From: "Deranged Mutant" <WlkngOwl@unix.asb.com>
Date: Tue, 18 Jun 1996 19:04:18 +0800
To: Vipul Ved Prakash <vipul@pobox.com>
Subject: pretty bad idea (was Re: pretty good reputation)
Message-ID: <199606180215.WAA28846@unix.asb.com>
MIME-Version: 1.0
Content-Type: text/plain


On 16 Jun 96 at 12:32, Vipul Ved Prakash wrote:

[..]
> > That's the point, though. PGP *does not* try to model a reputation 
> > system.  When you sign keys, you only attest that you are sure that 
> > that key belongs to the person whose name is on it.  A signature says 
> > *nothing* about a person's reputation.
[..]
> in that case if alice receives a key signed by bob, she would know
> the key is good.

Pretty-good, yes. (Assuming she trusts anything signed by Bob.)

>                                ... and there is no need for the two other fields. 
> infact the reason why i felt pgp is trying to model a reputation system 
> if because of these two fields that carry trust values.

Those are private/personal trust values only. Alice rates Bob on how 
well she'd trust a key signed by Bob.

This is subjective for Alice, based on her relationship with Bob. She 
may not know him well and assign him a low rating.  Or she may know 
him all too well, and give him a low rating.  This has no meaning for 
Carol, or Don, or anyone else, since they have different (if any at 
all) relationships with Bob.

> > You can't set up a global web of trust. It's computationally 
> > infeasable, esp. with contradictions, to resolve.  It's also 
> > meaningless.  Say Alice trusts Bob.  Bob trusts Carol.  Carol trusts 
> > Don.  Should Alice trust Don? No... subjective factors like "trust" 
> > don't commute.
> 
> this is precisely what i wanted to know. can there be a model wherein one
> can compute a trust-like parameter? if yes in what ways should this parameter

No, at least not one with meaning.

There are many factors that would go into one's rating of another: 
how well one knows the other; part history; bias for or against 
another because of thing like race, religion, political viewpoint, 
favorite OS, skills/talent or lack thereof, etc.  People's ratings of 
each other change as well... just imagine global ratings of a once 
overly well-liked famous person who after several years is now 
infamous (or an infamous person who is now forgiven).  And again, 
social pressure to give someone a certain rating...

[..]
> lets say a reputation system involves alice, bob, carol and don. Computing
> trust parameter with respect to each other can lead to situtions where
> alice trusts bob x% and has y% proof that he is a liar, after considering 
> all other relations. If this x% > y% she'll believe him, if not she won't.

Generally x+y=1 in this scheme. 

But lets say for a moment they are separate: x = trust level; y = mixtrust
level, independent of x... that makes little sense, since if y>.5, then Bob
isn't trustworthy, but why is x > y? x and y would always have to be
adjusted for each other... (so it's better off to use something like
x+y=1).

> thats why i feel the liar paradox doesn't really pose much of a
> threat.    

But even then, For Alice, x(Bob) > y(Bob), so Bob is trustworthy. For 
Bob though, x(Alice) < y(Alice).  So what of Carol? She does not yet 
know Alice and Bob, and sees their ratings about each other [Note: 
some people would assign ratings based on others; others would assign 
ratings based on personal experience... another problem with a global 
rating scheme].

Carol asks Alice about Bob, who says he's trustworthy.  She asks Bob
about Alice, who says Alice is untrustworthy.  If Alice is correct, 
then according to Bob she's not worth listening to, but then why 
listen to Bob.  If Bob is correct, Alice isn't worth listening to, 
but then again, maybe Bob isn't since she says Bob is... so why 
should Carol bother with any of them.

Ok, in real life it's possible to meet two people, one of him is a 
liar but not so much as to admit another is more truthful... and that 
truthful person agrees.  It's actually quite common... but how does a 
third person who doesn't know any of them figure it out? How does a 
computer program using a formula?

But given a system with several variables (ie, several people rating each
other) you'll have something that's even harder to resolve.  Given a 
global system with thousands or millions of variables... how does one 
make sense of all those ratings about a particular aspect of one 
person?

[..]
> There will be inconsistency only if trust/untrust values are absolute.
> that is you either trust someone or you don't. i am talking of a system
> wherein trust values lie between 0..1 which are derived from a set of

Oh no.  There's also a lot of inconsistency there too.  Doesn't 
matter if you use boolean or fuzzy values.

> tags. in that case bob alice trusts bob x% bob trusts carol y% and carol
> trusts alice z%. if z < x, z < y simply implies alice trusts bob, bob trusts
> carol more than carol trusts alice. so the system will be relative rather than

You're ignoring how much Alice trusts Bob, how much Carol trusts Bob 
and how much Alice trusts Carol.

> absolute. a person who is trusted by more people will carry greater
> weight

Why should that person carry more weight? (more weight than who else?)
What if all the people who trust that person are idiots?  Why should 
I, or you, or anyone else care about somebody more or less because "x 
number of people trust so-and-so"?

> and people trusted by him will carry higher weight too. this way snoopy

There's another flaw.  And what if those people trust him? 
Exponentially increasing trust?

> who is not trusted by many people will automatically have a weak relationship
> with others.

Not many people know Snoopy, so he has no trust level.  Nobody wants 
to get to know Snoopy just because he has no trust level.  He's 
locked out.

Bad idea.

> > There's the other point that someone else on the list (Perry?) brought up:
> > if people publicly rate each other, there may be social/political 
> > pressure to give some people lower or higher ratings.
> 
> again these ratings will be decided in a web rather that on one-to-one basis.
> that way the social/political pressure can be reduced though not completely
> eliminated.

No. Individuals put their ratings in the web to skew or confuse 
things.

Rob.


 
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