A portable device, a terminal, a system, and a method of storing data relating to transactions by terminals (1) of merchants in portable loyalty devices (3) of customers of at least one group comprising at least one merchant, said transaction being stored by the terminal (1) of said merchant in the portable device (3) by executing the following steps in any order: storing a first record corresponding to said transaction encrypted with an encryption key (C1) of the customer; and storing a second record corresponding to said transaction encrypted with a key (M1) associated with said group to which said merchant belongs.