Method and system for limiting the possibility of transforming data designed to constitute, in particular pre-payment tokens

The subject of the invention concerns a process and a system to limit the possibility to transform data, the transformation of TX-type data into TY-type data being carried out using an A-type transformation function, while the transformation of TY-type data into TX-type data is carried out using a B-type transformation function, inverse of the A-type transformation function, the data being in particular designed to constitute for instance pre-payment tokens.According to the invention, the system includes at least one A-type data processing system (STDA), at least one B-type data processing system (STDB), at least one link, at least once, between the system (STDA) and the system (STDB), at least one A-type processing and memorizing unit (UTMA) including at least the A-type transformation function, at least one B-type processing and memorizing unit (UTMB) including the B-type transformation function and not including the A-type transformation function.

The subject of the invention concerns the domain of technical means adapted to limit, through at least one processing and memorizing unit, the possibility to transform TX-type data into TY-type data and the possibility to transform TY-type data into TX-type data, the transformation of the TX-type data into TY-type data being carried out using an A-type transformation function, while the transformation of the TY-type data into TX-type data is carried out using a B-type transformation function, inverse of the A-type transformation function.

The subject of the invention finds a particularly advantageous but non-exclusive application in the domain of generation and use of data designed to constitute pre-payment tokens, such as prepayment cards for instance.

In the state of technology, appears the need, for certain applications, to attribute to at least three categories of persons or users, different transformation capabilities for data. The first user category is able to transform TX-type data into TY-type data using an A-type transformation function. The second user category is able to transform TY-type data into TX-type data using the B-type transformation function inverse of the A-type transformation function, but is not able to transform TX-type data into TY-type data using the A-type transformation function. The third user category is able neither to transform TX-type data into TY-type data using the A-type transformation function, nor to transform TY-type data into TX-type data using the B-type transformation function, inverse of the A-type transformation function.

For instance, such a need to distinguish three user categories exists for data designed to constitute pre-payment tokens. Thus, a first user category is able to generate from known initial identifiers corresponding each to a client possessing a resource consumption credit, tamperproof pre-payment token identifiers. A second user category is able to restore, from a prepayment token identifier, the known initial identifier, and therefore the client, with the intention of affecting his resource consumption to him. The third user category is able neither to generate pre-payment token identifiers, nor to determine the client corresponding to a token identifier.

For the implementation of such a process, is known in the previous art, the technique which uses a public keys—private keys encryption system, applied twice. The first user category has a public key #1and a private key #2. The second user category has a public key #2and a private key #1.

The first user category is able to transform TX-type data DXinto TY-type data DY. To that end, the data DXis encrypted using the public key #1to obtain intermediate data which is decrypted using the private key #2to form data DY.

The second user category is able to transform the TY-type data DYinto TX-type data DX. The data DYis encrypted using the public key #2to obtain intermediate data which is decrypted using the private key #1to constitute the data DX. However, the second user category is not able to transform the data DXinto data DY, because it does not have the private key #2.

The third user category is able to transform neither the data DXinto data DY, nor the data DYinto data DX.

The implementation of that technique of limitation of the possibility to transform data requires the setting up of a public keys certification infrastructure. Such infrastructure is relatively complex and costly.

The subject of the invention aims at remedying the drawbacks of the previous art by proposing a technique enabling to limit the possibilities of data use for three user categories, by implementing simple and inexpensive means.

So as to reach such a goal, the subject of the invention concerns a process to limit, through at least one processing and memorizing unit, the possibility to transform TX-type data into TY-type data and the possibility to transform TY-type data into TX-type data, the transformation of the TX-type data into TY-type data being carried out using an A-type transformation function, while the transformation of the TY-type data into TX-type data is carried out using a B-type transformation function, inverse of the A-type transformation function, the data being in particular designed to constitute for instance, pre-payment tokens, and being implemented on at least one data processing system.

