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Versions: 00                                                            
Internet Engineering Task Force (IETF)             Phillip Hallam-Baker
Internet-Draft                                         Comodo Group Inc.
Intended Status: Standards Track                         David Chadwick
Expires: November 14, 2014                            University of Kent
                                                            May 13, 2014


               Web PKI Operations: Revocation and Status
                    draft-ietf-wpkops-revocation-00

Abstract

   This document describes the certificate status mechanisms supported
   in the Web PKI

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.












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Table of Contents

   1.  Certificate Status . . . . . . . . . . . . . . . . . . . . . .  4
      1.1.  Operational Certificate Lifecycle Model . . . . . . . . .  4
         1.1.1.  Direct and Indirect Status Assertions  . . . . . . .  4
         1.1.2.  Trust Path Processing  . . . . . . . . . . . . . . .  5
         1.1.3.  Revocation Reasons . . . . . . . . . . . . . . . . .  5
         1.1.4.  Operational Certificate States . . . . . . . . . . .  7
      1.2.  Client Behavior . . . . . . . . . . . . . . . . . . . . .  8
   2.  Status Assertion Mechanisms  . . . . . . . . . . . . . . . . .  8
      2.1.  CRLs  . . . . . . . . . . . . . . . . . . . . . . . . . .  8
         2.1.1.  Status Model"  . . . . . . . . . . . . . . . . . . .  8
         2.1.2.  Revocation Reasons . . . . . . . . . . . . . . . . .  9
      2.2.  OCSP  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
         2.2.1.  CRL Responder  . . . . . . . . . . . . . . . . . . . 10
         2.2.2.  Lightweight Distribution . . . . . . . . . . . . . . 11
         2.2.3.  OCSP Stapling  . . . . . . . . . . . . . . . . . . . 11
      2.3.  Other . . . . . . . . . . . . . . . . . . . . . . . . . . 11
         2.3.1.  Hardcoded/Indirect Revocation List . . . . . . . . . 11
         2.3.2.  DANE . . . . . . . . . . . . . . . . . . . . . . . . 11
         2.3.3.  Certificate Transparency . . . . . . . . . . . . . . 12
   3.  Status Acquisition Mechanisms  . . . . . . . . . . . . . . . . 12
      3.1.  CRLSets . . . . . . . . . . . . . . . . . . . . . . . . . 12
      3.2.  SCVP  . . . . . . . . . . . . . . . . . . . . . . . . . . 12
      3.3.  XKMS  . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   4.  Cryptography Platforms (to be completed once the survey is
        finished) . . . . . . . . . . . . . . . . . . . . . . . . . . 13
      4.1.  Checklist . . . . . . . . . . . . . . . . . . . . . . . . 13
      4.2.  cryptlib  . . . . . . . . . . . . . . . . . . . . . . . . 13
      4.3.  Microsoft Windows . . . . . . . . . . . . . . . . . . . . 13
      4.4.  Network Security Services . . . . . . . . . . . . . . . . 13
      4.5.  OpenSSL . . . . . . . . . . . . . . . . . . . . . . . . . 13
   5.  Web Server Status (TBC once the survey is finished)  . . . . . 13
      5.1.  Checklist . . . . . . . . . . . . . . . . . . . . . . . . 13
      5.2.  Apache  . . . . . . . . . . . . . . . . . . . . . . . . . 13
      5.3.  IIS . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
      5.4.  LiteSpeed   . . . . . . . . . . . . . . . . . . . . . . . 13
      5.5.  nginx . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   6.  Web Client Status (TBC once the survey is finished)  . . . . . 14
      6.1.  Checklist . . . . . . . . . . . . . . . . . . . . . . . . 14
      6.2.  Chrome  . . . . . . . . . . . . . . . . . . . . . . . . . 14
      6.3.  Firefox . . . . . . . . . . . . . . . . . . . . . . . . . 14
      6.4.  Internet Explorer . . . . . . . . . . . . . . . . . . . . 15
      6.5.  Opera . . . . . . . . . . . . . . . . . . . . . . . . . . 15
      6.6.  Safari  . . . . . . . . . . . . . . . . . . . . . . . . . 15
   7.  CA Status  . . . . . . . . . . . . . . . . . . . . . . . . . . 16
      7.1.  Checklist . . . . . . . . . . . . . . . . . . . . . . . . 16
      7.2.  CA-Browser Forum Requirements . . . . . . . . . . . . . . 16
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   10.  References  . . . . . . . . . . . . . . . . . . . . . . . . . 16



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      10.1.  Normative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17




















































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1. Certificate Status

   A certificate is issued with a predetermined validity interval. It is
   common practice to specify a validity interval that starts a few
   hours or days before the instant of issue so as to avoid rejection by
   machines with clocks running behind the current time or otherwise
   mis-set. In normal operation the certificate will remain valid until
   it expires.

