From c4536dc4bdc9e922352c1e851cef43308c2393a5 Mon Sep 17 00:00:00 2001
From: Lee Miller <lee.miller@tutanota.com>
Date: Fri, 23 Dec 2022 18:50:33 +0200
Subject: [PATCH] Compute alternative MAC

---
 index.js | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/index.js b/index.js
index 4b257ad..17ef472 100644
--- a/index.js
+++ b/index.js
@@ -3,6 +3,7 @@
 // var objects = require('bitmessage').objects;
 // var Address = require('bitmessage').Address;
 // var TcpTransport = require('bitmessage-transports').TcpTransport;
+var crypto = require("crypto");
 var eccrypto = require("eccrypto");
 
 var assert = exports.assert = function(condition, message) {
@@ -16,6 +17,9 @@ var assert = exports.assert = function(condition, message) {
 	'0409d4e5c0ab3d25fe048c64c9da1a242c7f19417e9517cd266950d72c755713585c6178e97fe092fc897c9a1f1720d5770ae8eaad2fa8fcbd08e9324a5dde1857', 'hex'
     );
     const message = Buffer.from('The quick brown fox jumps over the lazy dog.');
+    const ephemPublicKey = Buffer.from(
+	'02ca00200293213dcf1388b61c2ae5cf80fee6ffffc049a2f9fe7365fe3867813ca812920020df94686c6afb565ac6149b153d61b3b287ee2c7f997c14238796c12b43a3865a', 'hex'
+    );
     const sampleOpts = {
 	iv: Buffer.from('bddb7c2829b08038753084a2f3991681', 'hex'),
 	ephemPrivateKey: Buffer.from(
@@ -24,8 +28,29 @@ var assert = exports.assert = function(condition, message) {
 	)
     };
     const sampleCiphertext = '64203d5b24688e2547bba345fa139a5a1d962220d4d48a0cf3b1572c0d95b61643a6f9a0d75af7eacc1bd957147bf723';
+    const sampleMACKey = 'f83f1e9cc5d6b8448d39dc6a9d5f5b7f460e4a78e9286ee8d91ce1660a53eacd';
     const sampleMAC = 'f2526d61b4851fb23409863826fd206165edc021368c7946571cead69046e619';
+    var mackey = await eccrypto.derive(
+	sampleOpts.ephemPrivateKey, encPublicKey
+    ).then(function(Px) {
+	var hash = crypto.createHash("sha512").update(Px).digest();
+	// var encryptionKey = hash.slice(0, 32);
+	var macKey = hash.slice(32);
+	return macKey
+    });
+    console.log('[mackey]', mackey.toString('hex'));
+
+    assert(mackey.toString('hex') == sampleMACKey, 'Bad MAC key!')
+    
+    var alt_mac = crypto.createHmac("sha256", mackey).update(
+	Buffer.concat([
+	    sampleOpts.iv, ephemPublicKey, Buffer.from(sampleCiphertext, 'hex')])
+    ).digest()
+
+    console.log('[alt mac]', alt_mac.toString('hex'));
+    
     var encrypted = await eccrypto.encrypt(encPublicKey, message, sampleOpts);
+    console.log('[ephem pubkey]', encrypted.ephemPublicKey.toString('hex'));
     console.log('[ciphertext]', encrypted.ciphertext.toString('hex'));
     console.log('[mac]', encrypted.mac.toString('hex'));