According to the invention, the process comprises:using an A-type data processing system and a B-type data processing system,setting up at least once, at least one link between the A-type data processing system and the B-type data processing system, so as to provide the transfer of at least TY-type data from the A-type data processing system to the B-type data processing system and/or to provide the transfer of at least TX-type data from the B-type data processing system to the A-type data processing system,during an A-type customization phase, creating at least one A-type processing and memorizing unit including at least the A-type transformation function,during an A-type transformation phase:for a user possessing at least one A-type processing and memorizing unit, enabling:to transfer at least one piece of TX-type data from the A-type data processing system to the A-type processing and memorizing unit,to transform in the A-type processing and memorizing unit, each piece of TX-type data into a piece of TY-type data, using the A-type transformation function,to transfer each piece of TY-type data from the A-type processing and memorizing unit to the A-type data processing system,for a user not possessing any A-type processing and memorizing unit, not being able to transform a piece of TX-type data into a piece of TY-type data, using the A-type transformation function,during a B-type customization phase, creating at least one B-type processing and memorizing unit including the B-type transformation function and not including the A-type transformation function,and during a B-type transformation phase:for a user possessing a B-type processing and memorizing unit, and not possessing an A-type processing and memorizing unit,enabling:to transfer at least one piece of TY-type data from the B-type data processing system, to the B-type processing and memorizing unit,to transform in the B-type processing and memorizing unit each piece of TY-type data into a piece of TX-type data, using the B-type transformation function,to transfer each piece of TX-type data from the B-type processing and memorizing unit to the B-type data processing system,not being able to transform a piece of TX-type data into a piece of TY-type data using the A-type transformation function.

FIG. 1illustrates an embodiment of a system1to limit the possibility of data transformation. The system1includes an A-type data processing system STDA. Generally speaking, such an A-type data processing system STDAincludes at least one processor A10enabling the execution of an implementation software A11. The A-type data processing system STDAcan be a computer, a server or be part, for instance, of various machines, devices, fixed or mobile products, or vehicles in the general sense. The A-type data processing system STDAis connected, using transfer means A12, by a link A20, to an A-type processing and memorizing unit UTMA.

For the sake of simplification in the rest of the description, the A-type data processing system STDAshall be refereed to as system STDAand the A-type processing and memorizing unit UTMAshall be refereed to as unit UTMA.

The link A20between the system STDAand the unit UTMAcan be realized in any possible way, such as for instance a serial link, a USB bus, a radio link, an optical link, a network link or a direct electric connection to a circuit of the system STDA, etc. It should be observed that the unit UTMAcan possibly be physically located inside the same integrated circuit than the processor of the system STDA. In this case, the unit UTMAcan be considered as a co-processor in relation to the processor of the system STDAand the link A20is internal to the integrated circuit.

The unit UTMAincludes transfer means A30and processing and memorizing means A31. It must be considered that the transfer means A12and A30are of software and/or hardware nature and are capable of providing and optimizing the data communication between the system STDAand the unit UTMA. Said transfer means A12, A30are adapted to enable to have at one's disposal an implementation software A11which is, preferably, independent from the type of link A20used. Said transfer means A12, A30are not part of the subject of the invention and are not described more precisely as they are well known by the Man of art.

Said unit UTMAis able to:using the transfer means A30:accept data provided by the system STDA,return data to the system STDA,using the processing and memorizing means A31:to store data possibly in secret and to retain at least a part of said data even if the unit UTMAis switched off,and to carry out algorithmic processing on data, part or all of said processing being possibly secret.

As non-limiting example, said unit UTMAcan be constituted by a material key on the USB bus of the system STDAor preferably by a chip card and its interface commonly called card reader linked up to the system STDA.

In the case where the unit UTMAis constituted by a chip card and its interface, the transfer means A30are split into two parts, one being on the interface and the other one being on the chip card. In this embodiment, the absence of the chip card is considered as equivalent to the absence of the unit UTMA, inasmuch as the processing and memorizing means A31contained in the chip card are missing.

The system1also includes a B-type data processing system STDB. Generally speaking, such a B-type data processing system STDBincludes at least one processor B10enabling the execution of an implementation software B11. The B-type data processing system STDBcan be a computer, a server or be part, for instance, of various machines, devices, fixed or mobile products, or vehicles in the general sense. The B-type data processing system STDBis connected, using transfer means B12, by a link B20, to a B-type processing and memorizing unit UTMB.

For the sake of simplification in the rest of the description, the B-type data processing system STDBshall be referred to as system STDBand the B-type processing and memorizing unit UTMBshall be referred to as unit UTMB.

