Cryptography Reference
In-Depth Information
All three versions of X.509 certificates contain the items 1 through 7 listed.
Only version 2 and version 3 certificates may additionally contain items 8 and 9,
whereas only version 3 may contain item 10.
The trust model employed by ITU-T X.509 is hierarchical.
7
This basically
means that a user must define a number of root CAs and corresponding root
certificates (i.e., certificates that are trusted by default) from which trust may extend.
Typically, a root certificate is self-signed, meaning that the root CA has issued its
own certificate (i.e., the subject and issuer are identical). Note that from a theoretical
point of view, self-signed certificates are not particularly useful. Anybody can
claim something and issue a certificate for this claim. Consequently, a self-signed
certificate basically says: “Here is my public key, trust me.”
Having established a number of root CAs and corresponding root certificates,
ausercantrytofinda
certification path
(or
certification chain
) that leads from a root
certificate to a leaf certificate (i.e., a certificate that is issued for a user or system).
Formally speaking, a certification path or chain is defined in a tree or wood of CAs
(root CAs and intermediate CAs) and refers to a sequence of one or more certificates
that lead from a root certificate to a leaf certificate. Each certificate certifies the
public key of its successor. Finally, the leaf certificate is typically issued for a person
or a system. Let's assume that
CA
root
is a root certificate and
B
is an entity for
which a certificate must be verified. In this case, a certification path or chain with
n
intermediate CAs (i.e.,
CA
1
,CA
2
,...,CA
n
) would look as follows:
CA
root
CA
1
CA
1
CA
2
CA
2
CA
3
...
CA
n−
1
CA
n
CA
n
B
The simplest model one may think of is a certification hierarchy representing a
tree with a single root CA. However, more general structures and graphs (including
mutually certifying CAs, cross-certificates, and multiple root CAs) are possible, as
well. A PKI structure or graph among multiple CAs generally provides one or more
certification paths between two entities.
7
Note, however, that ITU-T X.509 does
not
embody a hierarchic trust model. The existence of cross-
certificates, as well as forward and reverse certificates, makes the X.509 model a mesh, analogous
in some ways to PGP's web of trust. The X.509 model is often erroneously characterized as a
hierarchic trust model because it is usually mapped to the directory information tree (DIT), which
is hierarchic, more like name schemes.
