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Versions: (draft-linus-trans-gossip) 00 01 02 03 04 05

TRANS                                                        L. Nordberg
Internet-Draft                                                  NORDUnet
Intended status: Experimental                                 D. Gillmor
Expires: April 22, 2016                                             ACLU
                                                               T. Ritter

                                                        October 20, 2015


                            Gossiping in CT
                       draft-ietf-trans-gossip-01

Abstract

   The logs in Certificate Transparency are untrusted in the sense that
   the users of the system don't have to trust that they behave
   correctly since the behaviour of a log can be verified to be correct.

   This document tries to solve the problem with logs presenting a
   "split view" of their operations.  It describes three gossiping
   mechanisms for Certificate Transparency: SCT Feedback, STH
   Pollination and Trusted Auditor Relationship.

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 April 22, 2016.

Copyright Notice

   Copyright (c) 2015 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



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Defining the problem  . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Terminology and data flow . . . . . . . . . . . . . . . . . .   5
   5.  Who gossips with whom . . . . . . . . . . . . . . . . . . . .   6
   6.  What to gossip about and how  . . . . . . . . . . . . . . . .   6
   7.  Gossip Mechanisms . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  SCT Feedback  . . . . . . . . . . . . . . . . . . . . . .   6
       7.1.1.  HTTPS client to server  . . . . . . . . . . . . . . .   7
       7.1.2.  HTTPS server to auditors  . . . . . . . . . . . . . .   9
       7.1.3.  SCT Feedback data format  . . . . . . . . . . . . . .  10
     7.2.  STH pollination . . . . . . . . . . . . . . . . . . . . .  10
       7.2.1.  HTTPS Clients and Proof Fetching  . . . . . . . . . .  12
       7.2.2.  STH Pollination without Proof Fetching  . . . . . . .  13
       7.2.3.  Auditor and Monitor Action  . . . . . . . . . . . . .  13
       7.2.4.  STH Pollination data format . . . . . . . . . . . . .  13
     7.3.  Trusted Auditor Stream  . . . . . . . . . . . . . . . . .  14
       7.3.1.  Trusted Auditor data format . . . . . . . . . . . . .  14
   8.  3-Method Ecosystem  . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  SCT Feedback  . . . . . . . . . . . . . . . . . . . . . .  15
     8.2.  STH Pollination . . . . . . . . . . . . . . . . . . . . .  15
     8.3.  Trusted Auditor Relationship  . . . . . . . . . . . . . .  16
     8.4.  Interaction . . . . . . . . . . . . . . . . . . . . . . .  17
   9.  Security considerations . . . . . . . . . . . . . . . . . . .  17
     9.1.  Censorship/Blocking considerations  . . . . . . . . . . .  17
     9.2.  Privacy considerations  . . . . . . . . . . . . . . . . .  19
       9.2.1.  Privacy and SCTs  . . . . . . . . . . . . . . . . . .  19
       9.2.2.  Privacy in SCT Feedback . . . . . . . . . . . . . . .  19
       9.2.3.  Privacy for HTTPS clients performing STH Proof
               Fetching  . . . . . . . . . . . . . . . . . . . . . .  20
       9.2.4.  Privacy in STH Pollination  . . . . . . . . . . . . .  20
       9.2.5.  Privacy in STH Interaction  . . . . . . . . . . . . .  21
       9.2.6.  Trusted Auditors for HTTPS Clients  . . . . . . . . .  21
       9.2.7.  HTTPS Clients as Auditors . . . . . . . . . . . . . .  22
   10. Policy Recommendations  . . . . . . . . . . . . . . . . . . .  22
     10.1.  Mixing Recommendations . . . . . . . . . . . . . . . . .  22
     10.2.  Blocking Recommendations . . . . . . . . . . . . . . . .  24
       10.2.1.  Frustrating blocking . . . . . . . . . . . . . . . .  24
       10.2.2.  Responding to possible blocking  . . . . . . . . . .  24



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   11. IANA considerations . . . . . . . . . . . . . . . . . . . . .  25
   12. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  25
   13. ChangeLog . . . . . . . . . . . . . . . . . . . . . . . . . .  25
     13.1.  Changes between ietf-00 and ietf-01  . . . . . . . . . .  25
     13.2.  Changes between -01 and -02  . . . . . . . . . . . . . .  25
     13.3.  Changes between -00 and -01  . . . . . . . . . . . . . .  25
   14. Normative References  . . . . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   The purpose of the protocols in this document, collectively referred
   to as CT Gossip, is to detect certain misbehavior by CT logs.  In
   particular, CT Gossip aims to detect logs that are providing
   incosistent views to different log clients and logs failing to
   include submitted certificates within the time period stipulated by
   MMD.

   [TODO: enumerate the interfaces used for detecting misbehaviour?]

   One of the major challenges of any gossip protocol is limiting damage
   to user privacy.  The goal of CT gossip is to publish and distribute
   information about the logs and their operations, but not to leak any
   additional information about the operation of any of the other
   participants.  Privacy of consumers of log information (in
   particular, of web browsers and other TLS clients) should not be
   undermined by gossip.

   This document presents three different, complementary mechanisms for
   non-log elements of the CT ecosystem to exchange information about
   logs in a manner that preserves the privacy of HTTPS clients.  They
   should provide protective benefits for the system as a whole even if
   their adoption is not universal.

2.  Defining the problem

   When a log provides different views of the log to different clients
   this is described as a partitioning attack.  Each client would be
   able to verify the append-only nature of the log but, in the extreme
   case, each client might see a unique view of the log.

   The CT logs are public, append-only and untrusted and thus have to be
   monitored for consistency, i.e., they should never rewrite history.
   Additionally, monitors and other log clients need to exchange
   information about monitored logs in order to be able to detect a
   partitioning attack (as described above).





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   Gossiping about log responses to queries helps address the problem of
   detecting malicious or compromised logs with respect to a
   partitioning attack.  We want some side of the partitioned tree, and
   ideally both sides, to see the other side.

