FR2632101A1 - Transaction system of the electronic wallet type - Google Patents

Abstract

A card 2 having a memory microcircuit 20 with access controlled by hard-wired logic to the microprocessor 21 may interact with a terminal 1. The memory, in epROM (or e<2>pROM) technology, includes value words (for example, of thirty-two bits, of which twenty-five are value bits, four bits serve to code the value unit allocated to each of these bits, and a validation bit). By loading the memory with a plurality of words possessing the series of value units graded according to the successive powers of 2, calculating means may simply break down any transaction amount below the maximum value of the card into binary form, by consuming, for each transaction, at most one single bit for the value units, except possibly the highest.

Description

Electronic purse type transaction system.

The invention relates to the technical field of mid-point cards
crocircuits (chips), here named more briefly
"card (s)". The invention is of particular interest
their application as a means of payment.

For this purpose, bank cards are widely used.
magnetic stripe. First steps are taken for not
ser to microcircuit or chip bank cards. In
this last type of cards, -the memory processes a sum
of money as a number, encoded in several bytes or
memory cells, like an accounting entry
or a check. There is therefore no simple link between
each bit of memory is a unit of monetary value.

It is then necessary to ensure the trust (of the
credit) that is made to the card user as well
than on the card itself. The most common way used
for this is the confidential code. And we know that the
verification of a confidential code involves a procedure
which weighs down the handling of the card.

It is possible, to some extent, to avoid the re
systematic course with confidential code, with cards
say "prepaid". In these, each bit of memory
has a value fixed in advance. As the name indi
that, the owner of such a card pays the opposite
them before using it. Confidence in the card
it itself is therefore total. And, as the sums at stake are
relatively low, it is not necessary to check
that its holder is the legitimate possessor.

The so-called "tele-cards" cards, of the pre-paid type, function
currently in France with telephone devices
public. Their implementation is simple, because any telephone debt is defined as a number of telephone units to be paid.

There is presently a need for cards of the prepaid type, but which are suitable for applications broader than these "calling cards". In other words, it is about turning a card into an electronic wallet or wallet.

One of the problems that we then encounter is to optimize the card, knowing that its memory capacity is limited, and that the number of reloads that can be carried out in the card is also limited. Other factors intervene, such as the fact that the memory is of epROM technology, or of eepROM technology, also noted e2pROM. A distinction should also be made between cards which contain memory associated with wired logic, generally on a single chip, and cards which contain memory and a microprocessor in a single chip or else in two separate chips.

On the other hand, it is necessary in practice to avoid any "change of change" during transactions. This makes it possible to reserve the recharging of the card only in the event that the user wishes to acquire new liquidity (in the card), this operation being able to require a check of the credit granted to this user.

The object of the present invention is to provide solutions to these problems.

The invention therefore relates to a transaction system, of the type comprising at least one terminal and a microcircuit card suitable for cooperating with this terminal. The card includes a permanent write memory, possibly renewable, (e2pROM), at least certain bits of which are assigned a unit of value, which can be consumed by writing.

According to a general characteristic of the invention, the memory contains a coding zone making it possible to define at least one base value, from which can be established a series of value units, scaled according to the successive powers of 2. This memory also contains a plurality of value words, each associated with one of the value units, and each containing a predetermined number of value bits having this same value unit. Finally, the system comprises calculation means suitable for breaking down a monetary amount in binary with respect to said base value, and to establish commands to write or not a value bit in each of said value words according to the corresponding significant bit in said decomposition, the remaining higher order bits serving , if applicable, when writing to the most significant value word.

The Applicant has observed that statistically, the binary decomposition of a monetary amount gives on average as many zeros as there are ones. Consequently, a card constituted as it has just been defined must allow on average a number of transactions equal to the double of the said predetermined number of value bits which each value word possesses.

Very advantageously, the coding zone defines at least four, preferably at least six consecutive value units. However, this number is not limiting.

It may depend on the number of value words associated with the least significant value units. As will be seen later, it is possible to obtain a unit of auxiliary value equal to half of the unit of base value, by consuming bits of value already established in the card. It will also be seen that it is possible to associate several words of value with the same unit of value. Finally, it is possible to provide several sets of values operating as has just been indicated from different base values, with or without overlap.

