Abstract:
The invention relates to a method for sharing the authorization to use specific resources among multiple devices, which resources are accessible via messages on which a secret key operation was applied with a predetermined secret master key d available at a master device  11 . In order to provide an optimized sharing of authorization, it is proposed that the master device  11  splits the secret master key d into two parts d 1 , d 2 . A piece of information relating to the first part d 1  of the secret master key d is forwarded to the slave device  13  for enabling this slave device to perform a partial secret key operation on a message m. The second part d 2  of the secret master key d is forwarded to a server  12  for enabling the server  12  to perform partial secret key operations on a message m received from the slave device  13.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority under 35 USC §119 to European Patent Application No. 02015842.4 filed on Jul. 16, 2002. 
   FIELD OF THE INVENTION 
   The invention relates to a method for sharing the authorization to use specific resources among multiple devices, which resources are accessible via messages on which a secret key operation was applied with a predetermined secret key available at one of these devices. The invention relates equally to such devices and to a server supporting a sharing of authorization. 
   BACKGROUND OF THE INVENTION 
   It is known from the state of the art to provide an access to specific resources via a network only upon messages on which a secret key operation was performed. Such a secret key operation can be in particular signing the message digitally with a secret key or decrypting a received encrypted message based on a secret key. For example, bank account payment transactions or the purchase of rights for a piece of digital content may be enabled on-line with digitally signed messages. 
   Methods for generating digital signatures on messages in a distributed manner are proposed for example in the document “Networked cryptographic devices resilient to capture” in Proceedings of the 2001 IEEE Symposium on Security and Privacy, pp. 12-25, May 2001, by P. MacKenzie and M. K. Reiter. The presented methods are aimed at minimizing the impact of stolen devices by using a network server. They are based more specifically on function sharing between a device and a network server, e.g. on sharing a secret RSA signing key. For sharing a secret RSA (Rivest, Shamir and Adelman encryption) signing key d available at a device, the device provides a half-key d 2  to an untrusted server. Whenever needed, the device can recover the complementary half-key d 1  by asking the user to enter a password. The half-keys d 1  and d 2  satisfy the relation d=d 1 +d 2 (mod (N)), where N=pq is the RSA modulus, where p and q are different secret prime numbers available at the device, and where (N)=(p−1)(q−1). After the initialization process, the secret values d, p and q will be deleted at the device. The user can then generate a signature on a message m by requesting a partial signature m d2 (mod N) from the server. Thereafter, the device can compute the entire signature based on the generated second half-key d 1  according to the equation m d =m d1 *m d2 (mod N). 
   It is an underlying assumption of this method that there is only one device that uses the authorizations granted with a key pair d 1 , d 2 . 
   In some situations it might be desirable, however, to be able to use specific resources from several devices and/or by several users. An owner of a bank account which can be accessed on-line might wish to be able to access the account via several devices, for instance via a small mobile phone and a larger PDA (personal digital assistant). An owner of such a bank account might further wish to allow another person to access the account for a limited time at least to a limited extent. 
   A general approach for enabling a sharing of authorization is to define an authorization domain consisting of several personal devices. The authorization for a service is then granted to the domain, rather than to a specific device. A device is allowed access to the service if its membership in the authorization domain can be verified. 
   A more specific approach for enabling the use of resources from several devices has been proposed by the IETF sacred working group in “http://www.ietf.org/html.charters/sacred-charter.html”. 
   The IETF proposal aims at allowing users to utilize different user devices from which their authorizations can be used. To this end, two approaches are presented. 
   In the first approach, a user is enabled to create his/her credentials on one device and to securely upload them to a credential server. Thereafter, the user may download these credentials from the credential server to any device and use them there. The download process is controlled by an authentication of the user to the credential server. The authentication can be based in particular on passwords, since the user is not required to possess any personal device. 
   This first approach has the disadvantage that the credential server is an attractive point for attack. Further, depending on the details of the protocol, the credential server itself may have to be trusted to a high degree. For example, if the credentials are stored on the server encrypted with the user&#39;s password, the server will be able to mount a dictionary attack to recover the credentials. Moreover, in order to share the same resources among different users, the user to whom the credentials belong has either to enter his/her password personally to the device of another user, which is usually not possible, or to impart the password, which is usually not desired, since the password might be used also for other applications. 
