TECHNICAL FIELD 
     The present disclosure generally relates to a method for managing the access to a memory by using passwords, and an integrated circuit implementing this method. 
     The present disclosure particularly but not exclusively relates to integrated circuits with a “multi-user”-type memory, such as the contactless integrated circuits present in tags. 
     BACKGROUND INFORMATION 
     Contactless tags are often used as a product identification and traceability means. Within this context, different parties can be required to write or read data saved in the tags. For example, after affixing a contactless tag on the product he has just manufactured, a manufacturer writes in the memory of the integrated circuit of the tag the serial number of the product, the model and the type of product, its manufacturing date, etc. The distributor of the same product then writes data of his own in the tag, for example the purchase date, the trade references of the product, the destination of the product, etc. Finally, the retailer may also wish to write data of his own in the tag, for example the date of receipt of the product, the purchase price and the retail price of the product, etc. Therefore, three different users must share the memory space of the integrated circuit. 
     So that the data written by each user are protected against access attempts by unauthorized third parties, provision is usually made for a password-based protection system. Many contactless integrated circuits thus have a single password that must be presented before each access to the memory. However, to avoid the same password being shared by several users, one well-known method also involves dividing the memory space into several sectors which can each be protected by a determined password. 
       FIG. 1  schematically shows the structure of an RFID-type (Radio Frequency Identification) contactless integrated circuit IC 1  using several passwords. The integrated circuit comprises a contactless communication interface circuit ICT, a control circuit CCT 1  and an electrically erasable and programmable memory MEM 1 . The interface circuit ICT is linked to an antenna circuit ACT for contactlessly receiving or sending data. The control circuit CCT 1  receives, via the interface circuit ICT, read or write commands CMD for writing or reading the memory, and returns responses RSP via the circuit ICT. The memory MEM 1  comprises a user memory area UMEM and a system memory area SMEM, represented by a hatched area. The system area generally comprises data used by the control circuit CCT 1  to implement the contactless communication protocol. 
     The user memory area UMEM is split into several sectors of the same size, for example three sectors S 1 , S 2 , S 3  which can generally each receive from a few tens to a few hundred bytes. In each sector S 1 , S 2 , S 3  there is a reserved location receiving a password, respectively pw 1 , pw 2 , pw 3 . 
     Each memory is accessed, for example for reading or writing a word, by sending to the integrated circuit a read or write command comprising the address of the word (including the address of the sector in which the word is located) and the password of the sector. Before executing the command, the circuit CCT 1  reads the password in the sector and compares it with the password present in the command. If the result of the comparison is positive, the command is executed. In the opposite case, the circuit CCT 1  returns an error code. 
     Upon commissioning the integrated circuit IC 1 , the memory area UMEM is blank and the passwords are equal to a default value, generally 0. The first user chooses one of the sectors, generally the first sector S 1 , writes his password and then the data of the application in it. The second user does the same with the sector S 2  and the third user does the same with the last sector S 3 . Each user thus has a reserved memory space protected by a password and can access it at will to read or write data. 
     However, this method for sharing the memory space has various disadvantages. Firstly, partitioning the memory space into sectors of the same size does not enable the memory space to be optimally occupied, due to the fact that each user does not necessarily have the same number of data to write. Thus, one user may only use a small portion of the sector he was allocated, while another user may be restricted by the lack of space of the sector he is allocated. In this case, the memory space unused by the first user cannot be used by the second user, hence a significant waste of memory space. 
     Furthermore, a user (for example, the manufacturer) may want another user (for example, the wholesaler) to be able to selectively read certain data he has saved in the memory, but not all the data. 
     To meet this need, it is possible to subdivide the user memory space UMEM into smaller sectors that are greater in number, for example fifteen sectors instead of three. Thus, if the needs of the first user are slight in terms of memory space, the first user will only use a small number of sectors, the rest of the memory space being made available to the other users. Moreover, the first user can reveal the passwords of certain sectors to the other users, so that the latter can read these sectors. 
     However, this solution has a major disadvantage: when the number of sectors increases, the number of passwords increases proportionately as well as the percentage of the memory space occupied by the passwords that may each include several bytes. 
     BRIEF SUMMARY 
     Thus, one embodiment of the present invention provides a method enabling several users to share a memory without increasing, in an unacceptable manner, the memory space occupied by passwords. 
     One embodiment of the present invention provides a method for managing the access rights to a shareable memory space that is very flexible to use and particularly enables read- and write-access rights to be managed distinctly. 
     At least one embodiment of the present invention provides a method for managing the access to a memory space shareable by several users, by using a plurality of passwords, comprising: defining a maximum number of passwords, providing a password storage zone, dividing the shareable memory space into a plurality of blocks greater in number than the maximum number of passwords, providing, in each block, a parameterization field for parameterizing the protection of the block, providing, in each parameterization field, a binary index smaller in size than the password and designating a password assigned to the protection of the block, a same password possibly being used to protect several blocks, and allocating, to each block, access rights requiring a password to be presented that corresponds to the password designated by the index present in the block parameterization field. 
