Cryptography Basics: DES and AES

○ takes key and fixed size input block to generate fixed size output block

• Stream ciphers (e.g., RC4) ○ takes key and generates a stream of pseudorandom bits, XOR’d into data

• Triple-DES alternative to DES – but slow, has small blocks
• US NIST issued call for ciphers in 1997
• 15 candidates accepted in Jun 98
• 5 were shortlisted in Aug-99
• Rijndael was selected as the AES in Oct-2000
• issued as FIPS PUB 197 standard in Nov-2001

• designed by Rijmen-Daemen in Belgium
• has 128/192/256 bit keys, 128 bit data
• an iterative rather than feistel cipher processes data as block of 4 columns of 4 bytes ○ ○ operates on entire data block in every round
• designed to be: ○ resistant against known cryptanalysis attacks ○ speed and code compactness on many CPUs design simplicity ○

• AES is a non-Feistel cipher that encrypts and decrypts a data block of 128 bits arranged in a matrix of 4 columns of 4 bytes named state.
• It uses 10, 12, or 14 rounds. The key size, which can be 128, 192, or 256 bits, depends on the number of rounds. Key is expanded to array of words (state).
• In each round, the state undergoes: 1 Byte substitution (1 S-box used on every byte) 2 Shift rows (permute bytes between groups/columns) 3 Mix columns (subs using matrix multiply of groups) 4 Add round key (XOR state with key material)

e r u t c u r t S S E A

• Objective: To create round keys for each round, AES uses a key-expansion process.
• If the number of rounds is N, the key-expansion routine creates N 128-bit round keys from one single 128-bit cipher key. r r + 1
• takes 128-bit (16-byte) key and expands into array of 44/52/60 32-bit words

• Shift Rows
  • a circular byte shift to left in each row
  • decrypt inverts using shifts to right
  • since state is processed by columns, this step permutes bytes between the columns

• Add Round Key
  • XOR state with 128-bits of the round key
  • again processed by column (though effectively a series of byte operations)
  • inverse for decryption identical
  • since XOR own inverse, with reversed keys
  • designed to be as simple as possible
  • a form of Vernam cipher on expanded key
  • requires other stages for complexity / security

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• has a simple structure
• only AddRoundKey uses key
• AddRoundKey a form of Vernam cipher
• each stage is easily reversible
• decryption uses keys in reverse order
• decryption does recover plaintext
• final round has only 3 stages
• Review: www.securityfit.cz/download/kib/ rijndael_ingles2004. swf