Question 1

Which one of the following are characteristics of a hash function? (Choose two)

Accepted Answer

B, C

Explanation: Correct answers: One-way, Fixed length output https://en.wikipedia.org/wiki/Cryptographic_hash_function A cryptographic hash function is a mathematical algorithm that maps data of arbitrary size (often called the "message") to a bit array of a fixed size (the "hash value", "hash", or "message digest"). It is a one-way function, that is, a function which is practically infeasible to invert. Incorrect answers: Symmetric. Cryptographic algorithms can be categorized into three classes: Hash functions, Symmetric and Asymmetric algorithms. Differences: purpose and main fields of application. Requires a key. Well, technically, this is the correct answer. But in the hash-function, “key” is input data. Fast. Fast or slow is a subjective characteristic, there are many different algorithms, and here it is impossible to say this unambiguously like "Symmetric encryption is generally faster than asymmetric encryption."

Question 2

Denis is looking at an older system that uses DES encryption. A colleague has told him that DES is insecure due to a small key size. What is the key length used for DES?

Accepted Answer

C

Explanation: 56 https://en.wikipedia.org/wiki/DES The Data Encryption Standard is a symmetric-key algorithm for the encryption of digital data. Although its short key length of 56 bits makes it too insecure for applications, it has been highly influential in the advancement of cryptography.

Question 3

When learning algorithms, such as RSA, it is important to understand the mathematics being used. In
RSA, the number of positive integers less than or equal to some number is critical in key generation.
The number of positive integers less than or equal to n that are coprime to n is called ______.

Accepted Answer

C

Explanation: Euler's totient https://en.wikipedia.org/wiki/Euler%27s_totient_function In number theory, Euler's totient function counts the positive integers up to a given integer n that are relatively prime to n. Incorrect answers: Fibonacci number - commonly denoted Fn, form a sequence, called the Fibonacci sequence, such that each number is the sum of the two preceding ones, starting from 0 and 1. Fermat's number - named after Pierre de Fermat, who first studied them, is a positive integer of the form Fn = 2^2^n+1 where n is a non-negative integer. The first few Fermat numbers are: 3, 5, 17, 257, 65537, 4294967297, 18446744073709551617, … Mersenne prime – prime number that is one less than a power of two. That is, it is a prime number of the form Mn = 2^n − 1 for some integer n. They are named after Marin Mersenne, a French Minim friar, who studied them in the early 17th century.

Question 4

Which of the following is a cryptographic protocol that allows two parties to establish a shared key over an insecure channel?

Accepted Answer

D

Explanation: Diffie-Hellman https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange Diffie-Hellman key exchange is a method of securely exchanging cryptographic keys over a public channel and was one of the first public-key protocols as originally conceptualized by Ralph Merkle and named after Whitfield Diffie and Martin Hellman. DH is one of the earliest practical examples of public key exchange implemented within the field of cryptography. Incorrect answers: Elliptic Curve - Asymmetric Key Algorithm, provides encryption, digital signatures, key exchange, based on the idea of using points on a curve to define the public/private key, used in wireless devices and smart cards. The security of the Elliptic Curve cryptography is based on the fact that finding the discrete logarithm of a random elliptic curve element with respect to a publicly known base point is difficult to the point of being impractical to do so. (y2 = x3 + Ax + B) - Developed by Victor Miller and Neil Koblitz in 1985 MD5 - hash function - Created by Ronald Rivest. Replaced MD4. 128 bit output size, 512 bit block size, 32 bit word size, 64 rounds. Infamously compromised by Flame malware in 2012. Not collision resistant - Not Reversible - RFC 1321 RSA - is a public-key cryptosystem that is widely used for secure data transmission.

Question 5

What is the largest key size that AES can use?

Accepted Answer

A

Explanation: 256 https://en.wikipedia.org/wiki/Advanced_Encryption_Standard For AES, NIST selected three members of the Rijndael family, each with a block size of 128 bits, but three different key lengths: 128, 192 and 256 bits.

Question 6

Which of the following would be the weakest encryption algorithm?

