I'm John Berthels. I am a systems designer and coder. You can find details of my past employment on LinkedIn.
I'm currently available for freelance work. If you've got an interesting problem, please contact me to discuss it.
Basic cryptography. I'm on set #6 of the excellent Matasano crypto challenges as of 14 Jul 2014. "The most fun you can have with a compiler".
Google's go language ('golang'). I've developed and deployed production services in golang. I'm not yet sure if it's a "better C", a "static python" or an "erlang for the rest of us", but it's a lot of fun and I very much like the approach it takes to concurrency. I've worked on a go module called gop which has as it's goal the 'productionisation' of go servers, to allow them to fit in with prevailing devops practices.
Storage and sync. I've worked in this area for a while and I think Camlistore is very interesting.
Performance tuning. I recently had the chance to revisit a codebase with which I was familiar, but which had evolved since my last involvement and was being used at large scale. This was a 'perfect storm' of optimisation opportunities. There's nothing quite as satisfying as getting a 5x speedup on a real-world problem.
It's currently obsolete and orphaned, but I am still quite proud of the exmap memory analysis tool, which used a kernel loadable module to find exact memory usage (and account for page sharing) and reversed the ELF loader's mappings to work out exactly which bits of which executable where in which page of memory.
You can get hold of me by email or google talk using my firstname.lastname at gmail. You could also try tweeting at me.