The link B20between the system STDBand the unit UTMBcan be realized in any possible way, such as for instance a serial link, a USB bus, a radio link, an optical link, a network link or a direct electric connection to a circuit of the system STDB, etc. It should be observed that the unit UTMBcan possibly be physically located inside the same integrated circuit than the processor of the system STDB. In this case, the unit UTMBcan be considered as a co-processor in relation to the processor of the system STDBand the link B20is internal to the integrated circuit.

The unit UTMBincludes transfer means B30and processing and memorizing means B31. It must be considered that the transfer means B12and B30are of software and/or hardware nature and are capable of providing and optimizing the data communication between the system STDBand the unit UTMB. Said transfer means B12, B30are adapted to enable to have at one's disposal an implementation software B11which is, preferably, independent from the type of link B20used. Said transfer means B12, B30are not part of the subject of the invention and are not described more precisely as they are well known by the Man of art.

Said unit UTMBis able to:using the transfer means B30:accept data provided by the system STDB,return data to the system STDB,using the processing and memorizing means B31:to store data possibly in secret and to retain at least a part of said data even if the unit UTMBis switched off,and to carry out algorithmic processing on data, part or all of said processing being possibly secret.

As non-limiting example, said unit UTMBcan be constituted by a material key on the USB bus of the system STDBor preferably by a chip card and its interface commonly called card reader linked up to the system STDB.

In the case where the unit UTMBis constituted by a chip card and its interface, the transfer means B30are split into two parts, one being on the interface and the other one being on the chip card. In this embodiment, the absence of the chip card is considered as equivalent to the absence of the unit UTMB, inasmuch as the processing and memorizing means B31contained in the chip card are missing.

The system1according to the invention also includes at least once, at least one link L between the system STDAand the system STDB. Said link L constitutes an information transfer channel and can be realized in all known ways. Said link L can be provided by a computer network and/or by a material transmission of information (personal delivery, postal delivery, etc.). Depending on the applications, the link L can transmit information from the system STDAto the system STDB, from the system STDBto the system STDAor in both directions. As non-limiting example, the transfer by said link L between the system STDAand the system STDBcan take the following heterogeneous channel: transmission of files from the system STDA, then printing on a physical support, then transfer of said physical support, then keyboarding data on a computer, then lastly transfer through a computer network to the system STDB.

FIG. 2illustrates the data transformations carried out by the process according to the invention. Two data types are defined, namely the type TXand the type TY. Each of said types TXand TYis a computer type of data, such as for instance, a 8-bit character, a 32-bit integer, a 64-bit integer, a 512-bit integer, a 64-bit float. In a preferred variant embodiment, the 64-bit integer type is used as data type TX, as well as as data type TY.

The invention uses an A-type transformation function FAand a B-type transformation function FB. The A-type transformation function FAis a bijection having as starting set the type TXand as ending set the type TY. The B-type transformation function FBis a bijection having as starting set the type TYand as ending set the type TX. The A-type transformation function FAand the B-type transformation function FBare inverse of each other. For the sake of simplification in the rest of the description, the A-type transformation function FAshall be referred to as function FAand the B-type transformation function FBshall be referred to as function FB.

Thus, the function FAtransforms a piece of TX-type data DXinto a piece of TY-type data DY, namely DY=FA(DX), while the function FBtransforms a piece of TY-type data DY′ into a piece of TX-type data DX′, namely DX′=FB(DY′).

Since the two functions FAand FBare inverse of each other:by applying successively the two functions FA, then FBto a piece of data DX, one finds again the piece of data DX, namely DX=FB(FA(DX)),by applying successively the two functions FB, then FA, to a piece of data DY′, one finds again the piece of data DY′, namely DY′=FA(FB(DY′)).

FIG. 3illustrates the spot where the functions FAand FBare executed. So as to implement the invention, the functions FAand FBmust remain confidential. To this end, the function FAis carried out only inside the unit UTMAand the function FBis carried out only inside the unit UTMBand, possibly, inside the unit UTMA. Thus, in the unit UTMA, a piece of TX-type data DXis transformed by the function FAinto a piece of TY-type data DYand, possibly, a piece of TY-type data DY′ is transformed by the function FBinto a piece of TX-type data DX′. Furthermore, in the unit UTMB, a piece of TY-type data DY′ is transformed by the function FBinto a piece of TX-type data DX′.

