In this example you see a key group containing two keys, two key pairs and three public keys. The third column shows the trust you have in the keys. The first key pair is ultimately trusted and is also set as the default key (bold font) while the second one has expired. Two of the public keys are fully trusted while the trust of the last key is marginal. The last key is expanded, showing it's ElGamal subkey, an additional user id, both also with marginal trust, and some of it's signatures.

Signatures allow navigating through your keyring. Double clicking on a signature or a key shown as member of a group will jump directly to the corresponding primary key.

While the key manager allows you to do general actions with one or multiple keys, key groups or signatures, the key properties window gives you access to a single key. You can reach it by pressing enter in the key manager or double clicking the key.

In this window you can change the key passphrase and expiration of your secret keys. For all keys you can also set the owner trust value.

This value indicates how much you trust the owner of this key to correctly verify the identity of the keys he signs. Taking the owner trust into account gpg creates your own web of trust. You trust the keys you signed. If you assign owner trust to these persons you will also trust the keys they have signed without the need that you first have to sign their keys too.

When you sign a key of someone else (let's call her Alice) you announce that you are sure that this key really belongs to that person and the key can be trusted. Of course you really should have checked that. This usually means that you have to meet Alice, check at least one identity card and get the full key fingerprint or a copy of her key. Then you go home and sign that key. Usually you will later upload the newly signed key to a key server so everyone knows you have checked that key and the owner may be trusted. Alice will likely do the same so you both will have your keys signed by the other one. If one of you has no identity card at hand it's no problem if the signing happens in only direction.

But think about what happens if Alice lives on the other end of the world. You communicate with her regularly but there is no chance you will see her anytime soon. How do you trust her key?

When you select her key and then choose Sign Key... you will get the dialog that allows you to choose the options how you would like to sign that key.

First you can choose the key you will use to sign the key. Then you can enter how carefully you checked that she really is the person she pretends to be. This information will be stored together with the signature so it is a guidance for everyone else who might need that signature (more on this below). And then comes the option that would help you if you can't meet Alice in person: Local signature (cannot be exported). When you activate that option a special version of a signature will be created that can never even by accident leave you keyring.

But why is it important how carefully you checked Alice's identity? Who should care? There is a different way to solve your problem with the identity of Alice. If you can't visit Alice anytime soon just think of Trent. You know Trent has a keypair, too. And Trent is a globetrotter, being on a different continent at least twice a month. If you are lucky he will fly close to Alice soon. So you will go and meet with Trent to sign keys. Then you will drop Alice a note that Trent will be at her place soon and ask her if she can meet with him too to sign keys. After all this has happened you know that Trent's key can be trusted and Trent knows that Alice's key can be trusted. If you trust Trent that he has carefully checked Alice's identity then you can also trust her key.

These relationships between keys and their owners form a so called web of trust. Within that web there are some important values that define how trustworthy a particular key is. The first thing is how carefully the identity of the key owner was checked. That is the value you have seen above in the secret key selection window. For example you will likely know how to verify your local countries identity card but one from a completely different country may be hard to verify. So you could say that you have very carefully checked Trent's identity because you have seen his identity card and it looks very much the same as yours. But Trent, although he has seen both Alice's identity card and driver license might say he has only done casual checking of her identity as he is not absolutely sure about the documents from that part of the world.

The next important value is how much you trust the other person to verify documents. You know Trent is good at that. But George for example is no one you would call smart. He barely looked at your id card when you met him for key signing. You are sure that George is the person he pretends to be as you checked his documents carefully. But he doesn't seem to really care if he checks other people so you will have a high trust in the key of George but a very low trust in the signatures of George. If you open the properties of a key you will find the field Owner Trust. This is how much you trust the key owner when he signs keys. This value will not be exported, it is completely up to your personal preference.

Now you should have an idea how the web of trust is built, what the owner and key trust values are for, and why you always should be very careful when checking identities: other people might rely on you. But one element in the process is still unverified: the email addresses in the keys you signed. Creating a new user identity in your key with the email address of Alice or Trent will only take a few mouse clicks. You have verified that Trent really owns his key. But noone has checked until now that Trent really controls the email addresses of his user identities.

If you choose Sign and Mail User ID... from the menu instead you can close that gap. The idea is that you will sign the key as usual and afterwards it will be split into pieces. Every piece will only contain one user identity of Trent's key and your signature to it. This will be encrypted with Trent's key and sent only to the email address given in that identity. Only if Trent can receive this mail and decrypt the message he will be able to import that signature into his key ring. You will not upload your signatures, this is entirely up to him. If your signature will show up on a key server you can be sure that Trent really controls both his key as well as the email address you signed. The signatures you make in this process will also be not part of your keyring. So right after you signed Trent's key it will still be shown as untrusted in your keyring. Once Trent has received your mail and imported your signature into his keyring he can upload them to a keyserver. When you refresh his key from a keyserver you will get the new signatures. While that may sound inconvenient first it makes sure that you will not by accident see one of his identities as trusted that he does not control. Only the signatures that show up on a keyserver are those where everyone, including you, can be sure that he really controls the corresponding email addresses.