Computer Systems Security: Block Ciphers and DES

10 Questions

What is the main difference between block ciphers and stream ciphers?

Block ciphers process messages in blocks, while stream ciphers process messages a bit or byte at a time.

Why is a Feistel Cipher Structure needed in block ciphers?

To efficiently recover messages by decrypting ciphertext.

How many different plaintext blocks exist for a n-bit block in block ciphers?

$2^n$

Which cryptographic algorithms provide both secrecy and authentication services?

Block ciphers

What is the typical size of a block used in block ciphers?

64 or 128 bits

What is the main concept behind the Feistel cipher structure?

It partitions the input block into two halves and processes them through multiple rounds involving substitution and permutation operations.

What is the purpose of the confusion and diffusion principles in cipher design, as suggested by Claude Shannon?

Confusion aims to obscure the statistical properties of the plaintext in the ciphertext, while diffusion dissipates the statistical structure of the plaintext over the entire ciphertext.

What is the primary advantage of increasing the key size in a Feistel cipher design?

It improves the security of the cipher by making exhaustive key searching harder.

Which of the following statements about the Data Encryption Standard (DES) is true?

DES was developed by IBM and adopted as a Federal Information Processing Standard in 1977.

What is the primary concern in the design of the round function in a Feistel cipher?

Increasing the complexity of the round function can make cryptanalysis harder, but it may slow down the cipher.

Study Notes

Block Ciphers

Process messages in blocks, each of which is then en/decrypted
Typically use blocks of 64 or 128 bits
Most symmetric block ciphers are based on a Feistel Cipher Structure

Feistel Cipher Structure

Devised by Horst Feistel
Based on concept of invertible product cipher
Partitions input block into two halves
Processes through multiple rounds which perform a substitution on left data half based on round function of right half and subkey, then has a permutation swapping halves
Implements Shannon’s substitution-permutation network concept

Feistel Cipher Design Principles

Block size: increasing size improves security, but slows cipher
Key size: increasing size improves security, makes exhaustive key searching harder, but may slow cipher
Number of rounds: increasing number improves security, but slows cipher (typical 16)
Subkey generation: greater complexity can make analysis harder, but slows cipher
Round function: greater complexity can make analysis harder, but slows cipher
Fast software en/decryption and ease of analysis are more recent concerns for practical use and testing

Data Encryption Standard (DES)

Most widely used block cipher in the world
Adopted in 1977 by NBS (National Bureau of Standards) as FIPS (Federal Information Processing Standard) PUB 46
Encrypts 64-bit data using 56-bit key
Has widespread use, but has considerable controversy over its security

DES History

IBM developed Lucifer cipher (1971) by a team led by Feistel
Used 64-bit data blocks with 128-bit key
Redeveloped as a commercial cipher with input from NSA and others
In 1973, NBS issued a request for proposals for a national cipher standard
IBM submitted DES, which was selected as the standard