public static void Encrypt(byte[] key, byte[] data, int startIndex, int count)
{
int i, j, n, t;
byte c;
byte[] s = new byte[256]; //S-box
byte[] k = new byte[256];

for (i = 0; i < 256; i++)
{
s[i] = (byte)i;
}

//把 key 循环放进 K
j = 0;
for (i = 0; i < 256; i++)
{
k[i] = key[j % key.Length];
j++;
}

j = 0;
for (i = 0; i < 256; i++)
{
j = (j + s[i] + k[i]) % 256;
c = s[i];
s[i] = s[j];
s[j] = c;
}

i = 0;
j = 0;
for (n = startIndex; n < startIndex + count; n++)
{
i = (i + 1) % 256;
j = (j + s[i]) % 256;

//交换 S[i]和 S[j]
c = s[i];
s[i] = s[j];
s[j] = c;

t = (int)((s[i] + s[j]) % 256);
c = s[t];
data[n] = (byte)(data[n] ^ c); //异或加密

}