[openssl-users] problems with s_client recognizing revoked intermediate/subordinate ca

[Viktor Dukhovni]

On Thu, Mar 10, 2016 at 11:29:12PM +0100, Jakob Bohm wrote:

> >This is changing in OpenSSL 1.1.0, and may yet change in a future
> >OpenSSL 1.0.2 update.  Only the trust-anchor (top-most certificate
> >from the trust-store) is not checked for expiration or revocation
> >in OpenSSL 1.1.0.
> >
> >Intermediate certificates are checked, whether they are from the
> >trust-store, or acquired from the peer.  To get previous behaviour,
> >one needs to set the X509_V_FLAG_PARTIAL_CHAIN flag so that the
> >first certificate found in the trust store becomes the trust-anchor,
> >and chain construction stops there.
> >
> >Another way (in OpenSSL 1.1.0) to get an intermediate certificate
> >to terminate the chain is to decorate it with explicit auxiliary
> >trust EKUs via the "-trustout" and "-addtrust" options of "openssl
> >x509", and then add the decorated certificate to the trust store.
>
> This will cause a lot of grief when both OpenSSL versions
> are used on the same system, (since 1.1.0 is not a drop in
> replacement for OpenSSL 1.0.x), with the same default trust
> store directory.

I am not sure why there would be a lot of grief, by default the
partial chain flag is not set, and intermediate certificates are
not decorated with trust settings, as a result of which the only
user-visible change is that intermediate certificates that come
from the trust store will be checked in the same way as they would
had they arrived from the peer.  This fixes inconsistency of
behaviour.

The behaviour is *only* different if:

  * The trust store actually includes some intermediate certificates.
    Usually it contains just the root CAs.

AND either:

  * The application requests partial chains or the intermediate
    certificates have auxiliary trust EKUs.

    OR

  * Some of the trust-store intermediate CAs are expired or
    invalid.  (Don't do that).

> It would have been much better to use a separate directory
> for untrusted chain-building intermediary certificates, just
> like some other libraries do.

Intermediate certificates in the trust store are effectively
untrusted, unless decorated with auxiliary trust settings.  Only
the "partial chain" flag makes them trusted, applications should
not generally set that flag.

-- 
	Viktor.