#!@SHELL@
#
# amcrypt-ossl-asym.sh - asymmetric crypto helper using OpenSSL
# Usage: amcrypt-ossl-asym.sh [-d]
#
# Copyright © 2006  Ben Slusky <sluskyb@paranoiacs.org>
#
# 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; either version 2
# of the License, or (at your option) any later version.
#
# 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., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
#
# 
# Keys can be generated with the standard OpenSSL commands, e.g.:
#
# $ openssl genrsa -aes128 -out backup-privkey.pem 1024
# Generating RSA private key, 1024 bit long modulus
# [...]
# Enter pass phrase for backup-privkey.pem: <ENTER YOUR PASS PHRASE>
# Verifying - Enter pass phrase for backup-privkey.pem: <ENTER YOUR PASS PHRASE>
#
# $ openssl rsa -in backup-privkey.pem -pubout -out backup-pubkey.pem
# Enter pass phrase for backup-privkey.pem: <ENTER YOUR PASS PHRASE>
# Writing RSA key
#

# change these as needed
OPENSSL=			# whatever's in $PATH
CIPHER=aes-256-cbc		# see `openssl help` for more ciphers
AMANDA_HOME=~backup
RANDFILE=$AMANDA_HOME/.rnd
export RANDFILE
PASSPHRASE=$AMANDA_HOME/.am_passphrase	# optional
PRIVKEY=$AMANDA_HOME/backup-privkey.pem
PUBKEY=$AMANDA_HOME/backup-pubkey.pem

# where might openssl be?
PATH=/bin:/usr/bin:/usr/local/bin:/usr/ssl/bin:/usr/local/ssl/bin
export PATH
MAGIC='AmAnDa+OpEnSsL'
ME=`basename "$0"`
WORKDIR="/tmp/.${ME}.$$"

# first things first
if [ -z "${OPENSSL:=`which openssl`}" ]; then
	echo "${ME}: openssl not found" >&2
	exit 1
elif [ ! -x "${OPENSSL}" ]; then
	echo "${ME}: can't execute openssl (${OPENSSL})" >&2
	exit 1
fi

if [ -n "${PASSPHRASE}" ]; then
	# check the openssl version. if it's too old, we have to handle
	# the pass phrase differently.
	OSSL_VERSION=`eval \"${OPENSSL}\" version |cut -d\  -f2`
	case "${OSSL_VERSION}" in
	 ''|0.[0-8].*|0.9.[0-6]*|0.9.7|0.9.7[a-c]*)
		echo "${ME}: ${OPENSSL} is version ${OSSL_VERSION}" >&2
		echo "${ME}: Using pass phrase kluge for OpenSSL version >=0.9.7d" >&2
		PASS_FROM_STDIN=yes
		;;
	esac
fi

mkdir -m 700 "${WORKDIR}"
if [ $? -ne 0 ]; then
	echo "${ME}: failed to create temp directory" >&2
	exit 1
fi
# ignore SIGINT
trap "" 2
trap "rm -rf \"${WORKDIR}\"" 0 1 3 15

# we'll need to pad the datastream to a multiple of the cipher block size
# prior to encryption and decryption. 96 bytes (= 768 bits) should be good
# for any cipher.
pad() {
	perl -pe 'BEGIN { $bs = 96; $/ = \8192 } $nbytes = ($nbytes + length) % $bs; END { print "\0" x ($bs - $nbytes) }'
}

encrypt() {
	# generate a random printable cipher key (on one line)
	echo `"${OPENSSL}" rand -base64 80` >"${WORKDIR}/pass"

	# encrypt the cipher key using the RSA public key
	"${OPENSSL}" rsautl -encrypt -in "${WORKDIR}/pass" -out "${WORKDIR}/pass.ciphertext" -pubin -inkey "${PUBKEY}" -pkcs
	[ $? -eq 0 ] || return 1

	# print magic
	printf %s "${MAGIC}"

	# print the encrypted cipher key, preceded by size
	ls -l "${WORKDIR}/pass.ciphertext" | awk '{ printf("%-10d", $5) }'
	cat "${WORKDIR}/pass.ciphertext"

	# encrypt data using the cipher key and print
	pad | "${OPENSSL}" enc "-${CIPHER}" -nopad -e -pass "file:${WORKDIR}/pass" -nosalt
	[ $? -eq 0 ] || return 1
}

decrypt() {
	# read magic
	magicsize=`printf %s "${MAGIC}" | wc -c | sed 's/^ *//'`
	magic=`dd bs=$magicsize count=1 2>/dev/null`
	if [ "$magic" != "${MAGIC}" ]; then
		echo "${ME}: bad magic" >&2
		return 1
	fi

	# read size of encrypted cipher key
	n=`dd bs=10 count=1 2>/dev/null`
	[ $n -gt 0 ] 2>/dev/null
	if [ $? -ne 0 ]; then
		echo "${ME}: bad header" >&2
		return 1
	fi

	# read the encrypted cipher key
	dd "of=${WORKDIR}/pass.ciphertext" bs=$n count=1 2>/dev/null

	# decrypt the cipher key using the RSA private key
	if [ "${PASS_FROM_STDIN}" = yes ]; then
		"${OPENSSL}" rsautl -decrypt -in "${WORKDIR}/pass.ciphertext" -out "${WORKDIR}/pass" -inkey "${PRIVKEY}" -pkcs < "${PASSPHRASE}"
	else
		"${OPENSSL}" rsautl -decrypt -in "${WORKDIR}/pass.ciphertext" -out "${WORKDIR}/pass" -inkey "${PRIVKEY}" ${PASSARG} -pkcs 3< "${PASSPHRASE}"
	fi
	[ $? -eq 0 ] || return 1

	# use the cipher key to decrypt data
	pad | "${OPENSSL}" enc "-${CIPHER}" -nopad -d -pass "file:${WORKDIR}/pass" -nosalt

	# N.B.: in the likely event that we're piping to gzip, the above command
	# may return a spurious error if gzip closes the output stream early.
	return 0
}

if [ "$1" = -d ]; then
	if [ -z "${PRIVKEY}" ]; then
		echo "${ME}: must specify private key for decryption" >&2
		exit 1
	elif [ ! -r "${PRIVKEY}" ]; then
		echo "${ME}: can't read private key from ${PRIVKEY}" >&2
		exit 1
	fi

	if [ -n "${PASSPHRASE}" -a -e "${PASSPHRASE}" -a -r "${PASSPHRASE}" ]; then
		PASSARG='-passin fd:3'
	else
		PASSPHRASE=/dev/null
	fi

	decrypt
	if [ $? -ne 0 ]; then
		echo "${ME}: decryption failed" >&2
		exit 1
	fi
else
	if [ -z "${PUBKEY}" ]; then
		echo "${ME}: must specify public key for encryption" >&2
		exit 1
	elif [ ! -r "${PUBKEY}" ]; then
		echo "${ME}: can't read public key from ${PUBKEY}" >&2
		exit 1
	fi

	encrypt
	if [ $? -ne 0 ]; then
		echo "${ME}: encryption failed" >&2
		exit 1
	fi
fi