Security Analysis And Optimization Improvements Of AES Algorithm

Cryptography is an old technology, probably since the war of human society broke out, the cryptography has emerged. Because cryptography is only used for confidential communication of military, politics, diplomacy and other central department, so research in cryptography is conducted in secret, the knowledge of cryptography and related technology mainly controlled by the military, politics, diplomacy and other central department, it was not published. However, with the development of the computer science and technology, communications technology, microelectronic technology, the application of computer and networks is prevailing in people's daily life and works, bringing about the emergence of e-government, e-commerce, e-finance, which need to ensure information security. With the constant growth of cryptography in information security, cryptography gradually heads for commerce from the military science, becoming the subject which attracts more attention.As the application of the computer technology and network communication technology rapidly develops, the problems of network information security which have been brought about by the network attract more attention. A number of new subjects have emerged in theory and applications of cryptography. For example, in the field of block cipher, DES which was considered as an cryptography with adequate security by people in the past had been proved insecurity with the new analysis methods and computing technology. As a result, in October of 2000, Belgian cryptographer: Joan Daemen and Vincent Rijmen proposed "Rijndael encryption algorithm" which has been identified as the Advanced Encryption Standard (Advanced Encryption Standard, AES),it is a new generation of Data Encryption Standard.Firstly, the mathematical basis of AES and the detail of the main structure of the algorithm are described briefly, including the encryption algorithm, decryption algorithm and sub-key generation algorithm. The process of the encryption and decryption relates to wheel transform, which is composed of four transformations. After we analyse the security of AES cryptographic algorithm, we optimize and improve.In the security analysis part, the paper selects three kinds of analytical methods for attack: brute force attack with exhaustive keys, differential attack and Square attack. Force attack is the most fundamental and can be used for all types of attack algorithm with the method that is tried in turn. Because of the length of AES's key, the attack on AES is hardly invalid. Differential attack is one of the most commonly used block cipher analysis. It uses the imbalance of differential distribution of statistics of block cipher algorithm to find out certain information of the key to decipher that is one of the most effective ways for the iteration-based block cipher, but through our differential attack on AES algorithm we conclude that AES can effectively block this attack. Square attack is the most effective plaintext attack on cryptography with Rijndael round structure. It uses the characteristic that the Rijndael algorithm is based on the structure of bytes, and has some common with Square algorithm. The four-wheel fundamental attack on the algorithm can be extended to five or six, and we can obtain that the number of computing of the five-wheel is 232 times ,the number of six-wheel can achieve 246 times. If attack on AES algorithm is successful, we can obtain all the keys the correlation between the sub-keys, and it is an intimidation.In the view of potential threats, we refer to KHAZAD algorithm that is designed by V. Rijmen to improve the AES sub-key generation algorithm and enhance its security. Because of this sub-key improvements, the difficulty of decipher is expanded. When the attackers obtain the some-wheel key, they can not get the keys next to this wheel. Taking AES-128 for example, when an attacker get a key in some wheel, if he wants to guess the key of the next round, he not only needs to obtain the key of this round, but also assumes the four-wheel key in the front round. Each wheel key is 32 bits, so the number of guessing is 2128 times. Its time complexity is equal to the key quite exhausted. For the same reason, if you want to obtain the key through a forward projection, it also needs the guessing of the next round, its time complexity is 2128. Therefore, a key acquisition for the break out of the initial key is not helpful. A threat of some attack methods such as energy attacks on the AES algorithm effectively subsides.For a cipher algorithm, the evaluation of its advantages and weaknesses is mainly basis on its security and the cost of completing. Between the security and the cost of completing, the security of algorithm is undoubtedly the most important, if an algorithm is unsafe, it will no longer be considered. The cost refers to the efficiency of the various implementations (speed and storage requirements), including software, hardware, smart cards and so on. Therefore, this article also improves encryption and decryption algorithms of the current Advanced Encryption Standard AES. Improvement of the AES'encryption and decryption process is to adjust the sequence of four rounds transformation in the decryption process to make it the same with the encryption process, and then merge ShiftRow and MixColumn. The transformation after the merger is known as RowColumn, because it can achieve the effects of ShiftRow and MixColumn for byte, and reduce a calculation steps. A whole improvement for the encryption and decryption process aims at reducing costs, including software and hardware.In short, improvement of the AES algorithm is not only to improve the security of the algorithm, but also to harmonize and simplify the encryption and decryption process. The work that is carried out in this paper is the early stages of the study, there are a lot of work worthy of further study and exploration in order to try to improve the algorithm.