How do I delegate authentication to external API server?

I have two environments:

• A: Laravel-based application API server
  (I have full permission for this)
• B: External API server for A
  (I don't have permission to access user without his/her authentication)

users table have only two columns, id and external_user_id. id is created by A and external_user_id is created by B.

If I implement a custom provider, I need to follow two contracts:

<?php

interface UserProvider
{
    public function retrieveById($identifier);
    public function retrieveByToken($identifier, $token);
    public function updateRememberToken(Authenticatable $user, $token);
    public function retrieveByCredentials(array $credentials);
    public function validateCredentials(Authenticatable $user, array $credentials);
}

interface Authenticatable
{
    public function getAuthIdentifierName();
    public function getAuthIdentifier();
    public function getAuthPassword();
    public function getRememberToken();
    public function setRememberToken($value);
    public function getRememberTokenName();
}

However, there are very difficult problem: How do I implement retrieveByCredentials() before verifying?

via Chebli Mohamed