Add -rev flag to generate otpauth-migration:// QR-code

Resolves #22

README.md

@@ -26,7 +26,9 @@ to plain [otpauth links](https://github.com/google/google-authenticator/wiki/Key
   -link string
     	migration link (required)
   -qr
-    	generate QR-codes
+    	generate QR-codes (optauth://)
+  -rev
+    	reverse QR-code (otpauth-migration://)
 ```
 
 ## Example

main.go

@@ -1,7 +1,6 @@
 // Google Authenticator migration decoder
 //
 // convert "otpauth-migration" links to plain "otpauth" links
-//
 package main
 
 import (
@@ -32,7 +31,8 @@ func main() {
 		cache = flag.String("cache", "migration.bin", "cache file")
 		http  = flag.String("http", "", "serve http (e.g. localhost:6060)")
 		eval  = flag.Bool("eval", false, "evaluate otps")
-		qr    = flag.Bool("qr", false, "generate QR-codes")
+		qr    = flag.Bool("qr", false, "generate QR-codes (optauth://)")
+		rev   = flag.Bool("rev", false, "reverse QR-code (otpauth-migration://)")
 		info  = flag.Bool("info", false, "display batch info")
 	)
 	flag.Parse()
@@ -54,10 +54,14 @@ func main() {
 		}
 	case *qr:
 		for _, op := range p.OtpParameters {
-			if err := op.WriteFile(op.FileName() + ".png"); err != nil {
+			if err := migration.PNG(op.FileName()+".png", op.URL()); err != nil {
 				log.Fatal("write file: ", err)
 			}
 		}
+	case *rev:
+		if err := migration.PNG("otpauth-migration.png", migration.URL(data)); err != nil {
+			log.Fatal(err)
+		}
 	case *eval:
 		for _, op := range p.OtpParameters {
 			fmt.Printf("%06d %s\n", op.Evaluate(), op.Name)

migration/handler.go

@@ -3,7 +3,7 @@ package migration
 import "net/http"
 
 func (op *Payload_OtpParameters) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-	pic, err := op.QR()
+	pic, err := QR(op.URL())
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return

migration/qrcode.go

@@ -2,23 +2,21 @@ package migration
 
 import (
 	"io/ioutil"
+	"net/url"
 
 	"github.com/skip2/go-qrcode"
 )
 
 // QR image bytes as PNG
-func (op *Payload_OtpParameters) QR() ([]byte, error) {
-	return qrcode.Encode(op.URL().String(), qrcode.Medium, -3)
+func QR(u *url.URL) ([]byte, error) {
+	return qrcode.Encode(u.String(), qrcode.Medium, -3)
 }
 
-// WriteFile writes QR code as PNG to specified file
-func (op *Payload_OtpParameters) WriteFile(fname string) error {
-	pic, err := op.QR()
+// PNG writes QR code as PNG to specified file
+func PNG(filename string, u *url.URL) error {
+	pic, err := QR(u)
 	if err != nil {
 		return err
 	}
-	if err := ioutil.WriteFile(fname, pic, 0600); err != nil {
-		return err
-	}
-	return nil
+	return ioutil.WriteFile(filename, pic, 0600)
 }

migration/unmarshal.go

@@ -31,6 +31,16 @@ func Data(link string) ([]byte, error) {
 	return base64.StdEncoding.DecodeString(data)
 }
 
+func URL(data []byte) *url.URL {
+	v := make(url.Values)
+	v.Add("data", base64.StdEncoding.EncodeToString(data))
+	return &url.URL{
+		Scheme:   "otpauth-migration",
+		Host:     "offline",
+		RawQuery: v.Encode(),
+	}
+}
+
 // Unmarshal otpauth-migration data
 func Unmarshal(data []byte) (*Payload, error) {
 	var p Payload