Re: [IETF-PKIX] Terminology - Cross-Certification

[Bob Jueneman <BJUENEMAN@xxxxxxxxxx>]

I agree with the thrust of Al's comments, but would like to throw in an
additional comment.

To me, the primary difference between a hierarchical and a peer relationship
between CA's is not the network topology, but rather the degree of
legal/policy control and the (partial) assumption of liability for the
actions of the subordinate CAs.

If I am operating a corporate level CA for XYZ Corp,  I might choose to go
to a public CA such as VeriSign, GTE, etc., in order to have my CA's
certificate signed by a recognized root key.  (Sooner or later, users are
going to start to rebel about the number of root keys that browser vendors
are gratuitously including in their software, and then maybe some order will
arise from the current chaos.)

As a condition for having my corporate certificate listed under say GTE's
master certificate, I would expect that I would have to sign a binding
agreement concerning policy, perhaps including auditing and other oversight
functions which are intended to protect relying parties and the root CA from
undue liability.

I would also expect that a condition of receiving a certificate from GTE,
there would be obligatory, contractual flowdown requirements that would
govern whether or not my top-level corporate CA could issue certificates to
subordinate CAs within my organization, and if so what policies and policy
OIDs would be incorporated.

Now, what does it mean if two top-level CAs cross-certify each other.  Does
it mean that each will accept full financial liability for the other's
errors and omissions, or for the failure of any of their subordinate CA's
particular failures?  Not bloody likely!

Instead, I believe that the primary use of cross-certification will be to
ACCREDIT other CAs, in the sense of an ADVISORY rating service.  Is the
German government going to undertake any liability for what VeriSign does?
No.  But on the other hand, they might well represent to German consumers
that VeriSign is a well-recognized CA, and appears to meet certain accepted
standards of behavior with respect to their internal operations, auditing,
and other controls. Likewise, VeriSign might cross-certify the German
Government's CA, but without taking any responsibility for their actions.
The differences in the legal regimes makes it very unlikely that any legal
obligations will flow between the CAs.

In the case that someone mentioned, The University of Maryland might either
unilaterally or bilaterally certify Purdue, but again that will only imply
that Maryland recognizes Purdue as a bona fide institution that conforms to
a reasonable set of practices, but it would not imply any contractual or
other liability relationship between the two institutions.

In my mind, therefore, cross-certification is very much like a Standard and
Poor's bond rating service, or maybe just a Dun and Bradstreet confirmation
that a company exists and seems to meet the minimum criteria.  If VeriSign
cross-certifies Billy Bob's CA and Bait Shoppe as a legitimate CA when they
are obviously not legitimate, VeriSign might very well expect to get sued.
But if Billy Bob is in fact a reasonably competent CA that only occasionally
fails to exercise due diligence in issuing certificates, relying parties
should expect to go against Billy Bob, and not VeriSign.  VeriSign completed
their responsibility when they ascertained that Billy Bob was properly
incorporated and had at least the trappings of a viable commercial service,
but they should not be expected to cross-insure Billy Bob's operation.

I would therefore disagree slightly with Al's comments in one particular
area, where he says:

>This contrasts with a peer-based architecture, in which one CA
essentially tells another one "I'll accept your certificates if they
have the following properties.  What else you do is your own business; I
won't interfere, but things that interwork with my users must have the
following properties."  The degree of control is much less.

CA's don't accept other CA's certificates, at least normally -- relying
parties accept certificates.  So the degree of control is even less that Al
intimates. On the other hand, if the IT or MIS department of some
organization essentially dictates to its users what kind of certificates are
to be considered acceptable, then cross-certification may be a way of
controlling that. But outside of the military I would expect that kind of
operation to be the exception rather than the norm.

Bob



Robert R. Jueneman
Security Architect
Novell, Inc.
Network Services Division
122 East 1700 South
Provo, UT 84604
801/861-7387
bjueneman@novell.com

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