Systems for allowing consumers to print postage indicia on mail, rather than purchasing stamps from a post office, are well-known. An example of such a system is an Internet postage system solution that was developed by the assignee of the present application. As shown in FIG. 1, the system includes a United States Postal Service (USPS) certificate authority 10, an operations center 12, a postage generating device 14 coupled between a user's PC 16 and a printer 18, and multiple USPS distribution centers 20, which also act as postage verifiers upon receipt of the mail.
A combination of software running on the user's PC 16 and the postage generating device 14 enables the user to purchase postage from the operations center 12 via the Internet using a variety of payment options. Once obtained, the postage is secured and stored in the postage generating device 14. The user may then print a stamp in the form of a USPS-approved information based indicia (IBI) 22 onto envelopes, labels, or directly onto mail pieces while also printing the destination and return addresses. The IBI 22 is printed as a 2-D barcode that typically includes various information including the name of the user, the ID of the device, the amount of postage remaining, the zip code of the destination, and the date. Since digital imaging, printing, photocopying, and scanning technology make it fairly easy to counterfeit the IBI 22, cryptographic methods, such as asymmetric public key cryptography, have been employed to generate and validate the IBI 22. In the prior art system shown in FIG. 1, for example, the certificate authority 10 transfers a digital certificate, which is a digitally signed public key, and a certificate ID to the postage generating device 14 via the operations center 12. When generating the IBI 22, the postage generating device 14 uses an internally generated private key and the public key to digitally sign the indicia, thereby creating a digital signature. The digital signature and the certificate ID are then included in the IBI 22.
After printing the stamp and applying it to the mail piece, the mail piece is dropped in a local mailbox. The local post office then transfers the mail to a local or originating distribution center 20a. The originating distribution center 20a scans the IBI 22 using a barcode scanner to read the information on the stamp including the certificate ID and the digital signature. The originating distribution center 20a uses the certificate ID to request from USPS authorization center 10 the same digital certificate used to sign the indicia in order to verify whether the IBI 22 is acceptable or fraudulent. All mail pieces with acceptable IBI's 22 are then sorted by the first three digits of the zip code to determine the destination region. The sorted mail is then transferred from the origination distribution center 20a to the respective destination distribution center 20b located in the destination region. The destination distribution center 20b then finishes sorting the mail based on remaining digits of the zip code and the mail is delivered.
Many variations exist to the above scheme for evidencing and verifying postage. For example, U.S. Pat. No. 5,982,896 describes a symmetric fixed key set approach whereby instead of using a private key for each postage generating device 14, a set of keys is created where each key in the set is shared by multiple postage generating devices 14. In addition, the keys are made valid for only a limited amount of time to minimize the harm created by the theft of any of the keys and to limit the time for key attack.
Generating time-limited keys, however, requires that new keys be generated periodically and distributed to the postage generating devices 14. Because the step of distributing the keys typically occurs over the Internet or a private communications link, security for the keys becomes paramount. It is also important to ensure that only authorized devices use those keys.
The method described in U.S. Pat. No. 5,982,896 for securing the keys has several disadvantages. One disadvantage is that the set of the shared keys used by the postage generating devices 14 are downloaded to the originating distribution centers 20 or other postage verifier. The shared keys are individually identified by pointers, which are also downloaded to the postage verifier, but are not cryptographically protected. Thus, the postage verifier has in its possession the entire set of cryptographic keys used by the postage generating devices 14. This fact makes the postage verifier a single point of attack: if the verifier is broken into, a perpetrator may easily impersonate all postage generating devices 14 in the postal system.
Accordingly, what is needed is an improved method for evidencing and verifying postage indicia. The present invention addresses such a need.