Ubuntu Manpage: Tomb - the Crypto Undertaker

NAME

       Tomb - the Crypto Undertaker

SYNOPSIS

       tomb [options] command [arguments]

DESCRIPTION

       Tomb  is  an application to manage the creation and access of encrypted storage files: it can be operated
       from commandline and it can integrate with a user's graphical desktop.

       Tomb generates encrypted storage files to be opened and closed using their  associated  keys,  which  are
       also protected with a password chosen by the user. To create, open and close tombs a user will need super
       user rights to execute the tomb commandline utility.

       A  tomb  is like a locked folder that can be safely transported and hidden in a filesystem; it encourages
       users to keep their keys separate from tombs, for instance keeping a tomb file on your computer  harddisk
       and its key file on a USB stick.

COMMANDS

dig Generates a file that can be used as a tomb and will occupy as much space as its desired initial
size, the unlocked .tomb file can then be locked using a key. It takes a mandatory -s option which
is the size in megabytes (MiB). Tombs are digged using random data gathered from a non-blocking
source (/dev/urandom).

forge Creates a new key and prompts the user for a password to protect its usage using symmetric
encryption. This operation uses random data from a blocking source (/dev/random) and it may take
long when run on a server with low entropy; to switch using a non-blocking source the --use-
urandom flag can be used. The -g option switches on the use of a GPG key instead of a password
(asymmetric encryption), then the -r option indicates the recipient key; more recipient GPG ids
can be indicated (comma separated). The default cipher to protect the key is AES256, a custom one
can be specified using the -o option, for a list of supported ciphers use -v. For additional
protection against dictionary attacks on keys, the --kdf option can be used when forging a key,
making sure that the tomb-kdb-pbkdf2 binaries in extras/kdf were compiled and installed on the
system.

lock Initializes and locks an empty tomb (made with dig) using a key (made with forge), making it ready
for usage. After this operation, the tomb can only be opened in possession of the key and knowing
its password. As in any other command requiring a key, the option -k should be used to specify a
key file; in case of encryption to GPG recipients the -g flag should be used followed by -r and
the recipient's secret GPG key id. The -o option can be used to specify the cipher specification:
default is "aes-xts-plain64:sha256", old versions of Tomb used "aes-cbc-essiv:sha256". If you are
looking for something exotic, also try "serpent-xts-plain64". More options may be found in
cryptsetup(8) and Linux documentation. This operation requires root privileges to loopback mount,
format the tomb (using LUKS and Ext4), then set the key in its first LUKS slot.

open Opens an existing tomb file (first argument) using a key (-k) which can also be an jpeg image (see
bury/exhume). If a second argument is given it will indicate the mountpoint where the tomb should
be made accessible, else the tomb is mounted in a directory inside /media (if not available it
uses /run/media/$USER). The option -o can be used to pass mount(8) options (default:
rw,noatime,nodev). The -g option is needed when using GPG encryption to recipients.

list List all the tombs found open, including information about the time they were opened and the hooks
that they mounted. If the first argument is present, then shows only the tomb named that way or
returns an error if it's not found. If the option --get-mountpoint is used then print a simple
list of currently open tomb mountpoint paths.

ps List all the processes found running inside the tombs that are open, printing out their PIDs and
owners. This is useful to have an overview of programs that are keeping the tombs busy and would
eventually be killed by the slam command. The lsof(8) utility is used internally to enumerate
processes running in one or all tombs.

index Creates or updates the search indexes of all tombs currently open: enables use of the search
command using simple word patterns on file names. Indexes are created using mlocate's updatedb(8)
and swish-e(1) if they are found on the system. Indexes allow to search very fast for filenames
and contents inside a tomb, they are stored inside it and are not accessible if the Tomb is
closed. To avoid indexing a specific tomb simply touch a .noindex file in it.

search Takes any string as argument and searches for them through all tombs currently open and previously
indexed using the index command. The search matches filenames if mlocate is installed and then
also file contents if swish++ is present on the system, results are listed on the console.

close Closes a currently open tomb. If more tombs are open, the first argument should be used to
specify the name of the tomb to be closed, or all to close all currently open tombs. This command
fails if the tomb is in use by running processes (to force close, see slam below).

slam Closes a tomb like the command close does, but it doesn't fail even if the tomb is in use by other
application processes: it looks for and closes each of them (in order: TERM, HUP, KILL). This
command may provoke unsaved data loss, but assists users to face surprise situations. It requires
lsof else it falls back to close.

