[RFC 0149] Cache key rotation

rfcs/0149-cache-key-rotation.md

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+---
+feature: (fill me in with a unique ident, my_awesome_feature)
+start-date: (fill me in with today's date, YYYY-MM-DD)
+author: (name of the main author)
+co-authors: (find a buddy later to help out with the RFC)
+shepherd-team: (names, to be nominated and accepted by RFC steering committee)
+shepherd-leader: (name to be appointed by RFC steering committee)
+related-issues: (will contain links to implementation PRs)
+---
+
+# Summary
+[summary]: #summary
+
+Rotate cache.nixos.org signing key.
+
+If we tolerate that people using stuff older than *X* could get a validation failure by default, fully rotating the key in 2*X* time seems relatively straightforward.
+
+# Motivation
+[motivation]: #motivation
+
+Never rotating a key is bad security practice.
+The current one has been in use at least since 2015.
+
+# Detailed design
+[design]: #detailed-design
+
+Preliminary action plan:
+- generate a new key
+- make it trusted by default (nix+nixpkgs, perhaps with backports to some branches)
+- wait until enough people trust the new key (at least one year, probably)
+- switch to signing with the new key
+- wait - until paths not signed by new key aren't commonly needed anymore
+- make Nix not need signatures for fixed-output derivations
+  (this step could be completed anytime earlier, too)
+  FIXME: maybe this holds already:
+    https://nixos.org/manual/nix/unstable/command-ref/conf-file.html#conf-trusted-public-keys
+- stop trusting the old key (nix+nixpkgs)
+
+
+
+- - -
+
+
+# Examples and Interactions
+[examples-and-interactions]: #examples-and-interactions
+
+This section illustrates the detailed design. This section should clarify all
+confusion the reader has from the previous sections. It is especially important
+to counterbalance the desired terseness of the detailed design; if you feel
+your detailed design is rudely short, consider making this section longer
+instead.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Why should we *not* do this?
+
+# Alternatives
+[alternatives]: #alternatives
+
+- change nothing, obviously
+- also resign old `*.narinfo`.  Maybe it's not too hard.
+  It would help people wanting ot use old builds.
+- double-sign `*.narinfo` for some time. (also not an exclusive alternative)
+  I don't know if consumers support multiple signatures.
+  It doesn't seem to give us significant advantage though;
+  acceptance of multiple keys seems more advantageous.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+- confirm Nix's (non-)acceptance of FODs without signature
+- determine timing (e.g. the *X* above).
+  Consider aligning the regular schedule with NixOS releases.
+
+# Future work
+[future]: #future-work
+
+What future work, if any, would be implied or impacted by this feature
+without being directly part of the work?