Patent Publication Number: US-7716722-B2

Title: System and method of proxy authentication in a secured network

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This is a continuation of and priority is claimed to co-pending U.S. Patent Application having Ser. No. 09/490,199, a filing date of Jan. 24, 2000, for “S YSTEM AND  M ETHOD OF  P ROXY  A UTHENTICATION IN A  S ECURED  N ETWORK ” of Swift et al. This co-pending U.S. Patent Application is commonly assigned herewith and is hereby incorporated herein by reference for all that it discloses. 

   BACKGROUND OF THE INVENTION 
   In a computer network environment, access to services is often restricted for security reasons, and a user attempting to access a server has to be authenticated before access can be granted. In many network systems, the user authentication process is based on the password of the user. User authentication prevents a malicious attacker from illegally gaining access to services by pretending to be an authorized user. 
   The requirement of user authentication for network security, however, can conflict with the need for delegation or appointing a proxy. There are many occasions in which a user intends to have another user or a service perform tasks on her behalf when she is not logged onto the network. For example, a user who will be out of her office for an extended period of time may want another user to access her files during her absence. Access to those files may be restricted for only the first user and requires user authentication based on her password. Due to security concerns, the first user may not want to give her password to the second user. Without her password, however, the second user will not be able to access the files of the first user. If the first user does decide to give her password to the second user, she will be taking the risk that second user may use her password for other unauthorized purposes in or after the supposed duration of the proxy authorization. 
   As another example, a user may submit a batch job to a batch service and then log off, expecting the batch service to run the batch job in the background without further attention of the user. To run the batch job, it is likely that the batch service will have to access services that the user submitting the batch job is authorized to access. Due to the requirement of user authentication for accessing services, however, the batch service cannot gain access to those services by simply holding itself out as the batch user. 
   Existing batch services solve this problem by taking approaches that are not satisfactory. Under one approach, the requirement for user authentication is simply waived. The operating system of the service is told to perform subsequent actions on the batch user&#39;s account without requiring authentication from that user. The problem of this approach is that all the computers running the batch jobs must be highly trusted and secure, because they can act as the user without authentication while the batch job is running. An alternative is to modify permissions on the objects that the batch job needs to access. It is, however, difficult to know in advance which objects will be accessed. Furthermore, the user may not have the authority to change the access permissions of the objects, such as when the security of the objects is managed by someone else. 
   Under another existing approach, the user&#39;s password is given to the batch service, which stores the password and uses it to instantiate the batch Job. The aspect of requiring the user to give her password to another entity, in this case the batch service, causes serious security concerns. There may be many batch jobs submitted to the batch service by different users, and the batch service will store the passwords of all of those batch users. If an attacker breaks into the batch service, he will find out the passwords of all the batch users. He can then act as any of those batch users and authenticate properly because he knows the password of that user. 
   Besides the security concerns, this approach may also encounter problems when the user who submitted a batch job changes her password. Many batch jobs are run periodically or continuously for months or even years. During the lifetime of the batch job, it is possible that the user will change her password one or more times. If the user changes her password but forgets to notify the batch service of the new password, the batch service can no longer authenticate itself as the user. As a result, access to the services will be denied and the batch job will fail. 
   Accordingly, there is a need for a mechanism for one user in a secured network to allow another user or a service to act as her without requiring the first user to divulge her password or other secrets, and preferably such mechanism allows such delegation or proxy to operate for an extended period without being affected by the user&#39;s changing her password. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, the present invention provides a method and system of controlling access to services in a network that enables an authorized proxy client to access a service on behalf of a user. To permit the proxy client to function as a proxy, the user first registers with a trusted security server proxy authorization information that identifies the proxy client and specifies the extent of proxy authority granted to the proxy client. When the proxy client wants to access a target service on behalf of the user, it sends a proxy access request to the trusted security server. The trusted security server checks the proxy authorization information of the user to verify whether the request is within the proxy authority granted to the proxy client. If so, the trusted security server returns to the proxy client a data structure containing information recognizable by the target service to authenticate the proxy client for accessing the target service on behalf of the user. 