passwd Changes the password protecting a key file specified using -k. With keys encrypted for GPG
recipients use -g followed by -r to indicate the new recipient key, or a comma separated list..
The user will need to know the key's current password, or possess at least one of the current
recipients GPG secret keys, because the key contents will be decoded and reencoded using the new
passwords or keys. If the key file is broken (missing headers) this function also attempts its
recovery.

setkey Changes the key file that locks a tomb, substituting the old one with a new one. Both the old and
the new key files are needed for this operation and their passwords or GPG recipient(s) secret
keys must be available. The new key must be specified using the -k option, the first argument
should be the old key and the second and last argument the tomb file. Use the -g option to unlock
the tomb with a GPG key, the -r to indicate the recipient or a comma separated list for more than
one recipient.

resize Increase the size of a tomb file to the amount specified by the -s option, which is the new size
in megabytes (MiB). Full access to the tomb using a key (-k) and its password is required. Tombs
can only grow and can never be made smaller. This command makes use of the cryptsetup(8) resize
feature and the resize2fs command: its much more practical than creating a new tomb and moving
everything into it. There is no data-loss if a failure occurs during resize: the command can be
re-launched and the resize operation will complete.

engrave
This command transforms a tomb key into an image that can be printed on paper and physically
stored as backup, i.e. hidden in a book. It Renders a QRCode of the tomb key, still protected by
its password: a PNG image (extension .qr.png) will be created in the current directory and can be
later printed (fits an A4 or Letter format). To recover an engraved key one can use any QRCode
reader on a smartphone: save it into a file and then use that file as a key (-k).

bury Hides a tomb key (-k) inside a jpeg image (first argument) using steganography: the image will
change in a way that cannot be noticed by human eye and hardly detected by data analysis. This
option is useful to backup tomb keys in unsuspected places; it depends from the availability of
steghide. Use the -g flag and -r option followed by recipient id to use GPG asymmetric encryption.

exhume This command recovers from jpeg images the keys that were previously hidden into them using bury.
Exhume requires a key filename (-k) and a jpeg image file (first argument) known to be containing
a key. If the right key password is given, the key will be exhumed. If the password is not known,
it is very hard to verify if a key is buried in any image or not.

OPTIONS

-k <keyfile>
For all operations requiring a key, this option specifies the location of the key file to use.
Arguments can also be jpeg image files where keys have been hidden using the bury command, or text
files retrieved from engraved QR codes. If the keyfile argument is "-" (dash), Tomb will read the
key from stdin (blocking).

-n Skip processing of post-hooks and bind-hooks if found inside the tomb. See the HOOKS section in
this manual for more information.

-p When opening a tomb, preserves the ownership of all files and directories contained in it.
Normally the open command changes the ownership of a tomb's contents to the UID and GID of the
user who has succesfully opened it: it is a usability feature in case a tomb is used by a single
user across different systems. This flag deactivates this behaviour.

-o Manually specify mount options to be used when opening a tomb instead of the default
rw,noatime,nodev, i.e. to mount a tomb read-only (ro) to prevent any modification of its data. Can
also be used to change the symmetric encryption algorithm for keys during forge operations
(default AES256) or the LUKS encryption method during lock operations (default aes-xts-
plain64:sha256).

-f Force flag, currently used to override swap checks, might be overriding more wimpy behaviours in
future, but make sure you know what you are doing if you force an operation.

-s <MBytes>
When digging or resizing a tomb, this option must be used to specify the size of the new file to
be created. Units are megabytes (MiB).

-g Tell tomb to use a asymmetric GnuPG key encryption instead of a symmetric passphrase to protect a
tomb key. This option can be followed by -r when the command needs to specify recipient(s).

-r <gpg_id>[,<gpg_id2>]
Provide a new set of recipient(s) to encrypt a tomb key. gpg_ids can be one or more GPG key ID,
comma separated.

--kdf <itertime>
Activate the KDF feature against dictionary attacks when creating a key: forces a delay of
<itertime> times every time this key is used. The actual time to wait depends on the CPU speed of
the computer where the key is used. Using 5 or 10 is a sane amount for modern computers, the
value is multiplied by 1 million.

-h Display a help text and quit.

-v Display version and quit.

