Chapter 16. How to do Amanda-server-side gpg-encrypted backups.

Chapter 16. How to do Amanda-server-side gpg-encrypted backups.
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Stefan G. Weichinger

Original text

AMANDA Core Team

<[email protected]>

Table of Contents

Setup
Test
Plans

Note

THIS IS *NOT* YET INTENDED FOR PRODUCTION SERVERS !!!

Bruce Fletcher asked for a "simple" encryption method to be used with Amanda-server. gpg-amanda http://security.uchicago.edu/tools/gpg-amanda/ seems to create problems at restore-time, as it uses a wrapper for gzip.

My solution uses a wrapper for GNU-tar instead, so there are several disadvantages avoided.

Note

This is based on a Amanda-vtape-setup with the Amanda-release 2.4.5. As this is still in the testing-stage, I have coded the home-dir of the Amanda-user into my scripts (/var/lib/amanda). This should be done with variables later, I agree ...

What you need:

aespipe http://loop-aes.sourceforge.net/aespipe/aespipe-v2.3b.tar.bz2 and the bz2aespipe-wrapper that comes with it. It gets patched as described later.
the wrapper-script /usr/local/libexec/amgtar, as listed down below,
GNU-PG http://www.gnupg.org/(en)/download/index.html. This should be part of most current operating systems already.
Amanda ;)

Setup

Configure and compile aespipe:

tar -xjf aespipe-v2.3b.tar.bz2
cd aespipe-v2.3b
./configure
make
make install

Generate and store the gpg-key for the Amanda-user:
```
# taken from the aespipe-README
head -c 2925 /dev/random | uuencode -m - | head -n 66 | tail -n 65 | \
gpg --symmetric -a > /var/lib/amanda/.gnupg/am_key.gpg
```
This will ask for a passphrase. Remember this passphrase as you will need it in the next step.
Store the passphrase inside the home-directory of the Amanda-user and protect it with proper permissions:
```
echo my_secret_passphrase > ~amanda/.am_passphrase
chown amanda:disk ~amanda/.am_passphrase
chmod 700 ~amanda/.am_passphrase
```
We need this file because we don't want to have to enter the passphrase manually everytime we run amdump. We have to patch bz2aespipe to read the passphrase from a file. I have called that file ~amanda/.am_passphrase.
It should NOT ;) look like this:
```
# cat ~amanda/.am_passphrase
my_secret_passphrase
```
Note
Store the key and the passphrase in some other place as well, without these information you can't access any tapes that have been encrypted with it (this is exactly why we are doing all this, isn't it? ;) ).

Create the wrapper for GNU-tar:

Example 16.1. /usr/local/libexec/amgtar

#!/bin/sh
#
# Original wrapper by Paul Bijnens
#
# crippled by Stefan G. Weichinger
# to enable gpg-encrypted dumps via aespipe

GTAR=/bin/tar
AM_AESPIPE=/usr/local/bin/amaespipe
AM_PASSPHRASE=/var/lib/amanda/.am_passphrase
LOG=/dev/null
LOG_ENABLED=1

if [ "$LOG_ENABLED" = "1" ]
then 
LOG=/var/log/amanda/amgtar.debug
date >> $LOG
echo "$@" >> $LOG
fi

if [ "$3" = "/dev/null" ]
then 
	echo "Estimate only" >> $LOG
	$GTAR "$@"
else 
	echo "Real backup" >> $LOG
	$GTAR --use-compress-program="$AM_AESPIPE" "$@" 3< $AM_PASSPHRASE
fi

rc=$?
exit $rc

Copy the wrapper-script bz2aespipe, which comes with the aespipe-tarball, to /usr/local/bin/amaespipe and edit it this way:

Example 16.2. /usr/local/bin/amaespipe

#! /bin/sh

# FILE FORMAT
# 10 bytes: constant string 'bz2aespipe'
# 10 bytes: itercountk digits
# 1 byte: '0' = AES128, '1' = AES192, '2' = AES256
# 1 byte: '0' = SHA256, '1' = SHA384, '2' = SHA512, '3' = RMD160
# 24 bytes: random seed string
# remaining bytes are bzip2 compressed and aespipe encrypted

# These definitions are only used when encrypting.
# Decryption will autodetect these definitions from archive.
ENCRYPTION=AES256
HASHFUNC=SHA256
ITERCOUNTK=100
WAITSECONDS=1
GPGKEY="/var/lib/amanda/.gnupg/am_key.gpg"
FDNUMBER=3

