The information is encrypted by the client program using the AES-256 symmetric encryption algorithm. The 256-bit encryption key (as well as another 256-bit HMAC key) is generated by running the PBKDF2 algorithm on the pass phrase as described in RFC 2898. Each file is divided into 2K blocks. Each block is compressed using zlib deflate and is encrypted using AES-256 in CTR mode (each block uses a different crypto-random nonce), and an HMAC (using SHA-256) is appended (to guarantee integrity upon restore). The data is fully compressed and encrypted before it ever enters the network. Encrypting the data on the client is more secure, and it makes the server more efficient and scalable.

Filenames are currently not encrypted. Encryption of file and directory names is a planned future enhancement, and can be done by upgrading the client software – no changes need to be made by the server.