Early versions of the Microsoft PPTP virtual private network software used the same RC4 key for the sender and the receiver. Which attack is the network vulnerable to when a stream cipher like RC4 is used twice with the same key?

Early versions of the Microsoft PPTP virtual private network software used the same RC4 key for the sender and the receiver. Which attack is the network vulnerable to when a stream cipher like RC4 is used twice with the same key?

• forgery attack
• meet-in-the-middle attack
• ciphertext-only attack
• plaintext-only attack

Explanation: Early versions of Microsoft's PPTP virtual private network software used the same RC4 key for the sender and the receiver (later versions solved this problem but may still have other problems). In any case where a stream cipher like RC4 is used twice with the same key, it is open to ciphertext-only attack.

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