La chiave deve essere verificata. Se ritieni che la chiave pubblica di qualcuno appartenga effettivamente a quell'individuo e siano nel tuo portachiavi, puoi utilizzare la tua chiave privata per firmare la chiave pubblica del tuo corrispondente e convalidarla.
Quindi sei Bob e ti fidi che la chiave pubblica di Alice appartenga effettivamente ad Alice, quindi la firmi con la tua chiave privata. Quindi la chiave di Alice ti è stata affidata. Anche le chiavi di cui Alice si fida, dicono che qualcuno chiamato Chris sarà anche nella tua rete di fiducia. Quindi puoi fidarti anche di Chris, perché Alice lo fa. Quindi la chiave di Chris sarà certificata con una firma attendibile.
Ora, se Alice si fida che la tua chiave appartenga a te, può convalidare la tua chiave pubblica firmandola con la sua chiave privata, quindi la tua chiave sarà ora inclusa nella stessa rete di fiducia.
a procedure was given to validate your correspondents' public keys: a correspondent's key is validated by personally checking his key's
fingerprint and then signing his public key with your private key. By
personally checking the fingerprint you can be sure that the key
really does belong to him, and since you have signed they key, you can
be sure to detect any tampering with it in the future. Unfortunately,
this procedure is awkward when either you must validate a large number
of keys or communicate with people whom you do not know personally.
GnuPG addresses this problem with a mechanism popularly known as the
web of trust. In the web of trust model, responsibility for validating
public keys is delegated to people you trust. For example, suppose
Alice has signed Blake's key, and
Blake has signed Chloe's key and Dharma's key.
If Alice trusts Blake to properly validate keys that he signs, then
Alice can infer that Chloe's and Dharma's keys are valid without
having to personally check them. She simply uses her validated copy of
Blake's public key to check that Blake's signatures on Chloe's and
Dharma's are good. In general, assuming that Alice fully trusts
everybody to properly validate keys they sign, then any key signed by
a valid key is also considered valid. The root is Alice's key, which
is axiomatically assumed to be valid.
Trust in a key's owner
In practice trust is subjective. For example, Blake's key is valid to
Alice since she signed it, but she may not trust Blake to properly
validate keys that he signs. In that case, she would not take Chloe's
and Dharma's key as valid based on Blake's signatures alone. The web
of trust model accounts for this by associating with each public key
on your keyring an indication of how much you trust the key's owner.
There are four trust levels.
unknown
Nothing is known about the owner's judgement in key signing.
Keys on your public keyring that you do not own initially have this
trust level.
none
The owner is known to improperly sign other keys.
marginal
The owner understands the implications of key signing and
properly validates keys before signing them.
full
The owner has an excellent understanding of key signing, and his
signature on a key would be as good as your own.
A key's trust level is something that you alone assign to the key, and
it is considered private information. It is not packaged with the key
when it is exported; it is even stored separately from your keyrings
in a separate database. The GnuPG key editor may be used to adjust
your trust in a key's owner.
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