Hal Finney writes, in regard to web-of-trust:
But this is not quite right. The fundamental fact about PGP key signatures, which is often misunderstood, is this:
You can only communicate securely with someone whose key is signed by a person you know, either personally or by reputation.
In other words, if I want to communicate with joe@abc.com, I can only do so if one of the signators of his key is a person I know. If not, I have no way of judging the validity of his key.
There are, however, degrees of certainty here. The only person I trust implicitly to sign keys is myself. If I have a key which is separated from me by more than one hop in the web of trust, but still connected to me via a chain of signatures, I have more certainty that this key is valid than I do for an unsigned key. Granted, if I don't know the actual signator of a particular key, my level of trust in the key's validity is pretty low, but it's nonzero as long as it's connected by a chain of signatures. The ease of mounting of a man-in-the-middle attack decreases with increasing signature connectivity, no? Of course, the reality is that use of totally unverified PGP keys is widespread, even among people who are well educated on the subject. This is not a good thing in the long run. -- Will