Mac users began reporting problems Tuesday when they tried to revoke digital certificates issued by DigiNotar, a Dutch company whose servers were compromised last month and used to issue fraudulent digital certificates. Mac users revoked the certificates on their computers, but still saw some sites that used those certificates being marked as trustworthy.

Digital certificates are an important part of the way the Internet works, and are essential whenever two computers try to connect using the HTTPS protocol. The problem is that Apple's operating system does not allow users to revoke DigiNotar certificates properly, and marks some websites as trustworthy when it shouldn't.

Seth Bromberger noticed the issue Tuesday afternoon. After reading a news report about DigiNotar being compromised, he decided to take matters into his own hands and revoke DigiNotar's certificates on his Mac, using Apple's Keychain software. That meant that any time he tried to visit a site signed by DigiNotar or one of its intermediaries, he should have received a warning.

He didn't. A visit to DigiNotar's website soon confirmed that all kinds of HTTPS material on the page that should have been marked by his browser as untrusted looked exactly as it had before he'd revoked the certificate. "I just wanted to validate that the solution that was proposed fixed the problem. And it didn't."

Most users don't revoke digital certificates themselves; they let the browser makers handle it. Chrome, Firefox and Internet Explorer have all blocked DigiNotar certificates, but Apple hasn't said what it plans to do with its Safari browser. That means that, for now, Mac Safari users will have a hard time solving the problem.