Secure Remote Password (SRP) for Swift

Secure Remote Password is a authentication protocol to prove your identity to another party, using a password, but without ever revealing that password to other parties. Not even the party you are proving your identity. See Secure Remote Password protocol for more information on this protocol.

Example usage

// This is a database of users, along with their salted verification keys
let userStore: [String: (salt: Data, verificationKey: Data)] = [
    "alice": createSaltedVerificationKey(username: "alice", password: "password123"),
    "bob": createSaltedVerificationKey(username: "bob", password: "qwerty12345"),
]

// Alice wants to authenticate, she sends her username to the server.
let client = Client(username: "alice", password: "password123")
let (username, clientPublicKey) = client.startAuthentication()

let server = Server(
    username: username,
    salt: userStore[username]!.salt,
    verificationKey: userStore[username]!.verificationKey)

// The server shares Alice's salt and its public key (the challenge).
let (salt, serverPublicKey) = server.getChallenge()

// Alice generates a sessionKey and proofs she generated the correct
// session key based on her password and the challenge.
let clientKeyProof = try client.processChallenge(salt: salt, publicKey: serverPublicKey)

// The server verifies Alices' proof and generates their proof.
let serverKeyProof = try server.verifySession(publicKey: clientPublicKey, keyProof: clientKeyProof)

// The client verifies the server's proof.
try client.verifySession(keyProof: serverKeyProof)

// At this point, authentication has completed.
assert(server.isAuthenticated)
assert(client.isAuthenticated)

// Both now have the same session key. This key can be used to encrypt
// further communication between client and server.
assert(server.sessionKey == client.sessionKey)

More information can be found in the documentation.

Swift Compatibility

Swift 4 is required with version 3 of this package. Use version 2 if you need Swift 3 compatibility.

Compatibility with other implementations

I like to believe this implementation correctly implements the RFC. However not all implementations do and might result in not being able to authenticate accross implementations. And subtle differences might result in low failure rates due to the randomness this protocol includes.

Python: ❌ srp is not compatible; it doesn’t correctly calculate k.
Python: ✅ srptools is compatible.

Development

Testing

This project includes unit tests. A few compiler flags are required to run the tests swiftly:

swift test -c release -Xswiftc -enable-testing

References

Credits

This library was written by Bouke Haarsma.