]> SECOM CO., LTD.

tadahiko.ito.public@gmail.com

Apple, Inc.

clintw@apple.com

DigiCert, Inc.

corey.bonnell@digicert.com

sn3rd

sean@sn3rd.com

SEC lamps Revocation This specification defines a profile of the Online Certificate Status Protocol (OCSP) that addresses the scalability issues inherent when using OCSP in large scale (high volume) Public Key Infrastructure (PKI) environments and/or in PKI environments that require a lightweight solution to minimize communication bandwidth and client- side processing. This specification obsoletes RFC 5019. The profile specified in RFC 5019 has been updated to allow and recommend the use of SHA-256 over SHA-1.

Introduction The Online Certificate Status Protocol specifies a mechanism used to determine the status of digital certificates, in lieu of using Certificate Revocation Lists (CRLs). Since its definition in 1999, it has been deployed in a variety of environments and has proven to be a useful certificate status checking mechanism. (For brevity, the term "OCSP" is used herein to denote the verification of certificate status; however, it should be noted that this protocol is employed solely to ascertain the revocation status of a certificate.) To date, numerous OCSP deployments have been implemented to provide timely and secure certificate status information, crucial for high-value electronic transactions and the handling of highly sensitive information, such as within the banking and financial sectors. Therefore, the requirement for an OCSP responder to respond in "real time" (i.e., generating a new OCSP response for each OCSP request) has been important. In addition, these deployments have operated in environments where bandwidth usage is not an issue and have run on client and server systems where processing power is not constrained. As the use of PKI continues to grow and move into diverse environments, so does the need for a scalable and cost-effective certificate status mechanism. Although OCSP as currently defined and deployed meets the need of small to medium-sized PKIs that operate on powerful systems on wired networks, there is a limit as to how these OCSP deployments scale from both an efficiency and cost perspective. Mobile environments, where network bandwidth may be at a premium and client-side devices are constrained from a processing point of view, require the careful use of OCSP to minimize bandwidth usage and client-side processing complexity . PKI continues to be deployed into environments where millions if not hundreds of millions of certificates have been issued. In many of these environments, an even larger number of users (also known as relying parties) have the need to ensure that the certificate they are relying upon has not been revoked. As such, it is important that OCSP is used in such a way that ensures the load on OCSP responders and the network infrastructure required to host those responders are kept to a minimum. This document addresses the scalability issues inherent when using OCSP in highly scaled PKI environments by defining a message profile and clarifying OCSP client and responder behavior that will permit:

1. OCSP response pre-production and distribution.
2. Reduced OCSP message size to lower bandwidth usage.
3. Response message caching both in the network and on the client.

It is intended that the normative requirements defined in this profile will be adopted by OCSP clients and OCSP responders operating in very large-scale (high-volume) PKI environments or PKI environments that require a lightweight solution to minimize bandwidth and client-side processing power (or both), as described above. OCSP does not have the means to signal responder capabilities within the protocol. Thus, clients may need to use out-of-band mechanisms (e.g., agreed upon arrangements between operators of OCSP responders and OCSP clients) to determine whether a responder conforms to the profile defined in this document. Regardless of the availability of such out-of-band mechanisms, this profile ensures that interoperability will still occur between an OCSP client that fully conforms with and a responder that is operating in a mode as described in this specification.

Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

OCSP Message Profile This section defines a subset of OCSPRequest and OCSPResponse functionality as defined in .

OCSP Request Profile

OCSPRequest Structure A partial extract of the ASN.1 structure corresponding to the OCSPRequest with the relevant CertID as defined in is provided here for convenience: OCSPRequests that conform to the profile in this document MUST include only one Request in the OCSPRequest.requestList structure. The CertID.issuerNameHash and CertID.issuerKeyHash fields contain hashes of the issuer's distinguished name (DN) and public key, respectively. OCSP clients that conform with this profile MUST use SHA-256, as defined in , as the hashing algorithm for the CertID.issuerNameHash and the CertID.issuerKeyHash values. Older OCSP clients that provide backward compatibility with use SHA-1, as defined in , as the hashing algorithm for the CertID.issuerNameHash and the CertID.issuerKeyHash values. However, these OCSP clients MUST transition from SHA-1 to SHA-256 as soon as practical. Clients MUST NOT include the singleRequestExtensions structure. Clients SHOULD NOT include the requestExtensions structure. If a requestExtensions structure is included, it is RECOMMENDED by this profile that the structure contain only the nonce extension (id-pkix-ocsp-nonce). See for issues concerning the use of a nonce in high-volume OCSP environments.

