## Feistel轮函数
每次Feistel轮函数内部，均经过4种运算，即：
* 1、扩展置换：右侧32位做扩展置换，扩展置换将32位输入扩展成为48位输出，使得扩展后输出数据长度与48位子密钥等长。
* 2、异或运算：右侧32位扩展置换为48位后，与48位子密钥做异或运算。
* 3、S盒置换：将异或运算后的48位结果，分成8个6位的块，每块通过S盒置换产生4位的输出，8个块S盒置换后组成32位的输出。
S盒置换的过程为：6位中取第1位和第6位组成行号，剩余第2、3、4、5位组成列号，从S盒置换表中取出相应行、列的十进制数，并转化为4位二进制数，即为S盒输出。
* 4、P盒置换：S盒置换后的32位输出数据，进行P盒置换，仍然输出为32位数据。
go标准库中DES Feistel轮函数代码如下：
```go
func feistel(right uint32, key uint64) (result uint32) {
//右侧32位扩展置换为48位，并与48位子密钥做异或运算
sBoxLocations := key ^ expandBlock(right)
var sBoxResult uint32
for i := uint8(0); i < 8; i++ {
//sBoxLocations>>42、sBoxLocations <<= 6，按每6位分块
sBoxLocation := uint8(sBoxLocations>>42) & 0x3f
sBoxLocations <<= 6
//6位中取第1位和第6位组成行号
row := (sBoxLocation & 0x1) | ((sBoxLocation & 0x20) >> 4)
//剩余第2、3、4、5位组成列号
column := (sBoxLocation >> 1) & 0xf
//feistelBox包括了S盒置换和P盒置换的实现
sBoxResult ^= feistelBox[i][16*row+column]
}
return sBoxResult
}
var feistelBox [8][64]uint32
//P盒置换
func permuteBlock(src uint64, permutation []uint8) (block uint64) {
for position, n := range permutation {
bit := (src >> n) & 1
block |= bit << uint((len(permutation)-1)-position)
}
return
}
//初始化feistelBox
func init() {
for s := range sBoxes {
for i := 0; i < 4; i++ {
for j := 0; j < 16; j++ {
f := uint64(sBoxes[s][i][j]) << (4 * (7 - uint(s)))
f = permuteBlock(f, permutationFunction[:])
feistelBox[s][16*i+j] = uint32(f)
}
}
}
}
//代码位置src/crypto/des/block.go
```
附go标准库中使用的扩展置换表和P盒置换表：
```go
//扩展置换表
var expansionFunction = [48]byte{
0, 31, 30, 29, 28, 27, 28, 27,
26, 25, 24, 23, 24, 23, 22, 21,
20, 19, 20, 19, 18, 17, 16, 15,
16, 15, 14, 13, 12, 11, 12, 11,
10, 9, 8, 7, 8, 7, 6, 5,
4, 3, 4, 3, 2, 1, 0, 31,
}
//P盒置换表
var permutationFunction = [32]byte{
16, 25, 12, 11, 3, 20, 4, 15,
31, 17, 9, 6, 27, 14, 1, 22,
30, 24, 8, 18, 0, 5, 29, 23,
13, 19, 2, 26, 10, 21, 28, 7,
}
//代码位置src/crypto/des/const.go
```
附go标准库中使用的S盒置换表：
```go
var sBoxes = [8][4][16]uint8{
// S-box 1
{
{14, 4, 13, 1, 2, 15, 11, 8, 3, 10, 6, 12, 5, 9, 0, 7},
{0, 15, 7, 4, 14, 2, 13, 1, 10, 6, 12, 11, 9, 5, 3, 8},
{4, 1, 14, 8, 13, 6, 2, 11, 15, 12, 9, 7, 3, 10, 5, 0},
{15, 12, 8, 2, 4, 9, 1, 7, 5, 11, 3, 14, 10, 0, 6, 13},
},
// S-box 2
{
{15, 1, 8, 14, 6, 11, 3, 4, 9, 7, 2, 13, 12, 0, 5, 10},
{3, 13, 4, 7, 15, 2, 8, 14, 12, 0, 1, 10, 6, 9, 11, 5},
{0, 14, 7, 11, 10, 4, 13, 1, 5, 8, 12, 6, 9, 3, 2, 15},
{13, 8, 10, 1, 3, 15, 4, 2, 11, 6, 7, 12, 0, 5, 14, 9},
},
// S-box 3
{
{10, 0, 9, 14, 6, 3, 15, 5, 1, 13, 12, 7, 11, 4, 2, 8},
{13, 7, 0, 9, 3, 4, 6, 10, 2, 8, 5, 14, 12, 11, 15, 1},
