[xmlsec] Verify signature after certificate expired

Moultrie, Ferrell (ISSAtlanta) FMoultrie at iss.net
Wed Oct 9 10:28:04 PDT 2002

  I'll have to check the spec also -- but it simply *can't* work this
way. If I sign e-mail today and it can't be verified tomorrow even
though nothing has changed, that's *wrong*. I do know that signing of
certs works this way but that's handled by ensuring that the root cert
is valid as of the expiration of the child cert. While you are likely
correct about CRLs .. none of the other signing schemes other than certs
behave this way (i.e., code signing (JAVA and Authenticode) and S/Mime
e-mail don't behave this way).
  Depending on what the spec says about this and whether it's MUST,
SHOULD or MAY, will determine how to proceed here. If it's SHOULD or MAY
and you feel strongly that the current behavior is correct, what about
an option for applications where the data must be proved authentic
essentially forever and the only desired restriction is that the cert
must have been valid when the data was signed?
  One way to look at this is that I'm giving my customers a perpetual
license to my application -- if it suddenly stops working in 9 months
when my cert expires, they aren't going to be happy. The commercial CA's
won't even give you a cert greater than 12 (maybe 24 if you press hard)
months. There's no way to validate code and other application meta-data
(the XML issue at hand) for a greater period of time if your assumption
is correct. That just won't work ...
  Please check on this and reconsider if it's at all an optional
behavior (which I strongly believe it is).

	-----Original Message-----
	From: Aleksey Sanin [mailto:aleksey at aleksey.com] 
	Sent: Wednesday, October 09, 2002 11:48 AM
	To: Moultrie, Ferrell (ISSAtlanta)
	Cc: Roman Bouchner; xmlsec at aleksey.com
	Subject: Re: [xmlsec] Verify signature after certificate expired
	I believe you are wrong here. The certificate must be valid when
signature is verified.
	Where "valid" includes:
	    1) certificate is signed by another "valid" cert or trusted
root cert
	    2) certificate is not in the CRL 
	    3) the certificate purpose matches the action you are going
to do (sign, verify, etc.)
	    4) the certificate is not expired
	All 4 items are important. I do not have an X509 spec around so
I could not prove my point.
	However, RFC 2560 (http://rfc.sunsite.dk/rfc/rfc2560.html):
	   An OCSP responder MAY choose to retain revocation information
	   a certificate's expiration. The date obtained by subtracting
	   retention interval value from the producedAt time in a
response is
	   defined as the certificate's "archive cutoff" date.
	Please note that when certificate is expired the OSCP may remove
the CRL from memory!
	Moultrie, Ferrell (ISSAtlanta) wrote:

		  Very important question here -- I want to make sure I
understand your
		reply. In general, it is not permitted to *sign* data
after the signing
		certificate has expired but it is allowed to *verify*
data after
		expiration. An example:
		I sign my code today allowing the user to verify that it
is uncorrupted
		when he installs/runs it. My certificate expires in
three months. If you
		don't honor the signature any more at that point, then
his code stops
		working -- a very bad thing. 
		Another example:
		I sign an e-mail to you today. Three months later my
cert expires. Six
		months later you try to read my e-mail and you're told
the signature is
		invalid. A bad thing.
		As these two examples illustrate, it is *not* standard
practice to
		invalidate signatures because the signer's cert has
expired. The only
		date restrictions should be:
		1) Signers cert *must* be valid as of the date the data
was signed;
		2) Root cert(s) *must* be valid as of the date they
signed the
		subsidiary cert (in fact, I believe proper practice is
that CA's
		shouldn't sign certs that extend beyond their own
validity date).
		The key concept is that user signing's *don't*,
*shouldn't*, *mustn't*
		expire just because the user's cert has expired. At that
point, he can't
		sign anything else until he renews his cert but
everything he signed
		still remains valid forever because it was valid when he
signed it and
		it hasn't changed. 
		Is there a problem here or did I just misunderstand
		-----Original Message-----
		From: Aleksey Sanin [mailto:aleksey at aleksey.com] 
		Sent: Wednesday, October 09, 2002 11:05 AM
		To: Roman Bouchner
		Cc: xmlsec at aleksey.com
		Subject: Re: [xmlsec] Verify signature after certificate
		 From the general security point of view the data are
*not valid* if the
		cert is expired.
		If you really want to do this then you should take a
look at the OpenSSL
		verification function and remove date check. However,
this is
		Roman Bouchner wrote:

			I would like to verify signed data however
signer's certificate has
			already expired. I want only verify data
			If I use function xmlSecDSigValidate, it returns
negative value, so I
			cannot determine if data was changed or not.
			If I change local date it does work, however it
is not right way I
			How I can solve this problems?
			Roman Bouchner
			xmlsec mailing list
			xmlsec at aleksey.com

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