数字签名-MD5

MD5是信息摘要的意思，报文产生摘要是唯一的（1:1），而且是单向的（通过摘要反推不出源报文）

java中的java.security.MessageDigest类，参考MessageDigest的功能及用法

 public static byte[] getMD5Mac(byte[] bySourceByte) {
        byte[] byDisByte;
        MessageDigest md;
        try {
            md = MessageDigest.getInstance("MD5");
            md.reset();
            md.update(bySourceByte);
            byDisByte = md.digest();
        } catch (NoSuchAlgorithmException n) {
            return (null);
        }
        return (byDisByte);
    }

MessageDigest 是 statefull，电话 MessageDigest.update (byte [] 输入） 累积文摘更新，直到我们称之为 MessageDigest.digest。运行此测试，以确保:

    MessageDigest md1 = MessageDigest.getInstance("MD5");
    md1.update(new byte[] {1, 2});
    md1.update(new byte[] {3, 4});
    System.out.println(Arrays.toString(md1.digest()));

    MessageDigest md2 = MessageDigest.getInstance("MD5");
    md2.update(new byte[] {1, 2, 3, 4});
    System.out.println(Arrays.toString(md2.digest()));

输出:

[8, -42, -64, 90, 33, 81, 42, 121, -95, -33, -21, -99, 42, -113, 38, 47]
[8, -42, -64, 90, 33, 81, 42, 121, -95, -33, -21, -99, 42, -113, 38, 47]

生成的摘要是byte[]类型的，需要转换成String

public static String bintoascii(byte[] bySourceByte) {
        int len, i;
        byte tb;
        char high, tmp, low;
        len = bySourceByte.length;
        StringBuilder sb = new StringBuilder(len);
        for (i = 0; i < len; i++) {
            tb = bySourceByte[i];
            tmp = (char) ((tb >>> 4) & 0x000f);
            if (tmp >= 10)
                high = (char) ('a' + tmp - 10);
            else
                high = (char) ('0' + tmp);
            sb.append(high);
            tmp = (char) (tb & 0x000f);
            if (tmp >= 10)
                low = (char) ('a' + tmp - 10);
            else
                low = (char) ('0' + tmp);
            sb.append(low);
        }
        return sb.toString();
    }