What happens if a keystream is reused?

• A. It allows attackers to derive the XOR of the plaintexts
• B. It increases ciphertext length
• C. It improves encryption security
• D. It eliminates the need for an IV

Answer: (A)

Reusing a keystream breaks secrecy because the keystream is meant to be used once. If two messages are encrypted with the same keystream, the ciphertexts satisfy C1 = P1 XOR K and C2 = P2 XOR K. When you XOR the two ciphertexts, the keystream cancels out: C1 XOR C2 = (P1 XOR K) XOR (P2 XOR K) = P1 XOR P2. This reveals a direct relationship between the two plaintexts, and with additional information (like knowing one of the plaintexts or guessing parts of it), an attacker can recover the other plaintext or even recreate the keystream. That’s why reusing the keystream is devastating for security.

This doesn’t change the length of the ciphertexts, so you wouldn’t see an effect on size. It certainly doesn’t improve security, and the system still relies on using a fresh keystream for each message, which is typically achieved by using a unique IV or nonce for every encryption.