gocryptfs Cryptography

gocryptfs builts upon well-known cryptographic primitives: scrypt for key derivation, AES-GCM for file content encryption and, as a world's first for encrypted filesystems, EME wide-block encryption for file name encryption.

This page describes forward mode, the default mode of operation, where the files are stored encrypted on disk and the mounted filesystem provides a plaintext view.

Master Key Storage

The master key is used to perform content and file name encryption. It is stored in gocryptfs.conf, encrypted with AES-256-GCM using the Key Encryption Key (KEK). The KEK is generated from the user password using scrypt.

When mounting a filesystem, the user is prompted for the password and the master key is decrypted:

Derived Keys

Since gocryptfs v1.3, separate keys are derived from the master key for file content and file name encryption. HKDF-SHA256 is used for the derivation (source code: ref1 ref2).

File Contents

All file contents are encrypted using AES-256-GCM (Galois/Counter Mode).

Files are segmented into 4KiB blocks. Each block gets a fresh random 128 bit IV each time it is modified. A 128-bit authentication tag (GHASH) protects each block from modifications.

Each file has a header containing a random 128-bit file ID. The file ID and the block number are concatenated (source code ref) and mixed into the GHASH as additional authenticated data. The prevents blocks from being copied between or within files.

To support sparse files, all-zero blocks are accepted and passed through unchanged.

File Names

Every directory gets a 128-bit directory IV that is stored in each directory as gocryptfs.diriv.

File names are encrypted using AES-256-EME (ECB-Mix-ECB wide-block encryption, see github.com/rfjakob/eme for details) with the directory IV as initialization vector. EME fixes the prefix leak that occours with CBC encryption.

The Base64 encoding limits the usable filename length to 176 characters. Filenames that are longer than that (longer than 255 characters in Base64-encoded form) use long file name handling, introduced in gocryptfs v0.9.

Long File Name Handling

If the Base64-encoded encrypted name is longer than 255 characters, it cannot be used as the file name on disk, as Linux filesystems do not allow names longer than that.

Instead, the encrypted name is hashed, and the file content is stored in gocryptfs.longname.[hash]. The long file name is stored in a support file, gocryptfs.longname.[hash].name.

Example directory listing containing an 1 MiB encrypted file with a long name:

    Size Name
      16 gocryptfs.diriv
 1056786 gocryptfs.longname.nONaEDDZOrwtQdXPH1SxSFkPtOc8srIyB82ZuduqG10
     299 gocryptfs.longname.nONaEDDZOrwtQdXPH1SxSFkPtOc8srIyB82ZuduqG10.name

This method for storing long file names has zero performance impact for filenames that are <= 176 characters, incurs no extra disk accesses for opening a file with a long name, and just one extra file read for each long-name file on readdir(1).

Because the hash is only taken from the encrypted file name that is public anyway, there is no security penalty for using long names.