CSE 5/7353 – January 25th 2006

Cryptography

Conventional Encryption

• Shared Key

• Substitution

• Transposition

5 Types Cryptanalysis

Strength of Cipher

• Unconditionally Secure

• Computationally Secure

Steganography

• List Types

General Cipher Characteristics

• Key Size• Transposition / Substitution• Block / Stream• Avalanche Effect• Surviving Plain Text Structure – Attacks• Historical Uses• Practical Observations

Caesar Cipher

Caesar Cipher Characteristics

• Key Size• Transposition / Substitution• Block / Stream• Avalanche Effect• Surviving Plain Text Structure – Attacks• Historical Uses• Practical Observations

Letter Substitution

Cipher Characteristics

• Key Size• Transposition / Substitution• Block / Stream• Avalanche Effect• Surviving Plain Text Structure – Attacks• Historical Uses• Practical Observations

Play Fair Cipher

Play Fair Cipher Characteristics

• Key Size• Transposition / Substitution• Block / Stream• Avalanche Effect• Surviving Plain Text Structure – Attacks• Historical Uses• Practical Observations

Vigenere Cipher

Cipher Characteristics

• Key Size• Transposition / Substitution• Block / Stream• Avalanche Effect• Surviving Plain Text Structure – Attacks• Historical Uses• Practical Observations

Vernam Cipher

Vernam Cipher Characteristics

• Key Size• Transposition / Substitution• Block / Stream• Avalanche Effect• Surviving Plain Text Structure – Attacks• Historical Uses• Practical Observations

Transposition Ciphers

Transposition Cipher Characteristics

• Key Size• Transposition / Substitution• Block / Stream• Avalanche Effect• Surviving Plain Text Structure – Attacks• Historical Uses• Practical Observations

Rotor Machines

Rotor Cipher Characteristics

• Key Size• Transposition / Substitution• Block / Stream• Avalanche Effect• Surviving Plain Text Structure – Attacks• Historical Uses• Practical Observations

Shannon

Shannon

• Diffusion– Plain Text “Smearing”– Not Permutation

• Confusion– Key Obfuscation

Feistel Cipher

Fiestel Cipher Characteristics

• Key Size• Transposition / Substitution• Block / Stream• Avalanche Effect• Surviving Plain Text Structure – Attacks• Historical Uses• Practical Observations

Modern Ciphers

DES

• Currently the most widely used block cipher in the world

• IBM’s LUCIFER was the precursor• One of the largest users of the DES is

the banking industry, particularly with EFT

• Although the standard is public, the design criteria used are classified

DES Security

• Recent analysis has shown that DES is well designed (diffusion & confusion)

• Rapid advances in computing speed though have rendered the 56 bit key susceptible to exhaustive key search – 1999 in 22hrs! – 3 DES

• DES also theoretically broken using Differential or Linear Cryptanalysis

• In practice, unlikely to be a problem yet

Overview of DES Encryption

• Basic process consists of: – An initial permutation (IP) – 16 rounds of a complex key dependent

calculation F– A final permutation, being the inverse of IP

• 64-bit key (56-bits + 8-bit parity)• 16 rounds

Initial permutation

Round 1

Round 2

Round 16

56-bitkey

Final permutation

…

+

F

Li – 1 Ri – 1

Ri

Ki

Li

• Each Round

DES Cipher Characteristics

• Key Size• Transposition / Substitution• Block / Stream• Avalanche Effect• Surviving Plain Text Structure – Attacks• Historical Uses• Practical Observations

Advanced Encryption Standard

AES

Origins of AES

• In 1999, NIST issued a new standard that said 3DES should be used– 168-bit key length– Algorithm is the same as DES

• 3DES had drawbacks– Algorithm is sluggish in software– Only uses 64-bit block size

Origins of AES (Cont’d)

• In 1997, NIST issued a CFP for AES– security strength >= 3DES– improved efficiency– must be a symmetric block cipher (128-bit)– key lengths of 128, 192, and 256 bits

Origins of AES (cont’d)

• First round of evaluation– 15 proposed algorithms accepted

• Second round– 5 proposed algorithms accepted

• Rijndael, Serpent, 2fish, RC6, and MARS

• Final Standard - November 2001– Rijndael selected as AES algorithm

The AES Cipher

• Block length is 128 bits• Key length is 128, 192, or 256 bits• NOT a Feistel structure

• Processes entire block in parallel during each round using substitutions and permutations

• The key that is provided as input is expanded• Array of forty-four 32-bit words (w[i])• Four distinct words serve as round key (128 bits)

Decryption

• Not identical to encryption• Equivalent structure exists• May need different implementations if

encryption and decryption are needed• Quite often only encryption needed

– Digest

AES Cipher Characteristics

• Key Size• Transposition / Substitution• Block / Stream• Avalanche Effect• Surviving Plain Text Structure – Attacks• Historical Uses• Practical Observations