Abstract:
The method employs interrelated cryptographic processes ( 210 ) and protocols ( 202,204,206,208,212,214 ), specially designed to provide reliability to vote casting, ballots recount, and verification of vote or poll results. These cryptographic processes and protocols jointly constitute a cryptographic voting scheme capable of meeting the specific reliability requirements of an electronic voting where voters remotely cast their votes. More specifically, these reliability requirements include voter authentication and privacy; accurate results, the impossibility of coercion and sale of votes, verifying the final results and, if necessary, the secrecy of intermediate results before completing the vote or poll. The proposed cryptographic voting scheme allows minimizing the confidence level to be placed on any of the electronic voting individual party and participants. Moreover, the cryptographic voting scheme significantly improves the implementation practical aspects with regard to prior works in secure electronic voting methods and systems.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention refers to electronic voting, and more specifically to cryptographic protocols and processes as well as computer programs for implementing them, employed to provide preferably remote electronic voting system with the suitable digital safety measures.  
         [0002]     In this specification will be understood as cryptographic process any ordered set of steps for transforming the information which includes cryptographic operations. The term cryptographic protocol will be used herein to refer to a cryptographic process carried out between several participants who exchange information through any data communication system. On the other hand, cryptographic scheme will be named a number of interlinked cryptographic processes and protocols having a common aim.  
         [0003]     According to said nomenclature, the object of this invention is to provide a secure electronic voting method including a cryptographic scheme to conduct with the utmost confidence any electoral processes and poll through communication networks such as Internet, wireless data communication networks or any other kind of computer network.  
         [0004]     The description of the invention contains material which can be protected by copyright. The proprietors of said copyright have no objection to the reproduction of the description of this document of application of a patent as it will be published or final version at the Patent Office by third parties, but they reserve any right on the copyright in other aspect arising from said document.  
       BACKGROUND OF THE INVENTION  
       [0005]     In a remote electronic voting or poll system, a plurality of authorized voters cast their vote or opinion from given vote casting devices to a virtual voting centre, through a communication network. A server or a plurality of them constitute that virtual polling station and they are responsible for accepting connections from voters and recording their votes or opinion issued for their further Tallying and tabulation. After they are tabulated, the results of the poll are finally published, probably through the communication network itself.  
         [0006]     It is clear that some safety features must be included in this kind of remote electronic voting systems. Voting in the traditional way (i.e. casting a physical ballot at a true polling station) is done with confidence because of the use of physical safety measures such as paper envelopes, guards, or physical ballot boxes. However, these kinds of measures for the physical protections are no longer useful in remote electronic voting systems. They must be replaced by suitable digital security measures, a task which involves complex problems, with a set of specific safety requirements. The voter&#39;s privacy, for example, is a central safety requirement at any electoral process and in many kinds of polls. Nobody (not even the electoral authorities nor the administrators of the voting serves) must be capable of correlating the voters with the votes cast. However, when casting their vote, the voters must be suitably identified through some authentication mechanism to prevent that non authorized voters vote or that authorized voters vote more than once. The accuracy of the results is also very important. Nobody must be capable of adding false ballots (may be in the name of the voters who abstained to vote), nor to erase or manipulate the valid votes cast. Another quality wished is the secrecy on any intermediate result while the poll is not complete, unless the characteristics of the process requires otherwise. The object of said secrecy is to prevent that the current state of the poll could affect the decision of some voters. Another very important safety requirement is to verify the final results. The voters must be capable of verifying that their respective votes have been correctly recorded. In the event of detecting any problem in the recount, the voters involved must be able to publicly evidence (without interfering with their privacy so doing) that their validated votes were not correctly treated. Lastly, the use of electronic voting systems must not expose the voters to coercion nor either facilitate sale of votes.  
         [0007]     This kind of specific safety requirements for an electoral process cannot be solved only with the use of generic digital security measures current in the market, such as firewalls, demilitarized areas, coding of the data at the transfer level, etc. A correct solution must necessarily include a special purpose cryptographic scheme specifically designed for the problems that a remote electronic voting imposes. A cryptographic voting scheme accurately determines the steps and actions involved in the remote vote casting as well by the device issuing the votes used by the voter as by the corresponding voting server. The scheme also determines the cryptographic operations which must occur during the process of vote tallying and also for verifying the final results. Of course, a cryptographic voting scheme must also be completed with generic safety measures to maximise the safety and to best protect the full voting system.  
         [0008]     The first cryptographic voting scheme was proposed by Chaum in 1981 (Chaum, D.  Untraceable electronic mail, return addresses and digital pseudonyms . Communications of the ACM, v.24, n.2, pp. 84-88, 1981). Many other proposals followed, giving a large group of voting schemes, known as schemes based on mixing techniques or mixing. The fundamental principle of these schemes based on mixing consisted in casting votes to a virtual polling station through some kind of anonymous channel constructed in the communication network. Usually, said voting schemes used the technique of the blind digital signature disclosed in U.S. Pat. No. 4,759,063, to validate the votes without impairing the voters privacy. Among the voting schemes based on mixing, the most representative examples were disclosed in [Fujioka, A., Okamoto, T. &amp; Ohta, K.  A practical secret voting scheme for large scale elections . Proc. of Auscrypt &#39;92, LNCS 718, pp. 244-251, 1992] and [Park, C., Itoh, K. &amp; Kurosawa, K.  efficient anonymous channel and all/nothing election scheme . Proc. of Eurocrypt &#39;93, LNCS 765, pp. 248-259, 1993]. The invention disclosed in U.S. Pat. No. 6,317,833 is also based on mixing although in contrast with above mentioned proposals, the mixing process is not carried out in the communication channel but in the vote receiving centre. The voters protect their votes with the public key owned by the voting authority. Said authority is very similar to the character of the Electoral Board disclosed in [Borrell, J. Rifá, J. An implementable secure voting scheme. Computers &amp; Security, 15, pp. 327-338, 1996]. Each voter requires a pair of asymmetric keys in order to sign the coded vote. In the description of the invention, it is only referred to the cryptosystem ElGamal [ElGamal, T.,  A public key cryptosystem and a signature scheme based on discrete logarithms , Proc. of Crypto &#39;84, LNCS 196, pp 10-18, 1985] as a means for said signature. It is then assumed that the pair of keys of each voter is to be brought in line with the cryptosystem ElGamal. Said requirement means a limitation in the voting scheme. In addition, the scheme does not offer any important benefit: the verifiability or the possibility that the voters verify the correct record of their votes when the process is complete.  
         [0009]     A second group of cryptographic voting schemes, based on homomorphic coding techniques, was started with the work by Benaloh and Yung on 1986 [Benaloh, J. C. &amp; Yung, M.  Distributing the power of a government to enhance the privacy of voters . Proc. of 5 th  Annual ACM Symposium on Principles of Distributed Computing, pp. 52-62, 1986]. These voting schemes avoided the use of anonymous channels by splitting individual votes into a number of pieces and sending each piece to a separate Tallying agent. Said agents counted the votes without the need of decoding them (for this they accounted on cryptographic mechanisms which secured the accurateness of the recount). Some representative examples of voting schemes based on homomorphic coding techniques have been recently proposed in [Sako, K. &amp; Kilian, J.  Secure voting using partially compatible homomorphisms . Proc. of Crypto &#39;94, LNCS 839, pp. 411-424, 1994], [Cramer, R., Franklin, M., Schoenmakers, B. &amp; Yung, M.  Multi - authority secret - ballot elections with linear work . Proc. of Eurocrypt &#39;96, LNCS 1070, pp. 72-83, 1996] and [Cramer, R., Gennaro, R. &amp; Schoenmakers, B.  A secure and optimally efficient multi - authority election scheme . Proc. of Eurocrypt &#39;97, LNCS 1233, pp. 103-118, 1997]. The inventions of U.S. Pat. No. 5,495,532 and WO 0120562 both disclose voting schemes based on homomorphic coding techniques. Said schemes require computing resources significantly greater than those required for the schemes based on mixing, making impossible they are implemented in client devices having a low computing capacity. Another limitation to said schemes is that, due to their intrinsic characteristics they result absolutely incapable of supporting ballots having arbitrary formats.  
         [0010]     Inventions U.S. Pat. No. 6,081,793 and U.S. Pat. No. 6,021,200 disclose rather simplistic cryptographic voting schemes based on the known “two-agency model”. In general, these two voting schemes provides a solution to the safety requirements of elections only in the event that both agencies do not join in an unfair coalition and, in addition, it is necessary to place full confidence in some of the parts of the system. In the U.S. Pat. No. 6,081,793 the model uses an authenticator during the ballot casting process to detach the voter authentication of the encrypted ballot to prevent any vote-voter correlation of the results. This implementation has the limitation that the result counter needs to put trust in the authenticator, because the counter cannot check by itself that the ballot contents belong to a valid voter. In addition invention voters doesn&#39;t have a voting receipt to check the voting results. The method checks during the voting process that the ballot has been received by the authenticator. But this check doesn&#39;t provide to the voter a voting receipt to verify if the votes were received by the counter when it starts the counting process or so far.  
         [0011]     In any cryptographic voting scheme, measures have to be taken designed to guarantee the verifiability of the final result of the vote cast by the voters and at same time measures for protecting against coercion to the voter by third parties as well as against the sale of votes by the voters themselves. The two sets of measures evidenced to be contradictory to each other. In fact, by facilitating the verifiability, the possibility of coercion and sale of votes is increased and if the possibility of coercion and the sale of votes is hindered the verifiability is also hindered. Two prior works simultaneously tried to overcome the two questions, although the proposals resulting from them are not implementable on regular communication networks. In U.S. Pat. No. 6,092,051 and EP1017025, voting cryptographic segments are disclosed which use an anonymous channel as the one disclosed in U.S. Pat. No. 5,682,430. Said channel must be physically protected against passive assailants who can steal information therefrom therefore the resulting voting systems cannot employ conventional data communication networks such as Internet.  
       SUMMARY OF THE INVENTION  
       [0012]     This invention discloses a remote electronic voting method and the protocols and cryptographic processes used, which compose a cryptographic voting scheme.  
         [0013]     The invention also refers to computer programs for implementing the said cryptographic processes and protocols.  
         [0014]     A first object of this invention is to meet a comprehensive list of safety requirements associated to electronic elections and polls: voters privacy, authentication of authorized voters, accuracy of the results from the poll, secrecy of any intermediate results until poll is complete (if required), hindering voters coercion as well as the sale of votes, and verifying the end results. This invention allows minimizing the confidence level that must be placed in any of the elements and participants of the electronic voting thus meeting the list of safety requirements even in the case of malicious or corrupted system administrators, electoral authorities or voters.  
         [0015]     This invention assures privacy to the voters by means of digital envelopes for the ballots, a secret shared cryptographic scheme among several electoral authorities and techniques of blind signatures or alternative protocols to validate the votes. The authentication of voters is preferably achieved by means of public key technology, [ITU-T,  Recommendation X. 509 (August 1997)  Information Technology—Open Systems Interconnection—the Directory: Authentication Framework,  199]) enabling, if required, a mechanism to avoid the installation of keys and digital certificates in the devices for issuing the voters own vote. The verifiability of the final results is based on digitally signed voting receipts matched with a highly reliable method for the voters downloading results. Accuracy of final results is secured by means of digital envelopes and proof of authenticity, with the support of digitally signed voting receipts. The secrecy of intermediate results, if required, is secured by means of digital envelopes. Preventing the coercion and the sale of the votes is based on a special design of the voting receipts and, if required, in the use of tamper-proof devices to prevent voters manipulate them.  
         [0016]     It is another object of the invention to overcome the practical limitations and drawbacks of prior works on secure electronic voting methods. With regard to this, the invention includes only low-cost cryptographic processes and methods which advantageously allow the effective implementation in thin clients. Furthermore, the invention stands an absolute flexibility of ballot formats, including open-ended questions and write-in candidates which/who can be chosen just in the moment of voting. Furthermore, the invention removes the need for any kind of special communication channel such as a secure channel physically protected or an anonymous channel. Only regular data communication channels of any usual data communication network are needed.  
         [0017]     It is yet another object of the invention to describe protocols alternative to the use of blind digital signatures to secure the privacy of the voters in cryptographic voting schemes based in mixing.  
         [0018]     It is yet another object of this invention to describe secure mechanisms for the protocols of opening and closing the virtual polling stations.  
         [0019]     Summarizing, this invention discloses a secure remote electronic voting method and the cryptographic voting protocol employed. The invention allows that electronic voting or polls through a regular communication network reach similar safety and confidence levels and in some aspects greater than that of traditional elections through physical means. Said object is reached by means of a secure electronic voting method comprising the steps detailed in claim one. 
     
