Abstract:
An apparatus, method, and computer program product for high-availability multi-agent cryptographic key recovery. The present invention defines a key recovery block that specifies allowable subsets of the total set of key recovery agents that can participate in a key recovery. For each subset, key recovery information is computed and stored after the subset is specified. This key recovery information is only useable by that subset because it is computed using that subset of public keys of the agents. When key recovery is initiated, a trusted processor (a key recovery coordinator) validates the contents of the key recovery block and it uses and is allowed to use any of the subsets of the agents to process the key recovery request. Since many subsets could be specified, the likelihood of key recovery failure is greatly diminished.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a cryptographic key recovery system and, more particularly, to a high-availability multi-agent cryptographic key recovery system. 
     2. Description of the Related Art 
     Copending U.S. patent application filed herewith, Ser. No. 09/224,886 entitled “Apparatus, Method, And Computer Program Product For Achieving Interoperability Between Cryptographic Key Recovery Enabled And Unaware Systems,” assigned to the International Business Machines Corporation, is incorporated herein by reference. This cited patent application describes a key recovery system. 
     U.S. patent application of D. B. Johnson et al., Ser. No. 08/629,815, now U.S. Pat. No. 5,815,573 filed Apr. 10, 1996, entitled “Cryptographic Key Recovery System” (“Johnson et al. I”), assigned to the International Business Machines Corporation, is incorporated herein by reference. This cited patent application describes a key recovery system using multiple key recovery agents. 
     U.S. patent application of D. B. Johnson et al., Ser. No. 08/681,679, now U.S. Pat. No. 5,796,830 filed Jul. 29, 1996, entitled “Interoperable Cryptographic Key Recovery System” (“Johnson et al. II”), assigned to the International Business Machines Corporation, is incorporated herein by reference. This cited patent application describes another key recovery system. 
     U.S. patent application of S. Chandersekaran et al., Ser. No. 08/971,204, now U.S. Pat. No. 6,355,972 filed Nov. 14, 1997, entitled “Frame-Work Based Cryptographic Key Recovery System” (“Chandersekaran et al.”), assigned to the International Business Machines Corporation, is incorporated herein by reference. This cited patent application describes a key recovery system. 
     Data encryption systems are well known in the data processing art. In general, such systems operate by performing an encryption operation on a plaintext input block, using an encryption key, to produce a ciphertext output block. The receiver of an encrypted message performs a corresponding decryption operation, using a decryption key, to recover the plaintext block. 
     Encryption systems fall into two general categories. Symmetric (or private key ) encryption systems such as the Data Encryption Standard (DES) system use the same secret key for both encrypting and decrypting messages. In the DES system, a key having 56 independently specifiable bits is used to convert 64-bit plaintext blocks to ciphertext blocks, or vice versa. 
     Asymmetric (or public key ) encryption systems, on the other hand, use different keys that are not feasibly derivable from one another for encryption and decryption. A person wishing to receive messages generates a pair of corresponding encryption and decryption keys. The encryption key is made public, while the corresponding decryption key is kept secret. Anyone wishing to communicate with the receiver may encrypt a message using the receiver&#39;s public key. Only the receiver may decrypt the message, however, since only he has the private key. Perhaps the best-known asymmetric encryption system is the RSA encryption system, named after its originators Rivest, Shamir and Adleman. 
     Asymmetric encryption systems are generally more computationally intensive than symmetric encryption systems, but have the advantage that they do not require a secure channel for the transmission of encryption keys. For this reason, asymmetric encryption systems are often used for the one-time transport of highly sensitive data such as symmetric encryption keys. 
     Data encryption systems of all types have attracted the attention of government intelligence agencies and law enforcement agencies because the same cryptographic strength that prevents decryption by unauthorized third parties also prevents decryption by intelligence or law enforcement officials having a legitimate reason for wanting to access the plaintext data. Because of such concerns, governments have either prohibited the use or export of strong encryption systems or have conditioned their approval on the use of weakened keys that are susceptible to key-exhaustion attacks (that is, systematically testing all possible keys until the right one is found). Such weak encryption systems have the obvious disadvantage that they are just as vulnerable to unauthorized third parties as they are to authorized government officials. 
