openssl cms verification date

[Tomas Mraz]

On Thu, 2024-02-08 at 14:13 +0100, François Legal wrote:
> Le Jeudi, Février 08, 2024 11:46 CET, Tomas Mraz <tomas at openssl.org>
> a écrit: 
>  
> > On Thu, 2024-02-08 at 11:37 +0100, François Legal wrote:
> > > Hello,
> > > 
> > > I'm new to this list.
> > > 
> > > I'm using pkcs7 packages to embbed firmware, to procide
> > > authenticity
> > > verification before doing firmware upgrades.
> > > 
> > > I use the openssl cms command for verification purpose, but face
> > > the
> > > following problem :
> > > when doing verify, openssl cms -verify does check whether the
> > > signing
> > > certificate is valid today, not whether or not it was still valid
> > > when the package got signed.
> > > 
> > > I saw the -attime option to specify the verification date, but
> > > found
> > > no easy way to fetch the signature date from the package for each
> > > signature.
> > > 
> > > So I was wondering if it was the intended function that the
> > > certificate validity verification was made at the verification
> > > date
> > > and not the signature date.
> > 
> > Yes, this was certainly intentional. I could envision that a new
> > option
> > could be added to the cms command that would verify the signature
> > at
> > the date when the signature was made. However please note that
> > without
> > some kind of assurance that the signature was really made at the
> > time
> > that is recorded in the message, the signature could have been done
> > with a key that was already expired anyway. This assurance is done
> > usually by timestamping via a trusted timestamping authority but
> > there
> > might be other means.
> > 
> 
> Sure I get the point. You can't really be sure that the signature was
> made at the clained time as the signerInfo structure is not signed
> itself. Could you please comment however on why verifying the
> validity of the certificate at the verification date is better in
> that matter.

The expiration of the certificate means - albeit in an extreme case -
that the private key is no longer trusted - i.e., it can do anything
like malicious signatures of crafted data, etc. So unless you have
trusted timestamps on the signed document, after the certificate
expiration, the signature has no validity. Of course this is a little
bit extreme view but...

> I already have a patch to provide for verifying the signature at
> signature time. Shall I send a pull request ?

Yes, sure. I assume you'll have to sign a CLA unless you already did it
before.

> François
> 
> 
> > -- 
> > Tomáš Mráz, OpenSSL
> > 
> 

-- 
Tomáš Mráz, OpenSSL