Add ability to create Threema ID QR code content (#15)

Author: marchof

README.md

@@ -219,6 +219,21 @@ You can query the remaining credits via API:
 System.out.println("Remaining credits: " + gw.getRemainingCredits());
 ```
 
+### Threema QR Code
+
+To establich trust with your users you can create a QR code with your
+Threema ID and your public key. When the user scans that QR code your
+gateway ID will show "green" in their contact list. The `qrcode()`
+method creates the text content which need to be converted to a QR
+graphic with a library of your choice (e.g.
+[zxing](https://github.com/zxing/zxing)).
+
+```java
+PublicKey publicKey = KeyEncoder.getPublicKey(privateKey);
+String qrtext = KeyEncoder.qrcode(threemaid, publicKey);
+System.out.println(qrtext);
+```
+
 ### Callback Handling
 
 You can configure your own HTTP server to receive messages from the

src/main/java/com/mountainminds/three4j/KeyEncoder.java

@@ -128,7 +128,7 @@ public void nextBytes(byte[] bytes) {
 	 * @param publicKey public key
 	 * @return QR code text
 	 */
-	public String qrcode(ThreemaId threemaid, PublicKey publicKey) {
+	public static String qrcode(ThreemaId threemaid, PublicKey publicKey) {
 		return String.format("3mid:%s,%s", threemaid.getValue(), encode(publicKey));
 	}
 

src/test/java/com/mountainminds/three4j/KeyEncoderTest.java

@@ -26,4 +26,12 @@ public void getPublicKey_should_return_corresponding_public_key() {
 		assertEquals(pair.getPublic(), publicKey);
 	}
 
+	@Test
+	public void qrcode_should_create_threema_qr_code() {
+		var threemaid = ThreemaId.of("*GWYTEST");
+		var publicKey = KeyEncoder.decodePublicKey("1234567812345678123456781234567812345678123456781234567812345678");
+		var qr = KeyEncoder.qrcode(threemaid, publicKey);
+		assertEquals("3mid:*GWYTEST,1234567812345678123456781234567812345678123456781234567812345678", qr);
+	}
+
 }

src/test/java/com/mountainminds/three4j/guide/ApiGuide.java

@@ -80,6 +80,7 @@ public static void main(String[] args) throws Exception {
 		ThreemaId receiverId = lookups(gw);
 		sendtextmessage(gw, myPrivateKey, receiverId);
 		simplemessage(gw);
+		threemaidqrcode(from, myPrivateKey);
 
 		callbackserver(gw, secret, myPrivateKey);
 
@@ -252,6 +253,24 @@ static void accountinfo(Gateway gw) throws IOException {
 
 	}
 
+	private static void threemaidqrcode(ThreemaId threemaid, PrivateKey privateKey) {
+		// ### Threema QR Code
+		//
+		// To establich trust with your users you can create a QR code with your
+		// Threema ID and your public key. When the user scans that QR code your
+		// gateway ID will show "green" in their contact list. The `qrcode()`
+		// method creates the text content which need to be converted to a QR
+		// graphic with a library of your choice (e.g.
+		// [zxing](https://github.com/zxing/zxing)).
+
+		// <CODE>
+		PublicKey publicKey = KeyEncoder.getPublicKey(privateKey);
+		String qrtext = KeyEncoder.qrcode(threemaid, publicKey);
+		System.out.println(qrtext);
+		// </CODE>
+
+	}
+
 	static void callbackserver(Gateway gw, String secret, PrivateKey myPrivateKey) throws Exception {
 
 		// ### Callback Handling