-q Run more quietly

-D Print more information while running, for debugging purposes

DEV MODE

       --no-color
              Suppress colors in console output (needed for string parsing by wrappers).

       --unsafe
              Enable  using  dev-mode arguments, i.e. to pass passwords from commandline options. This is mostly
              used needed for execution by wrappers and testing suite.

       --use-urandom
              Use a non-blocking random source to improve the speed of the forge command (key generation):  tomb
              uses  /dev/urandom  instead of /dev/random. According to some people using the non-blocking source
              of Linux kernel doesn't degrades the quality of random.

       --tomb-pwd <string>
              Use string as password when needed on tomb.

       --tomb-old-pwd <string>
              Use string as old password when needed in tomb commands requiring multiple keys,  like  passwd  or
              setkey.

       -U     Switch to this user ID when dropping privileges.

       -G     Switch to this group ID when dropping privileges.

       -T     Switch to this TTY terminal when dropping privileges.

HOOKS

       Hooks  are  special  files  that  can be placed inside the tomb and trigger actions when it is opened and
       closed; there are two kinds of such files: bind-hooks and post-hooks can be placed in the  base  root  of
       the tomb.

       bind-hooks
              This  hook  file  consists  of a simple text file named bind-hooks containing a two column list of
              paths to files or directories inside the tomb. The files and directories will be be made  directly
              accessible by the tomb open command inside the current user's home directory. Tomb uses internally
              the  "mount -o bind" command to bind locations inside the tomb to locations found in $HOME. In the
              first column are indicated paths relative to the tomb and in the second column are indicated paths
              relative to $HOME contents, for example:
                mail          mail
                .gnupg        .gnupg
                .fmrc         .fetchmailrc
                .mozilla      .mozilla

       exec-hooks
              This hook file gets executed as user by tomb with the  first  argument  determining  the  step  of
              execution  (open  or  close)  and the second being the full path to the mountpoint. The exec-hooks
              file should be executable (ELF or shell script) and present inside the Tomb.  Tomb  executes  this
              hook  as  user  and  adds  the  name,  loopback  device  and dev-mapper device paths as additional
              arguments for the close command.

PRIVILEGE ESCALATION

       The tomb commandline tool needs to acquire super user rights to execute most of its operations: to do  so
       it  uses sudo(8), while pinentry(1) is adopted to collect passwords from the user. Tomb executes as super
       user only when required.

       To be made available on multi user systems, the superuser execution of the tomb script can be  authorized
       for users without jeopardizing the whole system's security: just add such a line to /etc/sudoers:

            username ALL=NOPASSWD: /usr/local/bin/tomb

       Password  input  is  handled  by  the  pinentry program: it can be text based or graphical and is usually
       configured with a symlink. When using Tomb in X11 it is  better  to  use  a  graphical  pinentry-gtk2  or
       pinentry-qt  because  it  helps preventing keylogging by other X clients. When using it from a remote ssh
       connection it might be necessary to force use of pinentry-curses for instance by  unsetting  the  DISPLAY
       environment var.

SWAP

       On  execution  of certain commands Tomb will complain about swap memory on disk when present and abort if
       your system has swap activated. You can disable this behaviour using  the  --force.  Before  doing  that,
       however, you may be interested in knowing the risks of doing so:

       •      During such operations a lack of available memory could cause the swap to write your secret key on
              the disk.

       •      Even while using an opened tomb, another application could occupy too much memory so that the swap
              needs  to be used, this way it is possible that some contents of files contained into the tomb are
              physically written on your disk, not encrypted.

       If you don't need swap, execute  swapoff -a. If you really need it, you  could  make  an  encrypted  swap
       partition. Tomb doesn't detect if your swap is encrypted, and will complain anyway.

DENIABILITY

       The  possibility  to  have  an  encrypted  volume  which  is  invisible  and cannot be detected is called
       "deniability". The cryptographic layer of the device  mapper  in  Linux  (dm-crypt)  does  not  implement
       deniability.  Tomb is just a wrapper on top of that and it doesn't add cryptographic deniability. However
       a certain way of using tomb can facilitate a weak sort of deniability outside of the scenario  of  seized
       devices and forensic analysis of files and blocks on disc.