     Various cryptographic key recovery systems have recently been proposed as a compromise between the demands of communicating parties for privacy in electronic communications and the demands of law enforcement agencies for access to such communications when necessary to uncover crimes or threats to national security. Generally, in such key recovery systems, all or part of the key used by the communicating parties is made available to one or more key recovery agents, either by actually giving the key portions to the key recovery agents (in which case the key portions are said to be “escrowed”) or by providing sufficient information in the communication itself (as by encrypting the key portions) to allow the key recovery agents to regenerate the key portions. Key recovery agents would reveal the escrowed or regenerated key portions to a requesting law enforcement agent only upon presentation of proper evidence of authority, such as a court order authorizing the interception. The use of multiple key recovery agents, all of which must cooperate to recover the key, minimizes the possibility that a law enforcement agent can improperly recover a key by using a corrupt key recovery agent. 
     Key recovery systems serve the communicants&#39; interest in privacy, since their encryption system retains its full strength against third parties and does not have to be weakened to comply with domestic restrictions on encryption or to meet export requirements. At the same time, key recovery systems serve the legitimate needs of law enforcement by permitting the interception of encrypted communications in circumstances where unencrypted communications have previously been intercepted (such as where a court order has been obtained). 
     In addition to serving the needs of law enforcement, key recovery systems find application in purely private contexts. Thus, organizations may be concerned about employees using strong encryption of crucial files where keys are not recoverable. Loss of keys may result in loss of important stored data. 
     The term “key recovery” encompasses mechanisms that allow authorized third parties to retrieve the cryptographic keys used for data confidentiality, with the ultimate goal of recovery of encrypted data. There are two classes of key recovery mechanisms based on the ways keys are held to enable key recovery: key escrow and key encapsulation. Key escrow techniques are based on the paradigm that the government or a trusted third party called an “escrow agent,” holds the actual user keys or portions thereof. Key encapsulation techniques, on the other hand, are based on the paradigm that a cryptographically encapsulated form of the key is made available to third parties that require key recovery; the encapsulation technique ensures that only certain trusted third parties called “recovery agents” can perform the unwrap operation to retrieve the key material buried inside. There may also be hybrid schemes that use some escrow mechanisms in addition to encapsulation mechanisms. 
     An orthogonal way to classify key recovery mechanisms is based on the nature of the key that is either escrowed or encapsulated. Some schemes rely on the escrow or encapsulation of long-term keys, such as private keys, while other schemes are based on the escrow or encapsulation of ephemeral keys such as session keys. 
     Since escrow schemes involve the actual archival of keys, they typically deal with long-term keys, in order to avoid the proliferation problem that arises when trying to archive myriad ephemeral keys. These long-term “escrowed” keys are then used to retrieve the ephemeral keys used for data confidentiality. 
     Key encapsulation techniques can also choose to archive the encapsulated keys, but usually they do not. Instead, these techniques usually operate on the ephemeral keys, and associate the encapsulated key with the actual enciphered message and thereby dispense with the archival process. The encapsulated key is put into a key recovery block that is generated by the party performing the data encryption, and associated with the encrypted data. To ensure the transmission and the integrity of the key recovery block, it may be required for processing by the party performing the data decryption. The processing mechanism ensures that successful data decryption cannot occur unless the key recovery block is processed successfully. Since the key recovery block has to be associated with the cryptographic session in some way, key encapsulation schemes may require the perturbation of the communication protocol used. 
     The process of cryptographic key recovery involves two major phases. First, parties that are involved in cryptographic associations have to perform an operation to enable key recovery (such as the escrow of use keys, or the generation of key recovery blocks, etc.)—this is typically called the “key recovery enablement” phase. Next, authorized third parties that desire to recover the data keys do so with the help of a recovery server and one or more escrow agents or recovery agents; this is the actual “key recovery service” phase. 
     One desirable characteristic of key recovery systems is referred to as “dispersion.” A key recovery system having this feature requires the cooperation of multiple key recovery agents to recover a cryptography key. Because the cooperation of multiple key recovery agents is required, the possibility of abuse is reduced. 
     Schemes have been developed to enable the recovery of cryptographic keys using multiple agents in a key recovery system. In these systems, a key recovery block is generated to make a key recoverable only if all of the agents participate in the recovery process. If any agent is not available for any reason, then key recovery fails. This causes problems when a large scale deployment requires the use of many agents over a wide area network and not all agents are available all the time. Recoveries frequently fail because of the unavailability of one or more of the multiple key recovery agents. 
