Introduction to Cryptography

Question 1

Block Ciphers

Answer 1

Encrypts the data in blocks.
64-bit blocks are quite common.
AES uses a 128-bit block.

Question 2

Stream Ciphers

Answer 2

Encrypts the data as a stream, one bit at a time.

Question 3

Symmetric Block Cipher Algorithms

Answer 3

The Feistel Network
DES, 3DES
AES
Blowfish
Serpent
Twofish
Skipjack
IDEA
CAST
TEA
SHARK

Question 4

Basic Facts of the Feistel Function

Answer 4

Larger Block sizes increase security
Larger Key sizes increase security
If the round function is secure, then more rounds increase security

Question 5

Cipher

Answer 5

The algorithm needed to encrypt and decrypt a message

Question 6

Key

Answer 6

The random bits used in encrypting a message

Question 7

Algorithm

Answer 7

The mathematical process used to alter a message and read it unintelligible by any but the intended party

Question 8

The Feistel Function

Answer 8

This function starts by splitting the block of plain text data (often 64 bits) into two parts ( traditionally termed L0 and R0)

The round function F is applied to 1 of the halves. The term ‘round function’ simply means a function performed with each iteration, or round, of the Feistel cipher.

The output of each round function F is then xor’d with the other half.

Question 9

Data Encryption Standard (DES)

Answer 9

One of the oldest modern symmetric ciphers is DES.

The basic concept of DES is as follows:

• Data is divided into 64-bit blocks.
• The data is then manipulated by 16 separate steps of encryption involving substitutions, bit-shifting, and logical operations using a 56-bit key.
• Data is then further scrambled using a swapping algorithm.
• Data is finally transposed one last time.