- What it is
- A short identifier for the market signing key, written in your own record, together with the sources you took it from and the date.
- How you get one
- You collect it from several places that do not depend on each other, compare what they say, and write down the value they agree on.
- Where to keep it
- On paper you can consult without a browser, because the moment you need it is the moment a browser is what you distrust.
- How it goes wrong
- The operator rotates the key, or you copied only part of the value, or every source you consulted was really one source.
- What fails without it
- A signed notice is just text, and any page can claim any address with nothing standing behind the claim.
- Time to acquire
- One unhurried session, spread over more than one sitting if you want the sources to be genuinely separate.
What a fingerprint is, briefly
A signing key has a long public form and a short identifier derived from it. That short identifier is the fingerprint. It is designed to be quoted in places where the whole key would not fit: written on paper, read aloud, printed in a list. Two keys with the same fingerprint are, for every practical purpose here, the same key.
That is all it does. It gives you a compact way of saying which key you mean, so that a signature can be checked against a specific key rather than against whatever key happens to be attached to the thing you are reading.
Collected by you against shown to you
The distinction this page turns on is where the value came from. A fingerprint printed on the page you are currently reading tells you what that page wants you to believe. If the page is honest, it is correct. If the page is not, it is whatever the person running it chose to put there. Either way the page is the sole witness, and a witness vouching for itself has told you nothing.
A fingerprint you collected is different in one respect that matters enormously: it existed in your record before you arrived. It cannot be adjusted to match whatever you are now looking at, because you wrote it down at a time when nobody knew which page you would be reading today. That is the whole of the advantage, and it is not a small one.
| A fingerprint shown on the page | A fingerprint you already had |
|---|---|
| Arrives at the same time as the claim it supports. | Existed before the claim did. |
| Can be edited by whoever controls the page. | Cannot be edited to fit what you are reading now. |
| Agrees with the page by construction. | Agrees or disagrees, and either answer is informative. |
| Tells you the page is internally consistent. | Tells you whether this key is the key you meant. |
More than one place, and what counts as another place
Collecting from several sources only helps when the sources are actually independent. Three directories that all copied each other are one source wearing three hats, and they will agree perfectly while being wrong together. Working out what independence means in practice is the job of the independent sources page, which sets out the questions worth asking about any place you take a value from.
The short version: prefer sources with different owners, different histories and different reasons to exist. A value that appears in a signed message from the operator, in a long running community record, and in an archived copy from a year ago is standing on three legs. A value that appears on four sites launched in the same month is standing on one.
Written down before you needed it
Timing is the part people get wrong. Fingerprints get collected during an emergency, when an address has changed and something needs checking right now. That is the worst possible moment, because in an emergency you take the first value you find, from a page you found in a hurry, and write it into your notes as though it were established fact.
The version worth holding was collected on an ordinary day, calmly, with time to compare. The notebook is the natural home for it. Note the value, note where each copy came from, note the date you looked. That last part matters, and the dates page explains why an undated value is a value you will end up re-checking anyway.
What it lets you do that a page cannot do for you
With a fingerprint of your own, a signed notice becomes something you can test. You verify the signature against the key, compare that key against the value in your notes, and get an answer that does not depend on the goodwill of whatever you are reading. That is how a saved signed notice earns its place in the inventory, and it is why the two items belong together.
Without one, you are left comparing an address on one page against the same address on another page, which is a test of how widely something has been copied. Copying is cheap. A signature checked against a key you noted independently is not, and the difference shows up precisely when a set of addresses starts changing and everyone has a slightly different list.
This site prints its addresses and tests nothing. It does not know whether any of them answers and does not check. That is not modesty, it is the honest position for a page with no privileged access, and it is exactly the gap this artifact fills for the reader rather than for the page.
When it stops matching
This item is listed as going stale for good reason. Operators rotate signing keys, sometimes on schedule, sometimes after an incident, occasionally with no announcement anyone can find. When that happens your noted value stops matching and everything signed with the new key looks wrong.
A mismatch is a reason to stop and look rather than a verdict. Check whether the change is announced somewhere signed with the old key, which is the graceful way to rotate. Check whether your independent sources have moved as well or only one of them has. Then update your note with the new value, the new date and the sources that carried it, and keep the old line rather than erasing it. A record of what the value used to be is useful the next time someone claims it never changed.
Questions that come up
Why not just trust the fingerprint printed on a market page?
Because it arrives from the same place as everything else you are trying to check, so it can only confirm that the page agrees with itself. A page under someone else's control shows whatever that person wants shown, and a fingerprint is easy text to change. Your own note is useful because it was written down at a different time, from different sources, before anyone could tailor it to the situation you are in now.
How many sources make a fingerprint worth writing down?
Fewer than people think, provided they are genuinely unrelated. Two sources with different owners, different ages and no sign of having copied one another are worth more than five that all trace back to a single list. Spend the effort on establishing that the sources are separate rather than on gathering more of them, and record which ones you used so that a future disagreement can be traced.
What should I do if my noted fingerprint does not match?
Treat it as a signal to slow down, not as proof of anything in either direction. Go back to the sources you originally used and see whether they now carry a different value. Look for a rotation notice signed with the key you already trusted. If the sources disagree with each other, that disagreement is itself the finding, and the sensible response is to wait rather than to pick whichever value is most convenient.