Abstract:
A method and apparatus for producing an enhanced CRL. In response to a request containing an identifier of the most recently owned CRL stored by the requested, a certificate authority generates a CRL spanning from the most recently owned CRL to the current CRL. This CRL is formatted as a delta CRL and transmitted as a reply to the requester. This has the advantage of not requiring transmission of the full CRL even though more than one generation of CRL has occurred since the most recently owned CRL by the requester.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to the field of digital certificates and certificate revocation lists (CRL). More particularly, this invention relates to creation of a delta CRL that spans changes over more than two CRLs. 
   BACKGROUND OF THE INVENTION 
   Digital certificates are in wide use on the Internet and in the field of electronic commerce for authentication of all sorts of electronic transactions. In general, such digital certificates are used to certify the identity of an entity in the digital world, particularly as defined by the public key infrastructure (PKI). As digital certificates are issued and used, they often are either revoked or expire after a predetermined amount of time. In other situations, a digital certificate may be revoked or placed on hold pending some event. In order for digital certificates to be useful, it is important that those entities using digital certificates to authenticate the identity of an entity presenting the digital certificate have confidence that the digital certificate is valid. Generally, the validity of a digital certificate can be determined by reference to a Certificate Revocation List (CRL) produced by an authority that generates the certificates (usually a Certificate Authority). 
     FIG. 1  depicts a simple exemplary computer network  100  that utilizes a digital certificate and a Certificate Revocation List. In system  100 , a user terminal  104  may request via a network (for example the Internet)  108 , a digital certificate from a Certificate Authority  112 . The Certificate Authority  112  generates and issues the certificate, which is returned to the user terminal  104 . The user terminal  104  can then utilize the digital certificate to carry out the transaction with another entity such as remote server  116 . Such transactions may include financial transactions or any other transaction in which the identity of the user terminal  104  should be reliably authenticated. 
   When user terminal  104  sends the digital certificate to remote server  116 , the remote server  116  can inspect the digital certificate against a list of revoked certificates (the Certificate Revocation List) stored by the remote server  116 . In the event remote server  116  has not obtained a recent CRL, one can be requested from the Certificate Authority  112 . Certificate Authority  112  then either generates a new CRL or sends the most recently generated CRL to the remote server  116 . Remote server  116  can then determine whether nor not the digital certificate sent by user terminal  104  is valid. Thus, remote server  116  can authenticate the user terminal  104  and determine whether or not to authorize particular transaction at hand. 
     FIG. 2  depicts a message flow diagram  200  for the transaction just described. In this message flow diagram, a certificate request  204  is sent from the user terminal  104  to the Certificate Authority  112 . The Certificate Authority  112  generates a certificate at  208  and returns the certificate at  212  to the user terminal  104 . The user terminal  104  can then submit a transaction using the certificate at  218  to the remote server  116 . Remote server  116  can then request a new CRL at  222  of the Certificate Authority. The Certificate Authority  112  then generates or retrieves a CRL at  226  and sends the CRL to the remote server  116  at  230 . Depending on the nature of the transaction, the remote server  116  may process the CRL at  232  by taking various actions including, for example, sorting, filtering or reformatting the CRL and storing information in its own database. At  234 , the certificate can be authenticated against the CRL data at the remote server  116 . At  238  the transaction can be either approved or rejected in accordance with the authentication at  234  and at  242  the approval or rejection can be confirmed with the user terminal  104 . Those skilled in the art will recognize that many other message flows are possible with the message flow  200  if  FIG. 2  being intended as exemplary of a simple use of a digital certificate and a Certificate Revocation List. 
   With reference to  FIG. 3  the Certificate Authority  112  may generate the Certificate Revocation List in accordance with process  300 . CRLs are generated at the Certificate Authority either on a periodic basis, or as a result of some event such as a certificate revocation, or some combination thereof. The process starts at  302  after which a database of certificates is queried for certificates meeting a particular criteria of inactivity. One example is for the query to request all certificates that have been revoked. Other certificates are assumed to still be valid and active. 
   At  304  the certificate database at the Certificate Authority responds to the query with certificates meeting the specified criteria. Header information is then generated, for example, in accordance with X.509 and RFC 2459 standards (or other applicable CRL standards) at  312  and at  316  the certificate is formatted (for example, as an ASN.1 or other format CRL.) The digital certificate is signed at  320  to assure its authenticity and is then stored at  322  within a computer residing at the Certificate Authority. The process returns at  326 . Whenever a request is made for a new digital certificate, process  300  is carried out or, in some instances, the most recently generated CRL may be retrieved and forwarded to the requester. 
   As digital certificates find wider use, the number of such certificates issued has increased dramatically. With this increase comes an associated increase in the number of entries in a Certificate Revocation List. Accordingly, the process  300  as just described can become an extremely time consuming process that can result in the CRL being untimely in that many minutes or even hours can pass before an updated CRL can be generated. This is obviously undesirable since the process of authentication using the CRL should preferably be carried out on the most recent information available. 