   Disseminating information about a log poses a potential threat to the
   privacy of end users.  Some data of interest (e.g.  SCTs) are
   linkable to specific log entries and thereby to specific sites, which
   makes sharing them with others privacy-sensitive.  Gossiping about
   this data has to take privacy considerations into account in order
   not to leak associations between users of the log (e.g., web
   browsers) and certificate holders (e.g., web sites).  Even sharing
   STHs (which do not link to specific log entries) can be problematic -
   user tracking by fingerprinting through rare STHs is one potential
   attack (see Section 7.2).

3.  Overview

   SCT Feedback enables HTTPS clients to share Signed Certificate
   Timestamps (SCTs) (Section 3.3 of [RFC-6962-BIS]) with CT auditors in
   a privacy-preserving manner by sending SCTs to originating HTTPS
   servers which in turn share them with CT auditors.

   In STH Pollination, HTTPS clients use HTTPS servers as pools sharing
   Signed Tree Heads (STHs) (Section 3.6 of [RFC-6962-BIS]) with other
   connecting clients in the hope that STHs will find their way to
   auditors and monitors.

   HTTPS clients in a Trusted Auditor Relationship share SCTs and STHs
   with trusted auditors or monitors directly, with expectations of
   privacy sensitive data being handled according to whatever privacy
   policy is agreed on between client and trusted party.

   Despite the privacy risks with sharing SCTs there is no loss in
   privacy if a client sends SCTs for a given site to the site
   corresponding to the SCT.  This is because the site's logs would
   already indicate that the client is accessing that site.  In this way
   a site can accumulate records of SCTs that have been issued by
   various logs for that site, providing a consolidated repository of
   SCTs that could be shared with auditors.  Auditors can use this
   information to detect logs that misbehaves by not including
   certificates within the time period stipulated by the MMD metadata.

   Sharing an STH is considered reasonably safe from a privacy
   perspective as long as the same STH is shared by a large number of
   other log clients.  This "safety in numbers" can be achieved by
   requiring gossiping of STHs only of a certain "freshness" while also




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   refusing to gossip about STHs from logs with too high an STH issuance
   frequency (see Section 7.2).

4.  Terminology and data flow

   This document relies on terminology and data structures defined in
   [RFC-6962-BIS], including STH, SCT, Version, LogID, SCT timestamp,
   CtExtensions, SCT signature, Merkle Tree Hash.

   The following picture shows how certificates, SCTs and STHs flow
   through a CT system with SCT Feedback and STH Pollination.  It does
   not show what goes in the Trusted Auditor Relationship stream.

      +- Cert ---- +----------+
      |            |    CA    | ----------+
      |   + SCT -> +----------+           |
      v   |                           Cert [& SCT]
   +----------+                           |
   |   Log    | ---------- SCT -----------+
   +----------+                           v
     |  ^                          +----------+
     |  |          SCT & Certs --- | Website  |
     |  |[1]           |           +----------+
     |  |[2]          STH            ^     |
     |  |[3]           v             |     |
     |  |          +----------+      |     |
     |  +--------> | Auditor  |      |  HTTPS traffic
     |             +----------+      |     |
     |             /                 |    SCT
     |            /            SCT & Certs |
   Log entries   /                   |     |
     |          /                   STH   STH
     v         /[4]                  |     |
   +----------+                      |     v
   | Monitor  |                    +----------+
   +----------+                    | Browser  |
                                   +----------+

   #   Auditor                        Log
   [1] |--- get-sth ------------------->|
       |<-- STH ------------------------|
   [2] |--- leaf hash + tree size ----->|
       |<-- index + inclusion proof --->|
   [3] |--- tree size 1 + tree size 2 ->|
       |<-- consistency proof ----------|
   [4] SCT, cert and STH among multiple Auditors and Monitors





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5.  Who gossips with whom

   o  HTTPS clients and servers (SCT Feedback and STH Pollination)

   o  HTTPS servers and CT auditors (SCT Feedback)

   o  CT auditors and monitors (Trusted Auditor Relationship)

   Additionally, some HTTPS clients may engage with an auditor who they
   trust with their privacy:

   o  HTTPS clients and CT auditors (Trusted Auditor Relationship)

6.  What to gossip about and how

   There are three separate gossip streams:

   o  SCT Feedback - transporting SCTs and certificate chains from HTTPS
      clients to CT auditors/monitors via HTTPS servers.

   o  STH Pollination - HTTPS clients and CT auditors/monitors using
      HTTPS servers as STH pools for exchanging STHs.

   o  Trusted Auditor Stream, HTTPS clients communicating directly with
      trusted CT auditors/monitors sharing SCTs, certificate chains and
      STHs.

7.  Gossip Mechanisms

7.1.  SCT Feedback

   The goal of SCT Feedback is for clients to share SCTs and certificate
   chains with CT auditors and monitors while still preserving the
   privacy of the end user.  The sharing of SCTs contribute to the
   overall goal of detecting misbehaving logs by providing auditors and
   monitors with SCTs from many vantage points, making it possible to
   catch a higher number of violations of MMD and also catch logs
   presenting inconsistent views.

   SCT Feedback is the most privacy-preserving gossip mechanism, as it
   does not directly expose any links between an end user and the sites
   they've visisted to any third party.

   [Here's an alternative to that paragraph: SCT Feedback is the most
   privacy-preserving gossip mechanism, as it does not create any
   potential cross-origin tracking mechanisms.  ]





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   HTTPS clients store SCTs and certificate chains they see, and later
   send them to the originating HTTPS server by posting them to a well-
   known URL (associated with that server), as described in
   Section 7.1.1.  Note that clients will send the same SCTs and chains
   to servers multiple times with the assumption that a potential man-
   in-the-middle attack eventually will cease, and an honest server will
   receive collected malicious SCTs and certificate chains.

   HTTPS servers store SCTs and certificate chains received from clients
   and later share them with CT auditors by either posting them to
   auditors or making them available via a well-known URL.  This is
   described in Section 7.1.2.

