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Versions: (draft-housley-web-pki-problems) 00 01 02 03 04 05

Internet Architecture Board                                   R. Housley
Internet-Draft                                            Vigil Security
Intended status: Informational                             K. O'Donoghue
Expires: November 13, 2016                              Internet Society
                                                            May 12, 2016


Problems with the Public Key Infrastructure (PKI) for the World Wide Web
                   draft-iab-web-pki-problems-02.txt

Abstract

   This document describes some of the challenges facing the current
   Public Key Infrastructure (PKI) used for the World Wide Web (Web PKI)
   and considers promising improvements to address these challenges.
   Technical, process, and policy improvements to the WebPKI are
   considered.  In addition, some technical considerations beyond WebPKI
   itself are considered.  Hopefully the content of this document will
   help drive the Internet community toward wide spread adoption of some
   of the highlighted recommendations.

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."

   This Internet-Draft will expire on November 13, 2016.

Copyright Notice

   Copyright (c) 2016 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Very Brief Description of the Web PKI . . . . . . . . . . . .   3
   3.  Technical Improvements to the Web PKI . . . . . . . . . . . .   4
     3.1.  Weak Cryptographic Algorithms or Short Public Keys  . . .   4
     3.2.  Certificate Status Checking . . . . . . . . . . . . . . .   5
       3.2.1.  Short-lived Certificates  . . . . . . . . . . . . . .   6
       3.2.2.  CRL Distribution Points . . . . . . . . . . . . . . .   6
       3.2.3.  Proprietary Revocation Checks . . . . . . . . . . . .   6
       3.2.4.  OCSP Stapling . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Surprising Certificates . . . . . . . . . . . . . . . . .   8
       3.3.1.  Certificate Authority Authorization (CAA) . . . . . .   9
       3.3.2.  HTTP Public Key Pinning (HPKP)  . . . . . . . . . . .   9
       3.3.3.  HTTP Strict Transport Security (HSTS) . . . . . . . .  10
       3.3.4.  DNS-Based Authentication of Named Entities (DANE) . .  10
       3.3.5.  Certificate Transparency  . . . . . . . . . . . . . .  11
     3.4.  Automation for Server Administrators  . . . . . . . . . .  12
   4.  Policy and Process Improvements to the Web PKI  . . . . . . .  13
     4.1.  Determination of the Trusted Certificate Authorities  . .  13
     4.2.  Price for Certificates  . . . . . . . . . . . . . . . . .  14
     4.3.  Governance Structures for the Web PKI . . . . . . . . . .  15
   5.  Additional Technical Considerations . . . . . . . . . . . . .  15
     5.1.  Browser Error Messages  . . . . . . . . . . . . . . . . .  15
     5.2.  Time Synchronization  . . . . . . . . . . . . . . . . . .  16
   6.  Recommendations for Improving the Web PKI . . . . . . . . . .  16
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  22
   Appendix B.  IAB Members at the Time of Approval  . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   There are many interrelated problems with the current Public Key
   Infrastructure (PKI) used for the World Wide Web (Web PKI).  The Web
   PKI makes use of certificates as described in RFC 5280 [RFC5280].
   These certificates are primarily used with Transport Layer Security
   (TLS) RFC 5246 [RFC5246].




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   The economics of the Web PKI value chain are discussed in [VFBH],
   [AV], and [AVAV].  This document does not discuss the economic issues
   further, but these economic issues provide motivation for correcting
   the other problems that are discussed in this document.

   This document describes technical, usability, process, and policy
   problems, considers some emerging technical improvements, discusses
   some evoling process and policy improvements, and provides some basic
   recommendations for the Internet community.

2.  Very Brief Description of the Web PKI

   Certificates are specified in RFC 5280 [RFC5280].  Certificates
   contain, among other things, a subject name, a public key, a limited
   valid lifetime, and the digital signature of the Certification
   Authority (CA).  Certificate users require confidence that the
   private key associated with the certified public key is owned by the
   named subject.

   The architectural model used in the Web PKI includes:

   EE:   End Entity -- the subject of a certificate -- certificates are
         issued to end entities including Web servers and clients that
         need mutual authentication.

   CA:   Certification Authority -- the issuer of a certificate --
         issues certificates for end entities including Web servers and
         clients.

   RA:   Registration Authority -- an optional system to which a CA
         delegates some management functions such as identity validation
         or physical credential distribution.

   A valid certificate may be revoked for any number of reasons.  CAs
   are responsible for indicating the revocation status of the
   certificates that they issue throughout the lifetime of the
   certificate.  Revocation status information may be provided using the
   Online Certificate Status Protocol (OCSP) RFC 6960 [RFC6960],
   certificate revocation lists (CRLs) RFC 5280 [RFC5280], or some other
   mechanism.  In general, when revocation status information is
   provided using CRLs, the CA is also the CRL issuer.  However, a CA
   may delegate the responsibility for issuing CRLs to a different
   entity.

   The enrollment process used by a CA makes sure that the subject name
   in the certificate is appropriate and that the subject actually holds
   the private key.  Proof of possession of the private key is often
   accomplished through a challenge-response protocol.



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3.  Technical Improvements to the Web PKI

   Over the years, many technical improvements have been made to the Web
   PKI.  This section discusses several problems and the technical
   improvements that have been made to address them.  This history sets
   the stage for suggestions for additional improvements in other
   sections of this document.

