Abstract:
A device for use in a system with multiple receiving units, and multiple intermediate units each configured to communicate with the device and at least some of the multiple receiving units, includes a communication module configured to send information toward and receive information from the receiving units and the intermediate units, a memory, and a processor coupled to the memory and the communication module. The processor is configured to: cause the communication module to send information toward each of the receiving units sufficient for the receiving units to obtain a key chain corresponding to that receiving unit, each key chain containing a plurality of keys, each key in each key chain being related to other keys in the respective key chains by at least one inverse of a one-way function; select a key from a key chain associated with a particular receiving unit and stored in the memory; and cause the communication module to send the selected key, and an indication of which receiving unit the selected key is associated with, toward the intermediate unit associated with the particular receiving unit.

Description:
FIELD OF THE INVENTION 
   The invention relates to secure communications and more particularly to reduced-communication sharing of secure communication keys. 
   BACKGROUND OF THE INVENTION 
   In today&#39;s technology-driven society, it is often desirable to have secure communications among a large group of members. For such communications, the Internet Engineering Task Force (IETF) has defined three problem areas, namely source authentication, group key management, and group policy distribution. Group key management includes distribution of keys used to encrypt data/communications to enable secure communications while inhibiting undesired access to, and undesired ability to calculate, these keys. Referring to  FIG. 1 , scalability issues in group key distribution can be addressed in a system  10  using a centralized group manager (GM)  12  that manages the group of members  16  by proxy via subordinate subgroup managers (SGMs)  14 . 
   The GM  12  delegates key management functions to designated SGMs  14 . Each SGM  14  distributes keys to members (M)  16  within the SGM&#39;s subgroup. Two categories of SGMs are: (1) trusted third-party entities in an infrastructure containing group management entities (the GM  12  and the SGMs  14 ); and (2) members designated as SGMs. For members as SGMs, the SGM for any member may change during a lifetime of the group or subgroup. If so, the replacement of the SGM may involve very large computation as well as communication overhead. The SGM  14  and each of its members  16  establish a shared secret during initialization of the SGM  14  and when changing SGMs  14 . Establishing the shared secret can be performed over a secure channel using asymmetric key operations, with one asymmetric key operation for each member  16  associated with the new SGM  14 . Asymmetric key operations use significant computational power (e.g., approximately 1,000-10,000 times more computational power than symmetric operations). 
   SUMMARY OF THE INVENTION 
   In general, in an aspect, the invention provides a device for use in a system with multiple receiving units, and multiple intermediate units each configured to communicate with the device and at least some of the multiple receiving units. The device includes a communication module configured to send information toward and receive information from the receiving units and the intermediate units, a memory, and a processor coupled to the memory and the communication module. The processor is configured to: cause the communication module to send information toward each of the receiving units sufficient for the receiving units to obtain a key chain corresponding to that receiving unit, each key chain containing a plurality of keys, each key in each key chain being related to other keys in the respective key chains by at least one inverse of a one-way function; select a key from a key chain associated with a particular receiving unit and stored in the memory; and cause the communication module to send the selected key, and an indication of which receiving unit the selected key is associated with, toward the intermediate unit associated with the particular receiving unit. 
   Implementations of the invention may include one or more of the following features. The processor is further configured to, for each of the receiving units: repeatedly apply a first one-way function initially using a primary seed as an operand, and thereafter using a result of a previous application as an operand, to determine a plurality seeds; and calculate the key chain using a second one-way function with the corresponding plurality of seeds as operands. The information comprises the primary seed and a number indicative of a number of keys in the key chain. The processor is further configured to communicate with each receiving unit via the communication module to agree upon the number of keys in the key chain for each receiving unit. 
   Implementations of the invention may also include one or more of the following features. The processor is further configured to determine a change of intermediate unit associated with the particular receiving unit from a first intermediate unit to a second intermediate unit, wherein the processor is configured to cause the communication module to send another selected key toward the second intermediate unit in response to determining the change of intermediate unit. Each key chain associated with each receiving unit has a sequence of the keys in the key chain, and wherein the another selected key is a more-senior key in the sequence of keys in the associated key chain than the selected key. The another selected key is the next-most-senior key in the sequence of keys in the associated key chain relative to the selected key. The information is the key chain. 
