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Filename : decrypt_derived
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#!/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.

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

unset -v keys count
keys="$(dmsetup table --target crypt --showkeys -- "$1" 2>/dev/null | cut -s -d' ' -f5)"
count="$(printf '%s' "$keys" | wc -l)"

if [ -n "$keys" ] && [ $count -le 1 ]; then
    if [ "${keys#:}" = "$keys" ]; then
        printf '%s' "$keys"
        exit 0
    else
        echo "$0: device $1 uses the kernel keyring" >&2
    fi
elif [ $count -eq 0 ]; then
    echo "$0: device $1 doesn't exist or isn't a crypto device" >&2
else
    echo "$0: more than one device match" >&2
fi
exit 1