     SUMMARY OF THE INVENTION 
     The present invention is a method, apparatus, and computer program product for multiple agent key recovery where not all of the agents are required for the recovery process. The present invention defines a key recovery block that specifies allowable subsets of the total set of key recovery agents that can participate in a valid key recovery. 
     For each subset, key recovery information is computed and stored after the subset is specified. This key recovery information is only useable by the listed subset because it is computed using the public keys of that subset of agents. 
     When key recovery is initiated, a trusted processor (a key recovery coordinator) validates the contents of the key recovery block and it uses and is allowed to use any of the subsets of the agents to process the key recovery request. Since many subsets could be specified, the likelihood of key recovery failure is greatly diminished. 
     According to one aspect of the present invention, a method is provided for key recovery for use in a key recovery system having a set of key recovery agents to recover a cryptography key. The method includes the steps of receiving a key recovery request from a key recovery client; receiving a key recovery block containing a plurality of key recovery agent subsets, each containing a different subset of the key recovery agents in the set; determining the availability of the agents in one of the key recovery agent subsets; and, when all of the agents in that subset are determined to be available, requesting key information from those agents; receiving key information from those agents; generating a key based on the key information; and sending the key to the key recovery client. 
     According to another aspect of the present invention, a method is provided for generating a key recovery block in for use in a key recovery system having a set of key recovery agents to recover a cryptography key. The method includes the steps of generating a plurality of key recovery agent subsets, each containing a different subset of the key recovery agents in the set; generating key recovery information for each key recovery agent in each subset; and populating a key recovery block with the key recovery agent subsets and the key recovery information. According to one embodiment, the step of generating key recovery information includes the steps of encrypting the cryptography key using the public key of one of the key recovery agents to produce a result; and encrypting that result using the public key of a different one of the key recovery agents. 
     According to another aspect of the present invention, a key recovery block is provided for use in a key recovery system having a set of key recovery agents to recover a cryptography key. The key recovery block includes a subset number field that specifies a number of subsets S of the key recovery agents that can recover the cryptography key, and S subset fields. Each subset field has a key recovery agent number field that specifies the number of key recovery agents in the subset, and a plurality of key recovery agent fields, each specifying a key recovery agent and key recovery information for that key recovery agent. 
     Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a conventional key recovery system. 
     FIG. 2 is a flowchart depicting the operation of a conventional key recovery enablement process in a conventional key recovery system. 
     FIG. 3 is a flowchart depicting a conventional process for generating a conventional key recovery block. 
     FIG. 4 depicts the format of a common key recovery block proposed by the Key Recovery Alliance. 
     FIG. 5 is a flowchart depicting the operation of a conventional key recovery service in recovering a cryptographic key. 
     FIG. 6 depicts a key recovery system according to a preferred embodiment of the invention. 
     FIG. 7 depicts the architecture of a sender or receiver in a key recovery system according to a preferred embodiment of the present invention. 
     FIG. 8 is a flowchart depicting the operation of a protocol handler according to a preferred embodiment of the present invention. 
     FIG. 9 is a flowchart depicting the operation of a cryptography handler according to a preferred embodiment of the present invention. 
     FIG. 10 is a flowchart depicting the operation of the present invention in generating a key recovery block for high availability multi-agent key recovery according to a preferred embodiment. 
     FIG. 11 depicts the format of a key recovery field in a key recovery block produced by the process of FIG.  10 . 
     FIG. 12 is a flowchart depicting the operation of the present invention in the key recovery service phase according to a preferred embodiment. 
     FIG. 13 depicts an example computer system in which the present invention can be implemented. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiment is discussed in detail below. While specific steps, configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the relevant art will recognize that other steps, configurations and arrangements can be used without departing from the spirit and scope of the present invention. 
     FIG. 1 shows a conventional key recovery system  100 . In system  100 , a sender  102  communicates with a receiver  104  by transmitting one or more encrypted messages (making up a communications session) over a communication channel  106 . Sender  102  and receiver  104  may each comprise computer workstations, suitably programmed to provide the encryption and key recovery functions described below. Sender  102  and receiver  104  may be located in different countries or within a single country. 
     The transmitted messages are encrypted by sender  102  using a cryptographic key and decrypted by receiver  104  using a corresponding cryptographic key. In addition, at least one of the transmitted messages contains a conventional key recovery block, as described below. 