   Additional features and advantages of the invention will be made apparent from the following detailed description of illustrative embodiments, which proceeds with reference to the accompanying figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the appended claims set forth the features of the present invention with particularity, the invention, together with its objects and advantages, may be best understood from the following detailed description taken in conjunction with the accompanying drawings of which: 
       FIG. 1  is a block diagram generally illustrating an exemplary computer system usable for forming a network system on which the present invention may be implemented; 
       FIG. 2  is a schematic diagram showing a system in which a proxy client is permitted to access a target service on behalf of a user in accordance with the invention; 
       FIG. 3  is a schematic diagram showing a network that implements network security based on the use of session keys and tickets for authentication; 
       FIG. 4  is a schematic diagram showing data exchange between a client and a key distribution center for establishing communication therebetween; 
       FIG. 5  is a schematic diagram showing a client requesting a session ticket for accessing a server; 
       FIG. 6  is a schematic diagram showing a client presenting a session ticket to a server for authentication. 
       FIG. 7  is a schematic diagram showing an embodiment of the invention in which a user registers proxy authorization information with a key distribution center; 
       FIG. 8  is a schematic diagram showing the embodiment of  FIG. 7  in which a proxy client obtains a ticket for accessing a target service on behalf of a user; and 
       FIG. 9  is a flowchart showing a process of proxy authentication in the embodiment of  FIGS. 7 and 8 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Turning to the drawings, wherein like reference numerals refer to like elements, the invention is illustrated as being implemented in a suitable computing environment. Although not required, the invention will be described in the general context of computer-executable instructions, such as program modules, being executed by a personal computer. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that the invention may be practiced with other computer system configurations, including hand-held devices, multi-processor systems, microprocessor based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. The invention may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules maybe located in both local and remote memory storage devices. 
   With reference to  FIG. 1 , an exemplary system for implementing the invention includes a general purpose computing device in the form of a conventional personal computer  20 , including a processing unit  21 , a system memory  22 , and a system bus  23  that couples various system components including the system memory to the processing unit  21 . The system bus  23  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The system memory includes read only memory (ROM)  24  and random access memory (RAM)  25 . A basic input/output system (BIOS)  26 , containing the basic routines that help to transfer information between elements within the personal computer  20 , such as during start-up, is stored in ROM  24 . The personal computer  20  further includes a hard disk drive  27  for reading from and writing to a hard disk  60 , a magnetic disk drive  28  for reading from or writing to a removable magnetic disk  29 ,and an optical disk drive  30  for reading from or writing to a removable optical disk  31  such as a CD ROM or other optical media. 
   The hard disk drive  27 , magnetic disk drive  28 , and optical disk drive  30  are connected to the system bus  23  by a hard disk drive interface  32 , a magnetic disk drive interface  33 , and an optical disk drive interface  34 , respectively. The drives and their associated computer-readable media provide nonvolatile storage of computer readable instructions, data structures, program modules and other data for the personal computer  20 . Although the exemplary environment described herein employs a hard disk  60 , a removable magnetic disk  29 ,and a removable optical disk  31 , it will be appreciated by those skilled in the art that other types of computer readable media which can store data that is accessible by a computer, such as magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, random access memories, read only memories, and the like may also be used in the exemplary operating environment. 
   A number of program modules may be stored on the hard disk  60 , magnetic disk  29 , optical disk  31 , ROM  24  or RAM  25 , including an operating system  35 , one or more applications programs  36 , other program modules  37 , and program data  38 . A user may enter commands and information into the personal computer  20  through input devices such as a keyboard  40  and a pointing device  42 . Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to the processing unit  21  through a serial port interface  46  that is coupled to the system bus, but may be connected by other interfaces, such as a parallel port, game port or a universal serial bus (USB). A monitor  47  or other type of display device is also connected to the system bus  23  via an interface, such as a video adapter  48 . In addition to the monitor, personal computers typically include other peripheral output devices, not shown, such as speakers and printers. 
