Having read this nice article, I decided I also want a digital signature certified by a certification authority (CA). Most of them you have to pay for, but Thawte is offering personal email signatures for free.
The process of applying for one is straightforward enough. You first register with Thawte, provide them with details like date of birth etc. and choose a password. Choosing a strong password is important, as you will be reminded anyway. Now you need to request the certificate and that’s where things get a wee bit complicated for Mac users. First, you should use Firefox or Mozilla for dealing with thawte.com, from now on. When you request a certificate, choose a netscape one and then follow the instructions. Once it’s ready an email will tell you so. Now, on the website in “view certificate status, click on the certificate. A new page will load, at the bottom of which you can press fetch. Firefox won’t appear to have done anything. Then in Firefox Preferences, go to Certificates, find your certificate, export it to a file and double click on it in Finder. This will import it into your Macs keychain manager. Now delete the file and the certificate from firefox.
And that’s it, you can now use Mail to sign your emails. For more details see this lengthy tutorial