From 4e8537bad8924253eb031900734a1f23635068bb Mon Sep 17 00:00:00 2001
From: Mikkel ALMONTE--RINGAUD <vexitofficial@gmail.com>
Date: Tue, 5 Nov 2024 23:56:40 +0100
Subject: [PATCH] fix(qr): probably the correct implementation (finally)

---
 src/api/qr-pay.ts | 71 ++++++++++++++++++++++++++++++++---------------
 1 file changed, 48 insertions(+), 23 deletions(-)

diff --git a/src/api/qr-pay.ts b/src/api/qr-pay.ts
index fe03935..7988d18 100644
--- a/src/api/qr-pay.ts
+++ b/src/api/qr-pay.ts
@@ -1,48 +1,73 @@
 import { AsnParser } from "@peculiar/asn1-schema";
-import { PrivateKeyInfo } from "@peculiar/asn1-pkcs8";
 import { ECPrivateKey } from "@peculiar/asn1-ecc";
-import { p256 } from "@noble/curves/p256";
+import { PrivateKeyInfo } from "@peculiar/asn1-pkcs8";
+
+import { hashWithHMAC } from "~/core/hmac";
 import { sha256 } from "@noble/hashes/sha256";
-import { base64 } from "@scure/base";
 import { bytesToHex } from "@noble/hashes/utils";
+
+import { base64 } from "@scure/base";
+import { p256 } from "@noble/curves/p256";
+
+import { otp } from "~/api/private/otp";
 import type { Identification } from "~/models";
-import { otp } from "./private/otp";
-import { hashWithHMAC } from "~/core/hmac";
 
+// NOTE: We're only using `IZLY` for now but
+// in the app there's also `SMONEY` as a mode.
 const QrCodeMode = {
   IZLY: "AIZ",
   SMONEY: "A"
 } as const;
 
-const signWithPrivateKey = (textToSign: string, pem: string): Uint8Array => {
-  const cert = AsnParser.parse(base64.decode(pem), PrivateKeyInfo);
-  const keys = AsnParser.parse(cert.privateKey.buffer, ECPrivateKey);
+/**
+ * Generates the signature for the last
+ * part of the QR code payload.
+ */
+const sign = (content: string, keyInfo: string): Uint8Array => {
+  const info = AsnParser.parse(base64.decode(keyInfo), PrivateKeyInfo);
+  const keys = AsnParser.parse(info.privateKey.buffer, ECPrivateKey);
   const privateKey = new Uint8Array(keys.privateKey.buffer);
 
-  const hash = sha256.create();
-  hash.update(textToSign);
+  const hash = sha256.create().update(content).digest();
+  const signed = p256.sign(hash, privateKey).toDERRawBytes();
+
+  // Here's how we can debug this function...
+  // Prerequisites: have the same inputs as the app when generating the QR code,
+  //                so make sure to have the same `identification` object
+  //                and `date` string as the payload.
+  //
+  // 1. Retrieve the public key from the private key
+  // -  Luckily, Izly provides the public key in the ECPrivateKey structure.
+  // const publicKey = p256.getPublicKey(privateKey);
+  //
+  // 2. Grab the last part of the QR code payload
+  // -  so the part we're generating in this function...
+  // const signedFromKotlin = base64.decode("MEUCIG1jEvjmNjx8PWK7u5BwaMverup7vvzSVkI6TYoyRh22AiEA4lmlaaOhu18E3oMo6uMmVQoFzShMZU0Sy8EhaOPbgQQ=");
+  //
+  // 3. Compare the app's generated signature with the hash we generated !
+  // const verified = p256.verify(signedFromKotlin, hash, publicKey);
+  // console.log("verified:", verified);
+  //
+  // When `verified` is true, the signature is valid.
+  // Othrwise, the signature is invalid and there's work to do...
 
-  const signature = p256.sign(hash.digest(), privateKey);
-  return signature.toDERRawBytes();
+  return signed;
 };
 
 /**
- * @returns data to show in a qrcode
+ * Generates the payload that are contained
+ * in the QR codes for the payment in the app.
  */
 export const qrPay = (identification: Identification): string => {
   // Replicate `SimpleDateFormat("yyyy-MM-dd HH:mm:ss")`
   const dateFormatter = new Intl.DateTimeFormat("fr-CA", { timeZone: "UTC", year: "numeric", month: "2-digit", day: "2-digit", hour12: false, second: "2-digit", minute: "2-digit", hour: "2-digit" });
   const date = dateFormatter.format(new Date()).replace(",", "");
+  const hotpCode = otp(identification);
 
-  const hotpCode = otp(identification.seed, identification.refreshCount);
-  identification.refreshCount++; // should increment because we called `otp()`
-  const substring = hotpCode.substring(0, hotpCode.length - 1);
-
-  const str = `${QrCodeMode.IZLY};${identification.userPublicID};${date};3`;
-  let sb = str + ";";
-  sb += bytesToHex(hashWithHMAC(`${str}+${identification.nsse}`, substring)) + ";";
-
-  const signed = base64.encode(signWithPrivateKey(sb, identification.qrCodePrivateKey));
+  const content = `${QrCodeMode.IZLY};${identification.userPublicID};${date};3`;
+  const hmac = bytesToHex(hashWithHMAC(`${content}+${identification.nsse}`, hotpCode));
+  const payload = `${content};${hmac};`;
 
-  return sb + signed;
+  // Concatenate payload with signature.
+  return payload + base64.encode(sign(payload, identification.qrCodePrivateKey));
 };