    
     SHORT DESCRIPTION OF THE DRAWINGS  
       [0020]      FIGS. 1   a ,  1   b  and  1   c  shows the main elements on which the electronic voting method of this invention is implemented. A plurality of Vote Casting Platforms  102  (where the Voter Agents  116  are executed a Polling Station  104 , a Publication Site  106  and a Tally Site  108  are connected to each other through a Communication Network  114 . Said elements can be found fully distributed and separated from each other, as shown in  FIG. 1   a . Also several levels of grouping can occur.  FIG. 1   b  shows the configuration with the Polling Station  104  and the Publication Site  106  grouped.  FIG. 1   c  shows a configuration with the Polling Station  104 , the Publication Site  106  and the Tally Site  108 . In any case, the Tally Site  108  is operated by an Electoral Board  110  constituted by several members and it is physically connected to the Communication Network  114  through a very secure network connection  112 .  
         [0021]      FIG. 2  shows the cryptographic processes and protocols comprised in this invention and the order in which said protocols and processes take place.  
         [0022]      FIGS. 3   a  and  3   b  show two possible configurations of the very secure network connection  112  used to connect the Tally Site  108  to the Communication Network  114 .  
         [0023]      FIG. 4  shows the format of the opening order  402  that the Electoral Board  110  releases to the Polling Station  104 . From the receipt of said order, the Polling Station  104  proceeds to accept connections from the Voter Agents  116 .  
         [0024]      FIG. 5  shows the seven steps constituting the Vote Casting Protocol  206 .  
         [0025]      FIG. 6  shows the format of the closing order  602  that the Electoral Board  110  releases to the Polling Station  104 . From the receipt of said order, the Polling Station  104  is no longer accepting connections from the Voter Agents  116 .  
         [0026]      FIGS. 7   a ,  7   b  and  7   c  show three examples of spliting the data list for verifying that the Verification Protocol  214  is used.  FIG. 7   a  shows the spliting of said list into two subassemblies having mutually excluding inputs, so that the partial downloading of results from the Publication Site  106  carried out by the Voter Agent  116  only affects half of the inputs.  FIG. 7   b  shows the spliting of said list into four mutually excluding subassemblies.  FIG. 7   c  shows the existence of an option of excessively minority vote which is not taking into account to carry out the splitting and which is fully included in every and each resulting subassemblies. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     This invention refers to a secure electronic voting method and the cryptographic scheme used, specially applicable to remote electronic voting through a communication network. Said cryptographic scheme includes cryptographic protocols and processes which take place before, during and after the voting or poll itself. The scope of the invention does not cover the tasks of constructing the roll and the possible management of asymmetric keys for the voters.  
         [0028]     Creating a roll of voters is in fact a prior step essential for any poll which seeks to meet the minimum safety requirements. Said roll will be useful during the Vote Casting Protocol  206  and/or the Tallying Process  210 , essentially for preventing that non allowed users can vote and that authorized voters record more votes, as well as for preventing the introduction of false ballots or the names of voters who abstained from voting.  
         [0029]     Preferably, the voting method will be supported on a public key infrastructure or PKI, a concept disclosed in specification X.509 [ITU-T,  Recommendation X. 509 (August 1997)  Information Technology—Open Systems Interconnection—the Directory: Authentication Framework,  1997], with the aim of managing asymmetric keys for the voting partners. The voter&#39;s asymmetric keys are used during the poll for authentication purposes and not for repudiating them. Each voter private key must be known or accessible only by the voter himself. This involves either that the voters generate by their own means their pairs of asymmetric keys or that the pairs of asymmetric keys are generated by a suitable Certifying Authority and are safely delivered to the voters. The private key of each voter can remain at the Polling Station  104  itself, protected by means of a system of symmetric encryption with a password meeting given safety parameters, namely those referring to its entropy. In this case, each voter only ought to know and keep his password. Some representative examples of symmetric encryption which could be used are disclosed in (Applied Cryptography, Protocols, Algorithms and Source Code in C (second edition), Bruce Sneier, editor John Wiley &amp; Sons, Inc., 1996) and (The Design of RijndaeL: AES—The Advanced Encryption Standard (Information Security and Cryptography), Joan Daemen and Vincent Rijmen, Springer Verlag, 2002). The protection of the voter&#39;s private keys by means of symmetric encryption prevents that they can be used without knowing the password. This way, the coded private key can be located at the Polling Station  104  with which the voter will interact and be sent to him through the Communication Network  114 , at the beginning of the Vote Casting Protocol  206 . This technique of storage of the voter&#39;s private keys allows preventing the drawbacks arising from installing private keys and digital certificates in the Vote Casting Platforms  102 . Alternatively, each voter can keep his private key on a secure physical support (diskette, CD, smart card or another kind of reliable personal key) or coded in his personal computer or in another computing platform he owns forming part of or interconnected with the Votes Casting Platform  102 .  
         [0030]      FIG. 1   a  shows the architecture of the voting system in the event of maximum scattering of the elements composing it. In said figure a plurality of voters can be noted, interacting through their respective Voter Agents  116  in the Vote Casting Platform  102 , a Polling Station  104 , a results Publication Site  106  and a Tally Site  108 , all of them connected through a Communication Network  114 , such as Internet. Due to the sensitivity of the Tally Site  108 , it is connected to the Communication Network  114  through a high secure network connection  112 . An Electoral Board  110  formed by several members controls the reliability of the Tally Site  108 , as well as the administrative and functional tasks linked to the electoral process. Opening and closing the Polling Station  104  would be two examples of said tasks.  FIGS. 1   b  and  1   c  show other possible configurations of the voting system, in which the grouping level of the different elements composing the system is higher.  FIGS. 3   a  and  3   b  show two possible configurations of the secure network connection  112  used to connect the Tally Site  108  to the Communication Network  114 . In  FIG. 3   a , the connection to the Tally Site  108  to the Communication Network  114  is protected with network computer safety measures, such as above mentioned firewalls, demilitarized areas, etc. This kind of measures significantly hinder the fraudulent remote access which have the aim of attacking the processes and tasks which are being carried out at the Tally Site  108 .  FIG. 3   b  shows an even more reliable configuration, with the Tally Site  108  physically disconnected from the Communication Network  114 . An additional device connected to the Communication Network  114  is in charge of receiving data that the Tally Site  108  requires to carry out its tasks (basically the digital ballot box). Said data are transferred from the additional device to the Tally Site  108  by means of a physical support such as a magnetic disk, a Zip, CD or DVD. The data which have to flow in the opposite sense also use same kind of support.  
         [0031]     The cryptographic processes and protocols included in this invention require from the voter complex mathematical calculations during his interactions with the Polling Station  104  and the Publication Site  106 . The complexity of said calculations causes that they are carried out on behalf of the voters by a Voter Agent  116  formed of a set of programs or software. It can be a conventional application, a plug-in for an Internet navigator, an application executed by the navigator himself or a module which is executed in a terminal of a mobile telephony or another computing mobile personal device. The Voter Agent  116  must be duly audited and certified to guarantee the voter that no operation is carried out which is not disclosed in the voting cryptographic processes and protocols. The Voter Agent  116  is executed at the Vote Casting Platforms  102 . They can take the form of different hardware or a match of them such as a personal computer, a voting terminal, a personal digital assistant (PDA), a mobile telephony terminal or an smart card.  
         [0032]     The Polling Station  104  has the mission of accepting the connections of the Voter Agents  116 , collaborate with them in the voting cryptographic processes and protocols and keep the casted votes for counting them thereafter. Optionally, different Polling Station  104  can be created as independent virtual polling stations. This kind of configuration could be given as an example in the case of very large electoral roll requiring distributing the voters in several independent polling stations. In the disclosed below, the existence of a single Polling Station  104  will be assumed (a single virtual polling station) although the method disclosed in this invention could easily be extrapolated to configurations having a plurality of them.  
         [0033]     When ending the vote or poll, the Tally Site  108  receives from the Polling Station  104 , the digital ballot box containing all the votes casted. Operated by an Electoral Board  110  composed of a plurality of members, the Tally Site  108  enters the votes and draw the list of results. The Tally Site  108  is a component specially sensitive of the voting system. This is why as well hardware as software components constituting it must be kept as simple as possible. Namely, the software must be duly audited and certified to guarantee that no operation is carried out which is not disclosed in the cryptographic voting processes and protocols. As it can be noted in  FIG. 3   a  and  3   b , the Tally Site  108  is connected to the Communication Network  114  by means of a high secure network connection  112 . The object of said high liability connection  112  is to prevent the non allowed remote access to the Tally Site  108 . In a first option, the high reliability connection  112  can be implemented by means of firewalls systems which filter the remote access to the Tally Site  108 . In addition, a permanent watching group can monitor with the assistance of intruders detection systems every incidence and anomaly. However, the preferred option to provide the Tally Site  108  with maximum protection with respect to remote access consists in completely insulating it from the Communication Network  114 , as shown in  FIG. 3   b . However, to allow that several data are sent (among them the digital ballot box) to the Tally Site  108  through the Communication Network  114 , an additional device is available, physically close to the Tally Site  108  and with connection to the Communication Network  114  (connection protected with the already mentioned safety measures). Said additional device is in charge of receiving the digital ballot box or other data which are thereafter transferred to the Tally Site  108  by means of a suitable physical support or a suitable point to point connection. The information flow in the opposite sense (data to be sent from the Tally Site  108  through the Communication Network  114 ) also follows a similar operative. It must be pointed out that for a best reliability of the system as a whole firewalls systems for filtering packages at the Polling Station  104  and the Publication Site  106  can also be used.  
         [0034]     The Publication Site  106  receives from the Tally Site  108  the voting results once they have been tabulated and it take charge of publishing them through the Communication Network  114 . In addition, it allows that the voters can verify the correct recording of their vote or, at least, the correct reception of them by the Electoral Board  110 . The Publication Site  106  can consist in one or a plurality of Publication Sites  106  connected to the Communication Network  114 .  
         [0035]     In the following explanations, it will be assumed that the Polling Station  104 , the Tally Site  108  and the Publication Site  106  are located split and well differentiated. However, the possibility of grouping them has already been discussed.  
         [0036]     The voting system disclosed uses a series of cryptographic processes and protocols which are represented in  FIG. 2 , jointly with the order in which they are being performed along the elections or the poll. First, the Initialization Protocol  202  begins executed by the members of the Electoral Board  110 . In this protocol, the tasks necessary to prepare voting are carried out. Then, it is proceeded to the Opening Protocol  204  of the Polling Station  104 . The voters can, from that moment on, execute the Vote Casting Protocol  206 , by means of their respective Voter Agents  116  and jointly with the Polling Station  104 , in order to release their votes. When the voting period is complete, Closing Protocol  208  is carried out. The following step consists in the Tallying Process  210 , in which the list of results is assessed. When the Tallying and tabulation of results is complete, they are transferred from the Tally Site  108  to the Publication Site  106  by means of the Results Transfer Protocol  212 . The voters finally verify the correct treatment of their votes through the Verification Protocol  214 .  
         [0037]     To allow a more detailed description of the listed cryptograph processes and protocols, following marking will be used: 
        V: A specific voter acting from his Vote Casting Platform  102 , through his Voter Agent  116 .     PS: Polling Station  104 .     EB: Electoral Board  110      elect.: Chain of data univocally identifying the elections or polls being held. It can be, for example, a sequential identifier, or the date and a text describing the elections.     ident.: A unique identifier for each vote cast, It can be, for example a chain of random data having a length sufficient with relation to the electoral roll of the voters which are participating in it.     vote: Chain of data identifying in an unique form a given voting option. It is a chain of data which supports arbitrary length and contents. Said characteristic allows a maximum flexibility relative to the format of the ballot papers. For example, open questions can be implemented to which the voter can state an opinion on the subject matter of the question or directly write the identity of the voted candidate.     M 1 |M 2 : Concatenation of two messages M 1  and M 2 .     H{M}: Summary of M message obtained through a cryptographic summary function.     K: Key of a given symmetric encrypted system. Subscripts will be used to distinguish among the different keys of this kind used in the protocols.     E K (M): Symmetric encryption of M message with K key.     P entity  and S entity : Pair of asymmetric keys (public key and private key, respectively) owned by the entity.     P* entity  and S* entity : Pair of asymmetric keys (public key and private key, respectively) owned by the entity, corresponding to a probabilistic encryption according to the proposal by Blum and Goldwasser [Blum, M. and Goldwasser, S.  An Efficient Probabilistic Public Key Encryption Scheme which Hides All Partial Information, Advances in Cryptology—CRYPTO &#39; 84 Proceedings, pp. 289-299, Springer Verlag, 1985].     P entity (M):Asymmetric M message encryption with the public key P entity .     P* entity (M): Probabilistic encryption M message with the public key Pentity entity .     E K (M)\P entity (K): Digital envelope of M message designed to entity. Said digital envelope comprises the M encryption with a K symmetric session key, concatenate to the encryption of said K session key with the addressee P entity  public key. The mechanism of digital envelope is disclosed in (PKCS#7,  Cryptographic Message Syntax Standard . A RSA Laboratories Technical Note, Version 1.5, 1 Nov. 1993).     S entity [H{M}]): Digital signature of M message created with the private key of entity. The signature carries a given cryptographic transformation determined by the private key entity on the summary of the M message obtained by means of a cryptographic summary function. [Rivest, R. L., Shamir, A. and Adleman, L. M.  A Method for Obtaining Digital Signatures and Public Key Cryptosystems.  Comm. of the ACM, V. 21, n. 2, pp. 120-126, 1978] can be taken as a reference of said cryptographic transformation. The format resulting from a digital signature is disclosed in [PKCS#7 , Cryptographic Message Syntax Standard , A RSA, Laboratories Technical Note, Version 1.5, 1 Nov. 1993].     M BF : M message covered by a BF blind factor, according to the proposal of U.S. Pat. No. 4,759,063.        
 
