Patent Publication Number: US-2009235069-A1

Title: Arrangement of and method for secure data transmission

Description:
This application is a national stage application that claims priority under 35 U.S.C. 371 to Patent Cooperation Treaty Application No. PCT/NL2006/050175, entitled “Arrangement of and method for secure data transmission,” inventors Marco Alexander Henk Sonnega et al., filed Jul. 13, 2006, and which has been published as Publication No. WO2007/117131, which application is herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of protecting data communications in which secret keys are used to encrypt/decrypt data, and possibly digitally sign data, which data is transmitted along a communication path and needs to be secured. 
     TECHNOLOGICAL BACKGROUND 
     Massachusetts Institute of Technology has developed a network authentication protocol that is known as “Kerberos”. Kerberos is available as open source software, however also as a commercial software product. Kerberos uses a secret-key cryptography. A Kerberos server distributes “tickets” to communication units after these communication units have authorized themselves to the Kerberos server. 
     Another known way of secret communications is by using Virtual Smartcard Services (VSS). VSS is a substitute for a physical smartcard in a full scale PKI solution (PKI=Public Key Infrastructure). 
     SUMMARY OF THE INVENTION 
     There is a need in the market to provide a high level of security in communications without having to wait for a full scale implementation of PKI which may take another 10 to 15 years when implemented by a central governmental organization. 
     To that end the invention provides some methods and arrangements as specified in the annexed claims. 
     The invention allows to exchange data in a secure way via, e.g., the Internet. The invention allows this by using digital certificates without the normal burden of issuing and managing digital certificates which makes other implementations in which private and public keys are used so expensive. 
     The invention is directed to a reliable way of authentication and, subsequently, securing a connection between two clients or between a client and a server using a centrally managed policy enforcer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be explained with reference to some drawings which are only intended to illustrate the invention and not to limit the invention in any way. The scope of the invention is defined by the annexed claims and their technical equivalents. 
         FIG. 1  shows a schematic overview of a network arrangement used in the present invention; 
         FIG. 2  shows a schematic overview of a computer arrangement; 
         FIG. 3  shows a schematic over of a network arrangement suitable for securely sending and receiving emails. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     1. Introduction 
     An overwhelming majority of services provided through the Internet are web-based applications. These applications rely on secure and reliable communications. The Internet uses TCP-IP as its lingua franca. IP&#39;s strength lies in it&#39;s easily and flexible routed packages. These packages are also the main weakness. The way IP routes these packages makes IP networks vulnerable for security risks. This document discloses a solution to reduce such weakness. 
     Rising Demand for Security 
     In the recent decades many traditional services have been transformed into digital and e-business services. Today millions of people order products and services on-line without any personal contact with their suppliers. Web shops, e-tickets, payments, track &amp; trace, product configuration and many other services are completely digitised. 
     As the amount of money involved in on-line business is growing, so is the interest of criminal organizations to use the Internet for their benefit. This situation calls for secure communication and/or strong authentication. In other words: whom are we talking to, and how do we make sure that nobody else is listening or manipulating the conversation. 
     Solutions 
     Solutions to this problem do exist. The two techniques used for this purpose are signing and encryption. And here we see the basics of digital certificates that contain key pairs with a public key and a private key, which digital certificates are designed for both signing and encryption. Digital certificates serve their purpose very well if and only if used in the correct way. This implies correct administration, user discipline and no calamities. If this is not the case, one can start all over again. It also involves high investments in infrastructure and education. Not to mention the time, effort and costs of maintaining this so-called Public Key Infrastructure (PKI). 
     PKI is the future, but it is only feasible if it is implemented on a large centralised scale. Like the way we all have a passport or other means of identification, which can be trusted because it has been issued by the state. But these PKI solutions will not be implemented on a large centralised scale for another five to ten years. So, until that moment has arrived an alternative is needed. The present invention provides such an alternative in the form of using a security server in a network environment which provides digital certificates with a limited lifetime, as will be explained below. 
     Security Server 
     In the present invention, a security server  4  is used which is e.g. shown in  FIG. 1 . This document explains how the security server  4  allows the use of the benefits of digital certificates without the hassle, for a fraction of the costs and with the option of migration to a full-blown PKI infrastructure in the future left open. The first chapter gives a broad outline of the general principle of the security server  4 . The following chapters deal with the more technical aspects of individual components and the way they address the issues involved in securing communication with digital certificates. 
     In order to better understand the present invention, first a brief explanation of digital certificates is provided. The explanation is as follows and is copied from  Computer Desktop Encyclopedia,  1981-2005, The Computer Language Company Inc. Ver. 18.4, 4 th  Quarter 2005: 
     “Digital Certificate 
     The digital equivalent of an ID card used in conjunction with a public key encryption system. Also called “digital IDs,” digital certificates are issued by a trusted third party known as a “certification authority” (CA) such as VeriSign (www.verisign.com) and Thawte (www.thawte.com). The CA verifies that a public key belongs to a specific company or individual (the “subject”), and the validation process it goes through to determine if the subject is who it claims to be depends on the level of certification and the CA itself. 
     Creating the Certificate 
     After the validation process is completed, the CA creates an X.509 certificate that contains CA and subject information, including the subject&#39;s public key (details below). The CA signs the certificate by creating a digest (a hash) of all the fields in the certificate and encrypting the hash value with its private key. The encrypted digest is called a “digital signature, “and when placed into the X.509 certificate, the certificate is said to be “signed.” 
     The CA keeps its private key very secure, because if ever discovered, false certificates could be created. See HSM. 
     Verifying the Certificate 
     The process of verifying the “signed certificate” is done by the recipient&#39;s software, which is typically the Web browser. The browser maintains an internal list of popular CAs and their public keys and uses the appropriate public key to decrypt the signature back into the digest. It then recomputes its own digest from the plain text in the certificate and compares the two. If both digests match, the integrity of the certificate is verified (it was not tampered with), and the public key in the certificate is assumed to be the valid public key of the subject. 
     Then What . . . . 
     At this point, the subject&#39;s identity and the certificate&#39;s integrity (no tampering) have been verified. The certificate is typically combined with a signed message or signed executable file, and the public key is used to verify the signatures ( . . . ). The subject&#39;s public key may also be used to provide a secure key exchange in order to have an encrypted two-way communications session ( . . . ). . . .
         Major Data Elements in an X.509 Certificate:   Version number of certificate format   Serial number (unique number from CA)   Certificate signature algorithm   Issuer (name of CA)   Valid-from/valid-to dates   Subject (name of company or person certified)   Subject&#39;s public key and algorithm   Digital signature created with CA&#39;s private key       

