Exam 200-201 topic 1 question 51 discussion

In a ciphertext-only attack, an attacker intercepts two ciphertexts encrypted using the same key and tries to obtain information about the plaintext or the key. With two ciphertexts encrypted using the same key, an attacker can XOR the two ciphertexts together to obtain the XOR of the two plaintexts. If the attacker can guess or obtain some of the plaintext, they can use it to recover the other plaintext by XORing it with the XOR of the two ciphertexts. This can allow the attacker to obtain the key and decrypt other messages encrypted using the same key. Therefore, it is important to use a unique key for each encryption operation when using a stream cipher like RC4 to avoid such vulnerabilities.

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. https://simple.wikipedia.org/wiki/Ciphertext-only_attack#:~:text=In%20cryptography%2C%20a%20ciphertext%2Donly,%2C%20even%20better%2C%20the%20key.

c is correct cipher text is known cipher attach

"C" is correct. RC4 is a Symmetric Algorithm (like DES, 3DES, AES, IDEA, RC2, RC4, RC5, RC6, Blowfish). The aim of the ciphertext-only attack is to discover the cifer key because it was used twice the same key.

C is the right answer. See the examples provided here: https://en.wikipedia.org/wiki/Ciphertext-only_attack