Block vs Stream Ciphers
• Stream ciphers process messages a bit or byte at
a time when en/decrypting.
• Block ciphers process messages in into blocks,
each of which is then en/decrypted.
– Like a substitution on very big characters: 64-bits or
more
• Many current ciphers are block ciphers, one of
the most widely used types of cryptographic
algorithms
DES (Data Encryption Standard)
DES (Data Encryption Standard)
DES (Data Encryption Standard)
One round in DES ciphers
DES (Data Encryption Standard)
DES (Data Encryption Standard)
DES (Data Encryption Standard)
DES (Data Encryption Standard)
DES (Data Encryption Standard)
DES (Data Encryption Standard)
DES (Data Encryption Standard)
DES (Data Encryption Standard)
DES (Key
Scheduling)
DES (Key Scheduling)
DES (Data Encryption Standard)
Strength of DES – Key Size
• 56-bit keys have 256 = 7.2 x 1016 values
• Brute force search looks hard
• Recent advances have shown is possible
– in 1997 on a huge cluster of computers over the Internet in a
few months
– in 1998 on dedicated hardware called “DES cracker” by
Electronic Frontier Foundation (EFF) in a few days ($220,000)
– in 1999 above combined in 22hrs!
DES Replacement
• Double-DES (2DES): Man-in-the-middle attack.
DES Replacement
DES Replacement
• Triple-DES (3DES)
– 168-bit key, no brute force attacks
– Underlying encryption algorithm the same, no
effective analytic attacks
– Drawbacks
• Performance: no efficient software codes for DES/3DES
• Efficiency/security: bigger block size desirable
Triple-DES
Triple-DES
Triple-DES
AES (Advanced Data Encryption
Standard)
• Advanced Encryption Standards (AES)
– US NIST issued call for ciphers in 1997
– Rijndael was selected as the AES in Oct-2000
• Private key symmetric block cipher
• Stronger & faster than Triple-DES
• In AES, all operations are performed on 8-bit
bytes. In particular, the arithmetic operations of
addition, multiplication, and division are
performed over the finite field GF(28).
AES (Advanced Data Encryption
Standard)
Note
AES has three different configurations with respect to
the number of rounds and key size.
AES (Advanced Data Encryption
Standard)
AES configuration
AES (Advanced Data Encryption
Standard)
AES (Advanced Data Encryption
Standard)
Structure of each round
Modes
Block Cipher Principles
• Most symmetric block ciphers are based on a
Feistel Cipher Structure
Substitution-Permutation Ciphers
• Substitution-permutation (S-P) networks [Shannon,
1949]
– modern substitution-transposition product cipher
• S-P networks are based on the two primitive
cryptographic operations
– substitution (S-box)
– permutation (P-box)
• provide confusion and diffusion of message
• These form the basis of modern block ciphers
Confusion and Diffusion
• Cipher needs to completely obscure statistical
properties of original message
• A one-time pad does this
• More practically Shannon suggested S-P networks
to obtain:
• Diffusion – dissipates statistical structure of
plaintext over bulk of ciphertext
• Confusion – makes relationship between
ciphertext and key as complex as possible
Feistel Cipher Structure
• Feistel cipher implements Shannon’s S-P
network concept
– based on invertible product cipher
• Process through multiple rounds which
– partitions input block into two halves
– perform a substitution on left data half
– based on round function of right half & subkey
– then have permutation swapping halves
Feistel
Cipher
Structure