3.1.  Weak Cryptographic Algorithms or Short Public Keys

   Over the years, the digital signature algorithms, one-way hash
   functions, and public key sizes that are considered strong have
   changed.  This is not a surprise.  Cryptographic algorithms age; they
   become weaker with time.  As new cryptanalysis techniques are
   developed and computing capabilities improve, the work factor to
   break a particular cryptographic algorithm will reduce.  For this
   reason, the algorithms and key sizes used in the Web PKI need to
   migrate over time.  A reasonable choice of algorithm or key size
   needs to be reevaluated periodically, and a transition may be needed
   before the expected lifetime expires.

   The browser vendors have been trying to manage algorithm and key size
   transitions, but a long-lived trust anchor or intermediate CA
   certificate can have a huge number of certificates under it.  So,
   removing one certificate because it uses a weak cryptographic
   algorithm or a short public key can have a significant impact on a
   large subtree.  In addition, if a certificate for a web site with a
   huge amount of traffic is in that subtree, it will increase the
   difficulty in removing the certificate with a weak cryptographic
   algorithm or a short public key.

   As a result, some valid trust anchors and certificates contain
   cryptographic algorithms long after weaknesses have been discovered
   and widely known.  Similarly, valid trust anchors and certificates
   contain public keys after computational resources available to
   attackers have rendered them too weak.  We have seen a very
   successful migration away from certificates that use the MD2 or MD5
   one-way hash functions.  However, there are still a number of
   certificates that use SHA-1 and 1024-bit RSA public keys [MB2015]
   [MB2016], and these certificates should be replaced.

   Today, the algorithms and key sizes used by a CA to sign end-entity
   certificates with a traditional lifespan of a year should offer 112
   to 128 bits of security.  SHA-256 is a widely studied one-way hash
   function that meets this requirement.  RSA with a public key of at
   least 2048 bits or ECDSA with a public key of at least 256 bits are
   widely studied digital signature algorithms that meet this
   requirement.



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   The algorithms and key sizes used by a CA to sign long-lived
   intermediate CA certificates that often have a lifespan of several
   decades should offer even greater security.  SHA-384 is a widely
   studied one-way hash function that meets this requirement.  RSA with
   a public key of at least 3072 bits or ECDSA with a public key of at
   least 384 bits are widely studied digital signature algorithms that
   meet this requirement.

   Obviously, additional algorithm transitions will be needed in the
   future as these algorithms age.  These algorithms, like the ones that
   were used earlier, will become weaker with time.  RFC 7696 [RFC7696]
   offers some guidelines regarding cryptographic algorithm agility.

3.2.  Certificate Status Checking

   Several years ago, many browsers did not perform certificate status
   checks by default.  That is, browsers did not check whether the
   issuing CA had revoked the certificate unless the user explicitly
   adjusted a setting to enable this feature.  This check can be made by
   fetching the most recent certificate revocation list (CRL) RFC 5280
   [RFC5280], or this check can use the Online Certificate Status
   Protocol (OCSP) RFC 6960 [RFC6960].  The location of the CRL or the
   OCSP responder is usually found in the certificate itself.  However,
   both of these approaches add latency.  The desire to provide a
   responsive user experience is a significant reason that this feature
   has not been turned on by default.  Mobile browsers simply do not
   bother to check revocation status [IMC2015].

   Certificate status checking needs to be used at all times.  Several
   techniques have been tried by CAs and browsers to make certificate
   status checking more efficient.  Many CAs are using Content Delivery
   Networks (CDNs) to deliver CRLs and OCSP responses, resulting in very
   high availability and low latency.  Yet, browser vendors are still
   reluctant to perform standard-based status checking by default for
   every session.

   Certificate status checking by the browser can reveal the web sites
   that the browser user is visiting to the OCSP Responder or the server
   providing the CRL.  This privacy concern can be eliminated by having
   the Web Server include the OCSP Response in the TLS handshake.  This
   is called OCSP Stapling, and it is discussed further in
   Section 3.2.4.

   Measurements in 2015 [IMC2015] show that a surprisingly large
   fraction of Web PKI certicates have been revoked.  Note that these
   measurements were taken around the time of the Heartbleed incident
   [HEARTBLEED], and the revocation activity may have been unusually
   high due to this incident.  In addition, this incident exacerbates



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   the problem of incomplete Web PKI revocation checking.  These
   measurements show that browsers are not obtaining current certificate
   revocation information because it is too expensive in terms of
   latency and bandwidth.  Finally, only a small number of CRL and OCSP
   servers are available over IPv6, and as more of the Web moves to IPv6
   [ABLOG] this is expected to become an increasingly significant issue.

   The following subsections identify some approaches for reducing the
   perceived and actual cost of revocation status checks.

3.2.1.  Short-lived Certificates

   Short-lived certificates are an excellent way to reduce the need for
   certificate status checking.  The shorter the life of the
   certificate, the less time there is for anything to go wrong.  If the
   lifetime is short enough, policy might allow certificate status
   checking can be skipped altogether.  In practice, implementation of
   short-lived certificates requires automation to assist web server
   administrators, which is a topic that is discussed elsewhere in this
   document.

