Intelligent caching for ocsp service optimization

BACKGROUND

0001In a conventional public key infrastructure (PKI) security system, public key certificates (also known as digital certificates) are issued by a certificate authority (CA) to bind the public key of the subject with the subject identity. The certificate can then be used by other parties to verify that a public key belongs to a certain entity, individual or organization. However, later on, the CA may decide to revoke some of the certificates it has issued for a variety of reasons. Thus, any party that relies on certificates for performing any security functions should verify that the certificate it is using has not been revoked. The CA typically puts the serial numbers of revoked certificates on a certificate revocation list (CRL). However many devices have difficulty using CRLs for checking revocation status, due to issues such as lack of network connectivity, or insufficient bandwidth or processing power when dealing with large CRLs. Thus many PKI systems provide an online certificate status checking protocol (OCSP). Any party that holds a certificate from another party can use OCSP for verifying revocation status of the certificate instead of downloading a full CRL. This is typically done by sending an OCSP request to the CA or a designated responder (an OCSP responder) and then receiving an OCSP response.

0002An OCSP request contains enough information to uniquely identify the certificate (e.g., a serial number for the certificate to be checked, along with the hashed version of the CA's name and/or key). For high volume applications, the OCSP request is not signed and thus the identity of the requestor is not included in the request either. This provides some flexibility in the way the OCSP request can be generated. An OCSP response provides a status value for the identified certificate. Unlike OCSP requests, OCSP responses must be signed by an authority that the requester trusts. This is either the CA that has issued the certificate and is acting as OCSP responder or a separate OCSP responder designated by that CA.

0003The OCSP response has a time stamp and a validity period. The responder is identified by a hash of its public key and its certificate. The certificate for which status is being reported is identified as it was done in the OCSP request (serial number and hash of issuer name and/or key).

0004Although OCSP provides for an easier method for checking certificate status, there are a number of issues relating to its implementation and scalability. First of all, OCSP responses must be signed with the OCSP responder's private key, which is typically stored inside a hardware security module (HSM). So every time a signature is required, this key inside the HSM must be accessed, and used to perform the signing function, both of which takes CPU time, which is a valuable resource for the HSM. Therefore, the OCSP responder performance is directly affected by the HSM signing performance. Another issue is since OCSP is provided by an online server (over HTTP), the bandwidth of the link between the OCSP requester and the OCSP responder can be a bottleneck and a major contribution to OCSP service costs. Third, in many cases, the consumer of the OCSP service has to spend real time waiting for an OCSP response in order to be able to complete a certain function. The roundtrip delay of requesting and then receiving an OCSP response may thus have a negative effect on the performance of that function.

0005To address these various issues, there are several optimizations to OCSP that can be implemented. One is caching of the OCSP response. An OCSP response may be kept in a cache up to the end of its validity period. In this way, within the validity period, there may not be a need to send any more requests to the OCSP responder, and thus bandwidth and signing CPU time can be saved. The validity period is set depending on the applications and entity for which the certificate is used, the likelihood for certificate revocation and other security considerations.

0006This caching of the OCSP response can be used in conjunction with another OCSP optimization, the use of an OCSP proxy. An OCSP proxy can interact with an OCSP responder on behalf of the end device by sending OCSP requests, and then caching the responses and responding to the end device accordingly. This is facilitated by the fact that OCSP requests do not require signatures and thus can be issued by entities other than the end device. In some cases, OCSP proxy can also passively observe the OCSP transaction between the end device and the OCSP responder and then cache certain information (e.g. parts of or the whole OCSP response) according to its policies. Since the use of an OCSP proxy (along with a cache of OCSP responses) reduces the number of interactions with the OCSP responder, the bandwidth and overall time required to obtain an OCSP response is greatly reduced. An example of this approach will now be discussed in reference to <figref idrefs="DRAWINGS">FIG. 1</figref>.

0007<figref idrefs="DRAWINGS">FIG. 1</figref> is a schematic illustrating a conventional system <b>100</b> implementing OCSP service using a proxy and a cache. System <b>100</b> includes a device <b>101</b>, an end device <b>102</b>, an OCSP proxy <b>104</b>, a cache <b>106</b>, a CRL <b>108</b>, an OCSP responder <b>110</b> and a CA <b>112</b>. In practice, a plurality of devices may potentially be interacting with a single end device, however to simplify explanation, only one device (device <b>101</b>) is shown. Similarly, in practice, a plurality of end devices and CAs may potentially be interacting with a single OCSP proxy, however for simplicity in explanation, only one of each is shown.

