Abstract:
A system using digital certificates having overlapping validity intervals. The overlapping certificates can be used in a hierarchical certificate authorities network in order to obtain benefits such as to increase the usage of all the certificates in the certificate chain; reduce/eliminate the certificate updates/downloads to a large population; only replace the minimum number of certificates in the trust hierarchy to re-establish the certificate chain; reduce the complexity of maintaining certificate nesting in certificate generation process; reduce the risk of service interruption; and control the extent of older technology in circulation and to reduce the risk associated with older products being more susceptible to attack. The certificate renewal process of a preferred embodiment is described.

Description:
CLAIM OF PRIORITY 
   This application claims priority from U.S. Provisional Patent Application Ser. No. 60/424,893, filed Nov. 8, 2002 entitled “OPTIMIZING CERTIFICATE RENEWAL PROCESS FOR LARGE SCALE PKI DEPLOYMENT IN A HIERARCHICAL CERTIFICATE AUTHROITY INFRASTRUCTURE” that is hereby incorporated for reference as if set forth in full in this document for all purposes. 

   BACKGROUND OF THE INVENTION 
   This invention relates in general to security of digital information and more specifically to a certification system for digital information transfer. 
   Digital information has become extremely important in all aspects of commerce, education, government, entertainment and management. In many of these applications, the ability to ensure the privacy, integrity and authenticity of the information is critical. As a result, several digital security mechanisms have been developed to improve security. 
   One approach to digital security that is commonly used is for a certificate authority (CA) to issue a certificate to a certificate holder. The holder can then provide the certificate to a third party as an attestation by the CA that the holder who is named in the certificate is in fact the person, entity, machine, email address user, etc., that is set forth in the certificate. And that a public key in the certificate is, in fact, the holder&#39;s public key. People, devices, processes or other entities dealing with the certificate holder can rely upon the certificate in accordance with the CA&#39;s certification practice statement. 
   A certificate is typically created by the CA digitally signing, with its own private key, identifying information submitted to the CA along with the public key of the holder who seeks the certificate. A certificate usually has a limited period of validity, and can be revoked earlier in the event of compromise of the corresponding private key of the certificate holder, or other revocable event. 
   One standardized approach to today&#39;s digital security is referred to as the Public Key Infrastructure (PKI). PKI provides for use of digital certificates to authenticate the identity of a certificate holder, or to authenticate or certify other information. Typically, a PKI certificate includes a collection of information to which a digital signature is attached. A CA that a community of certificate users trusts attaches its digital signature and issues the certificates to various users and/or devices within a system. 
   If a certificate has expired, another certificate can be obtained by going through the proper steps to contact a CA and obtain another valid certificate. This approach may work well where, for example, certificates are assigned to users and a user is responsible for obtaining, updating (e.g., when a user&#39;s identification information changes) and renewing that user&#39;s certificate. However, the generation, transmission and updating of certificates in association with hardware devices (i.e., the hardware devices are each associated with a certificate) may introduce problems in certificate management, transmission, control, use, etc., especially where the number of devices is large. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention allows certificates having overlapping validity to be used. The overlapping certificates can be used in a hierarchical certificate authorities network in order to obtain benefits such as to increase the usage of all the certificates in the certificate chain; reduce/eliminate the certificate updates/downloads to a large population; only replace the minimum number of certificates in the trust hierarchy to re-establish the certificate chain; reduce the complexity of maintaining certificate nesting in certificate generation process; reduce the risk of service interruption; and control the extent of older technology in circulation and to reduce the risk associated with older products being more susceptible to attack. The certificate renewal process of a preferred embodiment is described. 
   In one embodiment the invention provides a method for providing a certificate in a digital security system, the method comprising receiving a first certificate including a first validity period; receiving a second certificate including a second validity period, wherein the second validity period overlaps with the first validity period and wherein the periods are not the same; and providing one of the certificates to a device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a trust hierarchy according to a preferred embodiment of the present invention; 
       FIG. 2  illustrates a time line showing use of certificates with overlapping validity periods; 
       FIG. 3  shows a time line where a rekey operation is performed at the Root CA; and 