3.2.2.  CRL Distribution Points

   The certificate revocation list distribution point (CRLDP)
   certificate extension RFC 5280 [RFC5280] allows a CA to control the
   maximum size of the CRLs that they issue.  This helps in two ways.
   First, the amount of storage needed by the browser to cache CRLs is
   reduced.  Second, and more importantly, the amount of time it takes
   to download a CRL can be scoped, so that the amount of time needed to
   fetch any single CRL is reasonable.

   Few CAs take advantage of the CRLDP certificate extension to limit
   the size of CRLs.  In fact, there are several CAs that publish
   extremely large CRLs.  Browsers never want to suffer the latency
   associated with large CRLs, and some ignore the CRLDP extension when
   it is present.  Browsers tend to avoid the use of CRLs altogether.

3.2.3.  Proprietary Revocation Checks

   Some browser vendors provide a proprietary mechanism for revocation
   checking.  These mechanisms obtain revocation status information once
   per day for the entire Web PKI in a very compact form.  No network
   traffic is generated at the time that a certificate is being
   validated, so there is no latency associated with revocation status
   checking.  The browser vendor infrastructure performs daily checks of
   the Web PKI, and then the results are assembled in a proprietary
   format and made available to the browser.  These checks only cover




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   the trust anchor store for that browser vendor, so any trust anchors
   added by the user cannot be checked in this manner.

   Measurements in 2015 [IMC2015] show that proprietary status checking
   is not currently providing adequate coverage of the Web PKI.

3.2.4.  OCSP Stapling

   Browsers can avoid transmission of CRLs altogether by using the
   Online Certificate Status Protocol (OCSP) RFC 6960 [RFC6960] to check
   the validity of web server certificates.  The TLS Certificate Status
   Request extension is defined in Section 8 of RFC 6066 [RFC6066].  In
   addition, RFC 6961 [RFC6961] defines the TLS Multiple Certificate
   Status Request extension, which allows the web server to provide
   status information about its own certificate and also the status of
   intermediate certificates in the certification path.  By including
   this extension in the TLS handshake, the browser asks the web server
   to provide an OCSP response in addition to its certificate.  This
   approach greatly reduces the number of round trips by the browser to
   check the status of each certificate in the path.  In addition, the
   web server can cache the OCSP response for a period of time, avoiding
   additional latency.  Even in the cases where the web server needs to
   contact the OCSP responder, the web server usually has a higher
   bandwidth connection than the browser to do so.

   The provision of the time-stamped OCSP response in the TLS handshake
   is referred to as "stapling" the OCSP response to the TLS handshake.
   If the browser does not receive a stapled OCSP response, it can
   contact the OCSP responder directly.  In addition, the MUST_STAPLE
   feature [TLSFEATURE] can be used to insist that OCSP stapling be
   used.

   When every browser that connects to a high volume website performs
   its own OCSP lookup, the OCSP responder must handle a real-time
   response to every browser.  OCSP stapling can avoid enormous volumes
   of OCSP requests for certificates of popular websites, so stapling
   can significantly reduce the cost of providing an OCSP service.

   OCSP stapling can also improve user privacy, since the web server,
   not the browser, contacts the OCSP responder.  In this way, the OCSP
   responder is not able to determine which browsers are checking the
   validity of certificate for websites.

   Many web site are taking advantage of OCSP stapling.  At the time of
   this writing, browser venders report that about 12% the the
   transactions use OCSP stapling, and the number is on the rise.





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3.3.  Surprising Certificates

   All of the CAs in the trust store are equally trusted for the entire
   domain name space, so any CA can issue for any domain name.  In fact,
   there have been certificates issued by CAs that are surprising to the
   legitimate owner of a domain.  The domain name owner is surprised
   because they did not request the certificates.  They are initially
   unaware that a CA has issued a certificate that contains their domain
   name, and once the surprising certificate is discovered, it can be
   very difficult for the legitimate domain name owner to get it
   revoked.  Further, browsers and other relying parties cannot
   distinguish a certificate that the legitimate domain name owner
   requested from a surprising one.

   Since all of the CAs in the trust store are equally trusted, any CA
   can issue a certificate for any domain name.  There are known cases
   where attackers have thwarted the CA protections and issued
   certificates that were then used to mount attacks against the users
   of that web site [FOXIT].  For this reason, all of the CAs listed in
   the trust store must be very well protected.

   The Baseline Requirements produced by the CA/Browser Forum [CAB2014]
   tell CAs the checks that need to be performed before a certificate is
   issued.  In addition, WebTrust [WEBTRUST] provides the audit
   requirements for CAs, and browser vendors will remove a CA from the
   trust anchor store if successful audit reports are not made
   available.

   When a CA issues a certificate to a subordinate CA, the inclusion of
   widely supported certificate extensions can reduce the set of
   privileges given to the sub-CA.  This aligns with the traditional
   security practice of least privilege, where only the privileges
   needed to perform the envisioned tasks are provided.  However, many
   sub-CAs have certificates that pass along the full powers of the CA,
   creating additional high-pay-off targets for attackers, and these
   sub-CAs may not be held to the same certificate issuance requirements
   and audit requirement as the parent CA.