         [0055]     The first step of the voting scheme consists in the Initialization Protocol  202  which comprises the allocation of initial values to all the parameters necessary for the voting in progress and the generation of pairs of asymmetric keys necessary for the Electoral Board  110  as a whole and for the Polling Station  104 . Said initialization processes can be carried out during a shorter or longer period of time. In a secure device, two pairs of asymmetric keys are generated for the Electoral Board  110  as a whole. They are called P EB   (ADM)  and S EB   (ADM)  the public and private keys, respectively of the first of said pairs and P EB   (ENV)  and S EB   (ENV)  the keys of the second pair. Also, a pair of keys is generated for the Polling Station  104 , called P PS  and S PS . The public keys must be certified. Said certification can be achieved by a Certifying Authority having a wide representation among Internet conventional navigators. Alternatively, the public keys could also be certified by a Certifying Authority of the Public Key Infrastructure (PKI) associated to the voting system or another Certifying Authority of some PKI the root public key of which is in some way acknowledged by the voters.  
         [0056]     Keys P PS  and S PS  are going to be used by the Polling Station  104  for performing the digital signatures during the Vote Casting Protocol  206  and the Closing Protocol  208 . Namely, the private key S PS  will be used very often during the Vote Casting Protocol  206  and therefore, it will preferably installed in a hardware device such as, for example, a cryptographic accelerating card which will be associated to the Polling Stations  104 . If said hardware is not available, the private key will be protected by means of conventional protecting mechanisms offered by the file system of the Polling Stations  104 .  
         [0057]     Keys P EB   (ADM)  and S EB   (ADM)  are going to be used by the Electoral Board  110  for the Opening  204  and Closing Protocols  208  of the Polling Station  104 , optionally for the Vote Casting Protocol  206  as well as for all those operations which require that a digital signature is performed by the Electoral Board  110 . The private key S EB   (ADM)  will stay at the Tally Site  108  which will be connected to the Communication Network  114  by means of the High Reliability connection  112 .  
         [0058]     Keys P EB(   ENV)  and S EB   (ENV)  will be used only for the voting in progress. With the public key P EB   (ENV)  of the Electoral Board  110 , the voters will protect their votes during the Vote Casting Protocol  206 . The private key S EB   (ENV)  of the Electoral Board  110  is a piece of information having a great sensitivity because with it the votes can become unprotected to allow their recount. The knowledge of said key by a single voting authority would weaken the electronic voting for concentrating too much confidence at a single point. The person who would know said private key could sustain pressures or simply be dishonest and try to manipulate the poll. To prevent that an excess of confidence is concentrated, a cryptographic mechanism is used for sharing the confidence between the different members constituting the Electoral Board  110 . Said mechanism remove the possibility that a single member of the Electoral Board  110  or a given minority of them are acquainted with the private key S EB   (ENV) . In general, it is assumed that no majority coalitions having dishonest aims will occur between the members of the Electoral Board  110 , because each of its members may have conflicting interests as they represent different voting options, they pertain to third parties having interests in that the elections are correctly performed or simply be an independent auditor. However, even in the case that a majority coalition having dishonest aim occurs, this invention allows, if applicable, that the voters themselves are capable of detecting the manipulation of the digital ballot box (suppressing or modifying the votes cast as well as adding false ballots).  
         [0059]     Different cryptographic protocols for sharing secrets can be used in this invention to split the private key S EB   (ENV)  of the Electoral Board  110  between its members, meeting the mentioned characteristics of confidence sharing. In the cited work by Schneier a description of them can be found.  
         [0060]     When the private key S EB   (ENV)  of the Electoral Board  110  has been split into individual portions trough a cryptographic protocol for sharing secrets, each of the portions is stored, conveniently encrypted, at the Tally Site  108  used for performing said splitting. To encrypt the portions, a suitable encryption system (typically symmetric) is used as well as a secure random key generated by the Tally Site  108  itself and different for each of the portions. One of the portions is allocated to each member of the Electoral Board  110  so that he receives the random key used for encrypting said portion (generated with or without his intervention). The key received is stored in a secure memory personal device. The original private key, as well as the non coded portions are finally destroyed erasing any trace of them from the processing, storing and memory devices and systems of the Tally Site  108 . Alternatively, the members of the Electoral Board  110  could have directly received their respective portions so said portions would not be stored encrypted at the Tally Site  108  but in the secure memory personal devices.  
         [0061]     The following step after the Initialization Protocol  202  is the Opening Protocol  204  of the Polling Station  104 . The Electoral Board  110  is the one which determines the moment for opening the Polling Station  104 . For that, the opening order  402  (See  FIG. 4 ) is released to the Polling Station  104 . Said opening order  402  must contain following fields. First, the identifier of the elections in progress elect. Second, the identifier of the Polling Station  104  (typically the Internet IP address of the Polling Station  104  to which the voters have access, or its domain name DNS). Third, the state of the Polling Station  104  (“OPEN”). Fourth, the probability rate PR ACK . Said rate regulates the process of issuing voting confirmations from the Voter Agents  116  to the Tally Site  108 . This process is carried out at the end of the Vote Casting Protocol  206  as it will be disclosed later on. Optionally, the opening order  402  can also contain the opening date and time. Lastly, said order must incorporate an authenticity evidence that it has been generated by the Electoral Board  110 . Said evidence consists in the digital signature of the opening order  402 , using S EB   (ADM) . The generic form of the opening order  402  would be then: 
        elect|PS|“Open”|PR ACK |S EB   (ADM) [H{elect|PS|“Open”|PR ACK }]       
 