     A set of key recovery agents  110 A- 110 N is provided. Collectively, key recovery agents  110  possess sufficient information to generate the cryptographic key using the key recovery block. However, no single agent  110  has sufficient information to generate the key. It is contemplated that the establishment of key recovery agents could take place as part of the establishment of a general public key infrastructure. 
     Communications over communication channel  106  are assumed to be subject to interception by third parties. An authorized third party, referred to herein as a key recovery client  108 , can obtain the key by communicating with key recovery agents  110 . Examples of key recovery clients include law enforcement agents, enterprises network managers, and individuals. Unauthorized third parties intercepting the encrypted communications will be unable to decipher the communications unless they successfully use one or more cryptanalytic techniques. 
     As described above, key recovery proceeds in two phases: key recovery enablement and key recovery service. FIG. 2 is a flowchart depicting the operation of a conventional key recovery enablement process in a conventional key recovery system such as that shown in FIG.  1 . Key recovery enablement includes the generation by receiver  102  of a key recovery block and ciphertext produced using the key, and sending of this information to receiver  104 . The key, key recovery block and ciphertext may be sent as a single message, or as separate messages, as would be apparent to those skilled in the relevant art. 
     First, a cryptographic key is generated, as shown in step  204 . The generation of cryptographic keys is well known in the relevant arts. Sender  102  then sends this cryptographic key to receiver  102 , as shown in step  206 . The key may be sent using communication channel  106  or by some other means. Next, sender  102  generates a key recovery block as shown in step  208 . Generation of the key recovery block will be described in detail below with reference to FIG.  3 . Sender  102  transmits the key recovery block to receiver  104  using communication channel  106 , as shown in step  210 . Finally, sender  104  encrypts a plaintext message using the cryptographic key to generate ciphertext, as shown in step  212 . Such encryption methods are well known in the relevant art. Sender  102  then transmits the ciphertext over communication channel  106  to receiver  104 , as shown in step  214 . Because both the key recovery block and ciphertext were sent over communication channel  106 , they are available to key recovery client  108  because key recovery client  108  is monitoring communication channel  106 . 
     FIG. 3 is a flowchart depicting a conventional process for generating a conventional key recovery block, and corresponds to step  208  of FIG.  2 . First, sender  104  generates a set of key recovery agents that defines all of the key recovery agents required for key recovery, as shown in step  302 . In general, the set of key recovery agents is specified well in advance of the communication session on a permanent basis for multiple communication sessions. Next, sender  102  generates key recovery information for each key recovery agent in the set, as shown in step  304 . This key recovery information includes information sufficient for the key recovery agents to either collectively reconstruct the cryptographic key or to provide key recovery data that can be used to reconstruct the key. Finally, sender  102  populates a key recovery block with the key recovery agents and the key recovery information generated in steps  302  and  304 , as shown in step  306 . 
     FIG. 4 depicts the format of a common key recovery block  400  proposed by the Key Recovery Alliance and published in a document entitled “A Common Key Recovery Block Format: Promoting Interoperability Between Dissimilar Key Recovery Mechanisms, Version 1.1” by Sarbari Gupta, dated May 28, 1988, and available at the Key Recovery Alliance website, www.kra.org. For convenience, that information is summarized here. 
     KRB version number  402  specifies the version of the key recovery block format. KRB length  404  specifies the number of 32 bit words in the entire key recovery block  400 . Object identifier  406  is a variable-length field that specifies the organization that is responsible for the key recovery mechanism and corresponding key recovery fields (KRF) The object identifier is ASN.1-encoded using DER rules. Block  408  is reserved. KRF length  410  specifies the number of 32 bit words in the key recovery field. 
     Key recovery field  412  is of variable length and specifies the key recovery information required to recover the key. This information includes the identity of the key recovery agents and the key recovery information required by each agent. The format and contents of key recovery fields  412  are specified by object identifier  406 . 
     Validation field type  414  specifies the technique used to generate validation field  418 . Validation field length  416  specifies the number of 32 bit words in the validation field value. Validation field value  418  is used for to verify that the key recovery block was not tampered with during transmission. The validation field value is calculated over the entire key recovery block. 