   Some major implementations have not fully implemented the mechanisms
   necessary to reduce sub-CA privileges.  For example, RFC 5280
   [RFC5280] includes the specification of name constraints, and the CA/
   Browser Forum guidelines [CAB2014] encourage the use of dNSNames in
   permittedSubtrees within the name constraints extension.  Despite
   this situation, one major browser does not support name constraints,
   and as a result, CAs are reluctant to use them.  When they are used,
   the name constraints extension in not marked critical so that the
   browser that does not support this extension is free to ignore the
   extension.  This situation leads to enforcement of the name



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   constraint by some browsers and not others.  Further, global CAs are
   prepared to issue certificates within every top-level domain,
   including ones that are newly-approved.  It is not practical for
   these global CAs to use name constraints in their sub-CA
   certificates.

   As a result of procedural failures or attacks, surprising
   certificates are being issued.  Several mechanisms have been defined
   to avoid the issuance of surprising certificates or prevent browsers
   from accepting them.

3.3.1.  Certificate Authority Authorization (CAA)

   The Certificate Authority Authorization (CAA) [RFC6844] DNS resource
   record allows a domain administrator to specify one or more CAs
   authorized to issue certificates that include that domain name.
   Then, a trustworthy CA will refuse to issue a certificate for a
   domain name that has a CAA resource record that does not explicitly
   name the CA.

   To date, only one major CA performs this check, and there is no
   indication that other CAs are planning to add this check in the near
   future.

3.3.2.  HTTP Public Key Pinning (HPKP)

   HTTP Public Key Pinning (HPKP) [RFC7469] allows a web server to
   instruct browsers to remember the server's public key fingerprints
   for a period of time.  The fingerprint is a one-way hash of the
   subject public key information in the certificate.  The Public-Key-
   Pins header provides a maximum retention period, fingerprints of the
   web server certificate, and optionally fingerprints for backup
   certificates.  The act of saving the fingerprints is referred to as
   "pinning".  During the pin lifetime, browsers require that the same
   web server present a certificate chain that includes a public key
   that matches one of the retained fingerprints.  This prevents
   impersonation of the website with a surprising certificate.

   A website can choose to pin the CA certificate so that the browser
   will accept only valid certificates for the website domain that are
   issued by that CA.  Alternatively, the website can choose to pin
   their own certificate and at least one backup certificate in case the
   current certificate needs to be replaced due to a compromised server.

   Some browser vendors also pin certificates by hardcoding fingerprints
   of very well known websites.





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   When HPKP is used, browsers may be able to detect a man-in-the-
   middle.  Sometimes the man-in-the-middle is an attacker, and other
   times a service provider purposefully terminates the TLS at a
   location other than the web server.  One example became very public
   in February 2012 when Trustwave admitted that it had issued a
   subordinate CA certificate for use by a company to inspect corporate
   network traffic [LC2012].  When HPKP is used, the browser user will
   be notified if the key-pining is violated, unless the violating
   certificate can be validated to a locally installed trust anchor.  In
   this situation, the browser is assuming that the user intended to
   explicitly trust the certificate.

3.3.3.  HTTP Strict Transport Security (HSTS)

   HTTP Strict Transport Security (HSTS) [RFC6797] is a security policy
   mechanism that protects secure websites against downgrade attacks,
   and it greatly simplifies protection against cookie hijacking.  The
   presence of the Strict-Transport-Security header tells browsers that
   all interactions with this web server should never use HTTP without
   TLS, providing protection against both eavesdropping and active
   network attacks.  The protections can be tied to a domain or a domain
   and all of its sub-domains.

   When a web server includes the Strict-Transport-Security header, the
   browser is expected to do two things.  First, the browser
   automatically turns any insecure links into secure ones.  For
   instance, "http://mysite.example.com/mypage/" will be changed to
   "https://mysite.example.com/mypage/".  Second, if the TLS Handshake
   results in some failure, such as the certificate cannot be validated,
   then an error message is displayed and the user is denied access to
   the web application.  Any web server misconfiguration, such as a
   certificate expiration, will result no one being able to access the
   web site until the configuration is corrected.

   To date, HSTS ha seen very little deployment, and there is no
   indication that the browser vendors intend to add support for it.

3.3.4.  DNS-Based Authentication of Named Entities (DANE)

   The DNS-Based Authentication of Named Entities (DANE) [RFC6698]
   allows domain administrators to specify the raw public keys or
   certificates that are used by web servers in their domain.  DANE
   leverages the DNS Security Extensions (DNSSEC) [RFC4034][RFC4035],
   which provides digital signatures over DNS zones that are validated
   with keys that are bound to the domain name of the signed zone.  The
   keys associated with a domain name can only be signed by a key
   associated with the parent of that domain name.  For example, the
   DNSSEC keys for "www.example.com" can only be signed by the DNSSEC



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   keys for "example.com".  Therefore, a malicious actor can only
   compromise the keys of their own subdomains.  Like the Web PKI,
   DNSSEC relies on public keys used to validate chains of signatures,
   but DNSSEC has a single root domain as opposed to a multiplicity of
   trusted CAs.

   DANE binds raw public keys or certificates to DNS names.  The domain
   administrator is the one that vouches for the binding of the public
   key or the certificate to the domain name by adding the TSLA records
   to the zone and then signing the zone.  In this way, the same
   administrator is responsible for managing the DNS names themselves
   and associated public keys or certificates with those names.  DANE
   restricts the scope of assertions that can be made, forcing them to
   be consistent with the DNS naming hierarchy.

