We recently fielded a high volume of questions about how to obtain a SSL Certificate for a .onion address. This blog post should provide basic answers to queries regarding .onion certificates.
As of September, .onion is recognized as a special-use domain by the IESG, which means they can be secured with SSL Certificates. (Previously .onion was considered an internal name.) Publicly trusted certificates authenticate organizations to Tor users and are an essential part to fighting phishing and MITM attacks. The CA/B Forum outlined guidelines for vetting .onion names, which you can read here.
The Tor project is dedicated to helping users browse the web anonymously. However, getting a SSL Certificate to identify yourself (or an organization) to users is not about anonymity. This makes ordering a SSL Certificate for a .onion site a complicated process, which is why DigiCert adheres to the CA/B Forum guidelines for .onion certificates. When ordering a .onion certificate, make sure to remember the following:
To purchase a certificate for your .onion site, simply order an EV SSL or EV Multi-Domain SSL Certificate and fill out the order form.