     FIG. 5 is a flowchart depicting the operation of a conventional key recovery service in recovering a cryptographic key such as that generated in step  204  of FIG. 2 using a key recovery block such as that generated in step  208  of FIG.  2 . Conventionally, this service is carried out by key recovery client  108 . While monitoring communication channel  106 , key recovery client  108  receives a key recovery block, as shown in step  502 . Key recovery client  108  determines the key recovery agents specified in key recovery field  412  of key recovery block  400 , as shown in step  504 . In this example, the key recovery block specifies key recovery agents  110 A through  110 N. Key recovery client  108  then determines the availability of all of the key recovery agents  110 A through  110 N, as shown in step  506 . 
     If all of the agents are available, as indicated by the “yes” branch from step  508 , then the key recovery clients requests key recovery data from the key recovery agents as shown in step  510 . After receiving this key recovery data, as shown in step  512 , key recovery client  108  generates the key, as shown in step  514 . 
     However, if any key recovery agent  110  is not available, as indicated by the “no” branch from step  508 , then the key recovery operation fails, as shown in step  518 . In order for a conventional multi-agent key recovery service to successfully complete, all of the key recovery agents specified by the key recovery block  400  must be available. 
     In contrast to conventional systems such as that described above, the present invention provides a key recovery system that enables key recovery when one or more key recovery agents are unavailable. FIG. 6 depicts a key recovery system  600  according to a preferred embodiment of the invention. In system  600 , a sender  602  encrypts messages using a cryptographic key and generates a key recovery block. These are sent to receiver  604  using communication channel  606 . If necessary, the cryptographic key can be sent by communication channel  606 , or by any other means, as would be apparent to one skilled in the art. A key recovery client  608  monitors communication channel  606 . In response to key recovery requests from key recovery client  608 , a key recovery coordinator  612  provides key recovery service with the help of key recovery agents  610 A- 610 N. 
     In a preferred embodiment, sender  602  and receiver  604  each employ a Frame-Work Based Cryptographic Key Recovery System such as that described in Chandersekaran et al. For convenience, such a system is now briefly described with reference to FIG.  7 . 
     System  700  includes an application  702 . Application  702  can be any application that requires communication services, such as an electronic mail program. Application  702  is coupled to a protocol handler  704 . Protocol handler  704  provides communication services over communication channel  606  in response to commands and data passed from application  702 . In the preferred embodiment, the communication protocol employed is TCP/IP. 
     Protocol handler  704  is coupled to a cryptography handler  706 . Cryptography handler  706  fulfills requests from protocol handler  704  to generate cryptography keys, to encrypt plaintext using cryptography keys to create ciphertext, to decrypt ciphertext using cryptography keys to produce plaintext, and to provide key recovery services. 
     Cryptography handler  706  is coupled to a policy module  708 . Policy module  708  contains rules regarding the handling of messages encrypted with, or to be encrypted using, strong cryptographic methods. These polices can be established by law enforcement agencies, enterprises, individuals, and the like. For example, one policy mandated by the United States is to provide any cryptographic systems destined for export with a suitable key recovery method to be employed whenever strong cryptography is used. Cryptography handler  706  consults policy module  708  whenever strong cryptography is requested by protocol handler  704 . For example, in the system destined for export from the United States, any request for strong cryptography from protocol handler  704  will, in accordance with the rules in policy module  708 , be provided with key recovery. 
     In a preferred embodiment, the key recovery and cryptography services are modularized. Therefore, these services need not be provided by the same entity that provided cryptography handler  706 . This allows a purchaser of a cryptography system to purchase these three elements from different vendors if desired. 
     Cryptography handler  706  is coupled to key recovery service provider (KRSP)  710 . In response to requests from cryptography handler  706 , KRSP  710  generates key recovery blocks and provides other key recovery services as described below. 
     Cryptography handler  706  is also coupled to cryptography service provider (CSP)  712 . In response to requests from cryptography handler  706 , CSP  712  generates cryptographic keys and provides other cryptography services. 
     FIG. 8 is a flowchart depicting the operation of protocol handler  704  according to a preferred embodiment of the present invention. When application  702  requires that data be sent over communication channel  606 , it passes that data, along with suitable commands regarding transmission and the like, to protocol handler  704 . For example, an email application may pass a message and a “send” command to protocol handler  704  for transmission to a receiver. Protocol handler  704  receives the data and command, as shown in step  802 . 
     Protocol handler  704  checks the command to determine whether the data is to be encrypted, as shown in step  804 . If not, protocol handler  704  sends the data to the receiver, as shown in step  806 . 