{13, 6, 4, 9, 8, 15, 3, 0, 11, 1, 2, 12, 5, 10, 14, 7},
{1, 10, 13, 0, 6, 9, 8, 7, 4, 15, 14, 3, 11, 5, 2, 12},
},
// S-box 4
{
{7, 13, 14, 3, 0, 6, 9, 10, 1, 2, 8, 5, 11, 12, 4, 15},
{13, 8, 11, 5, 6, 15, 0, 3, 4, 7, 2, 12, 1, 10, 14, 9},
{10, 6, 9, 0, 12, 11, 7, 13, 15, 1, 3, 14, 5, 2, 8, 4},
{3, 15, 0, 6, 10, 1, 13, 8, 9, 4, 5, 11, 12, 7, 2, 14},
},
// S-box 5
{
{2, 12, 4, 1, 7, 10, 11, 6, 8, 5, 3, 15, 13, 0, 14, 9},
{14, 11, 2, 12, 4, 7, 13, 1, 5, 0, 15, 10, 3, 9, 8, 6},
{4, 2, 1, 11, 10, 13, 7, 8, 15, 9, 12, 5, 6, 3, 0, 14},
{11, 8, 12, 7, 1, 14, 2, 13, 6, 15, 0, 9, 10, 4, 5, 3},
},
// S-box 6
{
{12, 1, 10, 15, 9, 2, 6, 8, 0, 13, 3, 4, 14, 7, 5, 11},
{10, 15, 4, 2, 7, 12, 9, 5, 6, 1, 13, 14, 0, 11, 3, 8},
{9, 14, 15, 5, 2, 8, 12, 3, 7, 0, 4, 10, 1, 13, 11, 6},
{4, 3, 2, 12, 9, 5, 15, 10, 11, 14, 1, 7, 6, 0, 8, 13},
},
// S-box 7
{
{4, 11, 2, 14, 15, 0, 8, 13, 3, 12, 9, 7, 5, 10, 6, 1},
{13, 0, 11, 7, 4, 9, 1, 10, 14, 3, 5, 12, 2, 15, 8, 6},
{1, 4, 11, 13, 12, 3, 7, 14, 10, 15, 6, 8, 0, 5, 9, 2},
{6, 11, 13, 8, 1, 4, 10, 7, 9, 5, 0, 15, 14, 2, 3, 12},
},
// S-box 8
{
{13, 2, 8, 4, 6, 15, 11, 1, 10, 9, 3, 14, 5, 0, 12, 7},
{1, 15, 13, 8, 10, 3, 7, 4, 12, 5, 6, 11, 0, 14, 9, 2},
{7, 11, 4, 1, 9, 12, 14, 2, 0, 6, 10, 13, 15, 3, 5, 8},
{2, 1, 14, 7, 4, 10, 8, 13, 15, 12, 9, 0, 3, 5, 6, 11},
},
}
//代码位置src/crypto/des/const.go
```
## 3DES
DES是一个经典的对称加密算法，但也缺陷明显，即56位的密钥安全性不足，已被证实可以在短时间内破解。
为解决此问题，出现了3DES，也称Triple DES，3DES为DES向AES过渡的加密算法，它使用3条56位的密钥对数据进行三次加密。
为了兼容普通的DES，3DES并没有直接使用加密->加密->加密的方式，而是采用了加密->解密->加密的方式。
当三重密钥均相同时，前两步相互抵消，相当于仅实现了一次加密，因此可实现对普通DES加密算法的兼容。
3DES解密过程，与加密过程相反，即逆序使用密钥。
go标准中3DES加密算法的实现如下：
```go
type tripleDESCipher struct {
cipher1, cipher2, cipher3 desCipher
}
func NewTripleDESCipher(key []byte) (cipher.Block, error) {
if len(key) != 24 {
return nil, KeySizeError(len(key))
}
c := new(tripleDESCipher)
c.cipher1.generateSubkeys(key[:8])
c.cipher2.generateSubkeys(key[8:16])
c.cipher3.generateSubkeys(key[16:])
return c, nil
}
//3DES加密
func (c *tripleDESCipher) Encrypt(dst, src []byte) {
c.cipher1.Encrypt(dst, src)
c.cipher2.Decrypt(dst, dst)
c.cipher3.Encrypt(dst, dst)
}
//3DES解密
func (c *tripleDESCipher) Decrypt(dst, src []byte) {
c.cipher3.Decrypt(dst, src)
c.cipher2.Encrypt(dst, dst)
c.cipher1.Decrypt(dst, dst)
}
//代码位置src/crypto/des/cipher.go
```
## 后记
相比DES，3DES因密钥长度变长，安全性有所提高，但其处理速度不高。
因此又出现了AES加密算法，AES较于3DES速度更快、安全性更高，后续单独总结。
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