Preferably, each value word contains an encoding sub-zone defining the unit of value which is associated with it; That is, the aforementioned coding area consists of the combination of coding sub-areas incorporated into each value word.

It is, however, conceivable that the coding zone be distinct from the words, and define their respective units of value by pointing to them, or by another mode of designation, such as the direct association of addresses in memory with multiples of the base unit of value, which is then the only parameter to be written into memory.

The invention also provides that, in the absence of value bits for certain value units, they can be regenerated by consuming more significant value bits. The different means that can be used for this "automatic change" will be explained later.

The invention also provides a mode of loading the card, by any desired sum defined by the user. A complete set of value words is then created, with all their valid bits. To adjust the value pre-loaded in the card to the desired sum, a sum equal to the difference between the maximum value of the card and said desired sum is immediately consumed by writing.

Provision is also made for the card to be able to be reloaded, before all of its value bits are consumed, by a desired sum. This reloading mode consists (at least for epROM technology cards) in the creation of at least one valuable word (in principle in the highest weights), by excess, with immediate consumption by writing an equal sum to this excess.

In a particularly interesting embodiment, the value words each have an integer number of bytes, preferably 32 bits, from which it is possible to define two reserved bits, twenty five value bits, four bits to define the unit of value associated with the word, and a validation bit. This validation bit confirms that the sum indicated by the value word has indeed been prepaid, with certification by the entity which guarantees the payment. This validation is not necessarily incorporated into each value word.

Although the calculation means necessary for the implementation of the invention can be at least in part housed in the terminal, it is considered particularly advantageous that these calculation means are entirely housed in the card.

Other characteristics and advantages of the invention will emerge on examination of the detailed description below, and of the appended drawings, in which - Figures 1 and 2 illustrate two different embodiments of a transaction system allowing the implementation of the invention; FIG. 3 is a diagram illustrating the interaction between the coding zone and the value words according to the invention; FIG. 4 illustrates the structure of an electronic purse, according to a preferred embodiment of the present invention; FIG. 5 illustrates an example of a practical implementation of the electronic purse according to the invention, while FIG. 5A illustrates a variant thereof; FIG. 6 illustrates an example of the initial loading of the electronic purse of FIG. 5 ;; FIG. 6A illustrates four different transactions that can be carried out with the electronic purse of FIG. 6; FIG. 7 is the diagram of a mechanism for regenerating value bits in the purse according to the invention; FIG. 8 illustrates different conditions applicable for recharging the electronic purse according to the invention in units of value, while FIG. 8A compares two of these situations; and - Figures 9 and 9A illustrate two other application examples of the electronic purse according to the invention.

The accompanying drawings are essentially of a cer- tain character. They therefore form an integral part of the description, and can not only serve to make the latter better understood, but also, where appropriate, contribute to the definition of the invention.

In FIG. 1, a terminal 1 provided with processing and calculation means 12 interacts with a card 2 having a microprocessor 21 controlling a memory 20, for example of the epROM type. I1 could be a two-chip card, but it is currently preferred to integrate the memory and the microprocessor on the same chip.

In FIG. 2, a terminal 11 provided with processing and calculation means 112 is coupled to a card 22, provided with wired logic 221 controlling a memory 220, on the same chip.

The memory is for example of the eepROM type, also sometimes denoted e2pROM.

These are the main types of cards currently available on the market.

More generally, a card according to the invention comprises a permanent write memory (renewable in. E2pROM technology). This expression here means that writing to memory is substantially more difficult than reading from it. Such memories are often called programmable read only memories.

The memories which can be used according to the invention are electrically programmable read only memories, in which this electrically controlled programming operation corresponds to the permanent writing defined above. In a prepaid card, an unwritten bit is assigned a unit of value, which will be consumed by writing this bit.

Reprogramming the card assumes that we are able to erase it. Only e2pROM memories are electrically erasable. For their part, epROM technology memories would only be erasable by other means, such as ultraviolet radiation.

Cards incorporating such epROM memories are designed to prohibit any erasure.

