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I'm thinking about what happens when GitHub becomes unresponsive. If the URLs where the public keys are hosted die at the same time, then posting via the CLI would become impossible. Maybe GitHub serves only those URLs via a reverse proxy or similar, so it could return responses regardless of whether the backend is busy, or maybe the dev-null side caches the public keys. If that's the case, it seems pretty robust.

Since there seems to be no way for the dev-null side to immediately know that the user side has invalidated a key, would the cache strategy hinder damage control in the event of a private key leak?