         [0063]     When the opening order  402  has been received by the Polling Station  104 , the voters can start casting their votes. When the Vote Casting Protocol  206  is complete, the vote cast by each voter will be stored at the Polling Station  104 ,duly protected with cryptographic means which will be disclosed below. Also, the Vote Casting Platform  102  will store a voting receipt with the aim of verifying thereafter the vote.  
         [0064]     The Vote Casting Protocol  206  comprises seven steps, as detailed in  FIG. 5 . If the Vote Casting Platform  102  consists in a reliable personal device and has available a secure tamper-proof module, all the cryptographic operations that the Voter Agent  116  performs in the Vote Casting Protocol  206  (and also in the Verification Protocol  214 ) are carried out within it for a best security.  
         [0065]     The first step of the Vote Casting Protocol  206  is optional and contemplates the voter authentication downstream the Polling Station  104 . In the second step, the voter selects his voting option from the different options provided. In the third step, the Voter Agent  116  obtains a ballot identifier, unique for each ballot. Said identifier can be known (and even selected) by the voter in the configuration with non linked identifiers. The ballot identifier cannot be known by the voter in the configuration with linked identifiers. In the fourth step, the Voter Agent  116  obtains a voting receipt which validates the ballot and which will allow the verifiability of the results of the vote. The voting method allows two different levels of verifiability, depending on its configuration (linked or non linked identifiers). In the fifth step, the Voter Agent  116  protects the ballot and remaining related information by constructing a digital envelope which can only be open by a set of qualified members of the Electoral Board  110 . In the sixth step, after the Polling Station  104  received the digital envelope from the preceding step, it delivers to the Voter Agent  116  a voting voucher evidencing that the vote was actually delivered. In the seventh step, the Voter Agent  116  decides to totally or partly send said voucher to the Tally Site  108 , according to the probability PR ACK  specified in the opening order  402 , as a confirmation of correct finalization of the Vote Casting Protocol  206 . The Vote Casting Protocol  206  always requires the voter authentication. Said authentication can be carried out in the first step of said protocol or by using a proof of authenticity in the fifth step.  
         [0066]     In the first step of the Vote Casting Protocol  206 , the voter authentication allows the Polling Station  104  to obtain with a reasonable reliability a proof of the true identity of each voter which is remotely contacting. Ideally, the voters should have available a pair of asymmetric keys allowing them to perform strong authentication protocols, such as for example those disclosed in standard X.509 above mentioned or by means of the industry standards WTLS [WAP Forum,  Wireless Transport Layer Security Specification,  Version 06 April 2001, April 2001], TLS [Dierks, T. and Allen, C.  The TLS protocol , version 1.0 Request for Comments 2246, January 1999] or his predecessor SSL [Freier, A. O., Karlton, P. and Kocher, P. C.  The SSL protocol , version 3.0. Internet-Draft, November 1996], with bilateral authentication. The use of biometric identification systems could be matched with the strong authentication mechanism to add more reliability to the voter&#39;s remote authentication. This first step of the Vote Casting Protocol  206  is optional and in the event that it is not performed, the vote authenticity evidence to be achieved in the fifth step of the Vote Casting Protocol  206  will offer due guaranties on each voter identity.  
         [0067]     On its hand, the Polling Station  104  must also be authenticated to the voters using for it the industry standard authentication protocols, mentioned in above paragraph. Said protocols, in addition, guarantees the secure transport of data (the secrecy, reliability and authenticity of the data exchanged between the Vote Casting Platform  102  and the Polling Station  104 ).  
         [0068]     During the second step of the Vote Casting Protocol  206 , the voter, interacting with the Voter Agent  116  at the Vote Casting Platform  102  selects the sense of his vote. This invention allows the maximum flexibility with respect to the format of the ballots. The voter can state selecting a given option, a set of them or for some and against some others. Open questions can also be posed to collect the opinion of the voter in a free format, or allow also electing the selected candidates at the very moment of casting the vote. The voter choice is coded in the chain of vote data which admits any format either standardized or not.  
         [0069]     The third step of the Vote Casting Protocol  206  consists in obtaining a unique identifier for the ballot. Said value (ident) must be sufficiently high to allow to univocally identifying each of the vote cast. The obtaining of ident can be achieved by two different ways, depending on how the voting method is configured. A first configuration allows the voter to choose the ident value within a predetermined range or to delegate to his Voter Agent  116  the election of said value. A second configuration requires that the voter is not capable of determining the unique identifier of his ballot beforehand, nor to know it after it has been obtained. In this case, the ident value is obtained by the Voter Agent  116  at random with an even distribution.  
         [0070]     In the first mentioned configuration, the ballot identifiers appear published in the final results, without any link with the respective voting options (which will be published grouped as totals). This first configuration will allow the voters during the Verification Protocol  214 , to make sure of the correct receipt of their ballots by the Electoral Board  110 . Said configuration will be designated as “configuration with non linked ballot identifiers”. In the second configuration, “with linked ballot identifiers”, the final results incorporate a list of all the vote cast, individually and accompanied with their respective ballot identifiers to facilitate their location and identification. This second configuration will allow the voters to make sure not only the correct receipt of their ballots, but also of their correct recording by the Electoral Board  110 . In both cases, this invention secures the impossibility of coercion or sale of the votes based on the use and knowledge of the ballot identifiers.  
         [0071]     In the configuration with non linked ballot identifiers, the voter has no means available to evidence the link between his ballot identifier and the voting option chosed. For this reason, the coercion or sale of votes based on the search from the results published of the ballot identifier has no effect.  
         [0072]     However, in the configuration with linked ballot identifiers, the coercion or sale of votes could be based on the capacity of the voter to predetermine, or only be acquainted with the identifier of his ballot. This would mean a significant weakness because the coercion and the sale of votes would be no longer a confined problem but would become a significant threat to the reliability of the elections as a whole. In fact, the difference has to be set between the coercion or the individual sale of votes, because the fact to remotely voting from uncontrolled environments and the mass coercion or sale of votes based on the automated processing of the ballot identifiers or the voting receipts. By simply the voters being acquainted with the ballot identifiers before their publication in the final results this would mean an unequivocal evidence of the voting option chosen by each voter. This way, it would not be required to observe the voter while he is casting his vote but it would be enough to verify which actually the vote cast was when tallying its ballot identifier with the list of votes published.  
         [0073]     Therefore, in the configuration with linked ballot identifiers, the Voter Agent  116  must prevent the access of the voter to his ballot identifier, as well during its generation as when it is stored thereafter, Ideally, said control of access is carried out incorporating to the Vote Casting Platforms  102  some kind of hardware tamper-proof device such as a smart card. The ident value is generated within said device, stored in a protected memory area which prevents the access even to the voter himself. A first version of said concealing technique for sensitive data for the voters was introduced in [Riera, A. Borrell, J. and Rifá, J.  An incoercible verifiable electronic voting protocol.  Proc. of the IFIP SEC &#39;98 Conference, pp. 206-215, Austrian Computer Society, 1998]. Alternatively, said data concealment can be achieved by means of software protection. However, this kind of protection is too weak in front of given attacks. That is why the preferred implementation is based on the use of a hardware tamper-proof device. Even though, in the event of using software protection for concealing the ballot identifier, this invention includes a cryptographic protocol between the Voter Agent  116  and the Polling Station  104  to obtain the ballot identifier. The protocol proposed allows the joint generation of the ballot identifier so that the voter can in no way predetermine beforehand the identifier, while it prevents the Polling Station  104  to obtain any knowledge about the identifier after they have been jointly generated. Before starting the protocol, the Voter Agent  116  and the Polling Station  104  must agree the use of a given symmetric encryption system. The size of the encryption device block must coincide or be higher than the size of the ballots identifier to be generated (in the event it is lower, the protocol must be repeated as many times as required). The Voter Agent  116  generates at random a K i  key for the encryption system selected and, by means of a cryptographic summary function, it calculates the summary chain of said key, H{K i }. The Voter Agent  116  sends said summary to the Polling Station  104 . At the Polling Station  104 , a random value R is generated having a number of bits equal to the size of the agreed encryption device block. The Polling Station  104  can digitally sign the summary of the symmetric key it received jointly with the R value it generated. Said digital signature will be useful for verifying, at the Tally Site  108  the consistency of the ballot identifier which is obtained, preventing the falsification of said identifiers by the dishonest voters. In the following step of the protocol, the Polling Station  104  sends then to the Voter Agent  116  following data: 
        H{K i }|R|S PS [H{H{K i }|R}]       
 
