I recently took the new CAcert assurer test. Afterwards, one has to send a S/MIME-signed mail to get a PDF-certificate.

Having the same problem like Bernd, the answer came in an RC2-encrypted S/MIME-mail. I'm using kmail, kmail uses gpgsm for S/MIME and that doesn't support RC2.

While this opens some obvious questions (Why is anyone in the world still using RC2? Why is anyone using S/MIME at all?), I was able to circumvent that without the hassle of installing thunderbird (which was Bernd's solution).

openssl supports RC2 and can handle S/MIME. And this did the trick:

openssl smime -decrypt -in [full mail] -inkey sslclientcert.key

It needed the full mail, which took me a while, because I first tried to only decrypt the attachment.