IOSR Journal of Computer Engineering (IOSR-JCE) e-ISSN: 2278-0661,p-ISSN: 2278-8727, Volume 22, Issue 5, Ser. I (Sep. – Oct. 2020), PP 55-58 www.iosrjournals.org DOI: 10.9790/0661-2205015558 www.iosrjournals.org 55 | Page To A Differential Attack for Symmetric Block Cipher Juraev G.U. 1 , Djabborov A.Kh. 2 1 (Department of Information Security, National University of Uzbekistan named after Mirzo Ulugbek, Uzbekistan ) 2 (Department of mathematical modeling, Samarkand state university, Uzbekistan) Abstract: This article discusses in detail the issues related to the effective conduct of differential cryptanalysis for modern symmetric block data encryption algorithms. For this purpose, an additional stage is introduced to organize a differential attack for symmetric block ciphers. As the first stage of a differential attack, it is proposed to build an attack model, in this case, an action model, which will allow for a reasonable time and an acceptable number of cleartext - ciphertext pairs to calculate the encryption subkey used. Key Word: plaintext, ciphertext, symmetric block ciphers, differential attack, differential cryptanalysis, transformation, strength, differential characteristics. --------------------------------------------------------------------------------------------------------------------------------------- Date of Submission: 20-09-2020 Date of Acceptance: 05-10-2020 --------------------------------------------------------------------------------------------------------------------------------------- I. Introduction As known, ensuring the confidentiality of information when transmitting messages over an insecure communication channel is a traditional task of cryptography. For this purpose, symmetric and asymmetric ciphers are now successfully used. Symmetric ciphers have high encryption rates and are also much easier to implement in both software and hardware, for example the symmetric block cipher DES is about 1000 times faster than the asymmetric cipher RSA. For this reason, symmetric encryption is often used to encrypt messages with a longer length than asymmetric encryption. Symmetric ciphers are divided into two categories: stream and block ciphers [1]. It should also be noted that some classifications do not distinguish between block and stream encryption, considering that stream encryption is encryption of blocks of unit length. Stream ciphers are needed primarily in cases where information cannot be divided into blocks. For example, a data stream, each character of which must be encrypted and sent somewhere, without waiting for the rest of the data sufficient to form a block. For this reason, stream encryption algorithms encrypt data bit by bit or character by character [1]. Basically, stream ciphers are implemented in hardware in wireless data networks due to the need to immediately transfer data as it arrives. Therefore, stream ciphers are very efficient, they are often used to encrypt audio and video information [2]. Modern symmetric block data encryption algorithms encrypt an m-bit block of plaintext and decrypt an m-bit block of ciphertext. The same key is used for encryption and decryption, or the decryption key is easily calculated from the encryption key and vice versa. As a result, the decryption algorithm must be the inverse of the encryption algorithm. According to the Kerchhoff principle, the encryption and decryption algorithms must be fully known to the cryptanalyst, i.e. the secrecy of the cipher must be ensured by the secrecy of the encryption key. Only in some cases, for example, for military and intelligence purposes, the essence of the cryptosystem is kept secret. Symmetric block ciphers have the following basic requirements: • sufficient cryptographic strength; • fast encryption speed; • simplicity of encryption and decryption procedures. The general principles of building block ciphers have been defined by K. Shannon [3,4], on the basis of which it is necessary to use in the block cipher algorithm: a) substitutions (nonlinear transformations); b) permutations of symbols in blocks; c) iterating operations a) and b) (i.e., repeating them multiple times with different keys). Multiple application of the above algorithms, relatively simple cryptographic transformations, ensures the cryptographic strength of ciphers.