Chapter 36. Public key certificates in Identity Management

This chapter introduces X.509 public key certificates, which are used to authenticate users, hosts and services in Identity Management (IdM). In addition to authentication, X.509 certificates also enable digital signing and encryption to provide privacy, integrity and non-repudiation.

A certificate contains information about

  • the subject that the certificate authenticates
  • who has signed (validated) the certificate, that is the issuer
  • the start and end of the validity of the certificate
  • the valid uses of the certificate
  • the public key of the subject

A message encrypted by the public key can only be decrypted by a corresponding private key. Although a certificate and the public key it includes can be made freely available, a user, host or machine must keep their private key secret.

36.1. Certificate authorities in IdM

Certificate authorities operate in a hierarchy of trust. In an IdM environment with an internal Certificate Authority (CA), all the IdM hosts, users and services trust certificates that have been signed by the CA. Apart from this root CA, IdM supports sub-CAs to which the root CA has granted the ability to sign certificates in their turn. Frequently, the certificates that such sub-CAs are able to sign are certificates of a specific kind, for example VPN certificates.

From the certificate point of view, there is no difference between being signed by a self-signed IdM CA and being signed externally.

The role of the CA is the following:

  • It issues and verifies digital certificates
  • It signs the certificate to prove that the certificate belongs to the user, host or service that presents it
  • In an IdM environment with an internal CA, the CA which is the Certificate Renewal Master and which maintains the Certificate Revocation List (CRL) is the highest authority

36.2. Comparison of certificates and Kerberos

Certificates perform a similar function to that performed by Kerberos tickets. Kerberos is a computer network authentication protocol that works on the basis of tickets to allow nodes communicating over a non-secure network to prove their identity to one another in a secure manner. The following table shows a comparison of Kerberos and X.509 certificates:

Table 36.1. Comparison of certificates and Kerberos













Type of cryptography involved



Default validity

Short (1 day)

Long(2 years)

By default, Kerberos in Identity Management only ensures the identity of the communicating parties.

36.3. The pros and cons of using certificates to authenticate users in IdM

The advantages of using certificates to authenticate users in IdM include:

  • A PIN that protects the private key on a smart card is typically less complex and easier to remember than a regular password.
  • Depending on the device, a private key stored on a smart card cannot be exported. This provides additional security.
  • Smart cards can make logout automatic: IdM can be configured to log out users when they remove the smart card from the reader.
  • Stealing the private key requires actual physical access to a smart card, making smart cards secure against hacking attacks.
  • Smart card authentication is two-factor authentication: it requires both something you have (the card) and something you know (the PIN).
  • Smart cards are more flexible than passwords because they provide the keys that can be used for other purposes, such as encrypting email.
  • Using smart cards use on shared machines that are IdM clients does not typically pose additional configuration problems for system administrators. In fact, smart card authentication is an ideal choice for shared machines.

The disadvantages of using certificates to authenticate users in IdM include:

  • Users might lose or forget to bring their smart card or certificate and be effectively locked out.
  • Mistyping a PIN multiple times might result in a card becoming locked.
  • There is generally an intermediate step between request and authorization by some sort of security officer or approver. In IdM, the security officer or administrator must run the ipa cert-request command.
  • Smart cards and readers tend to be vendor and driver specific: although a lot of readers can be used for different cards, a smart card of a specific vendor might not work in the reader of another vendor or in the type of a reader for which it was not designed.
  • The learning curve to certificates and smart cards might seem daunting to admininstrators with no experience in the area.