   In the second approach presented by IETF, credentials are transferred directly from one user device to another user device. This approach has the disadvantage that it implies that a complete transfer of the credentials from one device to another is performed. That is, after the transfer, the credentials will not be usable in the original device any more. This prevents concurrent sharing of authorizations. 
   In both approaches, the devices receiving the credentials also have to be trusted to a large extent, since they receive the credentials in plain text. There is no transparent way to control what a client could do with the credentials, and it is not possible to revoke the authorizations granted to a client device. Thus, a partial sharing of authorization is not possible. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide an improved method for sharing the authorization to use specific resources among multiple devices. 
   It is in particular an object of the invention to enable a use of the same authorization concurrently from more than one device. 
   At the same time, the required level of trust on a server supporting the sharing of authorization is to be kept minimal. That is, the server by itself should not be able to use the shared authorization. 
   These objects are reached according to the invention with a method for sharing the authorization to use specific resources among multiple devices, which resources are accessible via messages on which a secret key operation was applied with a predetermined secret master key available at a master device. In the proposed method, the master device, which acts as a delegator of the authorization, splits in a first step the secret master key into a first part and a second part. The master device then forwards a piece of information relating to the first part of the secret master key to a slave device acting as a delegatee of the authorization. This piece of information enables the slave device to perform a partial secret key operation on messages based on the first part of the secret master key. Moreover, the master device forwards the second part of the secret master key to a server for enabling the server to perform a partial secret key operation on messages received from the slave device based on the second part of the secret master key. 
   The invention proceeds from the method presented in the above cited document by MacKenzie and Reiter. It is assumed that there is a master device that has a master secret key, e.g. a private key of a RSA key pair consisting of a private key and a public key. The master device acting alone will be able to fully utilize the authorizations granted to the public key by itself. But the master device is typically not expected to be used in day-to-day transactions. Instead, the master device delegates its authorizations fully or partially to one or more slave devices. These slave devices constitute an authorization domain. There is a network assistant (server) that helps slave devices to exercise the delegated authorization. Whenever a slave device is to be added to the authorization domain, the master device splits the available secret master key into two parts. The master device then transmits information on one part of the secret master key to the slave device and the other part directly or indirectly to the server. 
   With the presented method, a sharing of authorization is initialized. Now the slave device can transmit a request to the server that a partial secret key operation is to be performed on a message, and as a result the server returns a processed message, i.e. a partially signed or decrypted message. The slave is then able to compute the entire signed or decrypted message by combining the received message with a message on which the slave device applied its own part of the secret key in a partial secret key operation. Neither the server nor the slave device is able to obtain a signed or decrypted message when acting alone. 
   Compared to the above cited document by MacKenzie and Reiter and to the above mentioned second approach by the IETF, it is an advantage of the invention that resources may be used via several devices and by different users concurrently. The invention does not technically restrict the number of devices that can be members of the authorization domain. 
   Compared to the above mentioned first approach by the IETF, it is an advantage of the invention that the server acting alone cannot use the secret key. 
   It has to be noted that the server and the master functionalities can be placed physically into one device. 
   Preferred embodiments of the invention become apparent from the dependent claims. 
   In a preferred embodiment of the invention, a chained delegation of the authorization to access specific resources is enabled. That means that a slave device to which the authorization has been provided is able to further delegate the authorization to other slave devices. The rationale for such a feature is that even when the master device is currently unavailable, e.g. broken or lost, the user is able to expand his authorization domain as long as there is at least one slave device left to which the authorization was already delegated. Basically, the delegation between slave devices may take place in the same way as from the master device to a slave device. Since the slave device is not in possession of the entire secret master key, however, the server adds the part of the secret master key available at the server for the respective delegating slave device to a received part of the partial secret key available at the delegating slave device. 
   In either case, the server should verify the identity of a device requesting a partial secret key operation, e.g. based on an authentication key, and of the user using the requesting device, e.g. based on an entered password, before transmitting a message on which a partial secret key operation was applied to the requesting device. 
   In a further preferred embodiment of the invention, the key splitting performed by a delegator is made dependent on a randomized password provided by the delegatee. It is proposed more specifically, that the delegatee generates a password verification value based on a password input by a user of the delegatee and on a first random number. This password verification value is provided to the delegator. The delegator then determines the respective first part of the secret master key based on the received password verification value and on a second random number. The piece of information which is forwarded by the delegator to the delegatee may comprise in this case the second random number. The delegatee is thereby enabled to compute the respective first part of the secret master key whenever required based on the correct password entered by the user, on the first random number used for generating the password verification value and on the received second random number. 
