This article relies largely or entirely on a single source. (May 2012)
Proton was an electronic purse application for debit cards in Belgium. The system was introduced in February 1995 with the goal to replace cash primarily for small transactions around € 15. For security, the card was limited to storing 125,00 EUR of available electronic cash (originally 5,000 BEF).
The card was used for small payments without a pin code or signature, and ran the same risk as with ordinary cash in that if the card was lost the cash value allocated to the card would also be lost.
The advantage to merchants was that they could accept payments without the necessity for a bank terminal to be connected to a centralised system for approving the transaction (the transaction was approved by the card itself), and the transaction was very quick.
Proton saw limited success in Belgium, despite being available on commonplace on parking meters, pay phones, and within convenience stores; possibly due to a poor understanding of the system. Despite this, the system was also implemented in other countries such as Chipknip in the Netherlands, and has been considered for other markets such as Australia.
Security was based on the message authentication code.
The system has been retired since 31 December 2014. Customers are requested to offload the charged amount back onto their bank account before the expiration date.