   The personal computer  20  may operate in a networked environment using logical connections to one or more remote computers, such as a remote computer  49 . The remote computer  49  may be another personal computer, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to the personal computer  20 , although only a memory storage device  50  has been illustrated in  FIG. 1 . The logical connections depicted in  FIG. 1  include a local area network (LAN)  51  and a wide area network (WAN)  52 . Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets and the Internet. 
   When used in a LAN networking environment, the personal computer  20  is connected to the local network  51  through a network interface or adapter  53 . When used in a WAN networking environment, the person computer  20  typically includes a modem  54  or other means for establishing communications over the WAN  52 . The modem  54 , which may be internal or external, is connected to the system bus  23  via the serial port interface  46 . In a networked environment, program modules depicted relative to the personal computer  20 , or portions thereof, may be stored in the remote memory storage device. It will be appreciated that the network connections shown are exemplary and other means of establishing a communications link between the computers may be used. 
   In the description that follows, the invention will be described with reference to acts and symbolic representations of operations that are performed by one or more computers, unless indicated otherwise. As such, it will be understood that such acts and operations, which are at times referred to as being computer-executed, include the manipulation by the processing unit of the computer of electrical signals representing data in a structured form. This manipulation transforms the data or maintains it at locations in the memory system of the computer, which reconfigures or otherwise alters the operation of the computer in a manner well understood by those skilled in the art. The data structures where data is maintained are physical locations of the memory that have particular properties defined by the format of the data. However, while the invention is being described in the foregoing context, it is not meant to be limiting as those of skill in the art will appreciate that various of the acts and operation described hereinafter may also be implemented in hardware. 
   Referring now to  FIG. 2 , the present invention is directed to a mechanism by which a user  70  of a computer network  72  is able to give proxy authorization to a client  74  in the network, and the authorized client is able to authenticate itself for accessing a service  76  on behalf of the user, without having to give confidential security data of the user, such as the user&#39;s password, to the proxy client for the purpose of authenticating the client by the service. To give the proxy authorization, the user  70  submits a proxy registration request  78  to a trusted security server  80 . The proxy registration request  78  provides proxy authorization data that identify one or more clients permitted to act as a proxy of the user and specify the extent of the proxy, such as which services a proxy client is allowed to access on the user&#39;s behalf and the lifetime of the proxy authorization. The proxy authorization data  82  are stored by the trusted security server  80  in its database. 
   When the proxy client  74  acting as a proxy of the user  70  needs to access the target service  76 , it first sends a proxy access request  84  to the trusted security server  80 . The proxy access request  84  identifies the user  70  that authorized the proxy and the target service  76  the proxy client wants to access. After receiving the proxy access request  78 , the trusted security server  80  retrieves the proxy authorization data of the specified user and determines whether the proxy access request should be allowed. In doing so, the trusted security server  80  considers, for example, whether the client  74  or a group that includes the client is identified in the authorization data, whether restrictions specified in the authorization data apply to the client or the target service  76 , and whether the term of the proxy authorization has expired. 
   If the trusted security server  80  determines that the proxy client  74  should be permitted to access the target service  76  on behalf of the user  70 , it creates a data structure  90  containing authentication information recognizable by the target service  76  for authenticating the proxy client acting as the user to access the target service. The format and contents of the proxy authentication data structure  90  depends on the security protocols used and their implementations in the network system. For instance, the proxy authentication data structure may be in the form of a “ticket” containing data encrypted with a secret key of the target service as in a preferred embodiment, or in other forms that may be commonly referred to as “certificates” or “capabilities.” In a preferred embodiment, the proxy authentication data structure may be given to the proxy client. The proxy client  74  can then include the data structure in an access request  92  as part of its credentials presented to the target service  76  for authentication. Alternatively, the proxy authentication data structure may be given to the target service  76 , which stores it for authenticating the proxy client when the proxy client attempts to access the target service. 
