RE: Why is privateKeyUsagePeriod deprecated?

[Sharon Boeyen <sharon.boeyen@xxxxxxxxxxx>]

Just want to voice agreement and support for David Chadwick's views on this
:-)

-----Original Message-----
From: David Chadwick [mailto:d.w.chadwick@xxxxxxxxxxxxx]
Sent: Monday, July 21, 2003 1:29 PM
To: David A. Cooper
Cc: Peter Gutmann; ietf-pkix@xxxxxxx
Subject: Re: Why is privateKeyUsagePeriod deprecated?




"David A. Cooper" wrote:
> 
> Peter Gutmann wrote:
> 
> >PKIX disallows (strongly) this extension, but without giving any reason
for
> >it:
> >
> >  This extension SHOULD NOT be used within the Internet PKI.  CAs
conforming
> >  to this profile MUST NOT generate certificates that include a critical
> >  private key usage period extension.
> >
> >I've now run into several PKI users (including fairly large ones like
> >government departments and large corporations) who resort to ignoring the
cert
> >expiry date in order to get around this restriction, since they have a
> >requirement to validate signatures long after use of the private key has
been

Peter

the original idea in X.509 was to allow a certificate expiry date to be
significantly after the private key usage period to allow for signature
verification long after signatures could no longer be created. thus both
times are useful.

David Cooper wrote

> It is my understanding that use of this extension was deprecated since,
> unless signed messages are timestamped by a trusted time stamping
> service, there is no way of determining when a message was signed.

this is obviously not true. If I recieve a signed message BEFORE the
private key usage period has expired, I KNOW that the signature is time
valid. I dont need a TSA to tell me that. I can read my watch. I can
then add my own time stamp and signature and stick it in my archives, if
I am so concerned about time.

> Without this ability, the relying party can not verify that the
> signature was created before the end of the private key usage period, if
> the signature is being verified after the end of the private key usage
> period.  If relying parties can not make use of the information, then
> there is no reason to include it in the certificate.
> 

This is a illogical leap that has been taken. Just because there is a
very small time period during which a signer can validly use his private
signing key, but the RP cannot receive it and validate it before the
private key usage period expires, is no reason to state that the RP can
not make use of the information. If 1% (or less) of the time is
unusable, it is no reason to discard 99% of the time (or more) that is
usuable. Anyhow, any good PKI would have replaced the user's signing key
long before the private key usage period had expired. 


> Of course, there is no reason that one can not discontinue use of a
> private key before the expiration of the corresponding certificate even
> if there is no indication in the certificate of when use of the private
> key is supposed to end.  Is there any reason that the PKI users you
> mention need to include this information in the certificate?  If there
> is a need to verify signatures for up to 7 years after the signature was
> generated, why not just institute a policy that all subscribers must
> rekey 7 years before the expiration of their certificates? 

This is a good point, and a work around for not using the private key
usage period. But like all policy, we can not record it in the cert, or
we can record it in the cert. The tendency with X.509 has been to record
as much of the policy as possible and that is automatically checkable in
the cert. So this argues for keeping and using the private key usage
period

David


> Why does the
> private key usage period need to be specified in the certificate (except
> perhaps indirectly by mentioning it in the certificate policy)?
> 
> Dave

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David W. Chadwick, BSc PhD
Professor of Information Systems Security
IS Institute, University of Salford, Salford M5 4WT
Tel: +44 161 295 5351  Fax +44 01484 532930
Mobile: +44 77 96 44 7184
Email: D.W.Chadwick@xxxxxxxxxxxxx
Home Page:  http://www.salford.ac.uk/its024/chadwick.htm
Research Web site: http://sec.isi.salford.ac.uk
Seminars: http://www.salford.ac.uk/its024/seminars.htm
Entrust key validation string: MLJ9-DU5T-HV8J
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