I probably didn't backdoor this

This is a practical attempt at shipping a program and having reasonably solid evidence there's probably no backdoor. All source code is annotated and there are instructions explaining how to use reproducible builds to rebuild the artifacts distributed in this repository from source.

The idea is shifting the burden of proof from "you need to prove there's a backdoor" to "we need to prove there's probably no backdoor". This repository is less about code (we're going to try to keep code at a minimum actually) and instead contains technical writing that explains why these controls are effective and how to verify them. You are very welcome to adopt the techniques used here in your projects.

The author should be assumed to be your average software developer, who might be suspiciously good with computer security, but doesn't have nation-state capabilities.

Contents

• Preparing retroactive reviews
• Pinned external resources
• Reading the source code
• Reproducing the ELF binary
• Reproducing the Docker image
• Reproducing the Arch Linux package
• Notes on security patches
• How is this related to Reproducible Builds
• Similar work

Preparing retroactive reviews

Since "reading the source code" requires advanced domain knowledge, this section describes a pen-and-paper aproach that can be used to cryptographically ensure you can retro-actively review what you executed, even if you didn't review before you executed it. Pen-and-paper should be taken literally here to ensure this can't be modified by software. If done correctly, you don't need to read the other sections immediately, instead you're creating an immutable papertrail that can later be used by a subject matter expert. Note that the review needs to happen on a different computer than the one that executed the code, for safety reasons.

Because it's in the authors interest to prove there are no backdoors, all external resources that are not contained within this repository need to be referred to in a way that's addressing its content (more on this in the next section).

We're starting with the main repository by cloning it and showing the commit hash we're about to work with:

$ git clone https://github.com/kpcyrd/i-probably-didnt-backdoor-this
$ cd i-probably-didnt-backdoor-this/
$ git rev-parse HEAD
aabbccddeeff00112233445566778899aabbccdd

The hash in the last line is going to be different for you. This 40 character id is what you need for your paper trail, you need to write this down (preferably along with the current date) and keep it in a safe location. It needs to be protected from undetected tampering but isn't secret, so you may create copies or even post it publicly.

This id uniquely identifies all files in this repository with their content. If a file is modified/removed/added/renamed in this repository, this hash changes too.

If you want to read more about the cryptographic properties behind this, look into Merkel trees.

Pinned external resources

In the previous section we've described how git is automatically tracking the content of all files in this repository with a single hash. Software projects often rely on external resources downloaded from the internet, like libraries.

Downloading resources from the internet doesn't weaken what we've established in the previous section, as long as:

1. The content of the resource is pinned with a cryptographic hash and the hash is recorded in the git repository.
2. We can be reasonably sure the resource is not going to disappear. If they disappear you could attempt to use backup copies, as long as they match the cryptographic hash in the repository.

If at least one of those two doesn't apply we "broke the chain of custody".

We don't have to implement this ourselves, but cargo and docker implement this internally.

Reading the source code

The repository contains 6 source code files, there's a writeup for each of them. Files ending with .md are documentation.

• Cargo.toml - Contains metadata about the project and a list of dependencies (if any)
• Cargo.lock - Automatically generated, records sha256 checksums for all dependencies
• src/main.rs - The actual source code of our program
• Makefile - A wrapper script with build instructions
• Dockerfile - Contains build instructions for a container image
• PKGBUILD - Contains build instructions for an Arch Linux package

Reproducing the ELF binary

The binary is built in a docker container, the exact command can be found in the Makefile. Running make executes the build in a specific Docker image (the official rust 1.54.0 alpine 3.14 docker image).

Because the build environment is pinned and there's nothing introducing non-determinism to the build (like recording the build time), running the build on different computers (or even operating systems) should always result in the same binary.

Start the build with this command:

$ make

This command should finish quite quickly and produces a binary that matches this checksum:

$ b2sum target/x86_64-unknown-linux-musl/release/asdf
cd112870cdf12052e5604e7559e45f95cac4e52a45e91c9d9285a22a82c6392e95fbf0dc5f784837e7769a3ce14c898c866a85e4d60b051d3416875e301e28aa  target/x86_64-unknown-linux-musl/release/asdf

Downloading and hashing the pre-compiled binary from the releases page should give you an identical hash:

$ curl -LsS 'https://github.com/kpcyrd/i-probably-didnt-backdoor-this/releases/download/v0.1.1/asdf' | b2sum -
cd112870cdf12052e5604e7559e45f95cac4e52a45e91c9d9285a22a82c6392e95fbf0dc5f784837e7769a3ce14c898c866a85e4d60b051d3416875e301e28aa  -

If you get the same checksum you've successfully reproduced the binary. If there's no difference between the pre-compiled binary and the one you built yourself this means the pre-compiled binary is just as trustworthy as the one you built yourself.

