cassh/cassh.sh

#!/bin/sh
# cassh - Manager for an OpenSSH Certification Authority
#
# Written in 2022 by Lucas
#
# To the extent possible under law, the author(s) have dedicated all
# copyright and related and neighboring rights to this software to the
# public domain worldwide. This software is distributed without any
# warranty.
# You should have received a copy of the CC0 Public Domain Dedication
# along with this software. If not, see
# <http://creativecommons.org/publicdomain/zero/1.0/>.

usage()
{
	cat - <<EOF >&2
Usage:
	${0##*/} issue [-hqv] [-I key_id] [-n principals]
	    [-V validity_interval]
	${0##*/} mkfile [-n principals] authorized_keys | known_hosts
EOF
	exit 1
}

err()
{
	printf "%s: %s\n" "${0##*/}" "$*" >&2
	exit 1
}

strip_leading_zeros()
{
	_s=$1
	if [ -z "$_s" ]; then
		return
	fi
	while [ X"${_s#0}" != X"$_s" ]; do
		_s=${_s#0}
	done
	echo "${_s:-0}"
}

strcmp()
{
	_r=$(expr "X$1" "$2" "X$3")
	[ "${_r:-0}" -eq 1 ]
}

# Returns comment from the ssh-agent if any is returned, otherwise it
# returns the public key's fingerprint.
get_ca_comment_from_sk()
{
	ssh-keygen -lf "$1" | {
		read -r pk_sz pk_fp pk_extra
		_comment=$(ssh-add -l | while read -r sk_sz sk_fp sk_extra; do
			if [ "X$sk_fp" = "X$pk_fp" ]; then
				echo "${sk_extra% (*)}"
				break
			fi
		done)
		echo "${_comment:-${pk_fp#*:}}"
	}
}

_template_fmt()
{
	_allowed_chars=$1
	_char=$2
	if [ "X$_char" = X% ]; then
		echo %
		return $?
	fi

	case $_char in
	[$_allowed_chars])
		;;
	*)
		return 1
		;;
	esac

	_v=$(eval echo '${_template_fmt_'"$_char"':-}')
	if [ -z "$_v" ]; then
		return 1
	fi

	echo "$_v"
}

template()
{
	_allowed=$1
	_s=$2
	_out=

	while [ "${_s#*%}" != "$_s" ]; do
		_t=${_s#*%}
		_out=$_out${_s%"%"$_t}
		_s=$_t
		_c=${_s%${_s#?}}

		_t=$(_template_fmt "$_allowed" "$_c")
		if [ $? -ne 0 ]; then
			return 1
		fi
		_out=$_out$_t

		_s=${_s#$_c}
	done
	_out=$_out$_s

	echo "$_out"
}

main_issue()
{
	hflag=
	key_id_fmt=%C/%f
	nflag=false
	principals_fmt=
	qflag=
	validity_interval=always:forever
	vflag=
	while getopts hI:n:qV:v flag; do
		case $flag in
		h)	hflag=-h ;;
		I)	key_id_fmt=$OPTARG ;;
		n)	nflag=true principals_fmt=$OPTARG ;;
		q)	qflag=-q ;;
		V)	validity_interval=$OPTARG ;;
		v)	vflag=${vflag:--}v ;;
		*)	usage ;;
		esac
	done
	shift $(($OPTIND - 1))
	if [ $# -ne 0 ]; then
		usage
	fi

	if [ ! -f ca.pub ]; then
		err "no ca.pub found"
	fi
	if ! ssh-add $qflag $vflag -T ca.pub; then
		err "can't use CA key"
	fi
	if [ ! -d pubkeys/ ]; then
		err "no pubkeys directory found"
	fi

	if [ ! -f serial.txt ]; then
		date -u +%Y%m%d000000000 >serial.txt
	fi
	read -r serial <serial.txt
	# Remove NNNNNNNNN suffix
	serial_date=${serial%?????????}
	current_date=$(date -u +%Y%m%d)
	if strcmp "$current_date" ">" "$serial_date"; then
		serial_date=$current_date
		serial_counter=0
	else
		# Remove YYYYmmdd prefix and leading 
		serial_counter=$(strip_leading_zeros "${serial#????????}")
	fi
	serial=$(printf "%s%09u\n" "$serial_date" "$serial_counter")

	_template_fmt_C=$(get_ca_comment_from_sk ca.pub)
	find pubkeys/ -type f -name '*.pub' ! -name '*-cert.pub' | {
		rc=0
		while read -r pk; do
			pkname=${pk%.pub}
			pkname=${pkname#pubkeys/}
			_template_fmt_f=$pkname

			id=$(template Cf "$key_id_fmt")
			set -- -I "$id" -Us ca.pub $hflag $qflag $vflag \
			    -V "$validity_interval" -z "$serial"

			if $nflag; then
				principals=$(template f "$principals_fmt")
				ssh-keygen "$@" -n "$principals" "$pk"
			else
				ssh-keygen "$@" "$pk"
			fi || rc=1

			serial_counter=$(($serial_counter + 1))
			if [ $serial_counter -ge 1000000000 ]; then
				err "can't issue more certificates today"
			fi
			serial=$(printf "%s%09u\n" "$serial_date" \
			    "$serial_counter" | tee serial.txt)

			if [ $rc -ne 0 ]; then
				break
			fi
		done
		return $rc
	}
}

main_mkfile()
{
	while getopts : flag; do
		case $flag in
		*)	usage ;;
		esac
	done
	shift $(($OPTIND - 1))
	if [ $# -lt 1 ]; then
		usage
	fi
	file=$1
	shift

	if [ ! -f ca.pub ]; then
		err "no ca.pub found"
	fi

	case $file in
	authorized_keys)
		if [ $# -gt 1 ]; then
			usage
		fi
		options=cert-authority${1:+,$1}
		printf "%s " "$options"
		;;
	known_hosts)
		if [ $# -gt 1 ]; then
			usage
		fi
		hostnames=${1:-}
		if [ -n "$hostnames" ]; then
			printf "@cert-authority %s " "$hostnames"
		else
			printf "@cert-authority "
		fi
		;;
	*)
		err "unknown file \"$file\""
		;;
	esac

	cat ca.pub
}

set -u

if [ $# -lt 1 ]; then
	usage
fi
cmd=$1
shift

case $cmd in
issue)	main_issue "$@" ;;
mkfile)	main_mkfile "$@" ;;
*)	usage ;;
esac