draft-ietf-trans-gossip-03.txt

TRANS                                                        L. Nordberg
Internet-Draft                                                  NORDUnet
Intended status: Experimental                                 D. Gillmor
Expires: January 9, 2017                                            ACLU
                                                               T. Ritter

                                                           July 08, 2016


                            Gossiping in CT
                       draft-ietf-trans-gossip-03

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 behavior 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 January 9, 2017.

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



Nordberg, et al.         Expires January 9, 2017                [Page 1]

Internet-Draft               Gossiping in CT                   July 2016


   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  . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Pre-Loaded vs Locally Added Anchors . . . . . . . . . . .   5
   5.  Who gossips with whom . . . . . . . . . . . . . . . . . . . .   5
   6.  What to gossip about and how  . . . . . . . . . . . . . . . .   6
   7.  Data flow . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Gossip Mechanisms . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  SCT Feedback  . . . . . . . . . . . . . . . . . . . . . .   7
       8.1.1.  SCT Feedback data format  . . . . . . . . . . . . . .   8
       8.1.2.  HTTPS client to server  . . . . . . . . . . . . . . .   8
       8.1.3.  HTTPS server operation  . . . . . . . . . . . . . . .  11
       8.1.4.  HTTPS server to auditors  . . . . . . . . . . . . . .  13
     8.2.  STH pollination . . . . . . . . . . . . . . . . . . . . .  14
       8.2.1.  HTTPS Clients and Proof Fetching  . . . . . . . . . .  15
       8.2.2.  STH Pollination without Proof Fetching  . . . . . . .  17
       8.2.3.  Auditor Action  . . . . . . . . . . . . . . . . . . .  17
       8.2.4.  STH Pollination data format . . . . . . . . . . . . .  17
     8.3.  Trusted Auditor Stream  . . . . . . . . . . . . . . . . .  17
       8.3.1.  Trusted Auditor data format . . . . . . . . . . . . .  18
   9.  3-Method Ecosystem  . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  SCT Feedback  . . . . . . . . . . . . . . . . . . . . . .  19
     9.2.  STH Pollination . . . . . . . . . . . . . . . . . . . . .  20
     9.3.  Trusted Auditor Relationship  . . . . . . . . . . . . . .  21
     9.4.  Interaction . . . . . . . . . . . . . . . . . . . . . . .  22
   10. Security considerations . . . . . . . . . . . . . . . . . . .  22
     10.1.  Attacks by actively malicious logs . . . . . . . . . . .  22
     10.2.  Dual-CA Compromise . . . . . . . . . . . . . . . . . . .  23
     10.3.  Censorship/Blocking considerations . . . . . . . . . . .  24
     10.4.  Flushing Attacks . . . . . . . . . . . . . . . . . . . .  25
       10.4.1.  STHs . . . . . . . . . . . . . . . . . . . . . . . .  25
       10.4.2.  SCTs & Certificate Chains on HTTPS Servers . . . . .  26
       10.4.3.  SCTs & Certificate Chains on HTTPS Clients . . . . .  26
     10.5.  Privacy considerations . . . . . . . . . . . . . . . . .  27
       10.5.1.  Privacy and SCTs . . . . . . . . . . . . . . . . . .  27
       10.5.2.  Privacy in SCT Feedback  . . . . . . . . . . . . . .  27
       10.5.3.  Privacy for HTTPS clients performing STH Proof
                Fetching . . . . . . . . . . . . . . . . . . . . . .  28



Nordberg, et al.         Expires January 9, 2017                [Page 2]

Internet-Draft               Gossiping in CT                   July 2016


       10.5.4.  Privacy in STH Pollination . . . . . . . . . . . . .  28
       10.5.5.  Privacy in STH Interaction . . . . . . . . . . . . .  29
       10.5.6.  Trusted Auditors for HTTPS Clients . . . . . . . . .  29
       10.5.7.  HTTPS Clients as Auditors  . . . . . . . . . . . . .  30
   11. Policy Recommendations  . . . . . . . . . . . . . . . . . . .  30
     11.1.  Blocking Recommendations . . . . . . . . . . . . . . . .  31
       11.1.1.  Frustrating blocking . . . . . . . . . . . . . . . .  31
       11.1.2.  Responding to possible blocking  . . . . . . . . . .  31
     11.2.  Proof Fetching Recommendations . . . . . . . . . . . . .  32
     11.3.  Record Distribution Recommendations  . . . . . . . . . .  33
       11.3.1.  Mixing Algorithm . . . . . . . . . . . . . . . . . .  34
       11.3.2.  The Deletion Algorithm . . . . . . . . . . . . . . .  35
     11.4.  Concrete Recommendations . . . . . . . . . . . . . . . .  36
       11.4.1.  STH Pollination  . . . . . . . . . . . . . . . . . .  36
       11.4.2.  SCT Feedback . . . . . . . . . . . . . . . . . . . .  39
   12. IANA considerations . . . . . . . . . . . . . . . . . . . . .  53
   13. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  53
   14. ChangeLog . . . . . . . . . . . . . . . . . . . . . . . . . .  53
     14.1.  Changes between ietf-02 and ietf-03  . . . . . . . . . .  53
     14.2.  Changes between ietf-01 and ietf-02  . . . . . . . . . .  54
     14.3.  Changes between ietf-00 and ietf-01  . . . . . . . . . .  54
     14.4.  Changes between -01 and -02  . . . . . . . . . . . . . .  54
     14.5.  Changes between -00 and -01  . . . . . . . . . . . . . .  55
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  55
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  55
     15.2.  Informative References . . . . . . . . . . . . . . . . .  55
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  56

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
   inconsistent views to different log clients, and logs failing to
   include submitted certificates within the time period stipulated by
   MMD.

   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 expose
   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



Nordberg, et al.         Expires January 9, 2017                [Page 3]

Internet-Draft               Gossiping in CT                   July 2016


   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
   audited for consistency, i.e., they should never rewrite history.
   Additionally, auditors and other log clients need to exchange
   information about logs in order to be able to detect a partitioning
   attack (as described above).

   Gossiping about log behavior 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) is linkable
   to specific log entries and thereby to specific websites, which makes
   sharing them with others a privacy concern.  Gossiping about this
   data has to take privacy considerations into account in order not to
   expose 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 8.2).

3.  Overview

   This document presents three gossiping mechanisms: SCT Feedback, STH
   Pollination, and a Trusted Auditor Relationship.

   SCT Feedback enables HTTPS clients to share Signed Certificate
   Timestamps (SCTs) (Section 3.3 of [RFC-6962-BIS-09]) with CT auditors
   in a privacy-preserving manner by sending SCTs to originating HTTPS
   servers, who in turn share them with CT auditors.

   In STH Pollination, HTTPS clients use HTTPS servers as pools to share
   Signed Tree Heads (STHs) (Section 3.6 of [RFC-6962-BIS-09]) with
   other connecting clients in the hope that STHs will find their way to
   CT auditors.





Nordberg, et al.         Expires January 9, 2017                [Page 4]

Internet-Draft               Gossiping in CT                   July 2016


   HTTPS clients in a Trusted Auditor Relationship share SCTs and STHs
   with trusted CT auditors 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 a misbehaving log that fails to include a
   certificate within the time period stipulated by its 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 only
   allowing gossiping of STHs issued in a certain window of time, while
   also refusing to gossip about STHs from logs with too high an STH
   issuance frequency (see Section 8.2).

4.  Terminology

   This document relies on terminology and data structures defined in
   [RFC-6962-BIS-09], including MMD, STH, SCT, Version, LogID, SCT
   timestamp, CtExtensions, SCT signature, Merkle Tree Hash.

   This document relies on terminology defined in
   [draft-ietf-trans-threat-analysis-03], including Auditing.

4.1.  Pre-Loaded vs Locally Added Anchors

   Through the document, we refer to both Trust Anchors (Certificate
   Authorities) and Logs.  Both Logs and Trust Anchors may be locally
   added by an administrator.  Unless otherwise clarified, in both cases
   we refer to the set of Trust Anchors and Logs that come pre-loaded
   and pre-trusted in a piece of client software.

5.  Who gossips with whom

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

   o  HTTPS servers and CT auditors (SCT Feedback and STH Pollination)

   o  CT auditors (Trusted Auditor Relationship)





Nordberg, et al.         Expires January 9, 2017                [Page 5]

Internet-Draft               Gossiping in CT                   July 2016


   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 via HTTPS servers.

