RC4加密与解密

一、看出初始化代码中，对字符数组 s 进行了初始化赋值，且赋值分别递增。之后对 s 进行了 256 次交换操作。通过识别初始化代码，可以知道 rc4 算法

void rc4_init(unsigned char *s, unsigned char *key, unsigned long Len) //初始化函数
{
    int i =0, j = 0;
    unsigned char k[256] = {0}; // 必须为无符号类型，否则会导致密文部分错误
    unsigned char tmp = 0;
    for (i=0;i<256;i++) {
        s[i] = i;
        k[i] = key[i%Len];
    }
    for (i=0; i<256; i++) {
        j=(j+s[i]+k[i])%256;
        tmp = s[i];
        s[i] = s[j]; //交换s[i]和s[j]
        s[j] = tmp;
    }
 }

void rc4_crypt(unsigned char *s, unsigned char *Data, unsigned long Len) //加解密
{
    int i = 0, j = 0, t = 0;
    unsigned long k = 0;
    unsigned char tmp;
    for(k=0;k<Len;k++) {
        i=(i+1)%256;
        j=(j+s[i])%256;
        tmp = s[i];
        s[i] = s[j]; //交换s[x]和s[y]
        s[j] = tmp;
        t=(s[i]+s[j])%256;
        Data[k] ^= s[t];
     }
}