Certificate revocation list

Certificate revocation list

In the operation of some cryptosystems, usually public key infrastructures (PKIs), a certificate revocation list (CRL) is a list of certificates (or more specifically, a list of serial numbers for certificates) that have been revoked or are no longer valid, and therefore should not be relied upon.

There are two different states of revocation, defined in RFC 3280:
* Revoked: A certificate is irreversibly revoked if, for example, it is discovered that the certificate authority (CA) had improperly issued a certificate, or if a private-key is thought to have been compromised. Certificates may also be revoked for failure of the identified entity to adhere to policy requirements such as publication of false documents, mis-representation of software behavior, or violation of any other policy specified by the CA operator or its customer. The most common reason for revocation is the user no longer being in sole possession of the private key ("e.g.", the token containing the private key has been lost or stolen).
* Hold: This reversible status can be used to note the temporary invalidity of the certificate ("e.g.", if the user is unsure if the private key has been lost). If, in this example, the private key was found and nobody had access to it, the status could be reinstated, and the certificate is valid again, thus removing the certificate from future CRLs.

There are 10 reasons to revoke a certificate (according to RFC 5280 p69), and they are:

* unspecified (0)

* keyCompromise (1)

* cACompromise (2)

* affiliationChanged (3)

* superseded (4)

* cessationOfOperation (5)

* certificateHold (6)

* -- value 7 is not used

* removeFromCRL (8)

* privilegeWithdrawn (9)

* aACompromise (10)

A CRL is generated and published periodically, after a clearly defined timeframe. A CRL can also be published immediately after a certificate has been revoked. The CRL is always issued by the CA which issues the corresponding certificates. All CRLs have a lifetime during which they are valid; this timeframe is often 24 hours or less. During a CRL's validity period, it may be consulted by a PKI-enabled application to verify a certificate prior to use.

To prevent spoofing or denial-of-service attacks, CRLs usually carry a digital signature associated with the CA by which they are published. To validate a specific CRL prior to relying on it, the certificate of its corresponding CA is needed, which can usually be found in a public directory.

Certificate expiration dates are not a substitute for a CRL. While all expired certificates are considered invalid, not all unexpired certificates are necessarily valid. CRLs or other certificate validation techniques are a necessary part of any properly operated PKI, as mistakes in certificate vetting and key management are expected to occur in real world operations. In a noteworthy example, a certificate for Microsoft was mistakenly issued to an unknown individual, who had successfully posed as Microsoft to the CA contracted to maintain the ActiveX 'publisher certificate' system (VeriSign). Microsoft saw the need to patch their cryptography subsystem so it would check the status of certificates before trusting them. As a short-term fix, a patch was issued for the relevant Microsoft software (most importantly Windows) specifically listing the two certificates in question as "revoked".

The certificates for which a CRL should be maintained are often X.509/public key certificates, as this format is commonly used by PKI schemes.

Problems With All CRLs

Best practices require that wherever and however certificate status is maintained, it must be checked whenever one wants to rely on a certificate. Failing this, a revoked certificate may be incorrectly accepted as valid. This means that to use a PKI effectively, one must have access to current CRLs. This requirement of on-line validation negates one of the original major advantages of PKI over symmetric cryptography protocols, namely that the certificate is "self-authenticating". Symmetric systems such as Kerberos also depend on the existence of on-line services (a key distribution center in the case of Kerberos).

The existence of a CRL implies the need for someone (or some organization) to enforce policy and revoke certificates deemed counter to operational policy. If a certificate is mistakenly revoked, significant problems can arise. As the certificate authority is tasked with enforcing the operational policy for issuing certificates, they typically are responsible for determining if and when revocation is appropriate by interpreting the operational policy.

The necessity of consulting a CRL (or other certificate status service) prior to accepting a certificate raises a potential denial-of-service attack against the PKI. If acceptance of a certificate fails in the absence of an available valid CRL, then no operations depending upon certificate acceptance can take place. This issue exists for Kerberos systems as well, where failure to retrieve a current authentication token will prevent system access. No comprehensive solutions to these problems are known, though there are multiple workarounds for various aspects, some of which have proven acceptable in practice.

An alternative to using CRLs is the certificate validation protocol known as Online Certificate Status Protocol (OCSP). OCSP has the primary benefit of requiring less network bandwidth, enabling real-time and near real-time status checks for high volume or high value operations.

ee also

* Authority revocation list
* Trusted third party
* Web of trust

External links

* RFC 3280
* RFC 5280


Wikimedia Foundation. 2010.

Игры ⚽ Поможем написать курсовую

Look at other dictionaries:

  • Certificate Revocation List — Saltar a navegación, búsqueda Para otros usos de este término, véase CRL (desambiguación). CRL es la sigla de Certificate Revocation List , que significa lista de certificados revocados . En la operación de algunos sistemas criptográficos,… …   Wikipedia Español

  • Certificate Revocation List — Eine Zertifikatsperrliste (engl. Certificate Revocation List – CRL) ist eine Liste, die die Ungültigkeit von Zertifikaten beschreibt. Sie ermöglicht es, festzustellen, ob ein Zertifikat gesperrt oder widerrufen wurde und warum. Zertifikate werden …   Deutsch Wikipedia

  • Certificate Revocation List — Liste de révocation de certificats La liste de révocation de certificats (CRL, certificate revocation list) est la liste des identifiants des certificats qui ont été révoqués ou ne sont plus valables et qui ne sont plus dignes de confiance. Un… …   Wikipédia en Français

  • Certificate revocation list — Liste de révocation de certificats La liste de révocation de certificats (CRL, certificate revocation list) est la liste des identifiants des certificats qui ont été révoqués ou ne sont plus valables et qui ne sont plus dignes de confiance. Un… …   Wikipédia en Français

  • certificate revocation list — atšauktų liudijimų sąrašas statusas T sritis informatika apibrėžtis Nebegaliojančių ↑liudijimų (liudijimų, kurių galiojimo laikas baigėsi arba kurie buvo panaikinti dėl įvairių priežasčių, turėtojo prašymu ir pan.) sąrašas, kurį sukuria ir… …   Enciklopedinis kompiuterijos žodynas

  • Revocation list — In the operation of some cryptosystems, usually public key infrastructures (PKIs), a certificate revocation list (CRL) is a list of certificates (or more specifically, a list of serial numbers for certificates) that have been revoked, and… …   Wikipedia

  • Revocation List — Eine Zertifikatsperrliste (engl. Certificate Revocation List – CRL) ist eine Liste, die die Ungültigkeit von Zertifikaten beschreibt. Sie ermöglicht es, festzustellen, ob ein Zertifikat gesperrt oder widerrufen wurde und warum. Zertifikate werden …   Deutsch Wikipedia

  • Authority revocation list — An Authority revocation list (ARL) is a form of certificate revocation list (CRL) but containing certificates issued to Certifying Authorities, contrary to CRL which contain revoked end entity certificates …   Wikipedia

  • Certificate server — Certificate servers validate, or certify, keys as part of a Public key infrastructure. Keys are strings of text generated from a series of encryption algorithms that allow you to secure communication for a group of users. Many Web servers, such… …   Wikipedia

  • Certificate authority — In cryptography, a certificate authority, or certification authority, (CA) is an entity that issues digital certificates. The digital certificate certifies the ownership of a public key by the named subject of the certificate. This allows others… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”