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

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

   It is worthwhile to note that when an HTTPS client or CT auditor
   interacts with a log, they may equivalently interact with a log
   mirror or cache that replicates the log.

7.  Data flow

   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.
























Nordberg, et al.         Expires January 9, 2017                [Page 6]

Internet-Draft               Gossiping in CT                   July 2016


      +- Cert ---- +----------+
      |            |    CA    | ----------+
      |   + SCT -> +----------+           |
      v   |                           Cert [& SCT]
   +----------+                           |
   |   Log    | ---------- SCT -----------+
   +----------+                           v
     |  ^                          +----------+
     |  |         SCTs & Certs --- | Website  |
     |  |[1]           |           +----------+
     |  |[2]         STHs            ^     |
     |  |[3]           v             |     |
     |  |          +----------+      |     |
     |  +--------> | Auditor  |      |  HTTPS traffic
     |             +----------+      |     |
    STH                              |  SCT & Cert
     |                        SCTs & Certs |
   Log entries                       |     |
     |                             STHs   STHs
     v                               |     |
   +----------+                      |     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 ----------|

8.  Gossip Mechanisms

8.1.  SCT Feedback

   The goal of SCT Feedback is for clients to share SCTs and certificate
   chains with CT auditors 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 with SCTs from many
   vantage points, making it more likely to catch a violation of a log's
   MMD or a log presenting inconsistent views.  The sharing of
   certificate chains is beneficial to HTTPS server operators interested
   in direct feedback from clients for detecting bogus certificates
   issued in their name and therefore incentivizes server operators to
   take part in SCT Feedback.




Nordberg, et al.         Expires January 9, 2017                [Page 7]

Internet-Draft               Gossiping in CT                   July 2016


   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 visited to any third party.

   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 8.1.2.  Note that clients will send the same SCTs and chains
   to a server multiple times with the assumption that any man-in-the-
   middle attack eventually will cease, and an honest server will
   eventually receive collected malicious SCTs and certificate chains.

   HTTPS servers store SCTs and certificate chains received from
   clients, as described in Section 8.1.3.  They 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 8.1.4.

8.1.1.  SCT Feedback data format

   The data shared between HTTPS clients and servers, as well as between
   HTTPS servers and CT auditors, is a JSON array [RFC7159].  Each item
   in the array is a JSON object with the following content:

   o  x509_chain: An array of PEM-encoded X.509 certificates.  The first
      element is the end-entity certificate, the second certifies the
      first and so on.

   o  sct_data: An array of objects consisting of the base64
      representation of the binary SCT data as defined in
      [RFC-6962-BIS-09] Section 3.3.

   We will refer to this object as 'sct_feedback'.

   The x509_chain element always contains a full chain from a leaf
   certificate to a self-signed trust anchor.

   See Section 8.1.2 for details on what the sct_data element contains
   as well as more details about the x509_chain element.

8.1.2.  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.  SCTs are included in the
   TLS handshake using one or more of the three mechanisms described in
   [RFC-6962-BIS-09] section 3.4 - in the server certificate, in a TLS
   extension, or in an OCSP extension.  The client MUST discard SCTs
   that are not signed by a log known to the client and SHOULD store the
   remaining SCTs together with a locally constructed certificate chain



Nordberg, et al.         Expires January 9, 2017                [Page 8]

Internet-Draft               Gossiping in CT                   July 2016


   which is trusted (i.e. terminated in a pre-loaded or locally
   installed Trust Anchor) in an sct_feedback object or equivalent data
   structure for later use in SCT Feedback.

   The SCTs stored on the client MUST be keyed by the exact domain name
   the client contacted.  They MUST NOT be sent to any domain not
   matching the original domain (e.g. if the original domain is
   sub.example.com they must not be sent to sub.sub.example.com or to
   example.com.)  They MUST NOT be sent to any Subject Alternate Names
   specified in the certificate.  In the case of certificates that
   validate multiple domain names, the same SCT is expected to be stored
   multiple times.

   Not following these constraints would increase the risk for two types
   of privacy breaches.  First, the HTTPS server receiving the SCT would
   learn about other sites visited by the HTTPS client.  Second,
   auditors receiving SCTs from the HTTPS server would learn information
   about other HTTPS servers visited by its clients.

   If the client later again connects to the same HTTPS server, it again
   receives a set of SCTs and calculates a certificate chain, and again
   creates an sct_feedback or similar object.  If this object does not
   exactly match an existing object in the store, then the client MUST
   add this new object to the store, associated with the exact domain
   name contacted, as described above.  An exact comparison is needed to
   ensure that attacks involving alternate chains are detected.  An
   example of such an attack is described in
   [dual-ca-compromise-attack].  However, at least one optimization is
   safe and MAY be performed: If the certificate chain exactly matches
   an existing certificate chain, the client MAY store the union of the
   SCTs from the two objects in the first (existing) object.

   If the client does connect to the same HTTPS server a subsequent
   time, it MUST send to the server sct_feedback objects in the store
   that are associated with that domain name.  However, it is not
   necessary to send an sct_feedback object constructed from the current
   TLS session, and if the client does so, it MUST NOT be marked as sent
   in any internal tracking done by the client.

   Refer to Section 11.3 for recommendations for implementation.

   Because SCTs can be used as a tracking mechanism (see
   Section 10.5.2), they deserve special treatment when they are
   received from (and provided to) domains that are loaded as
   subresources from an origin domain.  Such domains are commonly called
   'third party domains'.  An HTTPS client SHOULD store SCT Feedback
   using a 'double-keying' approach, which isolates third party domains
   by the first party domain.  This is described in [double-keying].



Nordberg, et al.         Expires January 9, 2017                [Page 9]

Internet-Draft               Gossiping in CT                   July 2016


   Gossip would be performed normally for third party domains only when
   the user revisits the first party domain.  In lieu of 'double-
   keying', an HTTPS client MAY treat SCT Feedback in the same manner it
   treats other security mechanisms that can enable tracking (such as
   HSTS and HPKP.)

   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.  For domains that are only loaded as third party domains, the
   client may never perform SCT Feedback; however the client may perform
   STH Pollination after fetching an inclusion proof, as specified in
   Section 8.2.

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

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

   The data sent in the POST is defined in Section 8.1.1.  This data
   SHOULD be sent in an already-established TLS session.  This makes it
   hard for an attacker to disrupt SCT Feedback without also disturbing
   ordinary secure browsing (https://).  This is discussed more in
   Section 11.1.1.

   The HTTPS server SHOULD respond with an HTTP 200 response code and an
   empty body if it was able to process the request.  An HTTPS client
   who receives any other response SHOULD consider it an error.

   Some clients have trust anchors or logs that are locally added (e.g.
   by an administrator or by the user themselves).  These additions are
   potentially privacy-sensitive because they can carry information
   about the specific configuration, computer, or user.

   Certificates validated by locally added trust anchors will commonly
   have no SCTs associated with them, so in this case no action is
   needed with respect to CT Gossip.  SCTs issued by locally added logs
   MUST NOT be reported via SCT Feedback.

   If a certificate is validated by SCTs that are issued by publicly
   trusted logs, but chains to a local trust anchor, the client MAY
   perform SCT Feedback for this SCT and certificate chain bundle.  If
   it does so, the client MUST include the full chain of certificates
   chaining to the local trust anchor in the x509_chain array.
   Performing SCT Feedback in this scenario may be advantageous for the
   broader internet and CT ecosystem, but may also disclose information
   about the client.  If the client elects to omit SCT Feedback, it can



Nordberg, et al.         Expires January 9, 2017               [Page 10]

Internet-Draft               Gossiping in CT                   July 2016


   choose to perform STH Pollination after fetching an inclusion proof,
   as specified in Section 8.2.

   We require the client to send the full chain (or nothing at all) for
   two reasons.  Firstly, it simplifies the operation on the server if
   there are not two code paths.  Secondly, omitting the chain does not
   actually preserve user privacy.  The Issuer field in the certificate
   describes the signing certificate.  And if the certificate is being
   submitted at all, it means the certificate is logged, and has SCTs.
   This means that the Issuer can be queried and obtained from the log,
   so omitting the signing certificate from the client's submission does
   not actually help user privacy.

