You cannot refuse to hand over data when given a warrant, but if the data is encrypted (and you do not possess the key to decrypt it) the information is essentially worthless and they won't throw you in jail for refusing to comply.

Fun fact: Lavabit turned over an archive of encrypted emails from Snowdens account when he was first contacted by the feds. Those emails are still perfectly secure, because Snowden is the only person who knows the password required to decrypt the mail files. The reason the feds demanded the SSL keys (a few weeks after he first turned over the emails) is because they could not decrypt the mail files and wanted to try and capture Snowdens password the next time he accessed his account.