There are several methods that allow this. I disagree with all of them, since I believe transparency is more important than anything else in an electoral process, and even the simplest is beyond comprehension of about 1/4 of voters, and beyond the effort to comprehend of about 3/4.

Anonymity is not the hard part---the challenge lies in verifying that an account from which a vote was cast is legitimate. But it is possible.

Alongside secrecy, I wonder how you prevent people from selling their votes in this system. It could be a much bigger problem than it currently is because of the ease with which someone could do it.