   In addition, DNSSEC reduces opportunities for redirection attacks by
   binding the domain name to the public key or certificate.

   Some Web PKI certificates are being posted in TLSA records, but
   browsers expect to receive the server certificate in the TLS
   handshake, and there is little incentive for browsers to confirm that
   the received certificate matches the one posted in the DNS.  For this
   reason, work has begun on a TLS extension that will allow the DNSSEC-
   protected information to be provided in the handshake, which will
   eliminate the added latency [TLSCHAIN].

3.3.5.  Certificate Transparency

   Certificate Transparency (CT) [RFC6962] offers a mechanism to detect
   surprising certificates, and once detected, administrators and CAs
   can take the necessary actions to revoke the surprising certificates.

   When requesting a certificate, the administrator can request the CA
   to include an embedded Signed Certificate Timestamp (SCT) in the
   certificate to ensure that their legitimate certificate is logged
   with one or more CT logs.

   An administrator, or another party acting on behalf of the
   administrator, is able to monitor one or more CT logs to which a pre-
   certificate or certificate is submitted, and detect the logging of a
   pre-certificate or certificate that contains their domain name.  When
   such a pre-certificate or certificate is detected, the CA can be
   contacted to to get the surprising certificate revoked.

   In the future, a browser may choose to reject certificates that do
   not contain an SCT, and potentially notify the website administrator
   or CA when they encounter such a certificate.  Such reporting will




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   help detect mis-issuance of certificates and lead to their
   revocation.

   The IETF Certificate Transparency Working Group is in the process of
   updating RFC 6962.  Many data structures are changing, and CT logs
   will have to pick a format, choosing version 1 (v1) that conforms to
   RFC 6962 or version 2 (v2) that conforms to the new specification.
   Since the data structures are incompatible, a v2 log will not be able
   to issue a valid v1 SCT.  A CT client can support both v1 and v2 SCTs
   for the same certificate, simultaneously, since a v1 SCT will be
   carried in different extension than a v2 SCT.

3.4.  Automation for Server Administrators

   The IETF has developed several protocols for certificate management,
   including the Certificate Management Protocol (CMP) [RFC4210],
   Certificate Management over CMS (CMC) [RFC5272], and Enrollment over
   Secure Transport (EST) [RFC7030].  There are also some proprietary
   certificate management protocols.  None of these protocols enjoys a
   dominate position in the market.

   There have been several attempts to provide automation for routine
   tasks that are performed by web server administrators, such as
   certificate renewal.  For example, some commercial tools offer
   automated certificate renewal and installation [DCEI][SSLM].  Also,
   at least one proposal was brought to the IETF that allows a web
   server to automate obtaining and renewing certificates [PHBOB].
   Without automation, there are many manual steps involved in getting a
   certificate from a CA, and to date none of these attempts at
   automation have enjoyed widespread interoperability and adoption.
   There are at least two ways that this impacts web security.  First,
   many web sites do not have a certificate at all.  The cost, time, and
   effort are too great for the system administrator.  This is
   especially true if the web site is not involved in financial
   transactions or some other critical activity.  Second, once a
   certificate is obtained, a replacement is not obtained until the
   current one expires.  Automation can reduce the amount of time that
   an administrator needs to dedicate to certificate management, and it
   can make certificate renewal timely and automatic.  Both of these
   should lead to more widespread deployment and improved web security.

   The IETF ACME working group [ACMEWG] is working on protocols that
   will provide system administrators with an automated way to enroll
   and renew their certificates.  The expectation is that these
   specifications will lead to widely available and interoperable tools
   for system administrators.  The expectation is that these protocols
   and tools will be supported by all web server environments and CAs,
   which will greatly reduce complexity and cost.  In addition, the



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   easier renewal process provided by automation can be used to reduce
   certificate lifetimes, which in turn will reduce the time required to
   flush old algorithms out of the system when it is decided to
   transition to newer more secure algorithms.

4.  Policy and Process Improvements to the Web PKI

   As with many technologies, the issues and complexities associated
   with Web PKI use and deployment are just as much policy and process
   as technical.  These have evolved over time as well.  This section
   discusses the ways that business models and operational policies and
   processes impact the Web PKI.

4.1.  Determination of the Trusted Certificate Authorities

   A very basic question for users of the Web PKI is "Who do you trust?"
   The system for determining which CAs are added to or removed from the
   trust store in browsers has been perceived by some as opaque and
   confusing.  As mentioned earlier, the CA/Browser Forum has developed
   baseline requirements for the management and issuance of certificates
   [CAB2014] for individual CAs.  However, the process by which an
   individual CA gets added to the trust store for each of the major
   browsers is not straightforward.  The individual browser vendors
   determine what should and should not be trusted by including those
   trusted CAs in their trust store.  They do this by leveraging the
   AICPA/CICA WebTrust Program for Certification Authorities [WEBTRUST].
   This program provides auditing requirements and a trust mark for CAs.
   Failure to pass an audit can result in the CA being removed from the
   trust store.