     Signing and Verifying a Digital Certificate 
     The signed certificate is used to verify the identity of a person or organization.” 
     By using a public key and a private key, asymmetrical encryption can be applied in communications: one party uses a public key to encrypt a message addressed to a third party, and the third party who possesses a private key associated with this public key uses this private key to decrypt the encrypted message. Decryption cannot be done with the public key itself. 
     Moreover, a public key and a private key can be used in a digital signing process as follows: one party signs a message with a digital signature that is calculated from the content of the message itself using a private key. The digital signature has a unique relation with the content of the message. A third party who possesses the associated public key checks the relation between the digital signature and the content of the received message. If there is a match, the third party knows that the content of the message has not been tampered with. 
     The Product &amp; Services 
     The Basic Concept 
     In one embodiment, as shown in  FIG. 1 , the architecture according to the invention comprises three parties interacting in order to create a secure channel between a digital service and a client: i.e., the digital service supported by a server  6 , e.g. a webserver, a client  2 ( n ) (n=1, 2, . . . , N) and the security server  4 . A client is here defined as a computer arrangement in its role as a client. A client may be any kind of terminal, like a personal computer, a lap top, a PDA (personal digital assistant), a smart phone, etc., but may, alternatively, be a router. 
       FIG. 1  shows these entities connected to one another in a network environment.  FIG. 1  shows several clients  2 ( 1 ) . . .  2 (N), a security server  4 , a server  6  capable of running a service which supports the X.509 standard, or compatible, like a bank server or a webserver. The clients  2 ( n ) and the server  6  are provided with suitable software and, thus, arranged to perform secure communications via the network, e.g., using a SSL digital certificate or other secure network communication protocol. Moreover, the security server  4  is provided with suitable software such that the security server  4  can securely communicate with the server  6 , e.g., using a SSL digital certificate or other secure network communication protocol. It is observed that a client may be a personal computer of a private person located at his/her premises. Nowadays, most people have at least one such personal computer equipped with such suitable software, obtained via software from a bank, e.g. loaded on the personal computer from a CD-ROM or DVD, in order to allow “private banking” with a bank server via the internet. 
     In order to authenticate users of clients  2 ( n ), the security server  4  is arranged with an authentication module to perform authentication and that may be implemented as a software program on a suitable processor. Alternatively, security server  4  may be connected to an external authentication service  8  running on a suitable server that provides the desired authentication upon request from security server  4 . 
       FIG. 2  shows a schematic overview of a computer arrangement in general. Such a computer arrangement can be used as client  2 ( n ) but also the security server  4  and the server  6  may have most of the components of the computer arrangement shown in  FIG. 2 . Each one of the clients  2 ( n ), the security server  4  and the server  6  will have at least a processor and some form of memory storing data and instructions to let the processor run a predetermined program to perform functionality in accordance with the invention. 
     The computer arrangement shown in  FIG. 2  comprises a processor  1  for carrying out arithmetic operations. The processor  1  is connected to a plurality of memory components, including a hard disk  5 , Read Only Memory (ROM)  7 , Electrically Erasable Programmable Read Only Memory (EEPROM)  9 , and Random Access Memory (RAM)  11 . Not all of these memory types need necessarily be provided. Moreover, these memory components need not be located physically close to the processor  1  but may be located remote from the processor  1 . 
     The processor  1  is also connected to means for inputting instructions, data etc. by a user, like a keyboard  13 , and a mouse  15 . Other input means, such as a touch screen, a track ball and/or a voice converter, known to persons skilled in the art may be provided too. 
     A reading unit  17  connected to the processor  1  is provided. The reading unit  17  is arranged to read data from and possibly write data on a data carrier like a floppy disk  19  or a CDROM  21 . Other data carriers may be tapes, DVD, etc. as is known to persons skilled in the art. 
     The processor  1  is also connected to a printer  23  for printing output data on paper, as well as to a display  3 , for instance, a monitor or LCD (Liquid Crystal Display) screen, or any other type of display known to persons skilled in the art. 
     The processor  1  may be connected to a communication network  27 , for instance, the Public Switched Telephone Network (PSTN), a Local Area Network (LAN), a Wide Area Network (WAN), the Internet, etc. by means of I/O means  25 . The processor  1  is arranged to communicate with other communication arrangements through the network  27 . 
     The data carrier  19 ,  21  may comprise a computer program product in the form of data and instructions arranged to provide the processor with the capacity to perform a method in accordance with the invention. However, such computer program product may, alternatively, be downloaded via the telecommunication network  27 . 
     The processor  1  may be implemented as stand alone system, or as a plurality of parallel operating processors each arranged to carry out subtasks of a larger computer program, or as one or more main processors with several sub-processors. Parts of the functionality of the invention may even be carried out by remote processors communicating with processor  1  through the network  27 . 
     Features of the different entities in  FIG. 1  may be as follows: 
     Digital Service by Server  6 : 
     