7.1.1.  HTTPS client to server

   When an HTTPS client connects to an HTTPS server, the client receives
   a set of SCTs as part of the TLS handshake.  The client MUST discard
   SCTs that are not signed by a log known to the client and SHOULD
   store the remaining SCTs together with the corresponding certificate
   chain for later use in SCT Feedback.

   When the client later reconnects to any HTTPS server for the same
   domain, it again receives a set of SCTs.  The client MUST add new
   SCTs from known logs to its store of SCTs for the server.  The client
   MUST send to the server any SCTs in the store that are associated
   with that server but which were not received from that server.

   [TODO: fix the above paragraph - it is vague and confusing. maybe an
   example including a client caching at most one SCT per host+log would
   clarify]

   [TODO: define "same domain"]

   Note that the SCT store also contains SCTs received in certificates.

   The client MUST NOT send the same set of SCTs to the same server more
   often than TBD.

   [benl says: "sent to the server" only really counts if the server
   presented a valid SCT in the handshake and the certificate is known
   to be unrevoked (which will be hard for a MitM to sustain)]

   [TODO: expand on rate/resource limiting motivation]

   Refer to Section 10.1 for recommendations about strategies.

   An SCT MUST NOT be sent to any other HTTPS server than one serving
   the domain to which the certificate signed by the SCT refers.  Not



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   following this constraint would lead to two types of privacy leaks.
   First, the server receiving the SCT would learn about other sites
   visited by the HTTPS client.  Secondly, auditors or monitors
   receiving SCTs from the HTTPS server would learn information about
   the other HTTPS servers visited by its clients.

   If the HTTPS client has configuration options for not sending cookies
   to third parties, SCTs of third parties MUST be treated as cookies
   with respect to this setting.  This prevents third party tracking
   through the use of SCTs/certificates, which would bypass the cookie
   policy.

   SCTs and corresponding certificates are POSTed to the originating
   HTTPS server at the well-known URL:

   https://<domain>/.well-known/ct/v1/sct-feedback

   The data sent in the POST is defined in Section 7.1.3.

   HTTPS servers perform a number of sanity checks on SCTs from clients
   before storing them:

   1.  if a bit-wise compare of an SCT plus chain matches a pair already
       in the store, this SCT and chain pair MAY be discarded

   2.  if the SCT can't be verified to be a valid SCT for the
       accompanying leaf cert, issued by a known log, the SCT SHOULD be
       discarded

   3.  if the leaf cert is not for a domain for which the server is
       authoritative, the SCT MUST be discarded

   Check number 1 is for detecting duplicates and minimizing processing
   and storage by the server.  It's important to note that the check
   should be on pairs of SCT and chain in order to catch different
   chains accompanied by the same SCT.  This mis-matched chain
   information may be useful as a diagnostic tool for HTTPS server
   operators.

   Check number 2 is to prevent DoS attacks where an adversary can fill
   up the store prior to attacking a client, or a denial of service
   attack on the server's storage space.

   Check number 3 is to help malfunctioning clients from leaking which
   sites they visit and additionally to prevent DoS attacks.

   Note that an HTTPS server MAY choose to store a submitted SCT and the
   accompanying certificate chain even when the SCT can't be verified



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   according to check number 2.  One such case would be when a
   certificate chain validation is performed and the chain ends in a
   trust anchor configured on the server.  In this instance, the server
   could also be configured to not bother with known-to-be-good (i.e.
   administratively-vetted) leaf certificates, and only store unknown
   leaf certificates that chain to a known trust anchor.  The risk of
   spamming and denial of service can be mitigated by configuring the
   server with all known acceptable certificates (or certificate hashes)
   applicable to this server.  This information may enable a HTTPS
   server operator to detect attacks or unusual behavior of Certificate
   Authorities even outside the Certificate Transparency ecosystem.

7.1.2.  HTTPS server to auditors

   HTTPS servers receiving SCTs from clients SHOULD share SCTs and
   certificate chains with CT auditors by either serving them on the
   well-known URL:

   https://<domain>/.well-known/ct/v1/collected-sct-feedback

   or by HTTPS POSTing them to a set of preconfigured auditors.  This
   allows an HTTPS server to choose between an active push model or a
   passive pull model.

   The data received in a GET of the well-known URL or sent in the POST
   is defined in Section 7.1.3.

   HTTPS servers SHOULD share all SCTs and accompanying certificate
   chains they see that pass the checks in Section 7.1.1.  If this is an
   infeasible amount of data, the server may choose to expire
   submissions according to an undefined policy.  Suggestions for such a
   policy can be found in Section 10.1.

   HTTPS servers MUST NOT share any other data that they may learn from
   the submission of SCT Feedback by HTTPS clients, like the HTTPS
   client IP address or the time of submission.

   Auditors SHOULD provide the following URL accepting HTTPS POSTing of
   SCT feedback data:

   https://<auditor>/ct/v1/sct-feedback

   Auditors SHOULD regularly poll HTTPS servers at the well-known
   collected-sct-feedback URL.  The frequency of the polling and how to
   determine which domains to poll is outside the scope of this
   document.  However, the selection MUST NOT be influenced by potential
   HTTPS clients connecting directly to the auditor.  For example, if a
   poll to example.com occurs directly after a client submits an SCT for



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   example.com, an adversary observing the auditor can trivially
   conclude the activity of the client.

7.1.3.  SCT Feedback data format

   The data shared between HTTPS clients and servers, as well as between
   HTTPS servers and CT auditors/monitors, is a JSON object [RFC7159]
   with the following content:

   o  sct_feedback: An array of objects consisting of

      *  x509_chain: An array of base64-encoded X.509 certificates.  The
         first element is the end-entity certificate, the second chains
         to the first and so on.

      *  sct_data: An array of objects consisting of the base64
         representation of the binary SCT data as defined in
         [RFC-6962-BIS] Section 3.3.

   The 'x509_chain' element MUST contain at least the leaf certificate
   and SHOULD contain the full chain to a root accepted by all of the
   logs in the set of logs issuing all the SCTs in the 'sct_data'
   element.

