Fixing the Problem with Gitian

Today we continue with chapter 9 about reproducible builds and Guix.

Fixing the Problem with Gitian

In order to verify no shenanigans happened in the process of converting source code to a compiled binary, you need something called reproducible builds, or deterministic builds.

What deterministic implies is that, given a source, you’re going to get the same binary. And if you change one letter in the source, you’re going to get a different binary, but everybody will get the same result if they make the same change.

In addition, there’s the problem of slight differences in machine configuration leading to a different binary file.

Until mid 2021, the way Bitcoin Core did this was with Gitian. In short, you’d take a virtual or physical computer, download the installation DVD^[Long ago you might have ordered a CD by snail mail, whereas nowadays, you’ll probably download an image and put it on a USB stick. When you install Ubuntu on a virtual machine, your computer creates a virtual DVD player using the image.] for a very specific Ubuntu version, and install that. This ensures everyone has an identical starting position, and because Ubuntu is widely used, there’s some confidence that there isn’t a Bitcoin backdoor on the installation disk.

Inside that machine, you build another virtual machine, which has been tailormade to ensure it builds identical binaries for everyone using it. For example, it uses a fixed fake time so that if a timestamp ends up in the final binary, it’s going to be the same timestamp no matter what time you ran the compiler. It ensures all the libraries are the exact same versions, it uses a very specific compiler version, etc. And then you build Bitcoin Core inside that virtual machine and look at the checksum. This should now match the downloadable files on

About a dozen developers and other volunteers run this “computer within a computer.” Around each new released version, they all compile the binaries and publish the resulting hashes for others to see. In addition, they sign these hashes with their public PGP keys.

However, while this sounds easy in theory, in practice, it’s always been a huge pain to get the system working. There aren’t many open source projects that use Gitian — as far as we know, only Bitcoin Core and Tor do. Even most, if not all, altcoin forks of the Bitcoin Core software don’t bother with this process.