http://bugs.winehq.org/show_bug.cgi?id=17783
--- Comment #11 from Austin English austinenglish@gmail.com 2009-04-18 15:17:28 --- (In reply to comment #7)
I'm fairly sure it's just a matter of installing the two certificates on http://www.verisign.com/support/verisign-intermediate-ca/extended-validation... into whatever wine-gecko is using as a certificate store. One of them is signed by an existing trusted Verisign root certificate, the other is signed by the former, and the Steam certificate is signed by the latter, completing the chain of trust.
I believe crypt32 handles that, which checks your native keychain store. See CRYPT_knownLocations in crypt32/rootstore.c