   Some clients have trust anchors that are locally added (e.g. by an
   administrator or by the user themselves).  A local trust anchors is
   potentially privacy-sensitive since it may carry information about
   the specific computer or user.  If a certificate is covered by SCTs
   issued by publicly trusted logs, but it chains to a privacy-sensitive
   local trust anchor, the client SHOULD submit it as an "x509\_chain"
   consisting only of the leaf certificate.

   [TBD: Be strict about what sct_data may contain or is this
   sufficiently implied by previous sections?]

   [TBD: There was discussion about including a few field for
   client->server reporting, which is the exact set and order of
   certificates sent by the HTTPS server to the client.  This is
   additional diagnostic information that a HTTPS server could use to
   check it's deployment... but is pretty much useless to CT or gossip.
   Right now we're not including this, but we're polling server
   operators to see if they would welcome this data.]

7.2.  STH pollination

   The goal of sharing Signed Tree Heads (STHs) through pollination is
   to share STHs between HTTPS clients, CT auditors, and monitors in
   while still preserving the privacy of the end user.  The sharing of



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   STHs contribute to the overall goal of detecting misbehaving logs by
   providing CT auditors and monitors with SCTs from many vantage
   points, making it possible to detect logs that are presenting
   inconsistent views.

   HTTPS servers supporting the protocol act as STH pools.  HTTPS
   clients and CT auditors and monitors in the possession of STHs should
   pollinate STH pools by sending STHs to them, and retrieving new STHs
   to send to other STH pools.  CT auditors and monitors should perform
   their auditing and monitoring duties by retrieving STHs from pools.

   STH Pollination is carried out by sending STHs to HTTPS servers
   supporting the protocol, and retrieving new STHs.  In the case of
   HTTPS clients, STHs SHOULD be sent in an already established TLS
   session.  This makes it hard for an attacker to disrupt STH gossiping
   without also disturbing ordinary secure browsing (https://).  This is
   discussed more in Section 10.2.1.

   HTPS clients send STHs to HTTPS servers by POSTing them to the well-
   known URL:

   https://<domain>/.well-known/ct/v1/sth-pollination

   The data sent in the POST is defined in Section 7.2.4.

   The response contains zero or more STHs in the same format, described
   in Section 7.2.4.

   An HTTPS client may acquire STHs by several methods:

   o  in replies to pollination POSTs;

   o  asking logs that it recognises for the current STH, either
      directly (v2/get-sth) or indirectly (for example over DNS)

   o  resolving an SCT and certificate to an STH via an inclusion proof

   o  resolving one STH to another via a consistency proof

   HTTPS clients (who have STHs), CT auditors, and monitors SHOULD
   pollinate STH pools with STHs.  Which STHs to send and how often
   pollination should happen is regarded as undefined policy with the
   exception of privacy concerns explained in the next section.
   Suggestions for the policy may be found in Section 10.1.

   An HTTPS client could be tracked by giving it a unique or rare STH.
   To address this concern, we place restrictions on different
   components of the system to ensure an STH will not be rare.



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   o  HTTPS clients sliently ignore STHs from logs with an STH issuance
      frequency of more than one STH per hour.  Logs use the STH
      Frequency Count metadata to express this ([RFC-6962-BIS] sections
      3.6 and 5.1).

   o  HTTPS clients silently ignore STHs which are not fresh.

   An STH is considered fresh iff its timestamp is less than 14 days in
   the past.  Given a maximum STH issuance rate of one per hour, an
   attacker has 336 unique STHs per log for tracking.  Clients MUST
   ignore STHs older than 14 days.  We consider STHs within this
   validity window to be personally identifiable data, and STHs outside
   this window not personally identifiable.

   A log may cease operation, in which case there will soon be no STH
   within the validity window.  Clients SHOULD perform all three methods
   of gossip about a log that has ceased operation - it is possible the
   log was still compromised and gossip can detect that.  STH
   Pollination is the one mechanism where a client must know about a log
   shutdown.  A client who does not know about a log shutdown MUST NOT
   attempt any heuristic to detect a shutdown.  Instead the client MUST
   be informed about the shutdown from a verifiable source (e.g. a
   software update).  The client SHOULD be provided the final STH issued
   by the log and SHOULD resolve SCTs and STHs to this final STH.  If an
   SCT or STH cannot be resolved to the final STH... XXX?

   When multiplied by the number of logs from which a client accepts
   STHs, this number of unique STHs grow and the negative privacy
   implications grow with it.  It's important that this is taken into
   account when logs are chosen for default settings in HTTPS clients.
   This concern is discussed upon in Section 9.2.5.

7.2.1.  HTTPS Clients and Proof Fetching

   There are two types of proofs a client may retrieve.

   An HTTPS client will retrieve SCTs from an HTTPS server, and must
   obtain an inclusion proof to an STH in order to verify the promise
   made by the SCT.

   An HTTPS client may receive SCT bundled with an inclusion proof to a
   historical STH via an unspecified future mechanism.  Because this
   historical STH is considered personally identifiable information per
   above, the client must obtain a consistency proof to a more recent
   STH.

   If a client requested either proof directly from a log or auditor, it
   would reveal the client's browsing habits to a third party.  To



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   mitigate this risk, an HTTPS client MUST retrieve the proof in a
   manner that disguises the client.

   Depending on the client's DNS provider, DNS may provide an
   appropriate intermediate layer that obfuscates the linkability
   between the user of the client and the request for inclusion (while
   at the same time providing a caching layer for oft-requested
   inclusion proofs.)

   [TODO: Add a reference to Google's DNS mechanism more proper than
   http://www.certificate-transparency.org/august-2015-newsletter]

   Anonymity networks such as Tor also present a mechanism for a client
   to anonymously retrieve a proof from an auditor or log.

7.2.2.  STH Pollination without Proof Fetching

   An HTTPS client MAY participate in STH Pollination without fetching
   proofs.  In this situation, the client receives STHs from a server,
   applies the same validation logic to them (signed by a known log,
   within a validity window) and will later pass them to a HTTPS server.

