Network Vulnerability: Reusing RC4 Key

C.

When a stream cipher like RC4 is used twice with the same key, the network becomes vulnerable to a ciphertext-only attack.

A stream cipher generates a pseudorandom key stream that is combined with the plaintext message to produce the ciphertext. When the same key is used twice, the same pseudorandom key stream is generated, which means that the same keystream is used to encrypt both messages. This makes it easier for an attacker to determine the original plaintext of both messages by analyzing the ciphertext.

In a ciphertext-only attack, the attacker has access to the encrypted messages but not the corresponding plaintext or the key used to encrypt them. The attacker attempts to recover the plaintext by analyzing patterns in the ciphertext or using statistical analysis to make educated guesses about the contents of the message.

Using the same key with a stream cipher twice makes it easier for the attacker to recover the plaintext because the same keystream is used for both messages. If the attacker can determine the plaintext of one message, they can use that information to help decrypt the second message.

Other attacks that are possible with a stream cipher include a plaintext-only attack, in which the attacker has access to the plaintext message but not the key or ciphertext, and a forgery attack, in which the attacker attempts to generate a valid message without access to the key or plaintext.

A meet-in-the-middle attack is a type of attack that can be used against certain types of ciphers, such as block ciphers. It involves encrypting the plaintext with a key and then decrypting the resulting ciphertext with a different key in order to find a pair of keys that produce the same result. However, this type of attack is not applicable to stream ciphers like RC4.