gopass.go

// Author: johnsiilver@gmail.com (John Doak)

/*
gopass is a library for getting hidden input from a terminal.

This library's main use is to allow a user to enter a password at the
command line without having it echoed to the screen.

The libary currently supports unix systems by manipulating stty.

This code is based upon code by RogerV in the golang-nuts thread:
https://groups.google.com/group/golang-nuts/browse_thread/thread/40cc41e9d9fc9247
*/
package gopass

import (
	"bufio"
	"fmt"
	"os"
	"os/signal"
	"strings"
	"syscall"
)

const (
	sttyArg0   = "/bin/stty"
	exec_cwdir = ""
)

// Tells the terminal to turn echo off.
var sttyArgvEOff []string = []string{"stty", "-echo"}

// Tells the terminal to turn echo on.
var sttyArgvEOn []string = []string{"stty", "echo"}

var ws syscall.WaitStatus = 0

// GetPass gets input hidden from the terminal from a user.
// This is accomplished by turning off terminal echo,
// reading input from the user and finally turning on terminal echo. 
// prompt is a string to display before the user's input.
func GetPass(prompt string) (passwd string, err error) {
	sig := make(chan os.Signal, 10)
	brk := make(chan bool)

	// Display the prompt.
	fmt.Print(prompt)

	// File descriptors for stdin, stdout, and stderr.
	fd := []uintptr{os.Stdin.Fd(), os.Stdout.Fd(), os.Stderr.Fd()}

	// Setup notifications of termination signals to channel sig, create a process to
	// watch for these signals so we can turn back on echo if need be.
	signal.Notify(sig, syscall.SIGHUP, syscall.SIGINT, syscall.SIGKILL, syscall.SIGQUIT,
		          syscall.SIGTERM)
	defer signal.Stop(sig)
	go catchSignal(fd, sig, brk)

	// Turn off the terminal echo.
	pid, err := echoOff(fd)
	if err != nil {
		return "", err
	}

	// Turn on the terminal echo and stop listening for signals.
	defer close(brk)
	defer echoOn(fd)

	rd := bufio.NewReader(os.Stdin)
	syscall.Wait4(pid, &ws, 0, nil)

	line, err := rd.ReadString('\n')
	if err == nil {
		passwd = strings.TrimSpace(line)
	} else {
		err = fmt.Errorf("failed during password entry: %s", err)
	}

	// Carraige return after the user input.
	fmt.Println("")

	return passwd, err
}

func echoOff(fd []uintptr) (int, error) {
	pid, err := syscall.ForkExec(sttyArg0, sttyArgvEOff, &syscall.ProcAttr{Dir: exec_cwdir, Files: fd})
	if err != nil {
		return 0, fmt.Errorf("failed turning off console echo for password entry:\n\t%s", err)
	}
	return pid, nil
}

// echoOn turns back on the terminal echo.
func echoOn(fd []uintptr) {
  // Turn on the terminal echo.
  pid, e := syscall.ForkExec(sttyArg0, sttyArgvEOn, &syscall.ProcAttr{Dir: exec_cwdir, Files: fd})
  if e == nil {
    syscall.Wait4(pid, &ws, 0, nil)
  }
}

// catchSignal tries to catch SIGKILL, SIGQUIT and SIGINT so that we can turn terminal
// echo back on before the program ends.  Otherwise the user is left with echo off on
// their terminal.
func catchSignal(fd []uintptr, sig chan os.Signal, brk chan bool) {
	select {
	case <-sig:
		echoOn(fd)
		os.Exit(-1)
	case <-brk:
	}
}