   In a preferred embodiment, the proxy access control of the invention is built upon a network security system that implements the Kerberos version 5 authentication protocol. To facilitate a description of this embodiment, the basic operation of the Kerberos authentication process is first described below. 
   Referring to  FIG. 3 , the Kerberos authentication protocol provides a mechanism for mutual authentication between a client  100  and a server  102 , or between one server and another, before a network connection is opened between them. The protocol assumes that the initial transactions between the client  100  and the server  102  take place on an open network  104  where most computers are not physically secure and packets traveling between them may be monitored and modified by others. 
   The Kerberos protocol relies heavily on an authentication technique involving shared-key cryptography. Specifically, communication partners share a secret cryptography key. One party proves knowledge of the secret key by encrypting a piece of information, the other by decrypting the encrypted information and verifying the validity of the information. The secret key for the communication is distributed by a Kerberos key distribution center (KDC)  106 , which functions as a trusted entity in this authentication scheme. The KDC  106  is a service that runs on a physically secured server. It maintains a database  108  with account information for all security principals in its “realm” or domain. Along with other information about each security principal, the KDC stores a cryptography key known only to that security principal and KDC. This key is used in exchanges between the security principal and the KDC and is known as a “long-term key.” In most implementations, the long-term key is derived from a user&#39;s logon password. 
   Under the Kerberos protocol, when a client wants to establish a session with a server, it has to first obtain a session key and a session ticket for authentication purposes. The session key and the session ticket are issued by the KDC upon request by the client. Since the KDC is itself a server, the client has to obtain a session key and a session ticket for transactions with the KDC before it can request for session keys and tickets for other services. To that end, the user of the client has to be authenticated based on her password. When a user  70  logs on, the Kerberos client  100  on her computer accepts her password and converts it into a cryptographic key by passing the text of the password through a one-way hash function. The result is the user&#39;s long-term key. The KDC  106 , on the other hand, gets its copy of the user&#39;s long-term key  110  from the record about the user in its database. Immediately after accepting the user&#39;s password and deriving the user&#39;s long-term key, the Kerberos client  100  on the user&#39;s computer sends a request  112  to the KDC  106  for a session key and a session ticket for use in subsequent transactions with the KDC during this logon session. The request  112  identifies the user  70 . Depending on the specific implementation of the Kerberos protocol, the request  112  may also contain data encrypted with the user&#39;s long-term key for proving to the KDC that the user knows her password. 
   When the KDC  106  receives a request from the Kerberos client on the user&#39;s computer, it searches its database for the user, retrieves the user&#39;s account record, and takes the long-term key from a field in the record. If the request contains encrypted data, the KDC may also decrypt the data with the long-term key of the purported user. The KDC  106  responds to the request  112  by returning a session ticket for itself. This special session ticket  114  for communicating with the KDC is called a “ticket-granting ticket” (“TGT”). Like an ordinary session ticket, a TGT  114  contains a copy of the session key that the server, in this case the KDC  106 , will use to communicate with the client  100 . Like any other session key, the session key in the TGT is for temporary use, valid only until the TGT expires or the user logs off. For this reason, it is called a “logon session key.” The TGT is encrypted with the long-term key of the KDC. The response also includes a copy of the logon session key  116  encrypted with the user&#39;s long-term key. 
   When the client  100  receives the KDC&#39;s reply  118  to its initial request  112 , it uses its cached copy of the user&#39;s long-term key  120  to decrypt the encrypted copy of the logon session key  116  in the reply. It may then discard the long-term key derived from the user&#39;s password since it is no longer needed, and use the logon session key in all subsequent exchanges with the KDC  106 . 