         
         
           
             Any server  6  capable of running a service which supports the X.509 standard, or equivalent, and that requires a secure connection to a client  2 ( n ) can be used in combination with the security server  4 .
 
Client  2 ( n ):
 
             An end-user of the client  2 ( n ) may install suitable software from the security server  4 . Any known secure way, e.g. using code signing or using a traditional X509 technology, to download software from a server to a client may be used for that purpose. 
             When installed the software on the client  2 ( n ) controls the user interface and contains the logic to establish secure channels with the security server  4 . 
             the software on the client  2 ( n ) may use credentials retrieved from the security server to establish secure channels with digital services, using open and standards based technology, such as two-sided SSL/TLS. 
           
         
       
    
     Security Server  4 : 
     
         
         
           
             The central security server  4  handles incoming requests from clients  2 ( n ). Depending on the activated modules and the selected security and escalation levels the security server  4  deals with every incoming request based on the settings selected for the corresponding digital service on server  6 . When the service is under attack, the security level can be tuned right away to a higher level, to avoid damages. 
           
         
       
    
     2. Concepts 
     Securing Communication Lines 
     Nowadays, digital certificates are used on a regular basis. When one accesses a ‘secure’ website on the Internet, for instance when booking a flight on-line, a digital certificate is used at the side of server  6 . This digital certificate is used for two purposes. It is used for the encryption of the connection and it tells the web browser of the client  2 ( n ) that the website the browser is connecting to really is the website the browser thinks he is connecting to. In that sense, the digital certificate used by server  6  serves the purpose of authenticating server  6 , like a passport can be used to authenticate an owner of that passport. The browser of client  2 ( n ) checks the authenticity of every piece of information coming from the server  6  during the whole session. 
     When one looks at the information flowing from the browser to the server  6 , however, it is evident that the situation is not totally secure after all. While information coming from the server  6  is validated during the whole session, the authenticity of the information coming from the client  2  ( n ) is checked only once: i.e., when the user logs in. After that, the server  6  has no way of telling whether the next piece of information is coming from the same client  2 ( n ) and has not been tampered with. For checking the client  2 ( n ) continuously during the whole communication session, the client  2 ( n ) needs a digital certificate that can be checked by the server  6  too. If both server  6  and client  2 ( n ) have and use a valid digital certificate the communication is secure in both ways. 
     The same principle is valid for email communications. Digital certificates have to be used at both sides to encrypt and/or sign messages. If signed, a receiver of an email can check the authenticity of the message and if encrypted, it is unreadable to anyone but the receiver provided the receiver has a decryption key. 
     Managing Digital Certificates 
     For servers  6 , the management of digital certificates is not a big issue. The number of servers  6  is small, digital certificates can be arranged and installed as needed and incidents can be dealt with on an ad-hoc basis. The knowledge of the administrators and other people involved can easily be maintained. 
     For clients  2 ( n ), the situation is different. End-user PC&#39;s and other clients  2 ( n ) are more vulnerable to security breaches, viruses and other incidents that make the digital certificate lose its value. It might have been copied. If so, a new digital certificate must be arranged and installed. And the old digital certificate must be ‘blacklisted’ on the server  6 . The compromised digital certificate itself might be valid for another year or even longer. And how to teach each end-user how to handle digital certificates and deal with incidents? 
     In the invention, the security server  4  takes care of the end-user part of the digital certificate. The security server  4  authenticates the client  2 ( n ) in accordance with a predetermined reliability level, and, if successful, provides the client  2 ( n ) with the means to use a digital certificate, as will be explained below. Different ways of authentication may be selected to provide different levels of reliability. 
     Using the concept of the invention, communication between a client  2 ( n ) and a server  6  is completely secured without requiring any technical knowledge from the end-user. 
     As will be explained in detail below, issuing a digital certificate to a client  2 ( n ), installation of the digital certificate and management of the validity of the digital certificate, etc. is controlled by the security server  4 , every time it is needed. 
     Escalation 
     Within security, the level of security strongly depends on investments (costs) to prevent misuse versus damage (costs) when misuse occurs. This is also the case within the ICT environment. Whereby one can wonder: “Does everybody within an organization realize what damage means?” For a financial organization, intrusion could mean a (temporary) loss of money, which can be controlled by using transaction limits. Loss of imago however, can result in delayed introduction of new services, meaning loss of revenue and profit for the short and long term. With the concept of the present invention, different required levels of security can be recognized and organizations using this platform are allowed to escalate their security level when the stakes get higher or the need arises. This security level is associated with the authentication process performed by the security server  4  to authenticate a client  2 ( n ). 
     To understand the concept of escalation, first some general explanation is given as to possible levels of security in a client-server environment. 
     At the authentication level (when one logs in at a website) several options are available that each relate to a different security level:
         Using a password   Using a one-time password (i.e., with each log-in a new password is used)   Using a challenge-response protocol (a question from the server is received by a client  2 ( n ) which only the client  2 ( n ) can understand)   Using a hardware token (e.g. USB);   Using a smart card (e.g. GemPlus, Schlumberger);   Using a biometric feature (e.g. fingerprint, iris recognition).   In general: the more secure, the higher the costs.       

