Signing store paths

Setup

This is based on the 📖︎ Set up a Signing Key section of the Nix manual.

Signing Key

You need a signing key to sign store paths. The key name (cache.example.org-1 for example) can be anything, but it's suggested to use the hostname of your cache/store (e.g. cache.example.org or raspberrypi) with a suffix denoting the number of the key (to be incremented every time you need to revoke a key). You can create a signing key using the 📖︎ nix-store --generate-binary-cache-key or the 📖︎ nix key generate-secret commands.

# mkdir -pv /var/secrets
# cd /var/secrets

Using the old CLI:
# nix-store --generate-binary-cache-key cache.example.org-1 nix-cache-priv-key nix-cache-pub-key

Using the new nix CLI:
# nix key generate-secret --key-name cache.example.org-1 > nix-cache-priv-key
# chmod 0600 nix-cache-priv-key # make the private key inaccessible to other users
# nix key convert-secret-to-public < nix-cache-priv-key > nix-cache-pub-key

The keys will look like this:
# cat nix-cache-priv-key
cache.example.org-1:MKk58wILGij4+VKU5xXESsU+MnslTUKfCjX/9OEXq6c6lhrq8lbOw9UpHeAC46rkpXf7/jJ0KmhnjIXcpVrT0Q==
# cat nix-cache-pub-key
cache.example.org-1:OpYa6vJWzsPVKR3gAuOq5KV3+/4ydCpoZ4yF3KVa09E=

Auto-signing

To automatically sign store paths after building, you need to configure the Nix option 📖︎ secret-key-files with the path of the private key. Here is how you do it on NixOS:

❄︎ /etc/nixos/configuration.nix

{
  nix.settings.secret-key-files = [ "/var/secrets/nix-cache-priv-key" ];
}

And here on non-NixOS:

≡︎ /etc/nix.conf

secret-key-files = /var/secrets/nix-cache-priv-key

Signing already-built store paths

When you add a key to secret-key-files, Nix does not automatically sign store paths that have already been built. You must run the 📖︎ nix store sign command to do that.

# nix store sign --all --key-file /var/secrets/nix-cache-priv-key

Trusting store paths using a signing key

Read the public key from /var/secrets/nix-cache-pub-key and configure the Nix option 📖︎ trusted-public-keys with it. Here is how you do it on NixOS:

❄︎ /etc/nixos/configuration.nix

{
  # NixOS automatically includes the public key of the NixOS binary cache
  nix.settings.trusted-public-keys = [ "cache.example.org-1:BBBBBBB...=" ];
}

And here on non-NixOS:

≡︎ /etc/nix.conf

trusted-public-keys = cache.nixos.org-1:AAAAAAA...= cache.example.org-1:BBBBBBB...=

Usage

Now you can copy store paths without signatures from the NixOS binary cache to a remote!

$ # Note that we're not a trusted user as that could allow copying without valid signatures
$ nix store --store ssh-ng://anothermachine.local info
Store URL: ssh-ng://anothermachine.local
Version: 2.34.7
Trusted: 0

$ # Yet we can copy store paths built on this machine
$ nix build --impure --expr "let pkgs = import <nixpkgs> { }; in pkgs.hello.overrideAttrs { pname = \"hello-$SECONDS\"; }"
$ # Note how cache.nixos.org-1 isn't mentioned here:
$ nix path-info ./result --sigs
/nix/store/... ultimate cache.example.org-1:AAAAAAAAA...==
$ nix copy ./result --to ssh-ng://anothermachine.local