0008In this example, presume device <b>101</b> initiates secure communication with end device <b>102</b>. During initiation, device <b>101</b> provides end device <b>102</b> with a certificate of device <b>101</b>, wherein the certificate includes its public key. The public key will be used by both device <b>101</b> and device <b>102</b> for secure communication. The certificate is used to verify the identity of device <b>101</b>. Before end device <b>102</b> continues to securely communicate with device <b>101</b> (or very early on in the communication), it must determine whether the public key provided by device <b>101</b> is still valid. Presume in this case that device <b>101</b> has previously registered (or received) its public key with (from) CA <b>112</b>. Further, presume that CA <b>112</b> had provided device <b>101</b> with a certificate of validation for its public key. The certificate will have some sort of identifier, such as a serial number, that ties the certificate to the public key of device <b>101</b> and CA <b>112</b>. The certificate will additionally have a validation period, for which the certificate is “good.” After expiration of the validation period, the initial certificate is no longer usable, and the public key of device <b>101</b> may be issued a new certificate by CA <b>112</b>, if required.

0009If end device <b>102</b> can verify that the certificate provided by device <b>101</b> is still valid, then end device <b>102</b> may securely communicate with device <b>101</b>.

0010An example process <b>200</b> of secure communication of system <b>100</b> will now be described with reference to <figref idrefs="DRAWINGS">FIG. 2</figref>.

0011In operation, process <b>200</b> starts (S<b>202</b>) and device <b>101</b> provides end device <b>102</b> with a public key and a corresponding certificate (S<b>204</b>). As mentioned above, when device <b>101</b> initiates secure communication with end device <b>102</b>, device <b>101</b> sends the public key and a corresponding certificate to end device <b>102</b>. The certificate provides end device <b>102</b> with the capability to verify the identity of device <b>101</b>.

0012Upon receiving the certificate, end device <b>102</b> sends an OCSP request to OCSP proxy <b>104</b> (S<b>206</b>), which includes the certificate identifier and issuer of the certificate to be verified. Typically the end device <b>102</b> is not aware of the presence of OCSP proxy <b>104</b>, so end device <b>102</b> sends the OCSP request towards the address that end device <b>102</b> has for OCSP responder <b>110</b>. The OCSP request is either intercepted by OCSP proxy <b>104</b>, or the network will redirect the request to OCSP proxy <b>104</b>. For example, for purposes of discussion, presume that device <b>101</b> is attempting to securely communicate with end device <b>102</b>. Device <b>101</b> will send a public key having a public key certificate associated therewith to end device <b>102</b>. The certificate may have a serial number associated therewith. Further, presume in this example, that the certificate of device <b>101</b> has been registered with CA <b>112</b> and thereby indirectly with OCSP responder <b>110</b>.

0013OCSP proxy <b>104</b> then searches cache <b>106</b> for response for the certificate identifier (S<b>208</b>) to check if a response has been cached (S<b>210</b>). In this example, in the event that the certificate corresponding to the public key provided by device <b>101</b> had been recently requested, the OCSP response corresponding thereto would have been stored in cache <b>106</b>.

0014If an OCSP response is found in cache <b>106</b>, then first the status and validity period of the cached OCSP response is verified (S<b>212</b>). A valid (not expired) OCSP response will indicate whether the certificate corresponding to the public key provided by device <b>101</b> is still valid or has been revoked. Further, the validity of the OCSP response will indicate the time period that this OCSP response can be used to verify the status of the certificate corresponding to the public key provided by device <b>101</b>, e.g., a number of days.

0015If cached OCSP response is still valid and indicates that the certificate status is valid and not revoked, OCSP proxy <b>104</b> then searches the latest CRL <b>108</b> for the certificate identifier (S<b>214</b>) to check if the certificate which corresponds to the certificate identifier, has been revoked (S<b>216</b>) since the time the OCSP response has been cached. There may be situations where the public key provided by device <b>101</b> has been compromised. In such situations, CA <b>112</b> may revoke the certificate corresponding to the public key provided by device <b>101</b> and add the certificate identifier (e.g. serial number) to CRL <b>108</b>. In such an instance, the certificate corresponding to the public key provided by device <b>101</b> will be listed on the latest CRL <b>108</b> provided by CA <b>112</b>, even though a very recent OCSP response on the same certificate might have indicated the certificate as valid. This is why OCSP proxy <b>104</b> must check the latest CRL <b>108</b> before trusting its cached OCSP responses.