In this regard, the security arrangements considered to ensure the integrity of the chips inside the card and the monitoring of the correct use of the memory as a whole are considered to be known to those skilled in the art.

According to the invention (FIG. 2), the memory (20 or 220) contains a coding zone 252 of a series of value units, scaled according to the successive powers of 2, starting from a base value, as well as 'a plurality of value words 251 each associated with one of the value units, and containing a predetermined number of value bits having this same value unit.

FIG. 4 shows the words of value MV1 to MV8, with in addition an optional word MVO, the role of which will be seen later.

In a particular embodiment of the invention, each word occupies 32 bits. The structure of the word MV1 will be described by way of example; it comprises - two so-called system bits 250-1 reserved for control applications for example, - twenty five bits of value 251-1, - four unit bits of value 252-1, and - one validation bit 253- 1.

The four value unit bits allow encoding up to sixteen different value levels. In the current version of the invention, it is considered sufficient to use only eight different value levels, at least for the basic version of the electronic wallet.

Depending on whether they are in a virgin or unwritten state (state conventionally denoted 0) or in the written state, denoted 1, the twenty-five value bits will each have the value defined by zone 252-1, which is for the word MV1 considered here the "base value" of the electronic purse.

It is planned to perform validation by changing bit 253-1 to 1 by writing, after it has been possible to confirm (under the conditions defined above) the loading of the value bits according to the associated value unit. It will be noted that the validation bit 253-1 could be produced in a different technology making it possible to make the writing even more irreversible, for example in the form of fuses incorporated in the chip.

As a variant of the above, the value units 252 could not be incorporated into each value word, but defined in an encoding zone 252 placed in a place entirely separate from the memory. In this case, the value of the bits of each value word would be defined by a correspondence between the value word and a point of the coding zone, such a function being able to be carried out for example by pointers or by another system of direct, indirect or implicit addressing. The same can be done for the validation bit.

It will be better understood on the example of FIG. 5 the capacities of the electronic purse according to the invention. The rank 1 value word corresponds to the base unit of value (code UV = 0001), which corresponds for example to the value 0.10 of a monetary unit, such as the franc. Up to twenty-five bits can be loaded at this value unit, which corresponds to a maximum value of 2.5 francs. The word of rank 2 has the code UV = 0010, for a unit of value 0.20, which, for twenty five bits, gives 5 francs.

So on, we go to the rank 8 value word whose UV code is 1000 with a unit value of 12.80 francs and a maximum value of 320 francs.

With only one value word per value step, the maximum value of the electronic purse thus defined is 637.50 francs.

In FIG. 5A, there is shown a variant in which the rank 9 value word is also used, the unit value of which is 25.6 francs, for a maximum value of 640 francs of this word, which brings the value of the electronic purse at 1277.50 francs.

It will have been noticed in FIGS. 5 and 5A that the optional code word 0 makes it possible to break down bits of value of greater weight according to a unit of value equal to half of the base value (code 1), into the occurrence 0.05 cent. It is currently considered that it is not necessary to carry out an initial loading of this word with code value 0.

It is now assumed that the card is blank, that is to say that it is not loaded with any value, and we will describe with reference to FIG. 6 how we can introduce 400 francs to it.

This is the electronic purse of FIG. 5, that is to say in which neither the rank 9 value word nor the rank 0 code word is used a priori.

The value to be entered being 400 francs and the maximum value 637.50 francs, the difference is 237.50 francs. Divided by the base unit of UVB value, this difference comes to 2375 units, which converted to binary gives
10010 1000111.

All value words from rank 1 to 9 are completely filled for this map load. Next, we consider the binary value written above, from right to left. Each time a bit at 1 is encountered, a value bit is consumed by writing in the value word of corresponding rank. For example, in the case of FIG. 6, a value bit will be consumed in the value words MV1, MV2, MV3 and MV7.

What remains of the binary value expressed above corresponds to 18 units of the word of value of rank 8, and we will therefore consume 18 bits of this same word.

After that, the validation bits 253 of each of the value words are confirmed.

The invention provides a particularly simple and elegant means of loading a value into the electronic purse, while being sure to maintain a very high availability in bits of value for each of the levels or words of value forming part of the electronic purse.