     The used security level depends on the value of the services to be provided by server  6  as well as existing threats. Both parameters can change over time. Increasing the level of security is called escalation. 
     The same concept of authentication can be used when a client  2 ( n ) has to log-in to security server  4 . With the security server  4 , the administrator can do a number of escalations, without bothering the end-users. Some other enhancements of security require actions at the user side (e.g. Microsoft security updates, installation of client software necessary for the invention or maybe the use of a hardware token). The invention provides a platform and not so much a tool, and therefore allows the integration of all kinds of security measures without affecting the basic functionality. For instance, the method of authentication (password, challenge-response, hardware token) can be changed while all other implemented modules and functionalities continue to work as they did before. This will be explained in detail below. 
     3. Platform 
     The invention was developed bearing in mind that the product should not only handle the security needs of today but also those of the future. As a result, it was built as a platform with plug-in modules. The platform handles the generic functionality needed for all services. The modules handle specialised functionalities such as authentication and email support. This leaves room to upgrade functionalities while leaving the platform in place. New services can be easily integrated without disturbing the other services. 
     Seamless Integration Choice of Platforms 
     The importance of the fact that server components have to integrate seamlessly into existing ICT environments has been acknowledged. The central environment of the invention can be implemented on a choice of platforms varying from Linux to mission critical environments such as the HP NonStop platform. 
     Communication and integration with existing ICT systems, such as databases and authentication infrastructures, is made easy by the use of open and standards based technology. 
     PC&#39;s, PDA&#39;s and Future Devices 
     The benefits are not restricted to use only on PC&#39;s, but can be extended to mobile devices such as PDA&#39;s and smart phones. The use of open and standards based technology guarantees that future devices will also support the technology as described here. 
     Platform Architecture 
     In an embodiment, the authentication architecture consists of three parties interacting in order to create a secure connection between a client and a webserver. 
     Client Application Software 
     The client  2 ( n ), after the user is authenticated, receives client application software from the security server  4 . Any known technique to securely download such software from security server  4  may be used. Instead, the software may have been loaded from a suitable CD-ROM or the like. The application software is lightweight application software that is responsible for the retrieval of a valid temporary digital certificate, e.g. a X.509 digital certificate, from the security server  4 . The client application software provides the user interface to the solution of the present invention. It presents the user of client  2 ( n ) with the necessary dialog boxes and contains the logic to create a secure connection with the security server  4 . The secure connection may be based on a Diffie-Hellman protocol. It implements the encrypted communication between the security server  4  and client  2 ( n ). 
     Basic Process 
     The basic process is explained with reference to a situation in which client  2 ( n ) wishes to set up a secure connection with server  6  that is a bank server that supports secure communications by using its own bank server digital certificate including a bank public key BPuK and an associated bank server private key BPrK stored in its memory in a safe way. It is assumed that the client  2 ( n ) has stored suitable software in its memory to communicate with security server  4 , as explained in the preceding section. Then there are two separate stages in setting up the secure connection. These two stages are performed in sequence and are independent of each other: 
     First Stage:
         In the first stage, the client  2 ( n ) receives a digital certificate and a private key after being authenticated, as follows.   To that end, the client  2 ( n ) establishes a connection with the security server  4 . During setting up the connection, several data are exchanged between the client  2 ( n ) and the security server  4 . The client  2 ( n ) sends data so as to have him authenticated by the security server  4 , using one of the methods referred to above. Authentication may be based on an authentication service controlled by security server  4  itself or by external authentication service  8 . After a successful authentication, the security server  4  sends a temporary digital certificate to client  2 ( n ), as well as a private key that is associated with the public key within the temporary digital certificate. The private key associated with the public key can be transmitted from the security server  4  to the client  2 ( n ) using the software on the client  2 ( n ) that can support a secure connection, e.g., based on the Diffie-Hellman protocol, between the two. The digital certificate and the private key can, e.g., be securely transmitted in a so-called PCKS #12 packet. However, the invention is not restricted to this.       