0016If the certificate identifier is not found on CRL <b>108</b>, and the validity period of the cached OCSP response includes the present time, the cached OCSP response is still valid and thus OCSP proxy <b>104</b> forwards the cached response to end device <b>102</b> (S<b>218</b>) and process <b>200</b> ends (S<b>220</b>). In this situation, the secure communication with device <b>101</b> may commence or continue.

0017However, if an OCSP response for the certificate identifier cannot be found in cache <b>106</b>, if the cached OCSP response is expired, or if the certificate identifier is found in CRL <b>108</b>, then OCSP proxy <b>104</b> forwards the OCSP request to OCSP responder <b>110</b> (S<b>222</b>) so that OCSP responder <b>110</b> can issue a new OCSP response for the certificate.

0018OCSP responder <b>110</b> then provides a signed OCSP response with a validity period (S<b>224</b>). In this situation, OCSP responder <b>110</b> has indicated that the certificate corresponding to the public key provided by device <b>101</b> is valid.

0019OCSP proxy <b>104</b> caches this new OCSP response in cache <b>106</b> (S<b>226</b>) according to its policies regarding the certificate. There may be an instance where end device <b>102</b>, or some other device using OCSP proxy <b>104</b>, may make an inquiry relating to the certificate corresponding to the public key provided by device <b>101</b>. By caching the new OCSP response in cache <b>106</b>, OCSP proxy <b>104</b> will not need to check with OCSP responder <b>110</b>, as discussed above with reference to step S<b>210</b> for future queries.

0020Finally OCSP proxy <b>104</b> forwards the new response to end device <b>102</b> (S<b>218</b>) and process <b>200</b> ends (S<b>220</b>). If the OCSP response indicates that the certificate is valid, at this point end device <b>102</b> is that the certificate corresponding to the public key provided by device <b>101</b> is good, and secure communication with device <b>101</b> may commence. If the OCSP response indicates that the certificate was revoked, end device <b>102</b> will follow the prescribed policies regarding revoked certificates and typically this at least includes: not starting, if it has already started, ending communications with device <b>101</b>.

0021Concurrent with performance of process <b>200</b>, CRL <b>108</b> is updated on a regular basis by CA <b>112</b>, such that CRL <b>108</b> maintains a current list of the revoked certificates. OCSP responder <b>110</b> is also regularly updated with CRL data, such that it can provide the appropriate response when prompted by OCSP proxy <b>104</b>. To perform the optimizations described here, OCSP proxy <b>104</b> is also regularly updated with the CRL data from CA <b>112</b>.

0022As illustrated in <figref idrefs="DRAWINGS">FIGS. 1 and 2</figref>, OCSP proxy <b>104</b> manages the OCSP requests from end device <b>102</b> such that interaction with OCSP responder <b>110</b> is reduced. By storing OCSP responses in cache <b>106</b>, OCSP proxy <b>104</b> may provide end device <b>102</b> with OCSP responses without contacting OCSP responder <b>110</b>. Only when responses are expired, revoked, or not found in cache <b>106</b> is it necessary for OCSP proxy <b>104</b> to contact OCSP responder <b>110</b> such that a new response can be obtained and forwarded to end device <b>102</b>. In this manner, the bandwidth consumption between end device <b>102</b> and OCSP responder <b>110</b> is reduced. Further, the number of times OCSP responder <b>110</b> must sign an OCSP response (requiring access of private key stored in HSM, taking up CPU time) is reduced, thus improving the overall speed of the OCSP request/response process.

0023While this use of response caching and proxies is effective in improving the performance of OCSP systems, there is still a need for further improvement.

0024What is needed is a system and method which implements “intelligent” caching of OCSP responses such that the performance of OCSP service can be further optimized.

BRIEF SUMMARY

0025The present invention provides a system and method for implementing “intelligent” caching of OCSP responses such that the performance of OCSP service can be further optimized.

0026In accordance with an aspect of the present invention, an online certificate status checking protocol (OCSP) system is provided for use with a first device, an end device and a certificate authority. The first device can provide a certificate. The end device can provide an OCSP request based on the certificate and process an OCSP response. The certificate authority can provide a CRL update. The certificate has a validity period. The OCSP system includes an OCSP responder, and OCSP proxy and a cache. The OCSP responder can provide the OCSP response. The OCSP proxy can receive the OCSP request from the end device, can send the OCSP request to the OCSP responder, can receive the OCSP response from the OCSP responder and can send the OCSP response to the end device. The cache can store information based on the OCSP response. The OCSP proxy can further store, in the cache, information based on the OCSP response and can send a proactive OCSP request to the OCSP responder based on a predetermined policy. The OCSP responder can further send a proactive OCSP response to the OCSP proxy in response to the proactive OCSP request. The OCSP proxy can further update the information in the cache based on the proactive OCSP response. The OCSP proxy can additionally provide, using the updated information in the cache, a second OCSP response to the end device in response to a subsequent request from the end device related to information of the certificate.