8.1.3.  HTTPS server operation

   HTTPS servers can be configured (or omit configuration), resulting
   in, broadly, two modes of operation.  In the simpler mode, the server
   will only track leaf certificates and SCTs applicable to those leaf
   certificates.  In the more complex mode, the server will confirm the
   client's chain validation and store the certificate chain.  The
   latter mode requires more configuration, but is necessary to prevent
   denial of service (DoS) attacks on the server's storage space.

   In the simple mode of operation, upon receiving a submission at the
   sct-feedback well-known URL, an HTTPS server will perform a set of
   operations, checking on each sct_feedback object before storing it:

   1.  the HTTPS server MAY modify the sct_feedback object, and discard
       all items in the x509_chain array except the first item (which is
       the end-entity certificate)

   2.  if a bit-wise compare of the sct_feedback object matches one
       already in the store, this sct_feedback object SHOULD be
       discarded

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

   4.  if an SCT in the sct_data array can't be verified to be a valid
       SCT for the accompanying leaf cert, and issued by a known log,
       the individual SCT SHOULD be discarded

   The modification in step number 1 is necessary to prevent a malicious
   client from exhausting the server's storage space.  A client can
   generate their own issuing certificate authorities, and create an
   arbitrary number of chains that terminate in an end-entity
   certificate with an existing SCT.  By discarding all but the end-
   entity certificate, we prevent a simple HTTPS server from storing



Nordberg, et al.         Expires January 9, 2017               [Page 11]

Internet-Draft               Gossiping in CT                   July 2016


   this data.  Note that operation in this mode will not prevent the
   attack described in [dual-ca-compromise-attack].  Skipping this step
   requires additional configuration as described below.

   The check in step 2 is for detecting duplicates and minimizing
   processing and storage by the server.  As on the client, an exact
   comparison is needed to ensure that attacks involving alternate
   chains are detected.  Again, at least one optimization is safe and
   MAY be performed.  If the certificate chain exactly matches an
   existing certificate chain, the server MAY store the union of the
   SCTs from the two objects in the first (existing) object.  If the
   validity check on any of the SCTs fails, the server SHOULD NOT store
   the union of the SCTs.

   The check in step 3 is to help malfunctioning clients from exposing
   which sites they visit.  It additionally helps prevent DoS attacks on
   the server.

   [ Note: Thinking about building this, how does the SCT Feedback app
   know which sites it's authoritative for?  It will need that amount of
   configuration at least. ]

   The check in step 4 is to prevent DoS attacks where an adversary
   fills up the store prior to attacking a client (thus preventing the
   client's feedback from being recorded), or an attack where an
   adversary simply attempts to fill up server's storage space.

   The above describes the simpler mode of operation.  In the more
   advanced server mode, the server will detect the attack described in
   [dual-ca-compromise-attack].  In this configuration the server will
   not modify the sct_feedback object prior to performing checks 2, 3,
   and 4.

   To prevent a malicious client from filling the server's data store,
   the HTTPS server SHOULD perform an additional check in the more
   advanced mode:

   o  if the x509_chain consists of an invalid certificate chain, or the
      culminating trust anchor is not recognized by the server, the
      server SHOULD modify the sct_feedback object, discarding all items
      in the x509_chain array except the first item

   The HTTPS server MAY choose to omit checks 4 or 5.  This will place
   the server at risk of having its data store filled up by invalid
   data, but can also allow a server to identify interesting certificate
   or certificate chains that omit valid SCTs, or do not chain to a
   trusted root.  This information may enable an HTTPS server operator




Nordberg, et al.         Expires January 9, 2017               [Page 12]

Internet-Draft               Gossiping in CT                   July 2016


   to detect attacks or unusual behavior of Certificate Authorities even
   outside the Certificate Transparency ecosystem.

8.1.4.  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-gossip/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 8.1.1 with the following difference: The
   x509_chain element may contain only he end-entity certificate, as
   described below.

   HTTPS servers SHOULD share all sct_feedback objects they see that
   pass the checks in Section 8.1.3.  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 11.3.

   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.

   As described above, HTTPS servers can be configured (or omit
   configuration), resulting in two modes of operation.  In one mode,
   the x509_chain array will contain a full certificate chain.  This
   chain may terminate in a trust anchor the auditor may recognize, or
   it may not.  (One scenario where this could occur is if the client
   submitted a chain terminating in a locally added trust anchor, and
   the server kept this chain.)  In the other mode, the x509_chain array
   will consist of only a single element, which is the end-entity
   certificate.

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

   https://<auditor>/ct-gossip/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



Nordberg, et al.         Expires January 9, 2017               [Page 13]

Internet-Draft               Gossiping in CT                   July 2016


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

8.2.  STH pollination

   The goal of sharing Signed Tree Heads (STHs) through pollination is
   to share STHs between HTTPS clients and CT auditors while still
   preserving the privacy of the end user.  The sharing of STHs
   contribute to the overall goal of detecting misbehaving logs by
   providing CT auditors with STHs 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 in the possession of STHs can pollinate STH
   pools by sending STHs to them, and retrieving new STHs to send to
   other STH pools.  CT auditors can improve the value of their auditing
   by retrieving STHs from pools.

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

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

   The data sent in the POST is defined in Section 8.2.4.  This data
   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 11.1.1.

   On a successful connection to an HTTPS server implementing STH
   Pollination, the response code will be 200, and the response body is
   application/json, containing zero or more STHs in the same format, as
   described in Section 8.2.4.

   An HTTPS client may acquire STHs by several methods:

   o  in replies to pollination POSTs;

   o  asking logs that it recognizes 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




Nordberg, et al.         Expires January 9, 2017               [Page 14]

Internet-Draft               Gossiping in CT                   July 2016


   HTTPS clients (that have STHs) and CT auditors 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 below.  Suggestions for the policy can be found in
   Section 11.3.

   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.

   o  HTTPS clients silently 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-09]
      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 not to be personally identifiable data, and STHs
   outside this window to be personally identifiable.

   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 10.5.5.

   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 since 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, clients SHOULD follow
   the requirements and recommendations set forth in Section 11.1.2.

8.2.1.  HTTPS Clients and Proof Fetching

   There are two types of proofs a client may retrieve; inclusion proofs
   and consistency proofs.




Nordberg, et al.         Expires January 9, 2017               [Page 15]

Internet-Draft               Gossiping in CT                   July 2016


   An HTTPS client will retrieve SCTs together with certificate chains
   from an HTTPS server.  Using the timestamp in the SCT together with
   the end-entity certificate and the issuer key hash, it can obtain an
   inclusion proof to an STH in order to verify the promise made by the
   SCT.

   An HTTPS client will have STHs from performing STH Pollination, and
   may obtain a consistency proof to a more recent STH.

   An HTTPS client may also receive an 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 needs to obtain a consistency proof
   to a more recent STH.

   A client SHOULD perform proof fetching.  A client MUST NOT perform
   proof fetching for any SCTs or STHs issued by a locally added log.  A
   client MAY fetch an inclusion proof for an SCT (issued by a pre-
   loaded log) that validates a certificate chaining to a locally added
   trust anchor.

   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
   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).  See [draft-ct-over-dns] for an example of how
   this can be done.

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

   Even when using a privacy-preserving layer between the client and the
   log, certain observations may be made about an anonymous client or
   general user behavior depending on how proofs are fetched.  For
   example, if a client fetched all outstanding proofs at once, a log
   would know that SCTs or STHs received around the same time are more
   likely to come from a particular client.  This could potentially go
   so far as correlation of activity at different times to a single
   client.  In aggregate the data could reveal what sites are commonly
   visited together.  HTTPS clients SHOULD use a strategy of proof
   fetching that attempts to obfuscate these patterns.  A suggestion of
   such a policy can be found in Section 11.2.




Nordberg, et al.         Expires January 9, 2017               [Page 16]

Internet-Draft               Gossiping in CT                   July 2016


   Resolving either SCTs and STHs may result in errors.  These errors
   may be routine downtime or other transient errors, or they may be
   indicative of an attack.  Clients SHOULD follow the requirements and
   recommendations set forth in Section 11.1.2 when handling these
   errors in order to give the CT ecosystem the greatest chance of
   detecting and responding to a compromise.

8.2.2.  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 the validity window) and will later pass them to another 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).

8.2.3.  Auditor Action

   CT auditors 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.

8.2.4.  STH Pollination data format

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

   o  sths - an array of 0 or more fresh SignedTreeHeads as defined in
      [RFC-6962-BIS-09] Section 3.6.1.