From there, one can perform transactions as illustrated in Figure 6A. A purchase of 10 francs corresponds to 100 times the basic unit of value. The number 100 is then written in binary, and FIG. 6A shows that it is necessary to consume a value bit in each of the words MV3, MV6 and
MV7. Figure 6A contains two other examples, for transactions of amounts 15.5F, 25.9F and 49.9F.

The above applies equally to epROM or e2pROM memory cards, with access controlled by wired logic or microprocessor.

The example which has just been described assumes that most of the transactions carried out in France are expressed in decimes, it being noted that the coins currently in progress stop at 5 cents.

If it becomes necessary to have the value word MV0, each bit of which is worth 5 cents, it can be generated in the form of twenty four value bits representing overall 1.2 francs by consumption of a value bit of the word. MV3 and a value bit of word MV4. There again, we will initially admit twenty five value bits in the word MVO, then we will immediately consume a value bit to bring the number back to twenty four, after which the validation bit 253-0 of the word MV0 will be confirmed. This can be done in the microprocessor 21, or by cooperation of the wired logic 221 with a secure terminal.

After loading 400 francs, as described with reference to FIG. 6, at least twenty four bits are available on each of the words of value MV1 to MV7, and of nine bits for the word of value MV8.

Whatever the amount of the purchase, the result is that we can guarantee at least the execution of twenty four transactions, provided of course that the cumulative amount of these does not exceed the total amount charged.

A non-negligible advantage of the invention is that each transaction or purchase can be done without giving change, and by consuming at most one value bit of each value unit, except in the highest value unit.

If we take into account the fact that in practice, the decomposition into binary of a given purchase or transaction has as much chance of giving 0 bits as 1 bits, the number of possible transactions may be higher .

However, it is not certain that the sequence of particular transactions that a particular user will carry out obeys this statistic.

The invention therefore also provides for means of internal regeneration of valuable words which would be emptied prematurely, with the help of those which are not yet emptied. This is a kind of "automatic change of value units" which does not modify the value contained in the card (unlike the "reloading" described later).

The basic situation is when the total residual amount of the card is not used up, while the word or words associated with a given unit of value are. This occurs after at least twenty four transactions, in the case of the initial load of Figure 6.

As a general rule, a word of twenty four bits of value of the unit of value of rank I can be validated, in return for the consumption of a unit of 1 bit of value of the unit of value of rank I + 3 , as well as 1 value bit of the value unit of rank I + 4.

FIG. 7 illustrates a preferred embodiment of this mechanism.

The starting point is the observation that the unit of value of UVi is empty.

If it is the unit of value of rank 1, the mechanism described above is systematically applied.

If it is one of the least significant value units, here UV2 to UV4, we first find out whether the number of residual bits on the value unit of the immediately lower rank is less than thirteen (c 'that is to say at most equal to half of twenty-four).

If therefore more than half of the tokens of the lower value unit remain, the easiest way will be to consume two bits of this lower value unit rather than one bit of the missing value unit.

Otherwise, the mechanism described above will be applied.

For units with a higher weight value, an automatic change will only be carried out if these do not reach the maximum weight.

In the example of figure 7, nothing will be done for the units of value UV6, UV7 and UV8.

On the other hand, for the unit of value UV5, it is first determined whether more than three value bits remain at level W8.

This is indeed a necessary condition to be able to regenerate value units at the UV5 level. If this condition is verified, we also test whether row UV6 is empty. In this case, a word of twenty four units of value of rank W 5 will be created in exchange for three bits of value of rank UV8.

If, on the contrary, there were value bits at row UV6, it will be preferable to create a word of twenty bits with a value of row
UV4 in return for a bit of value of rank UV8 and of a bit of value of rank UV6.

Finally, if there was only one bit of rank value left
UV8, nothing will be done, because it is then inappropriate to consume this value bit.

Overall, the automatic change of value units is therefore carried out according to the aforementioned mechanism, with modulations which can take into account - for units with a value of less weight than the presence of half of the value bits in the rank immediately below, and - for the units of value of greater weight, the presence of insufficient number of bits in the units of value even higher to carry out the automatic exchange operation.

