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Diffstat (limited to 'Documentation/SECURITY.md')
| -rw-r--r-- | Documentation/SECURITY.md | 102 |
1 files changed, 26 insertions, 76 deletions
diff --git a/Documentation/SECURITY.md b/Documentation/SECURITY.md index 4db4c24..e9e1015 100644 --- a/Documentation/SECURITY.md +++ b/Documentation/SECURITY.md @@ -1,92 +1,42 @@ 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 +Master Key Storage +------------------ -### Metadata +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 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. +The KEK is generated from the user password using `scrypt`. -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 +------------- -### File Contents +All file contents are encrypted using AES-256-GCM. -* 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 +Files are segmented into 4KB blocks. Each block gets a fresh random +96 bit IV each time it is modified. A 128-bit authentication tag (GHASH) +protects each block from modifications. -### File Names +Each file has a header containing a random 128-bit file ID. The +file ID and the block number are mixed into the GHASH as +*additional authenticated data*. The prevents blocks from being copied +between or within files. -* 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 +To support sparse files, all-zero blocks are accepted and passed through +unchanged. -* 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. +File Names +---------- -Master Key Storage ------------------- +Every directory gets a 128-bit directory IV that is stored in each +directory as `gocryptfs.diriv`. -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*. +File names are encrypted using AES-256-CBC with the directory IV. -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. + |