- How often
- Every session, before typing anything.
- How long
- Under a minute, assuming the fingerprint is pinned.
- What it prevents
- A copied login page, which is the most common attack in this whole subject and the least sophisticated.
- If you skip it
- Nothing, for months. Then one evening, everything.
- How you notice you stopped
- You will not, which is the problem. Put it before the password field rather than after, so skipping it requires a decision.
The procedure
- Find the signed block. It begins and ends with the armour lines. No signed block means you are finished and the answer is no.
- Copy all of it. Nothing trimmed, no whitespace altered, no quotation marks converted. A partial copy fails in a way that looks identical to tampering.
- Check against your pinned fingerprint. Not a key the page offers, and not one you fetched because the check complained.
- Read the output. Both which key signed and the date inside the signed text.
The four results
| Result | Meaning | What to do |
|---|---|---|
| Bad signature | Text altered after signing, or fabricated | Leave. Unambiguous, no judgement needed. |
| No public key | Signed by a key you do not hold. A gap in your keyring. | Import the right key after checking its fingerprint. Never one the page offers. |
| Good, unexpected key | The maths works against a key that is not yours | Leave. The word good appears, which is why this catches people. |
| Good, old date | Verifies, statement is months old | Treat as unconfirmed. Signatures never expire, so genuine old ones get replayed. |
The habit inside the habitThe last two rows are what catch experienced readers, and one habit defeats both. Read which key signed and when, rather than stopping at the word good. Two seconds.
What it does not tell you
Whether the signer is honest, whether your own machine is trustworthy, or anything about what happens after you log in. It answers one question completely and no others at all. That narrowness is exactly why it is worth doing and why the rest of the schedule exists.