Data Encryption Standard (DES) © 2000 Gregory Kesden.

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Presentation transcript:

Data Encryption Standard (DES) © 2000 Gregory Kesden

Overview Initialization Termination Round … Plaintext Cyphertext Key 16 rounds of permutations and substitution DES is a 64-bit block cipher. Both the plaintext and ciphertext are 64 bits wide. The key is 64-bits wide, but every eighth bit is a parity bit yielding a 54-bit key.

Initialization Plaintext Initial Permutation Right Half Left Half Round Key

Termination Ciphertext Reverse Initial Permutation Right Half Left Half Round Key

A Round Right Half Left Half Transformed Key (48-bit) Permutations and substitution XOR Left Half Right Half

Permutations and Substitutions Right Half Transformed Key (48-bit) S-Box Substitution P-Box Permutation XOR E-Box Permutation 8 S-Boxes are used by the P-Box

Key Transformation 64-bit Key Remove parity bit 56-bit Key split 28-bit left semikey 28-bit right semikey shift 28-bit shifted left semi-key 28-bit shifted right semi-key Compression Permutation 48-bit semi-key

E-Box (Expansion Box)

S-Box (Substitution Box) There are 8 different S-Boxes, each of which provides a different 6:4 mapping. Where’d they come from? Some combination of IBM and NSA. The mappings are based on cryptanalysis and are ostensibly free of weaknesses, back-doors, &c.

P-Box (Permutation Box) S-Box 1 S-Box 2S-Box 3S-Box 4S-Box 5 S-Box 6S-Box 7S-Box 8 48-bit input 32-bit output

Block Cipher Modes “How do we break a message down into blocks for encryption by DES or another block cipher?”

Electronic Code Book (ECB) Mode Block 1 Block 2 Block 3 Block 4 Block 5 … E(block) Block 1 Block 2 Block 3 Block 4 Block 5 … Plaintext Ciphertext Pad last block, if necessary Block Encryption

Cipher Block Chaining (CBC) Mode Random Block 1 Block 2 Block 3 Block 4 … E(block) Block 2 Block 3 Block 4 Block 5 … Plaintext Ciphertext Pad last block, if necessary Random Block called IV can be sent in plain text. Not a secret – just prevents a codebook. Often times a timestamp. XOR Block Encryption

Leftmost Cipher Feedback Mode (CFB) Mode C I-6 C I-5 C I-4 C I-3 C I-2 C I-1 Shift Register (1 Block wide) Next unit of Ciphertext E(register) 1 unit is 1/N block Block Encryption Encrypted Register XOR Next unit of Plaintext After each unit, shift input register and insert the most recently generated unit of ciphertext output

Leftmost Output Feedback Mode (OFB) Mode C I-6 C I-5 C I-4 C I-3 C I-2 C I-1 Shift Register (1 Block wide) Next unit of Ciphertext E(register) 1 unit is 1/N block Block Encryption Encrypted Register XOR Next unit of Plaintext After each unit, shift input register and insert the leftmost unit of the encrypted register. output