03 November 2013

Before I get about talking old stuff, here’s a few important suggestions for anyone who came here looking for my old MD5 Java implementation (in case you don’t know, MD5 is a cryptographic hash function):

  • Firstly, do not use it. There are plenty of good alternatives starting from Java’s standard library’s java.security.MessageDigest up to separate open-source implementations such as org.bouncycastle.crypto.digests.MD5Digest.

  • Secondly, do not use MD5 at all. MD5 has not been considered secure enough for new applications for over a decade now. Use SHA256 or better instead. It’s of course another issue if you’re working with legacy protocols, but for any new implementation you just do not use MD5.

  • Thirdly, if you are planning to do what seems to be every new web site programmers thing, that is, when you’ve come to realize that storing plaintext passwords is bad and have come up with the idea of storing passwords as hashes, and were thinking of using MD5 for that — and of course now you’d be thinking of SHA256 instead — do not use a cryptographic hash function for password hashing. Use a hash function specifically designed for long-term secure password hash storage such as PBKDF2. Just don’t use a plain hash function for password hashing. Trust me.

  • And of course if all of the above is new news for you, please please please get some education. I pathologically hate closet cryptographers e.g. people who think they know everything about cryptography since they’ve finally succeeded in breaking out of a wet paper bag.

The reason for this post is that I still keep getting e-mails for support and questions about an MD5 Java implementation I wrote a looooooong time ago (1996). It is so long time ago that in a few years people starting their professional programming careers will be younger than that piece of code.

(Oh boy, am I old.)

So if you have come here to report a bug in my MD5 implementation or have a question about how to use it, now you have the full story why it is effectively abandonware (which it sort of isn’t, since it is under LGPL and has thus been incorporated to many other codebases) — and that it is usable in and relevant to only a very narrow set of programming problems, which yours most likely is not one of them.

I don’t even have the source online anymore. I find keeping it online pointless for the reasons listed above. If you are super-interested in the source, just search for md5 paavolainen, although a lot of the hits are actually derived (mostly better!) implementations.

Why did I write an MD5 Java implementation in the first place?

(Imagine the following spoken with a hoarse, oldtimer voice, worn rough by the dust inhaled by years in solitude service among racks and racks of ancient servers.)

Well, this happened in 1996 when Java was still at version 1.0.2 and did not have java.security package at all (it came along in 1997). I just needed MD5 in a browser Java applet for the purpose of signing a request sent to backend server (implemented in C as a module to CERN httpd, oh the times) on a research project into micropayments. I didn’t find a ready-to-use MD5 Java class (the Internet was much smaller back then) so I wrote one following almost 1:1 the RSA C-language reference implementation. (The micropayment project never got anywhere practical, but that’s a whole another story…)

(Back to normal voice.)

P.S. I’m not saying that I’m offended by people e-mailing me about the MD5 class — on the contrary, it does feel good to know you have made - albeit very small - but a lasting contribution to the “great internet of buzzwords”. What I sincerely hope for is that those e-mail would stop, not because I don’t like them, but because MD5 in general, and definitely not my unmaintained implementation of it should not be used anymore. Move along, don’t dwell in the cryptographic past.

P.P.S. I’ll be more positive on the next post, I promise.

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