CISSP Question #1167: Real Exam Question with Answer & Explanation

Question

Using the cipher text and resultant clear text message to derive the non-alphabetic cipher key is an example of which method of cryptanalytic attack?

Options

• AFrequency analysis
• BCiphertext-only attack
• CProbable-plaintext attack
• DKnown-plaintext attack

Explanation

A known-plaintext attack is a type of cryptanalytic attack where the attacker has access to both the ciphertext and the corresponding plaintext, and tries to derive the key or the algorithm used to encrypt the message. This type of attack can be effective against some symmetric ciphers, such as substitution ciphers, where the key is non-alphabetic and the ciphertext is a permutation of the plaintext. For example, if the attacker knows that the plaintext "HELLO" corresponds to the ciphertext "QNUUX", they can infer that the key is "Q-H, N-E, U-L, X-O". A frequency analysis attack is a type of cryptanalytic attack where the attacker analyzes the frequency of letters or symbols in the ciphertext and compares them with the expected frequency of the language of the plaintext. A ciphertext-only attack is a type of cryptanalytic attack where the attacker only has access to the ciphertext and tries to guess the plaintext or the key by using statistical methods, brute force, or other techniques. A probable-plaintext attack is a type of cryptanalytic attack where the attacker has access to the ciphertext and some information about the probable plaintext, such as the format, the length, or some common words or phrases, and tries to recover the key or the algorithm used to encrypt the message.

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