8.3.  Trusted Auditor Stream

   HTTPS clients MAY send SCTs and cert chains, as well as STHs,
   directly to auditors.  If sent, this data MAY include data that
   reflects locally added logs or trust anchors.  Note that there are




Nordberg, et al.         Expires January 9, 2017               [Page 17]

Internet-Draft               Gossiping in CT                   July 2016


   privacy implications in doing so, these are outlined in
   Section 10.5.1 and Section 10.5.6.

   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 may also do 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-09]
   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 8.2.1, 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.
   A specification of such a protocol can be found in
   [draft-ct-over-dns].

8.3.1.  Trusted Auditor data format

   Trusted Auditors expose a REST API at the fixed URI:

   https://<auditor>/ct-gossip/v1/trusted-auditor

   Submissions are made by sending an HTTPS POST request, with the body
   of the POST in a JSON object.  Upon successful receipt the Trusted
   Auditor returns 200 OK.

   The JSON object consists of two top-level keys: 'sct_feedback' and
   'sths'.  The 'sct_feedback' value is an array of JSON objects as
   defined in Section 8.1.1.  The 'sths' value is an array of STHs as
   defined in Section 8.2.4.

   Example:







Nordberg, et al.         Expires January 9, 2017               [Page 18]

Internet-Draft               Gossiping in CT                   July 2016


   {
     'sct_feedback' :
       [
         {
           'x509_chain' :
             [
               '----BEGIN CERTIFICATE---\n
                AAA...',
               '----BEGIN CERTIFICATE---\n
                AAA...',
                ...
             ],
           'sct_data' :
             [
               'AAA...',
               'AAA...',
               ...
             ]
         }, ...
       ],
     'sths' :
       [
         'AAA...',
         'AAA...',
         ...
       ]
   }

9.  3-Method Ecosystem

   The use of three distinct methods for auditing 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.

9.1.  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-provided 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 maintaining a list of known SCTs
      and certificate chains (or hashes thereof)



Nordberg, et al.         Expires January 9, 2017               [Page 19]

Internet-Draft               Gossiping in CT                   July 2016


   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 CT auditors

   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.

   It is worthwhile to note that an attacker may be able to prevent
   detection of an attack on a webserver (in all cases) if SCT Feedback
   is not implemented.  This attack is detailed in Section 10.1).

   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 is not deployed by a webserver, malicious logs will
   be able to attack all users of the webserver (who do not have a
   Trusted Auditor relationship) with impunity.  Additionally, users who
   wish 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.

9.2.  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.

   Logs (or a third party mirroring the 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 Proof Fetching (such as the
   DNS mechanism discussed).  Unfortunately, any manner that is




Nordberg, et al.         Expires January 9, 2017               [Page 20]

Internet-Draft               Gossiping in CT                   July 2016


   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 requests expose a considerable amount of sensitive
   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, even when 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.

9.3.  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 auditing 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-
   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.





Nordberg, et al.         Expires January 9, 2017               [Page 21]

Internet-Draft               Gossiping in CT                   July 2016


   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.

9.4.  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 both SCT Feedback and STH Pollination
   with Proof Fetching or if they have a Trusted Auditor relationship.
   (Participating in SCT Feedback is required to prevent a malicious log
   from refusing to ever resolve an SCT to an STH, as put forward in
   Section 10.1).  Additionally, participating in SCT Feedback enables
   an HTTPS client to assist in detecting the exact target of an attack.

   HTTPS servers that omit SCT Feedback enable malicious logs to carry
   out attacks without risk of detection.  If these servers are targeted
   specifically, even if the attack is detected, without SCT Feedback
   they may never learn that they were specifically targeted.  HTTPS
   servers without SCT Feedback do gain some measure of herd immunity,
   but only because their clients participate in STH Pollination (with
   Proof Fetching) or have a Trusted Auditor Relationship.

   When HTTPS servers omit SCT feedback, it allows their users to be
   attacked without detection by a malicious log; the vulnerable users
   are those who do not have a Trusted Auditor relationship.

10.  Security considerations

10.1.  Attacks by actively malicious logs

   One of the most powerful attacks possible in the CT ecosystem is a
   trusted log that has actively decided to be malicious.  It can carry
   out an attack in two ways:

   In the first attack, the log can present a split view of the log for
   all time.  The only way to detect this attack is to resolve each view
   of the log to the two most recent STHs and then force the log to
   present a consistency proof.  (Which it cannot.)  This attack can be
   detected by CT auditors participating in STH Pollination, as long as




Nordberg, et al.         Expires January 9, 2017               [Page 22]

Internet-Draft               Gossiping in CT                   July 2016


   they are explicitly built to handle the situation of a log
   continuously presenting a split view.

   In the second attack, the log can sign an SCT, and refuse to ever
   include the certificate that the SCT refers to in the tree.
   (Alternately, it can include it in a branch of the tree and issue an
   STH, but then abandon that branch.)  Whenever someone requests an
   inclusion proof for that SCT (or a consistency proof from that STH),
   the log would respond with an error, and a client may simply regard
   the response as a transient error.  This attack can be detected using
   SCT Feedback, or an Auditor of Last Resort, as presented in
   Section 11.1.2.

10.2.  Dual-CA Compromise

   [dual-ca-compromise-attack] describes an attack possible by an
   adversary who compromises two Certificate Authorities and a Log. This
   attack is difficult to defend against in the CT ecosystem, and
   [dual-ca-compromise-attack] describes a few approaches to doing so.
   We note that Gossip is not intended to defend against this attack,
   but can in certain modes.

   Defending against the Dual-CA Compromise attack requires SCT
   Feedback, and explicitly requires the server to save full certificate
   chains (described in Section 8.1.3 as the 'complex' configuration.)
   After CT auditors receive the full certificate chains from servers,
   they MAY compare the chain built by clients to the chain supplied by
   the log.  If the chains differ significantly, the auditor SHOULD
   raise a concern.  A method of determining if chains differ
   significantly is by asserting that one chain is not a subset of the
   other and that the roots of the chains are different.

   [Note: Justification for this algorithm:

   Cross-Signatures could result in a different org being treated as the
   'root', but in this case, one chain would be a subset of the other.

   Intermediate swapping (e.g. different signature algorithms) could
   result in different chains, but the root would be the same.

   (Hitting both those cases at once would cause a false positive
   though, but this would likely be rare.)

   Are there other cases that could occur?  (Left for the purposes of
   reading during pre-Last Call, to be removed by Editor)]






Nordberg, et al.         Expires January 9, 2017               [Page 23]

Internet-Draft               Gossiping in CT                   July 2016


10.3.  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
   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 11.3 and in Section 11.1.2.

   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 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
   scenarios (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



Nordberg, et al.         Expires January 9, 2017               [Page 24]

Internet-Draft               Gossiping in CT                   July 2016


   protect against these attacks; strategies are suggested in
   Section 11.3.

   Handling censorship and network blocking (which is indistinguishable
   from network error) is relegated to the implementation policy chosen
   by clients.  Suggestions for client behavior are specified in
   Section 11.1.

10.4.  Flushing Attacks

   A flushing attack is an attempt by an adversary to flush a particular
   piece of data from a pool.  In the CT Gossip ecosystem, an attacker
   may have performed an attack and left evidence of a compromised log
   on a client or server.  They would be interested in flushing that
   data, i.e.  tricking the target into gossiping or pollinating the
   incriminating evidence with only attacker-controlled clients or
   servers with the hope they trick the target into deleting it.

   Flushing attacks may be defended against differently depending on the
   entity (HTTPS client or HTTPS server) and record (STHs or SCTs with
   Certificate Chains).

10.4.1.  STHs

   For both HTTPS clients and HTTPS servers, STHs within the validity
   window SHOULD NOT be deleted.  An attacker cannot flush an item from
   the cache if it is never removed so flushing attacks are completely
   mitigated.

   The required disk space for all STHs within the validity window is
   336 STHs per log that is trusted.  If 20 logs are trusted, and each
   STH takes 1 Kilobytes, this is 6.56 Megabytes.

   Note that it is important that implementors do not calculate the
   exact size of cache expected - if an attack does occur, a small
   number of additional STHs will enter into the cache.  These STHs will
   be in addition to the expected set, and will be evidence of the
   attack.

   If an HTTPS client or HTTPS server is operating in a constrained
   environment and cannot devote enough storage space to hold all STHs
   within the validity window it is recommended to use the below
   Deletion Algorithm Section 11.3.2 to make it more difficult for the
   attacker to perform a flushing attack.







