Internet DRAFT - draft-wpkops-revocation
draft-wpkops-revocation
Internet Engineering Task Force (IETF) Phillip Hallam-Baker
Internet-Draft Comodo Group Inc.
Intended Status: Standards Track David Chadwick
Expires: November 13, 2014 University of Kent
May 12, 2014
Web PKI Operations: Revocation and Status
draft-wpkops-revocation-00
Abstract
This document describes the certificate status mechanisms supported
in the Web PKI
Status of This Memo
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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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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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