     Second Stage:
         In the second stage, the client  2 ( n ) carries out the secured communication session with the bank server  6 . This communication can now be secured from both sides since, now, both the client  2 ( n ) and the bank server  6  possess their own digital certificate and their own private keys. So, they can setup a so-called two-sided SSL (SSL=Secure Sockets Layer). This is controlled by the application software and key pair as downloaded from the security server  4  in the first stage. The second stage is not controlled by the security server  4 .       

     In accordance with the invention, the certificate as sent by the security server  4  to the client  2 ( n ) has only a predetermined limited lifetime. Such a predetermined lifetime may be expressed in the form of a time period, e.g., a number of hours such as a maximum of 24 hours or 1 hour, a number of minutes less than 60 minutes, or a number of seconds less than 60 seconds. Alternatively, the duration may be defined as being coupled to a predetermined number of communication sessions with a third party, e.g., 1 session. As a further alternative, the lifetime may be defined as being coupled to a predetermined number of actions, like retrieving one or more messages from a web-server or an email server, as will be explained later. As a further alternative, the limited lifetime may be defined as a predetermined maximum number of usages, like 1 or less than 10. The limited lifetime is included in one of the attributes of the temporary digital certificate. E.g., if the validity is expressed in units of time, the valid-from/valid-to attribute can be used. Alternatively, an additional attribute may be used to indicate the limited lifetime. The software as installed in the client  2 ( n ) is arranged to recognize this attribute and use it to remove the digital certificate after the validity has expired. 
     On-Line Service 
     One of the major benefits of the architecture is that the on-line service can usually be made available without modification. As long as the security is dealt with using standard digital certificates (which is the case for most on-line services), the solution of the invention integrates with the on-line service without the need to modify the service. Of course, in the case of a non standards based on-line service, a custom module could be developed to make the security available for this service as well. 
     Servers and Platform 
     The security server  4  processes incoming requests from clients  2 ( n ) as controlled by the client application software installed on the client  2 ( n ). The security server  4  controls the protocol that is required to generate the required key pairs on request. Optionally the security server  4  can also manage a database which stores accounts, available key pairs and valid digital certificates, process database queries from other daemons and manage a stock of available encryption keys. When the amount of available keys drops below a certain lower threshold, the security server  4  may generate new keys until an upper threshold is reached. 
     4. Platform Options 
     Digital Certificate Authority 
     The platform is used for the distribution and management of digital certificates. These digital certificates need to be generated first. This is done using a so-called Digital certificate Authority. This is basically a digital certificate itself and the key is used to generate and sign new digital certificates. Most companies do not have a Digital certificate Authority (CA) of their own. Therefore, the security server  4  has an option to use an internal CA to generate the digital certificates. But some companies do have their own CA (from Baltimore for example). Therefore, the security server  4  also has the option to use digital certificates generated by such a third party CA. 
     Authentication 
     Two groups of user authentication may be distinguished:
         a) User authentication using an internal authentication mechanism. This internal authentication mechanism may be based on any of the authentication mechanisms explained above.   b) User authentication using an existing external authentication system, like via authentication service  8 , and using the results from the external authentication in the symmetric key generation process. (e.g. Vasco Digipass with one-time password). To that end, the security server  4  connects to an authentication processor associated with the bank server  6  which does the actual authentication after the user of client  2 ( n ) has sent his/her credentials. In this case an existing user administration can be used and integrated in the platform.       

