the invention is illustrated here , nonlimitingly , within the framework of a ring - signature technique as described in the aforesaid publication by r . rivest , et al . each entity e i ( 1 ≦ i ≦ r ) of the set e is furnished with a public key pub 1 of rsa type , of modulus n i and exponent e i , which defines a permutation f i over { 0 , 1 , 2 , . . . , n i - 1 ): f i ( x )= x e i modulo n i ( this permutation is the so - called rsa cipher function ). the domain of the keys is widened to a common domain [ 0 , 2 b - 1 ], this defining the integer b chosen such that b ≧ log 2 ( n i ) for any i and the set b ={ 0 , 1 , 2 , . . . , 2 b - 1 }. typically , b = 1024 or 2048 depending on the rsa keys of the users . this widening makes it possible to have keys of different sizes depending on the members of the group . the functions g i are defined from b into b . denoting by q i ( x ) and r i ( x ) the quotient and the remainder of the euclidian division of x by n i ( x = q i ( x ). n i + r i ( x ) with 0 ≦ r i ( x )& lt ; n i ), we have : if ( q i ( x )+ 1 ). n i & lt ; 2 b , then g i ( x )= q i ( x ). n i + f i ( r i ( x )); and if ( q i ( x )+ 1 ). n i ≧ 2 b , then g i ( x )= x . this function g i dependent on the public key pub i is invertible and its inverse h i depends on the private key pr i associated with pub i : if ( q i ( y )+ 1 ). n i & lt ; 2 b , then h i ( y )= q i ( y ). n i + f i − 1 ( r i ( y )); and if ( q i ( y )+ 1 ). n i ≧ 2 b , then h i ( y )= y , f i − 1 being the rsa decipher function , the inverse of f i , evaluated by means of the private key pr i . the combination c is defined by means on the one hand of a symmetric cipher function cs k , constructed for example on the basis of a conventional algorithm such as des (“ digital encryption system ”), aes (“ advanced encryption system ”), etc ., and defining a permutation over b on the basis of a key k , and on the other hand of a hash function h , constructed for example on the basis of the conventional sha - 1 function and able to produce all the valid keys k . the key k is generated by hashing the message m to be signed : k = h ( m ). the combination function c m , v is then given by : c m , v ( y 1 , y 2 , . . . y r )= cs k ( y r ⊕ cs k ( y r − 1 ⊕ cs k ( y r − 2 ⊕ . . . ⊕ cs k ( y 1 ⊕ v ) . . . ))) this combination function is invertible for any position s , the solving of the ring equation ( 1 ) consisting simply in calculating : u = cs k ( y s − 1 ⊕ cs k ( y − 2 ⊕ . . . ⊕ cs k ( y 1 ⊕ v ) . . . )); u ′= ds k ( y s + 1 ⊕ ds k ( y s + 2 ⊕ . . . ⊕ ds k ( y r ⊕ ds k ( v )) . . . )) by means of the symmetric deciphering function associated with cs k ; then the invention modifies this known ring - signature mechanism by replacing one at least of the randomly drawn values of variability parameters , i . e . v and / or x i for i ≠ s , by an identity trace of the signatory e s . anonymity withdrawal data ( awd ) are produced so as to be stored and kept secret by at least one anonymity withdrawal entity ( awe ) in relation with an identification id s of the signatory e s . the identity trace , determined as a function of these awd , subsequently makes it possible , if the need is felt , to revoke the anonymity afforded to the signatory by the ring - signature mechanism . the awe can be constituted by the signatory himself or by a trusted server supervised by an independent third party . it may also include both the signatory and such a trusted server . with reference to the figures , depicted therein are the computer devices held by the possible signatories e i of the group e , which consist for example of computers or terminals capable of communicating with one another through one or more telecommunication networks ( not shown ) ( for example an ip type network such as the internet or an intranet ). these devices are equipped with programs suitable for implementing the steps described hereinbelow . it is assumed that each member of the group e knows the public keys pub i of all the others . the identification of each member e i is effected by means of an identity id i , which may consist of the index i , the public key pub i , and a certificate of x . 509 type or other , etc ). fig2 to 4 , 6 and 8 also show a trusted server s usable in certain embodiments of the method . this server is linked to the telecommunication network and is also equipped with programs suitable for implementing the steps described hereinbelow . it may for example deploy a secure web server of sslv3 type ( see “ the ssl protocol - version 3 . 