tssrp6a

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.

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, SRPConfig, SRPParameters, SRPRoutines,
} from "tssrp6a"

const srp6aNimbusConfig = new SRPConfig(
  new SRPParameters(),
  (p) => new SRPRoutines(p),
);
const userId = "[e-mail]";
const userPassword = "password";
const { s: salt, v: verifier } = createVerifierAndSalt(
  srp6aNimbusConfig,
  userId,
  userPassword,
);
// store salt and verifier in a data base

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, SRPConfig,
 SRPParameters, SRPRoutines, SRPServerSession
} from "tssrp6a"

const srp6aNimbusConfig = new SRPConfig(
  new SRPParameters(),
  (p) => new SRPRoutines(p),
);

const username = "[e-mail]";
let password = "password";

// Sign up
const { s: salt, v: verifier } = createVerifierAndSalt(
  srp6aNimbusConfig,
  username,
  password,
);

// Sign in
const srp6aNimbusClient = new SRPClientSession(srp6aNimbusConfig);
srp6aNimbusClient.step1(username, password);
// erase password at this point, it is no longer stored
password = ""

const server = new SRPServerSession(srp6aNimbusConfig);
// server gets identifier from client, salt+verifier from db (from signup)
const B = server.step1(username, salt, verifier);

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

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

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

Recomendations

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 chose 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").

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.