   It is an advantage of this embodiment of the invention that the necessity is avoided that users have to reveal their long-term secrets to other users or to transfer them from one device to another, while it is at the same time ensured that only authorized users can access specific resources. The user of the device which requests an introduction into the authorization domain can choose a new password or use an old password based on which the secret master key is to be split, since the password itself is never revealed to the server or to the device from which an introduction to the authorization domain is requested. It is further an advantage that the respective first part of the secret master key does not have to be stored itself at the delegatee. 
   Advantageously, the master device and the server share a security association. This is an important feature, because otherwise a slave device can masquerade as the server and obtain both halves of the secret key. The security association between a master device and a server may consist of an authentication key associated to the master device, a confidentiality key associated to the master device and the lifetimes of these keys. The authentication key can be in particular a key of a symmetric authentication algorithm or a public digital signature algorithm, and the confidentiality key can be in particular a key of either a symmetric or an asymmetric algorithm. Preferably, both keys are keys of symmetric algorithms, since this increases the protocol speed and decreases the size of the message. 
   Further advantageously, a security association between the respective slave device and the server is also established. If this security association is based as well on symmetric mechanisms, the computation workload on the server side is decreased, and moreover, the slave device and the master device may now share exactly the same types of security associations. This allows extending the capability of the proposed authorization delegation to the slave device in a particularly simple way. 
   Moreover, a confidential channel between a respective delegator and a respective delegatee should be provided, for example, by PKI (Public key infrastructure), shared keys, a physical connection, etc. This ensures that only an authorized device can be the delegatee and receive the secret information sent by the delegator. In particular, it prevents a server from masquerading as a delegatee, and obtain both halves of the secret key. 
   The proposed delegation of authorization may be restricted in several ways. 
   A first type of restrictions is aimed at protecting the interests of the owner of the delegator. For this type of restrictions, a delegator may be enabled to define bounds of further delegations permitted to a delegatee. 
   A second type of restrictions, in contrast, is aimed at protecting the rights of third parties, for instance the rights of a copyright owner. 
   For the second type of restriction, the delegator must verify for each intended delegation that the involved server and/or the intended delegatee are compliant with restriction protocols associated to a particular authorization by checking whether the server and/or the intended delegatee comprise a certificate indicating this compliance. The certificate of a delegatee may indicate for instance that the delegatee is from a specific manufacturer guaranteeing a compliance. The certificate of a server may indicate for instance that the server is either from a specific manufacturer guaranteeing a compliance or is operated by a specific operator guaranteeing a compliance. 
   The restriction protocols may be for example DRM (digital rights management) protocols. DRM is a technology that is used for enabling and controlling copyright protected digital content usage and distribution. According to known DRM protocols, a rights issuer provides a device with a DRM voucher containing rules regarding a fair use of a content. According to an embodiment of the present invention, a respective delegator forwards the contents of a received voucher to each delegatee. A compliant delegatee will then copy these rules from the voucher to each request to a server to perform a partial key operation. Because the delegatee is compliant, it can be enforced to copy these rules from the voucher. 
   Based on the received rules, the server is able to check whether the request is within the limits assigned to the authorization of the master device. The server will only perform the requested partial key operation, in case the request is determined to be within the limits assigned to the authorization of the master device. The server will obey the policies from the voucher, because it is compliant. 
   One restriction of the second type might be that there is an overall limit on the number of devices that can be brought into the authorization domain. The number should be high enough not to bother ordinary users trying to make fair use of an authorization they obtained. But it should be low enough to discourage anyone from trying to make a business even in the absence of any limits in individual DRM vouchers, or if the limits have been somehow modified. 
   For realizing the invention, the steps of the proposed method associated to a delegating device are implemented in a delegator, i.e. in a master device and possibly in addition in one or more slave devices. The steps of the proposed method associated to a delegating device are implemented in a delegatee, i.e. in one or more slave devices. The steps of the proposed method associated to a server are implemented in a server, in particular in a network server. 
   Delegator and delegatee can be any electronic device that is suited to establish a communication with other electronic devices and with a server, e.g. mobile phones, PDAs, PCs, etc. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     Other objects, features and advantages of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. 