     Key Generation 
     When using the internal Digital certificate Authority, the keys that are generated for use in the digital certificates can be generated in advance and stored in the database controlled by the security server  4 . This can be done at times when the security server  4  is not handling much traffic. Then, later when the security server  4  is busy no processing power is needed for key generation as the key can be retrieved from the database. 
     An other option is to get the key at the moment it is needed. If security server load is no issue, this gives the easiest implementation. It is also possible to retrieve the digital certificate from an external source at the moment it is needed. In that case, the key generation takes no processing power either. 
     5. Further embodiments 
     Now, some further embodiments of the present invention will be explained. 
     a. Email Module 
     Concept 
     Email is one of the most commonly adopted functionalities offered by the Internet. Although the use is straightforward and widespread, this is also one of the least secure ways of communication. This is the case unless encryption and signatures are used to secure the email conversation. Just as with web browsing this can be accomplished using digital certificates issued by the security server  4 . 
     To that end, in an embodiment, clients  2 ( n ) are provided with a special secure email module. 
     In an embodiment, a key pair including a public key PuK(i) and an associated private key PrK(i) that, together, are associated with an issued digital certificate are stored in the database of the security server  4 . This is done to prevent a sent and/or received email protected by this digital certificate from becoming unreadable due to the digital certificate not being valid anymore. If the digital certificate has become invalid, based on this stored key pair PuK(i), PrK(i) the security server  4  can generate a new temporal digital certificate that can be used to read the email concerned. An additional advantage is that secured email can be transmitted from any system provided with this secure email module. 
     Functionality 
     The secure email module will be explained with reference to  FIG. 3 . Components with the same reference number as in  FIGS. 1 and 2  refer to the same components.  FIG. 3  shows an email server  10  connected to clients  2 ( n ) and  2 ( n ′) (n≠n′). Assume, a user of client  2 ( n ) wishes to send an email message to client  2 ( n ′) in a secure way. Then, the user starts an email application (e.g. Windows Outlook). In the same way as explained above, he receives a temporary digital certificate including a public key PuK( 1 ) from the security server  4 . Moreover, as explained above, he receives an associated private key PrK( 1 ) from security server  4 . 
     In the same way, client  2 ( n ′) collects a temporary digital certificate from security server  4 . Then, client  2 ( n ′) has a public key PuK( 2 ) and a private key PrK( 2 ). 
     Now, secure email exchange between clients  2 ( n ) and  2 ( n ′) can be performed. E.g., client  2 ( n ) may wish to digitally sign an email to be sent to client  2 ( n ′). Then, client  2 ( n ) signs the email while using his private key PrK( 1 ). Client  2 ( n ) sends his public key PuK( 1 ) to client  2 ( n ′) who uses this public key PuK( 1 ) to verify that the content of the email has not been tampered with. 
     If client  2 ( n ) wishes to send an encrypted email to client  2 ( n ′) he requests client  2 ( n ′) to send him his public key PuK( 2 ). Upon receipt of this public key PuK( 2 ) he encrypts the email with the public key PuK( 2 ) and then send the email to email server  10 . Client  2 ( n ′) read the email from email server  10  and decrypts the email with his private key PrK( 2 ). 
     The digital certificate as used by client  2 ( n ) may be valid only for a predetermined short time period, as explained above. Alternatively, the digital certificate is only valid for one email message. As a further alternative, the digital certificate may be valid as long as the email client software is active. Then, as soon as the user closes the email client software, the digital certificate is removed from the client  2 ( n ). The validity of the digital certificate of client  2 ( n ′) is also limited in time, e.g., that digital certificate is only valid for a single email and/or during one hour. 
     Again, the way the validity of the digital certificate is limited in time is defined by an attribute in the digital certificate itself. The software as installed in the clients  2 ( n ),  2 ( n ′) is arranged to recognize this attribute and use it to remove the digital certificate after the validity has expired. 
     b. Signing Module 
     Concept 
     Just like in the email module, the client  2 ( n ) can be provided with a digitally signing module used to digitally sign digital documents. 
     Functionality 
     In the same way as explained with reference to the email module, client  2 ( n ) receives a temporary digital certificate including a public key PuK( 1 ) from the security server  4 . Moreover, as explained above, he receives an associated private key PrK( 1 ) from security server  4 . 
     Then, client  2 ( n ) signs the document while using his private key PrK( 1 ). Client  2 ( n ) may store the signed document in his own memory. Alternatively, client  2 ( n ) may send the signed document to another client  2 ( n ′). If so, client  2 ( n ) sends his public key PuK( 1 ) to client  2 ( n ′) who uses this public key PuK( 1 ) to verify that the content of the email has not been tampered with. 