FIG. 4makes explicit the three categories of persons or users C1, C2, C3discriminated between by the subject of the invention, depending on the possession or not of the units UTMAand/or UTMB.

Each user of the first category C1is able to transform a piece of TX-type data into a piece of TY-type data using the function FAand, possibly to transform a piece of TY-type data into a piece of TX-type data using the function FB. Each user of the first category C1can thus use a unit UTMAand, possibly, a unit UTMB.

Each user of the second category C2is able to transform a piece of TY-type data into a piece of TX-type data using the function FB. However, each user of the second category C2is not able to transform a piece of TX-type data into a piece of TY-type data using the function FA. Each user of the second category C2can use a unit UTMB, but cannot use a unit UTMA.

Each user of the third category C3possesses neither a unit UTMA, nor a unit UTMB. No user of said third category C3is able to transform a piece of TX-type data into a piece of TY-type data using the function FA, or to transform a piece of TY-type data into a piece of TX-type data using the function FB.

Naturally, the functions FAand FBare interesting only if they are not trivial and are difficult to infer from the observation of data coming in and out of the units UTMAand/or UTMB.

FIG. 5illustrates a first variant embodiment of the functions FAet FBusing the known technique of secret keys encryption. According to this variant, the function FAis carried out in the form of a secret key encryption function CS using as secret key, a secret piece of information ICS.

The secret key encryption function CS is a standard encryption function such as for instance DES, inverse DES, triple DES, or IDEA. The secret piece of information ICSis a key for the chosen encryption function. As such, the secret piece of information ICSbelongs to the type KCS, i.e. to the set of the keys for said function. For instance, said KCS-type secret piece of information ICSis a 56-bit integer when the chosen secret key encryption function CS is DES.

In other words, the transformation of a piece of TX-type data DXinto a piece of TY-type data DYusing the function FAamounts to encrypt the piece of data DXusing the secret key encryption function CS, using as secret key, the KCS-type secret piece of information ICS.

Similarly, the function FB, inverse of the function FA, is also carried out in the form of a secret key encryption function CSI, called inverse, using as secret key, a secret piece of information ICSI.

The secret key inverse encryption function CSI is a standard encryption function such as for instance DES, inverse DES, triple DES, or IDEA.

The secret piece of information ICSIis a key for the chosen encryption function. As such, the secret piece of information ICSIbelongs to the type KCSI, i.e. to the set of the keys for said function.

In other words, the transformation of a piece of TY-type data DY′ into a piece of TX-type data DX′ using the function FBamounts to encrypt the piece of data DY′ using the secret key inverse encryption function CSI, using as secret key, the KCSI-type secret piece of information ICSI.

The secret key inverse encryption function CSI using the secret key ICSI, is the inverse of the secret key encryption function CS using the secret key ICS. For instance, in the case where the secret key encryption function CS is carded out by the function DES, the secret key inverse encryption function CSI must be carried out by the function inverse DES, while the KCS-type secret piece of information ICSand the KCSI-type secret piece of information ICSImust be identical.

FIGS. 6 and 7illustrate a second variant embodiment of the functions FAand FB, using the known technique of public key-private key encryption.

FIG. 6illustrates a first embodiment in which the function FAis carried out in the form of a public key encryption function CPU using as public key, a secret piece of information ICPU.

The public key encryption function CPU is a standard encryption function, for instance RSA. The secret piece of information ICPUis a key for the chosen encryption function. As such, the secret piece of information ICPUbelongs to the type KCPU, i.e. to the set of the public keys for said function. For instance, said KCPU-type secret piece of information ICPUcan be formed by a “module” and a “public exponent” when the chosen public key encryption function CPU is RSA.

In other words, the transformation of a piece of TX-type data DXinto a piece of TY-type data DYusing the function FAamounts to encrypt the piece of data DXusing the public key encryption function CPU, using as public key, the KCPU-type secret piece of information ICPU.

Similarly, the function FB, inverse of the function FA, is for its part carried out in the form of a private key decryption function CPUI, using as private key, a secret piece of information ICPUI.

The private key decryption function CPUI is a standard function, for instance RSA.

The secret piece of information ICPUIis a key for the chosen decryption function. As such, the secret piece of information ICPUIbelongs to the type KCPUI, i.e. to the set of the private keys for said function.

