.NET Core  

Top Backend Developer Interview Guide 2025: .NET, Node.js, API Security, DevOps & Cloud

In today’s digital landscape, building secure, scalable, and high-performance backend systems is crucial. Whether you're working with .NET Core, Node.js, or deploying APIs in the cloud, a solid foundation in REST API design, encryption, and DevOps practices is essential.

This article covers essential interview-level knowledge and practical implementation techniques for backend developers working across .NET, Node.js, cryptography, cloud security, and API design. It concludes with hands-on tasks to evaluate readiness in real-world scenarios.

.NET (C#) Development Essentials

  • .NET Core vs. .NET Framework
    • .NET Core: Cross-platform, open-source, ideal for microservices and modern applications.
    • .NET Framework: Windows-only, better suited for legacy enterprise apps.
  • Dependency Injection in .NET Core: Leverage IServiceCollection in Startup.cs using methods like AddSingleton, AddScoped, and AddTransient.
  • Building REST APIs with ASP.NET Core
    • Use controllers, define [HttpGet], [HttpPost] routes.
    • Return ActionResult<T> and inject dependencies via constructor injection.
  • Async/Await for Scalability
    • Enables non-blocking I/O operations.
    • Crucial for handling concurrent requests with fewer threads.
  • Exception Handling in Web APIs: Use try/catch blocks, custom middleware, and return meaningful HTTP status codes.
  • Securing API Endpoints: Implement JWT or OAuth2, enable HTTPS, validate input, and use [Authorize].
  • HttpClient Usage: Prefer IHttpClientFactory to prevent socket exhaustion and centralize configuration.

Node.js Development Essentials

  • Asynchronous Operations & Event Loop: Single-threaded event loop with async callbacks handles concurrency efficiently.
  • Streams: Handle data in chunks for memory efficiency. Useful in file processing and large payload APIs.
  • Securing Express.js APIs: Use helmet, JWT, input sanitization, HTTPS, and validation.
  • Environment Variables & Secrets: Use .env and dotenv. In production, integrate secret managers like AWS Secrets Manager.
  • Middleware in Express.js: Use app.use() to inject logging and authentication logic.

Cryptography & Secure Data Handling

  • Symmetric vs. Asymmetric Encryption
    • Symmetric: Same key for encryption/decryption.
    • Asymmetric: Public/private key pair (e.g., RSA).
  • RSA & ECC
    • RSA: Widely used for secure communication.
    • ECC: Smaller keys, faster, same security level as RSA.
  • SHA-256 Hashing: Used for password hashing and data integrity checks.
  • Digital Signatures: Sign with private key, verify with public. Ensures authenticity and integrity.
  • Certificate Storage: Use secure storage (Azure Key Vault, AWS Secrets Manager). Avoid hard-coding.
  • Key Rotation: Maintain backward compatibility temporarily. Monitor, then remove old keys.
  • Preventing MITM Attacks: Use HTTPS, enforce HSTS, and validate certificates.

Secure Token Handling & Certificate Management

  • JWT Structure & Verification
    • Consists of: Header, Payload, and Signature.
    • Verify using public key or shared secret.
  • OAuth 2.0 vs OpenID Connect
    • OAuth: Authorization protocol.
    • OpenID Connect: Extends OAuth to include authentication.
  • Token Issuance in Distributed Systems: Use a trusted token authority. Sign tokens securely and manage expiration.
  • Certificate Signing & Trust Chain: CA-signed certificates are trusted by clients. Self-signed certificates are not.
  • Managing Private Keys in Cloud: Store in secure services and use managed identities.

Cloud & DevOps Best Practices

  • Secrets & Certificate Management
    • Use Azure Key Vault or AWS Secrets Manager.
    • Avoid storing secrets in source control.
  • CI/CD Deployments: Automate builds/tests using GitHub Actions, Azure DevOps, or AWS CodePipeline.
  • CI/CD Security: Scan code, restrict access, use signed build artifacts.
  • Monitoring & Logging: Use Azure Monitor, AWS CloudWatch, ELK, or Datadog.

REST API Design & Security

  • REST Principles: Statelessness, resource-based URIs, and correct HTTP verbs.
  • Security Best Practices: Enforce HTTPS, validate inputs, use tokens, and avoid sensitive data in URLs.
  • API Versioning: Common practice: /api/v1/resource or use custom headers.
  • HTTP Status Codes: 401 Unauthorized, 404 Not Found, 400 Bad Request, 204 No Content.
  • Rate Limiting & Abuse Prevention: Use middleware or external tools like NGINX or Redis for token buckets.

System Design Scenarios

  • Secure Digital Pass System: Issue JWTs with expiration. Maintain a revocation list or blacklist.
  • Certificate Generation Microservice: Stateless service, secure key handling, async job queue, audit logs.
  • Secure Mobile/Web Backend Architecture: Use REST APIs + HTTPS + token auth. Separate identity and data APIs.

Bonus: Hands-On Tasks

.NET API with JWT Auth

services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
  .AddJwtBearer(options => { /* token validation config */ });

Node.js RSA Encryption

const { generateKeyPairSync, publicEncrypt, privateDecrypt } = require('crypto');
const { publicKey, privateKey } = generateKeyPairSync('rsa', { modulusLength: 2048 });
const encrypted = publicEncrypt(publicKey, Buffer.from("secret"));
const decrypted = privateDecrypt(privateKey, encrypted);

.PFX in Azure Key Vault

  • Upload via Azure Portal or CLI.
  • Access with Azure.Security.KeyVault.Certificates.

CI/CD with GitHub Actions

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - name: Setup .NET
        uses: actions/setup-dotnet@v3
        with:
          dotnet-version: '7.0.x'
      - run: dotnet build
      - run: dotnet test

Conclusion

This article is a compact yet powerful reference for backend engineers preparing for interviews or looking to enhance their expertise in secure backend development. From foundational principles to real-world scenarios, it bridges theory and practice in modern backend engineering.

Membership not found