Security  

mTLS (Mutual TLS)

A prerequisite to understand this

To understand mTLS (Mutual TLS) clearly, you should be familiar with:

  • Client–server communication

  • HTTPS and TLS basics

  • Public/private key cryptography

  • Certificate Authority (CA)

  • X.509 certificates

No deep cryptography knowledge is required.

Introduction

mTLS (Mutual Transport Layer Security) is a security protocol in which both the client and the server authenticate each other using digital certificates before establishing a secure connection.

Standard TLS → server authentication only

mTLS → both client and server authentication

After successful authentication:

  • Communication is encrypted

  • Identities are verified

  • Trust is established mutually

What problem can we solve with mTLS?

1. Insecure Client Authentication API keys, passwords, and tokens: Can be leaked or reused. Are difficult to rotate safely mTLS replaces them with certificate-based identity.

2. Man-in-the-Middle (MITM) Attacks

  • mTLS ensures: Server authenticity and Client authenticity

  • An attacker cannot impersonate either side.

3. Zero-Trust Architecture :

  • No implicit trust based on network location.

  • Every request is authenticated and verified

4. Secure Service-to-Service Communication

  • Strong identity for each service

  • Reliable authorization decisions

About Certificates

What is a Certificate?

A certificate is a digitally signed identity document that binds an identity (service/user) to a public key

Certificate Components

  • Private Key (never shared)

  • Public Certificate

  • Common Name (CN)

  • Subject Alternative Names (SAN)

  • Issuer (CA)

  • Expiry date

  • Digital signature

Certificate Authority (CA)

  • Signs and validates certificates

  • Establishes trust

  • Can be public or private

Both client and server trust the same CA or trusted chain.

Trust Stores

A Trust Store is a secure repository of trusted Certificate Authority (CA) certificates.

  • Client trust store → trusted server CAs

  • Server trust store → trusted client CAs

A trust store does NOT contain private keys. It only contains, Root CA certificates and Intermediate CA certificates

The trust store is used to verify the identity of the peer during TLS or mTLS communication.

How to Implement This?

Create a Certificate Authority (CA)

  • Issue client and server certificates

  • Configure server to require client certificates

  • Configure client to present its certificate

  • Perform authorization based on certificate identity

  • Establish encrypted communication

mTLS

Identity & Authorization

After authentication:

  • Identity is extracted from certificate (CN, SAN, SPIFFE ID)

  • Authorization rules are applied (RBAC / ABAC)

Advantages

  • Strong mutual authentication

  • No shared secrets (passwords, API keys)

  • End-to-end encryption

  • Resistant to MITM attacks

  • Ideal for zero-trust networks

  • Suitable for microservices and internal APIs

Summary

mTLS = TLS + Client Authentication.

mTLS can be understood as TLS with added client authentication, where both the client and the server verify each other’s identity using digital certificates rather than relying on passwords or shared secrets. Trust is established through a Certificate Authority (CA) that both parties recognize, ensuring that only valid and authorized identities can communicate. Once verification is complete, all communication is fully encrypted and identity-aware, allowing systems to make secure authorization decisions based on cryptographic identity. Because of these properties, mTLS serves as a foundational building block for zero-trust architectures and secure service-to-service communication, especially in modern distributed and microservices-based systems.

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