Symmetric Key Block Ciphers

Classical Ciphers Substitution Transposition

Modern Cryptography Data Encryption Standard 1977 “New Directions in Cryptography” 1976

Ciphers Symmetric key block ciphers Public key ciphers Stream ciphers Post-quantum ciphers

Stream Ciphers Stream ciphers

Data Encryption Standard 1977 Horst Feistel (1915 – 1990) Simplified DES: Wade Trapp and Lawrence Washington

Data Encryption Standard 1976

DES DES Simplified DES 64-bit block 56-bit key (8 parity bits) 8 S-boxes 6 x 4 16 rounds Feistel cipher 12-bit block 9-bit key 2 S-boxes 4 x 3 4 rounds Feistel cipher

Simplified DES – Feistel Cipher

Key and Message

Key Schedule 9 bits of key 111 010 110 Use the 8 bits on the left for k1 1110 1011 Rotate to the left one bit 110 101 101 Use the 8 bits on the left for k12 1101 0110 Rotate to the left 1 bit 101 011 011 Use the 8 bits on the left for k3 1010 1101 Rotate to the left 1 bit 010 110 111 Use the 8 bits on the left for k4 0101 1011

Expansion Function and XOR

S-Box

DES Encryption

DES Decryption Encryption Decryption

DES Decryption Encryption Decryption

Secrecy of Algorithm Backdoor?

3DES 112-bit security

Security Level Perfect Security Random key, as long as the message, and used only once. Computational Security Today 128

Block Cipher Modes

Block Cipher Modes

Skipjack/Clipper Chip 64-bit block 80-bit key 32 rounds Unbalanced Feistel cipher Skipjack/Clipper Chip Skipjack declassified 1998 Clipper Chip 1993 -1996

Substitution-Permutation Networks Claude Shannon (1916 – 2001)

Claude Shannon “A Mathematical Theory of Communication” 1948 “The Theory of Secrecy Systems” 1949

“The Theory of Secrecy Systems 1949 Diffusion: “… the statistical structure of [the message] which leads to its redundancy is dissipated into long range statistics.” P-box Confusion: “… make the relation between the simple statistics of [the ciphertext] and the simple description of [the key] a very complex and involved one.” S-box

SPN

PRESENT 2007 Many

PRESENT 64-bit block 80- or 128-bit key 1 S-box 4 x 4 31 Rounds Substitution-Permutation Network

Substitution Layer

Permutation Layer

Substitution and Permutation

Key Schedule

Advanced Encryption Standard 2001 Joan Daemen and Vincent Rijmen Simplified AES: Mohammad Musa, Edward Schaefer, and Stephen Wedig

AES (Rijandael) AES Simplified AES 128-bit block 128-bit, 192-bit, or 256-bit key 10, 12, or 14 rounds SPN 16-bit block 16-bit key 2 rounds SPN

S-Box

S-Box

Add Key

SN, SR, MC

S-AES

Simplify the Other AES Finalists Serpent Twofish RC6 MARS