1, data encryption standard
The original idea of DES can refer to Engelmar machine in Germany during World War II, and the basic idea is basically the same. Traditional cryptographic encryption originated from the ancient idea of cyclic shift, and Enigma machine diffused fuzziness on this basis. But the essential principle is the same. Modern DES does the same thing at the binary level: it replaces fuzziness and increases the difficulty of analysis.
2. Encryption principle
DES uses a 56-bit key and an additional 8-bit parity bit to generate packets with a maximum of 64 bits. This is an iterative block cipher, which uses a technique called Feistel, in which the encrypted text block is divided into two halves. Using subkeys to apply a cyclic function to half of them, and then XOR the output with the other half; Then the two halves are exchanged, and this process will continue, but the last cycle will not be exchanged. DES uses 16 cycles and uses four basic operations: XOR, permutation, substitution and shift.
3. The common variant of DES is triple DES, which encrypts data three times with a key of 168 bits. It usually (but not always) provides very strong security. Triple DES is backward compatible with DES if all three 56 subelements are the same.
4. Heat splitting method
The main form of attacking DES is called brute force cracking or thorough key search, that is, trying various keys repeatedly until there is a match. If DES uses a 56-bit key, the number of possible keys is 2 to the 56th power. With the continuous development of computer system capabilities, the security of DES is much weaker than when it first appeared. But from a non-critical point of view, it can still be considered sufficient. But now only DES is used to identify the old system, and more new encryption standard-Advanced Encryption Standard (AES) is selected.