Subject: RE: TSA Response serialNumber Field

From: [email-masked]
Date: Fri, 2 Jun 2000 10:36:50 -0400

One can recast Chebyshev's  inequality from
    Pr(|x - mean| >= n * sd) <= 1/n**2
     to
     Pr(|x - mean| >= (sd / (1 - c)**.5)) <= (1 - c)
     or
     Pr(|x - mean| < (sd / (1 - c)**.5)) > c

     Where c is just 1 - 1/n**2.

     If c can be described as a confidence level, this answers your
question 2) below.  I am not sure that one would describe c here as a
confidence level, but I think it's reasonable to say that if one has
probability > C that a measurement actually lies within a range R, then one
has at least confidence C that the measurement lies within that range.

          Tom Gindin

"Manger, James H" <[email-masked]> on 06/02/2000 01:13:25
AM

To:   "[email-masked]'" <[email-masked]>
cc:
Subject:  RE: TSA Response serialNumber Field

Thanks Ed,

(snip)

If a relying party is happy with a 95% confidence interval it does not need
to know the exact distribution.  If a relying party wants a better
confidence level than the standard specifies (e.g. 99.7% instead of 95%) it
has a couple of choices:
(1) assume, say, a normal distribution and adjust the uncertainty
accordingly (e.g. multiple by 3/2); or
(2) use Chebyshev's inequality to get an upper bound of the required
adjustment [can Chebyshev's inequality be reformulated for this, i.e. for
confidence levels (e.g. 95%) instead of standard deviations (e.g. 2 sd)?];
or
(3) determine the actual distribution from the TSA's policy document and
adjust the uncertainty by the amount appropriate to that distribution.
We can leave the choice to the relying party.

---

From: Denis Pinkas <[email-masked]>
Date: Mon, 05 Jun 2000 15:16:23 +0200

I have not followed all the rational for using or not using
Chebyshev's inequality and since a relying party will NOT be happy
with a 95% confidence (nor 99,99%), I propose that we all look back
at the definition we have in the current document.
 
" By adding the XXXX value to the GeneralizedTime, an upper limit of
the time at which the timestamp has been created by the TSA can be
obtained. In the same way, by subtracting the XXXX to the
GeneralizedTime, a lower limit of the time at which the timestamp 
has been created by the TSA can be obtained."

The single remaining question is how do you call XXXX, while
sticking to THIS definition. It is currently called "accuracy". If
this term is not adequate, please make a proposal. I would like to
close this issue soon.

Denis

---

From: Ed Gerck <[email-masked]>
Date: Mon, 05 Jun 2000 20:25:38 -0700

Denis Pinkas wrote:

> I propose that we all look back

Dennis:

The question is not so much what it is called (even though it would be
nice to use an already accepted name for it) but that THIS definition is
broken.  Pls see the former emails.  So, one should not stick to THIS
definition.

Cheers,

Ed Gerck

---

From: Denis Pinkas <[email-masked]>
Date: Wed, 14 Jun 2000 16:50:40 +0200

Ed,

Sorry for the delay to answer.

> broken.  

The *concept* is quite clear and is *not* broken. Since you have not
proposed an alternative to define this concept, I assume that the
word chosen to define the concept is acceptable.

> Pls see the former emails.  So, one should not stick to THIS
> definition.

The definition says what it says and is very clear. It might not be
what you want (we are not making measurements of a value), but we
are seeking 100 % GUARANTEES for the value we get, even if the
"accuracy", as we call it, is pretty large. We are not seeking a
probabilistic answer, even with 99,99 % confidence.

I have just posted a new draft to the web editor. This new draft
should be available soon. 

Regards,

Denis

---

From: Ed Gerck <[email-masked]>
Date: Wed, 14 Jun 2000 10:13:34 -0700

> Sorry for the delay to answer.

No problem, my comment was not timestamped ;-)

Let me retain the thread, for continuity.

> > Ed Gerck wrote:

Please see the WG archives -- the alternative I suggested and was
accepted by some is "uncertainty", and was also defined.

> probabilistic answer, even with 99,99 % confidence.

My comment is not whether you are seeking a probabilistic answer,
but that seeking 100% guarantees is moot in this case -- this is NOT
a digital signature, this represents a measurement of a value.  So, if the
standard wants to warrant that which cannot be warranted then I
suggest that this be made clear to this WG and in the standard itself
for those that might not have time to wade through all PKIX WG
messages.

IMO, though, this should NOT be done and the standard should
make clear that there is no 100% guarantee on the time stamp -- which
does not mean it is useless either, because if the deviation is known
(as I suggested must be) then it is possible to get assurances which
approach 100% as much as you please as a function of the error bar
you want to consider.

Cheers,

Ed Gerck