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
	$signature_content   =  "" ;
	
	//sorting fields alphabetically
	
	ksort($params);
     foreach ($params as $name =>$value)
	{  
	// Recovery of vads_ fields

       if (substr($nom,0,5)=='vads_') {                   
           // Concatenation with  "+"       
           $signature_content .= $value."+";
	  }
	}
// Adding the certificate at the end
$signature_content  .= $key;
// Applying SHA-1 algorythm
$signature = sha1($signature_content );

return $signature ;
}