DES加密算法
DES加密算法
本文来自 ：https://blog.csdn.net/baidu_36856113/article/details/53558795
1997年数据加密标准DES正式公布，其分组长度为64比特，密钥长度为64比特，其中8比特为奇偶校验位，所以实际长度为56比特。现在DES已经被AES所取代。
1）DES的加密过程
明文64位->初始置换IP->16轮加密变换->逆初始置换IP-1->密文
a.初始置换IP
这里的初始IP置换表的意思为把64位明文按照表中的规则替换，比如第一行，把64位明文的第1位换为其58位，第2位换为50位，第3位换位42位…这表仔细观察有一定的规律，比如我们从右到左一列一列的分别读前4行，后4行。是不是就是2 4 6 8 10 12…和1 3 5 7 9 11…
b.16轮加密变换
看似很复杂，其实16轮的过程都是一样的。经过上面说的初始置换IP,接下来就是要把64位分为L0和R0各32位。然后算出f(R0,k1),其结果与L0异或作为R1.而L1直接是和R0相等的。
后面都一样： Li=Ri-1
Ri=Li-1异或f(Ri-1,ki)
i=1,2,3,…16,
这时我们就会问那K1~K16从哪来，加密函数f()怎么计算，这些问题我在后面会继续写的。
c.初始逆置换IP-1
方法同初始置换IP。
b1.密钥生成
是不是一下子就能看明白，这里的PC-1为选择置换，可以去掉奇偶校验位。PC-2也是选择置换，它是用于从Ci和Di中选取48位作为密钥Ki。C0和D0是把密钥中实际的56位分成左右28位，LSi是表示对Ci-1和Di-1进行循环左移变换，其中LS1,LS2,LS9,LS16是循环左移1位，其余为2位。
选择置换PC-1
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，
选择置换PC-2
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
b2.加密函数f()
这里的E是把32比特（即上面所提到的64比特分为俩个32比特。）的输入扩展为48比特。将E置换后的结果与48位的Ki异或的结果平均分为8组B1,B2…B8，每组Bi用Si盒处理。这样你会发现进去的是48位，出来的是32位。因为P为32位置换函数，它的输入为32位。把Bi{x1,x2,x3…x6}(48比特平均分为8组，每组就是6比特)中的x1x6和x2x3x4x5分别作为Si盒的行和列（比如x1x2x3…x6=101011,则x1x6=11把它转化为十进制为3。x2x3x4x5=0101转换为十进制为5，所以为3行5列，但是要注意Si盒的行列从0开始算起），找到这个十进制数，把它转化为4位二进制。这4位就是输出。
扩展变换E
这个也是有规律的把第一列和最后一列去掉看看。
S盒
S1
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,
S2
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,
S3
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,
S4
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,
S5
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,
S6
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,
S7
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,
S8
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置换
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