   When operating in this fashion, the HTTPS client is promoting gossip
   for Certificate Transparency, but derives no direct benefit itself.
   In comparison, a client who resolves SCTs or historical STHs to
   recent STHs and pollinates them is assured that if it was attacked,
   there is a probability that the ecosystem will detect and respond to
   the attack (by distrusting the log).

7.2.3.  Auditor and Monitor Action

   Auditors and Monitors participate in STH pollination by retrieving
   STHs from HTTPS servers.  They verify that the STH is valid by
   checking the signature, and requesting a consistency proof from the
   STH to the most recent STH.

   After retrieving the consistency proof to the most recent STH, they
   SHOULD pollinate this new STH among participating HTTPS Servers.  In
   this way, as STHs "age out" and are no longer fresh, their "lineage"
   continues to be tracked in the system.

7.2.4.  STH Pollination data format

   The data sent from HTTPS clients and CT monitors and auditors to
   HTTPS servers is a JSON object [RFC7159] with the following content:

   o  sths - an array of 0 or more fresh SignedTreeHead's as defined in
      [RFC-6962-BIS] Section 3.6.1.



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   [XXX An STH is considered fresh iff TBD.]

7.3.  Trusted Auditor Stream

   HTTPS clients MAY send SCTs and cert chains, as well as STHs,
   directly to auditors.  Note that there are privacy implications in
   doing so, these are outlined in Section 9.2.1 and Section 9.2.6.

   The most natural trusted auditor arrangement arguably is a web
   browser that is "logged in to" a provider of various internet
   services.  Another equivalent arrangement is a trusted party like a
   corporation to which an employee is connected through a VPN or by
   other similar means.  A third might be individuals or smaller groups
   of people running their own services.  In such a setting, retrieving
   proofs from that third party could be considered reasonable from a
   privacy perspective.  The HTTPS client does its own auditing and
   might additionally share SCTs and STHs with the trusted party to
   contribute to herd immunity.  Here, the ordinary [RFC-6962-BIS]
   protocol is sufficient for the client to do the auditing while SCT
   Feedback and STH Pollination can be used in whole or in parts for the
   gossip part.

   Another well established trusted party arrangement on the internet
   today is the relation between internet users and their providers of
   DNS resolver services.  DNS resolvers are typically provided by the
   internet service provider (ISP) used, which by the nature of name
   resolving already know a great deal about which sites their users
   visit.  As mentioned in Section XXX, in order for HTTPS clients to be
   able to retrieve proofs in a privacy preserving manner, logs could
   expose a DNS interface in addition to the ordinary HTTPS interface.
   An informal writeup of such a protocol can be found at XXX.

7.3.1.  Trusted Auditor data format

   [TBD specify something here or leave this for others?]

8.  3-Method Ecosystem

   The use of three distinct methods for monitoring logs may seem
   excessive, but each represents a needed component in the CT
   ecosystem.  To understand why, the drawbacks of each component must
   be outlined.  In this discussion we assume that an attacker knows
   which mechanisms an HTTPS client and HTTPS server implement.








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8.1.  SCT Feedback

   SCT Feedback requires the cooperation of HTTPS clients and more
   importantly HTTPS servers.  Although SCT Feedback does require a
   significant amount of server-side logic to respond to the
   corresponding APIs, this functionality does not require
   customization, so it may be pre-provides and work out of the box.
   However, to take full advantage of the system, an HTTPS server would
   wish to perform some configuration to optimize its operation:

   o  Minimize its disk commitment by whitelisting known SCTs and
      certificate chains

   o  Maximize its chance of detecting a misissued certificate by
      configuring a trust store of CAs

   o  Establish a "push" mechanism for POSTing SCTs to Auditors and
      Monitors

   These configuration needs, and the simple fact that it would require
   some deployment of software, means that some percentage of HTTPS
   servers will not deploy SCT Feedback.

   If SCT Feedback was the only mechanism in the ecosystem, any server
   that did not implement the feature, would open itself and its users
   to attack without any possibility of detection.

   If SCT Feedback was not deployed, users who wished to have the
   strongest measure of privacy protection (by disabling STH Pollination
   Proof Fetching and forgoing a Trusted Auditor) could be attacked
   without risk of detection.

8.2.  STH Pollination

   STH Pollination requires the cooperation of HTTPS clients, HTTPS
   servers, and logs.

   For a client to fully participate in STH Pollination, and have this
   mechanism detect attacks against it, the client must have a way to
   safely perform Proof Fetching in a privacy preserving manner.  The
   client may pollinate STHs it receives without performing Proof
   Fetching, but we do not consider this option in this section.

   HTTPS Servers must deploy software (although, as in the case with SCT
   Feedback this logic can be pre-provided) and commit some configurable
   amount of disk space to the endeavor.





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   Logs must provide access to clients to query proofs in a privacy
   preserving manner, most likely through DNS.

   Unlike SCT Feedback, the STH Pollination mechanism is not hampered if
   only a minority of HTTPS servers deploy it.  However, it makes an
   assumption that an HTTPS client performs anonymized Proof Fetching
   (such as the DNS mechanism discussed).  However, any manner that is
   anonymous for some (such as clients who use shared DNS services such
   as a large ISP), may not be anonymous for others.

   For instance, DNS leaks a considerable amount of information
   (including what data is already present in the cache) in plaintext
   over the network.  For this reason, some percentage of HTTPS clients
   may choose to not enable the Proof Fetching component of STH
   pollination.  (Although they can still request and send STHs among
   participating HTTPS servers, as mentioned earlier this affords them
   no direct benefit.)

   If STH Pollination was the only mechanism deployed, users that
   disable it would be able to be attacked without risk of detection.

   If STH Pollination was not deployed, HTTPS Clients visiting HTTPS
   Servers who did not deploy SCT Feedback could be attacked without
   risk of detection.

