Move nix-cache's binary cache signing key into sops

nix-serve's secretKeyFile was a manual, undocumented-outside-a-comment
`nix-store --generate-binary-cache-key` step per host -- easy to miss on
a fresh nix-cache instance (as lxc-nix-cache testing just found: systemd
fails the unit with EXIT_CREDENTIALS when LoadCredential can't find the
source file, which nginx then reports as a 502 from clients). It also
can't be regenerated per-host safely: modules/nix-cache/client.nix
hardcodes every client's trust in one specific public key, so every
nix-cache instance has to share the exact same keypair.

Sourced from secrets/nix-cache.yaml's new cache-priv-key entry instead,
via the same sops-nix pattern every other secret in this repo already
uses. Verified the added value derives to the exact public key
modules/nix-cache/client.nix already trusts before committing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot

docs/nix-cache.md

@@ -11,19 +11,38 @@ This repository configures `nix-cache` as a **binary cache server** and a **remo
 - Client hosts import `modules/nix-cache/client.nix` and, when remote building is enabled, `modules/nix-cache/remote-builder-client.nix`.
 - The `nix-cache` host imports `modules/nix-cache/server.nix`.
 
-## Binary cache signing keys (on nix-cache)
+## Binary cache signing key
+
+`modules/nix-cache/client.nix` hardcodes every client's trust in one
+specific public key (`cache.local-1:usoWYanY3Kpq2+kDIS2nhWoLZiRxanmdysdzqCFBHW4=`).
+That means whichever host is currently playing the `nix-cache` role has to
+use that *exact* keypair — not a freshly generated one — or no client will
+accept substitutes from it (they'd just silently fall back to building
+from source). So unlike most per-host secrets, this one can't be
+self-generated on first boot; it's managed via sops-nix like every other
+secret in this repo, sourced from `secrets/nix-cache.yaml`'s
+`cache-priv-key` entry (`modules/nix-cache/server.nix`).
+
+**Adding or rotating the value:**
 
 ```bash
-sudo install -d -m 0700 /etc/nix
+nix-shell -p sops --run 'sops secrets/nix-cache.yaml'
-sudo nix-store --generate-binary-cache-key nix-cache-1   /etc/nix/cache-priv.pem   /etc/nix/cache-pub.pem
-sudo chmod 0600 /etc/nix/cache-priv.pem
-sudo chmod 0644 /etc/nix/cache-pub.pem
-cat /etc/nix/cache-pub.pem
 ```
 
-Do not commit private keys.
+Add (or replace) a `cache-priv-key` entry with the private key file's exact
-Do not commit new password hashes or live credentials. Existing committed hashes
+contents. If you don't have it yet, generate a keypair once:
-should be rotated and moved to host-local secret management.
+
+```bash
+nix-store --generate-binary-cache-key nix-cache-1 cache-priv.pem cache-pub.pem
+```
+
+— paste `cache-priv.pem`'s contents into the `cache-priv-key` entry above,
+delete both local files afterward, and update
+`trusted-public-keys` in `modules/nix-cache/client.nix` (and every already-built
+client) to match `cache-pub.pem` if this is a genuine rotation rather than
+a first-time bootstrap. Any `nixos-configurations.*-nix-cache` host picks
+the new key up automatically on next activation — no more manual
+`/etc/nix/cache-priv.pem` install step.
 
 ## Remote builder SSH keys
 

modules/nix-cache/server.nix

@@ -1,18 +1,20 @@
 { config, pkgs, vars, ... }:
 