     According to one embodiment, the method comprises providing, in the parameterization field of each block, a first binary parameter defining the access rights to the block after presentation of a password corresponding to the password designated by the index, distinguishing between read- and write-access rights to the block. 
     According to one embodiment, the first binary parameter also defines access rights to the block without presentation of the password. 
     According to one embodiment, the method comprises providing, in the parameterization field of each block, a second binary parameter having a first value that authorizes the read- and write-access to the block whatever the value of the first binary parameter, and a second value that validates the effects of the first binary parameter on the access rights to the block. 
     According to one embodiment, a value of the index means that no password is allocated to the block. 
     According to one embodiment, the block is write-locked when the second binary parameter has the second value and when the index indicates that no password is allocated to the block. 
     According to one embodiment, the total size of the parameterization field of a block is smaller than the size of a password. 
     According to one embodiment, the write-access rights given to the parameterization field of a block are identical to the write-access rights that the parameterization field gives to the block in which it is situated. 
     According to one embodiment, the password storage zone comprises a plurality of password blocks each comprising a password field and a parameterization field defining the access rights to the password block. 
     According to one embodiment, the password storage zone is only accessible by using specific write commands for writing the passwords. 
     According to one embodiment, the blocks of the shareable memory space are accessed by session, the opening of a session comprising presenting, by the user, a user password, at least one comparing the password presented with at least one password present in the password storage zone, and storing the result of the comparison, at least if this result is positive. 
     According to one embodiment, a session is interrupted when a user presents a new password or when the memory space is switched off. 
     According to one embodiment, the storing of the result of the comparison of the passwords comprises loading into a reference register a binary value equal to an index designating a password block, and the processing of a request for read- or write-access to a block of the shareable memory space comprises comparing the index present in the parameterization field of the targeted block with the content of the reference register, the password of the block targeted by the request for access being considered to have been validly presented if the result of the comparison is positive. 
     One embodiment of the present invention also relates to an integrated circuit comprising a memory and a control circuit for controlling the access to the memory, the memory comprising a memory space shareable by several users, the control circuit comprising means for comparing passwords supplied by users and passwords present in the memory, wherein the memory comprises a password storage zone defining a maximum number of passwords, the memory space is divided into a plurality of blocks greater in number than the maximum number of passwords, each block comprises a parameterization field for parameterizing the protection of the block, each parameterization field of a block comprises a binary index smaller in size than a password and designating a password assigned to the protection of the block, a same password possibly being used to protect several blocks, and the control circuit is arranged for allocating, to each block, access rights requiring a password to be presented that corresponds to the password designated by the index present in the block parameterization field. 
     According to one embodiment, the parameterization field of each block further comprises a first binary parameter defining the access rights to the block after presentation of a password corresponding to the password designated by the index, and distinguishing between the read- and write-access rights to the block. 
     According to one embodiment, the first binary parameter also defines access rights to the block without presentation of any password. 
     According to one embodiment, the parameterization field of each block comprises a second binary parameter having a first value that authorizes the read- and write-access to the block whatever the value of the first binary parameter, and a second value that validates the effects of the first binary parameter on the access rights to the block. 
     According to one embodiment, the index of each block can be taken to a value meaning that no password is allocated to the block. 
     According to one embodiment, a block is write-locked when the second binary parameter has the second value and when the index indicates that no password is allocated to the block. 
     According to one embodiment, the total size of the parameterization field of a block is smaller than the size of a password. 
     According to one embodiment, the write-access rights of the parameterization field of a block are identical to the write-access rights that the parameterization field gives to the block in which it is situated. 
     According to one embodiment, the password storage zone comprises a plurality of blocks each receiving a password and each comprising a parameterization field defining the access rights to the block. 
     According to one embodiment, the password storage zone is arranged in a system area of the memory, and is only accessible to the users by using specific write commands for writing the passwords. 
     According to one embodiment, the blocks of the shareable memory space are accessed by session, the control circuit being arranged for executing a command for presenting a user password, and, if the user password is identical to a password present in the password storage zone, authorizing the user to access the blocks the index of which designates this password for the entire duration of a session, without requiring the password to be presented at each access to the blocks. 
     According to one embodiment, the control circuit is arranged for interrupting a session when a user presents a new password or when the integrated circuit is switched off. 
