#!/bin/sh

# Wrapper around openssl to generate self-signed X.509 server
# certificates or Certificate Signing Requests, or DKIM private keys.
# Inspired from make-ssl-cert(8) and opendkim-genkey(8).
#
# Copyright © 2014 Guilhem Moulin <guilhem@fripost.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 3 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, see <http://www.gnu.org/licenses/>.

set -ue
PATH=/usr/bin:/bin

# Default values
type=rsa
bits=
hash=

force=
config=
pubkey=pubkey.pem
privkey=privkey.pem
dns=

usage() {
    cat >&2 <<- EOF
		Usage: $0 command [OPTIONS]

		Command:
		    x509:       generate a self-signed X.509 server certificate
		    csr:        generate a Certificate Signing Request
		    dkim:       generate a private key (to use for DKIM signing)

		Options:
		    -t type:    key type (default: rsa)
		    -b bits:    key length or EC curve (default: 2048 for RSA, 1024 for DSA, secp224r1 for ECDSA)
		    -h digest:  digest algorithm
		    --dns CN:   common name (default: \$(hostname --fqdn); can be repeated
		    -f force:   overwrite key files if they exist
		    --config:   configuration file
		    --pubkey:   public key file (default: pubkey.pem)
		    --privkey:  private key file (default: privkey.pem; created with og-rwx)

		Return values:
		    0  The key pair was successfully generated
		    1  The public or private key file exists, and -f is not set
		    2  The key generation failed
	EOF
}

[ $# -gt 0 ] || { usage; exit 2; }
cmd="$1"; shift
case "$cmd" in
    x509|csr|dkim) ;;
    *) echo "Unrecognized command: $cmd" >&2; exit 2
esac

while [ $# -gt 0 ]; do
    case "$1" in
        -t) shift; type="$1";;
        -t*) type="${1#-t}";;

        -b) shift; bits="$1";;
        -b*) bits="${1#-b}";;

        -h) shift; hash="$1";;
        -h*) hash="${1#-h}";;


        -f) force=1;;
        --pubkey=?*) pubkey="${1#--pubkey=}";;
        --privkey=?*) privkey="${1#--privkey=}";;

        --dns=?*) dns="${dns:+$dns,}${1#--dns=}";;
        --config=?*) dns="${1#--config=}";;

        --help) usage; exit;;
        *) echo "Unrecognized argument: $1" >&2; exit 2
    esac
    shift;
done

rand=/dev/urandom
case "$type" in
    # XXX: genrsa and dsaparam have been deprecated in favor of genpkey.
    # genpkey can also create explicit EC parameters, but not named.
    rsa) genkey=genrsa; genkeyargs="-f4 ${bits:-2048}";;
    dsa) genkey=dsaparam; genkeyargs="-noout -genkey ${bits:-1024}";;
    # See 'openssl ecparam -list_curves' for the list of supported
    # curves. StrongSwan doesn't support explicit curve parameters
    # (however explicit parameters might be required to make exotic
    # curves work with some clients.)
    ecdsa) genkey=ecparam
           genkeyargs="-noout -name ${bits:-secp224r1} -param_enc named_curve -genkey";;
    *) echo "Unrecognized key type: $type" >&2; exit 2
esac

cn=
if [ "$cmd" = x509 -o "$cmd" = csr ]; then
    case "$hash" in
        md5|rmd160|sha1|sha224|sha256|sha384|sha512|'') ;;
        *) echo "Invalid digest algorithm: $hash" >&2; exit 2;
    esac

    [ "$dns" ] || dns="$(hostname --fqdn)"
    cn="${dns%%,*}"
    [ ${#cn} -le 64 ] || { echo "CommonName too long: $cn" >&2; exit 2; }
fi

[ -s "$privkey" -a -z "$force" ] && force=0
if [ "$cmd" != dkim ]; then
    for file in "$pubkey" "$privkey"; do
        [ "$force" != 1 -a -s "$file" ] || continue
        echo "Error: File exists: $file" >&2
        exit 1
    done
fi

if [ -z "$config" -a \( "$cmd" = x509 -o "$cmd" = csr \) ]; then
    config=$(mktemp) || exit 2
    trap 'rm -f "$config"' EXIT

    names=
    until [ "$dns" = "${dns#*,}" ]; do
        names=", DNS:${dns##*,}$names"
        dns="${dns%,*}"
    done

    # see /usr/share/ssl-cert/ssleay.cnf
    cat >"$config" <<- EOF
		[ req ]
		distinguished_name  = req_distinguished_name
		prompt              = no
		policy              = policy_anything
		req_extensions      = v3_req
		x509_extensions     = v3_req
		default_days        = 3650

		[ req_distinguished_name ]
		countryName         = SE
		organizationName    = Fripost
		commonName          = $cn

		[ v3_req ]
		subjectAltName      = email:admin@fripost.org, DNS:$cn$names
		basicConstraints    = critical, CA:FALSE
	EOF
fi

if [ "$force" != 0 ]; then
    # Ensure "$privkey" is created with umask 0077
    mv "$(mktemp)" "$privkey" || exit 2
    chmod og-rwx "$privkey" || exit 2
    openssl $genkey -rand /dev/urandom $genkeyargs >"$privkey" || exit 2
fi

if [ "$cmd" = x509 -o "$cmd" = csr ]; then
    [ "$cmd" = x509 ] && x509=-x509 || x509=
    openssl req -config "$config" -new $x509 ${hash:+-$hash} -key "$privkey" >"$pubkey" || exit 2
elif [ "$cmd" = dkim ]; then
    echo "Add the following TXT record to your DNS zone:"
    echo "${dns:-$(date +%Y%m%d)}._domainkey\tIN\tTXT ( "
    # See https://tools.ietf.org/html/rfc4871#section-3.6.1
    # t=s:      the "i=" domain in signature headers MUST NOT be a subdomain of "d="
    # s=email:  limit DKIM signing to email
    openssl pkey -pubout <"$privkey" | sed '/^--.*--$/d' \
    | { echo -n "v=DKIM1; k=$type; t=s; s=email; p="; tr -d '\n'; } \
    | fold -w 250 \
    | { sed 's/.*/\t"&"/'; echo ' )'; }
    [ "$force" != 0 ] || exit 1
fi