   In addition to the certificate revocation list just described, certificate authorities commonly generate a certificate revocation list that is referred to as a delta CRL or ΔCRL. A delta CRL is simply a type of CRL that reflects changes made between two consecutive CRLs. Delta CLRs can be generated, for example, using process  300  wherein the query of  304  is a query that further limits the selection criterion to digital certificates that have been changed since the most recently generated CRL (or between two adjacent CRLs). 
   The concept of delta CRLs is illustrated in  FIG. 4  by a sequence of CRLs numbered 1, 2, 3 and 4 with delta CRLs ( 504 ,  506  and  508 ) spanning CRL #1 and CRL #2, CRL #2 and CRL #3, and CRL #3 and CRL #4. With reference to  FIG. 2 , when a delta CRL is sent at  230 , one portion of the processing of the delta CRL at  232  is to retain the data from the most recent CRL while appending the appropriate delta CRL to the existing CRL to update the list of revoked certificates. 
   SUMMARY OF THE INVENTION 
   The present invention relates generally to digital certificates and CRLs. Objects, advantages and features of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the invention. 
   In one embodiment of the present invention a method and apparatus for producing an enhanced CRL is provided. In response to a request containing an identifier of the most recently owned CRL stored by the requester, a certificate authority generates a CRL spanning from the most recently owned CRL to the current CRL. This CRL is formatted as a delta CRL and transmitted as a reply to the requester. This has the advantage of not requiring transmission of the full CRL even though more than one generation of CRL has occurred since the most recently owned CRL by the requester. 
   A method of creating a digital certificate revocation list (CRL) consistent with an embodiment of the present invention includes determining a latest owned CRL stored by a CRL recipient; creating a delta CRL comprising a list of digital certificates with a status of satisfying at least one inactive criterion, wherein said status has changed since the latest owned CRL; and sending the delta CRL to the CRL recipient. 
   A method of creating a digital certificate revocation list (CRL) consistent with another embodiment of the invention includes receiving a request for a CRL, the request including an indication of a latest owned CRL; creating a delta CRL comprising a list of digital certificates satisfying at least one inactive criterion since the latest owned CRL; and sending the delta CRL as a reply to the request. 
   A data structure, stored on a computer readable storage medium or transported over an electronic communication medium, for a digital certificate revocation list (CRL) consistent with an embodiment of the invention includes a list of digital certificates representing changes to a CRL that have occurred since generation of at least two additional CRLs. The CRL includes a CRL identifier wherein the CRL is formatted as a delta CRL. 
   The above summaries are intended to illustrate exemplary embodiments of the invention, which will be best understood in conjunction with the detailed description to follow, and are not intended to limit the scope of the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself however, both as to organization and method of operation, together with objects and advantages thereof, may be best understood by reference to the following detailed description of the invention, which describes certain exemplary embodiments of the invention, taken in conjunction with the accompanying drawings in which: 
       FIG. 1  illustrates a simple exemplary system using digital certificates. 
       FIG. 2  is a signal flow diagram describing one use of a digital certificate and certificate revocation list in the system of  FIG. 1 . 
       FIG. 3  is a flow chart describing generation of a CRL. 
       FIG. 4  illustrates the generation of delta CRLs. 
       FIG. 5  illustrates the generation of delta CRLs spanning multiple delta CRLs. 
       FIG. 6  is a signal flow diagram describing use of a delta CRL spanning multiple delta CRLs. 
       FIG. 7  is a flow chart describing one method consistent with an embodiment of the present invention for generation of a delta CRL spanning multiple delta CRLs. 
       FIG. 8  is a flow chart describing another method consistent with an embodiment of the present invention for generation of a delta CRL spanning multiple delta CRLs. 
       FIG. 9  illustrates a computer system suitable for use in conjunction with embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be recognized by one skilled in the art that the present invention may be practiced without these specific details or with equivalents thereof. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention. 
   Notation and Nomenclature 
   Some portions of the detailed descriptions which follow are presented in terms of procedures, steps, logic blocks, processing, and other symbolic representations of operations on data bits that can be performed on computer memory. These descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. A procedure, computer executed step, logic block, process, etc., is here, and generally, conceived to be a self-consistent sequence of steps or instructions leading to a desired result. The steps are those requiring physical manipulations of physical quantities. 
   Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated in a computer system. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. 
   It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the present invention, discussions utilizing terms such as “processing” or “querying” or “formatting” or “merging” or “determining” or “receiving” or “requesting” or “signing” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
   Delta CRL Enhancement in Accordance with the Invention 
   While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail specific embodiments, with the understanding that the present disclosure is to be considered as an example of the principles of the invention and not intended to limit the invention to the specific embodiments shown and described. In the description below, like reference numerals are used to describe the same, similar or corresponding parts in the several views of the drawings. 