     According to one embodiment, the control circuit is arranged for, upon receiving the command for presenting a password, comparing the user password with a password present in a password block and, if the result of the comparison is positive, loading into a reference register a binary value equal to an index designating the password block, and upon receiving a read or write command for reading or writing a block of the shareable memory space, comparing the index present in the block targeted by the read or write command with the content of the reference register, and considering that the password of the block targeted by the command has been validly presented if the result of the comparison is positive. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       These and other features shall be presented in greater detail in the following description of one or more embodiments of the method of the present invention and of one or more embodiments of an integrated circuit implementing the method(s), given in relation with, but not limited to the following figures, in which: 
         FIG. 1  described above schematically represents the structure of a contactless integrated circuit having a shareable memory, 
         FIG. 2  schematically represents the structure of a contactless integrated circuit according to an embodiment of the present invention, having a shareable memory and implementing an embodiment of the method of the present invention, 
         FIG. 3  represents an example of an embodiment of the integrated circuit in  FIG. 2  in which the integrated circuit comprises a management circuit for managing the hard-wired logic memory, and represents certain elements of this management circuit, 
         FIG. 4  is a flowchart describing one embodiment of the execution by the integrated circuit in  FIG. 3  of a command for presenting a password, 
         FIG. 5A  is a flowchart describing one embodiment of the execution by the integrated circuit in  FIG. 3  of a write command for writing a data block, and 
         FIG. 5B  is a flowchart describing one embodiment of the execution by the integrated circuit in  FIG. 3  of a read command for reading a data block. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are given to provide a thorough understanding of embodiments. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     The headings provided herein are for convenience only and do not interpret the scope or meaning of the embodiments. 
       FIG. 2  schematically represents the structure of a contactless integrated circuit IC 2  according to one embodiment of the present invention. In an embodiment, the integrated circuit IC 2  comprises a contactless communication interface circuit ICT, a control circuit CCT 2  and an electrically erasable and programmable memory MEM 2 . The interface circuit ICT is linked to an antenna circuit ACT of the inductive coupling type (comprising an antenna coil) or of the UHF type (comprising a dipole antenna) for receiving data by inductive coupling or by electric field modulation, and sending data by load modulation or by modulating the reflection coefficient of the antenna circuit (“backscattering”). The control circuit CCT 2  is a hard-wired logic or microprocessor circuit, and the memory MEM 2  is of the EEPROM or FLASH type in some embodiments. 
     The format of the commands received and of the responses sent via the circuit ICT, and the contactless communication protocol used, are determined by a standard, for example the ISO 15693 standard, the ISO 14443 standard, the ISO 18000-3 standard, etc. or several standards if the integrated circuit is of the multiprotocol type. 
     In a manner classic in itself, the memory MEM 2  comprises a user memory area UMEM and a system memory area SMEM. The system area SMEM comprises data used by the circuit CCT 2  to implement the communication protocol. These data are for example the single identifier UID used by various anti-collision protocols, here coded on 8 bytes UID 0  to UID 7 , a parameter AFI and a parameter DSFID of 1 byte each, provided for by the ISO 15693 and ISO 18000-3 standards. 
     According to one embodiment of the present invention, and as described by Table 1 below, the user memory area UMEM is divided into a plurality of data blocks of the same size, here n+1 blocks B 0 , B 1 , B 2  . . . Bn. Each block Bi (B 0 , B 1 , . . . Bn) is designated by an address Ai (A 0 , A 1 , . . . An) and comprises a data field Di (D 0 , D 1 , . . . Dn) and a field PDi (PD 0 , PD 1 , . . . PDn) enabling the read- and write-protection of the block to be parameterized. 
     The data field Di of each block Bi here has an example size of 4 bytes, i.e., 32 bits, and the example parameterization field PDi here contains 5 useful bits. For purely technical reasons arising during the design of the memory, the size of the parameterization field PDi can be greater than 5 bits, and be one byte for example. The unused bits of the parameterization field, reserved for future use, will here be considered inexistent, for the sake of simplicity. 
     The memory MEM 2  also comprises a password zone PZ for storing passwords, which is arranged here in the system memory area SMEM. The zone PZ is divided into data blocks SBi of the same size as the blocks Bi of the memory area UMEM and here comprises three blocks SB 1 , SB 2 , SB 3  or “password blocks”. Like the blocks Bi, each block SBi (SB 1 , SB 2 , SB 3 ) comprises a data field Pi (P 1 , P 2 , P 3 ) and a parameterization field PPi (PP 1 , PP 2 , PP 3 ) for the protection of the block to be parameterized. The data field Pi is a password field and receives a password pwi (pw 1 , pw 2 , pw 3 ). As the fields Pi of the blocks SBi are here of the same size as the fields Di of the blocks Bi of the memory area UMEM, the passwords are coded on 32 bits, for example. 
     The content of the parameterization fields PDi and PPi is described by Table 2 below. The fields PDi and PPi each comprise an index IDX coded on two bits b 4 , b 3 , a parameter PR coded on two bits b 2 , b 1 , and a lock bit b 0 . This content is a sort of coding defining access rights to the data blocks or to the password blocks, to which the control circuit CCT 2  refers when a user sends the integrated circuit a read or write command for reading or writing a block, to determine whether or not this command can be executed. 
     If different from 0, the index IDX of the data blocks Bi designates a password block SBi, in accordance with the coding convention described by Table 3 below. The password pwi present in the field Pi of the password block SBi designated by the index IDX is therefore considered to be the password allocated to the data block Bi to manage the access rights to this block. If different from “00”, the index IDX of the password blocks SBi also designates a password block SBi, the user being given the choice of making the index of a password block designate the password block in which it is located, or another password block. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Organization of the Memory MEM2 
               
