Out of curiosity, why don't/didn't you start a new version of nixpkgs with hardened source? You could forgo the build server, forcing users to build from scratch (at least to start). You could leverage the plentiful, albeit, less secure, packaging code in the nixpkgs to quickly build out your hardened versions.
Effectively, you're building out an audited copy of nixpkgs on the same build engine, but with hardened configs. Write wrappers to validate git signatures when users update, and you got yourself a chain of trust on the source code distribution for your hardened nixpkg.
I'm sure you had reasons, I'm just interested to know your thought process.
I ultimately thought out what would be easier, a decade political fight to make massive changes to nix, or a fork of it written solo to improve auditability and security, or starting over from the top with a design that checks every dream box I wanted from a linux distro.
I had many RFCs that would have followed this rejected one if there was any change tolerance... so my fastest path to prove out my ideas for a distro with decentralized trust was to start one with that explicit goal.
If I wanted to make things maximally auditable and portable to different build engines, a published dead simple spec with multiple competing implementations that most software engineers already know how to write would be ideal. People could review an engine they use, or ensure all existing implementations on any operating system get identical results and are thus trusted that way. If it natively supports a ton of features to make deterministic builds wildly simpler, major bonus.
OCI/Containerfile was a check on all fronts, and some early maintainers and I riffed on design patterns and realized the OCI ecosystem already had specified multi party signing and verification, artifact integrity, smart layer by layer caching etc etc. This fit our dev experience and threat model perfectly and we could just skip implementing the package build and distribution layer and just start writing packages, like that day. None of us needed to learn or invent a new language or ask auditors to do so or fork nix ecosystem to have proper signing support and write a sane spec... that could be years of wheel spinning.
The time saved by choosing an existing widely used and implemented spec meant we were able to put all energy into full source bootstrapping, universal multi party hardware signing on every build, change, review, and reproduction. Just full source bootstrapped linux from scratch in containerfiles with OCI native multi party signing if all parties independently get the same oci hashes from local builds. Oh and we are going LLVM native like Chimera next week. Big sweeping changes like that are easy with our ultralight setup.
I would note that the features we need for deterministic builds in docker, the most popular OCI implementation, only landed a couple of months before we started stagex, and the full source bootstrapping work by the bootstrappable builds team only got a complete bootstrap for the first time a few months before that and Guix shortly after. Tons of reference material.
If stagex had started before 2022 I imagine we might have used a heavily trimmed down nix clone or tried to convince guix to adopt our threat model, which is much further along in supply chain security than nix but scheme would have been a very isolating choice. I think stagex got lucky starting at exactly the right time when two huge pieces of the puzzle were done for us.
Effectively, you're building out an audited copy of nixpkgs on the same build engine, but with hardened configs. Write wrappers to validate git signatures when users update, and you got yourself a chain of trust on the source code distribution for your hardened nixpkg.
I'm sure you had reasons, I'm just interested to know your thought process.