FOSDEM: Gnash Developer Deciphers RTMP

Savoye, developer with the Open Media Now project funded by Red Hat founder Bob Young, has worked on compilers and debuggers in the past. With Gnash he's been involved in enhancing support for ActionScript 9. Version 8 of the software already serves as a plug-in for Firefox and Konqueror, according to Savoye.

For him, reverse-engineering RTMP for Gnash has not least of all been a mental challenge. He would stare at hex code long enough until a revelation hit him, he said with a smile. He gave an example of his process: at first he searched for certain byte sequences of ASCII code in a collection of as many sample artifacts as possible. He could then determine the boundaries of other data fields and tried to locate a protocol data header. Based on his experience he could then often establish field lengths and checksums for the RTMP protocol.

Numbers are almost always coded in big-endian order in network protocols and checksums rarely consist of anything other than a simple sum or result of a cyclic redundancy check (CRC), according to Savoye. He wrote a log of test code to check his assumptions and warns prospective reverse engineers that they'd have to throw away much of their assumptions if proved false.

Not least of all he cautions imitators about possible legal implications: simple disassembling of proprietary software can lead to claims from respective originators. He therefore recommended involving legal counsel as early as possible in the process. It's also useful, according to Savoye, when a developer reimplements the code, to extract the specification from the network packet and leave just the protocol description for another independent developer in his implementation. Apparently the newest YouTube videos are already viewable using the current Gnash version based on Savoye's work.