             
          
           
               
                   
                   
                   
                   
                 Parameteriza- 
               
               
                   
                   
                   
                   
                 tion fields 
               
               
                   
                   
                   
                 Data fields Di and Pi 
                 PDi and PPi 
               
               
                 User Area 
                   
                 Blocks 
                 (32 bits) 
                 (8 bits) 
               
               
                   
               
               
                 UMEM 
                   
                 B0 
                 D0 
                 PD0 
               
               
                   
                   
                 B1 
                 D1 
                 PD1 
               
               
                   
                   
                 B2 
                 D2 
                 PD2 
               
               
                   
                   
                   
                 . . .  
                   
               
               
                   
                   
                 Bn 
                 Dn 
                 PDn 
               
             
          
           
               
                 SMEM 
                   
                   
                 UID0 
                 UID1 
                 UID2 
                 UID3 
                 (unused) 
               
               
                   
                   
                   
                 UID4 
                 UID5 
                 UID6 
                 UID7 
                 (unused) 
               
               
                   
                   
                   
                 AFI 
                 DSFID 
                   
                   
                 (unused) 
               
             
          
           
               
                   
                 PZ 
                 SB1 
                 P1 (pw1) 
                 PP1 
               
               
                   
                   
                 SB2 
                 P2 (pw2) 
                 PP2 
               
               
                   
                   
                 SB3 
                 P3 (pw3) 
                 PP3 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Format of the Fields PDi (PD0 to PDn) and PPi (PP1 to PP3) 
               
             
          
           
               
                 index IDX 
                 parameter PR 
                 lock bit 
               
               
                   
               
             
          
           
               
                 b4 
                 b3 
                 b2 
                 B1 
                 b0 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Coding of the Index IDX 
               
             
          
           
               
                 IDX 
                 Password 
               
               
                   
               
               
                 00 
                 the block is not protected by a password 
               
               
                 01 
                 the block is protected by the password pw1 present in the field 
               
               
                   
                 P1 of the block SB1 
               
               
                 10 
                 the block is protected by the password pw2 present in the field 
               
               
                   
                 P2 of the block SB2 
               
               
                 11 
                 the block is protected by the password pw3 present in the field 
               
               
                   
                 P3 of the block SB3 
               
               
                   
               
             
          
         
       
     
     The parameter PR defines access rights to the block in the manner described by Table 4 below, by distinguishing firstly between the read-access rights and the write-access rights, and secondly between the access rights given to a user having presented a valid password and the access rights given to a user not having presented the password allocated to the block or having presented an invalid password (a password is valid if it is equal to the password allocated to the block, i.e., the password designated by the index IDX present in the block). 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Coding of the Fields PDi (PD0 to PDn) and PPi (PP1 to PP3) 
               
             
          
           
               
                   
                 Password 
                   
               
               
                   
                 not presented 
                 Password presented 
               
             
          
           
               
                   
                 b2 b1 
                 b4 b3 
                 Read- 
                 Write- 
                 Read- 
                 Write- 
               
               
                 b0 
                 (PR) 
                 (IDX) 
                 accessible 
                 accessible 
                 accessible 
                 accessible 
               
               
                   
               
               
                 0 
                 xx 
                 xx 
                 YES 
                 YES 
                 YES 
                 YES 
               
               
                 1 
                 xx 
                 00 
                 YES 
                 NO 
                 YES 
                 NO 
               
               
                 1 
                 00 
                 01, 10, 11 
                 YES 
                 NO 
                 YES 
                 YES 
               
               
                 1 
                 01 
                 01, 10, 11 
                 YES 
                 YES 
                 YES 
                 YES 
               
               
                 1 
                 10 
                 01, 10, 11 
                 NO 
                 NO 
                 YES 
                 YES 
               
               
                 1 
                 11 
                 01, 10, 11 
                 NO 
                 NO 
                 YES 
                 NO 
               
               
                   
               
             
          
         
       
     