         [0075]     To obtain the ident value, the Voter Agent  116  encrypt with the K i  key the R value send by the Polling Station  104 , obtaining ident=E Ki  [R].  
         [0076]     The fourth step of the Vote Casting Protocol  206  consists in obtaining a voting receipt. In the configuration with non linked ballot identifiers, said voting receipt grants validity to the ballot identifier selected at the third step, and allows that the voter publicly files a claim in the event that said ballot identifier is not published in the final results. In the configuration with linked ballot identifiers, said voting receipt grants validity to the voting option selected at the second step, linking it to the ballot identifier obtained at the third step. This allows that the voter publicly files a claim in the event that said ballot identifier to which it is linked is not published in the final results. In both cases, the claim can be publicly filed proving the recording problem and without disclosing which was the voting option.  
         [0077]     In the configuration of non linked ballot identifiers, the voting receipt is obtained by means of the digital signature of the Polling Station  104  from the concatenating of the ballot identifier and the elect identifier: 
        ident|elect|S PS [H{ident|elect}]       
 
         [0079]     In the configuration with linked ballots identifiers, the voting receipt must contain information somewhat closely linked to the voting option selected by the voter, ballot. This way, further manipulation of the voting option is prevented, keeping valid same voting receipt. The voting receipt however cannot contain non coded the ballot value because this would prevent the voter from publicly filing a claim (the voter, when he shows his validated voting receipt, would disclose which was actually his voting option). The technique of the voting tags introduced in [Sako, K.  Electronic voting scheme allowing open objection to the tally , IEICE Tras. Fund. Of Electronics, Communication in Computer Science, E77-A, pp. 24-30, 1994] and improved in its practical implementation aspects in [Riera, A. Rifá, J. and Borrell, J.  Efficient construction of vote-tags to allow open objection to the tally in electronic elections . Information Processing Letters, Vol. 75, n.4, pp. 211-215, Elsevier Science, October 2000] assists to achieve the objectives to be met by the voting receipts. In this invention, a voting receipt for the configuration with linked identifiers is constructed applying a cryptographic summary function on the voter voting option, vote, and the ballot identifier ident, and concatenating each summary chain with the identifier of the elections in progress, elect, finally obtaining H{ident|vote}| elect. To provide validation to said voting receipt, the Polling Station  104  will use its private key S PS . However, the use of the digital signature conventional mechanism would allow that the administrators of the Polling Station  104  (or who has analogous privileges) to correlate the votes published in the results with the voting receipts previously validated during the Vote Casting Protocols  206 . To prevent said drawback, which would directly impair the voter&#39;s privacy, this invention uses variations of the digital signature mechanism. Said variations allow providing validity to the voting receipt and at same time it prevents at the Polling Station  104  to correlate the validated receipts and the votes which will be published later in the results.  
         [0080]     The first variation of the digital signature mechanism which can be used in this invention consists in the so-called blind digital signature, disclosed in U.S. Pat. No. 4,759,063. This way, the Voter Agent  116  prepares the data to be signed with a blind factor BF generated at random (H{H{ident |vote}|elect}) BF  and sends said data to the Polling Station  104 . The Polling Station  104  digitally signs the information received with its private key, obtaining S PS [(H{H{ident|vote}|elect}) BF ] and sends said signature to the Voter Agent  116 . Due to the random nature of the blind factor, the Polling Station  104  obtains no useful information about the voting receipt. The Voter Agent  116 , however, is capable of erasing the blind factor from the signature he receives, recovering the digital signature on the original voting receipt, which constitutes the validated voting receipt: 
        H{ident|vote}|elect|S PS [H{H{ident|vote}|elect}]       
 
