7.7.2. Trust levels

In addition to this, it is possible to subjectively decide the level of trust assigned to a particular key signer. In the previous example, Alice could decide that she does not trust Bob, because he is known for happily signing any public key he gets his hands on without caring to verify the owner's identity. In this case she sets the trust level of key 0xBBBBBBBB (the key Bob uses to sign other people's public keys) to None.
Here, “trust” refers solely as Bob's capacity to properly validate public keys. It does not infer anything else concerning Bob as a person, such as his trustworthiness, his being a law-abiding citizen, or any of his moral qualities. It does not concern, either, the content of Bob's messages being truthful or not.

There are five levels of trust:

• Unknown. Nothing can be said about the owner's judgement in key signing. This is the trust level initially associated to other people's public keys in your keyring.
• None. The owner is known to improperly sign keys.
• Marginal. The owner is known to properly sign keys.
• Full. The owner is known to put great care in key signing.
• Ultimate. The owner is known to put great care in key signing, and is allowed to make trust decisions for you.

You can set the level of trust of a particular key by selecting that key and choosing the option Set Owner Trust from Key Management, or from Key Properties itself.
You alone decide which level of trust to assign to a key, and the trust is assigned only locally. This is considered private information: it is not included with the key when it is exported, and is stored in a separate place from your keyring.
You should set the trust level of your own key pair to the maximum (I trust ultimately).