   Referring now to  FIG. 5 , when the client  100  wants to access to a server  102  other than the KDC, it sends a request  126  to the KDC  106 . The KDC  106  responds to the request  126  by sending two copies of a cryptographic key, one for the requesting client and the other for the server, to be used in a session between the client and the server. This key is accordingly referred to as the “session key.” The client&#39;s copy of the session key  128  is encrypted with the logon session key  116  that the client obtained earlier from the KDC. The server&#39;s copy of the session key is embedded along with information about the client in a data structure referred to as a session ticket  130 . The session ticket  130  is encrypted with the long-term key that is shared by the KDC with the server. In this way, the client is given a session key and an encrypted session ticket that can be used to establish communication with the server. The session ticket  130 , with the server&#39;s copy of the session key safely embedded therein, is the client&#39;s responsibility to manage. In this regard, the KDC  106  is simply providing a ticket-granting service and is not concerned with whether the server ever receives the session key. 
   When the client  100  receives the KDC&#39;s reply  132  to its request  126 , it extracts its copy of the session key  128  and the session ticket from the reply and puts both aside in a secure cache. Turning now to  FIG. 6 , when the client  100  wants to access the server  102 , it sends the server a connection request  134  that includes the session ticket  130 , which is still encrypted with the server&#39;s logon session key shared with the KDC, and an authenticator  136 , which contains data that can be validated by the server. For instance, the authenticator may include a time stamp encrypted with the session key  128 . The session ticket  130  and the authenticator  136  together form the client&#39;s credentials presented to the server  102  for authentication purposes. 
   When the server  102  receives the connection request  134  from the client  100 , it decrypts the session ticket therein with its logon session key  138  issued by the KDC  106  to extract the session key in the ticket. It then uses the session key to decrypt the authenticator  136 . If the decryption is successful and the contents of the authenticator are valid, the server  102  knows that the client&#39;s credentials were issued by a trusted authority, namely the KDC  106 . If the client  100  has asked for mutual authentication, the server  102  uses the session key to encrypt a portion of the data in the client&#39;s authenticator, such as the time stamp, and sends the encrypted portion to the client as the server&#39;s authenticator. The server is authenticated when the client successfully decrypts the server&#39;s authenticator and verifies the data therein. 
   The Kerberos protocol includes three subprotocols. The subprotocol in which the KDC gives a client a logon session key and a TGT is known as the Authentication Service (AS) Exchange. The subprotocol in which the KDC distributes a service session key and a session ticket for a requested service to the client is known as the Ticket-Granting Service (TGS) Exchange. The subprotocol in which the client presents the session ticket for admission to a service is called the Application Protocol (AP) Exchange. 
   With an understanding of the basic security scheme under the Kerberos version 5 authentication protocol as described above, the implementation of a preferred embodiment of the invention built upon the Kerberos system will now be described. In this embodiment as shown in  FIGS. 7 and 8 , a user  70  intends to give proxy authority to a client  74  or a group of clients to act on her behalf to perform a task that requires access to a target service  76 . In this embodiment, a Kerberos key distribution center (KDC)  106  functions as the trusted security server  80  in the architecture shown in  FIG. 2 . The proxy authentication procedure begins when the user  70  sends a proxy registration request  78  to a directory service  140  to create a proxy entry. The request  78  contains information to be included in the proxy entry to specify who is allowed to be a proxy and what the proxy is allowed to do. Specifically, the proxy entry preferably includes the following data:
         an identification of the user;   the security identifier (SID) of a security principal that is permitted to act on behalf of the user (i.e., be a proxy of the user);   the duration of the permission; and   restrictions on the permission.       