   In general, in another aspect, the invention provides a computer program product stored on a computer-readable medium, for use with a computer configured to communicate with a subgroup management device and a group management device, the computer program product including computer-executable instructions for causing the computer to: store a key chain comprising a plurality of keys, it being computationally difficult to determine any key in the key chain from another key in the key chain; use a first key in the key chain in association with a first subgroup management device with which the computer is associated; detect a change in association between the computer and an associated subgroup management device from the first subgroup management device to a second subgroup management device; and select a second key in the key chain, different from the first key, for use in association with the second subgroup management device. 
   Implementations of the invention may include one or more of the following features. The computer program product further includes computer-executable instructions for causing the computer to: receive a primary seed from the group management device; compute a seed chain, comprising a plurality of seeds, using the primary seed and a first one-way function; and compute the key chain using the plurality of seeds and a second one-way function. The computer program product further includes computer-executable instructions for causing the computer to: establish a secure communication channel with the group management device; and agree to a number of keys to be computed from the primary seed. The computer-executable instructions for causing the computer to select the second key cause the computer to select as the second key a more-senior key than the first key. The computer-executable instructions for causing the computer to select the second key cause the computer to select as the second key a next-most-senior key in the key chain relative to the first key. 
   Implementations of the invention may also include one or more of the following features. The computer-executable instructions for causing the computer to use the first key cause the computer to securely communicate with the subgroup management device using symmetric key operations using the first key. The computer-executable instructions for causing the computer to use the first key cause the computer to verify authenticity of the subgroup management device with the first key. The computer program product further includes computer-executable instructions for causing the computer to use the second key to at least one of securely communicate with the second subgroup management device and authenticate the second subgroup management device. 
   In general, in another aspect, in a system for communicating data securely from a data source, through intermediaries, to receivers, or for which authentication of data sources is desired, the invention provides a method including providing information related to a key chain from the source to a desired receiver, storing the key chain such that the key chain is accessible by the source and by the desired receiver, providing a particular key, from the key chain, by the source to a desired intermediary associated with the desired receiver, and using the particular key by the desired intermediary and the desired receiver for at least one of authentication of the desired intermediary and secure symmetric-key-operation communication between the desired intermediary and the desired receiver. 
   Implementations of the invention may include one or more of the following features. Providing the information related to the key chain comprises providing a primary seed to the desired receiver, the method further including, at the desired receiver, computing a seed chain, containing a plurality of seeds, from the primary seed using a first one-way function, with an operand of the first one-way function being a previous output of the one-way function, and computing the key chain using the plurality of seeds as operands of a second one-way function. The method further includes, at the source, computing the seed chain from the primary seed using the first one-way function, with an operand of the first one-way function being the previous output of the one-way function, and computing the key chain using the plurality of seeds as operands of the second one-way function. The desired intermediary is a first intermediary, the method further including detecting, by the source and the desired receiver, a change in intermediaries associated with the desired receiver from a first intermediary to a second intermediary, providing a more-senior key than the particular key from the source to the second intermediary, and using the more-senior key by the second intermediary and the desired receiver for at least one of authentication of the second intermediary and secure symmetric-key-operation communication between the second intermediary and the desired receiver. The more-senior key is a next-most senior key in the key chain relative to the particular key. 
   Implementations of the invention may also include one or more of the following features. Storing the key chain comprises storing the key chain at the source and at the selected receiver. Providing the information related to the key chain comprises providing the key chain. The method further includes establishing a secure communication channel between the source and a desired receiver, agreeing between the source and the desired receiver as to a number of keys to compute for the key chain, and tearing the secure communication channel down after providing the information related to the key chain from the source to the desired receiver, and after agreeing as to the number of keys. 
