Hi Mike,
The point of an off-line CA (usually the root) is that this is the 'trust
anchor' and often what is distributed to any relying parties as your trust
point. All sub-ordinate CAs can be compromised by compromising the root and
so it deserves the highest protection.
If a sub-ordinate (on-line) CA is compromised, the root can revoke that CA
and re-issue another and your PKI continues to function and the trust anchor
is still valid. If the root was compromised you have to through it all away
and tell any relying parties to remove the root.
Using an HSM just improves the overall security of a CA and if the volume of
certificates to be issued is very low then keeping the CA off-line also
improves security.
The actual architecture employed is usually determined by your security
policy as reflected in your certificate policy. You could use a low
assurance level policy for an on-line / software CA with minimal
registration requirements and a high assurance policy for a CA with rigorous
registration, CA with HSM and subscriber keys stored/created on hardware
tokens.
So really the answer depends on what you want to achieve. If I were a
relying party and audited your PKI and found an on-line root, ambiguous
policy, with software crypto and keys stored in users' profiles I would map
that onto one of my low assurance policies and give the certificates you
issue the due regard :-)
If you are really building a PKI then the architecture comes last after
determining the certificate policy - you build a system to provide the level
of assurance you need!
See RFC 3647
Cheers
Si
On 26/01/07, Mike Wiseman <[EMAIL PROTECTED]> wrote:
Hi,
I see that other CA designs often consist of an offline CA + online
subordinate CA. The
subordinate CA can issue certs quickly due to the online dataexchange
which is very nice
but what about the security risk to the online CA and its signing cert?
Why bother with an
offline CA when unauthorized access to the online CA compromises
everything in that part
of the hierarchy?
Also, does the use of an HSM to store private keys remove the need for the
attached CA to
be offline?
Thanks,
Mike
Mike Wiseman
Computing and Networking Services
University of Toronto
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