       FIG. 1  illustrates a basic delegation of authorization in an embodiment of the method according to the invention; 
       FIG. 2  illustrates a chained delegation of authorization in the embodiment of  FIG. 1 ; and 
       FIG. 3  illustrates a combination of the delegation according to the embodiment of  FIG. 1  with DRM protocols. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates the delegation of an authorization in an embodiment of the method according to the invention. The figure presents to this end a master device  11 , a network server  12  a slave device  13  between which messages are transmitted. To the master device  11  and the slave device  13 , a respective user  14 ,  15  is associated. 
   The master device  11  is in possession of a secret key d which can be used as a secret RSA exponent for signing messages in order to obtain access to specific resources, e.g. to a bank account, or to decrypt messages encrypted using the corresponding RSA public key. The authorization to make use of the secret key d at least to some extent is to be delegated to the slave device  13  by introducing the slave device  13  into an authorization domain. 
   It is assumed that a security association between the master device  11  and the server  12 , has been established. This may be done as part of an enrolling procedure with the server. The details of how the security association is set up is out of scope for this invention. This security association, which enables a secure transmission of data between the master device  11  and the server  12 , consists of an authentication key A(master), a confidentiality key K(master) and the lifetimes of these keys. Both keys, A(master) and K(master), are keys of symmetric algorithms. 
   The messages transmitted between master device  11 , server  12  and slave device  13  belong to a master-slave delegation protocol and are indicated in  FIG. 1  by arrows I-V. Messages I, II and III represented by arrows with solid lines are employed for delegating an authorization from the master device  11  to the slave device  13 , while messages IV and V represented by arrows with dashed lines are employed for using a delegated authorization. 
   In order to obtain a membership in an authorization domain, the slave device  13  first requests the user  15  to enter a password and generates a random number t′. The slave device  13  then computes a password verification value b by applying a function g on values t′ and, i.e. b=g(t′, ). The applied function g is a keyed hash function, for example HMAC-SHAl. Next the slave device  13  transmits a membership request along with value b to the master device  11 . Due to random value t′, the password verification value b reveals no information about the password to the master device  11 . This allows the user  15  of the slave device  13  to use the same long-term password for other purposes, too. 
   Upon receipt of the membership request, the master device  11  asks its user  14  whether the request is to be granted. The user  14  can consent to the request by entering a valid password. 
   In case the user  14  consents to the request, the master device  11  then generates an identity value ID by which the server  12  can identify a specific security association that will be established between the server  12  and the requesting slave device  13 . The master device  11  further generates a random authentication key A(ID) and a random confidentiality key K(ID). Keys A(ID) and K(ID) form the cryptographic parameters of the security association that will be shared between the slave device  13  and the server  12 . 
   The master device moreover generates a random number v. The master device  11  then computes a first half-key d 1  by using generated random number v and received random number b as variables in a keyed hash function f, i.e. d 1 =f(v,b). By using the random number v in addition to received random number b for calculating first half-key d 1 , the master device  11  does not have to trust the pseudorandom generator of the slave device  13 . The master device  11  further calculates a second half-key d 2  as the difference between the available key d and the computed first half-key d 1 , i.e. d 2 =d−d 1 . Finally, the master device  11  generates a disabling key u. The disabling key u can be generated for example by applying a cryptographic hash function on some random number t. If t is sent to the server  12 , it will mark the half-key d 2  as revoked. 
   Next, the private values that are intended for the server  12  are encrypted at the master device  11  by the key K(master) to form a token. The included values comprise slave authentication key A(ID), slave confidentiality key K(ID), password verification value b, disabling key u, second half-key d 2  and RSA modulus N. 
   Based on token, a dedicated membership ticket for slave device  13  is created. The membership ticket is generated by authenticating the generated ID value, token and, optionally, policy data with the authentication key A(master). 
   The optional policy data has a structure comprising, for example, a delegation bound and a content bound. The delegation bound indicates the maximum number of allowed further delegations from the slave device  13  to other slave devices, as will be explained further below. The content bound, on the other hand, is used if the message to be signed or the encrypted message contains some pre-defined structure including attributes whose values can be compared against this bound. One example of usage of this bound is fixing the allowed amount of a transaction. 
