example how to convert Azure AD B2C key info to PEM format

azure-ad-b2c-jwt-rsa.php

<?php                                                                                                                   
                                                                                                                        
// composer require phpseclib/phpseclib                                                                                 
require_once('vendor/autoload.php');                                                                                    
                                                                                                                        
                                                                                                                        
$tenant = 'cosmocode';                                                                                                  
$policy = 'b2c_1_signupandsignin';                                                                                      
                                                                                                                        
$json = file_get_contents("https://$tenant.b2clogin.com/$tenant.onmicrosoft.com/$policy/discovery/v2.0/keys");          
$data = json_decode($json, true);                                                                                       
$keydata = $data['keys'][0]; // FIXME keyid should be compared with kid from token                                      
                                                                                                                        
$rsa = new phpseclib\Crypt\RSA();                                                                                       
$rsa->loadKey(                                                                                                          
    array(                                                                                                              
        'e' => new phpseclib\Math\BigInteger(urlsafeB64Decode($keydata['e']), 256),                                     
        'n' => new phpseclib\Math\BigInteger(urlsafeB64Decode($keydata['n']), 256),                                     
    )                                                                                                                   
);                                                                                                                      
                                                                                                                        
var_dump($rsa->getPublicKey());                                                                                         
                                                                                                                        
                                                                                                                        
                                                                                                                        
// @FIXME use the function from a JWT library                                                                           
function urlsafeB64Decode($input)                                                                                       
{                                                                                                                       
    $remainder = strlen($input) % 4;                                                                                    
    if ($remainder) {                                                                                                   
        $padlen = 4 - $remainder;                                                                                       
        $input .= str_repeat('=', $padlen);                                                                             
    }                                                                                                                   
    return base64_decode(strtr($input, '-_', '+/'));                                                                    
}