Sep 21, 20214 min read

Implement Secure Remote Password Protocol (SRP) in your web app

Passwordless Authentication for Better Security

The Secure Remote Password (SRP) protocol is an augmented password-authenticated key exchange protocol.

It is a zero-knowledge-proof authentication protocol, which means both the server and the client don’t need to transmit or store password-related information. But still, they can securely verify the identity of each other.

SRP eases off the complexities of traditional authentication since you don’t need to worry about securing password-related information within the server or in between client-server communication. So, let’s see how SRP works and address some of the core security challenges related to passwords.

How SRP works

Let’s see how SRP achieves the goal of passwordless authentication. I have described the protocol in three steps for comprehension. For a more in-depth understanding, please refer to the official documentation.

Step 1: Registration

When the user registers with the username and password, a salt (random string) is generated. This salt is used alongside the user credentials to generate a secret key.

Then a verifier is generated using the secret key and a specified SRP group. Finally, the verifier, salt, and the SRP group are sent to the server instead of the actual username and password.

So, in a nutshell, no confidential information is sent to the Server. So, no man in the middle can use the verifier salt and SRP group to cause harm.

salt = client.generateRandomSalt() 
secret_key = client.KDF(username, password, salt) 
verifier = client.genVerifier(secret_key,SRP_group)
client.send_to_server(verifier, salt, SRP_group)

The registration process in the SRP protocol - graphic by author

Step 2: Authentication

When it’s time for authentication, the client requests the server to get the salt and the SRP group for the given username. Using that information, the client calculates a secret key and a public key. The public key is sent to the server, keeping the private key on the client-side.

salt, SRP_group = client.getData(username) 
c_secret_key = client.KDF(username, password, salt) 
c_public_key, c_private_key = client.generateKeyPair(SRP_group)
client.sendToServer(c_public_key)

The server also will calculate a key pair using the SRP group and send the public key back to the client.

s_public_key, s_private_key = server.generateKeyPair(SRP_group)
server.sendToClient(s_public_key)

After all, the client and the server will have the following information.

client-side: c_secret_key, c_private_key, s_public_key
server-side: verifier, s_private_key, c_public_key

The authentication process in the SRP protocol - graphic by author

Again, no confidential information is sent via wire between the client and server.

Step 3: Verification

Using the information gained during the authentication, the client and the server independently calculate a similar key, called the session encryption key. This is used in the subsequent requests to exchange information between the client and the server.

// client side
session_key = client.genSessionKey(c_secret_key, c_private_key, s_public_key)// server side
session_key = server.genSessionKey(verifier, s_private_key, c_public_key)

These are called Verifier-based key exchange protocol based on Diffie-Hellman.

The client encrypts the message using the session encryption key and sends it to the server. Then the server decrypts the message and verifies it. If the verification fails, the client’s request is denied.

Otherwise, the server encrypts a message using its session encryption key and sends it back to the client to decrypt it and verify it.

I think now you understand how SRP works, and let’s see how we can implement it in practice.

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Using SRP in Practice

Note: Since SRP involves complex cryptography operations, it is encouraged to use existing tried-and-true implementation instead of programming from scratch. There are already many implementations done in different languages to use one of them in your projects.

For this example, I will be using the thinbus-srp-npm-starter project, which uses thinbus-srp-js, a standard JavaScript SRP implementation.

First, you need to clone the project, install the NPM dependencies and run the project locally.

git clone https://github.com/simbo1905/thinbus-srp-npm-tester.git
cd thinbus-srp-npm-tester
npm install
npm start

Then, try registering at http://localhost:8080/register with a username and a password. If we look at the registration network request at this point, it will look like this.

registration network request - screenshot by the author

When logging in at http://localhost:8080/login, the client will first request the user’s salt and the SRP group from the server via /challenge request.

Using that information, the client computes its public key and encrypted message. Then, the client calls the /authenticate endpoint with the calculated data and the username.

/authenticate network request - screenshot by the author

A - Public (ephemeral) key that is generated on the client-side
M1 - Message that is encrypted on the client-side with the generated session encryption key.

The server will send a message back (M2), encrypted with the same session encryption key, responding to the client’s request. Then the client decrypts and verifies it using the client’s private key.

In the end, the client and server have confirmed each other’s authenticity and continue to share information between them.

encrypted message from the server - screenshot by the author

Final Thoughts

Passwords are the most commonly used authentication mechanism used on web applications. However, it's not easy to implement a flawless password authentication mechanism. Despite all the secure transmission and storing techniques, attackers can still access the passwords with enough patience and resources.

SRP protocol provides a solution for this by introducing a method to authenticate without transmitting or storing any form of password-related information.

Source: Medium - Pavindu Lakshan

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