The Piggy Bank Cryptographic Trope

This paper presents applications of the trope of the locked and sealed piggy-bank into which the secret can be easily inserted but from which it cannot be withdrawn without opening the box. We present a basic two-pass cryptographic scheme that can serve as template for a variety of implementations. Together with the sealed piggy-bank is sent a coded letter that lists and certifies the contents of the box. We show how this idea can help increase the security of cryptographic protocols for classical systems as well as those based on “single-state” systems. More specifically, we propose the use of a hashing digest (instead of the coded letter) to detect loss of key bits to the eavesdropper and use in communication systems where error correction is an important issue. Introduction The idea of locking a secret in a box and letting it be carried to the destination by an unreliable courier (Figure 1) (where it is unlocked by the recipient who has the key to unlock the box) is at the basis of most cryptographic protocols. This scheme assumes that the key has somehow been transported to the recipient in advance of the communication. The lock of the box is protected by placing a seal across it that ensures that it is not tampered with by the courier. In the case of the use of this scheme in data communication, the key may be transmitted over a side channel. If the rate at which the key is transmitted over the side channel equals the data rate, then this constitutes the unbreakable one-time pad [1].