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Table of contents
Mutual Authentication 
How Mutual Authentication Works
Mutual Authentication Process
Mutual Authentication Usecase
Attacks that Mutual Authentication Stops
Frequently Asked Questions
What is authentication?
What kinds of attacks does mutual authentication help stop?
How many types of authentication are present?
What is API mutual authentication?
Last Updated: Mar 27, 2024

It’s Mutual Authentication

Author Shiva
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Ashwin Goyal
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Hello, readers. In this article, we will learn about mutual authentication, how it works, the whole process of mutual authentication, the use case of its various fields, and finally attack that mutual authentication helps solve. 

introductory image

There are two types of authentication, i.e., one-way and mutual authentication. Let’s start this by understanding authentication first. 


A general and simple definition of authentication is the process of showing that a fact or document is genuine. The term authentication is often linked with proving a user's identification in computer science. And that is typically done by user establishing their identity by giving credentials, which are agreed-upon pieces of information communicated between the user and the system.

In one-way authentication, servers do not ask for certificates from the client, whereas in mutual authentication, it does.

one way authentication
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Mutual Authentication 

The term “mutual” indicates that communication is happening between two sides of the channel. And in this case, it’s for verification. 

mutual authentication

Consider an example of ola, uber and how they operate. The driver confirms your identity by name, and you can also confirm the driver’s identity by the car's number plate. You and the driver are exchanging information to confirm each other’s identity. That’s the overall analogy of mutual authentication for you.

Customers secure their information by adding SSL(secure socket layer) certificates to their websites. When a browser connects to a secure server, the SSL protocol is used to connect and verify the server's certificate. 

Customers can, however, use Mutual Authentication to have both the client and server authenticate each other using signed certificates. Mutual Authentication mandates the submission of signed certificates by both the client and the server for verification.

How Mutual Authentication Works

For mutually authenticating the endpoints of a communication systems connection, there are three options:

how mutual authentication works

1. Public Key Authentication: It is based on public key cryptography. A key is used to encrypt or digitally sign information. A public key and a private key are used. The private key is used to decrypt data encrypted with the public key.

Both sides of the conversation advertise a public key, and both must confirm they own the private key that accompanies their public key. Each side sends the other side a digital signature. If the signature can be confirmed using the public key, it means that the right private key was used and the party who supplied the signature is genuine.

2. Certificate Authentication: This method is identical to public key authentication in that both parties have a public key certificate rather than just a public key. The certificate provides extra information that aids in verifying the parties' identities, such as who produced the certificate and public key, to whom the certificate applies, when the certificate expires, and so forth. If both parties have TLS certificates, they can be used for mutual authentication.

3. Username and Password: Mutual authentication requires a certificate on the server side. The server shows a certificate to the client, who confirms it. On the client side, it works similarly to traditional username/password authentication in that the client submits its login and password combination to the server, which confirms the inputs.

Mutual Authentication Process

The following procedures are necessary for mutual authentication:

mutual authentication process

Step 1: First, the client sends a “Client Hello” message with SSL options. The server answers with a “Server Hello” message, with the SSL parameters selected.

Step 2: The server sends a message with the server's certificate. In the CertificateRequest message, the server asks for the client's certificate so the connection can be mutually authenticated.

Step 3: With the “Server Hello Done” message, the server closes its part of the negotiation. The client responds with a Certificate message that includes the client's certificate.

Step 4: In ClientKeyExchangemessage, the client transmits session key information (encrypted with the server's public key). The client sends a CertificateVerify message to the server to confirm ownership of the provided certificate.

Step 5: the client sends a “ChangeCipherSpec” message to activate the agreed choices for all future messages. The client sends the Finished message to allow the server to verify the newly active options.

Step 6: The server sends the ChangeCipherSpec message to enable the negotiated settings for all future messages. The server sends the Finished message to allow the client to inspect the newly activated options.

Mutual Authentication Usecase

Mutual authentication's main use cases include the following:

1. Internet of things: Many times in IoT, a "smart" device may need to verify itself over the internet(which is insecure) to access protected services on a server. Mutual authentication ensures that the information they get is authentic and comes from a valid source, lowering the possibility that an attacker has infiltrated their connections.

2. API Security: Authentication ensures that API calls come from a trusted source. Mutual authentication ensures that an API does not allow attacks and that an API user does not accept counterfeit API replies.

3. Zero Trust Security: Zero Trust is a security framework that requires all users, both inside and outside of an organization's network, to be verified, approved, and continuously validated for config before being permitted or maintaining permissions and data. Zero Trust believes there is no conventional network edge; networks can be local, in the cloud, or a combination or hybrid, with resources and workers located anywhere.

Attacks that Mutual Authentication Stops

Types of attacks mutual authentication can help to prevent:

1. On-path attacks: An attacker is in the center of a connection of two parties in an on-path attack. The attacker intercepts both directions of communication and impersonates both ends of the discussion to each other. Mutual authentication aids in the prevention of this form of attack because the attacker cannot authenticate at both ends of the conversation.

2. Impersonation and spoofing: These attacks are used by attackers to fool a server or a user into thinking they are a known and trustworthy entity. An attacker could impersonate a web server or a user. When both sides must verify, such attacks become significantly more complicated.

3. Credentials theft: Some versions of mutual authentication rely on passwords, which are nevertheless vulnerable to credential theft (when an attacker takes a valid user's password). However, because mutual authentication is typically based on a public key, credential theft is impossible because there are no credentials to steal. This can prevent phishing attacks from being successful.

Frequently Asked Questions

What is authentication?

Verifying whether or not someone or something is who or what they claim to be is known as authentication.

What kinds of attacks does mutual authentication help stop?

Mutual authentication can help avoid MITM(Man in the middle) attacks because both the sender and recipient authenticate each other before delivering their message keys; therefore, if one of the parties is not verified to be who they claim to be, the session will be terminated.

How many types of authentication are present?

There are 3 authentication factors:  password or personal identification number (PIN); a token, such as a passbook; or biometrics, such as fingerprints and voice recognition.

What is API mutual authentication?

The term "mutual authentication" refers to the requirement for the API (service) to authenticate itself to the client application (the service must present its identity to the client).


In this article, we looked into mutual authentication, and we also covered how mutual authentication is done, including steps. If you want to explore more, here are some related articles - 


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