The private key for each postage meter M is calculated, as shown in FIG. 2, to be the sum of the products AMy,x???mx where X varies from 1 to n and y is a number between 1 to Z. The term AMy,x is the coefficient value at the intersection of a particular row in the matrix 1 for a particular postage meter My and the corresponding column for a particular master private key (m1 to mn).