     As a further alternative, client  2 ( n ) sends the signed document to a central database where it is stored for administrative or legal reasons. Then, any third parties client  2 ( n ′) can retrieve this document from the central database and verify its content while using public key PuK( 1 ). 
     Again, the digital certificate as used by client  2 ( n ) may be valid only for a predetermined short time period, as explained above. Alternatively, the digital certificate is only valid for signing a predetermined limited number of documents, e.g. 1 document. 
     Again, the way the validity of the digital certificate is limited in time is defined by an attribute in the digital certificate itself. The software as installed in client  2 ( n ) is arranged to recognize this attribute and use it to remove the digital certificate after the validity has expired. By using criteria on one or more of the digital certificate attributes, the digital certificate can also be used for non-standard use, e.g. for a validity in the future. 
     c. Phishing/Pharming Module 
     Concept 
     The use of the Internet for financial and business transactions is growing day by day. With the growing amount of money involved in on-line transactions, the interest of organised crime is growing as well. And computer crimes get to be more creative as a result. Phishing attacks use ‘spoofed’ e-mails to lead consumers to counterfeit websites designed to trick recipients into divulging financial data such as credit card numbers, account usernames, passwords and social security numbers. Hijacking brand names of banks, e-retailers and credit card companies, phishers often convince recipients to respond. 
     Similar security issues arise in case of “pharming” in which somebody sets up a look-alike of an existing website. 
     The present invention, by means of a phishing/pharming module, can stop unsuspecting customers from giving their valuable personal data to criminals. The phishing/pharming module can block access to phishing and/or pharming websites, as will be explained below. 
     While establishing a connection between a client  2 ( n ) and security server  4  to obtain a temporary digital certificate, data is exchanged between the two. In this phase, security server  4  sends data to client  2 ( n ) regarding potential phishing attacks and pharming. Client  2 ( n ) may use this data to prevent such a phishing attack and/or pharming. 
     Functionality 
     Phishing schemes direct unsuspecting victims to a fake website posing to be a known company. Pharming schemes direct users to look-alike websites. A phishing/pharming module installed on client  2 ( n ) prevents this in the following way. 
     The phishing/pharming module is arranged to have the client  2 ( n ) contact security server  4 . The time this connection is made may be automatically triggered by the phishing/pharming module. The trigger may, e.g., be the moment client  2 ( n ) starts a webbrowser which is recognized by the phishing/pharming module. Upon detecting the webbrowser to start, the phishing/pharming module automatically retrieves actual data from security server  4  regarding actual phishing and/or pharming threats. This data contains data as to sites relating to the potential phishing and/or pharming threats. The phishing/pharming module informs the webbrowser of these potential phishing/pharming threats sites. The webbrowser uses this information to e.g. block access to these sites or to send a warning to the user of client  2 ( n ). 
     d. Central Storage of Digital Certificate and Private Key 
     In the embodiments above, it has been explained that the temporary digital certificate and associated private key are sent by security server  4  to client  2 ( n ) and stored in the memory of client  2 ( n ). However, in order to enhance security, in an alternative embodiment, the digital certificate and associated private key once generated or identified as being associated with client  2 ( n ) can be stored in a central database monitored and possibly controlled by security server  4 . Such a database may be located in the security server  4  or remote thereof. If so, such a centrally stored digital certificate and associated private key may be used in a digitally signing operation performed at a third parties server  6 , e.g., a server of a governmental organization. That third parties server can set up a secure connection with the security server  4  in order to access the centrally stored certificate and associated private key. The third parties server then uses the received private key to sign a message intended for client  2 ( n ) and sends the signed message to the client  2 ( n ). It also sends the public key present in the digital certificate to client  2 ( n ) which it uses to verify the content of the signed message. 
     In an alternative embodiment thereof, the client  2 ( n ) uses a temporary certificate associated with a public key and an associated private key both centrally stored under the control of security server  4 . So, in this case the private key and public key associated with the certificate are copies from the respective keys of a key pair that remains centrally stored and can be accessed afterwards by security server  4 . Such temporary certificate is received from security server  4  in the way as explained above. Then, the private key associated with the temporary certificate is used by client  2 ( n ) to sign an official form when sent to e.g. an official entity like the tax authorities. The public key associated with the temporary certificate is sent to the authorities too, and used by them to verify the content of the form received. This is an advantageous embodiment when entities like the tax authorities require one to use centrally stored keys. Moreover, the tax authorities can check with the security server  4  whether the used public key belongs to a key set that is centrally stored. 