   Various aspects of the invention may provide one or more of the following advantages. Asymmetric key operations can be avoided when a subgroup manager in a secure system is replaced or added, or system members otherwise become newly associated with a subgroup manager. Performance overhead can be avoided when members become associated with a new subgroup manager with which the members are to have secure communications. Efficient secret key downloads can be provided in a replicated server model. Efficient and secure key downloads can be provided in a hierarchical group key server model. Secure group communications can be provided with fewer operations performed than using current techniques. Performance overhead for secure group communications can be reduced. New subgroup managers to a secure communication system can be inhibited from determining prior security keys (i.e., backward secrecy may be provided). Departing subgroup managers of a secure communications system can be inhibited from determining future security keys (i.e., forward secrecy may be provided). Connection handoff between base stations in the presence of a home station can be done cheaper compared to current techniques. 
   These and other advantages of the invention, along with the invention itself, will be more fully understood after a review of the following figures, detailed description, and claims. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a simplified diagram of a group communications system. 
       FIG. 2  is a simplified diagram of a group communications system employing seeded key chains. 
       FIG. 3  is a simplified diagram of a seed chain and a corresponding key chain. 
       FIG. 4  is a block flow diagram of a process of using the system shown in  FIG. 2 . 
       FIG. 5  is a simplified diagram of an exemplary group communications system employing seeded key chains illustrating a subgroup manager being replaced. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring to  FIG. 2 , a secure group communications system  20  includes a group manager (GM)  22 , subgroup managers (SGMs)  24 , and members (M)  26 . While only two SGMs  24  and five members  26  are shown, many more SGMs  24  and members  26  are possible. For example, one GM  22  may be associated with tens or hundreds of SGMs  24 , and there may be millions of members  26  associated with the one GM  22 . These quantities shown and mentioned are exemplary only, and other numbers of SGMs  24  and members  26  are acceptable and within the scope of the invention. The system  20  is configured to provide secure communications among the GM  22 , the SGMs  24 , and the members  26 . The GM  22 , SGMs  24 , and members  26  may be implemented using computers that include processors and memory that store software code instructions for causing the processors to execute functions as described below. 
   The GM  22 , the SGMs  24 , and the members  26  can communicate over secure channels. These secure channels can be private lines, or public lines using asymmetric key operations (public/private key pairs) or symmetric key operations (a common key that has been securely agreed upon, e.g., using a private line or asymmetric key operations). Secure channels  28  between the GM  22  and the SGMs  24 , and secure channels  30  between the SGMs  24  and the respective members  26 , are permanent in that they are active for the life of the respective SGM  24 . Secure channels  32  are temporary in that they are active only during initialization of key chains, as discussed below, of the members  26 . Shared secrets of the secure channels  28 ,  30 , are periodically updated to help prevent attacks on information conveyed in the system  20 . Preferably, at least some updates are performed using asymmetric key operations. 
   The GM  22  is an apparatus with a high capacity for processing information and communicating with the SGMs  24  and the members  26 . For example, the GM  22  may be a server coupled to the SGMs  24  via high-speed communication lines such as T 1  lines, optical fibers, or other communication lines and/or networks. The GM  22  is configured to establish the relatively permanent secure communication channels  28  with the SGMs  24  (e.g., using IKE Phase1, SSL/TLS, or DH exchanges). The relatively permanent secure channels  28  to each of the SGMs  24  are maintained while each SGM  24  is part of the system  20 . 
   The GM  22  is configured to establish the temporary secure channels  32  with the members  26 , and to communicate with the members  26  to establish a key chain (a set of keys for encrypting information). Using techniques similar to those for establishing the channels  28 , the GM  22  can establish the channels  32 . The GM  22  can use the secure channels  32  to communicate a seed S (a value from which another seed and/or a key may be derived) to each of the members  26 . Seeds are preferably different for each member  26  and can be produced by the GM  22  using, e.g., a random number generator. The GM  22  is further configured to communicate with each member  26  to agree upon a number of keys, r, that can be extracted or otherwise determined using the provided seed. The number of keys r may be different for each member  26 , or at least some of the members  26  may have the same agreed-upon number of keys. 
