Using Certificate Revocation Lists
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.
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- Superseded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 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:
- The client initiates a session requiring a certificate, such as an HTTPS session.
- The server responds with a copy of the certificate that includes the public key.
- The client queries the CA for a copy of the CRL.
- The CA responds with a copy of the CRL.
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.
Full Security+ Course
SY0-501 Full Security+ Course
Helping you Pass the First Time
Online access includes all of the content from the
CompTIA Security+: Get Certified Get Ahead: SY0-501 Study Guide
- Introduction
- About the exam (including information on the number of questions, test duration, passing score, types of questions and more. Also includes a listing of the exam objectives)
- 75 question pre-assessment exam
- Mastering Security Basics (full content from Chapter 1 of the study guide including the exam topic review and 15 practice test questions)
- Understanding Identity and Access Management (full content from Chapter 2 of the study guide including the exam topic review and 15 practice test questions)
- Exploring Network Technologies and Tools (full content from Chapter 3 of the study guide including the exam topic review and 15 practice test questions)
- Securing Your Network (full content from Chapter 4 of the study guide including the exam topic review and 15 practice test questions)
- Securing Hosts and Data (full content from Chapter 5 of the study guide including the exam topic review and 15 practice test questions)
- Comparing Threats, Vulnerabilities, and Common Attacks (full content from Chapter 6 of the study guide including the exam topic review and 15 practice test questions)
- Protecting Against Advanced Attacks (full content from Chapter 7 of the study guide including the exam topic review and 15 practice test questions)
- Using Risk Management Tools (full content from Chapter 8 of the study guide including the exam topic review and 15 practice test questions)
- Implementing Controls to Protect Assets (full content from Chapter 9 of the study guide including the exam topic review and 15 practice test questions)
- Understanding Cryptography and PKI (full content from Chapter 10 of the study guide including the exam topic review and 15 practice test questions)
- Implementing Policies to Mitigate Risks (full content from Chapter 11 of the study guide including the exam topic review and 15 practice test questions)
- 75 question post-assessment exam
- Glossary
Get the SY0-501 Full Security+ Course Here
Test your readiness with these quality materials
Random 75-question tests
Random practice tests from the all of the practice test questions in the CompTIA Security+: Get Certified Get Ahead: SY0-501 Study Guide. All questions include explanations so you'll know why the correct answers are correct, and why the incorrect answers are incorrect.3 sets Performance-based Questions
Three new sets of performance-based questions with a total of 30 questions. These new questions use a new testing engine that includes realistic drag and drop, matching, sorting, and fill in the blank questions.
Flashcard Set
- 494 Online Security+ Glossary Flashcards
- 222 Online Security+ Acronyms Flashcards
- 223 Online Security+ Remember This Slide from the popular CompTIA Security+ Get Certified Get Ahead: SY0-501 Study Guide
Audio - SY0-501 Security+ Remember This Audio Files
Learn by Listening. Over one hour and 20 minutes of audio (MP3 downloads.)Audio - SY0-501 Security+ Question and Answer Audio Files
Learn by Listening. Over two hours hour and 53 minutes of audio (MP3 downloads.)Bonus #1
Audio from the end of chapter reviews from each of the chapters in the CompTIA Security+: Get Certified Get Ahead: SY0-501 Study Guide
. Over one hour and 40 minutes of additional audio.Bonus #2
Access to all of the online content that is available for free to anyone that purchases the CompTIA Security+ Get Certified Get Ahead: SY0-501 Study Guide
. This includes labs, extra practice test questions, and supplementary materialsBonus #3
Access the study materials for a total of 60 days because sometimes life happens.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.
