A system (10) for controlling the validity of printing of indicias on mailpieces from a potentially large number of users of postage meters (12) includes apparatus disposed in each said postage meter for generating a code and for printing the code on each mailpiece. The code is an encrypted code representative of the postage meter apparatus printing the indicia and other information uniquely determinative of the legitimacy of postage on the mailpieces. The keys (40, 42) for the code generating apparatus are changed to change its code generation at predetermined time intervals in each of the meters (12). A security center (16) includes apparatus for maintaining a security code database (30) and for keeping track of the keys for generating security codes in correspondence with the changes in each generating apparatus and the information printed on the mailpiece by the postage meter apparatus (12) for comparison with the code printed on the mailpiece. There may be two codes printed, one used by the Postal Service for its security checks and one by the manufacturer (22). The encryption key may be changed at predetermined intervals or on a daily basis or for printing each mailpiece.