     The write protection defined by the parameterization field PDi of a data block Bi or by the parameterization field PPi of a password block SBi, applies here to the entire block in question, and therefore also relates to the parameterization field PDi or PPi itself. Thus, a data block or a password block that is write-locked with or without password (PR=11) can no longer be made write-accessible, including by the user having the password. 
     Similarly, when the lock bit b 0  of a block has been set to 1 whereas the index IDX of the block indicates that no password has been allocated to the block (IDX=00, Cf. Table 3 and Table 4), the block is definitively write-locked but remains read-accessible. No password can therefore be subsequently allocated to this block. On the other hand, if a password has been allocated to the block (IDX=01, 10 or 11) or is allocated to the block when the bit b 0  is set to 1, the setting to 1 of the bit b 0  enables the access right allocation rule defined by the parameter PR to be validated. 
     However, it is preferred to keep the possibility of reading the parameterization fields PDi of the data blocks Bi, including when the corresponding data fields Di are not read-accessible. Thus, when b 2  b 1  b 0 =111(last line of Table 4) or b 2  b 1  b 0 =101(penultimate line of Table 4), the parameterization field PDi remains read-accessible to the authenticated or non-authenticated user. 
     Generally speaking, one embodiment of the present invention enables the number of passwords to be reduced while increasing the fineness of the breakdown of the user memory UMEM, and while offering very flexible management of the access rights. Thanks to an embodiment of the present invention, each password pwi can be used to protect any one of the data blocks Bi or any one of the passwords blocks Pi. In addition, the parameter PR enables different read and write rights to be defined and different rights for the user having the password and the user who does not have the password. 
     Each user can therefore use one or more passwords and grant the next user read rights to certain blocks by giving him one of the passwords used. When the integrated circuit is commissioned, the parameterization fields PDi of the data blocks Bi and the parameterization fields PPi of the password blocks SBi are all on 0 by default, as are the password fields Pi. Thus, the first user can occupy at will one or more data blocks and parameterize the protection of these blocks. The user can also write his password(s) in the password blocks and define the access rights to the passwords, particularly lock the password blocks so that no one can subsequently change the passwords, including himself. 
     Moreover, as the protection of the password blocks SBi can be indexed on passwords present in other password blocks, the user can decide to protect a secondary password by means of a main password (for example an administrator password) present in another password block. The user can then supply a third party with the secondary password without revealing the main password, without running the risk of the third party changing the secondary password and without being forced to write-lock the secondary password for the authenticated user, to reserve the possibility of subsequently changing it. 
     As the size of the data block parameterization fields (here 5 bits) is clearly smaller than the size of the passwords (here 32 bits), and as the number of passwords (here 3) is clearly smaller than the number of data blocks (for example n+1=256), the method for managing the access rights according to one embodiment of the present invention limits the memory space occupied by the passwords while offering a very fine granularity in the breakdown of the memory (breakdown on a block-by-block basis) enabling the filling of the user memory area UMEM to be optimized. For example, a first user having used the first password block SB 1  and having saved data in blocks B 0 , B 1 , B 2  can then access the memory and use for example blocks B 250 , B 251  (by designating its initial password as password allocated to these blocks, by means of the index IDX) if other users have occupied blocks B 3  to B 249  in the meantime. Therefore, an embodiment of the present invention enables user data having different passwords to be interlinked. 
     As the zone PZ is arranged here in the system memory area SMEM, the password blocks SBi are designated by system addresses SAi (here SA 1 , SA 2 , SA 3 ) that cannot be accessed by the user. According to one embodiment of the present invention, WRITE_PASSWORD and LOCK_PASSWORD commands are thus defined that are respectively dedicated to writing and to locking the passwords and which are executed by the control circuit CCT 2 . For these commands to be implemented, the password blocks SBi are designated by a block number NUMBi (here NUMB 1 , NUMB 2 , NUMB 3 ) and the block number is converted into system address by the circuit CCT 2 . 
     Write Password Command 
     This command is used to write or refresh a password of 32 bits in one of the fields Pi of the zone PZ. After writing the block, the password must be activated by using the LOCK_PASSWORD command described below. According to the access rights defined by the parameter PR of the corresponding parameterization field PPi, it is possible to change the value of the password after a valid WRITE_PASSWORD command. This command comprises at least the code of the command, or CODE wp , the number of the block targeted, or NUMBi, and the 32 bits of the password pwi (pw 1 , pw 2 , pw 3 ) to be written. It is therefore in the following form:
 
[CODE wp , NUMBi, pwi]
 
Lock Password Command
 
     This command is provided for acting on the parameterization field PPi of the password blocks SBi. The bits b 1  to b 4  are concerned by this command, i.e., the parameter PR and the index IDX. After receiving this command, the lock bit b 0  is automatically set to 1 by the circuit CCT 2  to activate the control, by the parameter PR, of the access rights to the password, or to disable the password block targeted if the index IDX is equal to 0. After application of this command, the protection of the block as defined by the parameter PR is therefore activated and it is not possible to put the lock bit b 0  back to 0. Thus, if the index IDX is taken to the value “00” or if the parameter PR is taken to the value “11”, the corresponding password is definitively write-locked. This command comprises at least the code of the command or CODE LP , the number of the password block targeted, or NUMBi, and the bits b 1  to b 4 . It is therefore in the following form:
 
[CODE LP , NUMBi, b1-b4]
 
     A read command for reading the passwords can also be provided, so that the user can check the proper execution of the WRITE_PASSWORD command. 
     Furthermore, so as to facilitate the management of the access rights as defined by Table 4, a command for presenting the user password, or PRESENT_PASSWORD command, is advantageously provided by an embodiment of the present invention. 
     Present Password Command 
     This command is used to present a user password pwu to the control circuit CCT 2  by indicating with which password pwi present in the zone PZ the password presented pwu must be compared. This command comprises at least the code of the command or CODE pp , the number of the block containing the targeted password pwi, or NUMBi, and the password presented pwu. It is therefore in the following form:
 