0027Additional advantages and novel features of the invention are set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF SUMMARY OF THE DRAWINGS

0028The accompanying drawings, which are incorporated in and form a part of the specification, illustrate an exemplary embodiment of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:

0029<figref idrefs="DRAWINGS">FIG. 1</figref> is a schematic illustrating a conventional system implementing OCSP service using a proxy and a cache;

0030<figref idrefs="DRAWINGS">FIG. 2</figref> illustrates an example method for the operation of the conventional system of <figref idrefs="DRAWINGS">FIG. 1</figref>;

0031<figref idrefs="DRAWINGS">FIG. 3</figref> illustrates an example method for the operation of system <figref idrefs="DRAWINGS">FIG. 1</figref>, in accordance with an aspect of the present invention; and

0032<figref idrefs="DRAWINGS">FIG. 4</figref> illustrates an example method for the maintenance of a cache and a CRL in accordance with an aspect of the present invention.

DETAILED DESCRIPTION

0033In accordance with an aspect of the present invention, a system and process are provided that implement intelligent caching (e.g. frequency-based predictive caching) within an OCSP system. By maintaining a cache of OCSP responses based on predetermined policies, the performance/cost of OCSP service can be further optimized. Non-limiting examples of predetermined policies in accordance with aspects of the present invention include a predetermined policy based on the types of devices and a predetermined policy based on a frequency of query for status checking on the device certificate.

0034There are many instances where hundreds of devices (for example, hundreds of devices like end device <b>102</b>) need to verify the validity of the certificate of devices such as device <b>101</b>, for example, on every given minute or less. These thousands of devices would rely on the OCSP response regarding the certificate for device <b>101</b>. This would mean that OCSP responder <b>110</b> will need to sign and return hundreds of OCSP responses on the same certificate. In a network having a large number of devices such as device <b>101</b>, will introduce a large load for OCSP responders responding to end devices such as device <b>102</b> querying the validity of certificates for devices such as device <b>101</b>. Caching the server certificate OCSP responses on OCSP proxy <b>104</b> could reduce the need for OCSP requests from OCSP proxy <b>104</b> by orders of magnitude. Furthermore, the daily routine maintenance of cache <b>106</b> can be done in off rush yours, e.g., nighttime, when the OCSP traffic is low.

0035In accordance with an aspect of the present invention, OCSP proxy <b>104</b> caches the OCSP responses based on policies and then monitors OCSP response entries in cache <b>106</b> on a regular basis. OCSP proxy <b>104</b> then compares the validity of the OCSP response entries based on their validity period. For example, some OCSP response entries may have been in cache <b>106</b> for 31 days, whereas their validity period is only 30 days. Such OCSP response entries would therefore be invalid. OCSP proxy <b>104</b> may then decide which entries should be dropped from cache <b>106</b> or refreshed within cache <b>106</b> based on a predetermined policy.

0036In one example policy, OCSP proxy <b>104</b> decides which OCSP response entries in cache <b>106</b> should be dropped or refreshed based on the number of OCSP requests for a certificate. For example, an OCSP response entry in cache <b>106</b> corresponding to a certificate that has not been queried in weeks since its initial query is unlikely to be queried within the next day. On the other hand, an OCSP response entry in cache <b>106</b> corresponding to a certificate that has been queried several times in the last day will very likely be queried again in the next day. Therefore, automatically refreshing the OCSP response entry in cache <b>106</b> corresponding to a certificate that has been queried a several times in the last day will preempt the need to refresh the OCSP response the first time it is queried after the validity period of that OCSP response expires.

0037In another example policy, OCSP proxy <b>104</b> decides which OCSP response entries in cache <b>106</b> should be dropped or refreshed based on whether the certificate is now on CRL <b>108</b>. For example, an OCSP response entry in cache <b>106</b>, indicating a valid certificate, corresponding to a certificate that was previously valid, but has been revoked by CA <b>112</b> can no longer be used. The certificate will now be listed on CRL <b>108</b> and thus OCSP response for that certificate should now indicate a revoked status. If such policy is in place, OCSP proxy <b>104</b> can request OCSP responder <b>110</b> to sign a new OCSP response for this revoked certificate and instead cache the new response that indicates the certificate is now revoked. Alternatively, OCSP proxy <b>104</b> may simply just remove all such OCSP response entries from cache <b>106</b> (for certificates that are on CRL <b>108</b>), and thereby the remaining number of entries in cache <b>106</b> will decrease thereby decreasing search time.