In other words the transformation of a piece of TY-type data DY′ into a piece of TX-type data DX′ using the function FBamounts to decrypt the piece of data DY′ using the private key decryption function CPUI, using as private key, the KCPUI-type secret piece of information ICPUI.

The private key decryption function CPUI using the private key ICPUI, is the inverse of the public key encryption function CPU using the public key ICPU. For instance, in the case where the public key encryption function CPU is carried out by the RSA encryption function, the private key decryption function CPUI must be carried out by the RSA decryption function, while the KCPU-type secret piece of information ICPUand the KCPUI-type secret piece of information ICPUImust be respectively an RSA public key and its associated private key.

FIG. 7illustrates a second embodiment in which the function FAis carried out in the form of a private key encryption function CPR using as private key, a secret piece of information ICPR.

The private key encryption function CPR is a standard encryption function, for instance RSA. The secret piece of information ICPRis a key for the chosen encryption function. As such, the secret piece of information ICPRbelongs to the type KCPR, i.e. to the set of the private keys for said function. For instance, said KCPR-type secret piece of information ICPRcan be formed by a “module” and a “private exponent” when the chosen private key encryption function CPR is RSA.

In other words, the transformation of a piece of TX-type data DXinto a piece of TY-type data DYusing the function FAamounts to encrypt the piece of data DXusing the private key encryption function CPR, using as private key, the KCPR-type secret piece of information ICPR.

Similarly, the function FB, inverse of the function FA, is for its part carried out in the form of a public key decryption function CPRI, using as public key, a secret piece of information ICPRI.

The public key decryption function CPRI is a standard function, for instance RSA.

The secret piece of information ICPRIis a key for the chosen decryption function. As such, the secret piece of information ICPRIbelongs to the type KCPRI, i.e. to the set of the public keys for said function.

In other words, the transformation of a piece of TY-type data DY′ into a piece of TX-type data DX′ using the function FBamounts to decrypt the piece of data DY′ using the public key decryption function CPRI, using as public key, the KCPRI-type secret piece of information ICPRI.

The public key decryption function CPRI using the public key ICPRI, is the inverse of the private key encryption function CPR using the private key ICPR. For instance, in the case where the private key encryption function CPR is carried out by the RSA encryption function, the public key decryption function CPRI must be carried out by the RSA decryption function, while the KCPR-type secret piece of information ICPRand the KCPRI-type secret piece of information ICPRImust be respectively an RSA private key and its associated public key.

In the two examples described in relation toFIGS. 6 and 7, the terms “encryption function” and “decryption function” are used to refer to two encryption operations inverse of each other. For the sake of clarity, the first function is called encryption function and the second function is called decryption function. That choice is arbitrary, so much so that the first function might as well be called decryption function and the second function might as well be called encryption function.

FIG. 8is a diagram illustrating the implementation of an additional transformation function in addition to the known encryption functions, as illustrated inFIGS. 5 to 7. Indeed, can be used as function FA, an additional transformation function Fadcombined with the secret key encryption function CS or with the public key encryption function CPU or with the private key encryption function CPR. Said additional transformation function Fadcan be combined in any way before and/or after the secret key encryption function CS, the public key encryption function CPU or the private key encryption function CPR. Naturally, said additional transformation function Fadcan also be formed by at least one encryption function.

Similarly, the function FBcan be formed by an additional transformation function, called inverse Fadi, which is combined with the secret key inverse encryption function CSI or with the private key decryption function CPUI or with the public key decryption function CPRI.

Whichever embodiment of the transformation functions, illustrated inFIGS. 5 to 8, it must be considered that the subject of the invention includes, besides, a customization phase of the processing and memorizing units during which the transformation functions are implanted in the processing and memorizing units.

FIG. 9illustrates an implementation example of customization apparatuses100Aand100Bfor processing and memorizing units UTM, with the intention of obtaining respectively, units UTMAand units UTMB.

In a preferred embodiment, it must be considered that each processing and memorizing unit UTM includes algorithmic means110necessary to carry out the function FAand algorithmic means120necessary to carry out the function FB. In the case where a secret key encryption function CS is used, the algorithmic means110correspond to means enabling, for instance, the carrying out of the DES function. In this case, the algorithmic means120correspond to means enabling the carrying out of the inverse DES function.