   The CA that issued a certificate has primary responsibility for
   maintaining the certificate life cycle and reporting changes to
   certificate status. But other parties can and in some cases do report
   status for third party certificates. In particular client and
   platform providers have revoked certificates known to have been mis-
   issued or in a case of a CA breach.

   [Introduce CRL Sets here, once I find a citation]

1.1. Operational Certificate Lifecycle Model

   PKIX does not describe a certificate lifecyle model. Instead the
   certificate lifecycle model is a consequence of the issue of PKIX
   Certificates and CRLs. While this is sufficient for describing PKIX
   it is not satisfactory as a reference model for describing
   operations. Not least because modern PKIX operations are frequently
   based on the use of OCSP rather than CRLs and differences in the
   semantics of CRLs and OCSP are one of the features we would want to
   measure. The distinction between an operational model and PKIX
   semantics is illustrated by considering the difference between the
   operational concept of direct/indirect status assertions and the PKIX
   semantics of direct/indirect CRLs.

1.1.1. Direct and Indirect Status Assertions

   PKIX CRLs may be marked as direct or indirect to indicate that they
   are issued by the same CA that issued the original certificate (a
   direct CRL) or by a third party (an indirect CRL).

   In the corresponding operational model we define a direct status
   assertion as being by the same CA that issued the original
   certificate and an indirect status assertion as being any status
   assertion that is not direct.

   The difference between the operational and PKIX models has important
   practical consequences. The CA that originally issued an assertion
   naturally holds a privileged position when it comes to revoking it. A
   direct CRL thus has a privileged position when considering the
   question of certificate validity. A direct status assertion thus has
   a privileged position when considering revocation status. A direct
   CRL carries an implicit claim that it is a direct status assertion
   but this is merely a claim unless the client validating the CRL takes



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   steps to verify it. For example by verifying that the CRL signature
   has valid trust chain to the same trust anchor as the certificate.

   CRLs introduce a further complication as a CRL contains a list of
   explicit statements declaring that a certificate is invalid. In the
   case of a direct CRL there is an implicit assertion that any issued,
   unexpired certificate not listed was valid at the time the CRL was
   issued. The processing rules specified in [RFC5280] appear to limit
   this implicit assertion to direct CRLs but this does not appear to be
   called out in the text.

   One of the main use cases that might motivate the issue of an
   indirect status assertion is the case where a third party notices
   that a certificate is being used for malicious purposes and intends
   to advise relying parties that they should not trust the certificate
   subject. Since it is the behavior of the subject rather than their
   identity that is at issue, there may not be sufficient reason for the
   CA to revoke the certificate. There is thus a case for parties other
   than the certificate subject and issuer having the ability to revoke
   certificates in certain circumstances. But does granting this ability
   also confer the ability to (implicitly) declare certificates valid?

   [Operational question: Do clients interpret indirect CRLs as
   substitutes for the direct CRL or as adjuncts providing additional
   information.]

1.1.2. Trust Path Processing

   One of the operational questions we would like to understand is the
   extent to which it is possible to revoke EE certificates by revoking
   one or more of the Certificate Signing Certificates in the
   certification path.

   Self Signed certificates used to transport Trust Anchors are not
   actually PKIX certificates and are not governed by the PKIX model
   (although they are X.509v3 certificates). One important consequence
   of this is that relying parties do not use PKIX mechanisms to check
   the validity of Trust Anchors.

   CSCs signed by the trust anchor are potentially subject to
   revocation. Do the status checking mechanisms employed in browsers
   support this in practice?

   [OCSP and CRLs raise separate issues here. In the case of an OCSP
   responder should we require signed OCSP tokens for each cert in the
   path? Is it possible to use a mix of CSCs and OCSP in stapled
   tokens?]







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1.1.3. Revocation Reasons

   A status declarer may declare a certificate invalid (i.e. revoke the
   certificate) before its scheduled expiry for a variety of reason that
   include:

      Subject requested revocation
         The certificate subject requested revocation.