if test x$1 = x-d ; then
    # decrypt
    n=`head -c 10 - | tr -d -c 0-9a-zA-Z`
    if test x${n} != xbz2aespipe ; then
        echo "bz2aespipe: wrong magic - aborted" >/dev/tty
        exit 1
    fi
    itercountk=`head -c 10 - | tr -d -c 0-9`
    if test x${itercountk} = x ; then itercountk=0; fi
    n=`head -c 1 - | tr -d -c 0-9`
    encryption=AES128
    if test x${n} = x1 ; then encryption=AES192; fi
    if test x${n} = x2 ; then encryption=AES256; fi
    n=`head -c 1 - | tr -d -c 0-9`
    hashfunc=SHA256
    if test x${n} = x1 ; then hashfunc=SHA384; fi
    if test x${n} = x2 ; then hashfunc=SHA512; fi
    if test x${n} = x3 ; then hashfunc=RMD160; fi
    seedstr=`head -c 24 - | tr -d -c 0-9a-zA-Z+/`
    #aespipe -K ${GPGKEY} -p ${FDNUMBER} -e ${encryption} -H ${hashfunc} -S "${seedstr}" -C ${itercountk} -d | bzip2 -d -q
    aespipe -K ${GPGKEY} -p ${FDNUMBER} -e ${encryption} -H ${hashfunc} -S "${seedstr}" -C ${itercountk} -d 
else
    # encrypt
    echo -n bz2aespipe
    echo ${ITERCOUNTK} | awk '{printf "%10u", $1;}'
    n=`echo ${ENCRYPTION} | tr -d -c 0-9`
    aesstr=0
    if test x${n} = x192 ; then aesstr=1; fi
    if test x${n} = x256 ; then aesstr=2; fi
    n=`echo ${HASHFUNC} | tr -d -c 0-9`
    hashstr=0
    if test x${n} = x384 ; then hashstr=1; fi
    if test x${n} = x512 ; then hashstr=2; fi
    if test x${n} = x160 ; then hashstr=3; fi
    seedstr=`head -c 18 /dev/urandom | uuencode -m - | head -n 2 | tail -n 1`
    echo -n ${aesstr}${hashstr}${seedstr}
    #bzip2 | aespipe -K ${GPGKEY} -p ${FDNUMBER} -e ${ENCRYPTION} -H ${HASHFUNC} -S ${seedstr} -C ${ITERCOUNTK} -w ${WAITSECONDS}
    aespipe -K ${GPGKEY} -p ${FDNUMBER} -e ${ENCRYPTION} -H ${HASHFUNC} -S ${seedstr} -C ${ITERCOUNTK} -w ${WAITSECONDS}
fi
exit 0

or apply this small patch

Example 16.3. bz2aespipe.patch

@@ -15,3 +15,5 @@
 ITERCOUNTK=100
-WAITSECONDS=10
+WAITSECONDS=1
+GPGKEY="/var/lib/amanda/.gnupg/am_key.gpg"
+FDNUMBER=3

@@ -36,3 +38,4 @@
     seedstr=`head -c 24 - | tr -d -c 0-9a-zA-Z+/`
-    aespipe -e ${encryption} -H ${hashfunc} -S "${seedstr}" -C ${itercountk} -d | bzip2 -d -q
+    #aespipe -K ${GPGKEY} -p ${FDNUMBER} -e ${encryption} -H ${hashfunc} -S "${seedstr}" -C ${itercountk} -d | bzip2 -d -q
+    aespipe -K ${GPGKEY} -p ${FDNUMBER} -e ${encryption} -H ${hashfunc} -S "${seedstr}" -C ${itercountk} -d
 else
@@ -52,3 +55,4 @@
     echo -n ${aesstr}${hashstr}${seedstr}
-    bzip2 | aespipe -e ${ENCRYPTION} -H ${HASHFUNC} -S ${seedstr} -C ${ITERCOUNTK} -T -w ${WAITSECONDS}
+    #bzip2 | aespipe -K ${GPGKEY} -p ${FDNUMBER} -e ${ENCRYPTION} -H ${HASHFUNC} -S ${seedstr} -C ${ITERCOUNTK} -w ${WAITSECONDS}
+    aespipe -K ${GPGKEY} -p ${FDNUMBER} -e ${ENCRYPTION} -H ${HASHFUNC} -S ${seedstr} -C ${ITERCOUNTK} -w ${WAITSECONDS}
 fi

Things I have changed:

Decreased WAITSECONDS: No need to wait for 10 seconds to read the passphrase.
Removed bzip2 from the pipes: Amanda triggers GNU-zip-compression by itself, no need to do this twice (slows down things, blows up size).
Added options -K and -p: This enables aespipe to use the generated gpg-key and tells it the number of the file-descriptor to read the passphrase from.

Note

You may set various parameters inside bz2aespipe. You may also call bz2aespipe with various command-line-parameters to choose the encryption-algorithm, hash-function etc. . For a start I have chosen to call bz2aespipe without command-line-options.

Reconfigure and recompile Amanda (yes, I'm sorry ...):
As described in How to use a wrapper you have to run configure again with the option --with-gnutar=/usr/local/libexec/amgtar, after that recompile and reinstall Amanda. These steps are described in the mentioned document.

Test

Still to come ...

Plans

There are several wishes:

Ability to switch encryption inside a dumptype. This HOWTO describes a method that enables/disables encryption for the whole installation. You might remove the amgtar-wrapper and simply link to plain GNU-tar again to disable encryption, but be aware that you also disable decryption with this step. You will hit problems when you then try to restore encrypted tapes.
Ability to switch encryption-parameters inside a dumptype. Choice of algorithm, hash-functions etc. I don't know if it makes sense to put it into a dumptype or if it would be enough to configure it once inside amaespipe (I assume the latter).
All this leads to the need to code this into Amanda itself: new dumptype-options and corresponding calls to GNU-tar etc. inside client-src/sendbackup-gnutar.c.

This is it so far. Release early, release often. Feel free to contact me with your thoughts on this paper.

Note

Refer to http://www.amanda.org/docs/howto-gpg.html for the current version of this document.

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