 AES is a block cipher intended to replace DES for commercial applications. It uses a 128bit block size and a key size of 128, 192, or 256 bits.

AES does not use a Feistel structure. Instead, each full round consists of four separate functions:
 Byte substitution.
 Permutation.
 Arithmetic operations over a finite field.
 XOR with a key.

Advantages of 3DES:
 Its 168bit key length overcomes the vulnerability to bruteforce attack of DES.
 Scrutiny on its encryption algorithm.

Disadvantages of 3DES:
 No support for efficient code and have slow performance.
 Uses 64key bit size. For reasons of both efficiency and security, a larger block size is desirable.

Initial criteria of evaluating candidate security algorithms:

Security:
 Refers to effort required to cryptanalyze an algorithm.

Aspects:
 Actual security: compared to other algorithms (at the same key and block size).
 Randomness: the extent of randomness in the algorithm output.
 Soundness: of the math basis of the algorithm’s security.

Cost:
 The algorithm should have high computational efficiency; so as to be usable is high speed applications.

Aspects:
 Licensing requirements: the used subalgorithms should be worldwide and nonexclusive.
 Computational efficiency: speed of the algorithm from software hardware wise.
 Memory requirements: memory required to implement the algorithm from software and hardware wise.

Algorithm and implementation characteristics:

Flexibility: flexibility can be evaluated from following:
 Ability to accommodate additional key and block sizes.
 Range platforms that the algorithm is acceptable.
 The algorithm can be implemented as stream cipher, message authentication code generator, pseudorandom number generator, hashing algorithm etc…
 Hardwar and software suitability: ability to be implemented efficiency on hardware and software.
 Simplicity: relative simplicity of its design.



Advanced criteria of evaluating candidate algorithms:

General security:
 Strength duo to known cryptographic (from math perspective) attacks such as differential and linear cryptanalysis.

Software implementations:
 Execution speed, performance across variety of platforms, variation of speed with keysize.
 Restrictedspace environments.

Hardware implementations:
 The amount of required hardware to implement the algorithm efficiency.

Attacks on implementations:
 Strength duo to physical attacks during algorithm execution to gather information about quantities such as keys.

Examples of such algorithms are:
 Time attacks.

Power analysis:
 Observation that the power consumed by a smart card at any particular time during the cryptographic operation is related to the instruction being executed and to the data being processed.
 For example, multiplication consumes more power than addition.
 Writing 1s consumes more power than writing 0s.

Encryption versus decryption:
 If the encryption and decryption differs, then extra space is needed for decryption.

Key agility:
 Ability to change keys quickly and with a minimum of resources.

Other versatility and flexibility:
 Parameter flexibility: ease of support for another key and block sizes and ease of increasing the number of rounds in order to cope with newly discovered attacks.
 Implementation flexibility: possibility of optimizing cipher elements for particular environments.
 Potential for instructionlevel parallelism.

The AES Cipher
 Block and key lengths can be independently specified to be: 128 (most common used), 192 or 256 bits.

Characteristics of AES Cipher:
 Resistance against all known attacks.
 Speed and code compactness over wide range of platforms.
 Design simplicity.

Notes about AES structure
 Not Feistel cipher and entire process data block in parallel during each round.
 The provided key is expanded into an array of fortyfour 32bit words, w[i]. Four distinct words (128 bits) serve as a round key for each round.

Four different stages are used, one of permutation and three of substitution:
 Substitute bytes: Uses an Sbox to perform a bytebybyte substitution of the block.
 ShiftRows: A simple permutation.
 MixColumns: A substitution that makes use of arithmetic GF(2^{8})
 AddRoundKey: A simple bitwise XOR of the current block with a portion of the expanded key.

Cipher Stages:
 Initial round with AddRoundKeyStage
 Followed by 9 rounds each one includes four stages.
 Tenth round that uses three stages only.
 Each stage is easily reversible.
 The decryption algorithm is not similar to the encryption algorithm. This is a consequence of the particular structures of AES.
 AES uses arithmetic in the finite field of GF(2^{8}), with irreversible polynomial
 The MixColumns step along with the ShiftRows step is the primary source of diffusion in Rijndael cipher.
 The expanded key can be seen as an array of 32bit words (columns) numbered from 0 to 43.
 For details about AES working mechanism see this video.
 One of the prerequisites of the MixColumn stage is to know how to apply multiplication in arithmetic field GF(2^{8}).
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