      Subject requested correction
         The certificate subject requested information in a certificate
         be corrected either because the original information is wrong
         or circumstances have changed. For example the subject's
         affiliation has changed. Such corrections are typically made by
         revoking the original certificate and issuing a replacement.

      Payment declined
         A CA may issue a certificate before payment has cleared. If the
         payment is subsequently declined, the certificate is revoked.

      Declined extension
         The certificate was originally issued on condition that use
         beyond an initial period would require an additional fee which
         the subject did not pay.

      Terms of Use
         The subject was determined to have breached the terms of use

      Fraudulent Request
         The application was determined to be fraudulent after issue

      CA compromise
         The certificate can no longer be trusted because the operations
         of the CA were compromised.

   The ability to provide a reason for revocation is defined, without
   explaining why a CA should provide this information or how relying
   parties should behave differently according to the revocation reason
   given. Revoked certificates are to be considered invalid regardless
   of the reason for revocation.

   PKIX does not define an order of severity. In cases where multiple
   reasons apply, the CA may pick any. There is no obligation to report
   a reason at all let alone report severity.

   Once a certificate is revoked the certificate lifecycle is complete
   as far as the CA is concerned and there is no obligation on the CA to
   update the revocation reason after the fact to reflect the discovery
   of a more serious cause.





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   In the case of a subject request the CA only has reliable knowledge
   of the fact of the request and not the reason(s) the request was
   made. A certificate subject might have requested the certificate be
   revoked because they have no further use for it or because they know
   the associated private key has been compromised. Even if the CA asks
   for the revocation reason there is no reason to expect the subject to
   answer. The subject may not wish to report that a private key has
   been compromised.

   The net effect of these limitations is that revocation reasons only
   provide a lower bound on the severity of the cause for which a
   certificate was revoked.

1.1.4. Operational Certificate States

   From an operational point of view, once issued, a PKIX certificate
   has five potential states, a single valid state and four invalid
   states:

      Nonexistent
         The certificate does not exist. This may be because the
         certificate has not yet been issued or it will never be issued.

      Valid
         The certificate was issued and is valid.

      Invalid
         No certificate was issued or the certificate issued is no
         longer valid. Hold The certificate exists but has been
         suspended with the possibility of reinstatement. Revoked The
         certificate exists but has been declared to be invalid with
         permanent effect. Expired The certificate existed in the past
         but the expiry date specified at issue has passed.

   The Hold state has been found to be of little or no practical value
   since issuing a new certificate is simpler and more effective than
   attempting to cancel a previous instruction to put the certificate on
   hold.

   CRLs and certain OCSP configurations do not permit a client to
   distinguish between the states Valid and Invalid/Nonexistent. The CRL
   mechanism was designed to allow a relying party to check the validity
   of a known certificate. It was thus unnecessary to distinguish the
   states Valid and Nonexistent as that would be verified by checking
   the signature. Accordingly a CRL contains only a list of invalid
   certificates.

   In the case of a CA Breach, key compromise or cryptanalytic attack, a
   certificate may be created that has a valid signature but was not
   issued by the CA. Such a certificate is 'Nonexistent' as far as the
   CA is concerned. Requiring a CA to distinguish these states in



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   reporting certificate status provides a limited degree of
   transparency in CA operations. A CA that reports 'Nonexistent' in
   response to a status request for an unexpired certificate that has a
   valid signature has a defective or breached issue process. A CA that
   reports valid in response to a status request for a non-existent
   certificate has a defective or breached revocation mechanism.

1.2. Client Behavior

   WebPKI clients are advised but not required to check certificate
   status before relying on the assertions they contain. Waiting to
   obtain status information from an external source before relying on a
   certificate may cause delay or even rejection of a valid certificate.

   Excluding the possibility that a client requests revocation status
   then ignores the result, the options available to a Web PKI client
   are therefore:

      Ignore
         The client does not process revocation status from any source

      Local
         The client only process revocation status that is available
         from local sources. For example hardcoded 'do not trust' lists
         or CRLSets.

      Soft-Fail
         The client attempts to obtain revocation status from external
         sources and will reject certificates reported as revoked but
         will accept a certificate as valid if the external source
         cannot be contacted, does not reply or rejects the request,
         etc.

      Hard-Fail
         The client attempts to obtain revocation status from external
         sources and will reject certificates unless either an
         affirmative assertion of validity or an affirmative assertion
         of not revoked is obtained.

