From a250ee9f912acb357d1eaf7f880070f7a9c57249 Mon Sep 17 00:00:00 2001
From: Daniel Huigens <d.huigens@protonmail.com>
Date: Thu, 1 Nov 2018 14:39:03 +0100
Subject: [PATCH] Clean up checksum calculation

---
 src/packet/public_key_encrypted_session_key.js |  9 ++++-----
 src/packet/secret_key.js                       | 10 +++-------
 src/util.js                                    | 16 +++++-----------
 3 files changed, 12 insertions(+), 23 deletions(-)

diff --git a/src/packet/public_key_encrypted_session_key.js b/src/packet/public_key_encrypted_session_key.js
index 895586da..260497da 100644
--- a/src/packet/public_key_encrypted_session_key.js
+++ b/src/packet/public_key_encrypted_session_key.js
@@ -108,8 +108,7 @@ PublicKeyEncryptedSessionKey.prototype.encrypt = async function (key) {
   let data = String.fromCharCode(enums.write(enums.symmetric, this.sessionKeyAlgorithm));
 
   data += util.Uint8Array_to_str(this.sessionKey);
-  const checksum = util.calc_checksum(this.sessionKey);
-  data += util.Uint8Array_to_str(util.writeNumber(checksum, 2));
+  data += util.Uint8Array_to_str(util.write_checksum(this.sessionKey));
 
   let toEncrypt;
   const algo = enums.write(enums.publicKey, this.publicKeyAlgorithm);
@@ -142,15 +141,15 @@ PublicKeyEncryptedSessionKey.prototype.decrypt = async function (key) {
   let decoded;
   if (algo === enums.publicKey.ecdh) {
     decoded = crypto.pkcs5.decode(result.toString());
-    checksum = util.readNumber(util.str_to_Uint8Array(decoded.substr(decoded.length - 2)));
+    checksum = util.str_to_Uint8Array(decoded.substr(decoded.length - 2));
   } else {
     decoded = crypto.pkcs1.eme.decode(result.toString());
-    checksum = util.readNumber(result.toUint8Array().slice(result.byteLength() - 2));
+    checksum = result.toUint8Array().slice(result.byteLength() - 2);
   }
 
   key = util.str_to_Uint8Array(decoded.substring(1, decoded.length - 2));
 
-  if (checksum !== util.calc_checksum(key)) {
+  if (!util.equalsUint8Array(checksum, util.write_checksum(key))) {
     throw new Error('Checksum mismatch');
   } else {
     this.sessionKey = key;
diff --git a/src/packet/secret_key.js b/src/packet/secret_key.js
index 497dce3c..c335114d 100644
--- a/src/packet/secret_key.js
+++ b/src/packet/secret_key.js
@@ -59,10 +59,6 @@ function SecretKey(date=new Date()) {
 SecretKey.prototype = new publicKey();
 SecretKey.prototype.constructor = SecretKey;
 
-function write_checksum(c) {
-  return util.writeNumber(util.calc_checksum(c), 2);
-}
-
 // Helper function
 
 function parse_cleartext_params(cleartext, algorithm) {
@@ -122,7 +118,7 @@ SecretKey.prototype.read = function (bytes) {
     //   key data.  These algorithm-specific fields are as described
     //   below.
     const cleartext = bytes.subarray(1, -2);
-    if (!util.equalsUint8Array(write_checksum(cleartext), bytes.subarray(-2))) {
+    if (!util.equalsUint8Array(util.write_checksum(cleartext), bytes.subarray(-2))) {
       throw new Error('Key checksum mismatch');
     }
     const privParams = parse_cleartext_params(cleartext, this.algorithm);
@@ -142,7 +138,7 @@ SecretKey.prototype.write = function () {
     arr.push(new Uint8Array([0]));
     const cleartextParams = write_cleartext_params(this.params, this.algorithm);
     arr.push(cleartextParams);
-    arr.push(write_checksum(cleartextParams));
+    arr.push(util.write_checksum(cleartextParams));
   } else {
     arr.push(this.encrypted);
   }
@@ -304,7 +300,7 @@ SecretKey.prototype.decrypt = async function (passphrase) {
     if (s2k_usage === 255) {
       hashlen = 2;
       cleartext = cleartextWithHash.subarray(0, -hashlen);
-      hash = write_checksum(cleartext);
+      hash = util.write_checksum(cleartext);
     } else {
       hashlen = 20;
       cleartext = cleartextWithHash.subarray(0, -hashlen);
diff --git a/src/util.js b/src/util.js
index e2eff20d..584394e5 100644
--- a/src/util.js
+++ b/src/util.js
@@ -378,20 +378,14 @@ export default {
    * Calculates a 16bit sum of a Uint8Array by adding each character
    * codes modulus 65535
    * @param {Uint8Array} Uint8Array to create a sum of
-   * @returns {Integer} An integer containing the sum of all character
-   * codes % 65535
+   * @returns {Uint8Array} 2 bytes containing the sum of all charcodes % 65535
    */
-  calc_checksum: function (text) {
-    const checksum = {
-      s: 0,
-      add: function (sadd) {
-        this.s = (this.s + sadd) % 65536;
-      }
-    };
+  write_checksum: function (text) {
+    let s = 0;
     for (let i = 0; i < text.length; i++) {
-      checksum.add(text[i]);
+      s = (s + text[i]) & 0xFFFF;
     }
-    return checksum.s;
+    return util.writeNumber(s, 2);
   },
 
   /**