Nordberg, et al.         Expires January 9, 2017               [Page 25]

Internet-Draft               Gossiping in CT                   July 2016


10.4.2.  SCTs & Certificate Chains on HTTPS Servers

   An HTTPS server will only accept SCTs and Certificate Chains for
   domains it is authoritative for.  Therefore the storage space needed
   is bound by the number of logs it accepts, multiplied by the number
   of domains it is authoritative for, multiplied by the number of
   certificates issued for those domains.

   Imagine a server authoritative for 10,000 domains, and each domain
   has 3 certificate chains, and 10 SCTs.  A certificate chain is 5
   Kilobytes in size and an SCT 1 Kilobyte.  This yields 732 Megabytes.

   This data can be large, but it is calculable.  Web properties with
   more certificates and domains are more likely to be able to handle
   the increased storage need, while small web properties will not seen
   an undue burden.  Therefore HTTPS servers SHOULD NOT delete SCTs or
   Certificate Chains.  This completely mitigates flushing attacks.

   Again, note that it is important that implementors do not calculate
   the exact size of cache expected - if an attack does occur, the new
   SCT(s) and Certificate Chain(s) will enter into the cache.  This data
   will be in addition to the expected set, and will be evidence of the
   attack.

   If an HTTPS server is operating in a constrained environment and
   cannot devote enough storage space to hold all SCTs and Certificate
   Chains it is authoritative for it is recommended to configure the SCT
   Feedback mechanism to allow only certain certificates that are known
   to be valid.  These chains and SCTs can then be discarded without
   being stored or subsequently provided to any clients or auditors.  If
   the allowlist is not sufficient, the below Deletion Algorithm
   Section 11.3.2 is recommended to make it more difficult for the
   attacker to perform a flushing attack.

10.4.3.  SCTs & Certificate Chains on HTTPS Clients

   HTTPS clients will accumulate SCTs and Certificate Chains without
   bound.  It is expected they will choose a particular cache size and
   delete entries when the cache size meets its limit.  This does not
   mitigate flushing attacks, and such an attack is documented in
   [gossip-mixing].

   The below Deletion Algorithm Section 11.3.2 is recommended to make it
   more difficult for the attacker to perform a flushing attack.







Nordberg, et al.         Expires January 9, 2017               [Page 26]

Internet-Draft               Gossiping in CT                   July 2016


10.5.  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-anonymization 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.

10.5.1.  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 NOT transmit that
   information in any other than the following two channels: to the
   server associated with the SCT itself; or to a Trusted Auditor, if
   one exists.

10.5.2.  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 and certificate chains 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 11.3.

   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
   client.  Given all the other opportunities for HTTPS servers to



Nordberg, et al.         Expires January 9, 2017               [Page 27]

Internet-Draft               Gossiping in CT                   July 2016


   fingerprint clients - TLS session tickets, HPKP and HSTS headers,
   HTTP Cookies, etc. - this is considered 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-09] 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 unique certificate or a variation of cert
   chains.  The risk for this attack is accepted on the same grounds as
   the unique SCT attack described above.

10.5.3.  Privacy for HTTPS clients performing STH Proof Fetching

   An HTTPS client performing Proof Fetching SHOULD NOT request proofs
   from a CT log that it doesn't accept SCTs from.  An HTTPS client
   SHOULD regularly request an STH from all logs it is willing to
   accept, even if it has seen no SCTs from that log.

   The time between two polls for new STH's SHOULD NOT be significantly
   shorter than the MMD of the polled log divided by its STH Frequency
   Count ([RFC-6962-BIS-09] section 5.1).

   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 exposes data
   in plaintext over the network (including what sites the user is
   visiting and what sites they have previously visited) and may not be
   suitable for some.

10.5.4.  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;



Nordberg, et al.         Expires January 9, 2017               [Page 28]

Internet-Draft               Gossiping in CT                   July 2016


   o  which software and software version is being used.

   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 10.5.2, 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.

10.5.5.  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 11.3 to attempt to
   minimize it, but follow-up testing with real world deployment to
   improve the policy will be required.

10.5.6.  Trusted Auditors for HTTPS Clients

   Some HTTPS clients may choose to use a trusted auditor.  This trust
   relationship exposes a large amount of information about 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



Nordberg, et al.         Expires January 9, 2017               [Page 29]

Internet-Draft               Gossiping in CT                   July 2016


   server to synchronize browser history across devices in a server-
   visible way, or when doing DNS lookups through a trusted DNS
   resolver.  For clients with such a relationship already established,
   sending SCTs to a trusted auditor run by the same organization does
   not appear to expose any additional information to the trusted third
   party.

   Clients who wish to contact a CT 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 exposure to the auditor by
   caching and delaying their SCT Feedback reports.  This is elaborated
   upon in Section 11.3.  This strategy is only as effective as the
   granularity of the timestamps embedded in the SCTs and STHs.

10.5.7.  HTTPS Clients as Auditors

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

   o  an Auditing HTTPS client potentially exposes its history to the
      logs that they query.  Querying the log through a cache or a proxy
      with many other users may avoid this exposure, but may expose
      information to the cache or proxy, in the same way that a non-
      Auditing HTTPS Client exposes information to a Trusted Auditor.

   o  an effective CT 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.

11.  Policy Recommendations

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







Nordberg, et al.         Expires January 9, 2017               [Page 30]

Internet-Draft               Gossiping in CT                   July 2016


11.1.  Blocking Recommendations

11.1.1.  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 its own HTTP 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-Alive 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.

   For these reasons, we recommend that, if at all possible, clients
   SHOULD send gossip data in an already established TLS session.  This
   can be done through the use of HTTP Pipelining, SPDY, or HTTP/2.

11.1.2.  Responding to possible blocking

   In some circumstances a client may have a piece of data that they
   have attempted to share (via SCT Feedback or STH Pollination), but
   have been unable to do so: with every attempt they receive an error.
   These situations are:

   1.  The client has an SCT and a certificate, and attempts to retrieve
       an inclusion proof - but receives an error on every attempt.

   2.  The client has an STH, and attempts to resolve it to a newer STH
       via a consistency proof - but receives an error on every attempt.

   3.  The client has attempted to share an SCT and constructed
       certificate via SCT Feedback - but receives an error on every
       attempt.





Nordberg, et al.         Expires January 9, 2017               [Page 31]

Internet-Draft               Gossiping in CT                   July 2016


   4.  The client has attempted to share an STH via STH Pollination -
       but receives an error on every attempt.

   5.  The client has attempted to share a specific piece of data with a
       Trusted Auditor - but receives an error on every attempt.

   In the case of 1 or 2, it is conceivable that the reason for the
   errors is that the log acted improperly, either through malicious
   actions or compromise.  A proof may not be able to be fetched because
   it does not exist (and only errors or timeouts occur).  One such
   situation may arise because of an actively malicious log, as
   presented in Section 10.1.  This data is especially important to
   share with the broader internet to detect this situation.

   If an SCT has attempted to be resolved to an STH via an inclusion
   proof multiple times, and each time has failed, this SCT might very
   well be a compromising proof of an attack.  However the client MUST
   NOT share the data with any other third party (excepting a Trusted
   Auditor should one exist).

   If an STH has attempted to be resolved to a newer STH via a
   consistency proof multiple times, and each time has failed, a client
   MAY share the STH with an "Auditor of Last Resort" even if the STH in
   question is no longer within the validity window.  This auditor may
   be pre-configured in the client, but the client SHOULD permit a user
   to disable the functionality or change whom data is sent to.  The
   Auditor of Last Resort itself represents a point of failure and
   privacy concerns, so if implemented, it SHOULD connect using public
   key pinning and not consider an item delivered until it receives a
   confirmation.

   In the cases 3, 4, and 5, we assume that the webserver(s) or trusted
   auditor in question is either experiencing an operational failure, or
   being attacked.  In both cases, a client SHOULD retain the data for
   later submission (subject to Private Browsing or other history-
   clearing actions taken by the user.)  This is elaborated upon more in
   Section 11.3.

