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Peter Gutmann wrote:
Yes, because they want to use the signing key (relatively) short-term, but be
able to verify sigs using the public portion years after the private portion
has been retired/destroyed/lost/whatever.
Again I'm astonished to see this discussion here. It shows like other
discussed topic how much disagreement is here about very basic topics.
Personally I see no use of PKUP extension. How long signatures created with
a private key associated with a PKC can be validated should not be specified
in a public-key certificate. The more natural solution to me is to specify
this in the policy and cryptographic protocol used for signing the message.