Using Certificate Revocation Lists

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If you’re planning to take the Security+ exam, you should have a basic understanding of PKI models and certificate management. This includes using Certificate Revocation Lists, the Online Certificate Status Protocol and Certificate signing requests.

For example, can you answer this question?

Q. Your organization is planning to implement an internal PKI. What is required to ensure users can validate certificates?

A. An intermediate CA

B. CSR

C. Wildcard certificates

D. CRL

More, do you know why the correct answer is correct and the incorrect answers are incorrect? The answer and explanation is available at the end of this post.

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Revoking Certificates

Normally, certificates expire based on the Valid From and Valid To dates. However, there are some instances when a CA will revoke a certificate before it expires.

For example, if a private key is publicly available, the key pair is compromised. It no longer provides adequate security because the private key is no longer private. Similarly, if the CA itself is compromised through a security breach, certificates issued by the CA may be compromised, so the CA can revoke certificates.

In general, any time a CA does not want anyone to use a certificate, the CA revokes it. Although the most common reasons are due to compromise of a key or the CA, there are others. A CA can use any of the following reasons when revoking a certificate:

  • Key compromise
  • CA compromise
  • Change of affiliation
  • Superseded
  • Cease of operation
  • Certificate hold

CAs use certificate revocation lists (CRL, pronounced “crill”) to revoke a certificate. The CRL is a version 2 certificate that includes a list of revoked certificates by serial number. For example, the figure shows a copy of a CRL.

Certificate Revocation Lists

Certificate revocation list

Validating Certificates

Before clients use a certificate, they first verify it is valid. The first check is to ensure that it isn’t expired. If the certificate is expired, the computer system typically gives the user an error indicating the certificate is not valid.

Clients also validate certificates through the CA. First, they verify that the certificate was issued by a trusted CA. Next, they query the CA to verify the CA hasn’t revoked the certificate. A common method of validating a certificate is by requesting a copy of the CRL. The following steps outline the process:

  1. The client initiates a session requiring a certificate, such as an HTTPS session.
  2. The server responds with a copy of the certificate that includes the public key.
  3. The client queries the CA for a copy of the CRL.
  4. The CA responds with a copy of the CRL.

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The client then checks the serial number of the certificate against the list of serial numbers in the CRL. If the certificate is revoked for any reason, the application gives an error message to the user.

Notice that the CA and the web server are both protected within demilitarized zones (DMZs). A DMZ is a buffered zone between an internal network and the Internet. The DMZ provides a layer of protection for Internet-facing servers, but servers in the DMZ are available on the Internet.

Another method of validating a certificate is with the Online Certificate Status Protocol (OCSP). OCSP allows the client to query the CA with the serial number of the certificate. The CA then responds with an answer of “good,” “revoked,” or “unknown.” A response of “unknown” could indicate the certificate is a forgery. OCSP can be more efficient, especially if the CRL includes a large number of revoked certificates.

Remember this

CAs revoke certificates for several reasons such as when the private key is compromised or the CA is compromised. An internal CA can revoke a certificate when an employee leaves. The certificate revocation list (CRL) includes a list of revoked certificates and is publicly available. An alternative to using a CRL is the Online Certificate Status Protocol (OCSP), which returns answers such as good, revoked, or unknown.


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Q. Your organization is planning to implement an internal PKI. What is required to ensure users can validate certificates?

A. An intermediate CA

B. CSR

C. Wildcard certificates

D. CRL

Answer is D. A certificate revocation list (CRL) includes a list of revoked certificates and it allows users to validate certificates.

Any CA can issue a CRL, so an intermediate CA is not needed.

Users request certificates with a certificate signing request (CSR).

Wildcard certificates reduce the administrative burden for certificates, but do not have anything to do with validating certificates.

See Chapter 10 of the CompTIA Security+: Get Certified Get Ahead: SY0-401 Study Guide for more information on PKI components and cryptography.

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