EDS is an Android encryption app which can be used to create and use encrypted file containers. The paid version has more features than the free, and if you're rooted you can even do more. EDS is compatible with TrueCrypt and VeraCrypt containers as well as several other formats. You do have to pay attention to the encryption and hash methods supported by EDS. Containers created by desktop software (i.e. TrueCrypt or VeraCrypt) are compatible with EDS as long as those methods are common. Not the most pretty or user friendly program you'll run into, but it does the job. The way it works is you open the encrypted container using a password and it then brings up a built-in file explorer showing what is in the container. You work on those files however you like, and then when you close the container everything is automatically buttoned back up. Technically, you asked about encrypting a folder, and what I'm describing is a container file which would sit in a synced folder. BitLocker is not compatible with EDS and I'm not aware of any compatible Android solution. TrueCrypt and VeraCrypt are both available on PCs, Macs and Linux. The author(s) of TrueCrypt stopped development earlier this year, but the downloads are still available and the public audit of TrueCrypt has yet to find any show stoppers which makes it a completely unsatisfactory solution. VeraCrypt is essentially the exact same UI as TrueCrypt, but is still being actively developed. VeraCrypt and TrueCrypt containers are not compatible with each other, and I've found VeraCrypt containers noticeably slower to open on both Windows and Android vs TrueCrypt. I believe there is also an implementation of the Linux LUKS setup on both Android and Windows, but don't know much about it.