#!/bin/sh

# WARNING: If you use the decrypt_derived keyscript for devices with
# persistent data (i.e. not swap or temp devices), then you will lose
# access to that data permanently if something damages the LUKS header
# of the LUKS device you derive from. The same applies if you luksFormat
# the device, even if you use the same passphrase(s). A LUKS header
# backup, or better a backup of the data on the derived device may be
# a good idea. See the Cryptsetup FAQ on how to do this right.

countlines() {
local IFS input count tmp
input="$1"
count=0
IFS='
'
for tmp in $input; do
count=$(( $count + 1 ))
done
echo $count
}

if [ -z "$1" ]; then
echo "$0: must be executed with a crypto device as argument" >&2
exit 1
fi

if ! device=$(dmsetup --showkeys table 2>/dev/null | grep "^$1:"); then
echo "$0: failed to find $1 in dmtable" >&2
exit 1
fi

if [ -z "$device" ]; then
echo "$0: device $1 doesn't exist" >&2
exit 1
fi

count=$(countlines "$device")
if [ $count -ne 1 ]; then
echo "$0: more than one device match $1" >&2
exit 1
fi

eval set -- $device
type="$4"
key="$6"

if [ "$type" != "crypt" ]; then
echo "$0: device $1 is not a crypto device" >&2
exit 1
fi

echo -n "$key"
exit 0