   Referring also to  FIG. 3 , the GM  22  is further configured use one-way functions in calculations. The GM  22  stores two different one-way functions, f and g (i.e., functions whose operands cannot be derived given the results of using the operands in the functions). These functions f and g may not be perfectly one way, in that an operand may be derivable from a result of either function, but doing so is so computationally intense as to allow the functions f and g to be considered to be one-way functions. The functions f and g are configured such that it is computationally infeasible to derive the operand from the result; The time needed to compute the operand from the result is longer than the lifetime of the result. For example, and not by way of limitation, under current computer technology, it could take 100 years or more to determine an operand from a result of either function for g. 
   Referring also to  FIG. 3 , the GM  22  is configured to use the functions f and g to calculate key chains  40  for each of the members  26 . The GM  22  is configured to apply the function f to the primary seed S (i.e., use the primary seed S as an operand in the function f) that the GM  22  downloads to the member  26  (each member  26  receives a primary seed, thus this discussion refers to only one of the members  26 ). The GM  22  applies the function f to the primary seed to obtain a first seed S 1 . The GM  22  applies the function f to the resulting seed S 1  to obtain a second seed S 2 , and continues applying the function f to the resulting seed until r seeds in addition to the primary seed S have been obtained. This produces a seed chain  42  according to S i =f(S i-1 ), where the primary seed S is S 0 , and the function is applied r times. Using each seed S x  in the seed chain obtained by applying the function f, the GM  22  applies the function g to obtain a corresponding key K. Thus, the GM  22  determines the key chain  42  including keys K 1 , K 2 , . . . K r-1 , K r , for each of the members  26  according to K i =g(S i ). The seeds S x  are related to the corresponding keys K x  by the inverse of the function g, and are thus computationally difficult to determine from the key K x . Similarly, prior seeds S x-1  are related to later seeds S x  by the inverse of the one-way function f and thus the prior seeds S x-1  are computationally difficult to determine from the later seeds S x . The keys are sequential in order from most senior key K 1  to most junior key K r  corresponding to most senior seed S 1  to most junior seed S r  (as seeds are produced in order from S 1  to S r ). 
   Further, the GM  22  is configured to store the key chains corresponding to the members  26 , to track the current key for each of the members  26 , and to provide the appropriate key to the appropriate SGM  24 . The GM  22  is configured to store the key chains in a memory of the GM  22  in association with the corresponding members  26 . At least each time an SGM  24  is changed, the GM  22  (that detects the SGM change) changes the current keys for all the members  26  whose SGM  24  changed. The current key can be tracked using a counter, e.g., decrementing the counter at each change and accessing a storage location indicated by the counter that stores the next key. Preferably, the GM  22  changes the current key K to the next key K in each member&#39;s key chain. The GM  22  preferably uses the keys in reverse order, such that the key K r  is used first by being downloaded to the appropriate SGM  24  first, followed by the key K r-1  and so on. The GM  22  is configured to download the appropriate key K to the appropriate SGM  24  with indicia associating the provided key K to the corresponding member  26 . 
   The SGMs  24  are configured to receive and use the keys K from the GM  22  corresponding to the SGMs&#39; associated members  26 . The SGMs  24  are configured to use the received keys K to securely communicate in a secure, symmetric manner with the members  26  associated with the SGMs  24 . Using the symmetric operation secure communications, the SGMs  24  can transmit data encryption keys, and data encrypted with the data encryption keys, to the members  26 . The SGMs  24  are computer systems that are typically, although not mandated, lower-powered (in a processing capacity sense) than the GM  22 , and higher-powered than the members  26  with which it is associated. The SGMs  24  preferably do not receive the seeds. 
   The SGMs  24  may be transient, being capable of leaving or ceasing to be an SGM  24 , and of replacing other SGMs  24 . As such, the SGMs  24  are configured to establish communications with members  26  previously associated with an SGMs  24  that the replacing SGMs  24  replace. The SGMs  24  may also discontinue communications with members  26  when the SGMs leave the system  20  or cease being an SGM  24 . SGMs  24  may expire, e.g., by existing for a predetermined amount of time. 