   Once the browser has shipped, how does a user know which CAs are
   trusted or what has changed recently?  For an informed user,
   information about which CAs have been added to or deleted from the
   browser trust store can be found in the release notes.  Users can
   also examine the policies of the various CAs which would have been
   developed and posted for the WebTrust Program.  However, this is a
   very high barrier for the average user.  There are options to make
   local modifications by educated users, but there is little
   understanding about the implications of these choices.  How does an
   individual, organization, or enterprise really determine if a
   particular CA is trustworthy?  Do the default choices inherited from
   the browser vendors truly represent the organization's trust model?
   What constitutes sufficiently bad behavior by a CA to cause removal
   from the trust store?

   In addition, it can be hazardous for users to remove CAs from the
   browser trust store.  If a user removes a CA from the browser trust
   store, some web sites may become completely inaccessible or require



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   the user to take explicit action to accept warnings or bypass browser
   protections related to untrusted certificates.

   One form of bad behavior by CAs is the mis-issuance of certificates.
   This mis-issuance can be either an honest mistake by the CA,
   malicious behavior by the CA, or a case where an external party has
   duped the CA into the mis-issuance.  When a CA has delegated
   authority to a sub-CA, and then the sub-CA issued bad certificates
   either unintentionally or maliciously, the CA is able to deny
   responsibility for the actions of the sub-CA.  However, the CA may be
   the only party that can revoke the sub-CA certificate to protect the
   overall Web PKI.

   Another complication with CAs and the trust store maintained by the
   browser vendor is an enterprise managed PKI.  For example, the US
   Department of Defense operates its own PKI.  In this case, the
   enterprise maintains its own PKI for the exclusive use of the
   enterprise itself.  A bridge CA may be used to connect related
   enterprises.  The complication in this approach is that the
   revocation mechanisms don't work with any additions that have been
   made by the enterprise.  See Section 3.2.3 on proprietary revocation
   checks.

   The guidelines provided by the WebTrust program [WEBTRUST] provide a
   framework for removing a CA from the trust store.  However, the
   implications of removing a CA can be significant.  There may be a few
   very large CAs that are critical to significant portions of Internet
   infrastructure.  Removing one of these trusted CAs can have a
   significant impact on a large cross section of Internet users
   resulting in potentially large numbers of websites no longer being
   trusted.  Users are already struggling to understand the implications
   of untrusted websites and often ignore the current warnings as
   discussed below.

4.2.  Price for Certificates

   Many CAs charge for each of the certificates that they issue.  This
   business model creates a hurdle for proper deployment.  In some
   cases, the cost causes people to deploy self-signed certificates that
   cannot be validated against the browser trust store.  In other cases,
   the cost is an incentive to use the same certificate and the
   associate private key on many different servers within a domain.
   This leads to greater exposure of the private key, and coordination
   is needed among the server administrators when it is time to renew
   the certificate.

   At least one CA is moving away from the charging-per-certificate
   business model.  The Let's Encrypt CA [LE] is offering free web



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   server certificates.  This zero-cost business model will
   significantly change the Web PKI, removing the cost concerns that
   leaded to insecure deployment.

4.3.  Governance Structures for the Web PKI

   There are a number of organizations that play significant roles in
   the operation of the Web PKI, including the CAB Forum, the WebTrust
   Program, and the browser vendors.  These organizations act on behalf
   of the entire Internet community.  Transparency in these operations
   is vital to basic trust in the Web PKI.  As one example, in the past
   the CAB Forum was perceived as being a closed forum; however, some
   changes were made to the operational procedures to allow more
   visibility if not actual participation in the process [CAB1.2].  How
   do we ensure that these processes continue to evolve in an open,
   inclusive, and transparent manner?  Currently, as the name implies,
   the CAB Forum members represent CAs and browser vendors.  How do we
   ensure that relying parties have a voice in this forum?

   Since the Web PKI is widespread, applications beyond the World Wide
   Web are making use of the Web PKI.  For example, the Web PKI is used
   to secure the connections between SMTP servers.  In these
   environments, the browser-centric capabilities are unavailable.  For
   example, see Section 3.2.3 on proprietary revocation checks.  The
   current governance structure does not provide a way for these other
   applications to participate.  How do we ensure that these other
   applications get a voice in this forum?

5.  Additional Technical Considerations

   Beyond the technical mechanisms that constitute the Web PKI itself,
   there are additional technical considerations that impact the success
   of the Web PKI.

5.1.  Browser Error Messages

   Many people find browser error messages related to certificates
   confusing.  Good man-machine interfaces are always difficult, but in
   this situation users are unable to understand the risks that they
   being asked to accept.  Even experts do not have the time or
   inclination to make an assessment of the situation that caused the
   error message.  As a result, browser users have learned to largely
   ignore the warning messages.

   Many different situations can cause an error message, where blocking
   the communication would not be in the interest of the browser user.
   There are many examples, including web sites that use self-signed
   certificates, captive portals that redirect intercepted HTTPS



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   connections, and certificates that appear expired because the local
   computer clock is wrong.

   Too often the warnings are due to web server misconfiguration.
   Further, solving the error message situation in isolation is probably
   not possible; instead, it needs to be considered along side the other
   issues raised in this document.

   If the risk to the user is high, then the session should be closed
   with a clear description of the problem that was encountered.  Doing
   so will lead to improved management of the overall infrastructure
   over time, especially as the tools that are being developed for web
   server administrators are rolled out.

   In some enterprises, avoiding these errors requires the addition of
   enterprise-specific trust anchors to the browser.  Additional tools
   are needed to allow administrations to easily add appropriate trust
   anchors, especially browsers on consumer-grade devices as more and
   more enterprises are embracing bring-your-own-device policies.