Reproducing the Docker image

There's a Dockerfile in the repository that always produces the same bit-for-bit identical image. It's a multi-stage build, so it builds the binary in one temporary image and then creates the real image with just FROM, COPY and ENTRYPOINT. The build environment is virtually identical to what we're using in the previous section, then we're copying it over into an Alpine image that's pinned by its sha256 hash.

$ make docker
sudo buildah bud --timestamp 0 --tag asdf
[1/2] STEP 1/4: FROM docker.io/rust@sha256:8463cc29a3187a10fc8bf5200619aadf78091b997b0c3941345332a931c40a64
[1/2] STEP 2/4: WORKDIR /app
[1/2] STEP 3/4: COPY . .
[1/2] STEP 4/4: RUN cargo build --release --locked --target=x86_64-unknown-linux-musl
    Finished release [optimized] target(s) in 0.02s
[2/2] STEP 1/3: FROM docker.io/alpine@sha256:eb3e4e175ba6d212ba1d6e04fc0782916c08e1c9d7b45892e9796141b1d379ae
[2/2] STEP 2/3: COPY --from=0 /app/target/x86_64-unknown-linux-musl/release/asdf /asdf
[2/2] STEP 3/3: ENTRYPOINT ["/asdf"]
[2/2] COMMIT asdf
Getting image source signatures
Copying blob bc276c40b172 skipped: already exists
Copying blob 7d377d49a080 done
Copying config f0b71b1591 done
Writing manifest to image destination
Storing signatures
--> f0b71b1591c
Successfully tagged localhost/asdf:latest
f0b71b1591cf50cf3609494187083741c1021fd99f6168ab8283c4390954cef1

The last line is the hash of the image we just built. We're using buildah to build the image because there's no way to set the layer timestamp with docker (causing the hash to vary). Unfortunately buildah records it's version, this image has been built with 1.22.3.

The pre-compiled images can be found on the container registry (also linked in the side-bar on the right). Pull the image with this command:

$ docker pull ghcr.io/kpcyrd/i-probably-didnt-backdoor-this:latest
latest: Pulling from kpcyrd/i-probably-didnt-backdoor-this
50341f5fa632: Already exists
163594b80890: Pull complete
Digest: sha256:11cc7ec2b907a325fa3565039d990a466a7d83a06aa7dffdebba38d495d1571d
Status: Downloaded newer image for ghcr.io/kpcyrd/i-probably-didnt-backdoor-this:latest
ghcr.io/kpcyrd/i-probably-didnt-backdoor-this:latest

You'll noticed the hash doesn't seem to match at first, but if everything worked the image id is indeed the same:

$ docker images --no-trunc ghcr.io/kpcyrd/i-probably-didnt-backdoor-this
REPOSITORY                                      TAG       IMAGE ID                                                                  CREATED        SIZE
ghcr.io/kpcyrd/i-probably-didnt-backdoor-this   latest    sha256:f0b71b1591cf50cf3609494187083741c1021fd99f6168ab8283c4390954cef1   51 years ago   9.38MB

Reproducing the Arch Linux package

There's a custom Arch Linux repository that's distributing a pre-built package:

[i-probably-didnt-backdoor-this]
Server = https://pkgbuild.com/~kpcyrd/$repo/os/$arch/

This package can be reproduced from source, the full writeup for this can be found in this document.

Notes on security patches

We've pinned very specific versions in multiple places (including the compiler). This is often considered bad style since we're now in charge of keeping all of this updated.

If you're adopting this in your own project you should periodically release new versions, even if you aren't making any changes to the code anymore. This also applies to many modern programming ecosystems these days due to lock files.

The following places need to be updated occasionally, causing the artifact hashes to change.

• Dependencies in Cargo.toml/Cargo.lock (if any, cargo update)
• FROM lines in Dockerfile (docker pull rust:alpine, docker pull alpine:latest)
• The build image in the Makefile (docker pull rust:alpine)

How is this related to Reproducible Builds

There's quite a bit of overlap with the reproducible builds project. The techniques used to rebuild the binary artifacts are only possible because the builds for this project are reproducible.

This project also attempts to exclusively use binaries distributed by high-profile targets like Alpine Linux and the Rust project. This is commonly accepted as "reasonable" in the wider tech industry, but makes their build servers and signing keys extremely valuable.

The reproducible builds effort attempts to reduce this risk by allowing independent parties to "reproduce" their packages with "confirmation rebuilds", just like you did when following the instructions here!

Similar work

• Verifying a Tails image for reproducibility
• Reproducing Monero Binaries
• i-probably-didnt-backdoor-this fork by @bureado using unprivileged build containers with podman
• A similar guide using NixOS and Bazel by @mlieberman85

Acknowledgments

This project was funded by Google, The Linux Foundation, and people like you and me through GitHub sponsors.

♥️♥️♥️

License

Licensed under either of Apache License, Version 2.0 or MIT license at your option.