Signed OCSPRequests Clients SHOULD NOT send signed OCSPRequests. Responders MAY ignore the signature on OCSPRequests. If the OCSPRequest is signed, the client SHALL specify its name in the OCSPRequest.requestorName field; otherwise, clients SHOULD NOT include the requestorName field in the OCSPRequest. OCSP responders MUST handle unsigned OCSP requests that contain the requestorName field, as if the requestorName field were absent.

OCSP Response Profile

OCSPResponse Structure A partial extract of the ASN.1 structure corresponding to the OCSPResponse with the relevant CertID as defined in is provided here for convenience: The value for response SHALL be the DER encoding of BasicOCSPResponse. Responders MUST generate a BasicOCSPResponse as identified by the id&nbhy;pkix&nbhy;ocsp&nbhy;basic OID. Clients MUST be able to parse and accept a BasicOCSPResponse. OCSPResponses that conform to this profile SHOULD include only one SingleResponse in the ResponseData.responses structure but MAY include additional SingleResponse elements if necessary to improve response pre-generation performance or cache efficiency and to ensure backward compatibility. For instance, to provide support to OCSP clients that do not yet support the use of SHA-256 for CertID hash calculation, the OCSP responder MAY include two SingleResponse elements in a BasicOCSPResponse. In that BasicOCSPResponse, the CertID of one of the SingleResponse structures uses SHA-1 for the hash calculation, and the CertID in the other SingleResponse uses SHA-256. OCSP responders SHOULD NOT distribute OCSP responses that contain CertIDs that use SHA-1 if the OCSP responder has no clients that require the use of SHA-1. Operators of OCSP responders may consider logging the hash algorithm used by OCSP clients to inform their determination of when it is appropriate to obsolete the distribution of OCSP responses that employ SHA-1 for CertID field hashes. See for more information on the security considerations for the continued use of SHA-1. The responder SHOULD NOT include responseExtensions. As specified in , clients MUST ignore unrecognized non-critical responseExtensions in the response. In the case where a responder does not have the ability to respond to an OCSP request containing an option not supported by the responder, it SHOULD return the most complete response it can. For example, in the case where a responder only supports pre-produced responses and does not have the ability to respond to an OCSP request containing a nonce, it SHOULD return a response that does not include a nonce. Clients SHOULD attempt to process a response even if the response does not include a nonce. See for details on validating responses that do not contain a nonce. See also for relevant security considerations. Responders that do not have the ability to respond to OCSP requests that contain an unsupported option such as a nonce MAY forward the request to an OCSP responder capable of doing so. The responder MAY include the SingleResponse.singleExtensions extensions structure.

Signed OCSPResponses Clients MUST validate the signature on the OCSPResponse. If the response is signed by a delegate of the issuing certification authority (CA), a valid responder certificate MUST be referenced in the BasicOCSPResponse.certs structure. It is RECOMMENDED that the OCSP responder's certificate contain the id-pkix-ocsp-nocheck extension, as defined in , to indicate to the client that it need not check the certificate's status. In addition, it is RECOMMENDED that neither an OCSP Authority Information Access (AIA) extension nor CRL Distribution Points (CRLDP) extension be included in the OCSP responder's certificate. Accordingly, the responder's signing certificate SHOULD be relatively short-lived and renewed regularly. Clients MUST be able to identify OCSP responder certificates using the byKey field and SHOULD be able to identify OCSP responder certificates using the byName field of the ResponseData.ResponderID choices. Older responders that provide backward compatibility with the protocol defined in MAY use the byName field to represent the ResponderID but should transition to using the byKey field as soon as practical. Newer responders that conform to this profile MUST use the byKey field to represent the ResponderID to reduce the size of the response.

OCSPResponseStatus Values As long as the OCSP infrastructure has authoritative records for a particular certificate, an OCSPResponseStatus of "successful" will be returned. When access to authoritative records for a particular certificate is not available, the responder MUST return an OCSPResponseStatus of "unauthorized". For example, OCSP responders that do not have access to authoritative records for a requested certificate, such as those that generate and distribute OCSP responses in advance and thus do not have the ability to properly respond with a signed "successful" yet "unknown" response, will respond with an OCSPResponseStatus of "unauthorized". Also, in order to ensure the database of revocation information does not grow unbounded over time, the responder MAY remove the status records of expired certificates. Requests from clients for certificates whose record has been removed will result in an OCSPResponseStatus of "unauthorized". Security considerations regarding the use of unsigned responses are discussed in .

thisUpdate, nextUpdate, and producedAt When pre-producing OCSPResponse messages, the responder MUST set the thisUpdate, nextUpdate, and producedAt times as follows:

thisUpdate:

The time at which the status being indicated is known to be correct.

nextUpdate:

The time at or before which newer information will be available about the status of the certificate. As described in , this field is optional. However, this field MUST be included in the profile specified in this document to help clients cache responses. See for additional information on caching.

producedAt:

The time at which the OCSP response was signed.

