Re: [pkix] Private key usage period extension

Peter Gutmann <pgut001@cs.auckland.ac.nz> Sat, 07 May 2016 05:09 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: Erik Andersen <era@x500.eu>, Directory list <x500standard@freelists.org>, PKIX <pkix@ietf.org>
Thread-Topic: [pkix] Private key usage period extension
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Date: Sat, 7 May 2016 05:09:28 +0000
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Subject: Re: [pkix] Private key usage period extension
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Erik Andersen <era@x500.eu> writes:

>This extension was included in RFC 3280 with a heavy health warning. It was
>omitted from RFC 5280 (except for A.2).

It's been deprecated since RFC 2459.  At that time no-one was ever able to
give a coherent explanation for this that got much beyond "PKIX doesn't do
that sort of thing" [0].
 
>In my mind, the validity of the private key should not spread outside the
>validity period of the certificate.

It's not meant for that, in fact it's the exact opposite, it's an extremely
useful extension for when you want to say that, for example, a signing key is
valid for one year but the certificate used to verify its signatures is valid
for ten years.  The lack of a capability for doing this has been plaguing
cert-based signatures for years, leading to all manner of workaround hacks to
deal with verifying signatures after the cert has expired.

Peter.

[0] Years later people retconned explanations for it, but none of them were 
    terribly credible.