5.2.  Time Synchronization

   Time synchronization is another factor that impacts the security and
   reliability of the Web PKI.  Reasonably accurate time is needed to
   check certificate expiration and to determine whether cached
   revocation status information is fresh.  There is ongoing work to
   improve the security of the time synchronization infrastructure, and
   it will use certificates to authenticate time servers.  Since the
   certificate infrastructure relies on quality time synchronization,
   this dependency creates a boot strapping issue.

6.  Recommendations for Improving the Web PKI

   To make the Web PKI more secure and more robust, the following
   priorities have been identified and are recommended for further
   development and deployment:

      Improve certificate status checking.
         Develop and deploy a standard solution for all relying parties
         is needed.  OCSP stapling seems to be a significant part of
         this solution.

      Automation for certificate enrollment and renewal.
         Develop and deploy standard protocols that provide system
         administrators with an automated way to enroll and renew their
         certificates.  This work is currently underway in the IETF.





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   In addition, solutions to these procedural and policy challenges are
   needed:

      Smooth transition between cryptographic algorithms.
         Develop best practices for smooth and timely transition between
         cryptographic algorithms.

      Eliminate surprising certificates.
         Develop best practices that use one or more of the several
         mechanisms that have been defined throughout the Web PKI to
         eliminate surprising certificates.

      Confidence in CA actions.
         Develop best practices for identifying and dealing with bad
         behavior by a CA that can be followed by all browser vendors.

      Open and transparent Web PKI governance.
         Develop a governance structure that allows relying parties to
         have a voice resulting in open and transparent governance.

7.  Security Considerations

   Not just the Web depends on the Web PKI.  For example, mail servers
   often use certificates that are validated using the trust store from
   a browser.  In addition, applications written in scripting languages
   that run in the browser environment do not have access to any
   alternative infrastructures.  The Web PKI is being leveraged to avoid
   the time and expense to establish an independent PKI.

   More and more Internet applications depend on the Web PKI.  For the
   most part, leveraging the Web PKI is improving the security of the
   Internet.  However, the Web PKI is being used in ways that were not
   envisaged in its design.  Care is needed to ensure that applications
   are not expecting security properties that cannot be delivered by the
   Web PKI.

   This document considers the weaknesses of the current Web PKI system
   and provides recommendations for improvements.  Some of the risks
   associated with doing nothing or continuing down the current path are
   articulated.  The Web PKI is a vital component of a trusted Internet
   and as such needs to be improved to sustain continued growth of the
   Internet.

8.  IANA Considerations

   None.

   {{{ RFC Editor: Please remove this section prior to publication. }}}



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9.  References

9.1.  Normative References

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,
              <http://www.rfc-editor.org/info/rfc6960>.

9.2.  Informative References

   [ABLOG]    Nygren, E., "Three years since World IPv6 Launch: strong
              IPv6 growth continues", June 2015,
              <https://blogs.akamai.com/2015/06/three-years-since-world-
              ipv6-launch-strong-ipv6-growth-continues.html>.

   [ACMEWG]   IETF, "Charter for Automated Certificate Management
              Environment (acme) Working Group", June 2015,
              <https://datatracker.ietf.org/doc/charter-ietf-acme/>.

   [AV]       Arnbak, A. and N. van Eijk, "Certificate Authority
              Collapse: Regulating Systemic Vulnerabilities in the HTTPS
              Value Chain", 2012 TRPC , August 2012,
              <http://dx.doi.org/10.2139/ssrn.2031409>.

   [AVAV]     Asghari, H., van Eeten, M., Arnbak, A., and N. van Eijk,
              "Security Economics in the HTTPS Value Chain", Workshop on
              Economics of Information Security (WEIS) 2013 , 2013,
              <http://www.econinfosec.org/archive/weis2013/papers/
              AsghariWEIS2013.pdf>.

   [CAB1.2]   CA/Browser Forum, "Bylaws of the CA/Browser Forum",
              October 2014, <https://cabforum.org/wp-content/uploads/CA-
              Browser-Forum-Bylaws-v.1.2.pdf>.

   [CAB2014]  CA/Browser Forum, "CA/Browser Forum Baseline Requirements
              for the Issuance and Management of Publicly-Trusted
              Certificates, v.1.2.2", October 2014,
              <https://cabforum.org/wp-content/uploads/BRv1.2.2.pdf>.





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   [DCEI]     DigiCert Inc, "Express Install(TM): Automate SSL
              Certificate Installation and HTTPS Configuration", August
              2015, <https://www.digicert.com/express-install/>.

   [FOXIT]    Prins, J., "DigiNotar Certificate Authority breach:
              "Operation Black Tulip"", September 2011,
              <http://www.rijksoverheid.nl/bestanden/documenten-en-
              publicaties/rapporten/2011/09/05/
              diginotar-public-report-version-1/
              rapport-fox-it-operation-black-tulip-v1-0.pdf>.

   [HEARTBLEED]
              Wikipedia, "Heartbleed", 2016,
              <https://en.wikipedia.org/wiki/Heartbleed>.