   The security principal identified in the proxy entry may be either a client on the network or a group of clients. The data on the restrictions on the proxy permission may indicate, among other things, which target services the proxy is permitted to access on behalf of the user  70 . By way of example, the proxy entries for the user may contain the following data: 
   Proxy Permission Entries for User ## 1   
   GrantedAccountSID 1 , 20-may-1999, all services 
   GrantedGroupSID 2 , 31-dec-2001, Idap/ntdev.Microsoft.com 
   where GrantedAccountSID 1  is the SID of a client permitted to be a proxy of the user, and GrantedGroupSID 2  is the SID of a group of clients, each of which is allowed to be a proxy. The dates in this example indicate the expiration dates for the proxy permissions for the respective principals. The third field of the proxy entry in this example identifies the services the proxy is allowed to access on behalf of the user. Such restriction data may be specified in many different ways with different granularity. For instance, this field may be used to specify groups or individual clients that the proxy can access. Alternatively, the field can provide a negative restriction by identifying those services the proxy should not be allowed to access. The field may identify specifically the directory the proxy is allowed to access, as in the second proxy entry in the example. 
   The request  78  by the user  70  to create a proxy entry is sent directly to the KDC  106 . In one embodiment, the transmission is over an RPC connection. Other network communication protocols, such as those based on sockets or datagrams, may be used for such transmission. After receiving the request, the KDC  106  creates the proxy entry  142  using the information in the request and stores the proxy entry in encrypted form under the requesting user&#39;s account in its directory. This proxy authorization information is secured in the sense that it is not accessible from outside the directory service or under any other protocol. This ensures that a user may grant proxy authorization only on her own account, and no user is allowed to grant proxy authorization on another user&#39;s account. In this regard, the authentication process by the KDC  106  on the user that sent the request ensures that the purported user making the request is in possession of his or her password. The authentication of the requesting user further prevents a user from maliciously granting herself proxy permissions on another user&#39;s account. The interface with the KDC allows a user to retrieve the current list of granted proxy permissions on her account and to manage the list by modifying the proxy entries. 
   Instead of having the users register proxy authentication for selected proxies, a system administrator may also be allowed to register proxy authentication regarding a given user. In this way, the administrator can enable certain trusted services to act as proxies for selected users. For example, the administrator can register a web server that supports a variety of authentication methods as a proxy for all of its Internet clients. The web server in its proxy capacity can then be used to allow Internet clients using different authentication methods to communicate with a target service that uses only one authentication protocol, such as the Kerberos protocol. 
   Once the proxy authorization for the client  74  or a group including the client has been registered with the KDC  106 , the client can request a ticket for accessing a target service  76  on behalf of the user. To that end, the client  74  first establishes authenticated communication with the KDC  106 . As implemented in a preferred embodiment, the client  74  constructs an authentication service (AS) ticket request  150  for the user&#39;s account according to the normal format of the Kerberos protocol. The client then inserts into the request  150  a piece of pre-authentication information that includes a ticket granting ticket  152  issued to the client  70  for the user&#39;s domain, an authenticator  154  for that ticket, and an annotation string  156  describing the purpose of the request, namely to obtain permission for the client to act as a proxy for the user. The client  74  then sends the AS ticket request with the pre-authentication data to the user&#39;s KDC  106 . It should be noted that at this time the user may no longer be logged onto the network. 
   When the KDC  106  receives the AS ticket request  150  from the client  74 , it pulls the user&#39;s name from the request and retrieves proxy entries of that user from its database. It then pulls out the requesting client&#39;s ticket-granting ticket  152  and verifies it according to the normal Kerberos protocol. The KDC  106  also verifies that the authenticator  154  in the request is valid. The authenticator proves that the requesting client is allowed to use the ticket-granting ticket included in the request. Thus, the client is authenticated, i.e., it is shown to be who it claims to be, rather than acting as someone else. 
   After the client  74  has been authenticated, the KDC  106  constructs the requesting client&#39;s SID as well as a list of SIDs of the groups of which the client is a member. The KDC then goes through the list  158  of proxy permissions of the user identified in the proxy ticket request  150 , checking each entry to determine: (1) whether the entry&#39;s SID matches the client&#39;s SID or one of its group SIDS; (2) whether the entry&#39;s allowed target list matches the target service specified in the request; (3) whether the entry is still valid, i.e., the lifetime of the proxy permission as specified in the entry has not expired. If no entry passes these inquiries, the client&#39;s request will be denied. 