[CODE LP , NUMBi, pwu]
 
     When the password presented corresponds to the targeted password, an access session is opened for the user for all the blocks having this password (i.e., the index of which IDX designates this password), the access rights to each block possibly being different however, according to the lock bit b 0  or to the parameter PR of each block. Thus, it is not necessary for the user to present his password every time he sends a read or write command. 
     After executing this command, the result of the comparison between the target password pwi and the user password pwu is stored by the control circuit CCT 2 , for the entire duration of the session. Thus, the session is interrupted when the integrated circuit is switched off (which occurs, for example, when the integrated circuit is of passive type and is outside the polling range of a reader, from which its supply voltage is extracted). The session is also interrupted when the user presents a new password, by means of the PRESENT_PASSWORD command. 
     Similar commands can be provided for managing the parameterization fields PDi of the data blocks Bi. In particular, a LOCK_DATABLOCK command can be provided. 
     Lock Datablock Command 
     This command is provided for acting on the field PDi of the data blocks Bi. As above, the bits b 1  to b 4  are concerned by this command, i.e., the parameter PR and the index IDX. This command acts on the field PDi in the same way as the LOCK_PASSWORD command acts on the field PPi, and will not therefore be described in detail. This command comprises at least the code of the command or CODE LD , the address Ai of the data block targeted in the memory UMEM, and the bits b 1  to b 4 . It is therefore in the following form:
 
[CODE LP , Ai, b1-b4]
 
       FIG. 3  shows one embodiment of the circuit IC 2  in which the control circuit CCT 2  is produced in hard-wired logic. The core of the circuit CCT 2  is a logic Finite State Machine FSM 2  that is designed to execute read and write commands for reading and writing the memory and the specific commands described above. The memory MEM 2  comprises a circuit PLCH one input of which is linked to a data bus DTB of 37 bits (32 bits for the data and 5 bits for the parameterization field) and one output of which is linked to the memory array of the memory MEM 2 , comprising a set of EEPROM-type memory cells arranged as a matrix. The circuit PLCH classically comprises programming latches and receives the data to be written in the memory. The memory MEM 2  also comprises a first group of sense amplifiers SAMP 1  to read the data fields Di or the password fields Pi, and a second group of sense amplifiers SAMP 2  provided for reading the 5 bits of the parameterization fields PDi or PPi of the data blocks or of the password blocks. The memory also comprises an address decoder (word line decoder) enabling a block to be read- or write-selected, which has not been represented for the sake of legibility of the Figure. 
     The group SAMP 2  thus supplies the bits b 0  to b 4  read in a block of the memory array. The bits b 0  to b 2  are sent to the logic machine FSM 2  to be tested. The bits b 3  and b 4 , forming the index IDX, are applied to a first input of a logic comparator COMP the second input of which receives the content of a reference register RREG controlled by the logic machine FSM 2 . The first input of the comparator COMP also comprises a zero detector ZDET that supplies a flag ZIDX that is equal to 1 when an index read in the memory is equal to 0(which means that the corresponding block has no allocated password). 
     The register RREG is used by the logic machine FSM 2  to save an index IDX corresponding to a password validly presented by a user by means of the PRESENT_PASSWORD command. 
     This will be better understood by referring to the flow chart in  FIG. 4 , which succinctly describes an embodiment of the steps executed by the logic machine FSM 2  to update the register RREG in response to a PRESENT_PASSWORD command. The following steps can be distinguished: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                   
                 Step S1: WAIT 
               
               
                   
                 Step S2: &lt;PRESENT_PASSWORD, NUMBi, pwu&gt; 
               
               
                   
                 Step S3: READ pwi(Pi) 
               
               
                   
                 Step S4: pwi(Pi) = pwu ? 
               
               
                   
                 Step S5: RREG=IDX(SBi) 
               
               
                   
                 Step S6: &lt;ERROR&gt; 
               
               
                   
               
             
          
         
       
     
     The step S 1  is a step of waiting for a command. In step S 2 , the logic machine receives and decodes the PRESENT_PASSWORD command that comprises the number NUMBi of the password block targeted and the user password pwu. In step S 3  the logic machine reads the password pwi(Pi) located in the field Pi of the password block SBi targeted by the block number NUMBi, i.e., the field Pi(SBi(NUMBi)). In step S 4 , the logic machine compares this password with the password pwu presented by the user. If the result of the comparison is positive, the logic machine goes to step S 5  where it loads, into the register RREG, an index value IDX designating the password block SBi in question, according to the coding convention defined by Table 3 above. For example, if the block targeted by the command is the block SB 1 , the index loaded into the register is “01”. In the opposite case, the logic machine goes to step S 6  where it returns an error message via the interface circuit ICT. 
     After processing a PRESENT_PASSWORD command validly presented, the register RREG thus contains the index of the password block for which a password has been presented, which then enables the logic machine not to check, upon each read or write command received, whether or not the user has the password of the blocks to which he requests access: the user is considered to be authenticated if these blocks have the same index as the index stored in the register RREG, and is considered to not have presented the password if the blocks do not have the same index. 
     For a better understanding,  FIG. 5A  is a flowchart succinctly describing an embodiment of the steps executed by the logic machine to process a command for writing a data block, by using the register RREG. The following steps can be distinguished: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                   
                 Step S1: WAIT 
               
