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Data Encryption Standard (DES)

Data Encryption Standard (DES). © 2000 Gregory Kesden. Initialization. Termination. …. Round. Round. Round. Overview. Plaintext. Key. Key. Key. Key. Cyphertext. 16 rounds of permutations and substitution.

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Data Encryption Standard (DES)

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  1. Data Encryption Standard (DES) © 2000 Gregory Kesden

  2. Initialization Termination … Round Round Round Overview Plaintext Key Key Key Key Cyphertext 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.

  3. Initial Permutation Initialization Key Right Half Plaintext Round Left Half

  4. Reverse Initial Permutation Termination Key Right Half Round Ciphertext Left Half

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

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

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

  8. E-Box (Expansion Box) 1 32 1 2 3 4 5 6 7 8 .. .. .. .. 29 30 31 32 48 1 2 3 4 5 6 7 8 9 10 11 .. .. .. .. 44 45 46 47 48 1

  9. 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.

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

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

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

  13. E(block) XOR XOR XOR XOR XOR E(block) E(block) E(block) E(block) Random Block 1 Block 2 Block 3 Block 4 … Block 2 Block 3 Block 4 Block 5 … Cipher Block Chaining (CBC) Mode Plaintext Block Encryption 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.

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

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

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