Digital signing and encrypting is available in Netscape Communicator, NS 6+, and Outlook Express. How about Opera's M2? I can't find anything relating to digital signing or digital certificate in Opera's website.
Also, does M2 available in Mac platform?
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.4) Gecko/20030624 Netscape/7.1 (ax) (CK-SillyDog)