         [0082]     A second variation for validating the voting receipts consists in encrypting them by means of a probabilistic cryptosystem, before transferring them to the Polling Station  104 . The probabilistic encryption has the property that a same message encrypted several times with a same public key gives rise to different encrypted texts so that the Polling Station  104  cannot relate the encrypted receipts it validated to the votes which are published in the results. The concept of probabilistic encryption was disclosed in [Goldwasser, S. and Micali, S.  Probabilistic encryption , Journal of Computer and System Sciences, Vol. 28, n° 2, April 1984], and later on, a possible implementation was disclosed in [Blum, M. and Goldwasser, S.,  An Efficient Probabilistic Public Key Encryption Scheme which Hides All Partial Information,  Advances in Cryptology—CRYPTO &#39;84 Proceedings, pp. 289-299, Springer Verlag, 1985]. The steps for validating the voting receipts using probabilistic encryption are as follows. First, the Voter Agent  116  generates a pair of session keys for the probabilistic cryptosystem, P* SESSION  and S* SESSION  (public key and private key, respectively) and probabilistically encrypts the receipt with the public key: 
        P* SESSION (H{ident|vote}|elect)        
 
         [0084]     Then the Voter Agent  116  transmits to the Polling Station  104  the encrypted receipt jointly with the recently generated public key, P* SESSION . The Polling Station  104  digitally signs this information with its private key, transmitting said signature to the Voter Agent  116 . The Voter Agent  116  stores the signature received jointly with the key S* SESSION , forming all of it the validated voting receipt: 
        P* SESSION (H{ident|vote}|elect)|P* SESSION ||S PS [H{P* SESSION (H{ident|vote}|elect)|P* SESSION }]||S* SESSION          
 
         [0086]     A third variation for validating the voting receipts consists in using symmetric encryption for concealing the receipt downstream the Polling Station  104 . To obtain the validation of his voting receipt, the Voter Agent  116  generates at random a K r  session symmetric key. The Voter Agent  116  encrypts the voting receipt with the key generated and sends to the Polling Station  104  said encryption jointly with the summary of K r  key, obtained by means of a cryptographic summary function: 
        E Kr [H{ident|vote}|elect]|H{K r }       
 
         [0088]     The Polling Station  104  digitally signs the message receipt with its private key, sending said signature to the Voter Agent  116 . The Voter Agent  116  stores the signature received with the K r  key, all of it forming the validated voting receipt: 
        E Kr [H{ident|vote}|elect]|H{K r }||S PS [H{E Kr [H{ident|vote}|elect]|H{K r }}]|K r  
 
 Once the Voter Agent  116  has available the voting receipt validated by the Polling Station  104  (according to any of the disclosed alternatives) he can proceed with the fifth step of the Vote Casting Protocol  206 . This step comprises the construction of a digital envelope to protect the ballot (and therefore the voter privacy). Only the Electoral Board  110  as a whole will be able to open said digital envelope, following this way an analogy with most of the conventional electoral systems. In the digital envelope, the Voter Agent  116  includes the vote chain, the ballot identifier ident and the validated voting receipt, as well as the information necessary to verify the correct obtaining of the ident in the event that said value has been generated jointly by the Voter Agent  116  and the Polling Station  104 . In addition, the digital envelope will contain all the information relevant for correctly administering the elections such as the kind of ballot, the date when the elections have to be held or the identifier of the language used. To construct said digital envelope, the Voter Agent  116  generates a session key K e  it encrypts it with the public key P EB   (ENV)  of the Electoral Board  110 . The digital envelope generic form would be then: 
    envelope=E Ke [vote|ident|voting receipt|relevant information]|P EB   (ENV) [K e ]       
 
         [0092]     After encrypting the contents of the envelope, the Voter Agent  116  attaches a proof of authenticity to said digital envelope. In this invention, four different options are contemplated to guarantee the authenticity of said digital envelope. In a first option, the Voter Agent  116  concatenates the digital envelope with the identity of the voter V, with the identifier of the elections elect, with the opening order  402  and optionally, with a mark of time (time) corresponding to the moment when the vote was issued and which is agreed upon between the Voter Agent  116  and the Polling Station  104 . The set of all those concatenated data is digitally signed with the private key S v  of the voter and is transmitted to the Polling Station  104 : 
        envelope|V|elect|Order 402|time|S V [H{envelope|V|elect|Order 402|time}]       
 
         [0094]     In a second option, the Voter Agent  116  concatenates the same previous data except the digital envelope, and digitally signs them with the private key S v  of the voter. 
        V|elect|Order 402time|S v [H{V|elect|Order 402|time}]       
 
         [0096]     Then the Voter Agent  116  prepare a second digital envelope envelope′ which he sends to the Polling Station  106 . To construct envelope he uses the public key P EB   (ADM)  of the Electoral Board  110 . This second digital envelope contains the first digital envelope and the digital signature on the specified values: 
        envelope′=E Ke′ [envelope|V|elect|Order 402|time|S V [H{V|elect|Order 402|time}]]|P EB   (ADM) [Ke′]       
 
         [0098]     The third option is indicated for elections in which the voters have no available a pair of asymmetric keys. In this kind of elections, each voter must have available a private voter identifier (PVI) different and secret which allows him to be authenticated. The Voter Agent  116  concatenates said piece of information, PVI, with the identity of the voter V, and with the identifier of the elections, elect, with the Opening Order  402  and optionally with a mark of time (time) and it constructs a second digital envelope, envelope′, with the public key P EB   (ADM)  of the Electoral Board  110 : 
        envelope′=E Ke′ [envelope|PVI|V|elect|Order 402|time|]|P EB   (ADM) [Ke′]       
 
         [0100]     In a fourth option, the Voter Agent  116 , based on an asymmetric cryptosystem selected, creates a pair of keys (P V   (AUT) , S v   (AUT)  where the public component or one of the components of the public key, is the summary of the digital envelope, envelope obtained by means of a cryptographic summary function. For example, using the public key cryptosystem RSA [Rivest, R. L. Shamir, A. Y. Adleman, L. M.  A method for obtaining digital signatures and public - key cryptosystems.  Communications of the ACM, v. 21, n. 2, pp. 120-126, February 1978], the public component, which usually receives the name of e, would correspond to the summary of the digital envelope, and the private component d, would be calculated, conventionally, in order it meets the properties of said cryptosystem. With the private component of the pair of keys created, the Voter Agent  116  obtains following signature: 
        V|elect|Order 402time|S V   (AUT) [H{V|elect|Order 402|time}]       
 
         [0102]     The Voter Agent  116  concatenates and sends to the Polling Station  104  the following information: 
        envelope|V|elect|Order 402|time|S V   (AUT) [H{V|elect|Order 402|time}]       
 
         [0104]     In the second and third options, in contrast with the first and fourth, it is not possible to deduct the identities of the voters who voted from the ballot box at the Polling Station  104 . Anyway, the proof of authenticity included with the digital envelope which contains the ballot prevents to add ballots in the name of voters who abstained to vote as well at the Polling Station  104  as at the Tally Site  108 .  
         [0105]     The sixth step of the Vote Casting Protocol  206  consists in obtaining a voting voucher. Said voucher, issued and validated by the Polling Station  104  by means of a digital signature, has the aim of thereafter allowing the voter to evidence that he finally deposited at the Polling Station  104  a digital envelope during the voting in progress. The voting voucher can then consist in the signature of the Polling Station  104  on the data delivered by the voter at the preceding step. For the first and fourth options disclosed above: 
        envelope|V|elect|Order 402|time|S PS [H{envelope|V|elect|Order 402|time}]       
 
         [0107]     For the second and the third options: 
        envelope′|S PS [H{envelope′}]       
 
         [0109]     The Polling Station  104  deposits in the digital ballot box the set of data delivered by the voter at the fifth step of the protocol, jointly with the voting voucher. This way, each ballot in the digital ballot box incorporates a proof of the authenticity from the voter himself and a proof of authenticity from the Polling Station  104 .  
         [0110]     The Voter Agent  116  stores the voting voucher obtained in the sixth step of the Vote Casting Protocol  206 . Said voucher will be required in the processes of filing a claim against the recount, to evidence that the voter actually casted his vote.  
         [0111]     The seventh and last step of the Vote Casting Protocol  206  consists in the Voter Agent  116  sending to the Tally Site  108 , of the chain of data corresponding to the digital signature included in the voting voucher obtained at the sixth step of the Vote Casting Protocol  206  S SP [H{envelope|V|elect|Order 402|time}]. This sending is not carried out in a deterministic way but the Voter Agent  116  sends according to the probability PR ACK  included in the opening order  402 . The objective of sending it is to be able to statistically detect the eventual erasure of the digital envelopes at the Polling Station  104  with a level of confidence as high as wished without overloading in excess the Tally Site  108 .  
         [0112]     The Vote Casting Protocol  206  could be stopped due to causes beyond the voting method (failures in the Communication Network  114 , for example). In the event that some of that unforeseeable events occurs, an agency for the survey of the elections in charge of overcoming said situations and attending the voters must exist. The specification and operation of said agency remains out of the scope of this invention.  
         [0113]     In  FIG. 2  it can be seen how the step following the Vote Casting Protocol  206  is the Closing Protocol  208  of the Polling Station  104 , which stops accepting new connections from Voter Agents  116 . The Closing Protocol  208  is started by the Electoral Board  110  which sends to the Polling Station  104  the closing order  602  (see  FIG. 6 ). The closing order  602  contains the elections identifier elect, the Polling Station  104  identifier and the state of the Polling Station  104  (“Closed”). Optionally, the closing order  602  can contain the closing date and hour. To prevent falsifying the closing order  602 , it is digitally signed with the private key S EB   (ADM)  of the Electoral Board  110 . The generic form of the closing order  602  would be then: 
        elect|PS|“Closed”|S EB   (ADM) [H{elect|PS|“Closed”}]       
 