0 ” internet draft , ietf , november 1996 ). requests are then sent to it according to the https protocol with client authentication . the signatures involved ( token , proof of signatory ) may all be stored in known structures of the type pkcs # 7 - signeddata or xml - dsig . signature verifications may if appropriate entail the verification of the certificates &# 39 ; trust chain . in the embodiments of the invention illustrated by fig1 to 6 , the variability parameter value taken equal to the identity trace is the value of the additional parameter v serving to close the signature ring . in the embodiment of fig1 the awe is constituted by the signatory e s . the latter &# 39 ; s computer device draws a random number μ ( step 1 ), which it stores as awd in step 2 , while keeping it secret . in step 3 , the identity trace v is calculated by cryptographic hashing of the concatenation of the random number μ and of the message m to be signed : in the present description , the notation w ∥ z designates the concatenation of any two messages w and z , h designates a cryptographic hash function taking its values in b , and h ′, h ″ designate hash functions that may be the same as or different from h . these hash functions h , h ′, h ″ may be chosen from among those conventionally employed in cryptographic applications ( sha - 1 , md - 5 , etc .). after having obtained the identity trace v , the signatory &# 39 ; s device randomly draws values x i in b for 1 ≦ i & lt ; s and s & lt ; i ≦ r in step 4 identical to the aforesaid step / b /, then executes the ring - signature in step 5 identical to the aforesaid steps / c /−/ e /, with the values x i ( i ≠ s ) for the parameters associated with the other members of the group e and the value v for the additional variability parameter . the ring - signature ( pub 1 , pub 2 , . . . , pub r ; v ; x i , x 2 , . . . , x r ) may be appended to the message m to form the signed message m communicated by the signatory e s . the ring - signature is verifiable by anybody according to the verification algorithm / f /−/ h / described previously . the anonymity withdrawal can be requested by a verifier v having a computer device able to communicate with those of the members of the group e by way of the telecommunication network and programmed in such a way as to be able to undertake the operations hereinbelow . in a first step 100 , the verifier v makes sure of the validity of the ring - signature . if the ring equation ( 1 ) is not satisfied for the signed message m , the signature is declared invalid . if it is valid , the verifier addresses at least one awd request to an awe , for example by e - mail . in the case of fig1 ( awe = e s ), this awd request is sent to the members of the group e , only e s being in a position to respond suitably thereto . if v guesses the identity of the signatory , he may send the request only to the latter . otherwise , he addresses it to all the members of the group , either in succession until a satisfactory response is obtained , or simultaneously . the awd request can include the message m or a part thereof so that the recipient can know for which message an anonymity withdrawal is required . it may in particular include a hash h ″ ( m ) of the message m , that the signatory e s has also preserved in memory in step 2 in relation with the awd . in the processing of the request , e s will thus be able to access the awd required . the signatory e s determines whether he accepts withdrawal of his anonymity vis - à - vis v , this possibly requiring prior authentication of v by e s . then , if he accepts , the stored awd are read in step 6 and addressed to v in a message which may possibly be signed with the private key of e s to guarantee its origin to v . receiving a value μ *(= awd ) in such a message originating from an identified member e i , v verifies in step 101 whether it is a satisfactory response , i . e . whether h ( μ *∥ m ) corresponds to the value v included in the ring - signature . if so , the anonymity of e s is withdrawn by v . otherwise , anonymity is maintained and another member of the group may possibly be interrogated in his or her turn . the signatory e s can also spontaneously withdraw his anonymity , by communicating μ to v ( with m ) or by publishing it on a website . the variant illustrated by fig2 allows the withdrawal of anonymity by virtue of an anonymity withdrawal third party supervising a trusted server s . the signatory e s generates a request for a random number μ ′ ( step 10 ), that it addresses to the server s . this request may contain a hash h ″ ( m ) of the message m for reference purposes . the server s authenticates e s ( step 200 ), for example within the framework of the establishing of an sslv3 session or in accordance with the content of the message transporting the request . if e s is properly authenticated , the processing of the request by s comprises the drawing of a first random number μ ( step 201 ) which is stored and kept secret in relation with the identity id s of the signatory ( step 202 ) and with h ″ ( m ). the value μ ′ is then calculated by hashing μ in step 203 ( μ ′= h ′( μ )) then returned to e s . the identity trace v is obtained in step 11 by the signatory e s by cryptographic hashing of the concatenation of μ ′ and of the message to be signed m ( or of just a part thereof ): v = h ( μ ′∥ m ). the aforesaid steps 4 and 5 are then executed to produce the ring - signature . after verification 100 of this ring - signature , v sends its awd request to the trusted server s . this request may be accompanied by the hash h ″ ( m ) for message identification purposes . the server s can then read the requested awd ( step 204 ) and return them to v . verification 101 allowing v to withdraw the anonymity then consists in testing whether h ( h ′( μ *∥ m ))= v , where μ * is the random number value received from s . if the test is positive , v identifies e s according to the identity obtained in the awd . it is noted that , in this embodiment , the server s has no knowledge of the message m before recording the awd and that it cannot therefore a priori censor e s ( a cryptographic hash such as h ″ ( m ) does not give access to the content of m ). fig3 to 6 illustrate embodiments of the invention making it possible , once anonymity has been withdrawn , to have proof ( authenticated withdrawals of anonymity ). in these embodiments , there is a proof collection phase in respect of anonymity withdrawal , during which an entity produces a proof p that may in particular include the identity id s of the signatory and the message m . this proof is preserved as awd by one or more awe ( trusted third parties and / or e s itself ). if e s is not an awe , an awe sends e s a hash q = h ( p ) which will serve as identity trace . after verification of the awd , v can apply an algorithm for verifying the proof of anonymity withdrawal which returns id s if the proof is valid and which denies the anonymity withdrawal if the proof is invalid . the proof can in particular be generated by electronic signing of a token j composed by concatenating a random number μ , the message m ( or a part of the latter , in particular a hash ) and of possible extensions . the signing of the token j can be performed by e s or by a trusted server s . in the latter case , it is judicious for the token j to also contain an identity id s of the signatory . the extensions of the token may comprise a serial number or other useful data . in an advantageous embodiment , these extensions include a timestamping token generated during composition of the token to be signed . this timestamping token can in particular be generated according to the tsp protocol (“ time stamp protocol ”, see rfc 3161 , “ internet x . 509 public key infrastructure time - stamp protocol ( tsp ), august 2001 , ietf ); it then contains the date and the time of the token j and it is signed by a timestamping third party . in the embodiment according to fig3 the awe consists of a trusted server s . the signatory e s generates a token request ( step 20 ), that it addresses to the server s together with the message m or a part of the latter ( this part may in particular consist of a hash h ″ ( m ) of the message m , thereby allowing s to generate the proof p without knowing the content of the message m ). the server s authenticates e s ( 200 ) and draws a random number μ ( 201 ) as previously . it then composes the token j in step 210 : j = μ ∥ m ∥ id s ∥ ext or j = μ ∥ h ″ ( m )∥ id s ∥ ext , where ext designates the optional extensions . the server s then signs the token j by means of its private key in step 211 , thereby producing the proof p stored in step 212 . in step 213 , it then calculates the value q = h ( p ), returned to e s as response to the latter &# 39 ; s request . the identity trace v is taken equal to this value q in step 21 by the signatory e s . the aforesaid steps 4 and 5 are then executed to produce the ring - signature . after verification 100 of this ring - signature , v sends his awd request to the trusted server s . this request can be accompanied by the message m or by the hash h ″ ( m ) for message identification purposes . the server s can then read the requested awd ( step 214 ) and return them to v . verification 101 allowing v to withdraw the anonymity then consists in testing whether h ( p *)= v , where p * is the proof received from s . if the test is positive , v proceeds to verifications 110 according to the proof p . the withdrawal of the anonymity of e s is effective only if these verifications are all positive . in step 110 , v verifies : consistency between the m found in p ( or its hash h ″ ( m )) and the message of the ring - signature m ; validity of id s and its consistency with pub s ; validity of the signature of the token j by s , by means of the public key of s . the embodiment illustrated by fig4 is similar to that of fig3 except that e s signs the message m by means of its private key pr s ( step 30 ), and sends s the signed message m * in the token request generated in step 31 . the server s verifies this signature by means of the public key pub s ( step 220 ), draws the random number μ then composes the token in step 221 with the signed message m *: j = μ ∥ m *∥ id s ∥ ext . the other operations are the same as those of fig3 . in step 110 , v will be able to perform a further verification on the proof p , as to the validity of the signature of the message m by e s , by means of the public key pub s of e s . this further verification makes it possible to render the withdrawal of the anonymity of e s non - repudiable . such non - repudiation can also be obtained by having the token j signed by the anonymous signatory e s , in an embodiment according to fig5 which does not involve any trusted third party . a random number μ is firstly drawn ( step 1 ) in the computer device of the signatory e s , which composes the token j in step 40 : j = μ ∥ m ∥ ext or j = μ ∥ h ″ ( m )∥ ext . this token j is then signed by means of the private key pr s ( step 41 ), and the result ( proof p ) is stored as awd in step 42 ( if necessary in relation with a hash h ″ ( m ) of the message ). in step 43 , e s determines the identity trace v = h ( p ), which it uses to