SMART CARD

Abstract: A smart card designed to be used as a pre-payment device comprises a multi-stage counter which includes a first stage which is decremented during payment. A second stage of the counter operates as set of one or more control bits designed to be switched by a recharging terminal to cause the first stage to be recharged. In this way a smart card having a predetermined capacity can be used as a card of lower capacity, but which can be recharged a limited number of times according to the number of control bits available.

Description: Smart Card This invention relates to smart cards designed to be used as pre-payment memory devices. A smart card basically comprises an integrated circuit which is embedded in the card. The circuit constitutes memory within which information is stored and is read when the card is in use. In the case of a card used for payment purposes, the information stored representing counter value is deducted from the memory as time progresses. One suitable available integrated circuit is known as "Eurochip" sold by Siemens. Such an arrangement can be useful when the card is used for paying the cost of a telephone conversation on a pay-phone or for some other pre-payment device, such as on a pay video channel or electricity meter. The information is stored in a plurality of bits constituting a counter which is decremented as time progresses. It is one object of the invention to provide a card which can be used more than once since this cuts down the considerably on the cost of the card in relation to its value. According to the present invention in one aspect, there is provided a smart card to be used as a pre-payment device comprising a multi-stage counter including a first stage which is decremented during payment; and a second stage which operates as a set of one or more control bits designed to be switched by a recharging terminal to cause the first stage to be re-charged. In this way, for example, a smart card of a certain pre-payment capacity can be used as a card of a lower capacity, but which is re-chargeable a limited number of times according to the number of control bits provided. Preferably the card includes a memory in which key-data from the recharging terminal is combined by means of an algorithm with data indicative of the respective card to generate a certificate which is read during a payment transaction to certify that the card has been recharged by an authentic recharging terminal. In this way, a way is provided of ensuring that the card has been recharged only by an authentic recharging terminal, as opposed to some fraudulent means. In a preferred embodiment, the card indicative data includes data indicative of the number of times that the card has been recharged and the certificate generated is different according to that number. In a preferred embodiment, the first stage of the counter is arranged in one or more ranks of bits, each rank corresponding to one power of the base of which the counter operates, a payment transaction being arranged to decrement the lowest order rank with the decrementing of a value greater than that in the lowest order rank being operative to decrement from a higher order rank and to recharge the lowest order rank, the control bits being located in the highest order rank. In another aspect there is provided a system for recharging with credits a smart card designed to be used as a pre-payment device, the smart card including a first stage which is decremented during payment, and a second stage which operates as a set of one or more control bits designed, when switched, to cause the first stage to be recharged; and a recharging terminal which is operative to switch the control bits on the card. In a further aspect there is provided a method of recharging with credits a smart card designed to be used as a pre-payment device, the smart card including a multi-stage counter including a first stage which is decremented during payment, and a second stage which operates as a set of one or more control bits designed to cause the first stage to be recharged, the method comprising introducing the smart card to a recharging terminal which is operative to switch the control bits so as to cause the first stage of the counter to be recharged. In order that the invention may be well understood, an embodiment thereof will now be described by way of example with reference to the accompanying diagrammatic drawings in which; Figures 1A, IB and IC show the constituent parts of the system required for the invention; Figure 2 shows the contents of the memory of the card shown in Figure 1 A; and Figure 3 shows arrangement of the counter which forms part of Figure 2. Referring to Figures 1 A to IC, a smart card 1 includes a memory 2, in particular as will be described with reference to the "Eurochip" manufactured and sold by Siemens, and an interface 3 by which the card can communicate with a recharge terminal 4, shown in Figure IB. In addition the system comprises a device, such as the telephone 5 shown in Figure IC, with which the card shown in Figure 1A can be used for payment purposes. As will be described in greater detail later on, the card 1 is designed to be sold with an initial predetermined number of credits which are decremented in use by the telephone 5. Once the predetermined number of credits have been used up the card can be introduced into the recharge terminal 4 which comprises an interface 6 for communicating with the card 1, a microprocessor 7 and memory 8. The method of operation will be described in greater detail later on, but essentially the recharge terminal functions, upon payment by the user of the card to the terminal operator, to recharge the card 1 with credits, this can be repeated for a limited number of times. The telephone 5 includes an interface 9 for communicating with the card, a microprocessor 10 connected to telephone circuitry 11 and a security module 12 which provides a means of verifying the authentication of an inserted card. A map showing the functions assigned to the various bits within the memory 2 on the card 1 is shown in Figure 2. In Figure 2, bits 0 to 63 comprise a read-only memory (in user mode) within which card identity, or ID, information is stored. Bits 64 to 104 comprise a counter which is decremented during payment . Figure 3 shows a base eight counter in which bits 72 to 103 are arranged in four rows or ranks R, where each rank corresponds to a power of the base which the counter operates. As shown, bits 96 to 103 represent the lowest value in the counter ie. each bit represents a value of 1 or 8°. Bits 88 to 95 each represent a value of 8, ie. 8 1 . Bits 64 and 69 - 71 are not available for the counter. Bits 65 to 68 are valid counter bits that can be written to once. The card is initially charged by writing a value to these bits and bits 72 to 103 giving a maximum charge of 20,480 units. According to the invention some of the highest value bits, e.g. bits 65 to 68 are used as revaluation flag and only bits 72 to 103 are used as the counter itself (giving a reduced maximum charge of 4,096 units). The counter operates by decrementing from the lowest order rank. Once those bits have been written to a value of zero, the next unit to be decremented causes the unit in the next highest available order rank to be written to a value of zero which thereby refigures all the succeeding bits. Bits 65 to 68 are designed to be swit hed in order to cause the counter to be recharged. In order to prevent the revaluation bits 65 to 68 being recharged fraudulently a certificate is written into the memory of the card each time a revaluation is performed. Referring back to Figure 2, bits 320 to 383 provide the location for four 16 bit recharge certificates, stored in the cards memory. Each recharge certificate is calculated as follows: a recharge algorithm R say, will take as an input a key k and some data d. It will produce as an output a certificate, C. More formally C = R (k,d). The input data, d, consists of a combination of the top counter stage (bits 65 - 68) and card identification data obtained from within bits 0 to 63. The key data k is stored within the memory 8 of the recharge terminal 4. In this way the data uniquely represents the card and its current recharge iteration. The algorithm is such that : is not feasible for an attacker to deduce the key data, k, from a knowledge of R, C and D, except by means of a "brute force" evaluation of all possible keys. The design such that only recharge terminals with valid keys will be able to consistently produce correct recharged certificates. No recharge certificate is required for the initial use. Whenever the card is inserted into a telephone 5 the security module 12 determines which certificate is currently active (e.g. if bits 65 - 68 have a value of 0011 then this implies that a second recharge has been performed). If the certificate stored on the card does not match that calculated by the security module 12, the card is rejected as having an invalid recharge certificate and cannot then be used for payment purposes. Each time the card is recharged, the appropriate bit is written to 0. For example, the first recharge will set bit 65 to 0. Referring back to Figure 2 the memory additionally includes an activation certificate area, bits 112 to 127. The Activation Certificate is generated in a similar way to the recharge certificates described previously. The activation certificate is designed to be written onto the card at the point of sale in order to verify that the card has been sold from an authorised supplier and, for example, has not been stolen. As an alternative the activation certificate could be written into one of the spaces occupied by one of the recharge certificates. This would reduce the number of available re-uses, but would allow bits 112 to 127 to be used for other purposes. In other alternative embodiments different numbers of recharges are possible if a smaller certificate is used. Similarly, if more than four counter bits are used as the top stage of the counter, more recharges would be available. For example if an 8 bit certificate were used, bits 72-75 could also be used to control the number of recharges, but this would mean bits 76 - 79 could not be used for counter value, thus reducing the maximum recharge allowed each time, but allowing 8 recharges, and hence a total of 9 uses.

