CRL stands for Certificate Revocation List.
A Certificate Revocation List (CRL) is a vital security component within a Public Key Infrastructure (PKI). It is a list maintained by a certification authority (CA) that specifies digital certificates that the CA has issued but subsequently revoked before their originally planned expiration date.
Understanding Certificate Revocation Lists
Digital certificates are used to verify the identity of individuals, websites, or devices in online communications. They have a defined validity period. However, circumstances can arise where a certificate needs to be invalidated prematurely. This is where a CRL comes into play.
- Purpose: The primary purpose of a CRL is to provide a mechanism for CAs to communicate to relying parties (like web browsers or applications) which certificates are no longer trustworthy and should not be accepted.
- Maintenance: CAs regularly update and publish CRLs, which can be accessed by clients to check the revocation status of certificates.
Why Are Certificates Revoked?
Certificates can be revoked for several critical reasons to maintain the security and integrity of a system:
- Compromised Private Key: If the private key associated with a certificate is stolen or compromised, the certificate must be revoked immediately to prevent unauthorized use.
- Change in Information: If information contained within the certificate, such as the organization's name or the user's affiliation, changes or becomes inaccurate.
- Loss of Trust: If the certificate holder no longer meets the requirements for trustworthiness or has violated policies.
- Superseded Certificate: When a new certificate is issued to replace an existing one, the old certificate may be revoked.
- Key Usage Cessation: If the certificate holder no longer needs or uses the certificate for its intended purpose.
How CRLs Work
When a client (e.g., a web browser) receives a digital certificate during an SSL/TLS handshake or other secure communication, it performs several checks to validate the certificate's authenticity and trustworthiness. One of these crucial checks involves consulting a CRL.
- Certificate Reception: A client receives a digital certificate from a server or entity.
- CA Identification: The client identifies the issuing Certification Authority (CA) from the certificate.
- CRL Download (or Cache Check): The client typically attempts to download the latest CRL from a distribution point specified within the certificate itself, or it may check its local cache if a recent version is available.
- Revocation Check: The client searches the downloaded CRL for the serial number of the certificate in question.
- Decision:
- If the certificate's serial number is found on the CRL, the client determines that the certificate has been revoked and should not be trusted. The connection or operation is then typically terminated.
- If the certificate's serial number is not found on the CRL, and all other validation checks pass, the certificate is considered valid, and the secure communication proceeds.
| Feature | Description |
|---|---|
| Purpose | Invalidate compromised or no longer valid digital certificates. |
| Issuer | Certification Authority (CA) |
| Content | List of revoked certificate serial numbers and revocation dates. |
| Distribution | Published periodically by CAs at designated distribution points (e.g., HTTP, LDAP URLs). |
| Alternative | Online Certificate Status Protocol (OCSP), which offers real-time status checks for individual certificates. |
While CRLs are effective, they can sometimes lead to latency issues or be slightly out of date depending on the publication schedule. This has led to the development and increased adoption of alternatives like the Online Certificate Status Protocol (OCSP), which provides real-time status checks for individual certificates. However, CRLs remain a foundational element in many PKI deployments.
For more information on Certificate Revocation Lists, you can refer to resources like TechTarget's definition: What Is a Certificate Revocation List (CRL) and How Is It Used?
