There is an interesting post describing the exploit of a weakness in MD5 via collisions, with a reproducible real-world example. The authors computationally found the collisions and were able to reliably and predictably produce two completely different postscript documents with the identical MD5 checksum. Their use-case story revolves around maliciously capturing a digital signature and using it for something other than it was intended. In the story, the MD5 checksum is relied upon to validate the authenticity of a document. The researchers wanted to show how this flaw could possibly be used in the real world.

"Recently, the world of cryptographic hash functions has turned into a mess. A lot of researchers announced algorithms ("attacks") to find collisions for common hash functions such as MD5 and SHA-1 (see [B+, WFLY, WY, WYY-a, WYY-b]). For cryptographers, these results are exciting - but many so-called "practitioners" turned them down as "practically irrelevant". The point is that while it is possible to find colliding messages M and M', these messages appear to be more or less random - or rather, contain a random string of some fixed length (e.g., 1024 bit in the case of MD5). If you cannot exercise control over colliding messages, these collisions are theoretically interesting but harmless, right? In the past few weeks, we have met quite a few people who thought so.

"With this page, we want to demonstrate how badly wrong this kind of reasoning is! We hope to provide convincing evidence even for people without much technical or cryptographical background."

Once again, security by obscurity defeated. Interesting read and might make you think. If anyone has comments on their test or process. I'd be interested to hear.