Exam CISSP topic 1 question 109 discussion

Answer is A, key word : cryptanalytic

A. Known-plaintext attack - KPA requires having plaintext and cipher text pairs in order to map out the key. (https://www.geeksforgeeks.org/cryptanalysis-and-types-of-attacks/) B. Ciphertext-only attack - COA has the cipher text only. This cannot be the answer. (https://www.geeksforgeeks.org/cryptanalysis-and-types-of-attacks/) C. Frequency analysis - frequency analysis is tempting since it mentions a monoaplphabetic key which is what frequency analysis is strong at breaking. However, as you have both the cipher and plaintext in this scenario, you don't need frequency analysis. (https://www.101computing.net/frequency-analysis/) D. Probable-plaintext attack - "A probable plaintext attack works by looking at certain bit positions for which a likely value can be predicted." (https://www.cs.columbia.edu/~smb/papers/probtxt.pdf) This attack is not relevant as you have the full plaintext.

known plain text attack is the answer: https://www.sciencedirect.com/topics/computer-science/plaintext-attack#:~:text=In%20the%20known%20plaintext%20attack,try%20to%20decrypt%20the%20ciphertext.

Answer B) Ciphertext-only attack The question says "resultant cleartext". this means you did not have the cleartext to start with....so it was a Ciphertext-Only Attack first...and then you used Frequency Analysis on the Plaintext Cipher to figure out the Monoalphabetic Cipher https://www.101computing.net/frequency-analysis/ https://www.quora.com/What-is-the-fundamental-reason-why-monoalphabetic-cipher-is-vulnerable-to-frequency-analysis-attack#:~:text=The%20fundamental%20reason%20why%20a,style%20or%20type%20of%20publication.

A. Known-plaintext attack In a known-plaintext attack, the attacker has access to both the cipher text (encrypted message) and the corresponding cleartext (decrypted message). By analyzing the relationship between the two, the attacker attempts to derive the encryption key or discover vulnerabilities in the encryption algorithm. In this case, using the cipher text and the resultant cleartext message to derive the monoalphabetic cipher key falls under the category of a known-plaintext attack. The attacker can compare the pairs of known cipher text and cleartext to deduce the correspondence between specific characters or patterns, which can lead to the recovery of the encryption key.

It should be C as it is monoalphabetic

A sounds good

In the known plaintext attack, the attacker has both a copy of the encrypted message along with the plaintext message that was used to generate the ciphertext.