During a B-type customization phase, the customization apparatus100Bcarried out through a data processing system of any type, includes means to customize at least one processing and memorizing unit UTM, with the intention of obtaining a unit UTMB. To that end, the algorithmic means120are used so as to obtain a unit UTMBwhich is able to carry out the function FB. However, the customization apparatus100Bmust also:either inhibit the algorithmic means110necessary to carry out the function FA,or not load to the processing and memorizing unit UTM, the customization information enabling the carrying out of the function FA, and possibly prevent its later loading.

Thus, a unit UTMBis obtained including the function FBand not including the function FA.

Similarly, the customization apparatus100Acarried out through a data processing system of any type, is used during an A-type customization phase, to customize at least one processing and memorizing unit UTM, with the intention of obtaining a unit UTMAincluding the function FAand, possibly, the function FB. In the case where the unit UTMAdoes not include the function FB, the algorithmic means120are inhibited or the customization information is not loaded, as explained above.

Naturally, the customization apparatuses100Aand100Bcan be carried out through a same data processing system. Moreover, processing and memorizing units UTM including only the algorithmic means necessary to carry out only one of the two transformation functions can be used. In that case, it is obviously not necessary to inhibit the inverse function.

The customization apparatuses100Aand100Bare also used to provide the generation of secret information used by the functions FAand FBand, possibly, to provide the generation of additional parameters for the additional functions Fad, Fadidescribed in the examples illustrated inFIGS. 5 to 8.

FIG. 10makes explicit the general principle of information generation. According to said figure, a principal secret SP is used by an algorithm Dpenabling to determine one of the pairs of secret pieces of information KCS-type ICSand KCSI-type ICSI, or KCPU-type ICPUand KCPU-type ICPUI, or KCPR-type ICPRand KCPR-type ICPRIand, possibly, parameters Padfor the additional transformation function Fadand parameters Padifor the additional inverse transformation function Fadi.

To increase security, it can be advantageous that the principal secret SP may be determined from shared secrets S1, S2, . . . , Sn, using a secret reconstruction algorithm Dps.

After the generation of that information, it can be considered proceeding to the customization of the units UTMAand UTMB. Thus, as it emerges more precisely fromFIG. 11, during the A-type customization phase, the customization apparatus100Ais used, to transfer to a processing and memorizing unit UTM, with the intention of obtaining a unit UTMA:the KCS-type secret piece of information ICSor the KCPU-type secret piece of information ICPUor the KCPR-type secret piece of information ICPRand, possibly, the parameters Padfor the additional transformation function Fadto enable the unit UTMAto carry out the function FA,and possibly, the KCSI-type secret piece of information ICSIor the KCPUI-type secret piece of information ICPUIor the KCPRI-type secret piece of information ICPRIand, possibly the parameters Padifor the additional inverse transformation function Fadito enable the unit UTMAto carry out the function FB.

Similarly, the customization apparatus100Bis used during the B-type customization phase to transfer to a processing and memorizing unit UTM, with the intention of obtaining a unit UTMB, the KCSI-type secret piece of information ICSIor the KCPUI-type secret piece of information ICPUIor the KCPRI-type secret piece of information ICPRIand, possibly the parameters Padifor the additional inverse transformation function Fadito enable the unit UTMBto carry out the function FB.

The subject of the invention aims at enabling to limit the possibility to transform TX-type data into TY-type data and the possibility to transform TY-type data into TX-type data. To that end, the subject of the invention aims at putting at the first user category C1's disposal, at least one unit UTMAto enable to transform a piece of TX-type data into a piece of TY-type using a function FA. Optionally, said unit UTMAincludes the possibility to transform a piece of TY-type data into a piece of TX-type data using the function FB.

The second user category C2has at least one unit UTMBable to provide the transformation of TY-type data into TX-type data using the function FB. However, no user of said second category C2is able to carry out the transformation of TX-type data into TY-type data using the function FA. It thus appears possible to limit the possibility to transform data between tie users of different categories.

The subject of the invention is particularly useful in the case where the two user categories C1and C2able to transform data, do not have access to the secret information characterizing those transformations. Such a goal is reached by using processing and memorizing units, such as material keys on the USB bus or chip cards. The only possibility for a user of a category to carry out a transformation attributed to a user of the other category, is to obtain the unit belonging to the latter.