   From the generated values, the values v, u, ID, A(ID) and K(ID) are now transmitted from the master device  11  to the slave device  13  in message II. Message II is transmitted via a confidential channel to the slave device  13 , since it contains secret keys A(ID) and K(ID). The confidential channel can be given by a physically secure connection or be based on a cryptographic security association between the master and the slave. This security association can be based on symmetric key algorithms or public key algorithms. When setting up such security associations users may perform the initial authentication of the devices using approaches described in the documents “Enhancements to Bluetooth baseband security”, in Proceedings of Nordsec 2001, Copenhagen, November 2001, by C. Gehrmann and K. Nyberg, or “The personal CA—PKI for a Personal Area Network”, IST Mobile &amp; Wireless Telecommunications Summit, Greece June 2002, by C. Gehrmann, K. Nyberg, and J. Mitchell. In case the security association is based on public key algorithms, the confidential channel is formed by encrypting message II using a public key belonging to the slave device  13 . The public key can be transmitted to the master device  11  for example in message I. The master device  11  must verify the authenticity of this public key before using it. In order to enable such a verification, methods described in the above mentioned two papers can be used. For a more straightforward approach, the slave device  13  may send message I including the public key and show a fingerprint of its public key on its display. The master device  11  then shows the fingerprint of the received public key on its display. Now the user(s)  14 ,  15  of the devices  11 ,  13  can check whether the two fingerprints match. If they do, the master device  11  is authorized to proceed with the delegation transaction. A user-friendly technique for displaying public key fingerprints is to use visual hashes. 
   The slave device  13  stores all values received in message II and the internally generated random value t′ to some secure persistent storage. Internally generated value b, in contrast, is deleted. The received and stored value v allows the slave device  13  to compute half-key d 1  with a keyed has function f(v, ) corresponding to the keyed hash function f(v,b) used by the master device  11  for computing half-key d 1 . A password verification value is calculated anew to this end each time it is required from a password supplied by user  15  and from random number t′ stored in the device  13 . 
   With another message III transmitted from the master device  11  to the server  12 , the required security association between the slave device  13  and the server  12  is established and the second half-key d 2  provided to the server  12 . Message III comprises to this end the generated ticket, which the server  12  verifies and stores into its database. Message III can be transmitted by the master device  11  before or after the transmission of message II. 
   Based on the values transmitted in messages II and III, the slave device  13  is now able to perform private key operations on messages independently of the master device  11 , in order to obtain access to specific resources associated to the public key of the master device  11 . 
   The usage of such a RSA private key operation will now be explained with reference to the fourth and a fifth message IV, V indicated in  FIG. 1 . 
   At the beginning of the private key operation, the user  15  of the slave device  13  is requested to enter a password, and the slave device determines a password verification value by applying the hash function g(t′, ) on stored random number t′ and received password. 
   The slave device  13  then determines a string containing the identification value ID, a label “priv_key_op” and an encryption of the message m on which the private key operation is to be performed, of an encoding value r and of password verification value. The encryption is performed using confidentiality key K(ID). The label “priv_key_op” indicates that the server  12  is to perform a private key operation as opposed to a further delegation operation, which will be explained further below. Next, the slave device applies the authentication algorithm using key A(ID) on the determined value, resulting in a value. 
   The slave device  13  then sends a partial private key operation request comprising the values and as message IV to the server  12 . 
   When the server  12  receives values and, it will search for the ID number associated to the slave device  13  in its database. Based on the ID number, the server obtains all the information that was transmitted within received from the master device for this specific slave device  13 , i.e. the values A(ID), b, u, d2, N and K(ID). Any further operation is aborted, in case the second half-key d 2  is disabled by disabling value u. 
   Subsequently, the server  12  authenticates the slave device  13 . To this end, the server  12  applies the authentication algorithm using key A(ID) to the received value and compares the result with received value. In case the compared values are not equal, the procedure is aborted. 
   The server may then decrypt the encrypted part of by means of the confidentiality value K(ID), in order to obtain message m, encoding value r and password verification value. Based on the obtained value, the server  12  now authenticates the user  15  by verifying that is equal to b, i.e. that the user  15  of the slave device  13  entered the correct password. If the server  12  can authenticate the slave device  13  but not the user  15 , the server  12  may keep count of successive incorrect password attempts. If the count exceeds a given bound, the server  12  may assume that the slave device  13  has been stolen and abort the procedure. 
   In case policy data with a content bound was comprised in the ticket provided to the server  12  for this slave device  13 , the server  12  also checks whether the values in the message m are within the limits provided for these values by the policy data. In case the values in message m are not within these limits, the procedure is aborted. 
