Script for loop-AES key generation

[Max Vozeler]

Hey all,

I've found that, whenever I want to create a new loop-AES key, I
always refer to README and look up/copy the exact command. More
than once I typoed and ended up with a bad key. So I thought, why
not make this into a short shell script.

Attached is one which does just that, along with a man page
that describes the available options. The script has been included
in the Debian package loop-aes-utils for about a year or so and
seems to generally work fine. It should work with any POSIX-
compliant shell (tested with bash, dash, posh, busybox sh). 

Let me know if you encounter any problems using it or have
suggestions/patches for improvement. :-)

Features:
 - Check for unsafe swap
 - Support for v1/v2/v3 type keys
 - Symmetric/asymmetric GnuPG encryption 
 - Configurable cipher

cheers,
Max
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#!/bin/sh
#
# loop-aes-keygen - Create loop-AES encryption keys
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; version 2 dated June, 1991.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program;  if not, write to the Free Software
# Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111, USA.
# 
# Copyright 2005-2006, Max Vozeler <xam at debian.org>
#
# $Id: loop-aes-keygen 1104 2006-03-17 15:43:11Z max $
#

set -e

umask 077

cipher=
userids=
rnd=/dev/random
version=3

usage()
{
	cat << USAGE
usage: loop-aes-keygen [opts] <keyfile>

  -v <1|2|3>	   Key format (Default: $version)
  -u userid        Encrypt for GnuPG pubkey <userid>
  -c cipher        Use GnuPG cipher <cipher>

USAGE
}

get_options()
{
	while getopts 'v:s:c:u:h' f
	do
		case $f in
		v) 
			version=$OPTARG
			;;

		c)
			cipher=$OPTARG
			;;

		s)
			rnd=$OPTARG
			;;

		u)
			userids="$userids $OPTARG"
			;;
			
		h)
			usage
			exit 0
			;;
		esac
	done
	shift `expr $OPTIND - 1`

	keyfile=$1
	
	if [ -z $keyfile ]; then
		echo No output file. Aborting
		usage
		exit 1
	fi

	if [ $version -lt 1 ] || [ $version -gt 3 ]; then
		echo Unsupported key format: $version
		exit 1
	fi
}

check_safe_loop()
{
	loopdev=$1

	opts=$(/sbin/losetup $loopdev 2>&1)
	if [ $? -ne 0 ]; then
		echo "Error: Check for $loopdev failed ($opts)"
		exit 1
	fi
		
	# If loop entry has an encryption= option assume it's safe
	if echo "$opts" | grep -q encryption=; then
		return 0
	fi

	return 1
}

check_safe_swap()
{
	if [ ! -r /proc/swaps ]; then
		echo Error: Cannot read /proc/swaps
		exit 1
	fi

	unsafe=
	while read line
	do
		set -- $line
		case $1 in
		/dev/loop*)
			if ! check_safe_loop $1; then
				unsafe=$1
				break
			fi
			;;
		Filename*)
			;;
		*)
			unsafe=$1
			break
			;;
		esac
	done < /proc/swaps

	if [ $unsafe ]; then
		echo Fatal: Unsafe swap detected: $unsafe
		exit 1
	fi

	return 0
}

keygen()
{
	version=$1
	keyfile=$2
	gpgargs=$3

	# These are the known loop-AES key formats:
	#  v1.x    1     45 bytes           AES key         
	#  v2.x   64   2880 bytes(45 * 64)  AES keys       
	#  v3.x   65   2925 bytes(45 * 65)  #65 is md5 seed
	case $version in
	1) nkeys=1 ;;
	2) nkeys=64 ;;
	3) nkeys=65 ;;
	*) return 1 ;;
	esac

	bytes=$((45*$nkeys))
	head -c $bytes $rnd | uuencode -m - | head -n $(($nkeys+1)) | tail -n $nkeys | gpg $gpgargs > $keyfile
}

get_options $*

if ! check_safe_swap; then
	exit 1
fi

if ! [ -x /usr/bin/gpg ]; then
	echo "Error: gpg not found"
	exit 1
fi

if ! [ -x /usr/bin/uuencode ]; then
	echo "Error: uuencode not found - see package sharutils"
	exit 1
fi

if [ -e $keyfile ]; then
	echo "Keyfile $keyfile exists. Aborting."
	exit 1
fi

gpgargs="--armor"

if [ "$userids" ]; then
	gpgargs="$gpgargs --encrypt"
	for id in $userids; do
		gpgargs="$gpgargs --recipient $id"
	done
else
	gpgargs="$gpgargs --symmetric"
fi

if [ $cipher ]; then
	gpgargs="$gpgargs --cipher-algo=$cipher"
fi

if ! keygen $version $keyfile "$gpgargs"; then
	echo An error occured while creating the key file.
	exit 1
fi

exit 0
-------------- next part --------------
\" $Id: loop-aes-keygen.1 1104 2006-03-17 15:43:11Z max $
.TH loop-aes-keygen 1 2006-01-19
.SH NAME
loop-aes-keygen
.
.SH SYNOPSIS
.B loop-aes-keygen
[
.I options
]
.I keyfile
.
.SH DESCRIPTION
Creates keys for loop-AES loopback encryption. 
.
.SH OPTIONS
.
.TP
.B \-v <1|2|3>
Key format (Default: 3).
.
.TP
.B \-c cipher
Encrypt using GnuPG cipher
.I cipher
.
.TP
.B \-u id
Encrypt using GnuPG pubkey
.I id
\&. Can be given more than once
(Default: Encrypt symmetrically with passphrase)
.
.TP
.B \-s device
Override random source (Default: /dev/random). 
.B Don't use this option unless you know what you are doing.
A bad source can silently create useless and trivially crackable keys. 
On the other hand, a known good source faster than /dev/random (eg. good
hardware RNG) can significantly speed up the key creation.
.
.SH EXAMPLES
.TP
Create key
$
.B loop-aes-keygen key.gpg
.TP
Create key for multiple users
$
.B loop-aes-keygen -u user1@$HOSTNAME -u \(dqUser 2\(dq key.gpg
.TP
Create blowfish encrypted key in loop-AES v2.x format
$
.B loop-aes-keygen -c BLOWFISH -v2 key.gpg
.
.SH AUTHOR
.TP
Max Vozeler <xam at debian.org>
.
.SH SEE ALSO
.BR losetup (8),
.BR /usr/share/doc/loop-aes-utils/README,
.BR /usr/share/doc/loop-aes-source/README*