Analysis And Optimization Improvement Of The Advanced Encryption Standard AES Algorithm

On October 2,2000,the National Institute of Standards and Technology(NIST)has officially determined the Rijndael algorithm as Advanced Encryption Standard(AES).The emergence of the AES algorithm is to replace Data Encryption Standard(DES),which is no longer secure.The AES algorithm is widely used in confidential communication and commercial encryption,which can be said to be the most representative encryption algorithm in the field of information security.This paper mainly studies the security and implementation of the AES algorithm,and on the basis of in-depth analyzing the algebraic properties and anti-attack ability of the AES algorithm,it puts forward the optimized and improved AES algorithm S-box and key expansion algorithm.Through the MATLAB simulation experiment,the correctness of each module function of the improved algorithm is verified.This paper proves that the proposed improved algorithm has better properties than AES algorithm in theory and practice.The main work and innovations are as follows:Firstly,detail introduced the mathematical basis and algorithm structure of the AES algorithm,including the operation of words and bytes over finite field,encryption and decryption algorithm,key expansion algorithm,equivalent MixColumns transformation and equivalent decryption algorithm.The common attack methods against cryptographic algorithms are studied,including strong attack,differential cryptanalysis,linear cryptanalysis,Square attack,algebraic attack and power analysis.Through the above research,the existing problems of the AES algorithm and the need for optimization and improvement are found.Secondly,deeply explored the construction principle and algebraic properties of the AES algorithm S-box.As the only non-linear component in the AES algorithm,its security determines the security strength of the AES algorithm.The results shows that there are some defects in the number of algebraic expression terms,affine transformation period,iterative output period and strict avalanche criterion distance of the S-box,which is likely to be the key to crack the AES algorithm.To solve this problem,a new S-box construction scheme is proposed,including the following three steps:(1)Choosing irreducible polynomials that can construct S-box with better algebraic properties;(2)Selecting affine transformation pairs that satisfy the affine transformation period,iterative output period and the best distance of strict avalanche criterion;(3)Adjust the construction steps of S-box and adopt the calculation sequence of "Affine transformation-Find inverse of multiplication-Affine transformation".Theexperimental analysis shows that the main algebraic properties of the new S-box are significantly improved compared with the AES algorithm S-box,and other performances are not reduced.But the generation time of the new S-box is increased by0.02 seconds,accounting for 2.1% of the total time.Furthermore,carefully studied the AES key expansion algorithm.The result shows that there is a strong correlation between the two adjacent keys.The attacker only needs to get any one round of sub keys to calculate all the keys including the initial key.To solve this problem,two improvement schemes is proposed:(1)Perform SubBytes,Rotword and wheel constant XOR operation on the first two bits of each wheel keys,and the strength of the improved algorithm after three rounds of operation is equivalent to brute-force cracking;(2)Adjust the key generation method of each wheel.From the second round,the key generation must be jointly operated by the sub-keys of the first two rounds,added compound operation and XOR operation,and the strength of the improved algorithm after two rounds of operation is equivalent to brute-force cracking.The data analysis shows that the security of the optimized key expansion algorithm has been significantly improved.However,the round key generation time increased by0.000176 seconds and 0.000464 seconds,accounting for 7.7% and 20.4% of the total time.Finally,the availability and correctness of the improved AES algorithm are analyzed in the experiment.MATLAB simulation experiment is used to implement the new s-box and the improved two key expansion algorithms.Use the improved algorithm to encrypt and decrypt strings and images,compared the data test set provided in the official standard of the AES algorithm(FIPS PUB 197).The experimental results prove that the improved AES algorithm proposed in this paper can correctly construct a new S-box and generate a new round key.The encryption and decryption functions can be normally implemented,and the encryption and decryption efficiency is equivalent to the AES algorithm.