0038In another example policy, OCSP proxy <b>104</b> decides which OCSP response entries in cache <b>106</b> should be dropped from or refreshed based whether the validity of the OCSP response is too close to expiration. For example, suppose there is an OCSP response entry in cache <b>106</b>, whose validity will expire in one minute or one hour or one day. In such a case, a policy might be in place to determine whether it is more efficient to let that OCSP response expire and delete it from cache <b>106</b> or have OCSP proxy <b>104</b> proactively create an OCSP request to replace the expiring cache with a fresh one. The policy for determination on use of proactive OCSP requests can be based a variety of factors, such as number of queries for that certificate validity, cost of certificate, size of cache, etc.

0039In accordance with another aspect of the present invention, OCSP proxy <b>104</b> may set a priority for proactive OCSP requests, based on policies. In one example policy, OCSP proxy <b>104</b> may set a priority based on which it would send the proactive OCSP requests. The priority may be based on the costs of the certificates (type of device) corresponding to the OCSP requests. In another example policy, OCSP proxy <b>104</b> may set a priority for proactive OCSP requests, based on the popularity of certificates corresponding to the OCSP requests, e.g., how often a certificate is queried. Another example policy for setting the priority may be the time left from validity period for the cached OCSP response.

0040In accordance with another aspect of the present invention, OCSP proxy <b>104</b> may, based on its policies, suggest specific validity periods for the OCSP response to OCSP responder <b>110</b>. The suggestion may take the form of an OCSP request extension or as a proprietary message. OCSP responder <b>110</b> may then, in response, set the validity period for the provided OCSP response based on the value hinted/requested by OCSP proxy <b>104</b> or ignore the hinted value.

0041In accordance with another aspect of the present invention, OCSP proxy <b>104</b> may interact with end device <b>102</b> in a non-OCSP manner. For example, OCSP proxy <b>104</b> may receive a certificate, e.g., from end device <b>102</b> for device <b>101</b>, as part of a proprietary or standard communication protocol. OCSP proxy <b>104</b> may then generate an OCSP request that includes an extension indicating to OCSP responder <b>110</b> that end device <b>102</b> does not understand OCSP. In this manner, OCSP responder <b>110</b> may provide minimum information to end device <b>102</b>, such as a certificate serial number, a status, a date and a signature by the OCSP responder.

0042An example embodiment in accordance with an aspect of the present invention will now be described with reference to <figref idrefs="DRAWINGS">FIGS. 1</figref>, <b>3</b> and <b>4</b>.

0043In accordance with an aspect of the present invention, system <b>100</b> of <figref idrefs="DRAWINGS">FIG. 1</figref> is used to implement an OCSP service, but with a different method of operation, such that the frequency of requests for each OCSP response is monitored and then predictive caching of the OCSP response is done so accordingly. First, an example process <b>300</b> for the general operation of system <b>100</b>, in accordance with an aspect of the present invention, will be described with reference to <figref idrefs="DRAWINGS">FIG. 3</figref>.

0044In operation, process <b>300</b> is similar to process <b>200</b> discussed above with reference to <figref idrefs="DRAWINGS">FIG. 2</figref> up to step S<b>216</b>, wherein if status the and validity period is good and not expired, OCSP proxy <b>104</b> then searches CRL <b>108</b> for the certificate identifier to check if the certificate has been revoked. Again, for purposes of discussion, presume that device <b>101</b> is attempting to securely communicate with end device <b>102</b>. In process <b>300</b>, after step S<b>216</b>, if the certificate is not found on CRL <b>108</b>, then the OCSP response is deemed to still be valid. At this point, OCSP proxy <b>104</b> increments the number of requests for the OCSP response corresponding to the public key provided by device <b>101</b> (S<b>302</b>).

0045OCSP proxy <b>104</b> then forwards the cached response to end device <b>102</b> (S<b>304</b>) and process <b>300</b> ends (S<b>306</b>). At this point end device <b>102</b> is assured that the certificate corresponding to the public key provided by device <b>101</b> is good and secure communication with device <b>101</b> may commence.

