212-81 Sample Questions & Answers
Symmetric block ciphers and real-world applications like digital signatures and Wi-Fi encryption share the top weighting, with cipher history, number theory behind asymmetric encryption, cryptanalysis methods, and quantum computing rounding things out.
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- Question 1Beginner
Introduction and History of Cryptography · Mono-Alphabet Substitution Ciphers
True or False: The ROT13 cipher is a specific implementation of the Caesar cipher that acts as its own inverse, meaning that applying the exact same ROT13 algorithm to the ciphertext will reveal the original plaintext without requiring a separate decryption key.
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Correct answer: A
ROT13 shifts each letter by exactly 13 places. Because the standard English alphabet has 26 letters, shifting a letter by 13 places twice (13 + 13 = 26) brings it back to its original starting position. This makes ROT13 a reciprocal cipher, where the encryption and decryption algorithms are identical.
- Question 2IntermediateSelect 2
Introduction and History of Cryptography · Mono-Alphabet Substitution Ciphers
Which TWO of the following characteristics represent the primary cryptanalytic weaknesses of mono-alphabetic single substitution ciphers? (Select TWO)
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Correct answers: A, C
Mono-alphabetic substitution ciphers replace each plaintext letter with exactly one ciphertext letter consistently. This means if 'E' is the most common letter in English, its substitute will be the most common letter in the ciphertext, making it highly vulnerable to frequency analysis.
Because the mapping is 1:1, structural patterns are preserved. For example, a word like 'HELLO' will always encrypt to a pattern like 'ABCCD', preserving the double letter at the end, which provides massive clues for cryptanalysts performing pattern word attacks.
- Question 3Intermediate
Introduction and History of Cryptography · Other Classical Ciphers
For a Vernam cipher (one-time pad) to maintain its mathematical property of perfect secrecy, the implementation must adhere to strict rules. Which of the following is an absolute requirement for the key in a true one-time pad?
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Correct answer: B
To achieve perfect secrecy, a one-time pad requires that the key is: 1) truly random (not pseudo-random), 2) at least as long as the plaintext message, 3) never reused in whole or in part, and 4) kept completely secret. If these conditions are met, the cipher is mathematically proven to be unbreakable.
- Question 4Beginner
Symmetric Cryptography and Hashes · Symmetric Cryptography Fundamentals
During a security architecture review, a consultant points out that a proprietary encryption algorithm developed in-house relies heavily on keeping the algorithm's internal workings completely secret from attackers. Which foundational cryptographic principle does this design directly violate?
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Correct answer: C
Kerckhoffs's Principle states that a cryptographic system should be secure even if everything about the system, except the key, is public knowledge. Relying on the secrecy of the algorithm itself is known as 'security through obscurity' and is widely considered a poor practice in modern cryptography.
- Question 5Intermediate
Symmetric Cryptography and Hashes · Symmetric Cryptography Fundamentals
A developer is writing a simple stream cipher and uses the binary XOR operation to combine a plaintext bit stream with a keystream. If the plaintext bit is '1' and the keystream bit is '1', what will the resulting ciphertext bit be, and why is this operation reversible?
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Correct answer: B
In a binary XOR (Exclusive OR) operation, if both bits are the same (1 and 1, or 0 and 0), the result is 0. If they are different, the result is 1. XOR is perfectly reversible; if you XOR the ciphertext (0) with the same keystream bit (1), you get the original plaintext (1) back. This property makes XOR the foundational mathematical operation for most symmetric ciphers.
flowchart TD P[Plaintext: 1] -->|XOR| C[Ciphertext: 0] K[Keystream: 1] -->|XOR| C C -->|XOR| P2[Decrypted: 1] K -->|XOR| P2 - Question 6Advanced
Symmetric Cryptography and Hashes · Block Cipher Modes of Operation
A telecommunications company is designing a secure video streaming service. The engineering team needs to select an AES block cipher mode of operation. The requirements dictate that the encryption must be highly parallelizable to handle 4K video streams without latency, and a dropped packet in transit must not corrupt subsequent blocks of video data.
Which of the following modes of operation is the OPTIMAL choice for this scenario?
flowchart LR Nonce --> Counter Counter --> Encrypt Encrypt --> XOR Plaintext --> XOR XOR --> CiphertextShow answer & explanation
Correct answer: C
Counter (CTR) mode turns a block cipher into a stream cipher by encrypting successive values of a counter. It is highly parallelizable because the keystream can be generated independently of the plaintext. Furthermore, because it operates like a stream cipher (XORing the keystream with plaintext), a dropped or corrupted packet only affects that specific data segment and does not propagate errors to subsequent blocks, making it ideal for high-speed video streaming.
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