New answers tagged nonce
For this your main concern is making sure that the information is only accessible to the intended recipient; which sounds a lot like what public cryptography / PGP tries to achieve. Unless you want to see if the target non-user has a PGP key published, or if you can get his certificate to encrypt the email with his public key; I think you'd just need to ...
What if instead you sent them a generic link that takes them to a page on your site which then generates a random, temporary url for them and forwards them to the registration page.
I don't think you can. You know nothing about the intended recepient, so whomever receives and uses the link is as likely as anyone else to be the right guy. A common half-measure is to make the link valid only for a short time.
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