   As great numbers of digital certificates are issued and revoked, a particular CRL can become extremely lengthy and therefore require substantial amounts of time to transmit, receive and process. The present invention addresses this problem by permitting the generation of a delta CRL that spans multiple generations of CRLs. This is illustrated in  FIG. 5  wherein, at the request of a requester, a delta CRL can be generated to span multiple CRLs. In this example, a delta CRL  502  is generated to span from CRL #1 to CRL #4. Thus, delta CRL  502  contains the certificate revocation list entries of delta CRL  504 , delta CRL  506  and delta CRL  508 . 
   Delta CRL  502  can be created using any number of techniques including simply appending the data from delta CRL  504 ,  506  and  508  together or by querying a database of digital certificate information for all changes in the certificate revocation list occurring between CRL #4 and CRL #1. The overall process is illustrated by the message flow diagram  600  of  FIG. 6 . This diagram is similar to message flow diagram  200  of  FIG. 2  until the point where the remote server requests a CRL of the certification authority. When this occurs at  604  of message flow  600 , the CRL request includes the number (or other identifier) of the latest CRL owned (stored) by remote server  116 . This CRL is designated CRL #L. When the request is received at the certificate authority, a delta CRL is generated that spans CRL #L to the current CRL at  608 . This delta CRL is then returned to the remote server at  612  and the remote server processes the delta CRL at  616  by appending its entries to the currently owned CRL #L. This can be literally interpreted to create a new CRL or the data from the delta CRL can simply be appended to the data from CRL #L and used for whatever purpose the CRL is being used for at remote server  116 . Once the processing is complete at  516 , the remote server now owns an equivalent of the most recent CRL. 
     FIG. 7  depicts a process  700  for creation of the delta CRL in accord with the present invention. At  704  the certificate authority or other entity generating the CRL receives a request for a CRL containing the most recent owned CRL (CRL #L). At  708 , entries are merged from all delta CRLs between the current CRL and CRL #L to retrieve the data necessary for creation of the delta CRL. This data is then formatted as a delta CRL at  716 , signed with a digital signature at  720  and sent to the requester as a reply at  728 . 
   In an alternative embodiment, depicted as process  800  of  FIG. 8 , when a request is received for a CRL, the request containing the most recently owned CRL (CRL #L), a certificate database is queried for the changes taking place between the current state and the state of the most recent CRL at  810 . Or, the current CRL (i.e., the most recently generated CRL) itself can be queried to obtain differences between it and CRL #L. This information is then formatted as a delta CRL at  716 , signed with a digital signature at  720  and sent as a reply at  728 . 
   In this manner, the delta CRL created in accordance with the present invention can be sent as a reply in lieu of sending a complete copy of the most recent CRL which may be much larger in size than the size of several conventional delta CRLs. Thus, transmission timesaving can be achieved as well as processing timesaving. 
   The processes previously described as carried out on a computer system, for example, a computer system residing at the certificate authority  112 . Such a computer system is depicted in  FIG. 9  as  900 . Computer system  900  includes a central processor unit (CPU)  910  with an associated bus  915  used to connect the central processor unit  910  to Random Access Memory  920  and Non-Volatile Memory  930  in a known manner. An output mechanism at  940  may be provided in order to display or print output for the computer administrator. Similarly, input devices such as keyboard and mouse  950  may be provided for the input of information from the computer administrator. Computer  900  also may include disc storage  960  for storing large amounts of information such as the list of certificates issued and the most recent Certificate Revocation List as well as any Certificate Revocation List cache and other information as required. Computer system  900  is coupled to the network (e.g., the Internet) using a network connection  970  such as an Ethernet adapter coupling computer system  900  through a fire wall and/or locally a network to the Internet. 
   Those skilled in the art will recognize that the present invention has been described in terms of exemplary embodiments based upon use of a programmed processor. However, the invention should not be so limited, since the present invention could be implemented using hardware component equivalents such as special purpose hardware and/or dedicated processors which are equivalents to the invention as described and claimed. Similarly, general purpose computers, microprocessor based computers, micro-controllers, optical computers, analog computers, dedicated processors and/or dedicated hard wired logic may be used to construct alternative equivalent embodiments of the present invention. 
   Those skilled in the art will appreciate that the program steps used to implement the embodiments described above can be implemented using disc storage as well as other forms of storage including Read Only Memory (ROM) devices, Random Access Memory (RAM) devices; optical storage elements, magnetic storage elements, magneto-optical storage elements, flash memory, core memory and/or other equivalent storage technologies without departing from the present invention. Such alternative storage devices should be considered equivalents. 
   The present invention is preferably implemented using a programmed processor executing programming instructions that are broadly described above in flow chart form, and that can be stored in any suitable electronic storage medium or that can be transmitted over any electronic communication medium. However, those skilled in the art will appreciate that the processes described above can be implemented in any number of variations and in many suitable programming languages without departing from the present invention. For example, the order of certain operations carried out can often be varied, and additional operations can be added without departing from the invention. Error trapping can be added and/or enhanced and variations can be made in user interface and information presentation without departing from the present invention. Such variations are contemplated and considered equivalent. 
   While the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications, permutations and variations will become apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended that the present invention embrace all such alternatives, modifications and variations as fall within the scope of the appended claims.