4.2 KiB
Usage of AES128 due to shorter keysize
2.4
All tomb keys forged using Tomb version 2.3 or preceeding are 256 bits large, which is insufficient to trigger usage of AES-256 encryption in XTS mode, which is the default. Therefore all tombs locked using smaller keys are silently encrypted using AES-128, according to the cryptsetup manual:
"By default a 256 bit key-size is used. Note however that XTS splits the supplied key in half, so to use AES-256 instead of AES-128 you have to set the XTS key-size to 512."
This problem has been noticed and corrected in Tomb version 2.4 where now the 'forge' command will automatically generate 512 bits keys. To switch to AES-256 encrypted tombs the only possibility is to create new keys, new tombs and copy the contents across, since the LUKS formatting occurs when the 'lock' command is issued using a new key. Using 'setkey' to switch key does not suffice to switch to AES-256.
This problem is minor and doesn't seem to heavily affect the security of Tombs created before 2.4 as the cryptographic strenght of AES-128 and AES-256 is comparable; yet it is reasonable to think that larger key sizes resist better to Quantum computing attacks.
Vulnerability to password bruteforcing
Issue affecting keys used in steganography
An important part of Tomb's security model is to make it hard for attackers to enter in possession of both key and data storage: once that happens, bruteforcing the password can be relatively easy.
Protection from bruteforcing is provided by the KDF module that can
be optionally compiled in extras/kdf-keys and installed.
If a key is buried in an image and then the image is stolen, the KDF protection does not works because attackers can bruteforce easily using steghide dictionary attacks: once found the password is the same for the steg crypto and the key crypto.
Users should keep in mind these issues when planning their encryption scheme and, when relying on steganography, keep the image always mixed in the same folder with many more images since that will be the multiplier making it slightly harder to bruteforce their password.
In most cases consider that password bruteforce is a feasible attack
vector on keys. If there are doubts about a key being compromised is
a good practice to change it using the setkey command on a secure
machine, possibly while off-line or in single user mode.
Ending newline in tomb keys
2.2
When used to forge new keys, Tomb version 2.2 incorrectly added a new line ('\n', 0x0A) character at the end of each key's secret sequence before encoding it with GnuPG. This does not affect Tomb regression and compatibility with other Tomb versions as this final newline is ignored in any case, but third party software may have problems. Those writing a software that supports opening Tomb files should always ignore the final newline when present in the secret material obtained after decoding the key with the password.
Versioning and stdin key
1.5
Due to distraction tomb version 1.5 displays its version as 1.4. Also version 1.5 did not work when using -k - to pipe keys from stdin, plus left the encrypted keys laying around in RAM (tmpfs). This was a minor vulnerability fixed in 1.5.1.
Key compatibility broken
1.3 and 1.3.1
Due to an error in the creation and decoding of key files, release versions 1.3 and 1.3.1 cannot open older tombs, plus the tombs created with them will not be opened with older and newer versions of Tomb.
This bug was fixed in commit 551a7839f5
Those who have created tombs with older versions can simply upgrade to release 1.4 (and any other following release) to fix this issue and be able to operate their tombs normally.
Those who have used Tomb 1.3 or 1.3.1 to create new tombs should use Tomb version 1.3.1 (available from https://files.dyne.org/tomb) to open them and then migrate the contents into a new tomb created using the latest stable Tomb version.
This bug was due to a typo in the code which appended a GnuPG status string to the content of keys. All users of Tomb 1.3.* should pay particular attention to this issue, however that release series was out as latest for less than a month.