In pursuit of validation

An emerging standard offers real-time certificate validation, necessary for any high-stakes e-commerce site.

By Ian Poynter
Network World, 02/26/01

Imagine the following scenario: You're operating an online service that trades currency futures. You've built a public-key infrastructure for distributing certificates to traders for authentication. When traders access your secure Web site, their browsers present the certificates you've issued and, based on this information, your site lets them place orders. Because your system supports transactions worth millions of dollars (and other currencies), you've made the certificates valid for six months, rather than the more common one year.

Now suppose one of your client companies dismisses a rogue trader whose certificate is valid for another three months. How can you ensure that the trader's access is immediately terminated? The answer is by using a procedure known as certificate revocation.

When a certificate is revoked, it is declared invalid before it has expired. The primary mechanism for this is the Certificate Revocation List (CRL), a digitally signed, time-stamped blacklist of revoked certificates that haven't expired. The certificate authority, which is the agency that originated the certificate, issues the CRL periodically. But CRLs have some major flaws. For one, they don't operate in real time. Many commercial certificate authorities issue CRLs only once per day at the most. This would certainly not be acceptable in our currency trading example.

Also, simply placing a certificate on the CRL isn't enough. The application requiring authentication must check the CRL each time a certificate is submitted. This is problematic for many reasons. First, a CRL quickly becomes unwieldy. Each certificate authority keeps only one CRL for each root certificate, which is a top-level certificate under which many individual certificates are issued. And the CRL is cumulative – every revoked certificate is added to the CRL and kept there until it expires.

So the CRL grows immense. For instance, CRLs for some root certificates issued by VeriSign, a major commercial certificate authority in Mountain View, Calif., can be a megabyte in size. If a certificate authority uses a single root certificate for each individual certificate it issues, as can be the case when a corporation is its own certificate authority, then all revoked certificates would be listed in one CRL. Hence, compiling, signing, transmitting, publishing and processing a CRL is a time-consuming process that eats CPU power. In the worst case, this can take seconds to complete. The time constraint and resource drain grow exponentially when a Web site must check certificates against multiple certificate authorities, as can happen after a merger when companies use different PKIs.

Another failing is that when a certificate authority updates its CRL, it overwrites the previous file, keeping no historical data.

But a mechanism that lets an application quickly verify a certificate's validity in real time is now available: the Online Certificate Status Protocol (OCSP). Standardized by the Internet Engineering Task Force in June 1999, OCSP became available in select PKI and independent validation products shortly afterward. However, because companies have only recently come to grips with PKIs, first adopters are only now beginning to implement it.

"Revocation has been the 5,000-pound elephant in the living room that everyone has been trying to ignore," says Jim Hewitt, director of consulting and technical services for CertCo, maker of OCSP validation software. "If you are relying on CRLs, you are only as good as your last CRL."

Early users say they can't live without OCSP. Take, for example, TC TrustCenter, a certificate authority in Hamburg, Germany, that services banks and other organizations in the European Union. While the company developed its PKI in-house, it purchased validation tools from KyberPass, in Ottawa.

OCSP adds the real-time status checking that TC TrustCenter's customers are beginning to demand, says Lutz Behnke, product manager for TC TrustCenter. "I think that validation should, must and will come over the next six to nine months, because people will realize that certificates are valid for too long a period. There must be a move to real-time status checks," he says.

Real-time validation via OCSP helps TC TrustCenter's clients immediately terminate online financial services to customers who haven't paid their bills. "Status checks are definitely required if you want to have any chance of stopping customers using a service without paying," Behnke says.

In an OCSP-based system, when a certificate needs validation, the application passes a request to an OCSP responder, such as KyberPass' Validation TrustPlatform or ValiCert's Validation Authority. The responder verifies the certificate, informing the client whether the certificate has been revoked. The responder can be a simple repository for the latest CRLs, but it adds more value when it allows revocation of certificates in real-time from an administrative interface.

The responder sits on the corporate network and answers queries from applications that need to check the validity of certificates. On the back end, the responder may contact a remote responder at the certificate authority's premises, although this isn't a necessity. Responders are sold as part of PKI products or as independent packages.

OCSP lets the power of PKI be available to all applications, especially those that have high-value, high-stake transactions. "As OCSP becomes more prevalent, revocation is something that all PKI-based applications can consider, without the need to process CRLs," CertCo's Hewitt says.

One area with an obvious, immediate need is online trading, he adds, where "the only assurance of each others' identities are various pieces of digital information, including certificates. Validation allows people to trust credentials they are presented with," Hewitt says.

To extend OCSP or not? The Internet Engineering Task Force defines Online Certificate Status Protocol Version 1 in RFC 2560 and is currently working on Version 2. This new version will add the ability to request information on the status of a certificate at some point in the past, a feature Certificate Revocation Lists and the current standard do not support. Click here for more...

Clearly, online status checking should be part of any application that relies on certificates for authentication and authorization, and part of every certificate-based architecture. "As OCSP has become available in applications, customers have realized that online status checking is something they should be doing," Behnke says.

When considering certificate-based authentication and authorization, IT should make sure online status checking is available in the products being evaluated. Right now, that basically means looking for OCSP Version 1 support.

Real-time certificate validation is an enabler for business-to-business and business-to-consumer e-commerce because it lets participants in any transaction be sure they're dealing with up-to-the-minute information on the validity of the certificates being used. Validation in general and OCSP in particular are definitely here to stay.

Poynter is founder and president of Jerboa, Inc., an Internet security consultancy based in Cambridge, Mass. He can be reached at ian@jerboa.com.

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