The author of Tarsnap, Colin Percival, can be contacted...

• ... by email to cperciva@tarsnap.com. (If desired, emails can be GPG-encrypted using Colin Percival's GPG key. If you send GPG-encrypted email, please include your public key so that we can send an encrypted reply.)
• ... on twitter as @cperciva.
• ... on EFnet as cperciva.

Actually, he's cperciva pretty much everywhere; but those three are generally the best ways to contact him.

Feel free to send questions, comments, suggestions, and bug reports — Tarsnap development is driven largely based on feedback from users, and the more input you can provide, the better.