2. Status Assertion Mechanisms

2.1. CRLs

   The PKIX CRL mechanism for asserting certificate status is described
   in [RFC5280]

2.1.1. Status Model"

   A CRL only provides a list of certificates that have been revoked. An
   issued, unexpired certificate is presumed to be valid if it does not
   appear in the CRL. The certificate states supported by the CRL



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   mechanism are thus:

      UNREVOKED
         Corresponds to operational states Valid, Nonexistent and
         Expired.

      UNDETERMINED
         Occurs when no CRL with a corresponding scope is available.

      REVOKED
         Corresponds to operational state Revoked.

      HOLD
         Corresponds to operational state Hold.

   The CRL result 'UNREVOKED' thus corresponds to three states in the
   Operational model of which one is Valid and the other two are Invalid
   states. A client that does not have a source of trusted time
   available may use the issue time of the CRL as the basis for checking
   expiry. The CRL mechanism does not provide a means of determining
   that a certificate was legitimately issued

2.1.2. Revocation Reasons

   [RFC5280] requires that a CRL entry specify a reason code but not the
   circumstances in which a code should be raised. [[This is however
   specified in X.509v3] The following reason codes are defined:

      *  unspecified

      *  keyCompromise

      *  cACompromise

      *  affiliationChanged

      *  superseded

      *  cessationOfOperation

      *  privilegeWithdrawn

      *  aACompromise

2.2. OCSP

   OCSP is defined in [RFC6960]. [RFC5019] (lightweight) and TLS
   Stapling [RFC6066] Section 8.






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   An OCSP service MAY return the following responses to a request:

      Success [[+ CRL Status Code]
         The OCSP status request succeeded and the service returned one
         of the CRL status codes described above.

      Refused
         The OCSP responder refused to answer the request.

      Unknown
         The certificate for which status was requested was not found or
         the status is not determined.

      Invalid
         The OCSP server returned an answer that was not understood.

      Fail
         The service failed to answer the request or the client was
         unable to contact the OCSP service.

   The OCSP response reflects the success or failure of the OCSP
   transaction rather than the status of the certificate being queried.
   Thus a client whose behavior is Soft-Fail will only reject a
   certificate if the OCSP response Success and an Invalid certificate
   status is returned. Thus an OCSP server that responds to a request
   for status for a certificate that is known to have never been issued
   with 'Invalid' will cause soft-fail clients to accept the
   certificate.

   Note that [RFC6960] does not differentiate the results Success/Valid
   and Unknown. CAs are however required to differentiate these
   responses under the CABForum Basic Requirements [TBS].

   The OCSP protocol permits responses to be signed in advance [static]
   or provide a proof of freshness by returning a nonce presented by the
   client.

   The protocol only permits static responses to report the status of
   individual certificates. There is no feature analagous to the NSEC3
   feature of DNSSEC which permits the non-existence of an entry in a
   particular range to be asserted.

2.2.1. CRL Responder

   An OCSP responder may generate responses from CRLs. Such a responder
   can generate most but not all the responses required in advance by
   generating revoked responses for all the certificates listed in the
   CRL and valid responses for all the certificate serial numbers
   presented in previous requests.





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   Such a responder cannot distinguish between Valid and nonexistent
   states unless provided with additional information not in the CRL.

2.2.2. Lightweight Distribution

   In the lightweight distribution mode of operation specified in
   [RFC5019], the CA generates OCSP responses for all unexpired
   certificates that it has issued. The signed tokens are then passed to
   a separate network for distribution. For example, a Content Delivery
   Network with a large number of delivery points.

   One of the main strengths of this model is that all the signing of
   OCSP tokens is done offline and no signing key is ever exposed to an
   external network. One consequence of this model is that responses for
   nonexistent certificates cannot be signed.

2.2.3. OCSP Stapling

   One of the principle limitations of the traditional OCSP model is
   that each TLS transaction becomes a three party communication. To
   complete the TLS connection the client must communicate with the
   server being contacted and the OCSP service. This approach introduces
   unnecessary delay and an additional potential point of failure and is
   therefore unsatisfactory.

   OCSP stapling permits a TLS server to provide a client that supports
   the stapling extension to provide the OCSP token together with the
   certificate it corresponds to. This permits a client to establish a
   TLS communication without the need for a three party communication in
   the case that the client and server both support stapling.

   The chief drawback to stapling is that support for stapling is
   optional. thus a client that does not receive a stapled token must
   attempt to obtain it from the OCSP service and is therefore subject
   to the same Softfail/hardfail dilemma described above.

