Example of implementation with PHP

// Function that computes the signature
// $params : table containing the fields to send in the payment form
function getSignature($params){
//Initialization of the variable that will contain the string to encrypt
$contenu_signature = "" ;

    // sorting fields alphabetically
    ksort($params);
        foreach ($params as $nom =>$valeur)
    { 
    // Recovery of vads_ fields 
        if (substr($nom,0,5)=='vads_') { 
            // Concatenation with "+"
            $contenu_signature .= $valeur."+";
        }
    }
// Adding the certificate at the end
$contenu_signature .= $key;
// Applying SHA-1 algorythm
$signature = sha1($contenu_signature);
return $signature ;
}