Crypto Undertaker

the Crypto Undertaker: Tomb

CyberPunk » Cryptography

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    Tomb aims to be a free and open source system for easy encryption and backup of personal files, written in code that is easy to review and links shared GNU/Linux components.

At present, Tomb consists of a simple shell script (Zsh) using standard filesystem tools (GNU) and the cryptographic API of the Linux kernel (cryptsetup and LUKS). Tomb can also produce machine parsable output to facilitate its use inside graphical applications.

How does it work?

To create a Tomb, do:

 $ tomb dig -s 100 secret.tomb
 $ tomb forge secret.tomb.key
 $ tomb lock secret.tomb -k secret.tomb.key

To open it, do

 $ tomb open secret.tomb -k secret.tomb.key

and after you are done

 $ tomb close

or if you are in a hurry

 $ tomb slam all

For the instructions on how to get started using Tomb, see INSTALL.

  Syntax: tomb [options] command [arguments]

  Commands:

   // Creation:
   dig     create a new empty TOMB file of size -s in MiB
   forge   create a new KEY file and set its password
   lock    installs a lock on a TOMB to use it with KEY

   // Operations on tombs:
   open    open an existing TOMB (-k KEY file or - for stdin)
   index   update the search indexes of tombs
   search  looks for filenames matching text patterns
   list    list of open TOMBs and information on them
   close   close a specific TOMB (or 'all')
   slam    slam a TOMB killing all programs using it
   resize  resize a TOMB to a new size -s (can only grow)

   // Operations on keys:
   passwd  change the password of a KEY (needs old pass)
   setkey  change the KEY locking a TOMB (needs old key and pass)

   // Backup on paper:
   engrave makes a QR code of a KEY to be saved on paper

   // Steganography:
   bury    hide a KEY inside a JPEG image (for use with -k)
   exhume  extract a KEY from a JPEG image (prints to stdout)

  Options:

   -s     size of the tomb file when creating/resizing one (in MiB)
   -k     path to the key to be used ('-k -' to read from stdin)
   -n     don't process the hooks found in tomb
   -o     options passed to commands: open, lock, forge (see man)
   -f     force operation (i.e. even if swap is active)
   -g     use a GnuPG key to encrypt a tomb key
   -r     provide GnuPG recipients (separated by coma)
   -R     provide GnuPG hidden recipients (separated by coma)
   --kdf  forge keys armored against dictionary attacks

   -h     print this help
   -v     print version, license and list of available ciphers
   -q     run quietly without printing informations
   -D     print debugging information at runtime

What is this for, exactly?

    This tool can be used to dig .tomb files (LUKS volumes), forge keys protected by a password (GnuPG symmetric encryption) and use the keys to lock the tombs. Tombs are like single files whose contents are inaccessible in the absence of the key they were locked with and its password.

Once open, the tombs are just like normal folders and can contain different files, plus they offer advanced functionalities like bind and execution hooks and fast search, or they can be slammed close even if busy. Keys can be stored on separate media like USB sticks, NFC, or bluetooth devices to make the transport of data safer: one always needs both the tomb and the key, plus its password, to access it.

The tomb script takes care of several details to improve user’s behaviour and the security of tombs in everyday usage: secures the typing of passwords from keyloggers, facilitates hiding keys inside images, indexes and search a tomb’s contents, lists open tombs and selectively closes them, warns the user about free space and last time usage, etc.

How secure is this?

    Death is the only sure thing in life. That said, Tomb is a pretty secure tool especially because it is kept minimal, its source is always open to review (even when installed) and its code is easy to read with a bit of shell script knowledge.

All encryption tools being used in Tomb are included as default in many GNU/Linux operating systems and therefore are regularly peer reviewed: we don’t add anything else to them really, just a layer of usability. The file KNOWN_BUGS.md contains some notes on known vulnerabilities and threat model analysis.

In absence or malfunction of the Tomb script it is always possible to access the contents of a Tomb only using a dm-crypt enabled Linux kernel, cryptsetup, GnuPG and any shell interpreter issuing the following commands as root:

lo=$(losetup -f)
losetup -f secret.tomb
pass="$(gpg -d secret.key)"
echo -n -e "$pass" | cryptsetup --key-file - luksOpen $lo secret
mount /dev/mapper/secret /mnt
unset pass

One can change the last argument /mnt to where the Tomb has to be mounted and made accessible. To close the tomb then use:

umount /mnt
cryptsetup luksClose /dev/mapper/secret

Stage of development

    Tomb is an evolution of the ‘mknest’ tool developed for the dyne:bolic 100% Free GNU/Linux distribution in 2001: its ‘nesting’ mechanism allowed the liveCD users to encrypt and make persistent home directories. Since then the same shell routines kept being maintained and used for dyne:bolic until 2007, when they were ported to work on more GNU/Linux distributions.

As of today, Tomb is a very stable tool also used in mission critical situations by a number of activists in dangerous zones. It has been reviewed by forensics analysts and it can be considered to be safe for military grade use where the integrity of information stored depends on the user’s behaviour and the strength of a standard AES-256 (XTS plain) encryption algorithm.

Compatibility

Tomb can be used in conjunction with some other software applications, some are developed by Dyne.org, but some also by third parties.

• Secrets is a software that can be operated on-line and on-site to split a Tomb key in shares to be distributed to peers: some of them have to agree to combine back the shares in order to retrieve the key.
• zuluCrypt is a graphical application to manage various types of encrypted volumes on GNU/Linux, among them also Tombs, written in C++.
• Mausoleum is a graphical interface to facilitate the creation and management of tombs, written in Python.
• pass-tomb is a console based wrapper of the excellent password keeping program pass that helps to keep the whole tree of password encrypted inside a tomb. It is written in Bash.

If you are writing a project supporting tomb volumes or wrapping tomb, let us know!

Compliancy

     Tomb qualifies as sound for use on information rated as “top secret” when used on an underlying stack of carefully reviewed hardware (random number generator and other components) and software (Linux kernel build, crypto modules, device manager, compiler used to built, shell interpreter and packaged dependencies).

Tomb volumes are fully compliant with the FIPS 197 advanced encryption standard published by NIST and with the following industry standards:

• Information technology — Security techniques — Encryption algorithms
  • ISO/IEC 18033-1:2015 — Part 1: General
  • ISO/IEC 18033-3:2010 — Part 3: Block ciphers

Tomb implementation is known to address at least partially issues raised in:

• Information technology — Security techniques — Key management
  • ISO/IEC 11770-1:2010 — Part 1: Framework
  • ISO/IEC 11770-2:2008 — Part 2: Mechanisms using symmetric techniques
• ISO/IEC 27005:2011 Information technology — Security techniques — Information security risk management
• ISO/IEC 24759:2014 Information technology — Security techniques — Test requirements for cryptographic modules

Any help on further verification of compliancy is very welcome, as the access to ISO/IEC document is limited due to its expensive nature.