diff --git a/README.md b/README.md
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+++ b/README.md
@@ -384,5 +384,6 @@ MySQL localhost:3306 ssl SQL> select * from store_test;
 C:\> "C:\Program Files\MySQL\MySQL Server 8.0\bin\mysqldump.exe" -u root -p nusbank > d:/nusbank.sql
 
 ```
-
-[Next](./server-side.md)
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+## Others
+* [Server Side](./server-side.md)
+* [Security & API](./security-api.md)
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+# Security and API
+
+* Defense on depth
+* Best Practices for web servers
+  + switch off unnecessary funtionality
+  + limit secure remote access
+  + use accounts with limited privileges
+  + segregate development, testing and live environment
+  + segregate data
+  + secrity patches
+  + monitor and audit servers and logs
+  + use security tools
+* Top 10 Application security risks
+  + Injection - 
+  + Broken authentication - compromised passwords
+  + sensitive data exposure
+  + xxe
+  + broken acess control
+  + security misconfiguration
+
+[Equifax case study](https://www.csoonline.com/article/3444488/equifax-data-breach-faq-what-happened-who-was-affected-what-was-the-impact.html)
+* security patches
+* broken authentication
+* broken access control
+* security misconfiguration
+* single point of failure
+* lack of management commitment
+
+## Security od Data in motion
+* HTTPS / FTPS
+* Type of SSL Certificates
+  + Signed by a Cerficate Authority (CA)
+  + Self-signed certificates (less trusted. For testing)
+  
+  + self sigend
+  ```
+  $ openssl genrsa -out key.pem
+  $ openssl req -new -key key.pem -out csr.pem
+  $ openssl x509 -req -days 9999 -in csr.pem -signkey key.pem -out cert.pem
+  $ rm csr.pem
+  ```
+
+  ```Javascript
+
+  ```
+* role based access control
+
+ ## Security date at rest
+ * encrypt
+   + sensitive files
+   + storage drive
+   + password
+* security of username and password 
+
+## Regulation and Standards
+* [international](https://www.pcisecuritystandards.org/)
+* [Singapore](https://www.mas.gov.sg/regulation/guidelines/technology-risk-management-guidelines)
+* [Personal Data Protection Act](https://www.pdpc.gov.sg/Overview-of-PDPA/The-Legislation/Personal-Data-Protection-Act)
+* [FintTech Service](https://singaporefintech.org/sfa-launches-digital-self-assessment-framework-to-fast-track-fintech-firms-partnerships-with-financial-institutions/)
+
+## API Security
+* Basic Authentication
+* API key
+* Bearer token
+* JSON Web Token (JWT)
+
+## SQL Injection attack
+
+* sending body `{ id = "5 or 1=1" }`, will return all records. Thus, data leak
+* need strong validation of inputs
+
+```javascript
+// GET route for /user/id query
+// with body = { "id": 2 }
+app.get("/user/id", (request, response) => {
+    console.log(request.body);
+    // !!! validation example !!!
+    if (request.body.id === null 
+        || Number.isNaN(request.body.id)
+        || !Number.isInteger(request.body.id)) {
+        response.send("Some id error occur");
+        return;
+    }
+    //
+    connection.query(`SELECT * FROM users WHERE user_id = ${request.body.id}`, 
+    (err, result) => {
+        if (err) {
+            response.send("Some id error occur");
+        }
+        else {
+            response.send(result);
+        }    
+    });
+});
+
+```
+
+## Code organisation & management
+* folder tree
+  + \server.js
+  + \database.js
+  + \routes  => store all routes
+  + \routes\customer.js
+  + \routes\seller.js
+    
+* modularise into individual module file
+  + server.js : setup server listen and route linkage
+  ```javascript
+  const express = require("express");
+  const bodyParser = require("body-parser");
+  
+  const CustomerRouter = require("./routes/customer");
+  const SellerRouter = require("./routes/seller");
+  
+  let app = express();  //uri mapper
+  app.use(bodyParser.json());
+  
+  app.use("/customer", CustomerRouter);
+  app.use("/seller", SellerRouter);
+  
+  app.listen(3000);
+  ```
+  + database.js : database connection
+  ```javascript
+  const mysql = require("mysql");
+  
+  let params = {
+    host: "localhost",
+    user: "root",
+    password: "root",
+    database: "sales",
+  };
+  
+  mysqlConnection = mysql.createConnection(params);
+  
+  mysqlConnection.connect((err) => {
+    if (err) {
+      console.log("Failed to Connect");
+    } else {
+      console.log("Connection successful");
+    }
+  });
+  
+  module.exports = mysqlConnection;  
+  ```
+  + customer.js : customer apis
+  ```javascript
+  const express = require("express");
+  const connection = require("../database");  // import database connection
+  
+  CustomerRouter = express.Router();
+  
+  // localhost:3000/customer/
+  CustomerRouter.get("/", (req, res) => {
+    connection.query("select * from customer", (err, rows) => {
+      if (err) {
+        console.log(err);
+        res.send("Some error occurred");
+      } else {
+        res.send(rows);
+      }
+    });
+  });
+  
+  // localhost:3000/customer/name/Zixuan
+  CustomerRouter.get("/name/:value", (req, res) => {
+    connection.query(
+      `select * from customer where customer_name = '${req.params.value}'`,
+      (err, rows) => {
+        if (err) {
+          console.log(err);
+          res.send("Some error occurred");
+        } else {
+          res.send(rows);
+        }
+      }
+    );
+  });
+  
+  // localhost:3000/customer/id/10
+  CustomerRouter.delete("/id/:value", (req, res) => {
+    connection.query(
+      `delete from customer where customer_id = ${req.params.value}`,
+      (err, rows) => {
+        if (err) {
+          console.log(err);
+          res.send("Some error occurred");
+        } else {
+          res.send("Deleted Successfully");
+        }
+      }
+    );
+  });
+  
+  module.exports = CustomerRouter;
+  ```
+  + seller.js : seller apis
+  ```javascript
+  const express = require("express");
+  const connection = require("../database");
+  
+  SellerRouter = express.Router();
+  
+  // localhost:3000/seller/
+  SellerRouter.post("/", (req, res) => {
+    connection.query(
+      `insert into seller (seller_name, seller_wallet) values ('${req.body.  name}',${req.body.wallet})`,
+      (err, rows) => {
+        if (err) {
+          console.log(err);
+          res.send("Some error occurred.");
+        } else {
+          res.send("Saved Successfully.");
+        }
+      }
+    );
+  });
+  
+  module.exports = SellerRouter;
+  ```
+* sample client fetch
+```javascript
+const fetch = require("node-fetch");
+
+fetch("http://localhost:3000/customer/name/Zixuan", {
+  method: "get",
+  //   body: JSON.stringify(
+  //     (body = {
+  //       name: "dixant",
+  //       email: "mittal",
+  //     })
+  //   ),
+  headers: { "Content-Type": "application/json" },
+})
+  .then((res) => res.json())
+  .then((json) => console.log(json));
+```
+
+* API key
+
+```javascript
+
+connection.query(
+  `SELECT * FROM client WHERE client_id = ${request.body.client_id} AND client_key = '${request.body.client_key}'`,
+  (err, result) => {
+    if (ok)
+      serve the request...
+      ...
+  }
+)
+```
+
+* Backend server
+  + formatting conflicting
+  + flexibility with separation
+  + separation on client-ui and backend
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