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DES cryptography in the history of the emergence of an innovation. Any previous designers and design details for cryptosystems are kept strictly confidential , and DES is published , anyone can study and analysis can be made without going through the permit and to DES DES chip -based security equipment . The security of DES is completely dependent on the key.

DES has come out since , although it has opened a lot of people worry and controversy, many people study it with a variety of attacks, but still no one really decipher it . The results from the present point of view , in addition to brute-force attacks, there is no better way to decipher the DES . The 56 long exhaustive key space is 256 , which means that if the speed of a computer is the first second test one million keys , then it searches for the key you need to complete part time nearly 2258 . However, the current high-speed computers , the DES key brute-force only tens of minutes , DES can not meet the security needs of modern encryption . In 2000 the U.S. government announced a new encryption algorithm AES replaces DES. To improve the security of the DES algorithm , we can select a method to increase the length of the DES key , implement this method is the simplest and most commonly used is a 128 -bit 3DES algorithm . Triple DES algorithm design idea is to use the same algorithm , encrypted with a different key twice the same ciphertext.

Diffie and Hellman public key in the envisioned two years later, first by Merkle and Hellman proposed the first public key algorithm MH- knapsack public key cryptosystem , followed by the United States at MIT three scholars Rivest, Shamir, and Adleman proposed RSA public key system . RSA cryptosystem is the n ame of the inventor of the three first letters of the name put together [ 21 ] . RSA encryption algorithm is an encryption key and a decryption key for two different cipher keys .

First, take two take two random large prime number p, q ( confidential ) , calculate the public modulus n = p × q ( public ) ; secret calculated from the p and q Euler function φ (n) = (p-1 ) × (q-1) ( confidentiality ) , discard p and q, do not let anyone know . Random integer e, satisfy gcd (e, (n)) = 1 ( e public encryption key ) . Calculating d satisfy d × e ≡ 1 (modφ (n)) ( d secret decryption key ) . Plaintext x by squaring modulo r e raised to the power to complete the encryption operation , resulting ciphertext y. (X, y values in the range 0 to n-1 within ), y = xe mod n. Decryption: the ciphertext y by squaring modulo n d th power . x = yd mod n But only on n and e ( not p and q) to calculate the d is impossible. Therefore , anyone can encrypt the plaintext , but only authorized users ( known d) can decrypt ciphertext .