cse 5/7353 – january 25 th 2006
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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
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