Validating Certificates

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If you plan to take the Security+ exam, it’s worth knowing the process used in validating certificates. As an example, can you answer this practice test question?

Validating Certificates Question

Q. Your organization requires the use of a PKI and it wants to implement a protocol to validate trust with minimal traffic. Which of the following protocols validates trust by returning short responses, such as “good” or “revoked”?

A. OCSP

B. CRL

C. CA

D. CSR

Answer below.


Validating Certificates Process

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 certificate authority (CA). First, they verify a trusted CA issued the certificate. 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 certificate revocation list (CRL).

The CRL is a version 2 certificate that includes a list of revoked certificates by serial number. For example, the following graphic shows a copy of a CRL.

Validating certificates with a CRL

In general, a CA revokes a certificate any time it does not want anyone to use 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  • Superseded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

The following steps (and graphic) outline the process a client takes when receiving and validating a certificate:

1. The client initiates a session requiring a certificate, such as an HTTPS session.

2. The Web 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.

Validating Certificates process

 

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 via the Internet.

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Validating Certificates with OCSP

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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Validating Certificates Answer

Q. Your organization requires the use of a PKI and it wants to implement a protocol to validate trust with minimal traffic. Which of the following is a protocol that validates trust by returning short responses, such as “good” or “revoked”?

A. OCSP

B. CRL

C. CA

D. CSR

A is the correct answer. Online Certificate Status Protocol (OCSP) validates certificates. Clients send the serial number of the certificate to the Certificate Authority (CA) within the Public Key Infrastructure (PKI) and the CA returns short responses such as good, unknown, or revoked.

A certificate revocation list (CRL) includes a list of revoked certificates listed by serial numbers and can become quite large after a while.

The CA isn’t a protocol.

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

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