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AES Encryption: Advanced Encryption Standard Overview

This article provides an overview of the Advanced Encryption Standard (AES), including its background, encryption and decryption processes, security issues, and implementation considerations.

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AES Encryption: Advanced Encryption Standard Overview

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  1. ICS 555Cryptography Advanced Encryption Standard (AES) Sultan Almuhammadi

  2. Outline • Background • AES Encryption and Decryption • Security Issues • Implementation Issues

  3. Background • In 1977, the National Bureau of Standards (NBS) adopted DES. • In 1994, the National Institute of Standards and Technology (NIST), reaffirmed DES for federal use for another 5 years. • In 1999, NIST adopted 3DES. • Pros: • 168-bit key  overcomes brute-force attack. • Cons: • Triple rounds  relatively slow in software. • 64-bit block size  larger is better. • 3DES is not a good candidate for long-term use.

  4. Background • In 1997, NIST called for a new Advanced Encryption Standard (AES) • AES Requirements: • Must have equal or better security than 3DES. • Must improve the efficiency. • Must be a 128-bit symmetric block cipher. • Must support 128/192/256 bit key lengths. • In 2001, NIST selected Rijndael (by Rijmen and Daemen) as the new AES. • AES will replace 3DES eventually. Until then, NIST approves 3DES for US government use.

  5. AES (Encryption) • Not a Feistel structure (data block is processed in parallel in each round) • Key Expansion provides 128-bit round keys (4 words each). • Each round hs 4 stages: • Substitute bytes: using an S-box to perform a byte-by-byte substitution of the block • ShiftRows: a simple permutation • MixColumns: substitution using arithmetic over GF(28) • AddRoundKey: bitwise XOR of the current block with a round key

  6. byte-by-byte substitution using S-box Simple permutation Substitution using arithmetic over GF(28) XOR with round key

  7. AES (Decryption) • Each stage is easily reversible: • Inv. Sub bytes: An inverse S-box is used. • Inv. ShiftRows: Inverse permutation • Inv. MixColumns: Inverse substitution using arithmetic over GF(28) • AddRoundKey: XOR with a round key in reverse order (B RK)  RK = B

  8. AES Security Issues • Only the AddRoundKey stage makes use of the key. • Other stages are reversible without the key  add no security. • AddRoundKey stage by itself is just an XOR scheme  attackable. • Other three stages provide confusion/diffusion/ nonlinearity (i.e. scrambling the block), but no security. • The four stages together in each round make it both efficient and highly secure. • The S-box is designed such that: • It is resistant to known cryptanalytic attacks • It has no fixed point (S-box(n) = n) • it is not self-inverse. Eg. S-box(95) = 2A, but Inv.S-box(95) = AD

  9. AES Implementation Issues • Very efficient on 8-bit processor: • AddRoundKey: bytewise XOR operation • ShiftRows: simple byte shifting • SubBytes: operates at byte-level • MixColumns: multiplies matrices in GF(28) • On 32-bit processor, a more efficient implementation can be achieved with operations defined on 32-bit words.

  10. AES Demo • Pls visit the link below for a full AES demo: http://www.formaestudio.com/rijndaelinspector/archivos/Rijndael_Animation_v4_eng.swf

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