For the purposes of this profile, ASN.1-encoded GeneralizedTime values, such as thisUpdate, nextUpdate, and producedAt, MUST be expressed Greenwich Mean Time (Zulu) and MUST include seconds (i.e., times are YYYYMMDDHHMMSSZ), even where the number of seconds is zero. GeneralizedTime values MUST NOT include fractional seconds.

Client Behavior

OCSP Responder Discovery Clients MUST support the AIA extension as defined in and MUST recognize the id-ad-ocsp access method. This enables CAs to inform clients how they can contact the OCSP service. In the case where a client is checking the status of a certificate that contains both an AIA extension pointing to an OCSP responder and a CRLDP extension pointing to a CRL, the client SHOULD attempt to contact the OCSP responder first. Clients MAY attempt to retrieve the CRL if no OCSPResponse is received from the responder after a locally configured timeout and number of retries.

Sending an OCSP Request To avoid needless network traffic, applications MUST verify the signature of signed data before asking an OCSP client to check the status of certificates used to verify the data. If the signature is invalid or the application is not able to verify it, an OCSP check MUST NOT be requested. Similarly, an application MUST validate the signature on certificates in a chain before asking an OCSP client to check the status of the certificate. If the certificate signature is invalid or the application is not able to verify it, an OCSP check MUST NOT be requested. Clients SHOULD NOT make a request to check the status of expired certificates.

Ensuring an OCSPResponse Is Fresh In order to ensure that a client does not accept an out-of-date response that indicates a "good" status when in fact there is a more up-to-date response that specifies the status of "revoked", a client must ensure the responses they receive are fresh. In general, two mechanisms are available to clients to ensure a response is fresh. The first uses nonces, and the second is based on time. In order for time-based mechanisms to work, both clients and responders MUST have access to an accurate source of time. Because this profile specifies that clients SHOULD NOT include a requestExtensions structure in OCSPRequests (see ), clients MUST be able to determine OCSPResponse freshness based on an accurate source of time. Clients that opt to include a nonce in the request SHOULD NOT reject a corresponding OCSPResponse solely on the basis of the nonexistent expected nonce but MUST fall back to validating the OCSPResponse based on time. Clients that do not include a nonce in the request MUST ignore any nonce that may be present in the response. Clients MUST check for the existence of the nextUpdate field and MUST ensure the current time, expressed in GMT time as described in , falls between the thisUpdate and nextUpdate times. If the nextUpdate field is absent, the client MUST reject the response. If the nextUpdate field is present, the client MUST ensure that it is not earlier than the current time. If the current time on the client is later than the time specified in the nextUpdate field, the client MUST reject the response as stale. Clients MAY allow configuration of a small tolerance period for acceptance of responses after nextUpdate to handle minor clock differences relative to responders and caches. This tolerance period should be chosen based on the accuracy and precision of time synchronization technology available to the calling application environment. For example, Internet peers with low latency connections typically expect NTP time synchronization to keep them accurate within parts of a second; higher latency environments or where an NTP analogue is not available may have to be more liberal in their tolerance (e.g., allow one day difference). See the security considerations in for additional details on replay and on-path attacks.

Transport Profile OCSP clients can send HTTP-based OCSP requests using either the GET or POST method. The OCSP responder MUST support requests and responses over HTTP. When sending requests that are less than or equal to 255 bytes in total (after encoding), including the scheme and delimiters (http://), server name, and base64-encoded OCSPRequest structure, clients MUST use the GET method (to enable OCSP response caching). OCSP requests larger than 255 bytes SHOULD be submitted using the POST method. In all cases, clients MUST follow the descriptions in when constructing these messages. When constructing a GET message, OCSP clients MUST base64-encode the OCSPRequest structure according to . Clients MUST NOT include whitespace or any other characters that are not part of the base64 character repertoire in the base64-encoded string. Clients MUST properly URL-encode the base64-encoded OCSPRequest according to . OCSP clients MUST append the base64-encoded OCSPRequest to the URI specified in the AIA extension . For example: In response to properly formatted OCSPRequests that are cachable (i.e., responses that contain a nextUpdate value), the responder will include the binary value of the DER encoding of the OCSPResponse preceded by the following HTTP header fields. Last-modified: < producedAt HTTP-date > ETag: "< strong validator >" Expires: < nextUpdate HTTP-date > Cache-control: max-age=< n >, public, no-transform, must-revalidate Date: < current HTTP-date > ]]> See for details on the use of these HTTP header fields.

