Encrypting External Drive

I need help figuring out if what I'd like to achieve is techically feasable. I need to encrypt my offsite backup drive. The problem I forsee is that this might be tricky since I don't want my master drive encryped. Is there a software out there that will encrypt my backup drive without interfearing with the backup software's (ChronoSync) ability to make normal backups from a master, non-encrypted drive?

Thanks!