establish the ring - signature as previously ( steps 4 and 5 ). the awd read in step 44 in response to the request of the verifier v comprise the proof p verified as previously in step 110 ( except in that the signature of p is verified by means of the public key pub s ). in the variant of fig6 the awe consists jointly of the trusted server s and of the anonymous signatory e s , and the awd are subdivided into a first part awd1 =( μ , id s ) preserved by s and a second part awd2 = p preserved by e s . the latter begins by asking s for a random number μ ′ as in the case of fig2 ( steps 10 and 200 - 203 ), this random number μ ′ being produced by hashing a first random number μ preserved and kept secret with id s ( and possibly h ″ ( m )) by s as awd1 . the signatory e s composes the token j in step 40 with the value μ ′ returned by s ( j = μ ′∥ m ′∥ ext ) , then he signs it in step 41 so as to obtain the proof p stored in step 42 as awd2 . the subsequent operations are similar to those of fig5 except that during the anonymity withdrawal , v firstly addresses itself to s to obtain awd1 =( μ , id s ), this allowing the latter to identify e s , then to es to obtain the proof p . the verification 110 can also pertain to the fact that the value μ ′ found in the signed token does indeed result from the hashing of the random number μ returned by s in the awd1s . in the embodiment of the invention illustrated by fig7 the value of variability parameter taken equal to the identity trace is not v but the value of one of the parameters x j associated with the other members e i of the group e ( j ≠ s ). the index j is then chosen arbitrarily by the signatory e s and appended to the awd . the variable v then again becomes a random number . in step 60 , e s draws two random numbers μ and v . the awd recorded in step 61 comprise the values of the random number μ and of the index j ( and possibly a hash h ″ ( m ) of the message m ). the value of the parameter x j associated with e j is calculated in step 62 according to : x j = h ( μ ∥ m ), and in step 63 , e s draws random values of the other x i , i . e . for i ≠ s and i ≠ j . the ring - signature is then executed as previously in step 5 . during anonymity withdrawal , e s reads the awd stored in step 64 and addresses them to v . on the basis of these data μ *, j *, the verification 120 allowing anonymity withdrawal consists in testing whether h ( μ *∥ m )= x j . if several signatories agree to sign a message in the name of a group e to which they belong , it is possible to use the method described hereinabove , and all its variants , in order to devise a multi - signature protocol involving one or more co - signatories in addition to e s . one of the signatories e s summons the other or others to obtain their agreement ; each co - signatory e t returns his own identity trace determined by one of the above variants to e s or has it sent back to the latter e s determines the identity trace of each co - signatory e t in place of the value of the parameter x j which is respectively associated therewith ( as in the case of fig7 ), and his own identity trace as variable v for executing the ring - signature . the anonymity withdrawal can then be partial : certain co - signatories may choose to reveal themselves , others not necessarily . if every one is passed by a third party , the latter can withdraw the anonymity of everyone simultaneously . [ 0120 ] fig8 illustrates an embodiment of the multi - signature , where there is a co - signatory e t in addition to the main signatory e s who executes the ring - signature . the signatories e s , e t confer so as to agree to sign the message m in common , with e s as main signatory , then they each send a token request to the trusted server s ( steps 300 ). this request contains the message m to be signed ( or h ″ ( m ) or else the message m signed with the private key of the sender ), as well as the designation s of the main signatory . before returning these requests , s can verify their compatibility , i . e . that the messages m transmitted with the respective requests of the various signatories are indeed the same and that they all designate the same main signatory . for each signatory , s then separately executes steps 200 , 201 , and 210 - 213 similar to those of fig3 to produce two distinct proofs p s , p t whose hashed versions q s = h ( p s ) and q t = h ( p t ) are returned to e s as identity traces of e s and e t , respectively . the main signatory e s then assigns the identity traces q s and q t to the variables v and x t ( step 301 ), then he randomly draws the values of the other parameters x i ( i ≠ s and i ≠ t ) in step 302 before executing the ring - signature in step 5 . the proofs p s and p t can then be verified separately or simultaneously by v at the server s . the signatories use unauthenticated identity traces ; this yields a “ common suggestion box ” concept , the signatories wishing to claim their collaboration ; the signatories use authenticated identity traces ( for example according to fig8 ): a cryptographic trace of a kind of group deliberation is thus retained . the ring - signature behaves like a commitment of the signatories who preserve their anonymity . anonymity can however be withdrawn if need be .