stalk -- encrypted chat using openssl

#!/bin/sh
pgm="${0##*/}" PORT=4242 KEYFILE="/tmp/stalk.$$" DELETE_KEYFILE=1 SERVER=
usage() { exec >&2
    printf "Usage (SERVER): %s [-p PORT] [-k keyfile] -s\n" "$pgm"
    printf "Usage (CLIENT): %s [-p PORT] host\n" "$pgm"
    exit 1
}
while getopts hk:p:s flag; do
    case "$flag" in
    k) KEYFILE="$OPTARG" DELETE_KEYFILE= ;;
    p) PORT="$OPTARG" ;;
    s) SERVER=1 ;;
    *) usage
    esac
done
shift $(( $OPTIND - 1 ))
[ $# -gt 0 ] && { openssl s_client -host "$1" -port "$PORT"; exit; }
[ "$SERVER" ] || usage
[ -e "$KEYFILE" ] || openssl req -nodes -new -x509 -batch \
    -subj / -out "$KEYFILE" -keyout "$KEYFILE" || exit
[ "$DELETE_KEYFILE" ] && trap "rm -f $KEYFILE" EXIT
openssl s_server -cert "$KEYFILE" -accept "$PORT"