Encrypting a signal, sending with an email

[Cardinal]

If I wanted to send a signal to my buddy that I am the actual person sending the email, is there a way to encrypt some kind of verification signal or serial number that rides with the email? This would also need to be able to be recognized by his computer automatically.

I know this is asking a lot, but I'm not looking for graphic details. I'd just like to know if it's possible, and the broad outlines if possible. Thanks.

[buckgully]

You can use many public key encryption systems (Like PGP, GPG, etc.) to digtially sign an email using your private key, so when he gets it he can (using compatible software) verify that it was you that sent it using your public key.

Here's the GPG handbook entry on doing just that:
http://www.gnupg.org/gph/en/manual.html#AEN153

That's if you want the message to be plain text. If you're going through all that trouble in the first place, you may as well encrypt and sign it.

[CurtC]

Then you have the issue of: how does he know that he has your public key, and not that of someone pretending to be you? You have to give him your public key yourself, or possibly keep it on a trusted server that's available to him.