Accepted Answer

A

Explanation: DES https://en.wikipedia.org/wiki/Data_Encryption_Standard DES is insecure due to the relatively short 56-bit key size. In January 1999, distributed.net and the Electronic Frontier Foundation collaborated to publicly break a DES key in 22 hours and 15 minutes. Incorrect answers: AES - has been adopted by the U.S. government and is now used worldwide. It supersedes the Data Encryption Standard (DES),which was published in 1977. The algorithm described by AES is a symmetric-key algorithm, meaning the same key is used for both encrypting and decrypting the data. RSA - The security of RSA relies on the practical difficulty of factoring the product of two large prime numbers, the "factoring problem". Breaking RSA encryption is known as the RSA problem. Whether it is as difficult as the factoring problem is an open question. There are no published methods to defeat the system if a large enough key is used. EC - Elliptic-curve cryptography (ECC) is an approach to public-key cryptography based on the algebraic structure of elliptic curves over finite fields. ECC allows smaller keys compared to non-EC cryptography (based on plain Galois fields) to provide equivalent security.

Question 7

Which of the following is not a key size used by AES?

Accepted Answer

D

Explanation: 512 bits https://en.wikipedia.org/wiki/Advanced_Encryption_Standard AES is a subset of the Rijndael block cipher developed by two Belgian cryptographers, Vincent Rijmen and Joan Daemen, who submitted a proposal to NIST during the AES selection process. Rijndael is a family of ciphers with different key and block sizes. For AES, NIST selected three members of the Rijndael family, each with a block size of 128 bits, but three different key lengths: 128, 192 and 256 bits.

Question 8

What is a TGS?

Accepted Answer

D

Explanation: The server that grants Kerberos tickets https://en.wikipedia.org/wiki/Kerberos_(protocol) The client authenticates itself to the Authentication Server (AS) which forwards the username to a key distribution center (KDC). The KDC issues a ticket-granting ticket (TGT), which is time stamped and encrypts it using the ticket-granting service's (TGS) secret key and returns the encrypted result to the user's workstation. This is done infrequently, typically at user logon; the TGT expires at some point although it may be transparently renewed by the user's session manager while they are logged in.

Question 9

DES has a key space of what?

Accepted Answer

D

Explanation: 2^56 https://en.wikipedia.org/wiki/Data_Encryption_Standard The Data Encryption Standard (DES) is a symmetric-key algorithm for the encryption of digital data. Although its short key length of 56 bits makes it too insecure for applications, it has been highly influential in the advancement of cryptography.

Question 10

Message hidden in unrelated text. Sender and receiver have pre-arranged to use a pattern to remove certain letters from the message which leaves only the true message behind.

Accepted Answer

B

Explanation: Null Ciphers https://en.wikipedia.org/wiki/Null_cipher A null cipher, also known as concealment cipher, is an ancient form of encryption where the plaintext is mixed with a large amount of non-cipher material. Today it is regarded as a simple form of steganography, which can be used to hide ciphertext. Incorrect answers: Caesar Cipher - Monoalphabetic cipher where letters are shifted one or more letters in either direction. The method is named after Julius Caesar, who used it in his private correspondence. Vigenère - method of encrypting alphabetic text by using a series of interwoven Caesar ciphers, based on the letters of a keyword. It employs a form of polyalphabetic substitution. Playfair Cipher - manual symmetric encryption technique and was the first literal digram substitution cipher. The scheme was invented in 1854 by Charles Wheatstone, but bears the name of Lord Playfair for promoting its use.

Question 11

Which of the following is a block cipher?

Accepted Answer

A

Explanation: AES https://en.wikipedia.org/wiki/Advanced_Encryption_Standard AES is a subset of the Rijndael block cipher developed by two Belgian cryptographers, Vincent Rijmen and Joan Daemen, who submitted a proposal to NIST during the AES selection process Incorrect answers: RC4. RC4 (Rivest Cipher 4 also known as ARC4 or ARCFOUR meaning Alleged RC4, see below) is a stream cipher. DH. Diffie–Hellman key exchange is a method of securely exchanging cryptographic keys over a public channel and was one of the first public-key protocols as conceived by Ralph Merkle and named after Whitfield Diffie and Martin Hellman. RSA. RSA (Rivest–Shamir–Adleman) is one of the first public-key cryptosystems and is widely used for secure data transmission.

