Midokura TSSRP6a

This library is a TypeScript implementation of Secure Remote Password SRP6a.

SRP allows a user to authenticate to a server without sending the password (zero-knowledge proof of password) using generated private/public keys.

See https://en.wikipedia.org/wiki/Secure_Remote_Password_protocol

https://tools.ietf.org/html/rfc5054

for all the details.

You can see a real-time demo here.

Requirements & Dependencies

The target of TypeScript output is es6.

This package has zero dependencies. It only needs BigInt native support, or a polyfill which is not included.

Note: This module makes use of Crypto.subtle and therefore only works on HTTPS.

Usage

Signup / registration

The user requests a registration page, the browser will generate a salt and take the user's identity and password and generate a verifier.
The browser sends email, salt, verifier to server. The server saves this to storage. Here is a complete example of signup:

import {
 createVerifierAndSalt, SRPParameters, SRPRoutines,
} from "tssrp6a"
(async ()=> {
    const srp6aNimbusRoutines = new SRPRoutines(new SRPParameters());
    const userId = "[email protected]";
    const userPassword = "password";
    const { s: salt, v: verifier } = await createVerifierAndSalt(
        srp6aNimbusRoutines,
        userId,
        userPassword,
    );
// store salt and verifier in a database
})()

Signin / login

The user starts an authentication session by entering his id and password.

The id is sent with a request to the server, which finds salt and verifier for that id. Server executes step1 to generate private key b and public key B, and responds to browser with salt and B.

The browser generates private key a, public key A and computes M1. Browser makes requests with A and M1.

Server verifies that the credentials were correct with step2, using b and M1. If successful, it also takes A and generates and responds with M2.

Browser may additionally verify the authority of the server from M2 with step3.

Note: a and b are generated for one authentication "session" and discarded immediately.

Here is a complete example of authentication session:

import {
 createVerifierAndSalt, SRPClientSession, SRPParameters, SRPRoutines,
 SRPServerSession
} from "tssrp6a"

(async ()=> {
    const srp6aNimbusRoutines = new SRPRoutines(new SRPParameters());

    const username = "[email protected]";
    let password = "password";

// Sign up
    const {s: salt, v: verifier} = await createVerifierAndSalt(
        srp6aNimbusRoutines,
        username,
        password,
    );

// Sign in
    const srp6aNimbusClient = new SRPClientSession(srp6aNimbusRoutines);
    await srp6aNimbusClient.step1(username, password);
// erase password at this point, it is no longer stored
    password = ""

    const server = new SRPServerSession(srp6aNimbusRoutines);
// server gets identifier from client, salt+verifier from db (from signup)
    const B = await server.step1(username, salt, verifier);

// client gets challenge B from server step1 and sends prove M1 to server
    const {A, M1} = await srp6aNimbusClient.step2(salt, B);

// servers checks client prove M1 and sends server prove M2 to client
    const M2 = await server.step2(A, M1);

// client ensures server identity
    await srp6aNimbusClient.step3(M2);
})()

Recommendations

SRP alone only prevents a man-in-the-middle attack from reading the password, but such an attack could also inject code into the browser to hijack the password.

Always use SRP in combination with HTTPS. Browsers can be vulnerable to: having malicious certificates installed beforehand, rogue certificates in the wild, server misconfiguration, bugs like the heartbleed attack, servers leaking password into errors and logs. SRP in the browser offers an additional hurdle and may prevent some mistakes from escalating.

The client can choose to exclude the identity of its computations or not. If excluded, the id cannot be changed. But this problem is better solved by an application schema that separates "identity" from "authentication", so that one identity can have multiple authentications. This allows to switch identity + password, and also to user more than one way of logging in (think "login with email+password, google, or facebook").

Serialization

The SRP protocol and therefore this library is stateful. Each step sets various internal state. Due to the randomness of some of this state (namely the public and private values), repeating the step methods with the same arguments is unlikely (almost definitely) to result in the same state. This proves to be an issue when using a stateless protocol such as HTTP (as opposed to websockets). The server "session" state (the server step 1 state) might not be easily kept in memory. Therefore, we provide a way to serialize and deserialize the step classes in order to restore state. serialize.test.ts shows some examples here's an explanation of how it works:

const serverStep1 = await new SRPServerSession(TEST_ROUTINES).step1(...); // Each step returns a class, in this case .step1 returns SRPServerSessionStep1

const serializedServerStep1 = JSON.stringify(serverStep1); // Some of the step methods (see below for which ones) have a .toJSON method that returns the internal state. JSON.stringify calls .toJSON
// you can now store serializedServerStep1 in a database or elsewhere. There are security implications, see below. 

// when you are ready to restore the state, call fromState on the same step class used to serialize the data (in this case SRPServerSessionStep1) to deserialize
const deserializedServerStep1 = SRPServerSessionStep1.fromState(
    TEST_ROUTINES, // first param is the routines
    JSON.parse(serializedServerStep1),
);

// deserializedServerStep1 is now functionally equivilent as serverStep1 because it contains the same state

Supported steps/classes for serialization are:

• SRPServerSessionStep1
• SRPClientSessionStep1
• SRPClientSessionStep2

While the password is never kept directly in the state, hashes of it are. If an adversary is able to access the serialized state it will likely open you up to some kind of MITM attack and depending on the step, may allow an attacker to perform a bruteforce and/or dictionary attack to retrieve the password. Do not expose the serialized data. For clients, this means do not send it over the network and be careful where you store it. For servers, only send it in encrypted form to parties you trust (such as your database). If you believe state at anytime may have been exposed, it is suggested you change passwords as soon as possible.

Notes

This package's default configuration matches the following Java's Nimbus SRP configuration:

SRP6CryptoParams.getInstance(2048, "SHA-512")

The default routines does not strictly follow SRP6a RFC because user identity is NOT included in the verifier generation. This makes possible for malicious server to detect if two users share the same password but also allows client to change it "identity" without regenerating password.

This example shows how to make SRP client strictly compliant with SRP6a specification.