gpg-transceiver

#!/bin/sh

usage() {
    cat >&2 <<EOF
usage: `basename $0` [-l] <host> <port> -r <user> [<args>...]

-l specifies "listen" mode as opposed to "connect" mode.
<user> is the user/key ID of the recipient on the other end.
<host> is the hostname to connect to or listen on.
<port> is the port to connect to or listen on.
<arg> specify additional arguments to gpg used during encryption.
EOF
    exit 1
}

listen=n
if [ "$1" = -l ]; then
    listen=y
    shift
fi
if [ $# -lt 2 ]; then
    usage
fi
host=$1
shift
port=$1
shift

# only render these characters in the message;
# this prevents weird control characters from being sent to stdout
chars='a-zA-Z0-9 ,./<>?:;"'\''\\|][{}[-]=_+`~!@#$%^&*()
'
# get the escape prefix for later use
esc=`printf '\033['`

send_lines() {
    while read -r line; do
        printf '%s\n' "$line" | gpg -e -s "$@";
        echo .
    done
}

run_netcat() {
    if [ $listen = y ]; then
        nc -l -p "$port" "$host"
    else
        nc "$host" "$port"
    fi
}

recv_lines() {
    while
        while :; do
            read -r line || exit
            if [ "$line" = . ]; then
                break
            else
                printf '%s\n' "$line"
            fi
        done | gpg -d; do
        :;
    done
}

{ { send_lines "$@" | run_netcat | recv_lines |
    # post-process stdout and stderr;
    # loop is needed to ensure line-buffering
    while read -r line; do printf '%s\n' "$line" | tr -d -c "$chars"; done |
    sed 's/^\(.*\)$/'"$esc"'34m> \1'"$esc"'0m/'; } 3>&1 1>&2 2>&3; } |
    sed 's/^\(.*\)$/'"$esc"'33m| \1'"$esc"'0m/'; 3>&1 1>&2 2>&3