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diff --git a/Documentation/SECURITY.md b/Documentation/SECURITY.md new file mode 100644 index 0000000..4db4c24 --- /dev/null +++ b/Documentation/SECURITY.md @@ -0,0 +1,92 @@ +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. |