         [0115]     After receiving the closing order  602  duly signed by the Electoral Board  110 , the Polling Station  104  makes it public. This way, it will be evidenced to the following Voter Agents  116  which try to connect, that the Polling Station  104  is already closed. The Electoral Board  110  waits, after sending the closing order  602 , the immediate receipt by the Polling Station  104  of the digital signature of the whole of the digital ballot box jointly with the identities of the voters that it is currently processing. Said digital signature prevents that the Polling Station  104  can accept new ballots once it is closed. After receiving said data, the Electoral Board  110  awaits a preset maximum time in order that the Voter Agents  116  who did not complete the Vote Casting Protocol  206  can complete it. After all the Voter Agents  116  pending have finished or that the maximum waiting time elapses, the Polling Station  104  transfers the complete digital ballot box to the Electoral Board  110 . This transfer is made with a secure connection such as that above detailed.  
         [0116]     After the digital ballot box is received, the Electoral Board  110  proceeds to start the Tallying Process  210 . The first task to carry out at the Tally Site  108  consists in verifying that the digital ballot box received tall yes with the digital signature received immediately after issuing the closing order  602 , adding anyway the digital envelopes of the voters who are completing the Vote Casting Protocol  206 . With this, the Electoral Board  110  can verify that no votes has been added beyond the term nor any possibly valid ballot has been suppresses. The Tally Site  108  also verifies the proofs of authenticity associated to each of the envelopes in the ballot box. Said verification allows assuring that votes have been only cast by the qualified voters of the electoral roll and that they did it only once. In the first, second and fourth authenticity options presented, the Tally Site  108  verifies the digital signature attached to the digital envelope (in the second option it must first open the external digital envelope with the private key S EB   (ADM)  of the Electoral Board ( 110 ). In the third option, after opening the external digital envelope with the private key S EB   (ADM)  of the Electoral Board  110 , it verifies that the private voter&#39;s identifier, PVI, corresponds to the voter identifier V. The Tally Site  108  also verifies the correct presence of the opening order  402  attached to each digital envelope. The Tally Site  108  also compares the ballot box received with the confirmations sent by the Voter Agent  116  during the last step of the Vote Casting Protocol  206 .  
         [0117]     In the event of finding two or more digital envelopes from a same voter (in the event that the electoral process allows it), only one of them will be considered valid. The mark of time, time, can be used signed by the voter, to determine the precedence order of the ballots from a same voter and chose for example the last of them. The possibility of issuing more than one digital envelope by each voter could be useful also to protect electronic voting against the individual coercion by means of same technique of coercion codes typical of the devices of electronic alarm. In this case, each digital envelope would have a coercion code attached. All the codes would have same format, but one of them would be the correct one (indicating lack of coercion) and the rest of them would be incorrect (indicating coercion). If the voter was coerced during the moment of casting the vote, he could chose to issue a false ballot, in the presence of the coercing people, associating to it an incorrect coercion code. This could be repeated as many times as it is required. In a given moment when the voter verifies that there is no coercion, he could issue his definitive vote , associating it to the correct code. Only said ballot would be included in the final recount, the rest being the digital envelopes with incorrect codes refused in the Tallying Process  210 . To prevent that a voter maliciously issues a high number of ballots having an incorrect coercion code, a minimum time can be set between the acceptance of two successive ballots from a same voter.  
         [0118]     To proceed to tabulating the ballots, at the Tally Site  108 , the digital must be open to have access to their content. Said digital envelopes only can be open when the minimum threshold of members of the Electoral Board  110  specified in the secrets sharing protocol contributes with his portions, and the private key S EB   (ENV)  of the Electoral Board  110  can be reconstructed. When opening the digital envelopes it must be accompanied with a random permutation of the position of the ballots, to prevent the correlation between the identities of the voters (proofs of authenticity which are attached to the ballots) and the respective ballots (data within the digital envelopes). This process of mixing ballots can be based on the techniques introduced in [Gülcü, C. and Tsudik, G.  Mixing E-mail with BABEL,  ISOC Symposium on Network and Distributed System Security, February 1996] and applied to electronic voting schemes in [Riera A. and Borrell J.  Practical approach to anonymity in large scale electronic voting schemes . Proc. of 1999 Network and Distributed System Security Symposium, pp. 69-82, Internet Society, 1999]. Anyway, the software implementing the process of mixing ballots must be duly audited and certified to guarantee that no operation will be carried out other than the cryptographic processes and protocols of this invention.  
         [0119]     The Tally Site  108  verifies the validity of each ballot when the digital envelope has been open which contained it. The integrity and coherence of every data contained in the digital envelope has to be verified. The voting receipt must be duly validated by the Polling Station  104  and must effectively correspond to the identifier of the ballot contained in the digital envelope, in the configuration with identifies of linked ballots). It is also verified, in the case that the ballot identifier has been obtained jointly between the Voter Agent  116  and the Polling Station  104 , that the Voter Agent  116  really used the correct ballot identifier. In general, the ballot as a whole must contain no formal defect. If it does, the ballot is considered a void ballot. A Voter who obtained a correct and validated voting receipt and despite it delivered a badly constructed ballot will be not able to file a valid claim due to the authenticity proof which is attached to the digital envelope containing the badly constructed ballot, and the voting voucher. The Electoral Board  110  will be capable to demonstrate the fraud committed by the voter without it is necessary to disclose any of his private keys S EB   (ENV) , or S EB   (ADM) . It is enough to disclose the session symmetric key or keys that the voter used to close the digital envelope or envelopes. Any independent auditing third party can take the digital envelope, the voting voucher, and the proof of authenticity, and verify the authenticity of the session symmetric key announced by the Electoral Board  110  by only encrypting it with the public key P EB   (ENV)  or P EB   (ADM)  depending on the digital envelope. With the corresponding symmetric key, the envelope can be open to verify that it does not contain a well-constructed ballot.  
         [0120]     After all the digital envelopes have been open and mixed, the private key S EB   (ENV)  of the Electoral Board  110  is destroyed, as well as the portions which allow its reconstruction (as well if it is located in the personal devices of the member of the Electoral Board  110  as well as if it is located at the Tally Site  108  itself). Optionally, said private key or the portions which allow its reconstruction are stored with a fully restricted access with the purpose of auditing they later.  
         [0121]     Then, the results to be published can be prepared from all the valid ballots. The results to be published consist in two differentiated data structures. First, the results in an abridged format (listing of vote option with the total number of ballots for each of them). Second, the Tally Site  108  prepares a list with an input for each valid ballot, which will be used for the purpose of the voters verifying the results. In the configuration with non linked ballot identifiers, said list of data to verify contains all the identifiers of ballots ident which resulted valid. In the configuration with ballot linked identifiers, the list of data for verifying contains all the vote identifiers with their respective voting options ident|vote attached. In addition, in the event that a blind digital signature has been used at the fourth step of the Vote Casting Protocol  206 , each input in the list of data for verifying can contain the corresponding validated voting receipt.  
         [0122]     As well the results in abridged format as the list of data for verifying must be digitally signed by the Electoral Board  110  with its key S EB   (ADM)  and finally transmitted from the Tally Site  108  to the Publication Site  106 . For this, the Electoral Board  110  activates the Results Transfer Protocol  212 . This sending is carried out by means of a secure connection as the one above detailed. After receiving the results to be published, the Publication Site  106  makes them publicly accessible through the Communication Network  114 . Said publication of results has two objectives, disclosing the electronic final results and allow the individual verifiability of the voters, through the list of data for verifying and the Verification Protocol  214 .  
         [0123]     The objective of the Verification Protocol  214  is to provide sufficient confidence to the voters with respect to the treatment their respective ballots received. For this, each voter who wishes it may have access to the Publication Site  106  by means of his Voter Agent  116  and carry out a search in the data list for verifying, with the aim of tracing his respective ballot identifier. Depending on how the voting method has been configured, the guaranties offered to the voters are different. When the ballot identifier has been traced in the list of data for verifying, the configuration with non linked ballot identifiers guarantees the voter the correct receipt of his ballot by the Tally Site  108  and the Electoral Board  110 . In the case of the configuration with linked ballot identifiers, the voter not only verifies the correct receipt of his ballot at the Tally Site  108 , but also the correct presence of his ballot in the final results, as the vote identifier has attached its own voting option (the coherence between the abridged results and the list of data for verifying can be universally controlled by any third party). Although the second of that configurations shows high characteristics with respect to verifiability, the first fully prevents the coercion and the sale of votes based on the voting receipt processing. In the second configuration, the coercion and the sale of votes based on the voting receipts processing are prevented by concealing to the voters themselves the ballot identifiers and the voting receipts, until the moment of the Verification Protocol  214 . This can be done with full guaranties only in the event that a suitable hardware device has been associated, such as a tamper-proof smart card, at each Vote Casting Platform  102 .  
         [0124]     The concrete actions a voter must perform during the Verification Protocol  214  can slightly differ depending on the configuration established by the fourth step of the Vote Casting Protocol  206  (see  FIG. 5 ). In the configuration with non linked ballot identifiers, the Voter Agent  116  obtains from the Publication Site  106  the list of data for verifying consisting in all the ballot identifiers processed at the Tally Site  108 . Said list must be preferably located indexed so that the search of a particular ballot identifier can be efficiently carried out by the voter (even by hand). Each voter is then capable of locating his corresponding ballot identifier in the list. In the event that the ballot identifier is not traced, the voter can file a public claim (which does not disclose which was actually the ballot selected) by producing the validated voting receipt, jointly with the voting voucher.  
         [0125]     In the configuration with linked ballot identifiers, the voter obtains from the Publication Site  106  the list of data for verifying, consisting in all the ballot identifiers with their respective voting options attached. Said list must be coherent with the totals for each voting option (abridged results). This coherence must be universally verified, by any third party. The voter does not know which the ballot identifier which corresponds to him is. His ballot identifier, stored in the Vote Casting Platform  102  is protected (by means of a tamper-proof hardware device or by means of a protecting software) and is not accessible to the voter. Said protection allows the access to the ballot identifier by the Voter Agent  116  only in the case that the list of data for verification corresponding to the elections in progress has already been published, digitally signed by the Electoral Board  10  with his key S EB   (ADM) . After the Voter Agent  116  has available the list of data for verifying and the ballot identifier, he can trace said identifier in said list. For this, the list must be preferably indexed so that the search of an identifier of a particular ballot can be efficiently carried out. In the event that the ballot identifier is not traced, the Voter Agent  116  facilitates the voting receipt, which until that moment was protected, to the voter. The voter can file a public claim, producing the validated voting receipt, jointly with the voting voucher. Said claim evidences the omission of a valid ballot in the results published without disclosing which the actual voting option was selected.  
         [0126]     The individual verifiability by each voter is not only suitable to provide guaranties to the voters about the correct treatment of their ballots, but it also results very effective for detecting general manipulations of the results. A small proportion of voter&#39;s verification is sufficient in order that the probability of a non detected manipulation is sufficiently minimized. To guaranty this minimum verifying the possibility is contemplated of randomly selecting a group of voters in order they carry out the Verification Protocol  214 .  
         [0127]     Within the frame of the Verification Protocol  214 , downloading from the Publication Site  106  the list of data for verifying can be highly inefficient because the list of data can have an arbitrarily big size. In the configuration with non linked ballot identifiers, the size is directly proportional to the number of voters. In the configuration with linked ballot identifiers, the size depends on the number of voters as well as the size of the voting options. In fact, for the verification it would be sufficient to partly unload the data, making sure that in said part unloading the ballot identifier involved is to be found. In the first configuration, said partial downloading can be easily structured, distributing all the ballot identifiers in mutually excluding sub-lists, each of which comprises given ranges for the ballot identifiers. The Verification Protocol  214  consists in downloading the suitable sub-list (the voter is capable of easily determining it due to its indexation by the ballot identifier) and tracing the right ballot identifier in the downloaded sub-list. However, in the second configuration, the partial data downloading must be carried out with more caution, because it can mean a threat for the privacy of the voters (no mere ballot identifiers are downloaded but they have their respective voting options attached). In an extreme case, for example, a partial downloading of a set of coincident inputs with respect to the selected ballot option, would completely interfere with the voter privacy.  
         [0128]     This invention includes a pair of mechanisms to allow the partial downloading of data for verifying them, without interfering at all with the voter&#39;s privacy. The first of them is based on splitting the list of data for verifying, at the Tally Site  108 , into subassemblies having an approximately same number of inputs and so that each of said subassemblies approximately contains the same proportion between voted options as in the global results. As it is dealt with subassemblies having same proportions as those disclosed in the global results, the voter&#39;s privacy when downloading one of said subassemblies is still guarantied. The simpler case of splitting consists in grouping the inputs in a unique subassembly which covers the full list (in this case, downloading the data in fact would be not partial but total). The following case would consist in splitting the data list into two subassemblies, so that downloading would be in fact partial affecting only half the inputs.  FIG. 7   a  graphically illustrates this possibility. The data list could also be split into three subassemblies, four subassemblies, and so on, achieving a higher efficiency when greater is the number of subassemblies (and therefore, smaller is the subassembly size).  FIG. 7   b  shows a splitting into four subassemblies. However, the efficiency of data partial downloading for verifying cannot be indefinitely improved. To fully guaranty the voter&#39;s privacy, it is required that all the voting options are represented in every and each resulting subassembly. In some cases, the efficiency of data partial downloading could be improved, by separately treating some of the voting options (the most minority options) without taking them into account to carry out the splitting. Inputs corresponding to said options would be fully included in every and each subassembly.  FIG. 7   c  graphically shows said possibility.  
         [0129]     Grouping the inputs in different subassemblies is carried out from the modular division of the ballot identifiers (they must be random values having an even distribution) by the number of subassemblies. Said modular division produces mutually excluding assemblies, with approximately the same number of inputs and with approximately the same proportion between the voted options and the global results. To accurately determine which of the input assemblies must be downloaded during the Verification Protocol  214 , the Voter Agent  116  only must perform the modular division of the corresponding ballot identifier by the number of subassemblies (information provided by the Publication Site  106  digitally signed by the Electoral Board  110 ). Said process does not threaten at all the voter privacy.  
         [0130]     The second mechanism of data partial downloading for verifying allows that the voter decides the approximate number of ballots he is willing to download N v . The Publication Site  106  makes that the maximum ballot identifier ident MAX , the minimum ballot identifier ident MIN  and the number of votes issued N TOTAL  are accessible. With this information, the Voter Agent  116  calculates an interval of about N v  ballots which contains his ballot identifier ident. The interval is defined by an initial identifier ident INI  and a final identifier ident END  where ident INI  is randomly selected so that it meets following restriction: 
 
