What is Mutual TLS (mTLS)?

What is Mutual TLS (mTLS)?
What is Mutual TLS (mTLS)?

You must be familiar with SSL (Secure Socket Layers) and TLS (Transport Layer Security) and their role in overseeing web security. SSL or TLS certificates are the most popular shields that protect the transmission of all data over the internet.

But there is a pressing need for more sophisticated security measures as cyber dangers emerge. This is where Mutual TLS or (mTLS authentication comes into the picture.

You have likely encountered TLS numerous times, especially when visiting websites that use the "https" protocol. Before we understand mTLS, it's important to grasp the concept of TLS.

What is TLS?

Transport Layer Security (TLS) is a widely used encryption protocol across the Internet. Initially known as SSL (Secure Socket Layer), TLS performs two crucial functions in a client-server connection.

Firstly, it authenticates the server, ensuring the client can trust its identity. Secondly, TLS encrypts the communication between the client and server, preventing unauthorised third parties from eavesdropping on the transmitted data.

How Does the TLS Protocol Work?

When setting up a web service with TLS, you must generate a pair of keys: a private key and a public key. The terminology commonly used refers to the private key as simply "key" and the public key as "certificate."

The private key, referred to as the "key," is kept confidential, while the public key, contained within the "certificate," is shared with the world.

When a client connects to a server, it requests the server to prove its identity. In response, the server presents its certificate to the client.

To verify the server's authenticity, the client challenges the server by encrypting a secret number using the public key from the certificate and asks the server to decrypt it.

Only someone who holds the corresponding private key can successfully decrypt the number. This process shows that the server holds the private key, confirming its certificate ownership.

You maintain control over your identity verification by never sharing your private key. This serves as the basic idea of how TLS achieves identity verification.

What is mTLS?

Simply put, Mutual TLS (mTLS) is a method of authentication that ensures both parties involved in a network connection are who they claim to be. This assurance is achieved by verifying their private keys and validating the information in their distinct TLS certificates.

How Does mTLS Work: Step-by-Step Guide

Mutual TLS closely resembles the TLS protocol but comprises an additional step before the key exchange. The client sends its public key and certificate to the server.

Next, the server verifies the authenticity of the request by confirming that the client holds the private key corresponding to the shared public key. The popular 'mTLS handshake' involves nine steps, outlined as follows:

  • Step 1: The client initiates the handshake by sending a 'Client Hello' message.
  • Step 2: Upon receiving the 'Client Hello,' the server responds with a 'Server Hello' message containing the same five pieces of information for the server, including a session ID.
  • Step 3: Next, the server sends its certificate, certificate chain, and public key to the client.
  • Step 4: The client verifies two aspects of the server's certificate: its validity and whether it belongs to the intended server. The validity is confirmed by checking the certificate's signature using the certificate authority's public key (CA) that signed it.
  • Step 5: The client generates a random secret key to ensure the certificate belongs to the intended server. It encrypts it using the server's public key and sends it to the server.
  • Step 6: The server decrypts the received random secret key using its private key.
  • Step 7: Eventually, the client sends its certificate, certificate chain, and public key to the server.
  • Step 8: The client performs a similar verification process for its certificate, checking its validity and whether it belongs to the intended client.
  • Step 9: The server generates a random secret key to confirm that the certificate indeed belongs to the client. It encrypts it using the client's public key and sends it to the client.

Applications and Use Cases of Mutual TLS

mTLS guarantees the trustworthiness and security of communication between clients and servers, particularly for connections that don't require user login, such as IoT devices. By employing mTLS, various types of attacks are mitigated, including:

  • Credential Stuffing: Attackers attempt to masquerade as legitimate users using stolen credentials, often obtained from data breaches.
  • Malicious API Requests: mTLS ensures that only authenticated and valid users can make API requests, preventing attackers from sending fraudulent API calls to exploit vulnerabilities.
  • Brute Force Attacks: In a brute force attack, an attacker systematically guesses a user's password through trial and error.
  • Phishing Attacks: Phishing attacks aim to collect user credentials, which can then be exploited to gain unauthorised access to networks or software.
  • Spoofing Attacks: Attackers attempt to mimic a web server or client, deceiving the other party in the communication.
  • On-path Attacks: Attackers position themselves between the client and server, intercepting or manipulating communications.

Mutual authentication through mTLS is applicable in various scenarios where the server needs to verify the authenticity and validity of a specific user or device. In practical applications, mTLS can be used for:

  • mTLS authenticates users who are accessing applications/websites.
  • It authorises devices to connect to corporate or private networks.
  • The protocol also creates secure connections between backend servers and cloud security services.
  • It enhances the security of Internet of Things (IoT) sensors and devices.
  • Additionally, it enables secure data exchanges between businesses through APIs (Business-to-Business or B2B exchanges).
  • mTLS is also known to facilitate trust and validation in places where each component must ensure the authenticity and validity of communication.

Conclusion: Leveraging mTLS Zero Trust

The use of mTLS authentication is common in security frameworks like Zero Trust, where it serves to authenticate an organisation's users, devices, and servers.

Zero Trust verification operates under the assumption that threats can exist both within and outside of the network. As a result, no connections are automatically trusted in the best Zero Trust Solutions.

It follows the principle of granting least-privilege access, meaning each party is only given the necessary level of access. This approach ensures that each user's access is limited to the specific resources they are authorised to access.

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