Section 97.113(a)(4) of the Amateur Service rules prohibits the transmission of “effectively encrypted or encoded messages, including messages that cannot be readily decoded over-the-air for true meaning.”

What _exactly_ that means is, of course, up for debate. Some say you can use encryption as long as the key is publicly available and obvious (i.e. my key is "password"). There's been a bunch of stuff happening in the last year around Winlink. Devil's in the details.