       FIG. 4  illustrates a high-level message flow of a certificate renewal process. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A preferred embodiment of the invention is designed to provide certificates in a digital content distribution system where many user devices are granted permission to access, play back, or otherwise present the digital content. For example, digital video can be distributed by satellite, cable, fiber optic or other communication links using digital networks such as the Internet, corporate or campus intranets, local area networks, etc. Ultimately, the digital video is presented to a user or viewer so that the user can play back the image and audio content on a home entertainment system, computer system, or other presentation system. 
   In the digital video application, the device that performs decoding, decryption and other processing of the content so that it can be output to audio speakers and to a display is referred to as a “set top box.” However, in general any type of device that processes content for presentation (e.g., personal digital assistant, cell phone, streaming video player, etc.) can be employed. Additionally, the device level can include control devices for facilitating the transfer of information, arbitrating network resources, updating processes and devices, etc. Although the invention is described primarily in reference to a digital video distribution scenario, it should be apparent that other scenarios and applications can benefit from the use of various features of the invention. 
     FIG. 1  illustrates the basic arrangement of a trust hierarchy suitable for use with the present invention. 
   In  FIG. 1 , trust hierarchy  100  includes three levels of participants in a network organization including manufacturer sites of set top boxes and the set top boxes, or devices, themselves. A first level, or root level, includes network operator  110 . The network operator is responsible for issuing the root level certificate to the manufacturing level of the hierarchy. In general, the network operator can be any entity sufficiently authorized and entrusted to carry out the certificate services. The network operator also must establish the physical identity of manufacturers and must ultimately establish, control and ensure the integrity of certificates distributed in the hierarchy. In the set-top box application, for example, a network operator can be established or licensed by a major media distributor. Second level participants can be manufacturers of set-top boxes or other playback devices. At the third level are the manufactured devices that are typically resident in a consumer&#39;s home or, as in the case of portable devices, are kept on a user&#39;s person. 
   Once manufacturers such as  120 ,  122  and  124  are physically identified the network operator (i.e., root certificate authority (“Root CA”)) certifies each manufacturer&#39;s public key by issuing a unique manufacturer certificate. The Root CA also publishes its own certificate to manufacturers. Each manufacturer uses their certified key to create device certificates for devices such as  130 - 140  in  FIG. 1 . A manufacturer is responsible for creating device certificates, and also for loading device key/certificate, the manufacturer&#39;s own certificate and the Root CA&#39;s certificate to a device at the manufacturing facility. For example, device  130  is issued a device certificate unique to device  130 . Device  130  will also have a certificate from manufacturer  120  and the root level certificate from network operator  110 . Similarly, devices  132  and  134  also include certificates from manufacturer  120  and network operator  110 . Devices  136 ,  138  and  140  include certificates from manufacturer  124  and from network operator  110 . 
   In a preferred embodiment, certificates at a given level have validity periods that overlap with certificates at other levels. Specifically, a subject certificate at a lower (Subject) level, such as a manufacturer level, can have a validity period that overlaps with one or more root (Issuer) level certificates from which the subject certificate did not issue. For example, assume a first root certificate is used to provide a key for a first manufacturer certificate. At a later time the first root certificate will be renewed so that a renewed root certificate is in place. However, the manufacturer certificate issued with the first root certificate is still valid even though a renewed root certificate has been issued and is now used to create all subsequent manufacturer and device certificates. Similarly, device certificates can overlap with manufacturer and root certificates. 
   In a preferred embodiment, devices are issued device certificates with validity periods that are anticipated to be longer than the operational lifetime of the device. For example, a period of 20 years is used in the preferred embodiment. When a device certificate expires, that device is no longer operable. The 20 year period is selected as a long enough time so that the device will most likely have been made obsolete by new technology. Or the device may fail either electronically or mechanically, or the device lifetime would be exceeded for other reasons. In other words, the preferred embodiment assumes that device certificates will not need to be renewed. Other embodiments can allow device certificates to be renewed, can have varying validity periods for certificates, or can provide other variations. 
   The validity period chosen can depend on a number of factors, such as the strength of the encryption technology used, e.g. the strength of the private key used to sign the certificate, and the desire to minimize traffic on the network and to reduce the burden of operation on certificate authorities. The longer and more often a certificate and its associated information (e.g., a private key) is used, the more susceptible it is to loss or discovery. Therefore, restricting certificate lifetime could be used to control the extent of older technology in circulation and to reduce the risk associated with older products being more susceptible to attack. However, the shorter lifetime is defined, the more often certificate renewal or re-enrollment would need to be performed, which would increase the operational cost of a certificate authority. This could also complicate the message protocol that needs to be built in the system to generate the alert for certificate expiration. 
