🔦5. Verify payment information

Verify the payment information for secure payment service implementation.

Based on the payment information from the client, the server verifies the payment amount for fraud and saves the payment information in the database if needed. The following are the steps for verifying the payment information.

• Server receives the i'mport payment ID (imp_uid) and order ID (merchant_uid)

• Call the Get payment API to get the payment details.

• Based on the response, compare the actual payment amount with the payment request amount (from merchant's database).

STEP 01 Server receives payment result

Node.js

Example of receiving a POST request to the merchant endpoint URL that receives the payment information

server-side

app.use(bodyParser.json());
  // "{Merchant endpoint that receives server's payment info}" POST request receiver
  app.post("/payments/complete", async (req, res) => {
    try {
      // Get imp_uid, merchant_uid from req.body
      const { imp_uid, merchant_uid } = req.body; 
    } catch (e) {
      res.status(400).send(e);
    }
  });

STEP 02 Get payment details

Node.js

Example of calling the Get payment API with the i'mport payment ID (imp_uid) to retrieve the payment info.

server-side

app.use(bodyParser.json());
    ...
    app.post("/payments/complete", async (req, res) => {
      try {
        // Get imp_uid, merchant_uid from req.body
        const { imp_uid, merchant_uid } = req.body; 
        ...
        // Get access token
        const getToken = await axios({
          url: "https://api.iamport.kr/users/getToken",
          method: "post", // POST method
          headers: { "Content-Type": "application/json" }, 
          data: {
            imp_key: "imp_apikey", // REST API key
            imp_secret: "ekKoeW8RyKuT0zgaZsUtXXTLQ4AhPFW3ZGseDA6bkA5lamv9OqDMnxyeB9wqOsuO9W3Mx9YSJ4dTqJ3f" // REST API Secret
          }
        });
        const { access_token } = getToken.data.response; // access token
        ...
        // Get payment info from i'mport server using imp_uid
        const getPaymentData = await axios({
          // Pass imp_uid
          url: \`https://api.iamport.kr/payments/\${imp_uid}\`, 
          // GET method
          method: "get", 
          // Add access toke to Authorization header
          headers: { "Authorization": access_token } 
        });
        const paymentData = getPaymentData.data.response; // Payment info
        ...
      } catch (e) {
        res.status(400).send(e);
      }
    });

STEP 03 Verify payment information

Why fraud detection is necessary

Since the payment request is made on the client side, a payment request can be forged or falsified by manipulating the client script. Therefore, you must compare the original requested amount with the actual processed amount after the payment process is complete.

For example, when paying for a product that costs 100,000 won, an attacker can manipulate the client script to change the amount property to a value lower than the actual amount.

Since you cannot prevent script manipulation on the client, you must check for fraud on the server after the payment is processed.

Node.js

Example of comparing the actual payment amount with the payment request amount, performing fraud check on the payment amount, and saving the data in the DB.

server-side

app.use(bodyParser.json());
  ...
  app.post("/payments/complete", async (req, res) => {
    try {
      // Get imp_uid, merchant_uid from req.body
      const { imp_uid, merchant_uid } = req.body; 
      // Get access token
      /* ...Omitted... */
      // Get payment info from iamport server using imp_uid
      /* ...Omitted... */
      const paymentData = getPaymentData.data.response; // Payment info
      ...
      // Get the requested payment amount from the DB
      const order = await Orders.findById(paymentData.merchant_uid);
      const amountToBePaid = order.amount; // Requested payment amount
      ...
      // Verify payment
      const { amount, status } = paymentData;
      // If amount matches. Processed amount === Requested amount
      if (amount === amountToBePaid) { 
        await Orders.findByIdAndUpdate(merchant_uid, { $set: paymentData }); // Save payment info to DB
        ...
        switch (status) {
          case "ready": // Issue virtual account
            // Save virtual account info in DB
            const { vbank_num, vbank_date, vbank_name } = paymentData;
            await Users.findByIdAndUpdate("/* customer id */", { $set: { vbank_num, vbank_date, vbank_name }});
            // Send virtual account issuance text message
            SMS.send({ text: \`Virtual account issued successfully. Account info \${vbank_num} \${vbank_date} \${vbank_name}\`});
            res.send({ status: "vbankIssued", message: "Virtual account issued successfully" });
            break;
          case "paid": // Payment complete
            res.send({ status: "success", message: "General payment successful" });
            break;
        }
      } else { // Amount mismatch. Forged/falsified payment.
        throw { status: "forgery", message: "Forged/falsified payment attempted" };
      }
    } catch (e) {
      res.status(400).send(e);
    }
  });

The original requested amount is queried from the database with the merchant_uid, and the actual processed amount is retrieved from the i'mport server with the imp_uid. The two values ​​are compared to verify that they match. If the verification is successful, the payment information is saved in the database and a response is returned based on the payment status (status). Otherwise, an error message is returned.

The payment result must be processed on the database based on the data received through a webhook for stable processing without any missing result data.