11.2.  Proof Fetching Recommendations

   Proof fetching (both inclusion proofs and consistency proofs) SHOULD
   be performed at random time intervals.  If proof fetching occurred
   all at once, in a flurry of activity, a log would know that SCTs or
   STHs received around the same time are more likely to come from a
   particular client.  While proof fetching is required to be done in a
   manner that attempts to be anonymous from the perspective of the log,
   the correlation of activity to a single client would still reveal
   patterns of user behavior we wish to keep confidential.  These



Nordberg, et al.         Expires January 9, 2017               [Page 32]

Internet-Draft               Gossiping in CT                   July 2016


   patterns could be recognizable as a single user, or could reveal what
   sites are commonly visited together in the aggregate.

11.3.  Record Distribution Recommendations

   In several components of the CT Gossip ecosystem, the recommendation
   is made that data from multiple sources be ingested, mixed, stored
   for an indeterminate period of time, provided (multiple times) to a
   third party, and 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 Certificate Chain for some amount of time, provide
      some of them back to the server at some point, and may 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, an
   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, an
   HTTPS server may choose to resolve 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 them is shared.  This problem has




Nordberg, et al.         Expires January 9, 2017               [Page 33]

Internet-Draft               Gossiping in CT                   July 2016


   previously been considered in the case of Mix Networks and Remailers,
   including papers such as [trickle].

   There are several concerns to be addressed in this area, outlined
   below.

11.3.1.  Mixing Algorithm

   When SCTs or STHs are recorded by a participant in CT Gossip and
   later used, it is important that they are selected from the datastore
   in a non-deterministic fashion.

   This is most important for servers, as they can be queried for SCTs
   and STHs anonymously.  If the server used a predictable ordering
   algorithm, an attacker could exploit the predictability to learn
   information about a client.  One such method would be by observing
   the (encrypted) traffic to a server.  When a client of interest
   connects, the attacker makes a note.  They observe more clients
   connecting, and predicts at what point the client-of-interest's data
   will be disclosed, and ensures that they query the server at that
   point.

   Although most important for servers, random ordering is still
   strongly recommended for clients and Trusted Auditors.  The above
   attack can still occur for these entities, although the circumstances
   are less straightforward.  For clients, an attacker could observe
   their behavior, note when they receive an STH from a server, and use
   javascript to cause a network connection at the correct time to force
   a client to disclose the specific STH.  Trusted Auditors are stewards
   of sensitive client data.  If an attacker had the ability to observe
   the activities of a Trusted Auditor (perhaps by being a log, or
   another auditor), they could perform the same attack - noting the
   disclosure of data from a client to the Trusted Auditor, and then
   correlating a later disclosure from the Trusted Auditor as coming
   from that client.

   Random ordering can be ensured by several mechanisms.  A datastore
   can be shuffled, using a secure shuffling algorithm such as Fisher-
   Yates.  Alternately, a series of random indexes into the data store
   can be selected (if a collision occurs, a new index is selected.)  A
   cryptographically secure random number generator must be used in
   either case.  If shuffling is performed, the datastore must be marked
   'dirty' upon item insertion, and at least one shuffle operation
   occurs on a dirty datastore before data is retrieved from it for use.







Nordberg, et al.         Expires January 9, 2017               [Page 34]

Internet-Draft               Gossiping in CT                   July 2016


11.3.2.  The Deletion Algorithm

   No entity in CT Gossip is required to delete records at any time,
   except to respect user's wishes such as private browsing mode or
   clearing history.  However, it is likely that over time the
   accumulated storage will grow in size and need to be pruned.

   While deletion of data will occur, proof fetching can ensure that any
   misbehavior from a log will still be detected, even after the direct
   evidence from the attack is deleted.  Proof fetching ensures that if
   a log presents a split view for a client, they must maintain that
   split view in perpetuity.  An inclusion proof from an SCT to an STH
   does not erase the evidence - the new STH is evidence itself.  A
   consistency proof from that STH to a new one likewise - the new STH
   is every bit as incriminating as the first.  (Client behavior in the
   situation where an SCT or STH cannot be resolved is suggested in
   Section 11.1.2.)  Because of this property, we recommend that if a
   client is performing proof fetching, that they make every effort to
   not delete data until it has been successfully resolved to a new STH
   via a proof.

   When it is time to delete a record, it can be done in a way that
   makes it more difficult for a successful flushing attack to to be
   performed.

   1.  When the record cache has reached a certain size that is yet
       under the limit, aggressively perform proof fetching.  This
       should resolve records to a small set of STHs that can be
       retained.  Once a proof has been fetched, the record is safer to
       delete.

   2.  If proof fetching has failed, or is disabled, begin by deleting
       SCTs and Certificate Chains that have been successfully reported.
       Deletion from this set of SCTs should be done at random.  For a
       client, a submission is not counted as being reported unless it
       is sent over a connection using a different SCT, so the attacker
       is faced with a recursive problem.  (For a server, this step does
       not apply.)

   3.  Attempt to save any submissions that have failed proof fetching
       repeatedly, as these are the most likely to be indicative of an
       attack.

   4.  Finally, if the above steps have been followed and have not
       succeeded in reducing the size sufficiently, records may be
       deleted at random.





Nordberg, et al.         Expires January 9, 2017               [Page 35]

Internet-Draft               Gossiping in CT                   July 2016


   Note that if proof fetching is disabled (which is expected although
   not required for servers) - the algorithm collapses down to 'delete
   at random'.

   The decision to delete records at random is intentional.  Introducing
   non-determinism in the decision is absolutely necessary to make it
   more difficult for an adversary to know with certainty or high
   confidence that the record has been successfully flushed from a
   target.

11.4.  Concrete Recommendations

   We present the following pseudocode as a concrete outline of our
   policy recommendations.

   Both suggestions presented are applicable to both clients and
   servers.  Servers may not perform proof fetching, in which case large
   portions of the pseudocode are not applicable.  But it should work in
   either case.

11.4.1.  STH Pollination

   The STH class contains data pertaining specifically to the STH
   itself.

   class STH
   {
     uint16   proof_attempts
     uint16   proof_failure_count
     uint32   num_reports_to_thirdparty
     datetime timestamp
     byte[]   data
   }

   The broader STH store itself would contain all the STHs known by an
   entity participating in STH Pollination (either client or server).
   This simplistic view of the class does not take into account the
   complicated locking that would likely be required for a data
   structure being accessed by multiple threads.  Something to note
   about this pseudocode is that it does not remove STHs once they have
   been resolved to a newer STH.  Doing so might make older STHs within
   the validity window rarer and thus enable tracking.









Nordberg, et al.         Expires January 9, 2017               [Page 36]

Internet-Draft               Gossiping in CT                   July 2016


   class STHStore
   {
     STH[] sth_list

     //  This function is run after receiving a set of STHs from
     //  a third party in response to a pollination submission
     def insert(STH[] new_sths) {
       foreach(new in new_sths) {
         if(this.sth_list.contains(new))
           continue
         this.sth_list.insert(new)
       }
     }

     //  This function is called to delete the given STH
     //  from the data store
     def delete_now(STH s) {
       this.sth_list.remove(s)
     }

     //  When it is time to perform STH Pollination, the HTTPS client
     //  calls this function to get a selection of STHs to send as
     //  feedback
     def get_pollination_selection() {
       if(len(this.sth_list) < MAX_STH_TO_GOSSIP)
         return this.sth_list
       else {
         indexes = set()
         modulus = len(this.sth_list)
         while(len(indexes) < MAX_STH_TO_GOSSIP) {
           r = randomInt() % modulus
           // Ignore STHs that are past the validity window but not
           // yet removed.
           if(r not in indexes
              && now() - this.sth_list[i].timestamp < TWO_WEEKS)
             indexes.insert(r)
         }

         return_selection = []
         foreach(i in indexes) {
           return_selection.insert(this.sth_list[i])
         }
         return return_selection
       }
     }
   }





Nordberg, et al.         Expires January 9, 2017               [Page 37]

Internet-Draft               Gossiping in CT                   July 2016


   We also suggest a function that will be called periodically in the
   background, iterating through the STH store, performing a cleaning
   operation and queuing consistency proofs.  This function can live as
   a member functions of the STHStore class.