               
                   
                 Step S10: &lt;WRITE DTi; Ai&gt; 
               
               
                   
                 Step S11: LOAD DTi and IREAD B(Ai) 
               
               
                   
                 Step S12: b0 = 0? 
               
               
                   
                 Step S13: ZIDX = 0? 
               
               
                   
                 Step S14: BCOMP = 1? 
               
               
                   
                 Step S15: b1 b2 = 11? 
               
               
                   
                 Step S16: IWRITE: B(Ai)= DTi 
               
               
                   
                 Step S17: &lt;ERROR&gt; 
               
               
                   
                 Step S18: b1 b2 = 01? 
               
               
                   
                 Step S19: &lt;RSP&gt; 
               
               
                   
               
             
          
         
       
     
     After the wait step S 1 , the logic machine receives a write command for writing the datum DTi at the address Ai, i.e., in the data block Bi of address Ai. In step S 11 , the logic machine loads (LOAD) the data DTi into the circuit PLCH ( FIG. 3 ) then carries out an internal reading (IREAD) of the address block Ai, i.e., the block B(Ai) (or block Bi according to the notation used above). The group of sense amplifiers SAMP 2  then supplies the bits b 0  to b 4 . The bits b 3  and b 4  (index IDX) are then applied to the input of the comparator COMP the output of which supplies a flag BCOMP that is equal to 1 if the comparison of the index IDX with the index loaded in the register RREG is positive. Furthermore, the flag ZIDX is equal to 0 if the index read is equal to “00”. 
     The logic machine first determines, in step S 12 , whether or not the lock bit b 0  is equal to 0. If b 0 =0, the targeted block can be accessed freely (refer to Table 4) and the logic machine goes to step S 16  that is an internal write step (IWRITE) of writing in the targeted block B(Ai) the data received DTi, which are present in the circuit PLCH and ready to be saved in the memory array. 
     If the bit b 0  is not equal to 0, the logic machine goes to step S 13  where it determines whether or not the index is equal to 0 by observing the value of the flag ZIDX. If the flag ZIDX is equal to 0, and as the bit b 0  is equal to 1, this means that no password has been declared for this block and that the block is write-locked (cf. Table 4). The logic machine then goes to step S 17  where it returns an error message through the interface circuit ICT. 
     If the flag ZIDX is not equal to 0, the logic machine goes to step S 14  where it determines whether or not the flag BCOMP is equal to 1. If the flag BCOMP is equal to 1, the logic machine goes to step S 15  where it determines whether or not the bits b 1  and b 2  are equal to 1 (only case of write-locking the block when the signature has been presented, cf. Table 4). 
     If the bits b 1  and b 2  are equal to 1, the logic machine goes to step S 17  and returns the error message. Otherwise, it goes to step S 16  to execute the write command for writing the data DTi in the block B(Ai). If the flag BCOMP is equal to 0, this means that the user having sent the command has not previously declared the password allocated to the block. The logic machine then goes to step S 18  to determine whether or not the bits b 1  and b 2  are respectively equal to 0 and to 1 (only case of authorizing writing of the block when the signature has not been presented, cf. Table 4). If the response is positive, the logic machine goes to step S 16  to execute the write command for writing the data DTi in the block B(Ai). Otherwise, it goes to step S 17  and returns the error message. After the write step, the logic machine goes through a step S 19  where it returns a message RSP indicating that the writing has been carried out (optional step), and then returns to step S 1 . 
       FIG. 5B  is a flowchart succinctly describing an embodiment of the steps executed by the logic machine to process a read command for reading a data block, using the register RREG. The following steps can be distinguished: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                   
                 Step S1: WAIT 
               
               
                   
                 Step S20: &lt;READ (Ai)&gt; 
               
               
                   
                 Step S21: IREAD B(Ai) 
               
               
                   
                 Step S22: b0 = 0? 
               
               
                   
                 Step S23: BCOMP = 1? 
               
               
                   
                 Step S25: b1 = 0? 
               