2.3. Other

2.3.1. Hardcoded/Indirect Revocation List

   Most browsers employ a 'blacklist' to block certificates known to be
   mis-issued. The number of entries supported in such lists is
   typically small. In some cases the list is hardcoded in the browser
   or platform code and is only updated with the browser or platform. In
   other cases the blacklist is updatable separately.

   Q: Which Web browsers support update of the list without updating the
   browser.






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2.3.2. DANE

   DANE assertions [RFC6698] may be used to cancel a certificate.
   [describe]

2.3.3. Certificate Transparency

   CT [RFC6962] provides a means of auditing the operation of a CA using
   only information that is available to the public from log servers.
   Moreover a client can determine that a certificate has been issued
   transparently (i.e. is in the log) or not. Any certificate the client
   receives that is not in the log should be treated as suspicious or
   invalid by the client.

   In order for CT to work, a CA registers a certificate in a log server
   and is returned a signed time stamp by the log server. This signed
   time stamp must be given to the certificate subject, and it must send
   this to RPs along with its certificate. This requires the TLS
   handshake to be enhanced to pass the signed time stamp, and clients
   and servers to be enhanced to receive and send it. This will take
   time to be rolled out to the entire Internet, so CT is not a short
   term solution.

   Monitor servers regularly scan the logs to look for suspicious or
   unauthorised certificates that have been deposited there.

   One potential problem with CT is that it is not described how the
   monitors will operate and determine whether a certificate is
   suspicious or unauthorised. How will they know if a certificate is
   suspicious or not? How will clients be notified of this? What does
   suspicious actually mean to a client? How should a client behave when
   it is told that a certificate is dodgy by a monitor? Who arbitrates
   on this if there is a disagreement between monitors or monitors and a
   CA? Is it the issuing CA? If so, this will not stop a compelled
   certificate creation attack since it is the subject that has to say
   which certificate is false and which is not, since both were issued
   by the same CA or its subordinate.

3. Status Acquisition Mechanisms

3.1. CRLSets

   [Working on getting a citable description]

   (Only supported in the Chrome Browser)

3.2. SCVP

   Not supported in any Web PKI application or service.





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3.3. XKMS

   Not supported in any Web PKI application or service.

4. Cryptography Platforms (to be completed once the survey is finished)

   [Should expand this noting that while support for a feature in the
   platfor is often a necessary for support in applications, it is not
   necessarily sufficient.]

4.1. Checklist

4.2. cryptlib

4.3. Microsoft Windows

4.4. Network Security Services

   Used in Firefox and Chrome

4.5. OpenSSL

5. Web Server Status (TBC once the survey is finished)

   Web Server support for revocation is fairly straightforward since the
   Web Server is only involved in revocation in the case of stapled OCSP
   tokens and this is supported in the latest versions of all the
   servers surveyed.

5.1. Checklist

      Support for OCSP Stapling?

5.2. Apache

      Support for OCSP Stapling?
         Since version 2.3.

5.3. IIS

      Support for OCSP Stapling?
         Yes

5.4. LiteSpeed

      Support for OCSP Stapling?
         Since version 4.2.4.







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5.5. nginx

      Support for OCSP Stapling?
         Since version 1.3.7.

6. Web Client Status (TBC once the survey is finished)

   [Need to consider further dimensions here. In particular Chrome
   behaves differently depending on the platform it is on and several
   browsers have different revocation checking for EV vs other
   certificate policies.]

6.1. Checklist

      Supported Revocation Checking Mechanisms
         To Do: Some browsers do not support CRL DP. Others give
         preference to OCSP, but fall back to CRL DP if the necessary
         AIA is missing. Some browsers give priority to OCSP, but switch
         to CRL DP when a particular issuer's revocation information is
         retrieved frequently.

      User Experience for Certificate Status Invalid

      User Experience for Certificate Status Unknown

      What sources are permitted to sign CRLs or OCSP responses for a
         certificate, can any trusted CA sign or only the CA that issued
         the certificate?

6.2. Chrome

   The Chrome browser automatically updates itself to the latest version
   unless this feature is explicitly disabled by the user.

      Supported Revocation Checking Mechanisms
         OCSP (at present).

      User Experience for Certificate Status Invalid

      User Experience for OCSP Fail
         Soft fail

6.3. Firefox

      Supported Revocation Checking Mechanisms
         OCSP checked by default since Firefox 3. OCSP Stapling has been
         added to nightly builds but is not yet in production releases.







