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Modelling trust


I am thankful to Carl for initially referencing here my MCG discussions on
the possible definition of trust in Information Theory terms. I am also
thankful for this forum's series of interesting list and private msgs that
resulted from the intellectual provocation posed by the definition and,
even, by the mere fact that such a definition has been proposed. 

I have tried to provide here, to the best of my ability and e-mail limits,
several examples and counter-examples on the consequences of the
definition that "trust is that which is essential to a communication
channel but which cannot be transferred from a source to a destination
using that channel".

Carl and the SPKI group have been champions on the locality of trust and
the proposed definition indeed anchors trust on the observer -- "trust is
in the eyes of the beholder" -- while rejecting a purely objectivist view.
As this work moves forward, it becomes more and more obvious that trust is
certification's ugly duck. Current protocols indeed take a leap of
ignorance on what trust is and start by defining means to convey it --
which leads to a series of inconsistencies as compared with the social use
of trust, such as ad hoc demanding transitive trust for communication

Given the very useful and practical questionings to the proposed trust
definition that came from this list, with some good devil's advocates and
also with encouraging support, I would like to ask for further
comments/support/criticism on the subject. To provide for a full context,
I add a few comments from other fora and summarize the main results below,
with added twists ;-) 

Some think that one cannot compare the "digital"  and "emotional" concepts
of trust -- the "digital" concept being the technical use of the word
trust as in communication processes, root-keys, digital signatures,
certificates, etc. and the "emotional" concept being the social
understanding of the word trust in commercial, legal and personal

First, the term digital trust is inapropriate when applied to a
communication process -- which can also be analogue. Similarly, technical
trust is also misleading, e.g. a technical argument in law is quite
different from a technical argument in engineering. The best word here
might be "process trust", which allows not only the protocol but also the
software, hardware, etc to be included in the trust concept -- e.g. a
modem can also be trusted in the communication technical sense. Similarly,
"social trust"  might also be better word to represent the emotional,
real-world, 3D or personal aspects of trust. So, I will use preferentially
both terms below: process trust and social trust.

The concept of process trust has several definitions, as I have located
them and there are possibly more (references welcome -- anyone? ):

1. NSA: "a trusted system or component is one with the power to break
one's security policy" [1] While some may consider that this definition
chimes in well with the relationship between a TTP and a TTP-subscriber,
it does have the merit that it considers trust to be subjective.

2. X.509: "Generally, an entity can be said to "trust" a second entity
when it (the first entity) makes the assumption that the second entity
will behave exactly as the first entity expects. This trust may apply only
for some specific function. The key role of trust in the authentication
framework is to describe the relationship between an authenticating entity
and a certification authority; an authenticating entity shall be certain
that it can trust the certification authority to create only valid and
reliable certificates." [2] This definition is the boolean negation of
NSA's reported definition -- thus X.509 is basically subjective in its
trust assumption, which is however later on denied by X.509 itself -- such
as when predicating certificate chains or by allowing CAs to define CPSs

3. ABA Guidelines I: trust is not defined per se, but indirectly, by
defining "trustworthy systems" (or, systems that deserve trust) as
"Computer hardware, software, and procedures that: (1)  are reasonably
secure from intrusion and misuse; (2)  provide a reasonably reliable level
of availability, reliability and correct operation; (3) are reasonably
suited to performing their intended functions; and (4) adhere to generally
accepted security principles. " [3] This definition is unfortunate in that
it confuses trust with fault-tolerance, especially so because
fault-tolerance is objective and can be quantitatively measured by
friends and foes alike -- whereas trust is the opposite.

4. ABA Guidelines II: the ABAG uses the word trust also in the legal sense
of something held in trust -- i.e, a property interest held by one person
for the benefit of another -- which has nothing to do with the issues
here, but may confuse the reader in a phrase such as "private key trust
service"  which is later on defined to be a legal trust concept in the
ABAG document. I note that a much better wording for such use of the word
trust in the ABAG would result from rephrasing everything in order to
highlight the expression "in trust" for this legal concept, such as using
"private key service in trust" instead of "private key trust service". [3]

BTW, the above two ABAG references are two examples where the ABAG would
need urgent revision, even though the Information Security Committee that
drafted those Guidelines reportedly (ref. Bob Jueneman) consider them

5. PGP: even though PGP uses the word trust extensively, such as in
web-of-trust, the concept of trust is NOT defined by PGP (AFAIK --
references?) and one has the impression that PGP uses the social concept
of trust. In fact, this would be appropriate because PGP was intended to
be an e-mail security software for a close group of friends and the
friends themselves would provide for the trust management issues -- in
their own socially acceptable way.