       For  instance  to  eliminate  any  trace  of tomb usage from the shell history ZSh users can activate the
       "HISTIGNORESPACE" feature and prefix all invokations of tomb with a blank space, including two  lines  in
       ".zshrc":

       export HISTIGNORESPACE=1
       alias tomb=' tomb'

PASSWORD INPUT

       Tomb  uses  the  external  program  "pinentry"  to  let  users type the key password into a terminal or a
       graphical window. This program works in conjunction with "gpg-agent", a daemon running in  background  to
       facilitate secret key management with gpg. It is recommended one runs "gpg-agent" launching it from the X
       session initialization ("~/.xsession" or "~/.xinitrc" files) with this command:

       eval $(gpg-agent --daemon --write-env-file "${HOME}/.gpg-agent-info")

       In the future it may become mandatory to run gpg-agent when using tomb.

SHARE A TOMB

       A  tomb  key  can  be  encrypted  with  more  than one recipient. Therefore, a tomb can be shared between
       different users. The recipients are given using the -r (or/and -R) option and if multiple each GPG key ID
       must be separated by a comma (,). Sharing a tomb is a very sensitive action and the user needs  to  trust
       that  all  the  GPG  public keys used are kept safe. If one of them its stolen or lost, it will be always
       possible to use it to access the tomb key unless all its copies are destroyed. The -r option can be  used
       in the tomb commands: open, forge setkey, passwd, bury, exhume and resize.

EXAMPLES

       •      Create a 128MB large "secret" tomb and its keys, then open it:

                   tomb dig -s 128 secret.tomb

                   tomb forge secret.tomb.key

                   tomb lock secret.tomb -k secret.tomb.key

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

       •      Open a Tomb using the key from a remote SSH shell, without saving any local copy of it:

                   ssh user@my.shell.net 'cat .secrets/tomb.key' | tomb open secret.tomb -k -

       •      Open  a Tomb on a remote server passing the unencrypted local key on stdin via SSH, without saving
              any remote copy of it:

                   gpg -d .secrets/tomb.key | ssh server tomb open secret.tomb -k cleartext --unsafe

       •      Create a bind hook that places your GnuPG folder inside the tomb, but makes it reachable from  the
              standard $HOME/.gnupg location every time the tomb will be opened:

                   tomb open GPG.tomb -k GPG.tomb.key
                   echo ".gnupg .gnupg" > /media/GPG.tomb/bind-hooks
                   mv ~/.gnupg /media/GPG.tomb/.gnupg && mkdir ~/.gnupg
                   tomb close GPG && tomb open GPG.tomb -k GPG.tomb.key

       •      Script  a  tomb  to  launch the Firefox browser every time is opened, keeping all its profile data
              inside it:

                   tomb open FOX.tomb -k FOX.tomb.key
                   cat <<EOF > /media/FOX.tomb/post-hooks
              #!/bin/sh
              if [ "$1" = "open" ]; then
                firefox -no-remote -profile "$2"/firefox-pro &
              fi
              EOF
                   chmod +x     /media/FOX.tomb/post-hooks

       •      Script a tomb to archive Pictures using Shotwell, launching it on open:

                   tomb open Pictures.tomb -k Pictures.tomb.key
                   cat <<EOF > /media/Pictures.tomb/bind-hooks
              Pictures Pictures
              EOF
                      cat <<EOF > /media/Pictures.tomb/post-hooks
              #!/bin/sh
              if [ "$1" = "open" ]; then
                which shotwell > /dev/null
                if [ "$?" = "0" ]; then
                  shotwell -d "$2"/Pictures/.shotwell &
                fi
              fi
              EOF
                   chmod +x /media/Pictures.tomb/post-hooks

BUGS

       Please report bugs on the Github issue tracker at https://github.com/dyne/Tomb/issues

       One can also try to get in touch with developers via the #dyne chat channel on https://irc.dyne.org.

COPYING

       This manual is Copyright (c) 2011-2017 by Denis Roio <jaromil@dyne.org>

       This manual includes contributions by Boyska and Hellekin O. Wolf.

       Permission is  granted to copy,  distribute and/or modify  this manual under the terms of the   GNU  Free
       Documentation  License,  Version  1.1  or  any   later    version   published   by   the   Free  Software
       Foundation.  Permission is granted  to make and distribute verbatim  copies of this manual page  provided
       the above  copyright notice and  this permission notice are preserved on all copies.

AVAILABILITY

       The most recent version of Tomb sourcecode and up to date documentation is available  for  download  from
       its website on https://tomb.dyne.org.