Swiftpack.co -  adam-fowler/swift-srp as Swift Package
Swiftpack.co is a collection of thousands of indexed Swift packages. Search packages.
Swift Secure Remote Password
.package(url: "https://github.com/adam-fowler/swift-srp.git", from: "0.4.0")

Swift SRP

This library provides a swift implementation of the Secure Remote Password protocol. Secure Remote Password (SRP) provides username and password authentication without needing to provide your password to the server. As the server never sees your password it can never leak it to anyone else.

The server is provided with a cryptographic verifier that is derived from the password and a salt value that was used in the generation of this verifier. Both client and server generate large private and public keys and with these are able to generate a shared secret. The client then sends a proof they have the secret and if it is verified the server will do the same to verify the server as well.

The SRP protocol is detailed in RFC2945. This library implements version 6a of the protocol which includes the username in the salt to avoid the issue where a malicious server attempting to learn if two users have the same password. I believe it is also compliant with RFC5054.


First you create a configuration object. This will hold the hashing algorithm you are using, the large safe prime number required and a generator value. There is an enum that holds example primes and generators. It is general safer to use these as they are the ones provided in RFC5054 and have been battle tested. The following generates a configuration using SHA256 and a 2048 bit safe prime. You need to be sure both client and server use the same configuration.

let configuration = SRPConfiguration<SHA256>(.N2048)

When the client wants to create a new user they generate a salt and password verifier for their username and password.

let client = SRPClient(configuration: configuration)
let (salt, verifier) = client.generateSaltAndVerifier(username: username, password: password)

These are passed to the server who will store them alongside the username in a database.

When the client wants to authenticate with the server they first need to generate a public/private key pair. These keys should only be used once. If you want to authenticate again you should generate a new pair.

let client = SRPClient(configuration: configuration)
let clientKeys = client.generateKeys()
let clientPublicKey = clientKeys.public

The contents of the clientPublicKey variable is passed to the server alongside the username to initiate authentication.

The server will then find the username in its database and extract the password verifier and salt that was stored with it. The password verifier is used to generate the servers key pair.

let server = SRPServer(configuration: configuration)
let serverKeys = server.generateKeys(verifier: values.verifier)
let serverPublicKey = serverKeys.public

The server replies with the serverPublicKey and the salt value associated with the user. At this point the server will need to store the serverKeys and the public key it received from the client, most likely in a database.

The client then creates the shared secret using the username, password, salt, its own key pair and the server public key. It then has to generate a proof it has the shared secret. This proof is generated from shared secret plus any of the public data available.

let clientSharedSecret = try client.calculateSharedSecret(
    username: username, 
    password: password, 
    salt: salt, 
    clientKeys: clientKeys, 
    serverPublicKey: serverPublicKey
let clientProof = client.calculateClientProof(
    username: username, 
    salt: salt, 
    clientPublicKey: clientKeys.public, 
    serverPublicKey: serverPublicKey, 
    sharedSecret: clientSharedSecret

This clientProof is passed to the server. The server then generates its own version of the shared secret and verifies the clientProof is valid and if so will respond with it's own proof that it has the shared secret.

let serverSharedSecret = try server.calculateSharedSecret(
    clientPublicKey: clientPublicKey, 
    serverKeys: serverKeys, 
    verifier: verifier
let serverProof = try server.verifyClientProof(
    proof: clientProof, 
    username: username, 
    salt: salt, 
    clientPublicKey: clientPublicKey, 
    serverPublicKey: serverKeys.public, 
    sharedSecret: serverSharedSecret

And finally the client can verify the server proof is valid

try client.verifyServerProof(
    serverProof: serverProof, 
    clientProof: clientProof, 
    clientKeys: clientKeys, 
    sharedSecret: clientSharedSecret

If at any point any of these functions fail the process should be aborted.


I believe this library is compliant with RFC5054 and should work with any server implementing this. I have verified this library against the example data in RFC5054, the Mozilla test vectors in https://wiki.mozilla.org/Identity/AttachedServices/KeyServerProtocol#SRP_Verifier and also against the Python library srptools.

Proof of secret

For generating the proof above I use the method detailed in RFC2945 but not all servers use this method. For this reason I have kept the sharedSecret generation separate from the proof generation, so you can insert your own version.

I have also supplied a simple proof functions server.verifySimpleClientProof and client.verifySimpleServerProof which use the proof detailed in the Wikipedia page on Secure Remote Password if you would prefer to use these.


Stars: 4
Last commit: 2 days ago

Ad: Job Offers

iOS Software Engineer @ Perry Street Software
Perry Street Software is Jack’d and SCRUFF. We are two of the world’s largest gay, bi, trans and queer social dating apps on iOS and Android. Our brands reach more than 20 million members worldwide so members can connect, meet and express themselves on a platform that prioritizes privacy and security. We invest heavily into SwiftUI and using Swift Packages to modularize the codebase.

Release Notes

2 days ago
  • Add simple version of client and server proofs detailed in the Wikipedia article on Secure Remote Password.
  • Move Error enums outside of SRPClient and SRPServer into enum SRPClientError and SRPServerError respectively

Swiftpack is being maintained by Petr Pavlik | @ptrpavlik | @swiftpackco | API