Question 12

John works as a cryptography consultant. He finds that people often misunderstand the reality of breaking a cipher. What is the definition of breaking a cipher?

Accepted Answer

A

Explanation: Finding any method that is more efficient than brute force. https://en.wikipedia.org/wiki/Cryptanalysis Bruce Schneier notes that even computationally impractical attacks can be considered breaks: "Breaking a cipher simply means finding a weakness in the cipher that can be exploited with a complexity less than brute force. Never mind that brute-force might require 2^128 encryptions; an attack requiring 2^110 encryptions would be considered a break...simply put, a break can just be a certificational weakness: evidence that the cipher does not perform as advertised."

Question 13

Changing some part of the plain text for some matching part of cipher text. Historical algorithms typically use this.

Accepted Answer

B

Explanation: Substitution https://en.wikipedia.org/wiki/Substitution_cipher In cryptography, a substitution cipher is a method of encrypting by which units of plaintext are replaced with ciphertext, according to a fixed system; the "units" may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing the inverse substitution. Incorrect answers: Decoding - the reverse process from encoding - converting the encoded message back into its plaintext format. Collision - occurs when a hash function generates the same output for different inputs. Transposition - a method of encryption by which the positions held by units of plaintext (which are commonly characters or groups of characters) are shifted according to a regular system, so that the ciphertext constitutes a permutation of the plaintext. That is, the order of the units is changed (the plaintext is reordered). Mathematically a bijective function is used on the characters' positions to encrypt and an inverse function to decrypt.

Question 14

Which of the following is an asymmetric cipher?

Accepted Answer

A

Explanation: RSA https://en.wikipedia.org/wiki/RSA_(cryptosystem) RSA (Rivest–Shamir–Adleman) is a public-key cryptosystem that is widely used for secure data transmission. It is also one of the oldest. The acronym RSA comes from the surnames of Ron Rivest, Adi Shamir, and Leonard Adleman, who publicly described the algorithm in 1977. An equivalent system was developed secretly, in 1973 at GCHQ (the British signals intelligence agency), by the English mathematician Clifford Cocks. That system was declassified in 1997. In a public-key cryptosystem, the encryption key is public and distinct from the decryption key, which is kept secret (private). An RSA user creates and publishes a public key based on two large prime numbers, along with an auxiliary value. The prime numbers are kept secret. Messages can be encrypted by anyone, via the public key, but can only be decoded by someone who knows the prime numbers. Incorrect answers: DES - is a symmetric-key algorithm for the encryption of digital data. Although its short key length of 56 bits makes it too insecure for applications, it has been highly influential in the advancement of cryptography. RC4 - RSA (Rivest–Shamir–Adleman) is one of the first public-key cryptosystems and is widely used for secure data transmission (stream cipher). AES - is a subset of the Rijndael block cipher developed by two Belgian cryptographers, Vincent Rijmen and Joan Daemen, who submitted a proposal to NIST during the AES selection process. Rijndael is a family of ciphers with different key and block sizes. For AES, NIST selected three members of the Rijndael family, each with a block size of 128 bits, but three different key lengths: 128, 192 and 256 bits.

Question 15

A measure of the uncertainty associated with a random variable.

Accepted Answer

D

Explanation: Entropy https://en.wikipedia.org/wiki/Entropy_(information_theory) In information theory, the entropy of a random variable is the average level of "information", "surprise", or "uncertainty" inherent in the variable's possible outcomes. The concept of information entropy was introduced by Claude Shannon in his 1948 paper "A Mathematical Theory of Communication". Incorrect answers: Diffusion - transposition processes used in encryption functions to increase randomness. Whitening - technique intended to increase the security of an iterated block cipher. It consists of steps that combine the data with portions of the key. Collision - situation where two different inputs yield the same output.

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