0046Similar to process <b>200</b>, in process <b>300</b>, if in step S<b>210</b> a response for the certificate identifier cannot be found in cache, or if in step S<b>212</b> the cached response is expired or if in step S<b>216</b> the certificate identifier is found in CRL <b>108</b>, then OCSP proxy <b>104</b> forwards the OCSP request to OCSP responder <b>110</b> (S<b>222</b>). Additionally similar to process <b>200</b>, in process <b>300</b>, at this point, OCSP responder <b>110</b> then provides a signed OCSP response with a validity period (S<b>224</b>) and OCSP proxy <b>104</b> caches this new OCSP response in cache <b>106</b> (S<b>226</b>).

0047In contrast with method <b>200</b>, in method <b>300</b>, number of requests of the new OCSP response is set to <b>1</b>, since this is the first time the new response is being requested (S<b>308</b>).

0048Finally, OCSP proxy <b>104</b> forwards the new OCSP response to end device <b>102</b> (S<b>304</b>) and process <b>200</b> ends (S<b>306</b>). At this point end device <b>102</b> is assured that the OCSP response on the certificate corresponding to the public key provided by device <b>101</b> is a valid one. If the OCSP response indicates a valid certificate, then end device <b>102</b> is assured the certificate is good, and secure communication with device <b>101</b> may commence. If on the other hand, the OCSP response indicates the certificate is revoked, then end device <b>102</b> will reject any attempt from device <b>101</b> to establish communications or associations or terminate any communications or associations if such is already in place.

0049Concurrent with performance of process <b>300</b>, CRL <b>108</b> is updated on a regular basis by CA <b>112</b>, such that CRL <b>108</b> maintains a current list of the revoked certificates. OCSP responder <b>110</b> is also regularly updated with CRL data, so it can provide the appropriate response when so prompted by OCSP proxy <b>104</b>. This is similar to the conventional implementation of OCSP service. However, unlike the conventional implementation, in accordance with an aspect of the present invention, OCSP proxy <b>104</b> also updates and maintains cache <b>106</b> in a certain manner to allow for predictive caching based on the frequency of response requests. This process will be described in further detail below.

0050An example process <b>400</b> for the maintenance of cache <b>106</b> and CRL <b>108</b> in accordance with an aspect of the present invention will now be described with reference to <figref idrefs="DRAWINGS">FIG. 4</figref>.

0051Process <b>400</b> starts (S<b>402</b>) and CRL <b>108</b> is updated with CRL data from CA <b>112</b> (S<b>404</b>). The CRL data includes an updated list of certificates corresponding to public keys, respectively, that have been revoked. The certificates may be listed as certificate identifiers, non-limiting examples of which include serial numbers.

0052If certificate identifier in OCSP response cache <b>106</b> indicating a good certificate is not on CRL <b>108</b>, then it the OCSP response is still considered to be valid and certificate status still valid (not revoked). For each certificate identifier with OCSP response indicating a good status in cache <b>106</b>, OCSP proxy <b>104</b> examines the OCSP response validity period (S<b>406</b>).

0053It is then determined whether the remaining validity period is below a predetermined expiration threshold (S<b>408</b>). This predetermined expiration threshold may be static or may be changed based on the needs of system <b>100</b>. For example, if it is determined that an original predetermined expiration threshold of 30 days is too long, the system may be configured to reduce the predetermined expiration threshold.

0054If it is determined that the remaining validity period of a certificate identifier is below the predetermined expiration threshold, it is then determined whether the number of requests for the certificate identifier is greater than a request frequency threshold (S<b>410</b>). For example, if a particular certificate identifier has been requested many times within a predetermined time period, there is an increased likelihood the particular certificate identifier will be requested again in the near future. Alternatively, if a particular certificate identifier has not been requested at all within a predetermined time period, there is a deceased likelihood the particular certificate identifier will be requested again in the near future. In such cases, there may be other policies that would dictate OCSP proxy <b>104</b> to create predictive OCSP requests for pre-signing. Non-limiting examples of such policies include predictive OCSP requests for pre-signing when the cost of the certificate is high, or predictive OCSP requests for pre-signing based on the type of device such that maintaining cache <b>106</b> is considered to have economic value.

0055If it is determined that the number of requests for a certificate identifier is greater than the request frequency threshold, this indicates that the certificate is one that is frequently queried, and thus keeping a response cached would be economical. Since the validity period of the response is about to expire, however, a new response is needed to replace it in cache <b>106</b>. Thus, OCSP proxy <b>104</b> creates a predictive OCSP request for pre-signing (S<b>412</b>), and sends this request to OCSP responder <b>110</b> (S<b>414</b>).

