The DES algorithm

The DES algorithm (Data Encryption Standard) a conventional (i.e., 1-key) substitution cipher represented by a simplified example

David Morgan

S-DES* process flow *simplified version of the DES algorithm for tutorial purposes, by Edward Schaefer, Santa Clara University

Credit to Cryptography and Network Security, Principles and Practice, William Stallings, Prentice Hall, 1999 for figure and precision of explanation.

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S-DES encryption component process flow

Credit to Cryptography and Network Security, Principles and Practice, William Stallings, Prentice Hall, 1999 for figures and precision of explanation.

ENCRYPT

Following slide images are screenshots from an exercise at http://homepage.smc.edu/morgan_david/vpn/assignments/assgt-sdes.htm

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Initial permutation IP 0 1 1 0 1 1 0 1 1 1 1 0 0 1 1 0

expansion/permutaion E/P

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XOR with subkey K1

...etc

s-box substitutions

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Permutation P4

XOR

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left-nibble replacement

Swap SW

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expansion/permutaion E/P

XOR with subkey K2

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s-box substitutions

Permutation P4

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XOR

left-nibble replacement

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Inverse initial permutation IP-1

DECRYPT

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Initial permutation IP

expansion/permutaion E/P

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XOR with subkey K2

...etc

s-box substitutions

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Permutation P4

XOR

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left-nibble replacement

Swap SW

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expansion/permutaion E/P

XOR with subkey K1

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s-box substitutions

Permutation P4

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XOR

left-nibble replacement

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Inverse initial permutation IP-1

Matches original input, decrypt succeeded

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