In the case of an e2pROM card, its valuable words can advantageously be reconfigured in an optimal manner, since their electrical erasure is possible.

The operation can then be assimilated to the reloading described below, but without modifying the value.

With reference to FIG. 8, the reloading of value units into the card according to the invention will now be described.

The automatic exchange which has just been described does not necessarily require the presence of the terminal (unless the calculation means incorporated in the card do not allow it to perform it alone). On the other hand, both the initial loading of value units and the reloading of value units involve cooperation between the card and the terminal.

With an e2pROM technology memory, reloading is similar to the initial loading: the residual value of the card is added to the value that you want to transfer to it (duly certified). After erasure, the memory can then be entirely reconfigured, as during the initial loading described above.

This erasure is not possible with an epROM technology memory. We will therefore now describe other methods, which moreover apply to the two technologies.

First of all, a reload can be considered as an initial load which would add new value words disjoint from the existing ones, as described above.

Advantageously, this initial loading type mechanism can be replaced by the generation of value bits at levels 8, 7 and / or 6, by searching for the closest excess value which makes it possible to obtain a maximum number of value bits generated. , while minimizing the number of validated value words. The adjustment of the reloaded amount to the desired sum is carried out by debiting the difference between the value of the validated word (s) and the value of the reloading amount.

This debit is made as if it were a payment transaction.

The top-up value is chosen by the bearer, and may be between 30 francs and 600 francs, for example, for the electronic purse of FIG. 5.

FIG. 8 summarizes the optimum recharging means, as a function of the interval in which the chosen recharged value is found. In this regard, as the value to be reintroduced increases when one reads FIG. 8 from top to bottom, this figure thus defines a series of intervals of possible reloading value.

If the value to be reloaded is less than or equal to 40 francs, we will validate a word of twenty five units of value of level 5 (for 40 francs), then we will consume the bits necessary to bring the reloaded value to the desired amount, and the the validation bit will of course be confirmed (this operation is carried out systematically when a value word is created, but it is not.

recalled each time in the present description).

The same technique is applied if the amount to be loaded is between 40 and 80, but with twenty five bits of UV6 level value.

The same could be done systematically if the amount to be recharged is between 80 and 160, with the UV7 value units. However, as shown in Figure 8, if the amount to be reloaded is between 80 and 120, and less than half of the value bits remain at the UV5 level or else at the UV6 level, it is preferable to load two words of these. same levels, rather than just one of level W7. However, this is an option which is not strictly necessary for the implementation of the present invention.

The rest of the table reads the same way, up to the 320 francs level. The aforementioned option about level 120 also exists for levels 200 and 240.

Beyond 320, it is currently considered preferential to find the minimum number of value words to load, then the combination of value units satisfying this condition while giving priority to the lowest value units. In the example described, for amounts greater than 320 francs, we will load - bits of value UV5 and UV8 if the amount is less than or equal to 360, - bits of value UV6 and UV8 if the amount is less than or equal to 400, - bits with value UV7 and UV8 if the amount is less than or equal to 480, - bits with value UV5, UV7 and UV8 if the amount is less than or equal to 520, - bits with value UV6, UV7 and UV8 if the amount is less than or equal to 560, and - bits of value UV5, UV6, UV7 and UV8 if the amount is less than or equal to 600.

FIG. 8A makes it possible to better understand the advantage of using intermediate levels such as that of 200 which appears in FIG. 8.

It is assumed that the state before recharging of the electronic purse is that illustrated on the first line of the figures of FIG. 8A, for a total value of 155.90 francs.

In the first case, 200 francs are charged in the form of twenty five units of value on the one hand for the UV5 rank, on the other hand for the UV7 rank. The card is then relatively enriched in currency ".

In the second case, sixteen value units of rank W8 are loaded, and one unit of value of ranks W5 and UV6 is consumed. These levels, which were already well filled, are not penalized too much. The bearer will be less hampered for high level transactions, since there are still many bits of rank UV8 to consume.

However, for most applications it will be rare to use the e-wallet for transactions of an amount close to its maximum value. This is why the first way of operating (line marked (A)) in FIG. 8A appears preferable, even if it requires the validation of two words.