//Just a suggestion:
#define MIN_PROOF_FAILURES_CONSIDERED_SUSPICIOUS 3

def clean_list() {
  foreach(sth in this.sth_list) {

    if(now() - sth.timestamp > TWO_WEEKS) {
      //STH is too old, we must remove it
      if(proof_fetching_enabled
         && auditor_of_last_resort_enabled
         && sth.proof_failure_count
            > MIN_PROOF_FAILURES_CONSIDERED_SUSPICIOUS) {
        queue_for_auditor_of_last_resort(sth,
                                        auditor_of_last_resort_callback)
      } else {
        delete_now(sth)
      }
    }


    else if(proof_fetching_enabled
            && now() - sth.timestamp > LOG_MMD
            && sth.proof_attempts != UINT16_MAX
            // Only fetch a proof is we have never received a proof
            // before. (This also avoids submitting something
            // already in the queue.)
            && sth.proof_attempts == sth.proof_failure_count) {
      sth.proof_attempts++
      queue_consistency_proof(sth, consistency_proof_callback)
    }
  }
}

   These functions also exist in the STHStore class.












Nordberg, et al.         Expires January 9, 2017               [Page 38]

Internet-Draft               Gossiping in CT                   July 2016


//  This function is called after successfully pollinating STHs
//  to a third party. It is passed the STHs sent to the third
//  party, which is the output of get_gossip_selection(), as well
//  as the STHs received in the response.
def successful_thirdparty_submission_callback(STH[] submitted_sth_list,
                                              STH[] new_sths)
{
  foreach(sth in submitted_sth_list) {
    sth.num_reports_to_thirdparty++
  }

  this.insert(new_sths);
}


//  Attempt auditor of last resort submissions until it succeeds
def auditor_of_last_resort_callback(original_sth, error) {
  if(!error) {
    delete_now(original_sth)
  }
}


def consistency_proof_callback(consistency_proof, original_sth, error) {
  if(!error) {
    insert(consistency_proof.current_sth)
  } else {
    original_sth.proof_failure_count++
  }
}

11.4.2.  SCT Feedback

   The SCT class contains data pertaining specifically to an SCT itself.

   class SCT
   {
     uint16 proof_failure_count
     bool   has_been_resolved_to_sth
     bool   proof_outstanding
     byte[] data
   }

   The SCT bundle will contain the trusted certificate chain the HTTPS
   client built (chaining to a trusted root certificate.)  It also
   contains the list of associated SCTs, the exact domain it is
   applicable to, and metadata pertaining to how often it has been
   reported to the third party.



Nordberg, et al.         Expires January 9, 2017               [Page 39]

Internet-Draft               Gossiping in CT                   July 2016


   class SCTBundle
   {
     X509[] certificate_chain
     SCT[]  sct_list
     string domain
     uint32 num_reports_to_thirdparty

     def equals(sct_bundle) {
       if(sct_bundle.domain != this.domain)
         return false
       if(sct_bundle.certificate_chain != this.certificate_chain)
         return false
       if(sct_bundle.sct_list != this.sct_list)
         return false

       return true
     }
     def approx_equals(sct_bundle) {
       if(sct_bundle.domain != this.domain)
         return false
       if(sct_bundle.certificate_chain != this.certificate_chain)
         return false

       return true
     }

     def insert_scts(sct[] sct_list) {
       this.sct_list.union(sct_list)
       this.num_reports_to_thirdparty = 0
     }

     def has_been_fully_resolved_to_sths() {
       foreach(s in this.sct_list) {
         if(!s.has_been_resolved_to_sth && !s.proof_outstanding)
           return false
       }
       return true
     }

     def max_proof_failures() {
       uint max = 0
       foreach(sct in this.sct_list) {
         if(sct.proof_failure_count > max)
           max = sct.proof_failure_count
       }
       return max
     }
   }



Nordberg, et al.         Expires January 9, 2017               [Page 40]

Internet-Draft               Gossiping in CT                   July 2016


   For each domain, we store a SCTDomainEntry that holds the SCTBundles
   seen for that domain, as well as encapsulating some logic relating to
   SCT Feedback for that particular domain.  In particular, this data
   structure also contains the logic that handles domains not supporting
   SCT Feedback.  Its behavior is:

   1.  When a user visits a domain, SCT Feedback is attempted for it.
       If it fails, it will retry after a month (configurable).  If it
       succeeds, excellent.  SCT Feedback data is still collected and
       stored even if SCT Feedback failed.

   2.  After 3 month-long waits between failures, the domain will be
       marked as failing long-term.  No SCT Feedback data will be stored
       beyond meta-data, but SCT Feedback will still be attempted after
       month-long waits

   3.  If at any point in time, SCT Feedback succeeds, all failure
       counters are reset

   4.  If a domain succeeds, but then begins failing, it must fail more
       than 90% of the time (configurable) and then the process begins
       at (2).

   If a domain is visited infrequently (say, once every 7 months) then
   it will be evicted from the cache and start all over again (according
   to the suggestion values in the below pseudocode).

   [ Note: To be certain the logic is correct I give the following test
   cases which illustrate the intended behavior.  Hopefully the code
   matches!

 Succeed 1 Time        num_submissions_attempted=1    num_submissions_succeeded=1  num_feedback_loop_failures=0
 Fail 10 Times         num_submissions_attempted=11   num_submissions_succeeded=1  num_feedback_loop_failures=0
 ... wait a month ...
 Fail 1 month later    num_submissions_attempted=12   num_submissions_succeeded=1  num_feedback_loop_failures=1
 ... wait a month ...
 Succeed 1 month later num_submissions_attempted=13   num_submissions_succeeded=2  num_feedback_loop_failures=0(r) indicates (Reset)
 -> Feedback is attempted regularly.

 Succeed 1 Time        num_submissions_attempted=1    num_submissions_succeeded=1  num_feedback_loop_failures=0
 Fail 10 Times         num_submissions_attempted=11   num_submissions_succeeded=1  num_feedback_loop_failures=0
 ... wait a month ...
 Fail 1 month later    num_submissions_attempted=12   num_submissions_succeeded=1  num_feedback_loop_failures=1
 ... wait a month ...
 Fail 1 month later    num_submissions_attempted=13   num_submissions_succeeded=1  num_feedback_loop_failures=2
 ... wait a month ...
 Succeed 1 month later num_submissions_attempted=14   num_submissions_succeeded=2  num_feedback_loop_failures=0(r)
 -> Feedback is attempted regularly.



Nordberg, et al.         Expires January 9, 2017               [Page 41]

Internet-Draft               Gossiping in CT                   July 2016


 Succeed 1 Time        num_submissions_attempted=1    num_submissions_succeeded=1  num_feedback_loop_failures=0
 Fail 10 Times         num_submissions_attempted=11   num_submissions_succeeded=1  num_feedback_loop_failures=0
 ... wait a month ...
 Fail 1 month later    num_submissions_attempted=12   num_submissions_succeeded=1  num_feedback_loop_failures=1
 ... wait a month ...
 Fail 1 month later    num_submissions_attempted=13   num_submissions_succeeded=1  num_feedback_loop_failures=2
 ... wait a month ...
 Fail 1 month later    num_submissions_attempted=14   num_submissions_succeeded=2  num_feedback_loop_failures=3
 ... clear_old_data() is run every hour ...
                       num_submissions_attempted=0    num_submissions_succeeded=0  num_feedback_loop_failures=3
                       sct_feedback_failing_longterm=True
 Fail 1 month later    num_submissions_attempted=1    num_submissions_succeeded=0  num_feedback_loop_failures=4
                       sct_feedback_failing_longterm=True
 ... clear_old_data() is run every hour ...
                       num_submissions_attempted=0(r) num_submissions_succeeded=0  num_feedback_loop_failures=3
                       sct_feedback_failing_longterm=True
 Succeed 1 month later num_submissions_attempted=2   num_submissions_succeeded=1  num_feedback_loop_failures=0(r)
                       sct_feedback_failing_longterm=False
 -> Feedback is attempted regularly.