   Manufacturer certificates are renewed after a two year period and are valid for 20 years. Note that any validity periods, intervals or other specific values or ranges discussed herein can vary with different embodiments. The validity periods can be eternal, tens of years, months, days, minutes, etc. The periods can be selected according to desired certificate policies, design choices or for other reasons. 
   At the end of each two year renewal period the manufacturer must obtain another certificate from the network operator. During the two year period the manufacturer can issue device certificates under the then-current manufacturer certificate. Similarly, the network operator, or Root CA, renews its root certificate every 2 years. At both the manufacturer and network operator levels, the certificates are issued with a different key every 30 years, or after fifteen 2-year renewal periods. As mentioned, this approach results in certificates at lower (Subject) levels overlapping with renewed certificates at the higher levels (Issuer). Details and implications of this overlapping approach are described below. 
   With the three-level approach of the preferred embodiment network operator and manufacturer certificates are renewed every 2 years and re-keyed every 30 years. The network operator is regarded as the root Certification Authority (CA) in the trust hierarchy and renews and/or reissues its root certificate every two years by self-authenticating with the same root key provided the key has been in use for less than 30 years. The root CA uses a rekeying renewal process. The manufacturers are “sub CA,” or mid level CA entities and renew their certificates from the root CA every two years without having to change the key as long as the key has been in use for less than 30 years. To limit the use of the same key for generating device certificates to no more than 50 years, the Manufacturer CA uses the rekeying renewal process after 15 renewals. This procedure results in the same key being used no more than 50 years as 30 years to rekey plus 20 years lifetime of a key issued at the end of the 30 year period. The end entity (device) certificate does not need to be renewed because the original overlapped validity period provides a sufficient validity period to cover the expected operational lifetime of the device. 
   Network Operator—Physical Identification 
   Physical identification and certification of manufacturers is performed by the network operator to establish the true identity of each manufacturer. Once physical identification is accomplished the network operator has some assurance that the known electronic communication channels (e.g., email address, Domain Name System (DNS) address, etc.) are reliable ways to contact the manufacturer. The certification process for the Network Operator who operates the root CA and certifies a Manufacturer CA includes (1) a registration (administrative) process and (2) a certificate management (technical) process. 
   In the registration process the network operator can receive and verify signed and notarized paper documents or other credentials. Any type of generally accepted, or official documents or other information can be obtained and used for this purpose. Approval and issuance of a manufacturer&#39;s certificate is contingent upon the manufacturer being identified to the satisfaction of the network operator. 
   In the certificate management process the electronic certificate is created and issued. The certificate is also recorded and tracked through the lifetime of the certificate. The management process includes control and management of the certificate such as renewing, revoking, and other maintenance. 
   In many cases a network operator might already have established business relationships with manufacturers prior to the deployment of a distribution network and end-user devices. Given these existing relationships, the registration and validation process can be simplified significantly. However, when a new manufacturer becomes a member of the network, when an existing manufacturer out-sources its secure activities to a third party, or in other instances where a new manufacturer is introduced the network operators will generally conduct a thorough physical identification and registration process. This process typically involves face-to-face presence with a manufacturer&#39;s representative and a network operator representative. 
   For example, a manufacturer&#39;s representative might visit a security officer or operator to submit the certificate request. A Public Key Cryptography Standard (PKCS), e.g., PKCS#10 Certification Request Syntax Standard format can be used to submit the certificate request and a PKCS#7 format can be used to receive the certificate. This registration process may require the representative of a manufacturer to prove the representative&#39;s personal identity, the identity of the company, and the representative&#39;s employment with and authorization by the company to perform the transaction. Although this approach is relatively straightforward it requires the representative of a manufacturer to personally visit a network operator&#39;s administration office and can be inconvenient. 
   Another approach uses electronic communications rather than in-person meetings. The network operator sends an application form to a new manufacturer via mail, fax or another channel. The manufacturer is required to supply the information shown in Table I, below. 
   
