QUESTION: 93

Which of the following would be the weakest encryption algorithm?

Answer(s): 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.

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QUESTION: 94

What is the name of the attack where the attacker obtains the ciphertexts corresponding to a set of plaintexts of his own choosing?

Chosen plaintext
Differential cryptanalysis
Known-plaintext attack
Kasiski examination

Answer(s): A

Explanation:

Chosen plaintext
https://en.wikipedia.org/wiki/Chosen-plaintext_attack
A chosen-plaintext attack (CPA) is an attack model for cryptanalysis which presumes that the attacker can obtain the ciphertexts for arbitrary plaintexts. The goal of the attack is to gain information that reduces the security of the encryption scheme.

Incorrect answers:
Differential cryptanalysis - is a general form of cryptanalysis applicable primarily to block ciphers, but also to stream ciphers and cryptographic hash functions. In the broadest sense, it is the study of how differences in information input can affect the resultant difference at the output. In the case of a block cipher, it refers to a set of techniques for tracing differences through the network of transformation, discovering where the cipher exhibits non-random behavior, and exploiting such properties to recover the secret key (cryptography key).
Known-plaintext attack - (KPA) is an attack model for cryptanalysis where the attacker has access to both the plaintext (called a crib), and its encrypted version (ciphertext). These can be used to reveal further secret information such as secret keys and code books.
Kasiski examination - (also referred to as Kasiski's test or Kasiski's method) is a method of attacking polyalphabetic substitution ciphers, such as the Vigenère cipher. It was first published by Friedrich Kasiski in 1863, but seems to have been independently discovered by Charles Babbage as early as 1846. In polyalphabetic substitution ciphers where the substitution alphabets are chosen by the use of a keyword, the Kasiski examination allows a cryptanalyst to deduce the length of the keyword. Once the length of the keyword is discovered, the cryptanalyst lines up the ciphertext in n columns, where n is the length of the keyword. Then each column can be treated as the ciphertext of a monoalphabetic substitution cipher. As such, each column can be attacked with frequency analysis.

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QUESTION: 95

Hash. 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.

Keccak
MD5
SHA-1
TIGER

Answer(s): B

Explanation:

MD5
https://en.wikipedia.org/wiki/MD5
The MD5 message-digest algorithm is a widely used hash function producing a 128-bit hash value. Although MD5 was initially designed to be used as a cryptographic hash function, it has been found to suffer from extensive vulnerabilities. It can still be used as a checksum to verify data integrity, but only against unintentional corruption. It remains suitable for other non-cryptographic purposes, for example for determining the partition for a particular key in a partitioned database.
MD5 was designed by Ronald Rivest in 1991 to replace an earlier hash function MD4, and was specified in 1992 as RFC 1321

Incorrect answers:
TIGER - hash. Created by Ross Anderson and Eli Baham. 192/160/128 bit output size, 512 bit block size, 53 bit word size, 24 rounds.
SHA-1 - Secure Hashing Algorithm. Designed by NSA. 160 bit output size, 512 bit block size, 40 bit word size, 80 rounds.
Keccak - SHA-3 (Secure Hash Algorithm 3) is the latest member of the Secure Hash Algorithm family of standards, released by NIST on August 5, 2015. SHA-3 is a subset of the broader cryptographic primitive family Keccak, designed by Guido Bertoni, Joan Daemen, Michaël Peeters, and Gilles Van Assche, building upon RadioGatún.

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QUESTION: 96

A technique used to increase the security of block ciphers. It consists of steps that combine the data with portions of the key (most commonly using a simple XOR) before the first round and after the last round of encryption.

Whitening
Key Exchange
Key Schedule
Key Clustering

Answer(s): A

Explanation:

Whitening https://en.wikipedia.org/wiki/Key_whitening
In cryptography, key whitening is a technique intended to increase the security of an iterated block cipher. It consists of steps that combine the data with portions of the key.
The most common form of key whitening is xor-encrypt-xor -- using a simple XOR before the first round and after the last round of encryption.
The first block cipher to use a form of key whitening is DES-X, which simply uses two extra 64-bit keys for whitening, beyond the normal 56-bit key of DES. This is intended to increase the complexity of a brute force attack, increasing the effective size of the key without major changes in the algorithm. DES-X's inventor, Ron Rivest, named the technique whitening.

Incorrect answers:
Key Clustering - different encryption keys generated the same ciphertext from the same plaintext message.
Key Schedule - an algorithm for the key that calculates the subkeys for each round that the encryption goes through.
Key Exchange - a method in cryptography by which cryptographic keys are exchanged between two parties, allowing use of a cryptographic algorithm.

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