   After a successful authentication procedures, the server  12  performs a partial private key operation on the received message m and the received encoding r based on the second half-key d 2  according to the formula=encode(m,r) d2 (mod N). 
   Since only the original master device  11  has access to the entire private key d, it cannot be assumed that slave devices  13  acting as delegators in a chained delegation, which will be described below, could perform computations modulo (N). Therefore, reduction modulo (N) proposed in the above cited document by MacKenzie and Reiter for computing the second half-key d 2  by the master device  11  was omitted in the presented embodiment of the invention. Since d 1  is generated as an output from a hash function, it may happen that d 2  is a negative integer. If this is the case, the server  12  computes first the private key operation with the positive integer −d 2 , and subsequently computes the inverse of the resulting number modulo N. With this convention, the server  12  can always perform partial private key operation, even if its exponent is a negative number. 
   Value resulting in the partial private key operation is encrypted based on confidentiality key K(ID) and provided to the slave device  13  as encrypted value in message V. 
   When the slave device  13  receives the partial private key operation response from the server  12 , it decrypts the received value with its confidentiality key K(ID). Further, it generates the first half-key d 1  using the stored value v and the recently generated value by applying the above mentioned function f(v, ). 
   The slave device applies the obtained half key d 1  on the message m and combines it by multiplication with the result of the partial private key operation received from the server according to the formula s=encode(m,r) d1 (mod N). The result of this computation is the desired result s, if s e ≡encode(m,r) (mod N). This provides also an implicit authentication of the server  12 . In case the last verification is positive, the slave device  13  may transmit the values s and r to the server providing the desired resources. 
   The protocol described with reference to  FIG. 1  allows the master device  11  to delegate its rights to a slave device  13 , which slave device  13  is thereby introduced into the authorization domain. There is no technical limitation on the number of slave devices that the master device  11  may introduce in this way into the authorization domain. 
   In the presented embodiment of the invention, a slave device  13  which is a member of the authorization domain may also introduce other slave devices into the authorization domain. This aspect of the embodiment of the invention will now be described with reference to  FIG. 2 . 
   In  FIG. 2 , master device  11 , server  12  and slave device  13  of  FIG. 1  are depicted again. In addition, a second slave device  23  is shown. 
   Based on the initialization procedure described with reference to  FIG. 1 , the first slave device  13  is able to calculate half-key d 1 , while the server  12  is in possession of a complementary half-key d 2 . 
   The first slave device  13  is allowed to further delegate the received authorization to the second slave device  23  without having to involve the master device  11 , unless the master device  11  transmitted policy data to the server  12  indicating that a further delegation is not allowed. 
   The procedure for the chained delegation corresponds basically to the procedure explained with reference to  FIG. 1 , except that the first slave device  13  takes the role of the master device  11 . Therefore, only the differences in the processing will be described in detail. A difference is due to the fact that the first slave device  13  is only able to calculate half-key d 1 , thus it is not in possession of the entire secret key d like the master device  11 . Further, the first slave device  13  has to be allowed to further delegate the authorization. 
   Upon a delegation request by the second slave device  23  with a message corresponding to message I of  FIG. 1 , the first slave device  13  generates a further first half-key d 11  based on a random number and provides this random number to the second slave device  23  in a message corresponding to message II of  FIG. 1 . Moreover, the first slave devices  13  calculates a value d′ 21  with d′ 21 =d 1 −d 11  and transmits it in a message corresponding to message III of  FIG. 1  to the server  12 . Next, the server  12  checks the number of delegations already made by the first slave device  13  and compares this number to the delegation bound which was received before as policy data from the master device  11 . If this number exceeds the delegation bound, then the server  12  does not allow the delegation. 
   In case the delegation is allowed, the server  12  adds the stored value of first half-key d 2  to the newly received value d′ 21  to obtain a value d 21  as further second half-key. Obviously, the resulting further second half-key d 21  is d 21 =d 2 +d′ 21 =d 2 +d 1 −d 11 =d−d 11 . Thereby, the second slave device  23  becomes a member of the authorization domain, because the second slave device  23  and the server  12  possess half-keys d 11 , d 21  which allow them to share the RSA private key function. A private key operation is performed exactly as with messages IV and V explained above, where values d 1  and d 2  are substituted by values d 11  and d 21 . 