     However, when encryption of the data is required, the data and a command to encrypt are sent to cryptography handler  706 , as shown in step  808 . In response, cryptography handler  706  generates a cryptography key and ciphertext encrypted with that key. In addition, if key recovery is required, cryptography handler  706  generates a key recovery block, as shown in step  810 , and as described in detail with respect to FIG. 9 below. Protocol handler  704  receives the key, key recovery block, and ciphertext from cryptography handler  706 , as shown in step  812 . 
     Protocol handler  704  then sends the ciphertext, and if necessary, the key and/or key recovery block, to the receiver, as shown in steps  814  and  816 . 
     FIG. 9 is a flowchart depicting the operation of cryptography handler  706  according to a preferred embodiment of the present invention. This operation corresponds to step  810  in FIG.  8 . Cryptography handler  706  receives data to be encrypted and a command specifying the encryption from protocol handler  704 , as shown in step  902 . Cryptography handler  704  then creates a cryptographic key, as shown in step  904 . In a preferred embodiment, this accomplished by requesting a cryptography key from a cryptography service provider  712 . Cryptography handler  704  also consults the key recovery policies embodied in policy module  708  to determine whether key recovery is required, as shown in step  906 . If key recovery is not required, processing moves to step  912 , as shown by the “no” arrow from step  908 . 
     However, if key recovery services are required, as shown by the “yes” branch from step  908 , then cryptography handler  706  creates a key recovery block, as shown in step  910 . In a preferred embodiment, this is accomplished by requesting a key recovery block from a key recovery service provider  710 . 
     Next, cryptography handler  706  uses the key created in step  904  to encrypt the data to produce ciphertext, as shown in step  912 . Cryptography handler then sends the cryptography key, ciphertext, and if necessary, key recovery block, to protocol handler  704 , as shown in step  914 . 
     FIG. 10 is a flowchart depicting an operation of the present invention in generating a key recovery block for high availability multi-agent key recovery according to a preferred embodiment. According to this process, sender  602  generates a plurality of key recovery agent subsets, each containing a different subset of the key recovery agents in the set, as shown in step  1002 . Then, for each subset, sender  602  generates key recovery information for each agent in that subset, as shown in step  1004 . 
     In a preferred embodiment, key recovery information for an agent subset is generated according to the following method. The key, or parts of the key, are encrypted using the public key of one of the key recovery agents in the subset. The result is then encrypted using the public key of another one of the key recovery agents in the subset. This process is repeated for each of the remaining key recovery agents in the subset. The final result is then encapsulated within the key recovery field of the key recovery block. To recover the key or key parts, each key recovery agent in the subset in turn decrypts the key recovery field of the key recovery block using its private key. Finally, sender  602  populates a key recovery block with the key recovery agent subsets and the key recovery information generated in steps  1002  and  1004 , as shown in step  1006 . 
     FIG. 11 depicts the format of a key recovery field  412  in a key recovery block  400  produced by the process of FIG.  7 . Field  1102  specifies the number of subsets of key recovery agents that follow in the key recovery field. Field  1102  is followed by S fields  1104 A- 1104 S, one for each agent subset. For example, returning to field  1104 A, each field  1104  includes the number of agents and in that subset, and key recovery information for each agent in the subset. 
     FIG. 12 is a flowchart depicting the operation of the present invention in the key recovery service phase according to a preferred embodiment. The process of FIG. 12 is performed by key recovery coordinator  612 . Key recovery coordinator  612  receives the key recovery block and a key recovery request from key recovery client  608 , as shown in step  1202 . In general, key recovery coordinator  612  authenticates the request to determine whether key recovery client  608  is authorized to proceed. Key recovery coordinator  612  then determines the key recovery agents&#39; subsets by examining the key recovery block, as shown in step  1204 . 
     Next, key recovery coordinator  612  examines each key recovery agent subset in turn to determine whether the agents in that subset are available. First, a key recovery agent subset is selected, as shown in step  1206 . Then, key recovery coordinator  612  determines the availability of the agents in the selected subset, as shown in step  1208 . If not all of the agents in the subset are available, as indicated by the “no” branch from step  1210 , then another key recovery agent subset is selected, as shown in step  1212 . When a subset is found that in which all of the agents are available, as shown by the “yes” branch from step  1210 , then key recovery coordinator  612  requests key recovery information from the key recovery agents in that subset, as shown in step  1214 . 
