fun performLogin(username: String, password: String) {
    repository
        .getNonce()
        .flatMap { nonce -> repository.getDeviceAttestation(nonce) }
        .flatMap { attestation -> repository.login(username, password, attestation) }
        .subscribe({
            loginResponse -> handleLoginSuccess(loginResponse)
        }, {
            error -> handleLoginFailure(error)
        })
}