Add new tool, promptsecret (decred#649)

promptsecret is used to read a secret from stdin without echoing and
write it to stdout.

.gitignore

@@ -1,5 +1,6 @@
-cmd/dcrd/dcrd
-cmd/dcrd/dcrctl
+dcrd
+cmd/dcrctl/dcrctl
+cmd/promptsecret/promptsecret
 *~
 *.pyc
 vendor/

cmd/promptsecret/promptsecret.go

@@ -0,0 +1,36 @@
+// Copyright (c) 2017 The Decred developers
+// Use of this source code is governed by an ISC
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"golang.org/x/crypto/ssh/terminal"
+)
+
+func zero(b []byte) {
+	for i := 0; i < len(b); i++ {
+		b[i] = 0x00
+	}
+}
+
+func main() {
+	fmt.Fprint(os.Stderr, "Secret: ")
+
+	secret, err := terminal.ReadPassword(int(os.Stdin.Fd()))
+	fmt.Fprint(os.Stderr, "\n")
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "unable to read secret: %v\n", err)
+		os.Exit(1)
+	}
+
+	_, err = os.Stdout.Write(secret)
+	zero(secret)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "unable to write to stdout: %v\n", err)
+		os.Exit(1)
+	}
+}