8.3.  Trusted Auditor Relationship

   The Trusted Auditor Relationship is expected to be the rarest gossip
   mechanism, as an HTTPS Client is providing an unadulterated report of
   its browsing history to a third party.  While there are valid and
   common reasons for doing so, there is no appropriate way to enter
   into this relationship without retrieving informed consent from the
   user.

   However, the Trusted Auditor Relationship mechanism still provides
   value to a class of HTTPS Clients.  For example, web crawlers have no
   concept of a "user" and no expectation of privacy.  Organizations
   already performing network monitoring for anomalies or attacks can
   run their own Trusted Auditor for the same purpose with marginal
   increase in privacy concerns.

   The ability to change one's Trusted Auditor is a form of Trust
   Agility that allows a user to choose who to trust, and be able to
   revise that decision later without consequence.  A Trusted Auditor
   connection can be made more confidential than DNS (through the use of
   TLS), and can even be made (somewhat) anonymous through the use of
   anonymity services such as Tor. (Note that this does ignore the de-




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   anonymization possibilities available from viewing a user's browsing
   history.)

   If the Trusted Auditor relationship was the only mechanism deployed,
   users who do not enable it (the majority) would be able to be
   attacked without risk of detection.

   If the Trusted Auditor relationship was not deployed, crawlers and
   organizations would build it themselves for their own needs.  By
   standardizing it, users who wish to opt-in (for instance those
   unwilling to participate fully in STH Pollination) can have an
   interoperable standard they can use to choose and change their
   trusted auditor.

8.4.  Interaction

   The interactions of the mechanisms is thus outlined:

   HTTPS Clients can be attacked without risk of detection if they do
   not participate in any of the three mechanisms.

   HTTPS Clients are afforded the greatest chance of detecting an attack
   when they either participate in STH Pollination with Proof Fetching
   or have a Trusted Auditor relationship.  Participating in SCT
   Feedback enables a HTTPS Client to assist in detecting the exact
   target of an attack, although they do not gain any direct benefit
   from it.

   HTTPS Servers that omit SCT Feedback may never learn about targeted
   attacks against them, even if the attack occurred and the log
   distrusted.  They do gain some herd immunity, enabling them to detect
   attacks, through their clients participating in STH Pollination or a
   Trusted Auditor Relationship.

   When HTTPS Servers omit SCT feedback, it allow a portion of their
   users to be attacked without detection; the vulnerable users are
   those who do not participate in STH Pollination with Proof Fetching
   and that not have a Trusted Auditor relationship.

9.  Security considerations

9.1.  Censorship/Blocking considerations

   We assume a network attacker who is able to fully control the
   client's internet connection for some period of time - including
   selectively blocking requests to certain hosts and truncating TLS
   connections based on information observed or guessed about client




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   behavior.  In order to successfully detect log misbehavior, the
   gossip mechanisms must still work even in these conditions.

   There are several gossip connections that can be blocked:

   1.  Clients sending SCTs to servers in SCT Feedback

   2.  Servers sending SCTs to auditors in SCT Feedback (server push
       mechanism)

   3.  Servers making SCTs available to auditors (auditor pull
       mechanism)

   4.  Clients fetching proofs in STH Pollination

   5.  Clients sending STHs to servers in STH Pollination

   6.  Servers sending STHs to clients in STH Pollination

   7.  Clients sending SCTs to Trusted Auditors

   If a party cannot connect to another party, it can be assured that
   the connection did not succeed.  While it may not have been
   maliciously blocked, it knows the transaction did not succeed.
   Mechanisms which result in a positive affirmation from the recipient
   that the transaction succeeded allow confirmation that a connection
   was not blocked.  In this situation, the party can factor this into
   strategies suggested in Section 10.1 and in Section 10.2.2.

   The connections that allow positive affirmation are 1, 2, 4, 5, and
   7.

   More insidious is blocking the connections that do not allow positive
   confirmation: 3 and 6.  An attacker may truncate a or drop a response
   from a server to a client, such that the server believes it has
   shared data with the recipient, when it has not.  However, in both
   scenatios (3 and 6), the server cannot distinguish the client as a
   cooperating member of the CT ecosystem or as an attacker performing a
   sybil attack, aiming to flush the server's data store.  Therefore the
   fact that these connections can be undetectably blocked does not
   actually alter the threat model of servers responding to these
   requests.  The choice of algorithm to release data is crucial to
   protect against these attacks, strategies are suggested in
   Section 10.1.

   Handling censorship and network blocking (which is indistinguishable
   from network error) is relegated to the implementation policy chosen




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   by clients.  Suggestions for client behavior are specified in
   Section 10.2.

9.2.  Privacy considerations

   CT Gossip deals with HTTPS Clients which are trying to share
   indicators that correspond to their browsing history.  The most
   sensitive relationships in the CT ecosystem are the relationships
   between HTTPS clients and HTTPS servers.  Client-server relationships
   can be aggregated into a network graph with potentially serious
   implications for correlative de-anonymisation of clients and
   relationship-mapping or clustering of servers or of clients.

   There are, however, certain clients that do not require privacy
   protection.  Examples of these clients are web crawlers or robots but
   even in this case, the method by which these clients crawl the web
   may in fact be considered sensitive information.  In general, it is
   better to err on the side of safety, and not assume a client is okay
   with giving up its privacy.

9.2.1.  Privacy and SCTs

   An SCT contains information that links it to a particular web site.
   Because the client-server relationship is sensitive, gossip between
   clients and servers about unrelated SCTs is risky.  Therefore, a
   client with an SCT for a given server should transmit that
   information in only two channels: to a server associated with the SCT
   itself; and to a trusted CT auditor, if one exists.

9.2.2.  Privacy in SCT Feedback

   SCTs introduce yet another mechanism for HTTPS servers to store state
   on an HTTPS client, and potentially track users.  HTTPS clients which
   allow users to clear history or cookies associated with an origin
   MUST clear stored SCTs associated with the origin as well.