   As becomes apparent, the described embodiment of the invention maintains the advantages of the method presented by MacKenzie and Reiter in the above cited document. As in the solution of this document, the presented method according to the invention involves minimal invasiveness, since it does not require an agreement from communication partners. Communication partners are not aware that a signature was constructed or that an encrypted message will be decrypted using the assistance of a network server. As in the solution by MacKenzie and Reiter, a minimal trust on the network servers is required, since the server by itself cannot use the private key. It only has to be trusted that the server will stop co-operating with a slave device if the disabling key for that slave device is disclosed and that the server obeys the requested policies. Since the server by itself cannot perform a complete private key operation, it is also a less attractive point of attack. Further, if a device is lost, stolen or removed from the authorization domain, it is not necessary to change the domain keypair. It is also not required to revoke the public key, i.e. to inform all peers who use the public key or certify it. As in the solution by MacKenzie and Reiter, the server verifies both, the user and the device, before the device is allowed to use an authorization. 
   In addition, the described method according to the invention does not put any technical restrictions on the number of devices that may become members of the authorization domain. In particular, a chained delegation between slave devices is enabled. The chained delegation does not require the availability of the master device. Still, the master device can restrict the usage of its secret key by providing appropriate policy data to the server. Each delegating party can add its own policies indicating whether it does or does not want to provide further delegation rights. The user of the respective delegatee can moreover choose a new password, or use an old password. The password itself is never revealed to the respective delegator or to the server. 
   It is to be noted that the described embodiment constitutes only one of a variety of possible embodiments of the invention, and also the described embodiment can be varied in many ways. A selection of possible variations will be presented in the following. 
   In the described embodiment of the invention, secret key d is split by the master device into half keys of equal size. In contrast to this approach, the workload of either the server or the slave device could be minimized by making its half-key particularly small, e.g. 1/10 th  of the size of the original key. 
   In the described embodiment of the invention, the master device chooses the values ID, u, A(ID) and K(ID). Alternatively, these values could be chosen as well by the server or by the slave device. If the server chooses these values, the protocol has to be interactive, i.e. the server must participate in the delegation process because these values have to be provided to the master device before message II. However, in case the master device chooses these values by itself as proposed, it does not have to rely on the quality of randomness available to the other entities. 
   In the described embodiment of the invention, the policy data is included directly in the membership ticket, i.e. without encryption. In case the policy data should remain confidential, it is also possible to include it in the data that is encrypted to token. 
   In the described embodiment of the invention, the membership ticket is provided directly from the respective delegator to the server. In an alternative approach, the membership ticket could also be provided to the server via the respective delegatee. In  FIG. 1 , for example, the membership ticket generated by the master device  11  could be transmitted to the slave device  13  in message II. The slave device  13  then forwards the membership ticket to the server  12  in message IV. In case is provided online, i.e. together with a request for a partial private key operation, the server must verify and decrypt every time when the slave device requests a partial private key operation. This can be avoided by storing the membership ticket in the server the first time the slave device transmits such a request to the server. Thereafter, does not have to be provided again. 
   In another alternative, the membership ticket could be provided from the respective delegator directly to the server each time the respective delegatee requests a partial private key operation from the server, i.e. not in an initializing step as in the above described embodiment of the invention. 
   In any case, the generation of the ticket is separated from the use of the ticket. 
   In the above cited document by MacKenzie and Reiter, a random string is employed, which is used as a one time pad for encrypting the result of the partial private key operation before it is sent from the server to the device. In the above described embodiment of the invention, instead an encryption of the result with a confidentiality key K(ID) is employed. This is not necessary. The computational workload of the server can be further reduced, if the slave provides the server with such a one time pad encrypted as part of the string in message IV to be used by the server to encrypt its reply message V to the slave device. 
     FIG. 3  illustrates a further embodiment of the invention which is integrated in a DRM system. 
   For the DRM system, it is assumed that each involved device has a public/private key pair. It is further assumed that a specific manufacturer provides each device with a device certificate for the public key of the device. The device certificate assures compliance to DRM specifications. Finally, the DRM relevant portions of each device are assumed to be tamper resistant. 
