Covers the history of cryptography, symmetric algorithms and hashing, number theory and asymmetric encryption, digital signatures and authentication protocols, and quantum-era cryptography.
12 free sample questions145 in the full practice test Other version: 212-81V3(253)
Introduction and History of Cryptography · Fundamentals of Cryptography
An organization is implementing a new digital contract system. The legal department requires that once a user signs a contract using their private key, they cannot later claim they did not authorize the transaction. Which cryptographic goal specifically addresses this legal requirement?
Answer and explanation
Correct answer: D
Non-repudiation is the cryptographic goal that ensures a sender cannot deny having sent a message or authorized a transaction. This is typically achieved through digital signatures, which bind the identity of the signer to the document. While authentication verifies identity, non-repudiation provides proof of origin that can be verified by a third party.
Q2Question 2
Intermediate
Introduction and History of Cryptography · Mono-Alphabet Substitution Ciphers
When configuring an Affine cipher using the mathematical formula E(x) = (ax + b) mod m, a cryptography student chooses a = 13 and m = 26 for the English alphabet. Why will this specific configuration fail to produce a reversible ciphertext?
Answer and explanation
Correct answer: B
For an Affine cipher to be reversible (decryptable), the key value 'a' must be coprime to the alphabet size 'm'. This means their greatest common divisor (GCD) must be 1. Since 13 and 26 share a common divisor of 13, 'a' does not have a modular multiplicative inverse mod 26. This causes multiple plaintext letters to map to the same ciphertext letter, destroying the data.
Q3Question 3
Advanced
Introduction and History of Cryptography · Multi-Alphabet Substitution Ciphers
A cybersecurity analyst is examining a historical ciphertext intercepted during a forensics investigation. The text appears to be English but makes no sense. The analyst runs a frequency analysis and finds that the letter 'E' appears at a frequency of 3%, 'X' at 11%, and 'Q' at 12%. Furthermore, the analyst identifies that the sequence 'XRQ' repeats at intervals of 12, 24, and 36 characters.
Based on these findings, the analyst determines this is a polyalphabetic substitution cipher.
Which of the following techniques should the analyst use next to determine the exact length of the encryption key?
Answer and explanation
Correct answer: A
The Kasiski examination is used to break polyalphabetic ciphers like the Vigenere cipher. By finding repeating sequences of characters in the ciphertext and calculating the distances between them (12, 24, 36), the analyst can find the Greatest Common Divisor (GCD). The GCD represents the probable length of the keyword (in this case, likely 3, 4, 6, or 12). Once the key length is known, the ciphertext can be broken down into mono-alphabetic blocks for standard frequency analysis.
flowchart TD
A[Identify Repeating Sequences] --> B[Calculate Distances Between Repeats]
B --> C[Find Factors of Distances]
C --> D[Determine GCD]
D --> E[Probable Key Length]
Q4Question 4
Beginner
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.
Answer and explanation
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.
Q5Question 5
IntermediateMultiple answers
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)
Answer and explanation
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.
Q6Question 6
Intermediate
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?
Answer and explanation
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.
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