   Auditors should treat all SCTs as sensitive data.  SCTs received
   directly from an HTTPS client are especially sensitive, because the
   auditor is a trusted by the client to not reveal their associations
   with servers.  Auditors MUST NOT share such SCTs in any way,
   including sending them to an external log, without first mixing them
   with multiple other SCTs learned through submissions from multiple
   other clients.  Suggestions for mixing SCTs are presented in
   Section 10.1.

   There is a possible fingerprinting attack where a log issues a unique
   SCT for targeted log client(s).  A colluding log and HTTPS server
   operator could therefore be a threat to the privacy of an HTTPS



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   client.  Given all the other opportunities for HTTPS servers to
   fingerprint clients - TLS session tickets, HPKP and HSTS headers,
   HTTP Cookies, etc. - this is acceptable.

   The fingerprinting attack described above would be mitigated by a
   requirement that logs MUST use a deterministic signature scheme when
   signing SCTs ([RFC-6962-BIS] Section 2.1.4).  A log signing using RSA
   is not required to use a deterministic signature scheme.

   Since logs are allowed to issue a new SCT for a certificate already
   present in the log, mandating deterministic signatures does not stop
   this fingerprinting attack altogether.  It does make the attack
   harder to pull off without being detected though.

   There is another similar fingerprinting attack where an HTTPS server
   tracks a client by using a variation of cert chains.  The risk for
   this attack is accepted on the same grounds as the unique SCT attack
   described above.  [XXX any mitigations possible here?]

9.2.3.  Privacy for HTTPS clients performing STH Proof Fetching

   An HTTPS client performing Proof Fetching should only request proofs
   from a CT log that it accepts SCTs from.  An HTTPS client should
   regularly [TBD how regularly?  This has operational implications for
   log operators] request an STH from all logs it is willing to accept,
   even if it has seen no SCTs from that log.

   The actual mechanism by which Proof Fetching is done carries
   considerable privacy concerns.  Although out of scope for the
   document, DNS is a mechanism currently discussed.  DNS leaks data in
   plaintext over the network (including what sites the user is visiting
   and what sites they have previously visited) - thus it may not be
   suitable for some.

9.2.4.  Privacy in STH Pollination

   An STH linked to an HTTPS client may indicate the following about
   that client:

   o  that the client gossips;

   o  that the client has been using CT at least until the time that the
      timestamp and the tree size indicate;

   o  that the client is talking, possibly indirectly, to the log
      indicated by the tree hash;

   o  which software and software version is being used.



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   There is a possible fingerprinting attack where a log issues a unique
   STH for a targeted HTTPS client.  This is similar to the
   fingerprinting attack described in Section 9.2.2, but can operate
   cross-origin.  If a log (or HTTPS Server cooperating with a log)
   provides a unique STH to a client, the targeted client will be the
   only client pollinating that STH cross-origin.

   It is mitigated partially because the log is limited in the number of
   STHs it can issue.  It must 'save' one of its STHs each MMD to
   perform the attack.

9.2.5.  Privacy in STH Interaction

   An HTTPS client may pollinate any STH within the last 14 days.  An
   HTTPS Client may also pollinate an STH for any log that it knows
   about.  When a client pollinates STHs to a server, it will release
   more than one STH at a time.  It is unclear if a server may 'prime' a
   client and be able to reliably detect the client at a later time.

   It's clear that a single site can track a user any way they wish, but
   this attack works cross-origin and is therefore more concerning.  Two
   independent sites A and B want to collaborate to track a user cross-
   origin.  A feeds a client Carol some N specific STHs from the M logs
   Carol trusts, chosen to be older and less common, but still in the
   validity window.  Carol visits B and chooses to release some of the
   STHs she has stored, according to some policy.

   Modeling a representation for how common older STHs are in the pools
   of clients, and examining that with a given policy of how to choose
   which of those STHs to send to B, it should be possible to calculate
   statistics about how unique Carol looks when talking to B and how
   useful/accurate such a tracking mechanism is.

   Building such a model is likely impossible without some real world
   data, and requires a given implementation of a policy.  To combat
   this attack, suggestions are provided in Section 10.1 to attempt to
   minimize it, but follow-up testing with real world deployment to
   improvise the policy will be required.

9.2.6.  Trusted Auditors for HTTPS Clients

   Some HTTPS clients may choose to use a trusted auditor.  This trust
   relationship leaks a large amount of information from the client to
   the auditor.  In particular, it will identify the web sites that the
   client has visited to the auditor.  Some clients may already share
   this information to a third party, for example, when using a server
   to synchronize browser history across devices in a server-visible
   way, or when doing DNS lookups through a trusted DNS resolver.  For



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   clients with such a relationship already established, sending SCTs to
   a trusted auditor run by the same organization does not appear to
   leak any additional information to the trusted third party.

   Clients who wish to contact an auditor without associating their
   identities with their SCTs may wish to use an anonymizing network
   like Tor to submit SCT Feedback to the auditor.  Auditors SHOULD
   accept SCT Feedback that arrives over such anonymizing networks.

   Clients sending feedback to an auditor may prefer to reduce the
   temporal granularity of the history leakage to the auditor by caching
   and delaying their SCT Feedback reports.  This elaborated upon in XXX
   Mixing.  This strategy is only as effective as the granularity of the
   timestamps embedded in the SCTs and STHs.

9.2.7.  HTTPS Clients as Auditors

   Some HTTPS Clients may choose to act as Auditors themselves.  A
   Client taking on this role needs to consider the following:

   o  an Auditing HTTPS Client potentially leaks their history to the
      logs that they query.  Querying the log through a cache or a proxy
      with many other users may avoid this leakage, but may leak
      information to the cache or proxy, in the same way that an non-
      Auditing HTTPS Client leaks information to a trusted auditor.

   o  an effective Auditor needs a strategy about what to do in the
      event that it discovers misbehavior from a log.  Misbehavior from
      a log involves the log being unable to provide either (a) a
      consistency proof between two valid STHs or (b) an inclusion proof
      for a certificate to an STH any time after the log's MMD has
      elapsed from the issuance of the SCT.  The log's inability to
      provide either proof will not be externally cryptographically-
      verifiable, as it may be indistinguishable from a network error.