FIGS. 12 and 13are respectively principle and application diagrams, illustrating an application example of the subject of the invention enabling the generation and use of data designed to constitute pre-payment tokens.

As it appears more precisely inFIG. 12, a starting set EDis defined, whose elements are pieces TX-type data. The staring set EDincludes, in the illustrated example, five elements, namely:3,4,5,6,7. Each element of the starting set EDcorresponds to an identifier of a client possessing a resource consumption credit, such as for instance a WEB pages viewing credit. During an A-type transformation phase, all the elements of the starting set EDare transformed using the function FAcontained in the unit UTMA, so as to obtain an ending set EAwhose elements are pieces of TY-type data. In the illustrated example, the elements3,4,5,6,7are transformed respectively into12850,85503,23072,70331,45082. The data thus obtained by the transformation carried out in the unit UTMAgives no indication on the elements of the starting set ED.

As it appears more precisely inFIG. 13, each user belonging to the first category C1, has a unit UTMAand can thus, from known initial identifiers, namely:3,4,5,6,7in the illustrated example, obtain pre-payment token identifiers, respectively12580,85503,23072,70331,45082. Such prepayment token identifiers can, for instance, be printed on tokens j which can be constituted by any appropriate support, such as plastic cards or coupons. Said pre-payment token identifiers are, preferably, masked to attest to the non-use of the resource corresponding to said tokens.

Moreover, information I enabling to characterize the starting set ED, is transmitted through a link L1, to at least one system STDBbelonging to a user of the second category C2. In the present case where the staring set EDis composed of successive integers, the information I enabling to characterize the starting set EDcan, for instance, be the value of the smallest element and the number of elements of the set, namely the pair (3,5).

In the illustrated example, one of the tokens j is transmitted to a user of the third category C3, who thus becomes a client possessing a resource consumption credit. That transmission is carried out by any appropriate mean, such as postal delivery or personal delivery (part of a link L2). Remember that each user of said third category C3has neither a unit UTMA, nor a unit UTMB.

After having uncovered the identifier of his pre-payment token, namely85503in the illustrated example, the user of the third category C3transmits through another part of the link L2to a user of the second category C2, the uncovered identifier, as well as a request Rqfor a resource Resto consume. Remember that each user of the second category C2has a unit UTMBlinked up to a system STDB. The identifier transmitted by the user of the third category C3to the system STDBis transferred to the unit UTMB, so as to provide its transformation, using the function FBcontained in the unit UTMB, with the intention of restoring the initial identifier. In the illustrated example, the unit UTMBthus transfers to the system STDBthe known initial identifier, namely4, corresponding to the prepayment token85503.

The system STDBuses the information I to verify that the transformed element, namely4in the illustrated example, belongs to the starting set ED. That verification enables to make sure that the pre-payment token has not been tampered with or invented. Thus, if the identifier is not recognized (N), the request Rqis refused, so much so that a negative reply Rpis sent to the user of the third category C3. If the request is accepted (O), the identifier is used as an index in an array T of resources. Said array T indicates the quantity of remaining credits (96 in the illustrated example) for the client possessing the pre-payment token corresponding to the identifier4. It is then verified that the remaining credits are sufficient for the request made. In the negative case (N), a negative reply Rpis sent to the user of the third category C3. In the case where the credits are sufficient (O), the array is updated by subtracting the cost of the requested resource, and a positive reply Rpcontaining the requested resource Resis prepared and then delivered to the user of the third category C3.

In the previous example, note that:a there are several links between the systems STDAand STDB, called L1, L2,the users of the first category C1correspond to persons able to issue pre-payment tokens,the users of the second category C2correspond to a provider of services wishing to charge the access to resources Res,the users of the third category C3possessing at least one pre-payment token, corresponds to clients possibly accessing charged-for resources,the users of the third category C3not possessing pre-payment tokens, cannot be clients and therefore cannot access charged-for resources.

In the preferred example described, the invention aims at limiting the possibility to transform data designed to constitute pre-payment tokens. Naturally, the subject of the invention can be implemented to limit the possibility to transform data of different kind, such as for instance, electronic mail messages, internet pages, etc.

The invention is not limited to the examples described and represented, as various modifications can be brought to it within its framework.