Words / Pair 38 of 41
One of these strings was chosen by somebody
Two strings sit near each other in every key listing and they look about equally official. One costs nothing to imitate. The other cannot be picked at all.
What is chosen and what is derived
Derived from the key itself
A long string computed from the key material. Nobody sat down and decided what it would say. Change anything about the key and it comes out different, so a match against a note you made earlier is a real match.
Typed in by whoever built the key
The name and address inside a key are free text supplied at the moment the key was made. They can say anything at all. A match tells you the two strings agree, and nothing about the two keys.
Both strings turn up in the same panel, in similar type, with similar labels. That layout is the trouble. It reads as one block of official detail about the key when in fact it is two kinds of thing stacked together, one written by a person and one produced by the arithmetic.
A reader who compares the name is doing something that feels like checking and is not. Whoever wants you to accept their key will make the name agree with whatever you expect, since that costs them one line of typing.
Why both strings look official
| What you are looking at | Who decides what it says |
|---|---|
| The name inside the key | Whoever built the key. Free text, any value. |
| The address inside the key | Also whoever built the key. It need not exist and nothing checks it. |
| Any comment line on the key | The same person again. |
| The creation time on the key | The machine that made it, set to whatever it was set to. |
| The fingerprint | Nobody. It falls out of the key material and cannot be aimed at a value. |
Read down that table and the pattern is plain enough. Four rows of things a person typed, one row of something a person could not. Every part of a key that carries a helpful human meaning is the part with nothing behind it.
Pick the string that nobody typed
Compare the derived string, all of it
Compare the whole fingerprint against a copy you obtained somewhere other than the message in front of you, and read it end to end rather than glancing at the tail.
- Get the fingerprint your software shows for the key in question.
- Get the fingerprint from your own earlier record, or from a channel that had no part in delivering this message.
- Compare every character. Short forms that show only the tail are a weaker comparison and are not worth leaning on.
When the derived string does not match
A mismatch is not automatically a forgery. Operators do replace keys. A key can be retired for good reasons and a new one put in its place, and when that happens the fingerprint you wrote down will not agree with the one in front of you. What separates a replacement from an impostor is not the new key, since the new key looks the same either way. It is whether the change was announced by the old key while the old key still spoke for anybody.
So a mismatch leaves you with a question rather than a verdict. Was there a statement signed by the key you already trusted, saying that a new key was coming and naming it? If there was, you have a chain you can follow. If the only thing telling you about the new key is the new key, the chain starts nowhere and the mismatch stands. Nothing about the name on either key contributes to that reasoning at any point.
Costs that are not symmetric
- If you reject a real key over a name that looks wrong
- You lose time and possibly a working line of contact. Operators change what they type into a key, or use a name you were not expecting, and none of that makes the key false.
- If you accept a key on a name that looks right
- You have trusted a string chosen by the person asking to be trusted. Everything that key then signs will read as authoritative to you, including anything about where to send money.
- If the fingerprint matches
- Carry on, and record where you got your copy of the fingerprint from. That record is the thing doing the work next time, and a note you cannot trace is not much better than none.
- If only the name matches
- Treat the key as unknown, which is what it is. Look for a fingerprint from a source that did not come bundled with the message, and see a signed list against a published one for what a page alone can vouch for.
Questions readers send about this pair
Can two keys have the same fingerprint?
For practical purposes no, which is the entire reason the fingerprint is the string worth comparing. Short forms showing only part of it are a different matter and give you far less to go on.
The email in the key looks right. Is that not something?
It is the same kind of thing as the name. Whoever built the key typed it in and nothing verified it. A key can carry any address, including one belonging to somebody else.
Where should my copy of the fingerprint come from?
Somewhere that was not the message you are checking. Anything printed inside or beside that message is the message vouching for itself, and it will always agree.