Why security must be designed in in HLD - Design It to Understand It

Design: Security-First System Design

Focus on integrating security principles from the start of system design. Out of scope: detailed cryptographic algorithm design and hardware security modules.

Functional Requirements

FR1: Ensure data confidentiality, integrity, and availability

FR2: Protect user privacy and sensitive information

FR3: Prevent unauthorized access and data breaches

FR4: Comply with relevant security standards and regulations

FR5: Enable secure authentication and authorization

FR6: Detect and respond to security incidents promptly

Non-Functional Requirements

NFR1: System must handle 10,000 concurrent users securely

NFR2: API response latency p99 under 200ms including security checks

NFR3: Availability target of 99.9% uptime

NFR4: Security measures must not degrade user experience significantly

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

❓ Question 6

Key Components

Authentication and Authorization services

Encryption for data at rest and in transit

Secure API gateways and firewalls

Audit logging and monitoring systems

Role-based access control (RBAC)

Security incident and event management (SIEM)

Design Patterns

Defense in Depth

Least Privilege

Fail Secure

Secure by Design

Zero Trust Architecture

Threat Modeling

Reference Architecture

  +-------------------+       +---------------------+       +-------------------+
  |                   |       |                     |       |                   |
  |   Client Devices   +------>+  API Gateway (WAF)  +------>+ Authentication &  |
  |                   |       |                     |       | Authorization Svc |
  +-------------------+       +---------------------+       +---------+---------+
                                                                       |
                                                                       v
                                                             +-------------------+
                                                             |                   |
                                                             |   Application     |
                                                             |     Servers       |
                                                             |                   |
                                                             +---------+---------+
                                                                       |
                                                                       v
                                                             +-------------------+
                                                             |                   |
                                                             |   Database with   |
                                                             |  Encryption at    |
                                                             |    Rest & Audit   |
                                                             +-------------------+

Additional components:
- Monitoring & Logging System
- Security Incident Response
- Role-Based Access Control enforcement

Components

API Gateway with WAF

Nginx + ModSecurity or AWS API Gateway with WAF

Filter and block malicious requests before reaching backend

Authentication & Authorization Service

OAuth 2.0 / OpenID Connect server

Verify user identity and enforce access control

Application Servers

Stateless microservices

Business logic with embedded security checks

Encrypted Database

PostgreSQL with Transparent Data Encryption

Store data securely with encryption and audit logs

Monitoring & Logging System

ELK Stack or Splunk

Collect logs and detect suspicious activities

Role-Based Access Control (RBAC)

Implemented in Auth service and app

Limit user permissions to minimum necessary

Request Flow

1. Client sends request to API Gateway

2. API Gateway filters request using Web Application Firewall rules

3. Request forwarded to Authentication Service to verify user identity

4. Upon successful authentication, Authorization Service checks user permissions

5. Authorized request proceeds to Application Servers for processing

6. Application Servers interact with encrypted Database for data operations

7. All actions are logged and monitored for anomalies

8. Security alerts trigger incident response if suspicious activity detected

Database Schema

Entities: - User(id, username, hashed_password, roles) - Role(id, name, permissions) - Permission(id, name, description) - AuditLog(id, user_id, action, timestamp, resource) Relationships: - User to Role: Many-to-Many - Role to Permission: Many-to-Many - AuditLog linked to User for traceability

Scaling Discussion

Bottlenecks

Authentication service overload under high concurrent login attempts

API Gateway becoming a single point of failure

Database encryption overhead impacting latency

Monitoring system overwhelmed by log volume

Complexity in managing fine-grained access control at scale

Solutions

Use distributed authentication with caching tokens and rate limiting

Deploy multiple API Gateway instances with load balancing

Optimize encryption with hardware acceleration and selective encryption

Implement log sampling and scalable log storage solutions

Adopt attribute-based access control (ABAC) and automate policy management

Interview Tips

Time: Spend 10 minutes understanding security requirements and risks, 20 minutes designing the architecture with security components, 10 minutes discussing scaling and trade-offs, 5 minutes summarizing key points.

Emphasize importance of integrating security from the start to avoid costly fixes later

Discuss defense in depth and least privilege principles

Explain how each component contributes to security goals

Highlight trade-offs between security, performance, and usability

Show awareness of scaling challenges and mitigation strategies