertext blocks for identical plaintext blocks. This makes it vulnerable to several attacks, including replay attacks. In a replay attack, an attacker intercepts and stores a ciphertext message, then sends it again at a later time, potentially bypassing any access control mechanisms that may be in place.

While cipher feedback (CFB), cipher block chaining (CBC), and output feedback (OFB) modes all use feedback to encrypt plaintext blocks and avoid this vulnerability, ECB mode encrypts each block of plaintext separately, so each block gets decrypted in exactly the same way. As a result, if an attacker intercepts and replays an encrypted message, the same plaintext block will be produced each time, allowing the attacker to bypass access control mechanisms.

Therefore, the use of ECB mode for encryption can make protocols without integrity protection even more susceptible to replay attacks. To mitigate this vulnerability, other block cipher modes of operation that provide stronger security, such as CBC or CFB, should be used instead of ECB. Additionally, integrity protection mechanisms, such as message authentication codes (MACs), can be used to detect and prevent replay attacks.