 {
-  # Generate the binary cache key pair on the nix-cache host:
+  # nix-serve's signing key has to be the *same* key on every host that
-  #   sudo install -d -m 0700 /etc/nix
+  # ever plays the nix-cache role -- modules/nix-cache/client.nix hardcodes
-  #   sudo nix-store --generate-binary-cache-key nix-cache-1 \
+  # every client's trust in one specific public key ("cache.local-1:..."),
-  #     /etc/nix/cache-priv.pem \
+  # so a freshly self-generated key here wouldn't be trusted by anyone.
-  #     /etc/nix/cache-pub.pem
+  # Managed via sops-nix like every other secret in this repo instead of
-  #   sudo chmod 0600 /etc/nix/cache-priv.pem
+  # the old manual `nix-store --generate-binary-cache-key` step -- see
-  #   sudo chmod 0644 /etc/nix/cache-pub.pem
+  # "Binary cache signing key" in docs/nix-cache.md for how to add/rotate
-  #   cat /etc/nix/cache-pub.pem
+  # the value in secrets/nix-cache.yaml.
+  sops.secrets."cache-priv-key".sopsFile = ../../secrets/nix-cache.yaml;
+
   services = {
     nix-serve = {
       enable = true;
-      secretKeyFile = "/etc/nix/cache-priv.pem";
+      secretKeyFile = config.sops.secrets."cache-priv-key".path;
     };
 
     nginx = {

secrets/nix-cache.yaml

@@ -1,4 +1,5 @@
 beszel-token: ENC[AES256_GCM,data:meuzUP/6wCssJDVTgbC0XwiLZPMGyDl55HEIiON9xOXCD9k6,iv:TDqWcp+8Mxd8wN09r5otQRQXq3XTeQphaTWxvvuLTAs=,tag:cRPZQGlwB/dTguBAheWPQg==,type:str]
+cache-priv-key: ENC[AES256_GCM,data:6vQKIf7eS0WNL2Eptoi4VWr18SRMZfN/H/aFUUtXdMYQY5LLyBp2EHRKqZcGFuh1nZhUdAxUztq/CVXx+QFxKW+ElHxCxUSp0QqI1fdSkBkKZb8hlit5SoX9JtLzZGg0HBNM3nJu,iv:0J+xmrPJhInHhFR/c41ACjuTfaIoMkQFSfbL2KkgFa8=,tag:f4s9Szs5oprVVRSyXaX48A==,type:str]
 sops:
     age:
         - enc: |
@@ -28,7 +29,7 @@ sops:
             2SEMsctdGC+5E3ilPXvPpZ5RONZHbXxn6kQRBlBv6AJERpGDzsfgfA==
             -----END AGE ENCRYPTED FILE-----
           recipient: age164px2a8e48ptsf9ngtan38aa6jls4jdl26mzrgzf6sn3vcvt49hqjrgr8w
-    lastmodified: "2026-07-19T02:30:40Z"
+    lastmodified: "2026-07-19T23:30:21Z"
-    mac: ENC[AES256_GCM,data:7+FeT6aeCGn+JFBXbPO0qP4BJ1nHPSennewv1kWkG+hOTIqs1ymuswUK1Hyfi6Z9h2umFX9HvK+o3qtmYvk6k7BUNe6w6QUHTNwm6lmNqrb7sgAE3iFVI2p9m14NGhgoTfnXx1M4JIZ1iuNYhCukpENI4+svIe+r7x5YeE5Evac=,iv:4AKAPI6upyAvHBr8BLWX7R/NupmJdcXdqiN8e0ZQ3ls=,tag:TkpfpCaLhn+Mx5cQZZuMdA==,type:str]
+    mac: ENC[AES256_GCM,data:kLGE2xawQT7mx+sfw68hmGk5nCEGiEjZrqTEl9B1dtQmTrMwmoVr/1RISi4LfJrwxy31mDgff4lcIL4wIJuM373uk3X8j4RNyYQNTfKEkORT6r8NHeepNs267O77pKGd7OmcM4MT/BqOnB8ELS7Wlf2ect7CAlvUUVyc8icxgZE=,iv:EYLDsHYHZ1XOQXafOTqHHWpk/OBNq/R6IJnOBYV33E4=,tag:thxrCPC5oGvDjhK7Dz87YA==,type:str]
     unencrypted_suffix: _unencrypted
     version: 3.13.1