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      User Experience for Certificate Status Invalid

      User Experience for Certificate Status Unknown
         Soft fail

6.4. Internet Explorer

      Supported Revocation Checking Mechanisms
         CRLs and OCSP.

      User Experience for Certificate Status Invalid
         IE5: If the certificate has a valid trust path, the user is
         presented with a dialog box that says "The Security certificate
         for this site has been revoked. This site should not be
         trusted"

         If however, a valid trust path cannot be found, the user
         receives the error message for the invalid trust path instead:
         "The user is presented with a dialogue box that tells them that
         'Information you exchange with this site cannot be viewed or
         changed by others. However there is a problem with the site's
         security certificate."

         IE??: Warning Page "There is a problem with this website's
         security certificate"

   CVE-2011-0199 : Chris Hawk and Wan-Teh Chang of Google MS ? ?the
   revocation date is determined by comparing the current date with the
   RevocationDate field in the CRL or the OCSP response? For Windows
   Vista with Service Pack 1 and Windows Server 2008, the OCSP signing
   certificate may chain up to any trusted root CA as long as the
   certificate chain includes the OCSP Signing EKU extension. CryptoAPI
   first determines whether a time valid version of the revocation
   object exists in the CryptoAPI disk cache.

6.5. Opera

      Supported Revocation Checking Mechanisms
         OCSP checked by default since Opera version 8

      User Experience for Certificate Status Invalid

      User Experience for Certificate Status Unknown
         Hard fail

6.6. Safari

      Supported Revocation Checking Mechanisms
         CRL: all, OCSP checking is enabled by default as of Mac OSX
         10.7 (Lion). Prior to that it had to be enabled in the Keychain
         preferences.



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Internet-Draft Web PKI Operations: Revocation and Status        May 2014


      User Experience for Certificate Status Invalid
         Dialog Box: Offers Continue/Cancel/Show Certificate

      User Experience for Certificate Status Unknown

      User Experience for OCSP-FAIL

   For Apple?s OS X, OCSP and CRL checking can be configured via
   Keychain Access -> Preferences ?> Certificates. A dialog box opens
   with three rows: OCSP, CRL, and Priority. Under OCSP and CRL, the
   three allowed options (a grayed-out one said something like ?always?)
   were: ?Off?, ?Best Attempt?, and ?Required if certificate indicates?.
   ?Best Attempt? was the default. Under Priority, the options were
   ?OCSP? ?CRL? and ?Require both?. ?OCSP? was the default.

   IE5: Popup dialog box: "Revocation information for the security
   certificate for this site is not available. Do you want to proceed?"

7. CA Status

   Historical behavior is only of interest to the extent that it affects
   current operations.

   Every PKIX certificate has a built in expiry date. Thus we are only
   interested in CA operations from the date at which their oldest
   unexpired certificate is still valid.

7.1. Checklist

      Are CRLs or OCSP supported

      Is the CDP extension filled?

      Is the AIA extension filled?

7.2. CA-Browser Forum Requirements

8. Security Considerations

   Put something here?

9. IANA Considerations

   None









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10. References

10.1. Normative References

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC6698]  Hoffman, P.,Schlyter, J., "The DNS-Based Authentication of
              Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.

   [RFC6962]  Laurie, B.,Langley, A.,Kasper, E., "Certificate
              Transparency", RFC 6962, June 2013.

   [RFC5280]  Cooper, D.,Santesson, S.,Farrell, S.,Boeyen, S.,Housley,
              R.,Polk, W., "Internet X.509 Public Key Infrastructure
              Certificate and Certificate Revocation List (CRL)
              Profile", RFC 5280, May 2008.

   [RFC6960]  Santesson, S.,Myers, M.,Ankney, R.,Malpani, A.,Galperin,
              S.,Adams, C., "X.509 Internet Public Key Infrastructure
              Online Certificate Status Protocol - OCSP", RFC 6960, June
              2013.

   [RFC5019]  Deacon, A.,Hurst, R., "The Lightweight Online Certificate
              Status Protocol (OCSP) Profile for High-Volume
              Environments", RFC 5019, September 2007.

Authors' Addresses

   Phillip Hallam-Baker
   Comodo Group Inc.

   philliph@comodo.com

   David Chadwick
   University of Kent

   d.w.chadwick@kent.ac.uk















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