               
                   
                 Step S24: RSP = DTi(Ai) 
               
               
                   
                 Step S26: &lt;ERROR&gt; 
               
               
                   
               
             
          
         
       
     
     In step S 20 , the logic machine receives a read command for reading a data block B(Ai) of address Ai. In step S 21 , it reads the data present in the block by means of the group of sense amplifiers SAMP 1  and simultaneously reads the bits b 0  to b 4  by means of the group SAMP 2 . The bit b 0  is tested in step S 22 . If b 0  is equal to 0, the logic machine goes to step S 24  and sends the data read DTi(Ai) in response to the command. If b 0  is equal to 1, the logic machine determines during the step S 23  whether or not the user has previously declared the password of the block by checking whether or not the flag BCOMP is equal to 1. If BCOMP is equal to 1, the logic machine goes to step S 24  and sends the data read. If the bit BCOMP is equal to 0, the logic machine goes to step S 25  where it determines whether or not the bit b 1  is on b0. If the bit b 1  is equal to 0, the logic machine goes to step S 24  and sends the data read. Otherwise, it goes to step S 26  to return an error message. 
     It will be understood by those skilled in the art that various alternative embodiments of the present invention are possible, such as concerning the coding of the parameterization field, the number of parameterization bits and the arrangement of the password blocks. The zone PZ containing the password blocks can particularly be arranged in the user memory UMEM and the password blocks are then accessible by using the read or write commands enabling the data blocks to be accessed. Furthermore, the use of several reference registers RREG and of several comparators COMP can enable several sessions with several passwords to be opened simultaneously. Moreover, the PRESENT_PASSWORD command can be provided without any indication of the target password block. In this case, the control circuit CCT 2  itself searches, among the password blocks, whether there is at least one password corresponding to the password presented by the user. 
     Finally, although managing the access rights by using the parameter PR (bit b 1 , b 2 ) and of the lock bit b 0  was proposed above, all or part of these controls can be removed within the scope of applications in which the management of the accesses can be simplified. For a better understanding, Table 5 below describes a simplified management of the access rights to the data blocks without using the parameter PR. Table 6 below describes a simplified management of the access rights to the data blocks without using the lock bit b 0 . Table 7 below describes a simplified management of the access rights to the data blocks without using either the parameter PR or the lock bit b 0 . In this last alternative, the protection is of the “all or nothing” type without differentiating the read-access rights from the write-access rights. The user can only read- and write-access a block if he has the password designated by the index. 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Coding of the Fields PDi and PPi Without Parameter PR 
               
             
          
           
               
                   
                 Password not 
                   
               
               
                   
                 presented 
                 Password presented 
               
             
          
           
               
                   
                   
                 Read- 
                 Write- 
                 Read- 
                 Write- 
               
               
                 b0 
                 IDX 
                 accessible 
                 accessible 
                 accessible 
                 accessible 
               
               
                   
               
               
                 0 
                 Xx 
                 YES 
                 YES 
                 YES 
                 YES 
               
               
                 1 
                 00 
                 YES 
                 NO 
                 YES 
                 NO 
               
               
                 1 
                 01, 10, 11 
                 NO 
                 NO 
                 YES 
                 YES 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Coding of the Fields PDi and PPi Without Bit b0 
               
             
          
           
               
                   
                 Password not 
                   
               
               
                   
                 presented 
                 Password presented 
               
             
          
           
               
                   
                   
                 Read- 
                 Write- 
                 Read- 
                 Write- 
               
               
                 PR 
                 IDX 
                 accessible 
                 accessible 
                 accessible 
                 accessible 
               
               
                   
               
               
                 xx 
                 00 
                 YES 
                 YES 
                 YES 
                 YES 
               
               
                 00 
                 01, 10, 11 
                 YES 
                 NO 
                 YES 
                 YES 
               
               
                 01 
                 01, 10, 11 
                 YES 
                 YES 
                 YES 
                 YES 
               
               
                 10 
                 01, 10, 11 
                 NO 
                 NO 
                 YES 
                 YES 
               
               
                 11 
                 01, 10, 11 
                 NO 
                 NO 
                 YES 
                 NO 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Coding of the Fields PDi and PPi Without Parameters PR and b0 
               
             
          
           
               
                   
                 Password not presented 
                 Password presented 
               
             
          
           
               
                   
                 Read- 
                 Write- 
                 Read- 
                 Write- 
               
               
                 IDX 
                 accessible 
                 accessible 
                 accessible 
                 accessible 
               
               
                   
               
               
                 00 
                 YES 
                 NO 
                 YES 
                 NO 
               
               
                 01, 10, 11 
                 NO 
                 NO 
                 YES 
                 YES 
               
               
                   
               
             
          
         
       
     
     All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety. 
     The above description of illustrated embodiments, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments and examples are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention and can be made without deviating from the spirit and scope of the invention. 
     These and other modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.
A method for managing the access to a memory space shareable by several users, by using passwords, comprises: defining a maximum number of passwords, providing a password storage zone, dividing the shareable memory space into a plurality of blocks greater in number than the maximum number of passwords, providing in each block a parameterization field for parameterizing the protection of the block, providing in each parameterization field a binary index smaller in size than a password and designating a password assigned to the protection of the block, and allocating, to each block, access rights requiring a password to be presented corresponding to the password designated by the index present in the block parameterization field. Application is provided particularly but not exclusively to multi-user integrated circuits.