Signatures, plainly

Signing gets described in ways that make it sound like a seal of approval. It is closer to a tamper strip that only one person can apply.

The mechanics in four lines

1. A person has two keys, one private and one public.
2. They run the message and the private key through an operation, producing a signature.
3. Anyone with the public key and the message can check the signature matches.
4. Change one character of the message and the check fails.

That is all of it. There is no third party, no registration, and no lookup.

The question that is left over

Whose key is it. Nothing in the mathematics answers this. A public key is a lump of data with a name attached by whoever handed it to you, and the name is not part of the key.

So the entire practical value of a signature rests on where you got the public key. A key you have held for a long time, obtained from a place you trusted before you needed anything, is worth something. A key that arrived attached to the message it is verifying is worth nothing at all.

The commonest mistake in one sentence

Downloading a key from the same page as the message and then feeling reassured when the check passes.

It will always pass. Anybody producing a fake announcement generates a keypair, signs with it, and publishes both. Verification confirms internal consistency and internal consistency is free.

Fingerprints

A fingerprint is a short hash of a public key, used so people can compare keys without pasting the whole thing. Comparing fingerprints is genuinely useful when you have two supposedly identical keys from different sources.

The same trap applies as with addresses. Compare the whole fingerprint, not the first and last groups. And a fingerprint from the same place as the key is not a second source.

Time

A signature does not carry a trustworthy time. It verifies today exactly as it did the day it was made. If a signed message says an address is current, current refers to whenever it was written, and you cannot find out when that was from the signature.

This matters for announcements. An old signed message about an awazon market address is genuine and possibly obsolete, and those two are not in tension.

Clearsigned and detached

A clearsigned message keeps the text readable with the signature wrapped around it. A detached signature is a separate file or block sitting beside the text. They differ in packaging and prove the same two things.

Both are frequently pasted around with the signature block quietly stripped, at which point they prove nothing and look exactly like a plain message. If somebody refers to a signed announcement and what you have is prose, you do not have a signed announcement.

What signatures are good for here

• Establishing that two messages came from the same key holder.
• Making tampering visible.
• Linking a new announcement to an old key you already had.

That third one is the useful case for addresses, and it only works if you kept the key from before. See whether you need PGP for a given step.

• What a signature proves
• Whether you need PGP
• A screenshot as proof