Web of Trust?

[Hal <hfinney@shell.portal.com>]

One of the key concepts widely used to describe PGP is the "web of trust".
This brings to mind a network of connections between people who know and
communicate with each other.  Two people who want to communicate can do
so securely if there is a path of connections in the form of signed keys
that joins them.

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.

This belies simple interpretations of the "web of trust".  I may have signed
A's key, A has signed B's, B has signed C's, C has signed D's, and D has signed
Joe's, but this is of no value unless I know D.  Only then can I trust Joe's
key.

This means that, in the "web" picture, I can only communicate securely with
people who are at most two hops away in the web of connections.  I can
communicate with the people I know, and I can communicate with the people they
know, and that is it.

This is unfortunate, because the simple web model ties into some famous
research which suggests that any two people chosen at random are only about
half a dozen steps apart in the web of who-knows-whom connections.  (This
result is where the title of the movie "Six Degrees of Separation" comes from.)
If you had a system which actually supported communications via such a web
model, it actually would have hope of letting two people communicate who did
not have a very long chain between them.  But PGP, with a maximum chain length
of two, will not allow this.

What would have to be added in order to allow a true web of trust model to be
used in a program like PGP?  Basically what is needed is some way to judge
the trustworthyness of signatures by people you don't know.  This would most
plausibly be provided by the people who had signed their keys.  For example,
if there were another type of key signature which did not only vouch for the
person's identity, but also for his trustworthyness and care in signing keys,
then a chain of such signatures could serve as the basis for a true web of
trust.  Obviously such signatures could not be given out nearly as easily as
the kind we have now, where a glance at some stranger's drivers' licence is
often all we get, but they could be given to close friends and those we know
and trust.

More elaborate systems might include numerical ratings of trustworthiness
which would help to estimate the strength of any given path.  The main point
is that some information of this kind would be needed in order to allow
communication with people distant in the web of connections.

Without this, I think we will continue to have problems with PGP being unable
to validate keys of people we want to communicate with.  People will collect
huge laundry lists of signatures in the hopes that whoever wants to commu-
nicate with them will know one of those people.  Centralized key validators
will appear (as in the case of the SLED service being started now, which will
sign a key based on a signed check with your name on it).  The result may be
a choice between using an unsigned key or using one signed by some faceless
bureaucracy, which is no better than the original PEM conception.

(People may be confused by this essay because they thought PGP worked this
way already.  PGP does have a follow-the-web model, but that is only for
following signatures.  In the example above, where I wanted to talk to Joe
and there was a chain to him through A, B, C, and D, we have to first sup-
pose that I know and trust all of A, B, C, and D.  Given that, what PGP can
do is to determine whether I have valid keys for all of those people.  It will
notice that A has signed B's key, so it is valid.  I know B and told PGP he
was trustworthy, and he signed C's key, so therefore that one is valid.  Sim-
ilarly, I know C and I know D so PGP can follow the chain through them.  Fin-
ally we come to Joe, whom I don't know, but because I know D and PGP followed
the web to determine that D's key is valid, PGP can determine that Joe's key
is valid.  But again, that was only because I knew D and everyone else in
the chain.  The bottom line is still that I can only communicate with people
who know someone I know.)

Hal