PGS is a lightweight framework for micro-payments based on two simple ideas:
- Use group selfies to represent all actors of a transaction and establish links among them
- Apply visual cryptography to produce secure shares of selfies, so that a single share has no value, but shares can be combined to reconstruct the transaction context using the human visual system only (no technological platform required).
Today, people of all backgrounds and cultures take selfies. Photo-taking is offered even by the cheapest smartphones and it requires virtually no training or technology awareness. The human visual system has always been used by humans to establish the context (purchaser/supplier roles, object of the transaction, price) of commercial transactions. In PGS schema, group selfies are used as context representations, where all the parties participating in a payment protocol can be represented in a self-validating way: the purchaser, the supplier, the good or service purchased, the amount, the time and place of the transaction. Each image is divided into shares by using a Visual Cryptographic (VC) scheme that gives strong security guarantees.
The most relevant property of VC schemes is that the reconstruction of the original image can be performed by simply stacking the shares, using the human visual system to perform the decoding operation. Also, the computation of the shares is simple procedures and requires low computational power. Finally, once the shares have been generated, any attempt to doctor or modify them impairs the original image’s reconstruction. Strong mathematical proofs of these properties have been given in the literature, including work by some of the authors of this proposal.