QUESTION: 77

A symmetric block cipher designed in 1993 by Bruce Schneier. Was intended as a replacement for DES. Like DES it is a 16 round Feistel working on 64bit blocks. Can have bit sizes 32bits to 448bits.

Skipjack
Blowfish
MD5
Serpent

Answer(s): B

Explanation:

Blowfish https://en.wikipedia.org/wiki/Blowfish_(cipher)
Blowfish is a symmetric-key block cipher, designed in 1993 by Bruce Schneier and included in many cipher suites and encryption products. Blowfish provides a good encryption rate in software and no effective cryptanalysis of it has been found to date. However, the Advanced Encryption Standard (AES) now receives more attention, and Schneier recommends Twofish for modern applications.
Blowfish has a 64-bit block size and a variable key length from 32 bits up to 448 bits. It is a 16-round Feistel cipher and uses large key-dependent S-boxes.

Incorrect answers:
Skipjack - symmetric algorithm. Designed by NSA for the clipper chip - a chip with built in encryption. The decryption key was kept in key escrow in case law enforcement needed to decrypt data without the owner's cooperation, making it highly controversial. Uses an 80 bit key to encrypt/decrypt 64 bit data blocks. It is an unbalanced Feistel network with 32 rounds.
Serpent - symmetric algorithm. Designed by Ross Anderson, Eli Biham, and Lars Knudsen. Has a block size of 128 bits. Key size is 128, 192, or 256 bits. Uses a substitution-permutation network instead of Feistel cipher. Uses 32 rounds working with a block of four 32-bit words. Each round applies one of eight 4-bit to 4-bit S-boxes 32 times in parallel. Designed so all operations can be done in parallel.
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.

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QUESTION: 78

A type of frequency analysis used to attack polyalphabetic substitution ciphers. It's used to try to discover patterns and use that information to decrypt the cipher.

Kasiski Method
Birthday Attack
Information Deduction
Integral Cryptanalysis

Answer(s): A

Explanation:

Kasiski Method
https://en.wikipedia.org/wiki/Kasiski_examination
In cryptanalysis, 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.

Incorrect answers:
Integral Cryptanalysis - uses lots of sets of plaintext that are similar with slight modifications. These are encrypted and then the variations are analyzed to determine if there's anything that can be zeroed in on.
Information Deduction - the attacker gains some Shannon information about plaintexts (or ciphertexts) not previously known.
Birthday Attack - cryptographic attack that exploits the mathematics behind the birthday problem in the probability theory forces collisions within hashing functions.

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QUESTION: 79

Basic information theory is the basis for modern symmetric ciphers. Understanding the terminology of information theory is, therefore, important. If a single change of a single bit in the plaintext causes changes in all the bits of the resulting ciphertext, what is this called?

Complete diffusion
Complete scrambling
Complete confusion
Complete avalanche

Answer(s): D

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QUESTION: 80

Which of the following encryption algorithms relies on the inability to factor large prime numbers?

Answer(s): A

Explanation:

Correct answers: 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.
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.

Incorrect answers:
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.
AES - Advanced Encryption Standard (AES), also known by its original name Rijndael, is a specification for the encryption of electronic data established by the U.S. National Institute of Standards and Technology (NIST) in 2001.
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.
MQV - (Menezes–Qu–Vanstone) is an authenticated protocol for key agreement based on the Diffie– Hellman scheme. Like other authenticated Diffie–Hellman schemes, MQV provides protection against an active attacker. The protocol can be modified to work in an arbitrary finite group, and, in particular, elliptic curve groups, where it is known as elliptic curve MQV (ECMQV).

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