public class RC4 {

    byte[] s = new byte[256];

    byte[] key;

    byte keylen;// 4 ~ 16

    int pi = 0;

    int pj = 0;

    public RC4() {

        this.keylen = 16;

        randKey();

        ksa();

    }

    public RC4(byte keylen) {

        this.keylen = keylen;

        randKey();

        ksa();

    }

    public RC4(byte[] key) {

        this.key = key;

        keylen = (byte) key.length;

        ksa();

    }

    private void randKey() {

//        SecureRandom r = new SecureRandom("abc".getBytes());

        Random r = new Random(0xdeadbeef);

        key = new byte[keylen];

        r.nextBytes(key);

    }

    public byte[] getKey() {

        return key;

    }

    private void ksa() {

        for (int i = 0; i < s.length; i++) {

            s[i] = (byte) i;

        }

        int j = 0;

        for (int i = 0; i < s.length; i++) {                    //使用密钥来交换s的元素

            j = ((j + s[i] & 0xff + key[i % keylen] & 0xff) % 256);    //重复使用密钥

            byte tmp = s[i];                                    //交换

            s[i] = s[j];

            s[j] = tmp;

        }

    }

    public byte prgaOneByte(byte in) {

        pi = pi + 1 % 256;

        pj = (pj + s[pi] & 0xff);

        byte tmp = s[pi];

        s[pi] = s[pj];

        s[pj] = tmp;

        int t = (s[pi] & 0xff + s[pj] & 0xff) % 256;

        int k = s[t] & 0xff;

        return (byte) ((k ^ in & 0xff) & 0xff);

    }

    public void prga(InputStream is, OutputStream os) throws IOException {

        int bufsize = 4 * 1024;

        byte[] buffer = new byte[bufsize]; // 4K

        byte[] outBuffer;

        int len = 0;

        while ((len = is.read(buffer, 0, bufsize)) != -1) { //读取一块

            outBuffer = prga(buffer, 0, len);

            os.write(outBuffer);

        }

    }

    public byte[] prga(byte[] buffer, int off, int len) throws IOException {

        byte[] outBuffer = new byte[buffer.length]; // 4K

        for (int i = 0 + off; i < len; i++) {                    //处理这块

            outBuffer[i] = prgaOneByte(buffer[i]);

        }

        return outBuffer;

    }

    public static void main(String[] args) throws IOException {

        String msg = "hello wold!";

        byte[] bmsg = msg.getBytes();

        byte[] key;

        System.out.println("bmsg :" + Arrays.toString(bmsg));   //加密前

        RC4 rc4 = new RC4();

        key = rc4.getKey();

        byte[] ebmsg = rc4.prga(bmsg, 0, bmsg.length);

        System.out.println("ebmsg :" + Arrays.toString(ebmsg)); //加密后

        RC4 rc42 = new RC4(key);

        byte[] debmsg = rc42.prga(ebmsg, 0, ebmsg.length);

        System.out.println("debmsg:" + Arrays.toString(debmsg));//解密后

        System.out.println("debmsg:" + new String(debmsg));

    }

}

bmsg :[, , , , , , , , , , ]

ebmsg :[-, , -, -, -, , -, -, -, , -]

debmsg:[, , , , , , , , , , ]

debmsg:hello wold!