     
       
             
           
             
             
           
         
             
               TABLE I 
             
             
                 
             
             
               Manufacturer CA Organizational Information: 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Organization name 
             
             
                 
               Organization address 
             
             
                 
               Organization Tax Identification Number or other documents that can 
             
             
                 
               prove the business identity 
             
             
                 
               Personal contact information: 
             
             
                 
               Company ID and/or driver&#39;s license, passport or other means of 
             
             
                 
               identification 
             
             
                 
               Email address 
             
             
                 
               Work phone number 
             
             
                 
               Employment references 
             
             
                 
               Authorization letter from an officer of the company 
             
             
                 
                 
             
           
        
       
     
   
   The complete application form is notarized and then sent to the network operators&#39; offices by a preferred means such as by fax, encrypted email, a secure link such as a virtual private network (VPN), etc. Naturally, any suitable communication means can be used, as desired. 
   The manufacturer should have a Manufacturer certificate authority (CA) to keep and safeguard copies of all the documents submitted. Some of information will be needed again to submit the certificate request to the Network Operator. The administrator (or a security officer) should validate all the information provided by a Manufacturer CA. The Manufacturer should electronically generate and transmit the certificate request to the network operator CA, or Root CA. A certificate request can include basic information such as the Manufacturer CA&#39;s public key and manufacturer identity information previously sent. The request is signed by the private key of the Manufacturer CA to prove it holds the corresponding public key. 
   The network operator compares the information received in the certificate request with the paper document that includes the information in Table I. If the information matches, the Root CA signs the manufacturer&#39;s certificate, thereby authenticating the identity of the requester. In addition, the Root CA can “stamp” the certificate with an expiration date. The certificate can be sent to the Manufacturer CA directly. 
   Certificate Validation Chain 
   A preferred embodiment of the invention uses digital certificates in compliance with Standard X.509 Version 3. See, e.g., RFC 2459, “Internet X.509 Public Key Infrastructure,” January 1999. Three types of certificates are used as follows: (1) self-signed Root CA certificates, (2) manufacturer CA certificates and (3) device certificates. The chain starts with an end-entity certificate at the lowest level of the trust hierarchy followed by a succession of one or more mid-level CA certificates (e.g., a manufacturer&#39;s certificate) where each CA certificate is the parent (Issuer) of the previous certificate (Subject), terminating in a self-signed root certificate at the top level, or first level. Other embodiments can use any number and type of levels in a hierarchy. 
   In order to limit ongoing risk to the system, the certificate validation process defined below is used to prevent unauthorized equipment and associated entities (e.g., equipment vendors) from re-entering the network. The certificate validation process tries to ensure that every signature in the certificate chain is verified, that every certificate validity period in the certificate chain is checked, and that certificate revocation checks are performed. 
   Certificate signature verification is used to check the validity of an end-entity device certificate. A Manufacturer certificate is used to verify the signature in the end-entity certificate and a root certificate is used to verify the signature in the Manufacturer certificate. 
   In a digital video content distribution application the distribution network usually only involves a small number of distribution nodes, or “headend” equipment. For example, the number of headend installations can be on the order of dozens. In this case, using a certificate revocation mechanism to address the problem associated with longer lifetimes will not cause the size of the certificate revocation database to increase significantly. Therefore, all the certificates can have a relatively long lifetime such as 20 years as the default. 