   A physical entity that is an SGM  24  may also be a member  26 , with the SGM  24  and the member  26  functionality being separate. For example, a high-powered computer in a housing complex may be both SGM  24  and member  26 , but the operation of the SGM  24  and the member  26  will be separate, and will operate as though the SGM  24  and the member  26  were physically different entities. 
   Each member  26  is configured to establish the temporary secure channels  32  with the GM  22 , and to communicate with the GM  22  to establish its key chain. The members  26  are typically computer systems such as personal computers, mobile devices, cell phones, or pagers, although other configurations of the members  26  are acceptable. Using techniques similar to those discussed for establishing the channels  28 , the members  26  can establish the channels  32 . The members  26  can use the secure channels  32  to receive a primary seed S from the GM  22 . Each member  26  is further configured to communicate with the GM  22  to agree upon the number of keys, r, that can be extracted or otherwise determined using the provided primary seed S. 
   Further, each of the members  26  is configured to store its key chain, to track the current key K, and to use the keys K to communicate with the corresponding SGM  24 . Each member  26  is configured to store the key chains in a memory of the member  26 . At least each time that the SGM  24  associated with the member  26  changes, the member  26  (that detects the change) changes the current key K, preferably to the next key K in the member&#39;s stored key chain. The current key can be tracked using a counter, e.g., decrementing the counter at each change and accessing a storage location indicated by the counter that stores the next key. The member  26  preferably uses the keys in reverse order, such that the key K r  is used first, followed by the key K r-1  and so on, and such that the GM  22  and the member  26  will have the same current key (i.e., be synchronized with respect to the keys K). Each member  26  uses the calculated keys K to securely communicate in a symmetric manner with its associated SGM  24 . Using the symmetric operation secure communications, the members  26  can receive data encryption keys, and data encrypted with the data encryption keys, and can decrypt the data using the data encryption keys. 
   The members  26  can also use the key chain  40 , or a key K from the chain  40 , for authentication purposes. The member  26  can calculate a derivative from the key chain  40  to serve as an authentication key (e.g., a data authentication key). The derivative may be determined similarly to how a key is derived from a seed. Also, the member  26  can use the fact that the SGM  24  provides an expected key from the chain  40  as an implicit authentication under the assumption that the GM  22  would not provide the key K to an unauthorized/unauthenticated SGM  24 . The members  26  can, e.g., compare a provided key with an expected key to verify authenticity. 
   In operation, referring to  FIGS. 4-5 , with further reference to  FIGS. 2-3 , a process  50  for synchronizing encryption keys between the GM  22  and the members  26  of the system  20  includes the stages shown. The process  50 , however, is exemplary only and not limiting. The process  20  can be altered, e.g., by having stages added, removed, or rearranged. 
   At stage  52 , the GM  22  establishes secure channels  28 ,  32  with the SGMs  24  and the members  26 , respectively. The secure channels  28  are established, e.g., using asymmetric key operations to agree upon a shared key for symmetric operations. The secure channels  32  between the GM  22  and the members  26  may be asymmetric operations. 
   At stage  54 , the members  26  are initialized and then the channels  32  are torn down. The GM  22  and the members  26  communicate over the secure channels  32 , with the GM  22  providing primary seeds S to the members  26  and the GM  22  and the members  26  agreeing upon the respective numbers r of seeds to be produced in their respective seed chains  42 . Each channel  32  is torn down once the primary seed S is downloaded and the number r of seeds to be produced is agreed upon. 
   At stage  56 , the keys K for each member  26  are determined by the GM  22  and the members  26 . The GM  22  and the members  26  apply the function f to the primary and subsequent seeds to produce the seed chains  42 , and apply the function g to the resulting seeds in the seed chain  42  to obtain the keys in the key chains  40 . The GM  22  stores the key chains in memory in association with the corresponding members  26  such that the GM  22  can access a key for a selected member  26 . The members  26  also store their key chains  42  for later retrieval, e.g., in numbered storage locations that can be identified by a counter. 
