Host key

How it works

RFC 4253 uses the host key in the transport handshake. A changed key is a failure until a human confirms the new fingerprint. Disabling the check makes every network attacker a valid destination.

A vendor rotates host keys on Thursday. Friday's job stops before login. The operator compares the new fingerprint to the vendor's mail and updates the pin. A job with checking off would have uploaded payroll to whoever answered on port 22.

How it differs

A host key names the server. A user key names the client. Rotating one does not rotate the other.

TOFU, trusting the first key you see, fails if the first connection was already hostile.

Pin the fingerprint out of band.

On the ticket

• The practical close is a log line: time, actor, byte count, result.
• Without that line the transfer is a story.
• With it, the next person can see whether this door did what the ticket claimed.
• If the path is shared, say so in the partner profile so a later change does not silently pick a different limit, key, or region.
• On a real ticket, write down the door, the byte count, and the clock.
• For host key, that means naming the host or bucket, the expected size, and the time the other side must have a complete file.
• A progress bar is not that record.
• A 200 response that arrives before the complete call is not that record.
• If a retry is allowed, say how many and whether it resumes.
• If a person must approve the send, name the person.
• Partners who receive host key files should match on hash or size before they import.
• A same-length corrupt file passes a size check and fails a hash.
• Keep the published hash off the only channel an attacker can edit, or treat it as a corruption check rather than a substitution check.
• When the path changes, new key, new region, new cap, update the profile the same day so the next run does not use a stale limit.