212-81 Practice Questions

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."

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.

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.

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.

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.

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.

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.

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.

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.

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.