# DES 算法实现

DES 加密算法看上去非常复杂，但是实际上大部分内容都是重复的，要实现 DES 加密，实际上只需要完成 $4$ 部分代码即可：

• S盒
• P盒
• 子密钥生成
• 轮函数

## S盒(S-box)

S盒全称是 substitution box ，在 DES 中用于实现置换操作，是唯一实现非线性的步骤，直接决定了算法的优劣

### S盒实现代码

func sbox(input bits.Bits) (output bits.Bits) {
// 48 bits
// pos := 0
for i := 0; i < 8; i++ {
offset := i * 6
row := input.Get(offset)*1 + input.Get(offset+5)*2
col := input.Get(offset+1)*1 + input.Get(offset+2)*2 + input.Get(offset+3)*4 + input.Get(offset+4)*8
v := bits.Bits(sp[7-i][row][col])
output = output | (v << (i * 4))
}
// 32 bits
return
}


### S盒代换表

// S盒代换表
sp = [][][]byte{
{
{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},
},
{
{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},
},
{
{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},
},
{
{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},
},
{
{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},
},
{
{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},
},
{
{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},
},
{
{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},
},
}


## P盒(P-box)

P 盒全称为 permutation box。相对于 S 盒，P 盒则相对较为简单
P 盒有三种类型：

• 压缩性：输出位元数比输入少
• 扩张性：输出位元数比输入多
• 平直性：输出位元数等于输入位元数

### P盒实现代码

// pbox P盒 置换盒
func pbox(input bits.Bits, key []int, inputLength int, outputLength int) (output bits.Bits) {
for i := 0; i < len(key); i++ {
v := input.Get(inputLength - key[i])
output.SetValue(outputLength-i-1, v)
}
return
}


### P盒置换表

// 初始置换
ip = []int{
58, 50, 42, 34, 26, 18, 10, 2,
60, 52, 44, 36, 28, 20, 12, 4,
62, 54, 46, 38, 30, 22, 14, 6,
64, 56, 48, 40, 32, 24, 16, 8,
57, 49, 41, 33, 25, 17, 9, 1,
59, 51, 43, 35, 27, 19, 11, 3,
61, 53, 45, 37, 29, 21, 13, 5,
63, 55, 47, 39, 31, 23, 15, 7,
}
// 轮函数拓展置换
ep = []int{
32, 1, 2, 3, 4, 5,
4, 5, 6, 7, 8, 9,
8, 9, 10, 11, 12, 13,
12, 13, 14, 15, 16, 17,
16, 17, 18, 19, 20, 21,
20, 21, 22, 23, 24, 25,
24, 25, 26, 27, 28, 29,
28, 29, 30, 31, 32, 1,
}
// 轮函数普通置换
pp = []int{
16, 7, 20, 21,
29, 12, 28, 17,
1, 15, 23, 26,
5, 18, 31, 10,
2, 8, 24, 14,
32, 27, 3, 9,
19, 13, 30, 6,
22, 11, 4, 25,
}
// 子密钥置换表1
pc1 = []int{
57, 49, 41, 33, 25, 17, 9,
1, 58, 50, 42, 34, 26, 18,
10, 2, 59, 51, 43, 35, 27,
19, 11, 3, 60, 52, 44, 36,
63, 55, 47, 39, 31, 23, 15,
7, 62, 54, 46, 38, 30, 22,
14, 6, 61, 53, 45, 37, 29,
21, 13, 5, 28, 20, 12, 4,
}
// 子密钥置换表2
pc2 = []int{
14, 17, 11, 24, 1, 5,
3, 28, 15, 6, 21, 10,
23, 19, 12, 4, 26, 8,
16, 7, 27, 20, 13, 2,
41, 52, 31, 37, 47, 55,
30, 40, 51, 45, 33, 48,
44, 49, 39, 56, 34, 53,
46, 42, 50, 36, 29, 32,
}


## 子密钥生成

// 子密钥左移位数
keyMove = []int{
1, 1, 2, 2, 2, 2, 2, 2,
1, 2, 2, 2, 2, 2, 2, 1,
}


### 子密钥生成实现代码

func getSubKey(key bits.Bits) (output []bits.Bits) {
output = make([]bits.Bits, 16)

// key 64 bit
temp := pbox(key, pc1, 64, 56) // 56 bits

C := temp.Mask(28)         // 28 bits
D := (temp >> 28).Mask(28) // 28 bits

for i := 0; i < 16; i++ {
C = C.LeftLoop(keyMove[i], 28)
D = D.LeftLoop(keyMove[i], 28)

output[i] = ((D << 28) | C)              // 56 bits
output[i] = pbox(output[i], pc2, 56, 48) // 48 bits
}

return
}


## 轮函数

### 轮函数实现代码

func f(input bits.Bits, key bits.Bits) (output bits.Bits) {
output = input                    // 32 bits
output = pbox(output, ep, 32, 48) // 48 bits
output = output ^ key             // 48 bits
output = sbox(output)             // 32 bits
output = pbox(output, pp, 32, 32) // 32 bits
return
}


## DES主函数

$64$ 位明文分割成 $2$$32$ 位明文，并按照费斯妥密码的要求，执行交换异或执行轮函数

### 主函数实现代码

func DES(input bits.Bits, key bits.Bits) (output bits.Bits) {
var temp bits.Bits
L := make([]bits.Bits, 2)
R := make([]bits.Bits, 2)

output = pbox(input, ip, 64, 64)

L[0] = (output >> 32).Mask(32) // 32 bits
R[0] = output.Mask(32)         // 32 bits

keys := getSubKey(key) // 48 bits

for i := 0; i < runCount; i++ {
this := i & 1
next := (i ^ 1) & 1

temp = f(R[this], keys[i]) // 32 bits

L[next] = R[this]
R[next] = temp ^ L[this]
}
output = (R[runCount&1] << 32) | L[runCount&1] // 64 bits
output = pbox(output, inverse(ip), 64, 64)
return
}