10.  Policy Recommendations

   This section is intended as suggestions to implementors of HTTPS
   Clients, HTTPS Servers, and Auditors.  It is not a requirement for
   technique of implementation, so long as privacy considerations
   established above are obeyed.

10.1.  Mixing Recommendations

   In several components of the CT Gossip ecosystem, the recommendation
   is made that data from multiple sources be ingested, mixed, provided
   to a third party, stored for an indeterminate period of time, and




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   eventually deleted.  The instances of these recommendations in this
   draft are:

   o  When a client receives SCTs during SCT Feedback, it should store
      the SCTs and Certificates for some amount of time, provide some of
      them back to the server at some point, and eventually remove them
      from its store

   o  When a client receives STHs during STH Pollination, it should
      store them for some amount of time, mix them with other STHs,
      release some of them them to various servers at some point,
      resolve some of them to new STHs, and eventually remove them from
      its store

   o  When a server receives SCTs during SCT Feedback, it should store
      them for some period of time, provide them to auditors some number
      of times, and may eventually remove them

   o  When a server receives STHs during STH Pollination, it should
      store them for some period of time, mix them with other STHs,
      provide some of them to connecting clients, may resolve them to
      new STHs via Proof Fetching, and eventually remove them from its
      store

   o  When a Trusted Auditor receives SCTs or historical STHs from
      clients, it should store them for some period of time, mix them
      with SCTs received from other clients, and act upon them at some
      period of time

   Each of these instances have specific requirements for user privacy,
   and each have options that may not be invoked.  As one example, a
   HTTPS client should not mix SCTs from server A with SCTs from server
   B and release server B's SCTs to Server A.  As another example, a
   HTTPS server may choose to resolve several STHs to a single more
   current STH via proof fetching, but it is under no obligation to do
   so.

   These requirements should be met, but the general problem of
   aggregating multiple pieces of data, choosing when and how many to
   release, and when to remove is shared.  This problem has been
   previously been considered in the case of Mix Networks and Remailers,
   including papers such as [X], [Y], and [Z].

   Certain common recommendations can be made:

   o  When choosing how many times to release data before expiring it
      from a cache, use a random number chosen from a distribution,
      rather than a fixed number.  This prevents an adversary from



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      knowing with certainty that it has successfully flushed a cache of
      a potentially incriminating piece of data.

   o  [TODO Enumerating the problems of different types of mixes vs
      Cottrell Mix]

   o  [TODO Integrating the IP address into the algorithm for releasing
      data]

   o  [TODO Prefer aggregating multiple piece of data into a single STH
      when possible]

   o  [TODO The importance of Flushing Attacks, and tying in network
      connection, and time interval]

10.2.  Blocking Recommendations

10.2.1.  Frustrating blocking

   When making gossip connections to HTTPS Servers or Trusted Auditors,
   it is desirable to minimize the plaintext metadata in the connection
   that can be used to identify the connection as a gossip connection
   and therefore be of interest to block.  Additionally, introducing
   some randomness into client behavior may be important - we assume
   that the adversary is able to inspect the behavior of the HTTPS
   client and understand how it makes gossip connections.

   As an example, if a client, after establishing a TLS connection (and
   receiving an SCT, but not making it's own HTTPS request yet),
   immediately opens a second TLS connection for the purpose of gossip -
   the adversary can reliably block this second connection to block
   gossip without affecting normal browsing.  For this reason it is
   recommended to run the gossip protocols over an existing connection
   to the server, making use of connection multiplexing such as HTTP
   Keep-Alives or SPDY.

   Truncation is also a concern -if a client always establishes a TLS
   connection, makes a request, receives a response, and then always
   attempts a gossip communication immediately following the first
   response - truncation will allow an attacker to block gossip
   reliably.

10.2.2.  Responding to possible blocking

   [Not sure here.  Maybe this section will get folded up into the
   above.  Or maybe it relates to the escape valve. -tjr]





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11.  IANA considerations

   [TBD]

12.  Contributors

   The authors would like to thank the following contributors for
   valuable suggestions: Al Cutter, Ben Laurie, Benjamin Kaduk, Karen
   Seo, Magnus Ahltorp, Steven Kent, Yan Zhu.

13.  ChangeLog

13.1.  Changes between ietf-00 and ietf-01

   o  Improve langugage and readability based on feedback from Stephen
      Kent.

   o  STH Pollination Proof Fetching defined and indicated as optional.

   o  3-Method Ecosystem section added.

   o  Cases with Logs ceasing operation handled.

   o  Text on tracking via STH Interaction added.

   o  Section with some early recommendations for mixing added.

   o  Section detailing blocking connections, frustrating it, and the
      implications added.

13.2.  Changes between -01 and -02

   o  STH Pollination defined.

   o  Trusted Auditor Relationship defined.

   o  Overview section rewritten.

   o  Data flow picture added.

   o  Section on privacy considerations expanded.

13.3.  Changes between -00 and -01

   o  Add the SCT feedback mechanism: Clients send SCTs to originating
      web server which shares them with auditors.

   o  Stop assuming that clients see STHs.



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   o  Don't use HTTP headers but instead .well-known URL's - avoid that
      battle.

   o  Stop referring to trans-gossip and trans-gossip-transport-https -
      too complicated.

   o  Remove all protocols but HTTPS in order to simplify - let's come
      back and add more later.

   o  Add more reasoning about privacy.

   o  Do specify data formats.

14.  Normative References

   [RFC-6962-BIS]
              Laurie, B., Langley, A., Kasper, E., Messeri, E., and R.
              Stradling, "Certificate Transparency", October 2015,
              <https://datatracker.ietf.org/doc/draft-ietf-trans-
              rfc6962-bis/>.

   [RFC7159]  Bray, T., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, March 2014.

Authors' Addresses

   Linus Nordberg
   NORDUnet

   Email: linus@nordu.net


   Daniel Kahn Gillmor
   ACLU

   Email: dkg@fifthhorseman.net


   Tom Ritter

   Email: tom@ritter.vg










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