ident&lt;ident INI +(((ident MAX −ident MIN )· N   V )/( N   TOTAL )) 
 
 The ident END  value is then calculated as: 
 
ident END =ident INI +(((ident MAX −ident MIN )· N   V )/( N   TOTAL )) 
 
         [0132]     In a trivial way it is then met that ident INI &lt;ident END . If the total number of ballots (N TOTAL ) is high, the ballot identifiers will be evenly distributed and it will be achieved that the number of ballot identifiers comprised between ident INI  and ident FIN  will be approximately equal to N v .  
         [0133]     If ident INI  is smaller than ident MIN , it is recalculated as follows: 
 
rest=(ident MIN −ident INI )− I  
 
         [0134]     The new ident INI  will be: 
 
ident INI =(ident MAX −rest) 
 
         [0135]     If ident END  is higher than ident MAX  it will be proceeded in same way. 
 
rest=(ident END −ident MAX )− I  
 
         [0136]     The new ident MAX  will be: 
 
ident END =(ident MIN +rest) 
 
         [0137]     The Voter Agent  116  sends ident INI  and ident FIN  to the Publication Site  106  and this later sends the ballot identifiers comprised in the interval. In the event that ident INI  is higher than ident FIN  the Publication Site  106  sends following intervals: (ident MIN , ident FIN ) and (ident INI , ident MAX ). This second option is indicated for the elections having a great number of votes cast, where the identifiers are evenly distributed. In the event that the number of votes is low, it is possible that the voter downloads more ballots than wished, or very little ballots, interfering this way against the voting privacy.  
         [0138]     The operations of the process for vote casting above disclosed will be carried out with the assistance of a computer program which can be directly loaded in the internal memory of a digital computer, which comprises parts of a code of computer program to carry out said operations, said program comprising parts thereof distributed between the Polling Station  104  and the Vote Casting Platform(s)  102 .  
         [0139]     Also the operations of the verification process and/or the claiming process as it was already explained will be carried out with the assistance of a computer program prepared to be directly loaded in the internal memory of a digital computer and which comprises parts of a computer program code to carry out said operations, said program comprising parts thereof distributed between the Publication Site  106  the Tally Site  108  and the Vote Casting Platform  102 .  
         [0140]     The invention also provides a computer program which can be directly loaded in the internal memory of a digital computer, the program being executed in said Tally Site  108  and which comprises parts of a computer program code to carry out the operations of the Tallying Process as it has been disclosed above.  
         [0141]     The invention in addition provides a computer program which can be directly loaded in the internal memory of a digital computer and which comprises parts of a computer program code distributed between the Polling Station  104  and the Tally Site  108 .