   In general, replacing the certificate after its expiration date could be a manual process. However, if a Manufacturer CA wishes to define its certificates with a short lifetime then an automated mechanism may need to be designed into its equipment to keep track of certificate expiration time such that all devices could obtain their new certificates in time before the old ones expire. An alert can also be originated at the manufacturer and sent over the network to the device so that a user of the device can take appropriate action such as by obtaining a renewed certificate, exchanging the device, etc. 
   In a preferred embodiment of the invention, the validation of validity period nesting is not required. However, a simple point-wise certificate validity check is performed to ensure all the certificates in the certificate chain are valid at the time when the check is performed. After 60 days of expiration of any certificate in the certificate chain any applications, processes, procedures, etc., whether manual or automatic, that use the expired certificate must cease. Note that other embodiments can use different approaches to a “grace” period for expiration such as not allowing expired certificates to be used for any length of time after expiration. One reason for allowing the grace period in the preferred embodiment is to prevent harsh results such as suddenly stopping a manufacturer from production. Another approach can provide an alert to a manufacturer when expiration is approaching. Other actions, such as additional notification to a manufacturer, restriction from participation in the distribution system, legal enforcement, etc., can be taken if certificates are not updated. 
   Usually the physical extraction of equipment with revoked certificates is only practical in small systems or networks with a manageable number of devices. In cases of larger networks with many devices a certificate revocation process can be used to electronically deactivate a device that is no longer trusted. A preferred embodiment uses a certificate revocation process that is part of standard PKI, in which the Certificate Authorities process revocation requests, and generate and maintain Certificate Revocation Lists (CRLs). A standard X.509 CRL is used. Note that the present invention attempts to minimize network traffic by reducing the need to transfer certificates, create CRLs and transfer other information. One way this is accomplished is with creation of new certificates at fixed, relatively long, intervals and by allowing certificates created with older Issuer certificates to remain valid. 
   A Manufacturer CA&#39;s certificate may become invalid before the normal expiration of its validity period. The network operator may suspend or revoke any certificate issued to the Manufacturer CAs, if the network operator believes, e.g., manufacturers&#39; private keys may have been lost, used in an authorized manner, or otherwise compromised. Under such circumstances, the root CA revokes the Manufacturer CA&#39;s certificate by including the certificate&#39;s serial number on the next scheduled CRL. Similarly, a Manufacturer CA can revoke any individual device if a device&#39;s key may have been compromised. The CRL can be stored and distributed to other parties who may wish to check the certificates against the CRLs. 
   If a Manufacturer CA&#39;s certificate is revoked, the manufacturer is required to go through the initial registration process with the network operator, as described above, to obtain a new certificate. After receiving a new certificate, the Manufacturer CA re-issues new certificates to all of the manufacturer&#39;s associated devices. Otherwise, all the device certificates are inherently revoked. 
   Certificate Renewal Process and Policy 
   There are two basic scenarios of certificate renewal as follows: (1) Creating a new certificate with the same name, key, and other information as the old one, but a new, extended validity period and a new serial number; and (2) creating a new certificate with a new, different public key (corresponding to a new, different private key) and a different serial number, and a different validity period. This second form of renewal is also known as a “re-key” process. 
   A simple point-wise certificate validity check is performed to ensure that all the certificates in the certificate chain are valid at the time when the check is performed. Therefore, maximizing the overlapping validity period of all the certificates could reduce the operational cost because a frequent certificate renewal process over a large population (such as the device population) is avoided. A preferred embodiment of the invention allows reduced complexity of certificate nesting by using the certificate policies in Table II. 
   
