SFTPGo supports data at-rest encryption via its `cryptfs` virtual file system, in this mode SFTPGo transparently encrypts and decrypts data (to/from the disk) on-the-fly during uploads and/or downloads, making sure that the files at-rest on the server-side are always encrypted.
So, because of the way it works, as described here above, when you set up an encrypted filesystem for a user you need to make sure it points to an empty path/directory (that has no files in it). Otherwise, it would try to decrypt existing files that are not encrypted in the first place and fail.
The SFTPGo's `cryptfs` is a tiny wrapper around [sio](https://github.com/minio/sio) therefore data is encrypted and authenticated using `AES-256-GCM` or `ChaCha20-Poly1305`. AES-GCM will be used if the CPU provides hardware support for it.
The only required configuration parameter is a `passphrase`, each file will be encrypted using an unique, randomly generated secret key derived from the given passphrase using the HMAC-based Extract-and-Expand Key Derivation Function (HKDF) as defined in [RFC 5869](http://tools.ietf.org/html/rfc5869). It is important to note that the per-object encryption key is never stored anywhere: it is derived from your `passphrase` and a randomly generated initialization vector just before encryption/decryption. The initialization vector is stored with the file.
The passphrase is stored encrypted itself according to your [KMS configuration](./kms.md) and is required to decrypt any file encrypted using an encryption key derived from it.
The encrypted filesystem has some limitations compared to the local, unencrypted, one:
- Upload resume is not supported.
- Opening a file for both reading and writing at the same time is not supported and so clients that require advanced filesystem-like features such as `sshfs` are not supported too.
- Truncate is not supported.
- System commands such as `git` or `rsync` are not supported: they will store data unencrypted.
- Virtual folders are not implemented for now, if you are interested in this feature, please consider submitting a well written pull request (fully covered by test cases) or sponsoring this development. We could add a filesystem configuration to each virtual folder so we can mount encrypted or cloud backends as subfolders for local filesystems and vice versa.
