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open, called the public key , and the other can not be disclosed , called the private key , only by holders of a person to know . When using asymmetric key algorithm , only use a matched pair of public and private keys , to complete the plaintext encryption and decryption process . Asymmetric key algorithm is the basic principle is that the two sides of the communication before the transceiver , a reception side must be had long been randomly generated public key given sender and the sender 's public key using the received encrypted text ; close recipient after receiving the encrypted ciphertext using his private key to decrypt the ciphertext. Its disadvantages are: algorithm is generally more complex, plus , the solution is slow. It involves algorithms such as RSA cryptographic algorithm .

Data Encryption Standard (DES) which was developed by the United States, IBM a data encryption algorithm, in 1975 the U.S. National Bureau of Standards (NBS) discloses the Data Encryption Standard , which is free to use it , and in 1977 it as a government data encryption preparation.

Data Encryption Standard is a symmetric cryptosystem , it uses the encryption and decryption keys are the same , and the use of an alternative method of shifting the data is encrypted by the DES encryption system , each encryption and decryption of the packet size or are 64 , which uses 64-bit keys for 64 -bit binary data block coding for 16 replacement and shift operations , the input plaintext into 64 different 64-bit ciphertext.

DES algorithm is a good performance of the data encryption algorithm , not only the random characteristics of a good , high linear complexity , computing speed, and easy to implement, plus the ability to standardize and universal. Therefore DES widely in the international application .

DES algorithm biggest drawback is the key too short to provide adequate security, in 1999 to use more than one hundred CPU, the use of parallel algorithms, with 23 hours or so successfully deciphered . To overcome this drawback , and later to propose a triple DES or 3DES system . Using the 168 's long key , perform the DES algorithm three times .

Data Encryption Standard to the input 64-bit plaintext block operation , the plaintext after an initial permutation IP is replaced m0, m0 clear the length are 32 bits long into the left and right portions of m0 = (L0, R0). Followed by 16 identical function f operation , during the process of data and the calculation function key combination . After 16 operations , the left and right part together also need to go through a late replacement .

In the first round , the key bits to be shifted , and then again from 56 selected 48 key . Through an extension of the right half of the data permutation expanded into 48 , and through an exclusive-OR operation into a new alternative 32-bit data at the time of its replacement . This four -step operation constitutes a function f. Then , through another XOR function f with the left half of the output , the results of which the right half of the new , original left right part of a new section. This operation was repeated 16 times, is realized.

Data Encryption Standard encryption and decryption using the same algorithm . It should be noted that the encryption and decryption keys used in reverse order 本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。