     
       
             
             
           
         
             
               TABLE II 
             
             
                 
             
           
           
             
               1. 
               The Root CA renews/reissues its root certificate every 
             
             
                 
               two years with the same root key provided the key has been in 
             
             
                 
               use for less than 30 years. Otherwise a rekeying renewal is 
             
             
                 
               performed. 
             
             
               2. 
               The Manufacturer CA renews its certificate from the Root 
             
             
                 
               CA every two years without having to change its key when the 
             
             
                 
               key has been in use for less than 30 years. Otherwise a rekeying 
             
             
                 
               renewal is performed. 
             
             
               3. 
               All CAs shall always use their most recent and updated 
             
             
                 
               key/certificate in the new certificate generation operation. 
             
             
               4. 
               Device certificates have a validity period of 20 years 
             
             
                 
             
           
        
       
     
   
     FIG. 2  illustrates overlapping certificate validity periods. 
   In  FIG. 2 , diagram  200  includes separate timelines for each of the three tiers, or levels, in the trust hierarchy. First level  210  corresponds to the Root CA, or network operator, certificates. Second level  220  corresponds to Manufacturer CA certificates. Third level  230  corresponds to device certificates. Certificate Rcert 1  at  212  includes the public key of RK 1  that can be used to verify the Root CA&#39;s signature on manufacturer certificates, such as on Mcert 1 . The public key of the manufacturer is provided with Mcert 1 . Mcert 1  is provided by the Root CA and is signed with the private key of RK 1 . The private key of MK 1  is used to sign the device certificates. Mcert 1  is also valid for 20 years. Finally, the private key of MK 1  is used to sign Dcert 1 . Dcert 1  includes the public key of DK 1 . 
   Both the public and private keys of DK 1  are created by the manufacturer and provided to the specific device at the time of manufacture. In a preferred embodiment the device certificate and private key can be updated, if necessary. 
   As shown in first level  210 , as each 2-year renewal period for Root CA certificates expires, a new certificate is created (RK 1 /Rcert 1 , RK 1 /Rcert 2 , RK 1 /Rcert 3 , . . . ) using the same key, RK 1 . At the end of the validity period of the 15th certificate, at  214 , a new key, RK 2 , is used to continue generating certificates (RK 2 /Rcert 16 , RK 2 /Rcert 17 , . . . ). 
   During the 2-year period before renewal of a root certificate the root certificate can be used to generate any number of manufacturer certificates. Other embodiments can place restrictions, such as a number or type limit, on certificates that are issued using the root certificates. As successive root certificates are renewed over time a root certificate currently being used by a Root CA will not necessarily be the root certificate in the chain of a manufacturer or device certificate since the manufacturer and/or device certificates may have been created using a prior root certificate. Similarly, a valid device certificate may not have the current corresponding manufacturer&#39;s certificate in its chain. However, even though the current root certificates may not be a particular manufacturer/device certificate chain, the prior root certificates (i.e., non-current root certificates) will still validate due to their ongoing validity periods that can overlap with validity periods of current certificates. A similar situation exists with manufacturer certificates with respect to device certificates. Other embodiments can exhibit certificate relationships among any number of levels. 
   Thus, a point-wise certificate validity check that is performed on a manufacturer or device certificate that uses prior root and/or manufacturer certificates in its chain will still validate. The point-wise check can be performed by software, hardware or a combination of both, (or another process such as manually, etc.), to determine whether the certificate chain is valid at a certain point in time. This is a fast way to perform a validity check that does not require extensive use of network bandwidth. In normal operation of the system, each device is associated with only one certificate chain that does not need to be updated since the certificate chain is expected to last beyond the lifetime of the device. Other embodiments can use different approaches to device certificates, including renewable certificates. 
   At  222  manufacturer certificate Mcert 15  uses root key RK 1 . The manufacturer uses the private key of MK 1  to sign Dcertm- 1 . At  214  a rekey at the root CA occurs. The new root key is designated as RK 2 . Manufacturer certificates signed using RK 2  include a new manufacturer&#39;s key, MK 2 , as shown beginning at  224 . Device certificates signed with MK 2  also include Root CA certificates signed with RK 2  in their certificate chain. Thus, the last root certificate using RK 1  is Rcert 15  having 2-year period  213  in which to issue certificates. The expiration of Rcert 15  is shown at  216 , after 20 years. Therefore, device certificate Dcertm- 1  at  232  must expire no later than at  216 . A new device key (DK) may be generated at the REKEY at  234 . 
   In a preferred embodiment, the Root CA rekey process is performed after the key has been in use for 30 years, because the longer and more often a key is used, the more susceptible it is to loss or discovery. New certificates are issued to Manufacturer CAs by the Root CA when the Root CA rekeys and begins issuing manufacturer certificates with a new manufacturer key. When a Root CA rekey occurs because of a compromise (that can happen at any time) each Manufacturer CA should immediately send a new certificate request to the Root CA. The Manufacturer CAs would then update all device certificates with the new manufacturer certificates issued by the Root CA as a result of the rekey operation. Upon a rekey, the Root CA can identify and authenticate Manufacturer CAs either by performing the physical identification and/or registration process described, above, or if it has been less than 10 years since a Manufacturer CA was identified as required then by using the currently valid certificate issued to the Manufacturer CA by the Root CA. 
     FIG. 3  illustrates a case where a rekey of the root takes place but does not require a manufacturer rekey. Such a situation can occur, for example, when the root key has been compromised and a scheduled root rekey will not take place for some time. 
   In  FIG. 3 , the Root CA rekeys the root key from RK 1  to RK 2  at point  250 . If the Manufacturer CA&#39;s key is not compromised then the Manufacturer can apply its new certificate from the root CA without changing its own key (assume its key is indexed by MK 1 ). If the rekey is a result of a root key being compromised, then the Manufacturer CA updates all the devices that have been created under MK 1  with the new Root CA certificate and new Manufacturer&#39;s certificate. If the “rekey” process is performed under the assumption that the key could be compromised in the future, not because it is already compromised, then no certificates need be updated in devices that are already created under RK 1  and MK 1 . 
   When the Manufacturer CA changes its key from MK 1  to MK 2  under the circumstance that the key is compromised, the device certificates created under MK 1  needs to be revoked. But if the “rekey” process for Man CA is performed under the assumption that the key could be compromised in the future, not because it is already compromised, then certificates do not need to be updated in devices that are already created under the MK 1 . 
   Device rekeying can also be used to allow a device to recover from a compromised key. The device re-key is performed by the device&#39;s associated Manufacturer CA. The Manufacturer CA generates the device key pair and issues a new device certificate. 
   Table III summarizes some use cases and requirements for certificate rekey scenarios. 
   