0056OCSP proxy <b>104</b> receives the new signed response from OCSP responder <b>110</b> and then caches the new response for future use (S<b>416</b>). The number of requests Nr for this new response is then set to 0 (S<b>418</b>). Then, OCSP proxy <b>104</b> checks if there are more certificate identifiers with a good status in cache <b>106</b> to check (S<b>424</b>). If not, process <b>400</b> ends (S<b>426</b>) and update of cache <b>106</b> is completed for that time period. If there are more responses to check, process <b>400</b> returns to step S<b>406</b> and the process repeats with the next serial number in cache <b>106</b>.

0057Returning back to step S<b>408</b>, if the remaining validity period of the response is not about to expire, then the next step is to search CRL <b>108</b> for the serial number (S<b>428</b>). OCSP proxy <b>104</b> checks if the certificate identifier is found in CRL <b>108</b> (S<b>430</b>) and if not, the current cached response for that certificate identifier is still useable and therefore the process moves on to check the response for the next certificate identifier in cache <b>106</b> (S<b>424</b>). However, if the certificate identifier is found in CRL <b>108</b>, then that means that the certificate is now revoked and a new response (indicating the certificates “revoked” status) is now needed. Thus OCSP proxy <b>104</b> deletes the current cached response from cache <b>106</b> (S<b>432</b>) and then goes on to create a pre-signing request to be sent to OCSP responder <b>110</b> (S<b>412</b>) such that an updated response (noting a “revoked” certificate status) can be obtained from OCSP responder <b>110</b>.

0058Returning back to step S<b>410</b>, for the case when the number of requests is actually less than the predetermined threshold, OCSP proxy <b>104</b> first checks if the response is already expired (S<b>420</b>). If so, this indicates that the certificate was not being checked frequently enough for OCSP proxy <b>104</b> to refresh its cached response, and thus the response is deleted from cache <b>106</b> (S<b>422</b>), and the process moves on to check the next certificate identifier (S<b>424</b>). However, if the response was not already expired, then it is simply left alone and the process moves on to check the next certificate identifier (S<b>424</b>).

0059The above process details how OCSP proxy <b>104</b> regularly maintains cache <b>106</b> and CRL <b>108</b> such that the performance of OCSP service is further optimized. Note that in order to be effective in improving performance, OCSP proxy <b>104</b> needs to perform process <b>400</b> on a regular, relatively frequent basis with an interval that is smaller than the OCSP response validity interval. For example, process <b>400</b> may be performed on a daily basis (such that cache <b>106</b> and CRL <b>108</b> are each updated daily), while the OCSP response validity interval may be 30 days.

0060Regarding the pre-signing requests being sent to OCSP responder <b>110</b> (such as in steps <b>5412</b> and step S<b>414</b> in process <b>400</b>), in accordance with an aspect of the present invention, OCSP proxy <b>104</b> may prioritize the pre-signing requests, so that requests with higher priority are sent to OCSP responder <b>110</b> first. High priority may be given based on a variety of factors. More frequently queried certificates (those responses with higher Nr) can be given higher priority. Responses with validity periods closer to expiration may be given higher priority. There may also be other policies that determine the priority, such as the cost of the certificate or type of certificate. In accordance with an aspect of the present invention, the priority may be implemented by the amount of time ahead of the expiry data the pre-signing request is issued. For example, very high priority certificates can be pre-signed 7 days prior to expiry, while medium-priority ones 5 days prior, and others 1 day prior.

0061Any of end device <b>102</b>, OCSP proxy <b>104</b>, CRL <b>108</b>, OCSP responder <b>110</b> and CA <b>112</b> may be embodied in hardware, software, or a combination of both. When embodied in software, any of end device <b>102</b>, OCSP proxy <b>104</b>, CRL <b>108</b>, OCSP responder <b>110</b> and CA <b>112</b> may be embodied in the form of program code (i.e., instructions). This program code may be stored on a computer-readable medium, such as a magnetic, electrical, or optical storage medium, non-limiting examples of which include a floppy diskette, a CD-ROM, a CD-RW, a DVD-ROM, a DVD-RAM, a magnetic tape, a flash memory, a hard disk drive, or any other machine-readable storage medium, wherein, when the program code is loaded into and executed by a machine, such as a computer or server, the machine becomes an apparatus for practicing aspects of the present invention, or portions thereof. A computer on which the program code executes will generally include a processor, a storage medium readable by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device, and at least one output device. The program code may be implemented in a high level procedural or object oriented programming language. Alternatively, the program code can be implemented in an assembly or machine language. In any case, the language may be a compiled or interpreted language.

