#!/bin/mksh
#-
# Copyright © 2015
#	Thorsten “mirabilos” Glaser <t.glaser@tarent.de>
#
# Provided that these terms and disclaimer and all copyright notices
# are retained or reproduced in an accompanying document, permission
# is granted to deal in this work without restriction, including un‐
# limited rights to use, publicly perform, distribute, sell, modify,
# merge, give away, or sublicence.
#
# This work is provided “AS IS” and WITHOUT WARRANTY of any kind, to
# the utmost extent permitted by applicable law, neither express nor
# implied; without malicious intent or gross negligence. In no event
# may a licensor, author or contributor be held liable for indirect,
# direct, other damage, loss, or other issues arising in any way out
# of dealing in the work, even if advised of the possibility of such
# damage or existence of a defect, except proven that it results out
# of said person’s immediate fault when using the work as intended.
#-
# Pipe a raw RFC822 message which is PGP/MIME encrypred through this
# script; optional argument is the target folder (‘+’ auto-prepended
# for dmail, otherwise dmail delivers to the INBOX) which uses dmail
# from uw-imapd; if not found, delivery is to #driver.unix/mail/x in
# unix/mbox format by hand, without locking.

rawinput=$(cat)
nl=$'\n'
unixpfx=$(date -u +'From MAILER-DAEMON %a %b %e %H:%M:%S %Y')

function die {
	print -ru2 -- E: $*
	exit 1
}

[[ $rawinput = *$nl$nl* ]] || die no RFC822 message
rawheader=$unixpfx$nl${rawinput%%$nl$nl*}
rawheader=${rawheader//$nl[	 ]/
 }
typeset -l loheader=$rawheader
rawbody=${rawinput#*$nl$nl}
rawbody=${rawbody//$'\r'}
[[ $loheader = *${nl}content-type:*([
	 ])multipart/encrypted*application/pgp-encrypted* ]] || \
    die no PGP/MIME encrypted message content type
[[ $rawbody = *$nl'-----BEGIN PGP MESSAGE-----'$nl*$nl'-----END PGP MESSAGE-----'$nl* ]] || \
    die no PGP/MIME encrypted message body

decoded=${rawbody#*$nl'-----BEGIN PGP MESSAGE-----'$nl}
decoded=${decoded%%$nl'-----END PGP MESSAGE-----'$nl*}
decoded=$(print -r -- "-----BEGIN PGP MESSAGE-----$nl$decoded$nl-----END PGP MESSAGE-----" | gpg)
[[ $decoded = *$nl$nl* ]] || die decoded message not RFC822

set -A transforms -- \
    -e '/^[Cc][Oo][Nn][Tt][Ee][Nn][Tt]-/d' \
    -e '/^[Ss][Uu][Bb][Jj][Ee][Cc][Tt]:/s//& **DECRYPTED**/'
set -A extraheaders
msgid=
[[ $loheader = *${nl}message-id:* ]] && msgid=$(print -r -- "$rawheader" | \
    sed -n '/^[Mm][Ee][Ss][Ss][Aa][Gg][Ee]-[Ii][Dd]:[
	 ]*\(<.*>\)[
	 ]*$/s//\1/p')
if [[ -n $msgid ]]; then
	transforms+=(-e '/^[Mm][Ee][Ss][Ss][Aa][Gg][Ee]-[Ii][Dd]:/s/>/.by.pgpdemime.mirbsd.org>/')
	if [[ $loheader = *${nl}references:* ]]; then
		transforms+=(-e '/^[Rr][Ee][Ff][Ee][Rr][Ee][Nn][Cc][Ee][Ss]:/s$'"
 $msgid")
	else
		extraheaders+=("References: $msgid")
	fi
fi

mda=$(whence -p dmail) || mda=
[[ -z $mda && -x /usr/mpkg/libexec/dmail ]] && mda=/usr/mpkg/libexec/dmail

if [[ -n $mda ]]; then
	transforms+=(-e '1d')
	target='your INBOX'
	[[ -n $1 ]] && mda="$mda ${1@Q}"
	[[ -n $1 ]] && target="the selected folder $1"
else
	decoded+=$nl
	target='“x” folder'
	mda='cat >>~/mail/x'
fi

if {
	print -r -- "$rawheader" | sed "${transforms[@]}" | tr '
' '\n'
	for x in "${extraheaders[@]}"; do
		print -r -- "$x"
	done
	print -r -- "$decoded"
    } | eval "$mda"; then
	print I: decoded message written to $target
else
	print W: decoded message maybe not written, error code $?
	exit 1
fi
exit 0