     e. Using Key Pairs More than Once 
     In a further embodiment, the security server  4  stores one or more sets of key pairs including a public key and an associated private key, and each time a client requests to send a temporary certificate, transmits such a certificate with a copy of one of those key sets. So, in this embodiment key sets can be used more than once. Note, that the certificate changes every time for the same key set since the certificate comprises more data than just the key set. 
     6. User Issues 
     The solution of the invention is technically sound and user friendly, as will be explained now. This paragraph describes the effects the invention will have on the users of services, which use the technology as described here. 
     Installation 
     Installation activities on the customer side are reduced to an absolute minimum. Installation of the required functionality on the client side can be performed while using state of the art technology and, in general, does not require hardware installation. 
     User Friendliness 
     The authentication mechanism as explained above, is very easy to use. Once installed on a client  2 ( n ) the authentication software on the client  2 ( n ) establishes a secure connection without any client interference other than entering credentials, such as USER ID and password, depending on the chosen authentication method. Also, since the client software is generic, it can be distributed via the Internet. 
     End-User Discipline 
     The authentication process is fully transparent and invisible to the end-user and does not require any end-user intervention. The authentication makes life not only easier for the (user of) client  2 ( n ) by making the client  2 ( n ) responsible for the authentication, but also for the service provider of a website because the webserver supporting the website can identify the client  2 ( n ) in a reliable way and can rely on the fact that no-one can mislead the client  2 ( n ). 
     Implementation 
     In general the introduction of a secure communication platform requires a substantial implementation period and budget, because keys need to be distributed to all clients  2 ( n ). In some cases, special hardware may be required. In the case of the invention, the only requirement is a small generic software client, which can be downloaded via the Internet or distributed via CD-ROM. The technical impact of the described platform is also minimal. The security server  4  can be placed either inside or outside the environment of the service provider of a website. 
     Organizational Impact 
     When using a PKI the service provider must not only administer which digital certificates have been issued, but also which digital certificates must be revoked. This requires a large and efficient administration. Another organizational impact is training. When new technology is introduced into a company, people must be trained. If a PKI project uses a smart card, then the help desk must also be trained in answering questions regarding this kind of devices. Maintenance must be set up, stocks must be watched and the product must be kept up-to-date. 
     Unlike full scale PKI solutions implementation of the invention does not require implementation of complex processes and extensive training within organizations. 
     Operations 
     The solution can easily be administered and maintained centrally: it supports real-time central management of all remote client access privileges, communication protection and digital certificate management. An administrator can dynamically react to changes in the security situation, enrol new users or services and delete old ones. 
     Financial 
     From a financial perspective, the invention distinguishes itself from other solutions when reviewing the price-performance ratio. The total initial cost per user is low since the client requires no special hardware. The recurring costs, which often are very high, usually are caused by comprehensive customer support. Authentication as explained above, requires minimal involvement from the end-user since his client  2 ( n ) does the work required. Once the client software is installed on the client  2 ( n ), end-user intervention is no longer required. There is also little impact on the organization. No new departments need to be set up. 
     Calamities 
     When using PKI the entire infrastructure relies on the integrity of central components such as a Digital certificate Authority (CA). In the event where the integrity of one of the essential components or procedures cannot be guaranteed any more, all issued digital certificates must be revoked and replaced by new digital certificates. The impact of such an event is substantial, especially when secure USB tokens, smart cards or any other hardware tokens are used for digital certificate storage. All tokens will have to be replaced. In today&#39;s insurance markets the lack of historic data makes it very difficult to insure against such risks. 
     When the security server  4  would be compromised, reinstalling the security server  4  will reinstate the integrity of the entire system. Reinstallation of the security server  4  has no impact on the users of services that use the described technology.