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We should taint untrusted link auth keys, and then downgrade connections using tainted keys to protocol warnings.

Link auth keys from the following sources are trusted:

• hard-coded authorities
• the consensus signed by hard-coded authorities

Link auth keys from the following sources are untrusted:

• hardcoded fallback dirs, because relay keys change over time
• our state file (if not confirmed in the consensus), because relay keys change over time
• onion service descriptors, because they come from untrusted services
• onion service introduce cells, because they come from untrusted clients

Split off #26924 (moved).