0062Any of end device <b>102</b>, OCSP proxy <b>104</b>, CRL <b>108</b>, OCSP responder <b>110</b> and CA <b>112</b> may also be embodied in the form of program code that is transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, over a network, including a local area network, a wide area network, the Internet or an intranet, or via any other form of transmission, wherein, when the program code is received and loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing aspects of the present invention, or portions thereof.

0063When implemented on a general-purpose processor, the program code may combine with the processor to provide a unique apparatus that operates analogously to specific logic circuits.

0064Although not required, any of end device <b>102</b>, OCSP proxy <b>104</b>, CRL <b>108</b>, OCSP responder <b>110</b> and CA <b>112</b> may be implemented via an operating system, for use by a developer of services for a device or object, and/or included within application or server software that operates in accordance with methods and apparatuses in accordance with aspects of the present invention, or portions thereof. Software may be described in the general context of computer-executable instructions, such as program modules, being executed by one or more computers, such as client workstations, servers or other devices. Generally, program modules include routines, programs, objects, components, data structures and the like that perform particular tasks or implement particular abstract data types. Typically, the functionality of the program modules may be combined or distributed as desired in various embodiments. Moreover, any of end device <b>102</b>, OCSP proxy <b>104</b>, CRL <b>108</b>, OCSP responder <b>110</b> and CA <b>112</b> may be practiced with other computer system configurations and protocols. Other well known non-limiting examples of computing systems, environments, and/or configurations that may be suitable for use with methods and apparatuses in accordance with aspects of the present invention, or portions thereof, include personal computers (PCs), automated teller machines, server computers, hand-held or laptop devices, multi-processor systems, microprocessor-based systems, programmable consumer electronics, network PCs, appliances, lights, environmental control elements, minicomputers, mainframe computers and the like.

0065Any of end device <b>102</b>, OCSP proxy <b>104</b>, CRL <b>108</b>, OCSP responder <b>110</b> and CA <b>112</b> may include a variety of computer readable media. Computer readable media can be any available media that can be accessed and includes both volatile and nonvolatile media, removable and non-removable media. By way of example, and not limitation, computer readable media may comprise computer storage media and communication media. Computer storage media include volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Non-limiting examples of computer storage media include RAM, ROM, EEPROM, flash memory or other memory technology, CDROM, digital versatile disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed. Communication media typically embody computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and include any information delivery media. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. Non-limiting examples of communication media include wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared and other wireless media. Combinations of any of the above should also be included within the scope of computer readable media.

0066As mentioned previously in reference to <figref idrefs="DRAWINGS">FIG. 3</figref>, when signing a new OCSP response, OCSP responder <b>110</b> sets the response validity period to a given time (e.g., 30 days). However, in accordance with an aspect of the present invention, OCSP proxy <b>104</b> may actually request OCSP responder <b>110</b> to set the validity period of the signed OCSP response to a specific amount, based on a policy such as frequency of requests, cost of certificate, or security environment. For example, if a certificate is less frequently requested or is of low cost, the OCSP response validity period can be requested to be long. This can be requested using a new validity period extension appended to the OCSP request, or a proprietary messaging method (non-OCSP messaging) between OCSP proxy <b>104</b> and OCSP responder <b>110</b>. OCSP responder <b>110</b> may, based on its own policy, accept or override the requested value.

0067In system <b>100</b> of <figref idrefs="DRAWINGS">FIG. 1</figref>, it was assumed that end device <b>102</b> has OCSP capability (e.g., capability to send OCSP requests and receive OCSP responses). However, in accordance with an aspect of a present invention, end devices without OCSP capability may also send requests to verify certificate status. For end devices that do not have OCSP capability, OCSP proxy <b>104</b> can accept a proprietary certificate status checking request (non-OCSP) from the client interested in certificate validation. OCSP proxy <b>104</b> then creates an OCSP request based on either OCSP standard (RFC 2560 or RFC 5019) but includes a non-standard OCSP_not_supported extension. OCSP responder <b>110</b> can create an OCSP response for OCSP proxy <b>104</b> to cache for other OCSP-capable end devices. Further, OCSP responder <b>110</b> can include an extension non_OCSP_response with enough information for end device <b>102</b> to assert the validity of the status checking results. This information could include: hash of OCSP responder <b>110</b>'s key, certificate serial number, date of status checking, and a signature calculated using OCSP responder <b>110</b>'s public key.

0068The foregoing description of various preferred embodiments of the invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The exemplary embodiments, as described above, were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.