Subject: RE: Re: Why is privateKeyUsagePeriod deprecated?

From: Stephen Kent <[email-masked]>
Date: Fri, 25 Jul 2003 13:44:51 -0400

At 12:25 +0900 7/25/03, bonny wrote:
>Dear Steve
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>The following are my comments for your views
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>only in the context of post facto evaluation for NR purposes is it
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>likely to be applicable
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>Yeah I agreed to this
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>it embodies the notion that we can declare that a signature
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>generated with a private key expires at a future date, relative to 
>NR concerns. this is not a good match for many real world contexts 
>on which NR is an issue,e.g., my wet signature does not expire 
>although an agreement may have a limited lifespan.
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>With the PKUP field we r assuring that the signatures should only be 
>created within the validity period of the private key.That doesn't 
>mean the signatures are not valid after the validity of the period.
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>So how does PKUP creates the notion , we can declare that a signature
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>generated with a private key expires at a future date.It only 
>suggests that no more signatures cant be generated.
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The PKUP does operate as you note, but the cert validity period is 
still present, and in the presence of the PKUP extension, this field 
is reinterpreted to state that after the notAfter date, the cert may 
no longer be used to verify any signature, period.  That too is part 
of the reason for not encouraging use of PKUP, i.e., the need to 
reinterpret the cert validity field.

Steve

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From: Denis Pinkas <[email-masked]>
Date: Wed, 30 Jul 2003 10:07:55 +0200

Steve,

(...)

 > The PKUP does operate as you note, but the cert validity period is still
 > present, and in the presence of the PKUP extension, this field is
 > reinterpreted to state that after the notAfter date, the cert may no
 > longer be used to verify any signature, period.

Hum ! The period is coming too early. :-)

The interpretation of validity period is independant of the presence or 
absence of the PKUP extension.

After the notAfter date, the cert may no longer be used to perform a new 
signature verification, unless such a verification has already been done 
before and the time of that verification can be demonstrated to be before 
the notAfter date (e.g. using time-stamping) and the certification path 
associated to all the revocation information close to that verification date 
has also be captured (and protected against key compromission).

 > That too is part of the reason for not encouraging use of PKUP,
 > i.e., the need to reinterpret the cert validity field.

This is an unrelated reason.

Denis

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From: Stephen Kent <[email-masked]>
Date: Wed, 30 Jul 2003 10:01:36 -0400

At 10:07 +0200 7/30/03, Denis Pinkas wrote:
>The interpretation of validity period is independant of the presence 
>or absence of the PKUP extension.
>
>After the notAfter date, the cert may no longer be used to perform a 
>new signature verification, unless such a verification has already 
>been done before and the time of that verification can be 
>demonstrated to be before the notAfter date (e.g. using 
>time-stamping) and the certification path associated to all the 
>revocation information close to that verification date has also be 
>captured (and protected against key compromission).
>

Denis,

When one reads through the set of conditions you describe for 
interpreting the validity of the validity period, I think I am still 
comfortable with my informal characterization of the issue. In the 
absence of PKUP, anyone trying to validate a cert after the cert has 
expired has to know, by context, that the purpose of the validation 
is NR, not authentication or authorization for realtime access. This 
has to be factored into the process, else the validation will always 
fail after the cert validity period has expired.

With PKUP, I believe the intent is that the validity period really 
becomes a hard stop, i.e., a post facto validation will now fail if 
the validity period has passed, irrespective of the use of a time 
stamp. Looking at the text in X.509 and in 3280, none of this is said 
explicitly, so I admit that I am interpreting what folks who support 
use of PKUP have said, but the same is true of your description above.

Steve