A system includes a managing center transmitting a data stream encrypted by control words included in control messages. The data stream is received by at least one user unit linked to a security module identified by a unique address. The security module contains a credit which is deducted according to the consumption of data from the stream. The security module can deduct the credit of an amount related to a product or an amount related to a duration, this amount and/or this duration being defined in the control message or in a management message. The user transmits to the managing center an identifier identifying the unique address and a value code representing an amount of credit to reload, the managing center dealing with and checking the value code and transmitting an encrypted message having the unique address and the amount to reload the credit with, to the security module.