Caching Recommendations The ability to cache OCSP responses throughout the network is an important factor in high volume OCSP deployments. This section discusses the recommended caching behavior of OCSP clients and HTTP proxies and the steps that should be taken to minimize the number of times that OCSP clients "hit the wire". In addition, the concept of including OCSP responses in protocol exchanges (aka stapling or piggybacking), such as has been defined in TLS, is also discussed.

Caching at the Client To minimize bandwidth usage, clients MUST locally cache authoritative OCSP responses (i.e., a response with a signature that has been successfully validated and that indicates an OCSPResponseStatus of "successful"). Most OCSP clients will send OCSPRequests at or near the nextUpdate time (when a cached response expires). To avoid large spikes in responder load that might occur when many clients refresh cached responses for a popular certificate, responders MAY indicate when the client should fetch an updated OCSP response by using the cache- control:max-age directive. Clients SHOULD fetch the updated OCSP response on or after the max-age time. To ensure that clients receive an updated OCSP response, OCSP responders MUST refresh the OCSP response before the max-age time.

HTTP Proxies The responder SHOULD set the HTTP header fields of the OCSP response in such a way as to allow for the intelligent use of intermediate HTTP proxy servers. See and for the full definition of these HTTP header fields and the proper format of any date and time values. HTTP Header Fields

HTTP Header Field	Description
Date	The date and time at which the OCSP responder generated the HTTP response.
Last-Modified	This value specifies the date and time at which the OCSP responder last modified the response. This date and time will be the same as the thisUpdate timestamp in the request itself.
Expires	Specifies how long the response is considered fresh. This date and time will be the same as the nextUpdate timestamp in the OCSP response itself.
ETag	A string that identifies a particular version of the associated data. It is RECOMMENDED by this profile that the ETag value be the ASCII HEX representation of the SHA-256 hash of the OCSPResponse structure.
Cache-Control	Contains a number of caching directives. max-age = < n > - where n is a time value later than thisUpdate but earlier than nextUpdate. public - makes normally uncachable response cachable by both shared and nonshared caches. no-transform - specifies that a proxy cache cannot change the type, length, or encoding of the object content. must-revalidate - prevents caches from intentionally returning stale responses.

OCSP responders MUST NOT include the "Pragma: no-cache", "Cache- Control: no-cache", or "Cache-Control: no-store" HTTP header fields in authoritative OCSP responses. OCSP responders SHOULD include one or more of these HTTP header fields in non-authoritative OCSP responses. For example, assume that an OCSP response has the following timestamp values: and that an OCSP client requests the response on March 20, 2023 01:00:00 GMT. In this scenario, the HTTP response may look like this: ]]> OCSP clients MUST NOT include a no-cache HTTP header field in OCSP request messages, unless the client encounters an expired response, which may be a result of an intermediate proxy caching stale data. In this situation, clients SHOULD resend the request specifying that proxies should be bypassed by including an appropriate HTTP header field in the request (i.e., Pragma: no-cache or Cache-Control: no-cache).

Caching at Servers In some scenarios, it is advantageous to include OCSP response information within the protocol being utilized between the client and OCSP responder. Including OCSP responses in this manner has a few attractive effects. First, it allows for the caching of OCSP responses on the OCSP responder, thus lowering the number of hits. Second, it enables certificate validation in the event the client is not connected to a network and thus eliminates the need for clients to establish a new HTTP session with the OCSP responder. Third, it reduces the number of round trips the client needs to make in order to complete a handshake. Fourth, it simplifies the client-side OCSP implementation by enabling a situation where the client need only the ability to parse and recognize OCSP responses. This functionality has been specified as an extension to the TLS protocol in but can be applied to any client-server protocol. It is RECOMMENDED by this profile that both TLS clients and servers implement the certificate status request extension mechanism for TLS. Further information regarding caching issues can be obtained from .

Security Considerations The following considerations apply in addition to the security considerations addressed in .

Replay Attacks Because the use of nonces in this profile is optional, there is a possibility that an out-of-date OCSP response could be replayed, thus causing a client to accept a good response when in fact there is a more up-to-date response that specifies the status of "revoked". In order to mitigate this attack, clients MUST have access to an accurate source of time and ensure that the OCSP responses they receive are sufficiently fresh. Clients that do not have an accurate source of date and time are vulnerable to service disruption. For example, a client with a sufficiently fast clock may reject a fresh OCSP response. Similarly, a client with a sufficiently slow clock may incorrectly accept expired valid responses for certificates that may in fact be revoked. Future versions of OCSP may provide a way for the client to know whether the responder supports nonces or does not support nonces. If a client can determine that the responder supports nonces, it MUST reject a reply that does not contain an expected nonce. Otherwise, clients that opt to include a nonce in the request SHOULD NOT reject a corresponding OCSPResponse solely on the basis of the nonexistent expected nonce but MUST fall back to validating the OCSPResponse based on time.

