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：

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：

//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：

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：

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。

感谢关注兄弟连区块链教程分享！