Hello,
Firstly there is no way to prove non-repudiation. There is no valid means to
prove encryption. For those who do not agree, please read up on computational
mathematics and the N vs NP problem (also see computational theory in general).
Micheal Sipser (from MIT) has some excellent papers on the topic.
Next lets get to prove. Prove is a mathematical determination of a rule. Even
in the case of a discovered and somehow proven encryption algorithm there is no
way to prove non-repudiation.
What does this mean? It comes down to a likelihood determination. This is a
probabilistic determination of the Cumulative distribution function (CDF)
associated with the survival and hazard functions of the plot of time against
likelihood of compromise.
Even in cases of a perfect algorithm there is an associated hazard function
associated with a brute force compromise of the key. In most cases this
Probability density Function (PDF) correlates to a Poisson distribution.
So what you are looking at in reality is a survival function that will be
acceptable in a court of law that will not be readily repudiable by the
opposing party.
To do this you need to look at proof beyond reasonable doubt. This is due to
the criminal standard of proof being used for deceit. As you wish to prove
against a person who may be lying this is the necessary level of proof. In
common law courts this is generally (though not exclusively) held at a
determined confidence level (CI) of 99%. That is an alpha set at 1%.
Now the determination needs to be complete in a cumulative manner which
includes the totality of the systems. In this you need to determine the
individual hazard function for each of the components. This is than
extrapolated into the total Survival function estimate for the system.
One property of the exponential distribution and hence the Poisson process is
that it is memory-less (This is the number of incidents occurring in any
bounded interval of time after time t is independent of the number of arrivals
occurring before time t).
Now this means that you are attempting to determine the lambda function λ(t)
associated with each hazard occurrence (being the likelihood of brute force or
other key compromise). The number of expected key compromises for each
component is than the integral of λ(t) for the period from 0 (start time) to a
determined safe time (i.e. promised non-repudiation of 5 years, 25 years etc).
So yes there are ways to achieve what you are asking. What you are looking at
is the expected "safe" time of your system.
Regards,
Craig
-----Original Message-----
From: Joe [mailto:bitshield@gmail.com]
Sent: Friday, 2 June 2006 4:32 AM
To: security-basics@securityfocus.com
Subject: Proving non-repudiation in e-Commerce App
Dear List-Members
I'm currently dealing with a review of an e-Commerce Application. One
goal is to prove that this application properly implements a
non-repudiation mechanism throughout the whole process-flow. This flow
starts at the user authentication, communication over the web to the
server component, then processing of the client requests and finally
logging.
The non-repudiation has similarities with e-Banking which points me to
the following keywords: digital signature, signed logging and time
stamp protocols. Using Google I also found various sources discussing
most of those points individually. However I'm looking for a more
general, broad and complete approach.
Do you guys have interesting sources and experiences about verifying
non-repudiation? Are there standards, defined processes, work-flows,
and implementation- or audit guidelines?
Thanks for your feedback
Joe
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