   In a conventional DRM system, a device first requests and receives a desired content from a content provider. In order to be able to make use of the content, the devices requires a content key. The device transmits its certificate to a rights issuer. The right issuer verifies the received certificate of the requesting device. In case it turns out to be valid, the rights issuer transmits a DRM voucher to the requesting device. The DRM voucher contains rules for DRM and the content key encrypted with the public key of the device. The device is then able to decrypt the key by means of a trusted DRM software that can access the RSA private key of the device. Finally, the device is able to decrypt the received content with the decrypted content key K and to make use of the received content within the scope of the DRM rules. 
   In the embodiment illustrated in  FIG. 3 , this DRM concept is combined with the approach presented with reference to  FIGS. 1 and 2 . 
     FIG. 3  shows a master device  11 , a network server  12 , a first slave device  13  and a second slave device  33 . 
   A user of the master device  11  has bought a permission to use a content available on a content server by transmitting the certificate of the master device  11 . Before a rights issuer gave permissions to a content, the compliance of the master device  11  was checked from its certificate. Similar as in the embodiments described with reference to  FIGS. 1 and 2 , the master device  11  is in possession of a secret key d which can be used as secret RSA exponent for decrypting a message which is encrypted using the corresponding RSA public key. In this case, the message is a content key which is required to decrypt the bought content. The authorization to make use of the secret key d at least to some extent can be delegated to a slave device  13 ,  33  by introducing it into an authorization domain. 
   Before the master device  11  performs the delegation as described above with reference to  FIG. 1 , including the splitting of key d, it first checks the certificates of the server  12  and of the respective slave device  13 ,  33 . More specifically, the master device  11  determines whether the slave device  13 ,  33  is from a predetermined manufacturer assuring compliance of all issued devices to the DRM specifications and whether the server  12  is from a predetermined manufacturer or operated by a predetermined operator assuring compliance of all servers to the DRM specifications. In case the checked certificates assure compliance of the slave device  13 ,  33  and of the server  12  to the DRM specifications, the master device  11  further indicates its own capability to render content in a DRM voucher and transmits this voucher to the server  12  and to the slave device  13 ,  33 . In addition, the master device  11  indicates to the slave device  13 ,  33  and to the server  12  delegation limits for the slave device  13 ,  33 . These limits may comprise the length of the remaining delegation chain and the number of delegations the slave device  13 ,  33  could make. 
   Only then, the master device  11  introduces the slave device  13 ,  33  into the authorization domain as described with reference to  FIG. 1 . The delegation operation again comprises creating a dedicated membership ticket for the slave device  13 ,  33  and sending this ticket to the server  12 , either directly or via the respective slave device  13 ,  33 . 
   When one of the slave devices  13  in the authorization domain wishes to use a copyright-protected, encrypted content, it retrieves this content from the content server. Since the content is encrypted, the slave device  13  requires a content key for the decryption. Before the slave devices  13  in the authorization domain can transmit a partial private key operation request to the server  12  as described above with reference to  FIG. 1 , however, the slave devices  13  copies the policy from the DRM voucher received from the master device  11  into its request. The request for a partial decryption transmitted from the slave devices  13  to the server  11  thus includes an encrypted content key as message m and in addition policies from the DRM voucher. 
   The server  12  receives the request. Before it replies as described above with reference to  FIG. 1 , the server  12  compares the policy included in the request with the current state, in order to check whether the slave device  13  is allowed to access the desired content. The server  12  may compare for instance how many devices are allowed to use the content concurrently according to the DRM policy and how many devices are already using the content. 
   In case the server  12  detects that the request is within the allowed limits, it transmits a reply to the requesting slave device  13  which includes the requested partial decryption of the content key. If the premises of the rules are not satisfied, the server  12  returns an error indication to the requesting slave device  13 . 
   In case the slave devices  13  receives a partially decrypted content key, it can decrypt the other part of the content key using its own half-key from the master device  11 . The slave device  13  then finalizes the decryption of the content key so that it can decrypt and access the content. 
   As in the embodiments described with reference to  FIGS. 1 and 2 , the server only has half-keys and is thus not able to access a content key and thereby a protected content by itself. At the same time, a fair use of protected content can be ensured with the embodiment described with reference to  FIG. 3 . 
   In each of the described embodiments, the master device  11 , the slave devices  13 ,  23 ,  33  and the server  12  obviously comprise a processing component for performing the processing described for the respective unit, a storage component for storing all values required at the respective unit for the described processing, and a communication component for performing the described exchange of data with a respective other unit. 
   Although the invention has been shown and described with respect to best mode embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention.