libgocryptfs/Documentation/SECURITY.md

GoCryptFS Security

"Security" can be split into "Confidentiality" and "Integrity". The security level gocryptfs provides for each is discussed in the next sections.

Confidentiality

Confidentiality means that information cannot be extracted from the encrypted data unless you know the key.

File Contents

• All file contents (even the last bytes) are encrypted using AES-256-GCM
• This is unbreakable in the foreseeable future. Attacks will focus on cracking the password instead (see section "Master Key Storage").
• Files are segmented into 4096 byte blocks
• Each block gets a fresh random 96 bit IV (none) each time it is written.
• This means that identical blocks can not be identified

File Names

• File names are encrypted using AES-256-CBC with a per-directory IV
• Each directory get a random 128 bit IV on creation
• Files with the same name in different directories are encrypted to different filenames and can not be identified
• File names are padded to multiples of 16 bytes
• This means that the exact length of the name is hidden, only length ranges (1-16 bytes, 17-32 bytes etc.) can be determined from the encrypted files

Metadata

• The size of the file is not hidden. The exact file size can be calculated from the size of the encrypted file.
• File owner, file permissions and timestamps are not hidden.

Integrity

Integrity means that the data cannot be modified in a meaningful way unless you have the key. The opposite of integrity is malleability.

File Contents

• The used encryption, AES-256-GCM, is a variant of authenticated encryption. Each block gets a 128 bit authentication tag (GMAC) appended.
• This means that any modification inside a block will be detected when reading the block and decryption will be aborted. The failure is logged and an I/O error is returned to the user.
• Every file has a header that contains a 16-byte random file id
• Each block uses the file id and its block number as GCM authentication data
• This means the position of the blocks is protected as well. The blocks can not be reordered or copied between different files without causing an decryption error.
• For technical reasons (sparse files), the special "all-zero" block is always seen as a valid block that decrypts to all-zero plaintext.
• This means that whole blocks can be zeroed out

File Names

• File names are only weakly protected against modifications.
• Changing a single byte causes a decode error in most of the cases. The failure is logged and the file is no longer visible in the directory.
• If no decode error is triggered, at least 16 bytes of the filename will be corrupted (randomized).
• However, file names can always be truncated to multiples of 16 bytes.

Metadata

• The file size is not protected against modifications
• However, the block integrity protection limits modifications to block size granularity.
• This means that files can be truncated to multiples of 4096 bytes.
• Ownership, timestamp and permissions are not protected and can be changed as usual.

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 unlock key.

The unlock key is generated from a user password using scrypt. A successful decryption of the master key means that the GMAC authentication passed and the password is correct. The master key is then used to mount the filesystem.