On-Path Attacks To mitigate risk associated with this class of attack, the client MUST properly validate the signature on the response. The use of signed responses in OCSP serves to authenticate the identity of the OCSP responder and to verify that it is authorized to sign responses on the CA's behalf. Clients MUST ensure that they are communicating with an authorized responder by the rules described in .

Impersonation Attacks The use of signed responses in OCSP serves to authenticate the identity of OCSP responder. As detailed in , clients must properly validate the signature of the OCSP response and the signature on the OCSP response signer certificate to ensure an authorized responder created it.

Denial-of-Service Attacks OCSP responders SHOULD take measures to prevent or mitigate denial- of-service attacks. As this profile specifies the use of unsigned OCSPRequests, access to the responder may be implicitly given to everyone who can send a request to a responder, and thus the ability to mount a denial-of-service attack via a flood of requests may be greater. For example, a responder could limit the rate of incoming requests from a particular IP address if questionable behavior is detected.

Modification of HTTP Header Fields Values included in HTTP header fields, as described in Sections and , are not cryptographically protected; they may be manipulated by an attacker. Clients SHOULD use these values for caching guidance only and ultimately SHOULD rely only on the values present in the signed OCSPResponse (). Clients SHOULD NOT rely on cached responses beyond the nextUpdate time.

Request Authentication and Authorization The suggested use of unsigned requests in this environment removes an option that allows the responder to determine the authenticity of incoming requests. Thus, access to the responder may be implicitly given to everyone who can send a request to a responder. Environments where explicit authorization to access the OCSP responder is necessary can utilize other mechanisms to authenticate requestors or restrict or meter service.

Use of SHA-1 for the Calculation of CertID Field Values Although the use of SHA-1 for the calculation of CertID field values is not of concern from a cryptographic security standpoint, the continued use of SHA-1 in an ecosystem requires that software that interoperates with the ecosystem maintain support for SHA-1. This increases implementation complexity and potential attack surface for the software in question. Thus, the continued use of SHA-1 in an ecosystem to maintain interoperability with legacy software must be weighed against the increased implementation complexity and potential attack surface.

IANA Considerations This document has no IANA actions.

References Normative References Independent Informative References Open Mobile Alliance Candidate Version V1.0

Differences from RFC 5019 This document obsoletes . defines a lightweight profile for OCSP that makes the protocol more suitable for use in high-volume environments. The lightweight profile specifies the mandatory use of SHA-1 when calculating the values of several fields in OCSP requests and responses. In recent years, weaknesses have been demonstrated with the SHA-1 algorithm. As a result, SHA-1 is increasingly falling out of use even for non-security-relevant use cases. This document obsoletes the lightweight profile as specified in to instead recommend the use of SHA-256 where SHA-1 was previously required. An OCSP client compliant with is still able to use SHA-1, but the use of SHA-1 may become obsolete in the future. Substantive changes to RFC 5019:

• requires new OCSP clients to use SHA-256 to support migration for OCSP clients.
• requires new OCSP responders to use the byKey field and support migration from byName fields.
• clarifies that OCSP clients MUST NOT include whitespace or any other characters that are not part of the base64 character repertoire in the base64-encoded string.

Examples

Root CA Certificate This is a self-signed certificate for the CA that issued the end-entity certificate and OCSP-delegated responder example certificates below. The key pair for the CA is the "testECCP521" key from .

End-Entity Certificate This is an end-entity certificate whose status is requested and returned in the OCSP request and response examples below. The key pair for the end-entity certificate is the "testECCP256" key from .

OCSP Responder Certificate This is a certificate for the OCSP-delegated response that signed the OCSP response example below. The key pair for the OCSP responder certificate is the "testECCP384" key from .

OCSP Request This is a base64-encoded OCSP request for the end-entity certificate above.

OCSP Response This is a base64-encoded OCSP response for the end-entity certificate above.

Acknowledgments The authors of this version of the document wish to thank and for their work to produce the original version of the lightweight profile for OCSP. The authors of this version of the document wish to thank , , , , and for their reviews, feedback, and suggestions. The authors wish to thank of RSA Security, Inc., of Vodafone Group R&D, and of CoreStreet, Ltd. for their contributions to the original specification. Listed organizational affiliations reflect the authors' affiliations at the time RFC 5019 was published.