This example shows the interest of the conditional-decompositions appearing on the levels 120, 200 and 240 of figure 8.

With reference to FIGS. 9 and 9A, another example of application of the electronic purse according to the invention will now be written.

Here, it is considered that transactions expressed in centimes will be rare, the unit considered being closer to the franc. In this case, the basic unit of value is 0.50 francs. By loading up to twenty five bits of value for levels 1 to 8, we obtain an overall value of the wallet equal to 3187.50 francs.

Those skilled in the art will understand that there may be more than one value word for each level of value units. This is illustrated in figure 9A, with the same scale of value units as for figure 9. Here we have two value words for unit UV1, three for unit UV2, two for unit UV3, and one word for each of the value units
UV4 to UV6. The total value of the card is 900 francs.

As already indicated, the invention applies particularly well to the card of FIG. 1, the latter having an internal microprocessor capable of carrying out itself all the operations relating to transactions as well as to the loading and reloading of. value units, and automatic exchange. He then only needs to receive from the terminal the validation of the credit of the cardholder for the amount to be loaded. In itself, - this validation will generally include a certificate which will allow the card to authenticate the entity authorized to guarantee the monetary values loaded, and may require the presentation of a confidential code. On the other hand, all transaction operations, as well as automatic exchange, can be carried out inside the card and without any need for confidential code control or other security measures.

When using a card according to FIG. 2, at least in its current technology, some of the functions necessary for loading and / or reloading words are not necessarily executable in the card itself.

The calculation means 112 of the terminal can then be used. It can be the same for the automatic exchange. It will of course be understood that the card can cooperate with two types of terminals, some highly secure installed for example in banks and having the ability to load or reload the card (as well as to participate in the automatic exchange if necessary), while the others, located in merchants or in all places where a simple transaction can be carried out, only having the function of accepting a transaction in exchange for the consumption of valuable bits in the card, this time without the same constraints of security.

In the foregoing, we have just indicated that the presentation of a confidential code is not necessary for the use of the electronic purse. It can however be considered.

It will be in particular when the electronic wallet becomes an electronic wallet, that is to say it contains a greater value.

By way of example, one could constitute an electronic wallet having the following characteristics - unit of value 1001 for bits of value of 50 francs each, - unit of value of 1010 for bits of value of 100 francs each, - unit of value of 1011 for bits with a value of 200 francs each, - unit of value 1100 for bits with a value of 400 francs each.

From there, it is possible to generate the following additional value levels - 1000 for bits with a value of 25 francs each and - 1101 for bits with a value of 800 francs each.

In this regard, it will be noted that one of the bits provided for the encoding of the value unit is not necessary, since three useful bits are sufficient to encode up to eight levels of value units. The fourth bit can be used to characterize the wallet or the electronic wallet of the invention, if necessary.

Of course, the present invention is not limited to the embodiment described. It can be extended by those skilled in the art in various ways.

In particular, provision can be made in the same card for the coexistence of several memory areas each defining an electronic wallet for a range of amounts of particular transactions, and an electronic wallet that obeys the same condition.

When the card contains only a very low residual value that cannot be suitable for a transaction, it can be reimbursed in cash, after implementation of security measures such as presentation of the confidential code.

It is also conceivable that the card is dedicated to transactions with a particular service provider, which can be identified in the card in a secure manner.

In practice, a “lifetime” of the card will most often be defined, the number of possible reloads in the card being for example fixed in advance. These data can be fixed when the card is personalized for the first time.

The value of the monetary unit can be encoded in memory in two's complement mode, or by any other means allowing the modification of the written code to alter its value only in the direction of a smaller monetary unit. .

During a transaction, after having consumed the value bits corresponding to the debit to be made, the microprocessor card will be able to send back to the merchant's terminal a transaction certificate which can be executed thanks to the calculation function of the card, using by example a diversified secret key, protected in the card.

For certain applications at least, the card will be authenticated by the terminal (debit) and / or the terminal (debit) by the card.

Authentication is systematic with the terminals.

highly secure.

Various encryption methods are available to perform the above functions securely.

Finally, the terminal may also have a list of prohibited cards, in the same way as is currently done for bank cards.

