Subject: RE: [Simple] Another idea for presence authorization

From: Jonathan Rosenberg <[email-masked]>
Date: Tue, 17 Jul 2001 17:42:23 -0400

-----Original Message-----
From: Sriram Parameswar [mailto:[email-masked]]
Sent: Tuesday, July 17, 2001 5:33 PM
To: 'Jonathan Rosenberg'; 'Michael Hammer'
Cc: 'Rosen, Brian'; 'Daniel Starin'; [email-masked]
Subject: RE: [Simple] Another idea for presence authorization

>Quick observation/question - if you always send off-line (say) and I am
indeed 
>offline (see Jonathans email below), haven't you actually divulged presence

>information? I contend that this is also an information leak.

Of course it doesn't! I can even mathemtically prove it. Here's the proof.

Let X represent the information conveyed in the NOTIFY. For simplicity, and
without loss of generality, X is a boolean random variable, which takes on
the value of a 1 if I'm online, and 0 otherwise. Since the NOTIFY always
contains the same value for this random variable (value 0), the probability
distribution function of X is readily defined as:

P(X=0) = 1
P(X=1) = 0

Now, based on Shannon's definition of information, the information conveyed
in a random variable is equal to its entropy. Entropy S is defined as:

S = - SUM(i=1 to N) OF (pi ln(pi))

plugging this in:

1 * ln(1) + 0 * ln(0) = 0 + 0 = 0

So, the information conveyed is ZERO. NADA. NOTHING.

In non-mathematical terms, if I always tell you its sunny outside,
irregardless of whether it is or isn't, can you ever know whether it really
is or isnt? Have I told you anything useful? No!

-Jonathan R.
---
Jonathan D. Rosenberg, Ph.D.                72 Eagle Rock Ave.
Chief Scientist                             First Floor
dynamicsoft                                 East Hanover, NJ 07936
[email-masked]                     FAX:   (973) 952-5050
http://www.jdrosen.net                      PHONE: (973) 952-5000
http://www.dynamicsoft.com

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From: Paul Kyzivat <[email-masked]>
Date: Tue, 17 Jul 2001 18:00:06 -0400

I agree with the concept of hiding the difference between not present
and not authorized. However, since an initial NOTIFY containing
not-present, immediately following the SUBSCRIBE, is information free,
why not just agree not to send it? Following a SUBSCRIBE, send nothing
until the response is something other than not-present. Do this even if
the subscriber *is* authorized and the subscribee is not present. Then
everyone just needs to understand that until they do receive a notify
they must assume the user in question is not present.

	Paul Kyzivat
	Cisco Systems

Jonathan Rosenberg wrote:
> _______________________________________________
> simple mailing list
> [email-masked]
> http://mailman.dynamicsoft.com/mailman/listinfo/simple

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From: James Undery <[email-masked]>
Date: Wed, 18 Jul 2001 09:20:36 +0100

> they must assume the user in question is not present.

The trouble is CPIM which says a successful SUBSCRIBE must be followed by a
NOTIFY, and it would be CPIM that needed to change.

James Undery

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From: Paul Kyzivat <[email-masked]>
Date: Wed, 18 Jul 2001 10:52:30 -0400

See below.

> NOTIFY, and it would be CPIM that needed to change.

Yes, I was more or less expecting that answer.

In reality, this entire discussion about what to divulge ought to take
place wrt CPIM, because it is applicable to any IM or presence system.

Alternately, one could argue that it is not really a subject for
standardization at all, but is merely a quality-of-implementation issue.

	Paul Kyzivat
	Cisco Systems