Cryptography Reference
In-Depth Information
A public-key certificate is not a proof of identity
. A public-key certificate can be
made available to anyone who needs to use the public key contained in it, so
presenting a public-key certificate is not a proof of identity. In order to identify
someone using their public-key certificate it is necessary to obtain evidence that
they know the private key that corresponds to the public key in the public-key
certificate. This technique is commonly used in entity authentication protocols
based on public-key cryptography. We saw an example in Section 9.4.2 when
we studied the STS protocol.
PUBLIC-KEY CERTIFICATE CREATORS
It should be clear that the creator of a public-key certificate plays an extremely
important role since this creator, by signing the certificate, is providing the
guarantee that all the data relating to the public key (including the name of the
owner) is correct.
A creator of a public-key certificate is referred to as a
certificate authority
(CA).
The certificate authority normally plays three important roles:
Certificate creation
. The CA takes responsibility for ensuring that the infor-
mation in a public-key certificate is correct before creating and signing the
public-key certificate, and then issuing it to the owner.
Certificate revocation
. The CA is responsible for revoking the certificate in the
event that it becomes invalid (see Section 11.2.3).
Certificate trust anchor
. The CA acts as the point of trust for any party relying on
the correctness of the information contained in the public-key certificate. To
fulfil this role, theCAwill need to activelymaintain its profile as a trusted organ-
isation. It may also need to enter into relationships with other organisations in
order to facilitate wider recognition of this trust (see Section 11.3.3).
We will shortly discuss all of these roles in more detail.
RELYING ON A PUBLIC-KEY CERTIFICATE
Recall from Section 11.1.1 that the motivation for public-key certificates is to
provide assurance of purpose of public keys. We thus need to establish exactly
how the use of a public-key certificate provides this assurance.
There are three things that someone who wishes to rely on a public-key
certificate (whom we will call a
relying party
) needs to be able to do in order
to obtain assurance of purpose of the public key:
Trust the CA
. The relying party needs to be able to trust (directly or indirectly) the
CA to have performed its job correctly when creating the certificate. We will
discuss exactly why a relying party might come to trust a CA in Section 11.3.
Verify the signature on the certificate
. The relying party needs to have access
to the verification key of the CA in order to verify the CA's digital signature
on the public-key certificate. If the relying party does not verify this signature
then they have no guarantee that the contents of the public-key certificate
