Ok the form bug was desepear but now the encrypt doesn't work !?
Fields of my SpecificTable are :
Este mensaje contiene información confidencial
The field name of my form are :
Este mensaje contiene información confidencial
$this->execPieceByName('ff_InitLib');
jimport( 'joomla.user.helper' );
$newpassword = ff_getSubmit('motdepasse');
$mynom = ff_getSubmit('nomstagiaire');
$myprenom = ff_getSubmit('prenomstagiaire');
$value = JUserHelper::hashPassword($newpassword);
$db = JFactory::getDBO();
$db->setQuery("UPDATE `#__SpecificTable` SET password = ".$db->quote($value)." WHERE nom = '$mynom' AND prenom='$myprenom' ");
$db->query();
a idea ?