   At stage  58 , the GM  22  sends the first keys K r  for the respective members  26  to the SGMs  24 , e.g., in  FIG. 5  the keys for members  26   1 ,  26   2  to the SGMs  24   1 ,  24   2 . The GM  22  can send encrypted data to the SGMs  24  and the SGMs  24  can send encrypted data (e.g., in the same format as received, or translated to another format) to the members  26 . Communications between the SGMs  24  and the members  26  are secure using symmetric operations using the keys downloaded by the GM  22  and the same keys calculated by the members  26 . The secure communications may be, e.g., to convey a data encryption key used by the GM  22  if the SGMs  24  relay encrypted data from the GM  22  without translation, or to convey SGM data encryption keys if the SGMs  24  do translate the data received from the GM  22 . 
   At stage  60 , the SGM  24   3  replaces the SGM  24   1 , and the GM  22  and the members  26   1 ,  26   2  detect the change in SGMs  24 . This detection can take a variety of forms and may be after the change, e.g., by receiving an indication from the new SGM  24   3 , or before the change, e.g., by recognizing or issuing a command from the GM  22  to replace the SGM  24   1  with the SGM  24   3 . 
   At stage  62 , in response to detecting the change, the GM  22  downloads the next keys, in the key chains  40  for the members  26   1 ,  26   2  associated with the new SGM  24   3 , to the new SGM  24   3 . The members  26   1 ,  26   2  associated with the new SGM  24   3  access their memories and retrieve the next keys in their respective key chains  40 . 
   At stage  64 , the new SGM  24   3  and its corresponding members  26   1 ,  26   2  communicate, and/or the members  26   1 ,  26   2  authenticate the SGM  24   3 . Secure communications are performed in a secure manner using symmetric operations by using the synchronized downloaded and retrieved keys. These communications can be, e.g., data encrypted using the synchronized keys, or a data encryption key encrypted with the synchronized keys, etc. Authentication may be a comparison of the key provided by the SGM  24   3  and the key selected next by the members  26   1 ,  26   2 . 
   Exemplary System 
   For example, as an illustration and not by way of limitation, the GM  22  could be a stock-quote server for providing streaming stock quotes, the SGMs  24  could be relays, and the members  26  could be end users&#39; machines such as personal computers, pagers, cell phones, or personal digital assistants (PDAs), for displaying stock quotes from the GM  22 . 
   The SGMs  24  could be relays disposed in close proximity to the members  26 , with the SGM  24   1  being a high-powered computer for a company and the SGM  24   2  being a high-powered computer residing in a housing complex. The members  26   1 - 26   2  are company employees and the members  26   3 - 26   5  are residents of the housing complex. The physical entities that are SGMs can themselves be members  26 , with the SGMs  24  and the members  26  being logically distinct within the same physical entities, and operating accordingly as described herein. 
   In this example, the GM  22  would provide encrypted stock quotes and the SGMs  24  would distribute the quotes to the members  26 . The SGM  24  could relay encrypted data from the GM  22  without translating the data, or could decrypt the data, re-encrypt it using a different data encryption key, and send the re-encrypted (translated) data to the members  26 . 
   Other Embodiments 
   Other embodiments are within the scope and spirit of the appended claims. For example, due to the nature of software, functions described above can be implemented using software, hardware, firmware, hardwiring, or combinations of any of these. Features implementing functions may also be physically located at various positions, including being distributed such that portions of functions are implemented at different physical locations. Further, the GM  22  may download all the keys K in a member&#39;s key chain  40 , e.g., if the member  26  does not have memory for storing the key chain  40  or storing the key chain  40  at the member  26  is undesirable. Also, for changes in SGM  24 , the key used by the member  26  and sent to the SGM  24  newly associated with the particular member  26  could be a key anywhere earlier in the chain  40 , but is preferably the next-most junior key (i.e., the key from the next-most-recently produced seed relative to the seed of the key used before the SGM change).