- How often
- Once. Again only if the key genuinely changes, which is rare and deserves suspicion.
- How long
- About an hour if you do it properly, which mostly means comparing carefully rather than quickly.
- What it prevents
- It is what makes the per session check mean anything. Without it that check proves nothing at all.
- If you skip it
- Every later verification is theatre. You will be checking a signature against whatever key the page handed you.
- How you notice you stopped
- You cannot stop. What happens instead is that people never start, and then believe they are protected because they run the check.
Why the fingerprint and not the key file
A key is a large blob you cannot meaningfully inspect. A fingerprint is a short hash of it, short enough to compare by eye and long enough that nobody can produce a second key matching it. You pin the fingerprint and treat key files as disposable.
That reverses the usual instinct, which is to save the key file carefully somewhere. The file needs no protection at all. It is public and you can fetch it from anywhere including somewhere hostile, then check it against the fingerprint you hold. Only the fingerprint has to be right.
The circular trapA signature checked against a key taken from the page you are checking proves nothing. The page, the key and the signature all came from one party, and internal consistency costs a forger nothing.
What counts as independent
| Source | Independent? |
|---|---|
| Different operators, different infrastructure | Yes. Rare in this space and worth seeking out. |
| Several sites run by one person | No, and this is very common and rarely disclosed. |
| Several sites that copied a fourth | No. Aggregation is the default here. |
| The same value across archived captures over years | Weakly. Persistence is harder to fake retroactively than a page is to edit now. |
Doing it
- Collect it from several genuinely independent places.
- Compare in full. The whole string, character by character, not the first four and last four, which is the check a forger designs around.
- Write it down somewhere you control. Paper is genuinely good here, because nothing running on your machine can alter it.
- If they disagree, stop and wait. At least one source is wrong or compromised and you cannot tell which from where you are standing. This is a reason to pause, not to take a vote.