Can someone explain how this MD5 MessageDigest code works?

Question

I am studying MD5 and also SHA with MessageDigest. Here is a code I have from class that implements MD5 with MessageDigest. I am having trouble understanding it.

So it gets the "instance" of MD5. I guess that is the MD5 algorithm? Then it updates the bytes. Why does it do this? Then it creates a variable bytes b with md.digest(), but I am not sure why it does this either? Then I think it uses the for statement to do the algorithm and maybe pad it (append 0?). If anyone could explain a little better, I'd appreciate!

   try {
                MessageDigest md = MessageDigest.getInstance("MD5"); // get the
                                                                        // instance
                                                                        // of md5
            md.update(bytes); // get the digest updated
            byte[] b = md.digest(); // calculate the final value
            int i;
            StringBuffer buf = new StringBuffer("");
            for (int offset = 0; offset < b.length; offset++) {
                i = b[offset];
                if (i < 0)
                    i += 256;
                if (i < 16)
                    buf.append("0");
                buf.append(Integer.toHexString(i));
            }
            message = buf.toString(); // output as strings

        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace(); // when certain algorithm is down, output the
                                    // abnormal condition
        }
        return message;
    }

score 2 · Answer 1 · answered Apr 18 '16 at 20:53

2

md.update(bytes) just puts the bytes through MD5. byte[] b = md.digest() gets out the result of the MD5 hash as a series of bytes.

Then the whole rest of the code is a dreadfully awkward way to convert bytes into a hexadecimal string.

answered Apr 18 '16 at 20:53

Louis Wasserman

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Could you explain a better way of how to convert the bytes into hex? – RockOn Apr 18 '16 at 20:54
1

http://stackoverflow.com/questions/9655181/how-to-convert-a-byte-array-to-a-hex-string-in-java – Andy Turner Apr 18 '16 at 20:55
I mean, I'd use Guava's `BaseEncoding.base16().lowerCase().encode(bytes)`. – Louis Wasserman Apr 18 '16 at 20:56
Or I'd rewrite the whole thing with Guava with the one line `Hashing.md5().hashBytes(bytes).toString()`. – Louis Wasserman Apr 18 '16 at 21:02

Can someone explain how this MD5 MessageDigest code works?

1 Answers1