diff --git a/test/checksum.js b/test/checksum.js
new file mode 100644
index 0000000..3a15dd4
--- /dev/null
+++ b/test/checksum.js
@@ -0,0 +1,33 @@
+const crypto=require("crypto");
+const assert=require("assert");
+const bufferEqual=require("buffer-equal");
+
+function writeLI64(value, buffer, offset) {
+  buffer.writeInt32LE(value[0], offset+4);
+  buffer.writeInt32LE(value[1], offset);
+  return offset + 8;
+}
+
+// based on https://s.yawk.at/QADm and https://confluence.yawk.at/display/PEPROTOCOL/Encryption
+describe("checksum",() => {
+  it("generate hash and checksum",() => {
+    let packetPlaintext = new Buffer("3C00000008","hex");
+    let sendCounter = [0,1];
+    let secretKeyBytes = new Buffer("ZOBpyzki/M8UZv5tiBih048eYOBVPkQE3r5Fl0gmUP4=","base64");
+
+    /////
+
+    let digest = crypto.createHash('sha256');
+    // sendCounter to little-endian byte array
+    let counter=new Buffer(8);
+    writeLI64(sendCounter,counter,0);
+    digest.update(counter);
+    digest.update(packetPlaintext);
+    digest.update(secretKeyBytes);
+    let hash = digest.digest();
+    assert(bufferEqual(hash, new Buffer("WkRtEcDHqlqesU6wdSnIz7cU3OCNKVMIsX3aXZMLRjQ=","base64")),hash.toString("base64"));
+
+    let checksum = hash.slice(0,8);
+    assert(bufferEqual(checksum, new Buffer("5A446D11C0C7AA5A","hex")));
+  })
+});
\ No newline at end of file
diff --git a/test/ecdh_key_exchange.js b/test/ecdh_key_exchange.js
index db3e88d..a0e95b7 100644
--- a/test/ecdh_key_exchange.js
+++ b/test/ecdh_key_exchange.js
@@ -4,6 +4,7 @@ const assert=require("assert");
 const bufferEqual=require("buffer-equal");
 
 // based on https://s.yawk.at/VZSf and https://confluence.yawk.at/display/PEPROTOCOL/Encryption
+// and https://github.com/mhsjlw/pocket-minecraft-protocol/issues/15
 describe("ecdh key exchange",() => {
   it("generate the secret",() => {