     
       
             
             
             
           
         
             
               TABLE III 
             
             
                 
             
             
               Use 
                 
                 
             
             
               Cases 
               Rekey Scenarios 
               Actions 
             
             
                 
             
           
           
             
               1 
               Only the ROOT CA requires rekey 
               The ROOT CA shall use/distribute its new certificate to 
             
             
                 
               Neither the Manufacturer/CA(s) nor 
               manufacturer CA(s), whenever it receives a new certificate 
             
             
                 
               device(s) requires rekey. 
               request or a certificate renewal request from a Manufacturer. 
             
             
               2 
               Only some Manufacturer/CA(s) 
               The ROOT CA shall always use its most recent and updated 
             
             
                 
               require(s) rekey 
               certificate in response to the manufacturer CAs&#39; rekeying 
             
             
                 
               Neither the ROOT CA nor the 
               renewal requests. The ROOT CA shall generate new 
             
             
                 
               devices require rekey 
               certificates for all re-keying manufacturer CAs. 
             
             
                 
                 
               The re-keying Manufacturer/CA(s) shall use the most recent 
             
             
                 
                 
               and updated key/certificates when they generate new device 
             
             
                 
                 
               certificates. 
             
             
               3 
               Only some devices require rekey 
               The Manufacturer/CAs shall issue new certificates to all re- 
             
             
                 
               Neither the ROOT CA nor the 
               keying devices 
             
             
                 
               Manufacturer/CA(s) requires rekey 
             
             
                 
             
           
        
       
     
   