The concept of social trust can be obtained from dictionaries, such as
Merriam Webster: " 1 a : assured reliance on the character, ability,
strength, or truth of someone or something b : one in which confidence is
placed. 2 a : dependence on something future or contingent : HOPE b :
reliance on future payment for property (as merchandise)  delivered : 
CREDIT 3 a : a property interest held by one person for the benefit of
another b : a combination of firms or corporations formed by a legal
agreement; especially : one that reduces or threatens to reduce
competition 4 archaic : TRUSTWORTHINESS 5 a (1) : a charge or duty imposed
in faith or confidence or as a condition of some relationship (2) : 
something committed or entrusted to one to be used or cared for in the
interest of another b :  responsible charge or office c : CARE, CUSTODY
<the child committed to her trust>" 

Having presented the various definitions found for "process trust" and
"social trust", we can easily observe that they are not even concordant
between themselves -- much less with one another.

However, even though the appearances are such, I made the (disparated?,
bold?) statement that they should *all* be basically the same and that
the apparent disagreement is just a matter of proper focus.

Thus, in this view, both "types" of trust are not apples and speedboats
and communication protocols shoud be based on social trust concepts -- not
on some ad hoc and academically unrealistic models (good for reaching the
publication quota, though). 

Further, I consider that it is a bad thing if one is using a model of
trust that divorces the digital world or communication concept of "process
trust" from the emotional, personal or 3D world concept of "social trust".
Instead of a "feature" of such a model, it is a bug.

In fact, the social and communication aspects of trust must be confortably
integrated if a socially useful communication protocol is to defined. In
other words, one must recognize that unless one arrives at a real-word or
social model of trust to be used in the electronic world, no logically
useful communication trust model can be set forth!

This idea is not entirely new. Shannon used it successfully 50 years ago
when modelling information, for example. In fact, for trust, Phill
Hallam-Baker declared the following in Nov/94: 

"We have two options either we can attempt to define wonderfull academic
forms of trust model de novo. Or we can observe the real world and attempt
to model the trust mechanisms that allow it to function. Since we do not
see a hierarchical trust model it is not the solution. We do not see
anarchy either, or at least in places where it has taken hold it is
disaster.What we see is binary interpersonal relationships heavily
qualified in manyways. The approach that has always seemed most promising
to me is to replicate those relationships allowing them full colour with
respect to the areas for which trust is granted (finacial, notarial,
reliability etc), the extent of such trust and the confidence with which
that trust is allowed." [4] 

Indeed, both the social and the communication process aspects of trust can
be equally taken care of in a single model, by adequately defining trust
using Information Theory tools and the IT's definition of information
together with a social model for trust's properties in the real-world. To

1. Information:

In Information Theory, information has nothing to do with knowledge or
meaning. In the context of Information Theory, information is simply that
which is transferred from a source to a destination, using a communication
channel. If, before transmission, the information is available at the
destination then the transfer is zero. Information received by a party is
that what the party does not expect -- as measured by the uncertainty of
the party as to what the message will be.

2. Trust:

Akin to the definition of information in Information Theory, trust in the
context of a communication process has nothing to do with friendship,
acquaintances, employee-employer relationships, loyalty, betrayal,
authorization and other hard to define concepts. In the concept of
Generalized Certification Theory, trust is simply "that which is essential
to a communication channel but which cannot be transferred from a source
to a destination using that channel".  [5]

As shown in [5] and its references, this definition of trust using
Information Theory terms allows the social aspects of trust to be
adequately represented and is also coherent with Information Theory's

Thus, a unique model of trust can represent "social trust" (ie, 3D world,
emotional, personal) when applied to a communication process (ie, digital
world) as well as represent "process trust"  (ie, digital or technical
trust) when applied to a social situation. 

To contrast, as an example, unrealistic models of trust such as used in
X.509 (albeit with a correct beginning) define certificate chains which
err in three aspects of social trust, because a certificate chain supposes
that trust is transitive, distributive and symmetric -- which it isn't.
Which will eventually invalidate any PKI scheme which grows beyond a
certain "critical radius". [reference 5 of ref. 1 below]. 

I thank you again for your interest on these matters. I think that the
ugly duck may be getting ready to repeat the fable and turn itself into
the most important aspect of security -- providing for useful reliance
tools -- both socially as well as processwise.

Perhaps, the mythical cert-o-meter?




[1]  reportedly, in

[2] ftp://ftp.bull.com/pub/OSIdirectory/Certificates/       

[3] http://www.abanet.org/scitech/ec/isc/dsgfree.html

[4] www-security mailing list archive, Re: Syncytial trust?  Sat, 12 Nov
    94 03:31:06 -0500
[5] http://www.mcg.org.br/trustdef.txt

Dr.rer.nat. E. Gerck                     egerck@novaware.cps.softex.br
    --- Meta-Certificate Group member, http://www.mcg.org.br ---