   [IMC2015]  Liu, Y., Tome, W., Zhang, L., Choffnes, D., Levin, D.,
              Maggs, B., Mislove, A., Schulman, A., and C. Wilson, "An
              End-to-End Measurement of Certificate Revocation in the
              Web's PKI", October 2015,
              <http://conferences2.sigcomm.org/imc/2015/papers/
              p183.pdf>.

   [LC2012]   Constantin, L., "Trustwave admits issuing man-in-the-
              middle digital certificate; Mozilla debates punishment",
              February 2012,
              <http://www.computerworld.com/article/2501291/internet/
              trustwave-admits-issuing-man-in-the-middle-digital-
              certificate--mozilla-debates-punishment.html>.

   [LE]       Internet Security Research Group, "Let's Encrypt", July
              2015, <https://letsencrypt.org/>.

   [MB2015]   Wilson, K., "Phase 2: Phasing out Certificates with
              1024-bit RSA Keys", January 2015,
              <https://blog.mozilla.org/security/2015/01/28/phase-2-
              phasing-out-certificates-with-1024-bit-rsa-keys/>.

   [MB2016]   Barnes, R., "Payment Processors Still Using Weak Crypto",
              February 2016,
              <https://blog.mozilla.org/security/2016/02/24/payment-
              processors-still-using-weak-crypto/>.

   [PHBOB]    Hallam-Baker, P., "OmniBroker Publication Protocol",
              draft-hallambaker-omnipublish-00 (work in progress), May
              2014.






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   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,
              <http://www.rfc-editor.org/info/rfc4034>.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
              <http://www.rfc-editor.org/info/rfc4035>.

   [RFC4210]  Adams, C., Farrell, S., Kause, T., and T. Mononen,
              "Internet X.509 Public Key Infrastructure Certificate
              Management Protocol (CMP)", RFC 4210,
              DOI 10.17487/RFC4210, September 2005,
              <http://www.rfc-editor.org/info/rfc4210>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC5272]  Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008,
              <http://www.rfc-editor.org/info/rfc5272>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <http://www.rfc-editor.org/info/rfc6066>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <http://www.rfc-editor.org/info/rfc6698>.

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797,
              DOI 10.17487/RFC6797, November 2012,
              <http://www.rfc-editor.org/info/rfc6797>.

   [RFC6844]  Hallam-Baker, P. and R. Stradling, "DNS Certification
              Authority Authorization (CAA) Resource Record", RFC 6844,
              DOI 10.17487/RFC6844, January 2013,
              <http://www.rfc-editor.org/info/rfc6844>.







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   [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
              Multiple Certificate Status Request Extension", RFC 6961,
              DOI 10.17487/RFC6961, June 2013,
              <http://www.rfc-editor.org/info/rfc6961>.

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
              <http://www.rfc-editor.org/info/rfc6962>.

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,
              <http://www.rfc-editor.org/info/rfc7030>.

   [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
              2015, <http://www.rfc-editor.org/info/rfc7469>.

   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
              <http://www.rfc-editor.org/info/rfc7696>.

   [SSLM]     Opsmate, Inc., "SSLMate: Secure your website the easy
              way", August 2015, <https://sslmate.com/>.

   [TLSCHAIN]
              Shore, M., Barnes, R., Huque, S., and W. Toorop, "X.509v3
              TLS Feature Extension", draft-shore-tls-dnssec-chain-
              extension-01 (work in progress), July 2015.

   [TLSFEATURE]
              Hallam-Baker, P., "X.509v3 TLS Feature Extension", draft-
              hallambaker-tlsfeature-10 (work in progress), July 2015.

   [VFBH]     Vratonjic, N., Freudiger, J., Bindschaedler, V., and J.
              Hubaux, "The Inconvenient Truth About Web Certificates",
              Workshop on Economics of Information Security (WEIS)
              2011 , 2011,
              <http://www.econinfosec.org/archive/weis2011/papers/The%20
              Inconvenient%20Truth%20about%20Web%20Certificates.pdf>.

   [WEBTRUST]
              CPA Canada, "WebTrust Program for Certification
              Authorities", August 2015, <http://www.webtrust.org/
              homepage-documents/item27839.aspx>.





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Appendix A.  Acknowledgements

   This document has been developed within the IAB Privacy and Security
   Program.  The authors greatly appreciate the review and suggestions
   provided by Rick Andrews, Mary Barnes, Richard Barnes, Marc Blanchet,
   Alissa Cooper, Nick Doty, Stephen Farrell, Joe Hall, Ted Hardie,
   Ralph Holz, Lee Howard, Christian Huitema, Eliot Lear, Xing Li, Lucy
   Lynch, Gervase Markham, Andrei Robachevsky, Thomas Roessler, Jeremy
   Rowley, Christine Runnegar, Jakob Schlyter, Wendy Seltzer, Martin
   Thomson, Brian Trammell, and Juan Carlos Zuniga.

Appendix B.  IAB Members at the Time of Approval

   {{{ RFC Editor: Please add the names to the IAB members at the time
   that this document is put into the RFC Editor queue. }}}

Authors' Addresses

   Russ Housley
   Vigil Security
   918 Spring Knoll Drive
   Herndon, VA  20170
   USA

   Email: housley@vigilsec.com


   Karen O'Donoghue
   Internet Society
   1775 Wiehle Ave #201
   Reston, VA  20190
   USA

   Email: odonoghue@isoc.org

















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