   On the other hand, if one or more matches are found, the KDC finds the most general proxy entry that matches the client&#39;s request, preferably first by range of services and second by expiration date. Based on that entry, the KDC constructs a ticket  160  for the client that allows the client to access the target service  76  on behalf of the user  70 . In this regard, the proxy client may either request a ticket-granting ticket (TGT) that allows it to make a subsequent request for a session ticket for accessing a target service, or may directly request a session ticket for a specific target service. When a TGT is requested, the KDC can store in the TGT for its own reference information regarding what the TGT may be used for. The ticket returned by the KDC in response to the AS request with the proxy client&#39;s TGT is the “proxy ticket.” The proxy ticket is set to expire no later than the lifetime of the proxy permission of the selected entry. In the authorization data field of the proxy ticket  160 , the KDC includes a record containing the following information:
         the annotation string from the client&#39;s request;   the name of the client that sent the request;   the realm of the client;   the security identifier (SID) of the client.
 
In an alternative embodiment, rather than including only the name, domain, and SID of the proxy client, all of the group SIDS for the proxy client may be included as well.
       

   The KDC then constructs a reply message  164  in response to the request. The reply message  162 , including the proxy ticket  160  and a session key  164 , is encrypted using the session key from the requesting client&#39;s ticket-granting ticket  152 . It is to be noted that under the regular Kerberos procedure a reply to an AS ticket request will be encrypted with a long-term key derived from the requesting user&#39;s password. By using the proxy client&#39;s logon session key in the TGT  152  to encrypt the reply, the client is able to use the proxy ticket  160  without knowing the user&#39;s password. 
   In one embodiment, the proxy ticket  160  is a ticket-granting ticket for the proxy client acting as the user. After the proxy client  74  has obtained a ticket granting ticket as a proxy of the user, it may then request additional tickets to additional services and authenticate itself to those services as the user. 
   In an alternative embodiment, the proxy ticket may be a session ticket that the proxy client  74  can use to directly authenticate to the target service  76 . The authentication data in the ticket allows the target service  76  to recognize that the access request is coming from an authorized proxy instead of the user, and to identify the proxy client. This information may be stored and later audited to assign responsibility to actions performed by the proxy under the name of the user. 
   The proxy authentication process in the embodiment described above is summarized in  FIG. 9 . To allow a proxy client to act on her behalf, a user first registers proxy authorization data with a KDC of its domain (step  178 ). When the proxy client wants to access a target service on behalf of the user, it sends a proxy permission request to the KDC of the user&#39;s domain (step  180 ). The request includes a TGT for the client so that the client can be authenticated by the KDC. When the KDC receives the request, it first authenticates the client based on the TGT (step  182 ). It then checks the proxy authorization information provided by the user to see whether the proxy client has been authorized to work as proxy to access the target service (step  184 ). If the proxy request should be granted, the KDC includes in its reply to the proxy client a proxy ticket encrypted with the session key in the TGT in the request (step  186 ). The client can then gain access to the target service by using the proxy ticket for authentication (step  186 ). In this way, the proxy client is able to gain access to the target service without the need to know the user&#39;s password or other secrets. 
   In view of the many possible embodiments to which the principles of this invention may be applied, it should be recognized that the embodiment described herein with respect to the drawing figures is meant to be illustrative only and should not be taken as limiting the scope of invention. For example, those of skill in the art will recognize that the elements of the illustrated embodiment shown in software may be implemented in hardware and vice versa or that the illustrated embodiment can be modified in arrangement and detail without departing from the spirit of the invention. Therefore, the invention as described herein contemplates all such embodiments as may come within the scope of the following claims and equivalents thereof.