Cryptography Reference
In-Depth Information
case for arguing that a top-level CA's business model involves them being in
such a position of trust that they have no motivation for providing incorrect
information about their public keys. Hence publication of the information on
their own website may suffice.
Note that wide distribution of the public key (certificate) of a CA is also extremely
important, since all relying parties of the public-key certificates signed by the CA
require this information. As an example, CAs who certify public keys that are used
in web-based commercial applications need to have their public-key certificates
incorporated into leading web browsers, or have them certified by a higher-level
CA who has done this.
11.2.3
Key pair change
The second phase of the key lifecycle that is significantly different for public keys
is key change.
REVOCATION OF PUBLIC-KEY CERTIFICATES
We explained in Section 10.6.2 that themain reasonwhy key change is challenging
for public keys is because it is almost impossible (and inmany cases undesirable) to
control who has access to a public key. This makes withdrawing an existing public
key very difficult. This process is normally referred to as
revoking
the public key,
since it involves 'calling back' information that has been released into the public
domain and is now no longer valid. In contrast, establishing a new public key is
relatively easy. Thus our discussion of key change for public keys will focus on
public-key revocation.
We observe that it does not suffice just to establish a new public key, since
we cannot always determine who has access to the old public key and hence we
cannot guarantee that all holders of the old public key will realise that a new public
key has been issued.
Revoking a public key essentially means revoking the public-key certificate.
With this in mind, it is worth observing that there may be situations where a
public-key certificate needs to be revoked and then a new public-key certificate
created for the
same
public-key value. We will assume that revocation of a
public-key certificate only takes place prior to its expiry date. A public-key
certificate should not be relied on by any relying parties if its expiry date has
been exceeded.
REVOCATION TECHNIQUES
Revocation of public-key certificates can only realistically be approached in one
of three ways:
Blacklisting
. This involves maintaining a database that contains serial numbers
of public-key certificates that have been revoked. This type of database is
often referred to as a
certificate revocation list
(or CRL). These CRLs need