     FIG. 4  illustrates a high-level message flow of a certificate renewal process. 
   In  FIG. 4 , a first step occurs when a Manufacturer CA (also referred to as a “Sub” CA) makes a request for a new certificate from the Root CA. The request is made using the Manufacturer CA&#39;s current key, MK 1 . The Root CA responds by generating and issuing a certificate response as “(MCert — 1, RCert — 1)”. This notation indicates that the issued certificate includes the new Mfr CA certificate, “MCert — 1,” and the Root CA certificate, “RCert — 1”. Next, a device certificate is needed (e.g., a new device has been manufactured) from the Mfr CA. The Mfr CA issues a certificate response as “(DCert — 1, SCert1, RCert1)” indicating that the response includes the Root CA, Mfr CA and device certificates. 
   At  302 , the white circle symbol on the Root CA&#39;s timeline indicates that a Root CA certificate renewal occurs without a re-key operation. In this case there is no impact to the Mfr CA&#39;s operation and the Mfr CA can continue to issue device certificates as usual. At  304  a Mfr CA renewal (without re-key) occurs and the Mfr CA obtains a new certificate from the Root CA to continue issuing device certificates with the newly obtained Mfr CA certificate and any current Root CA certificate. Subsequent certificate renewals by the Root CA and Mfr CA proceed in a similar manner. 
   As indicated by bracket  310 , at  306  a re-key renewal for the Mfr CA occurs. This causes a new Mfr CA certificate including a new Mfr CA key to be issued by the Root CA to the Mfr CA. At  308  a Root CA re-key operation occurs. 
   Although the invention has been discussed with reference to specific embodiments thereof, these embodiments are illustrative, and not restrictive, of the invention. For example, although the PKI system is used in a preferred embodiment, other embodiments may merely use portions of the PKI system or can use other systems. Although specific embodiments have been described in connection with a digital video distribution network, other applications are possible that use different devices, organization, number of levels, etc. 
   Any number of levels can be used in a trust hierarchy. The trust hierarchy need not follow the rigid level structure shown in  FIG. 1 . For example, it may be possible for multiple manufacturer levels (e.g., manufacturer subsidiary, affiliate), root and device levels to exist in a same hierarchy. Any number of CAs can exist at any of the levels. Other security approaches can be used in conjunction with the features of the present invention&#39;s embodiments. 
   In the description herein, numerous specific details are provided, such as examples of components and/or methods, to provide a thorough understanding of embodiments of the present invention. One skilled in the relevant art will recognize, however, that an embodiment of the invention can be practiced without one or more of the specific details, or with other apparatus, systems, assemblies, methods, components, materials, parts, and/or the like. In other instances, well-known structures, materials, or operations are not specifically shown or described in detail to avoid obscuring aspects of embodiments of the present invention. 
   A “computer-readable carrier” for purposes of embodiments of the present invention may be any medium or transmission that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, system or device. The computer readable carrier can be, by way of example only but not by limitation, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, system, device, propagation medium, or computer memory. 
   A “processor” or “process” includes any human, hardware and/or software system, mechanism or component that processes data, signals or other information. A processor can include a system with a general-purpose central processing unit, multiple processing units, dedicated circuitry for achieving functionality, or other systems. Processing need not be limited to a geographic location, or have temporal limitations. For example, a processor can perform its functions in “real time,” “offline,” in a “batch mode,” etc. Portions of processing can be performed at different times and at different locations, by different (or the same) processing systems. 
   Reference throughout this specification to “one embodiment”, “an embodiment”, or “a specific embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention and not necessarily in all embodiments. Thus, respective appearances of the phrases “in one embodiment”, “in an embodiment”, or “in a specific embodiment” in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics of any specific embodiment of the present invention may be combined in any suitable manner with one or more other embodiments. It is to be understood that other variations and modifications of the embodiments of the present invention described and illustrated herein are possible in light of the teachings herein and are to be considered as part of the spirit and scope of the present invention. 
   Embodiments of the invention may be implemented by using a programmed general purpose digital computer, by using application specific integrated circuits, programmable logic devices, field programmable gate arrays, optical, chemical, biological, quantum or nanoengineered systems, components and mechanisms may be used. In general, the functions of the present invention can be achieved by any means as is known in the art. Distributed, or networked systems, components and circuits can be used. Communication, or transfer, of data may be wired, wireless, or by any other means. 
   It will also be appreciated that one or more of the elements depicted in the drawings/figures can also be implemented in a more separated or integrated manner, or even removed or rendered as inoperable in certain cases, as is useful in accordance with a particular application. It is also within the spirit and scope of the present invention to implement a program or code that can be stored in a machine-readable medium to permit a computer to perform any of the methods described above. 
   Additionally, any signal arrows in the drawings/Figures should be considered only as exemplary, and not limiting, unless otherwise specifically noted. Furthermore, the term “or” as used herein is generally intended to mean “and/or” unless otherwise indicated. Combinations of components or steps will also be considered as being noted, where terminology is foreseen as rendering the ability to separate or combine is unclear. 
   As used in the description herein and throughout the claims that follow, “a”, “an”, and “the” includes plural references unless the context clearly dictates otherwise. Also, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. 
   The foregoing description of illustrated embodiments of the present invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed herein. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes only, various equivalent modifications are possible within the spirit and scope of the present invention, as those skilled in the relevant art will recognize and appreciate. As indicated, these modifications may be made to the present invention in light of the foregoing description of illustrated embodiments of the present invention and are to be included within the spirit and scope of the present invention. 
   Thus, while the present invention has been described herein with reference to particular embodiments thereof, a latitude of modification, various changes and substitutions are intended in the foregoing disclosures, and it will be appreciated that in some instances some features of embodiments of the invention will be employed without a corresponding use of other features without departing from the scope and spirit of the invention as set forth. Therefore, many modifications may be made to adapt a particular situation or material to the essential scope and spirit of the present invention. It is intended that the invention not be limited to the particular terms used in the following claims and/or to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include any and all embodiments and equivalents falling within the scope of the appended claims.