Free 212-81 sample questions

Correct answer: Non-repudiation

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.

Correct answer: The value 'a' (13) is not coprime to 'm' (26), meaning it does not have a modular multiplicative inverse.

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.

Correct answer: Calculate the Greatest Common Divisor (GCD) of the repeating sequence intervals using the Kasiski examination method.

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]

Correct answer: True

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.

Correct answers: They perfectly preserve the natural letter frequency distribution of the underlying language.; They preserve the structural patterns of words, such as double letters and word lengths (if spaces are kept).

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.

Correct answer: The key must be at least as long as the plaintext message and strictly truly random.

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.