Cryptography Reference
In-Depth Information
JOINING CA DOMAINS IN PRACTICE
While these techniques for joining CA domains appear to work in theory, it is
less clear how well they work in practice. Issues such as liability start to become
extremely complex when CA domains are joined in such ways. Our discussion of
certificate chains has shown that even verification of a public-key certificate chain
can be a complex process.
One of the highest profile examples of the connected certification model
in practice is the
web-based certification model
implemented by major web
browser manufacturers. This can be thought of as a fairly 'flat' certification
hierarchy, where commercial CAs have their root certificates embedded into a
web browser. Rather than 'cross-certifying' amongst these root CAs, the browser
manufacturer should ensure that CAs whose root certificates they accept have
met certain business practice standards. This allows relying parties to gain
some assurance of purpose of public-key certificates issued by CAs that they do
not have direct business relationships with. It also implements the reputation-
based model, since even relying parties who have no relationship with any
CA can still gain some degree of trust in a public-key certificate, so long as
they trust the web browser manufacturer to have vetted CAs to an appropriate
level.
One of the problems with the web-based certification model is that the trust
linkage between the root CAs is not particularly 'tight'. Arguably, a more serious
problem is that relying parties are left to conduct a significant portion of the
certificate chain verification process, since a web browser cannot automatically
check that all the fields of each certificate are what the relying party is expecting.
Relying parties cannot always be trusted to understand the importance of
conducting these checks. Indeed, even informed relying parties may choose, for
convenience, to omit these checks.
The connected certification model is probably most effective when the CAs
involved have strong relationships, such as when they operate in what we referred
to in Section 11.3.2 as a distributed environment. Some examples of this type of
environment arise in the financial and government sectors.
As we have seen from the discussion in this chapter, there are many complicated
issues to resolve when trying to implement a certificate-based approach to public-
key management. There are a number of alternative approaches that attempt to
resolve these by avoiding the use of public-key certificates. We now discuss two
such approaches.
Note that the use of public-key certificates is more common than either of
these alternative approaches. However, consideration of these approaches not
only indicates that certificates are not the only option for public-key management,