Furthermore, variants are conceivable at the level of the basic structure of the electronic "purse", depending on the applications. For example, the base unit of value being 0.5F, it is possible to provide 10 words of 16-bit value, at the rate of 2 words for each of the values 0.5F, IF, 2F, 4F, one word of value at 8F and a value word at 16F. Each 16-bit word here contains 16 bits of value and the associated value is simply defined by the addressing in memory.

Here, the "validation" processing no longer involves a validation bit in each value word. It is embodied by the address of the first value word, the number of these, and their respective values, the whole being able to be defined implicitly.

Finally, the cards according to the invention can be used for any monetary unit. They have the advantage of allowing processing in the European unit of account known as
ECU.

Claims (15)

Claims. 1. Transaction system, of the type comprising at least one terminal (1) and a microcircuit card (2) suitable for cooperating with this terminal, this card comprising a permanent write memory, at least certain bits of which are assigned a unit. value, comsummable by writing, characterized in that the memory (20; 220) contains an encoding zone (252) making it possible to define at least one base value from which a series of value units can be established, scaled according to successive powers of 2, as well as a plurality of value words (251), each associated with one of the value units, and containing a predetermined number of value bits having the same value unit, and in that the system comprises calculation means (21; 112) capable of breaking down a monetary amount into binary with respect to said base value, and of establishing commands for writing or not a value bit in each of said value words in bit function corresponding weight in said decomposition, the remaining higher-order bits serving, where appropriate, for writing to the highest-order value word. 2. System according to claim 1, characterized in that the coding zone (252) defines at least four, preferably at least six consecutive value units. 3. System according to one of claims 1 and 2, characterized in that each word contains a coding sub-area (252-1) defining the unit of value associated with it, the aforementioned coding area (252) consisting of the union of these coding sub-areas. 4. System according to one of claims 1 and 2, characterized in that the coding zone (252) is distinct from the words, the value units of the words being defined by pointing to them or by their address. 5. System according to one of the preceding claims, characterized in that the coding zone (252) is arranged to optionally define a base value half, as well as a corresponding value word, validable by consumption of units of. higher weight value. 6. System according to one of the preceding claims, characterized in that, in the absence of value bits for the base value unit (252-1), the calculation means are suitable for regenerating a value word. corresponding to this unit of value in return for writing a bit of value on at least one unit of value of greater weight. 7. System according to one of the preceding claims, -characterized in that, in the absence of value bits for a low-order value unit (252-2 to 252-4), and when the number of units of immediately lower weight value falls below a predetermined threshold, the calculation means are able to regenerate a value word corresponding to this value unit in return for writing a value bit on at least one higher weight value unit. 8. System according to one of the preceding claims, characterized in that, in the absence of value bits for a high weight value unit (252-5) and for the immediately higher weight value unit, and when the number of maximum weight value units is greater than a predetermined threshold, the calculation means are able to regenerate a value word corresponding to this value unit in return for writing a bit on at least one higher weight value unit. 9. System according to one of the preceding claims, characterized in that it has a mode of loading the card, by a desired sum, this mode consisting in the creation of all of said value words (251), with all of them. their valid bits, and in immediate consumption by writing a sum equal to the difference between the maximum values of the card and said desired sum. 10. System according to one of the preceding claims, characterized in that it has a mode of recharging the card, by a desired sum, this mode consisting in the creation of at least one value word (MV) of the weights. the highest, by excess, and in immediate consumption by writing a sum equal to this excess. 11. System according to claim 10, characterized in that the created value word (MV) is chosen as a function of the relationship between said desired sum and the maximum value of each word. 12. System according to claim 11, characterized in that the punctured value word (s) (MV) are also chosen as a function of the relationship between the desired sum and the sum of the maximum values of several words associated with different value units. , and as a function of the number of remaining value bits for these same value units. 13. System according to one of the preceding claims, characterized in that the value words (MV) each have an integer number of bytes. 14. System according to one of claims 1 to 13, characterized in that the calculation means (21) are entirely housed in the card. 15. System according to one of claims 1 to 13, characterized in that the calculation means (112) are at least partly housed in the terminal.