     In general, the key recovery agents authenticate the request to determine that key recovery coordinator  612  is authorized to proceed. The selected key recovery agents then provide the requested information. Key recovery coordinator  612  receives key recovery data, as shown in step  1218 , and uses this information to generate the key, as shown in step  1220 . In an alternative embodiment, the key recovery data includes the key. Key recovery coordinator  612  then sends the key to the key recovery client that requested it, as shown in step  1222 . 
     Thus, according to the present invention, multiple key recovery agent subsets can be defined within the specified set of key recovery agents, where my specified subset is allowed or is able to generate key recovery information sufficient to regenerate the key. 
     The present invention may be implemented using hardware, software or a combination thereof and may be implemented in a computer system or other processing system. In fact, in one embodiment, the invention is directed toward one or more computer systems capable of carrying out the functionality described herein. An example computer system  1300  is shown in FIG.  13 . The computer system  1300  includes one or more processors, such as processor  1304 . The processor  1304  is connected to a communication bus  1306 . Various software embodiments are described in terms of this example computer system. After reading this description, it will become apparent to a person skilled in the relevant art how to implement the invention using other computer systems and/or computer architectures. 
     Computer system  1300  also includes a main memory  1308 , preferably random access memory (RAM), and can also include a secondary memory  1310 . The secondary memory  1310  can include, for example, a hard disk drive  1312  and/or a removable storage drive  1314 , representing a floppy disk drive, a magnetic tape drive, an optical disk drive, etc. The removable storage drive  1314  reads from and/or writes to a removable storage unit  1318  in a well known manner. Removable storage unit  1318 , represents a floppy disk, magnetic tape, optical disk, etc. which is read by and written to by removable storage drive  1314 . As will be appreciated, the removable storage unit  1318  includes a computer usable storage medium having stored therein computer software and/or data. 
     In alternative embodiments, secondary memory  1310  may include other similar means for allowing computer programs or other instructions to be loaded into computer system  1300 . Such means can include, for example, a removable storage unit  1322  and an interface  1320 . Examples of such include a program cartridge and cartridge interface (such as that found in video game devices), a removable memory chip (such as an EPROM, or PROM) and associated socket, and other removable storage units  1322  and interfaces  1320  which allow software and data to be transferred from the removable storage unit  1318  to computer system  1300 . 
     Computer system  1300  can also include a communications interface  1324 . Communications interface  1324  allows software and data to be transferred between computer system  1300  and external devices. Examples of communications interface  1324  can include a modem, a network interface (such as an Ethernet card), a communications port, a PCMCIA slot and card, etc. Software and data transferred via communications interface  1324  are in the form of signals which can be electronic, electromagnetic, optical or other signals capable of being received by communications interface  1324 . These signals  1326  are provided to communications interface  1324  via a channel  1328 . This channel  1328  carries signals  1326  and can be implemented using wire or cable, fiber optics, a phone line, a cellular phone link, an RF link and other communications channels. 
     In this document, the terms “computer program medium” and “computer usable medium” are used to generally refer to media such as removable storage device  1318 , a hard disk installed in hard disk drive  1312 , and signals  1326 . These computer program products are means for providing software to computer system  1300 . 
     Computer programs (also called computer control logic) are stored in main memory  1308  and/or secondary memory  1310 . Computer programs can also be received via communications interface  1324 . Such computer programs, when executed, enable the computer system  1300  to perform the features of the present invention as discussed herein. In particular, the computer programs, when executed, enable the processor  1304  to perform the features of the present invention. Accordingly, such computer programs represent controllers of the computer system  1300 . 
     In an embodiment where the invention is implemented using software, the software may be stored in a computer program product and loaded into computer system  1300  using removable storage drive  1314 , hard drive  1312  or communications interface  1324 . The control logic (software), when executed by the processor  1304 , causes the processor  1304  to perform the functions of the invention as described herein. 
     In another embodiment, the invention is implemented primarily in hardware using, for example, hardware components such as application specific integrated circuits (ASICs). Implementation of the hardware state machine so as to perform the functions described herein will be apparent to persons skilled in the relevant art(s). In yet another embodiment, the invention is implemented using a combination of both hardware and software. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant arts that various changes in form and detail can be made without departing from the spirit and scope of the present invention. Thus the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. All cited patent documents and publications in the above description are incorporated herein by reference.