Claims: CLAIMS 1. A smart card to be used as a pre-payment device comprising a multi-stage counter including a first stage which is decremented during payment; and a second stage which operates as a set of one or more control bits designed to be switched by a recharging terminal to cause the first stage to be recharged. 2. A smart card according to claim 1 in which the card includes a memory within which a certificate is written by the recharging terminal, which certificate is readable during a payment transaction to certify that the card has been recharged by an authentic recharging terminal. 3. A smart card according to claim 2 in which key data from the recharging terminal is combined by means of an algorithm with data indicative of the card to generate the certificate. 4. A smart card according to any preceding claim in which the claim in which the card is rechargeable a limited number of times. 5. A smart card according to claim 4 as dependent on claim 3 in which the card indicative data includes data indicative of the number of times that the card has been recharged and in which the certificate generated is different according to that number. 6. A smart card according to any preceding claim in which the first stage of the counter is arranged in one or more ranks of bits, each rank corresponding to one power of the base at which the counter operates, a payment transaction being arranged to decrement the lowest order rank, with the decrementing of a value greater than that in the lowest order rank being operative to decrement from a higher order rank and to recharge the lowest order rank, the control bits being located in the highest order rank. 7. A system for recharging with credits a smart card designed to be used as a pre-payment device, the smart card including a first stage which is decremented during payment, and a second stage which operates as a set of one or more control bits designed, when switched, to cause the first stage to be recharged; and a recharging terminal which is operative to switch the control bits on the card. 8. A system according to claim 7 in which the recharging terminal is additionally operative to write a certificate onto the card, which certificate is readable during a payment transaction to certify that the card has been recharged by an authentic recharging terminal. A method of recharging with credits a smart card designed to be used as a pre-payment device, the smart card including a multi-stage counter including a first stage which is decremented during payment, and a second stage which operates as a set of one or more control bits designed to cause the first stage to be recharged, the method comprising introducing the smart card to a recharging terminal which is operative to switch the control bits so as to cause the first stage of the counter to be recharged. 10. A method according to claim 9 in which the recharging terminal writes a certificate onto the card which is readable during a payment transaction to certify that the card has been recharged by an authentic recharging terminal.

Inventors: HUMMERSTON WILLIAM, TIERNEY JOHN, HUMMERSTON, WILLIAM, TIERNEY, JOHN
Applicants: HUMMERSTON WILLIAM, PLESSEY TELECOMM, TIERNEY JOHN, GPT LIMITED, HUMMERSTON, WILLIAM, TIERNEY, JOHN
Application year: 1995
Main classification: G06K19
IPC codes: G06K19, G07F7