 Note above that the second run of clear_old_data() will reset num_submissions_attempted from 1 to 0.  This is
 CRITICAL. Otherwise, we would have the below bug (where after 10 months of failures, a success would not hit
 the required ratio to keep going)


 //The below represents a bug.
 Succeed 1 Time        num_submissions_attempted=1   num_submissions_succeeded=1  num_feedback_loop_failures=0
 Fail 10 Times         num_submissions_attempted=11  num_submissions_succeeded=1  num_feedback_loop_failures=0
 ... wait a month ...
 Fail 1 month later    num_submissions_attempted=12  num_submissions_succeeded=1  num_feedback_loop_failures=1
 ... wait a month ...
 Fail 1 month later    num_submissions_attempted=13  num_submissions_succeeded=1  num_feedback_loop_failures=2
 ... wait a month ...
 Fail 1 month later    num_submissions_attempted=14  num_submissions_succeeded=2  num_feedback_loop_failures=3
 ... clear_old_data() is run every hour ...
                       num_submissions_attempted=0   num_submissions_succeeded=0  num_feedback_loop_failures=3
                       sct_feedback_failing_longterm=True
 Fail 1 month later    num_submissions_attempted=1   num_submissions_succeeded=0  num_feedback_loop_failures=4
                       sct_feedback_failing_longterm=True
 Fail 9 times for 9 months
                       num_submissions_attempted=10  num_submissions_succeeded=0  num_feedback_loop_failures=13
                       sct_feedback_failing_longterm=True
 Succeed 1 month later num_submissions_attempted=11  num_submissions_succeeded=1  num_feedback_loop_failures=0(r)
                       sct_feedback_failing_longterm=False
 -> Feedback is NOT attempted regularly. \]

//Suggestions:
//  After concluding a domain doesn't support feedback, we try again



Nordberg, et al.         Expires January 9, 2017               [Page 42]

Internet-Draft               Gossiping in CT                   July 2016


//  after WAIT_BETWEEN_SCT_FEEDBACK_ATTEMPTS amount of time to see if
//  they added support
#define WAIT_BETWEEN_SCT_FEEDBACK_ATTEMPTS                     1 month

//  If we've waited MIN_SCT_FEEDBACK_ATTEMPTS_BEFORE_OMITTING_STORAGE
//  multiplied by WAIT_BETWEEN_SCT_FEEDBACK_ATTEMPTS amount of time, we
//  still attempt SCT Feedback, but no longer bother storing any data
//  until the domain supports SCT Feedback
#define MIN_SCT_FEEDBACK_ATTEMPTS_BEFORE_OMITTING_STORAGE      3

//  If this percentage of SCT Feedback attempts previously succeeded,
//  we consider the domain as supporting feedback and is just having
//  transient errors
#define MIN_RATIO_FOR_SCT_FEEDBACK_TO_BE_WORKING               .10

class SCTDomainEntry
{
  //  This is the primary key of the object, the exact domain name it
  //  is valid for
  string   domain

  //  This is the last time the domain was contacted. For client
  //  operations it is updated whenever the client makes any request
  //  (not just feedback) to the domain. For server operations, it is
  //  updated whenever any client contacts the domain. Responsibility
  //  for updating lies OUTSIDE of the class
  public datetime last_contact_for_domain

  //  This is the last time SCT Feedback was attempted for the domain.
  //  It is updated whenever feedback is attempted - responsibility for
  //  updating lies OUTSIDE of the class
  //  This is not used when this algorithm runs on servers
  public datetime last_sct_feedback_attempt

  //  This is the number of times we have waited an
  //  WAIT_BETWEEN_SCT_FEEDBACK_ATTEMPTS amount of time, and still failed
  //  e.g. 10 months of failures
  //  This is not used when this algorithm runs on servers
  private uint16   num_feedback_loop_failures

  //  This is whether or not SCT Feedback has failed enough times that we
  //  should not bother storing data for it anymore. It is a small function
  //  used for illustrative purposes
  //  This is not used when this algorithm runs on servers
  private bool     sct_feedback_failing_longterm()
    { num_feedback_loop_failures >= MIN_SCT_FEEDBACK_ATTEMPTS_BEFORE_OMITTING_STORAGE }

  //  This is the number of SCT Feedback submissions attempted.



Nordberg, et al.         Expires January 9, 2017               [Page 43]

Internet-Draft               Gossiping in CT                   July 2016


  //  Responsibility for incrementing lies OUTSIDE of the class
  //  (And watch for integer overflows)
  //  This is not used when this algorithm runs on servers
  public uint16    num_submissions_attempted

  //  This is the number of successful SCT Feedback submissions. This
  //  variable is updated by the class.
  //  This is not used when this algorithm runs on servers
  private uint16   num_submissions_succeeded

  //  This contains all the bundles of SCT data we have observed for
  //  this domain
  SCTBundle[] observed_records


  //  This function can be called to determine if we should attempt
  //  SCT Feedback for this domain.
  def should_attempt_feedback() {
    // Servers always perform feedback!
    if(operator_is_server)
      return true

    // If we have not tried in a month, try again
    if(now() - last_sct_feedback_attempt > WAIT_BETWEEN_SCT_FEEDBACK_ATTEMPTS)
      return true

    // If we have tried recently, and it seems to be working, go for it!
    if((num_submissions_succeeded / num_submissions_attempted) >
       MIN_RATIO_FOR_SCT_FEEDBACK_TO_BE_WORKING)
      return true

    // Otherwise don't try
    return false
  }

  //  For Clients, this function is called after a successful
  //  connection to an HTTPS server, with a single SCTBundle
  //  constructed from that connection's certificate chain and SCTs.
  //  For Servers, this is called after receiving SCT Feedback with
  //  all the bundles sent in the feedback.
  def insert(SCTBundle[] bundles) {
    // Do not store data for long-failing domains
    if(sct_feedback_failing_longterm()) {
      return
    }

    foreach(b in bundles) {
      if(operator_is_server) {



Nordberg, et al.         Expires January 9, 2017               [Page 44]

Internet-Draft               Gossiping in CT                   July 2016


        if(!passes_validity_checks(b))
          return
      }

      bool have_inserted = false
      foreach(e in this.observed_records) {
        if(e.equals(b))
          return
        else if(e.approx_equals(b)) {
          have_inserted = true
          e.insert_scts(b.sct_list)
        }
      }
      if(!have_inserted)
        this.observed_records.insert(b)
    }
    SCTStoreManager.update_cache_percentage()
  }

  //  When it is time to perform SCT Feedback, the HTTPS client
  //  calls this function to get a selection of SCTBundles to send
  //  as feedback
  def get_gossip_selection() {
    if(len(observed_records) > MAX_SCT_RECORDS_TO_GOSSIP) {
      indexes = set()
      modulus = len(observed_records)
      while(len(indexes) < MAX_SCT_RECORDS_TO_GOSSIP) {
        r = randomInt() % modulus
        if(r not in indexes)
          indexes.insert(r)
      }

      return_selection = []
      foreach(i in indexes) {
        return_selection.insert(this.observed_records[i])
      }

      return return_selection
    }
    else
      return this.observed_records
  }

  def passes_validity_checks(SCTBundle b) {
    //  This function performs the validity checks specified in
    //  {{feedback-srvop}}
  }
}



Nordberg, et al.         Expires January 9, 2017               [Page 45]

Internet-Draft               Gossiping in CT                   July 2016


   The SCTDomainEntry is responsible for handling the outcome of a
   submission report for that domain using its member function:

//  This function is called after providing SCT Feedback
//  to a server. It is passed the feedback sent to the other party, which
//  is the output of get_gossip_selection(), and also the SCTBundle
//  representing the connection the data was sent on.
//  (When this code runs on the server, connectionBundle is NULL)
//  If the Feedback was not sent successfully, error is True
def after_submit_to_thirdparty(error, SCTBundle[] submittedBundles,
                               SCTBundle connectionBundle)
{
  // Server operation in this instance is exceedingly simple
  if(operator_is_server) {
    if(error)
      return
    foreach(bundle in submittedBundles)
      bundle.num_reports_to_thirdparty++
    return
  }

  // Client behavior is much more complicated
  if(error) {
    if(sct_feedback_failing_longterm()) {
      num_feedback_loop_failures++
    }
    else if((num_submissions_succeeded / num_submissions_attempted)
            > MIN_RATIO_FOR_SCT_FEEDBACK_TO_BE_WORKING) {
      // Do nothing. num_submissions_succeeded will not be incremented
      // After enough of these failures, the ratio will fall beyond
      // acceptable
    } else {
      // The domain has begun its three-month grace period. We will
      // attempt submissions once a month
      num_feedback_loop_failures++
    }
    return
  }
  // We succeeded, so reset all of our failure states
  // Note, there is a race condition here if clear_old_data() is called
  // while this callback is outstanding.
  num_feedback_loop_failures     = 0
  if(num_submissions_succeeded != UINT16_MAX )
    num_submissions_succeeded++


  foreach(bundle in submittedBundles)
  {



Nordberg, et al.         Expires January 9, 2017               [Page 46]

Internet-Draft               Gossiping in CT                   July 2016


    // Compare Certificate Chains, if they do not match, it counts as a
    // submission.
    if(!connectionBundle.approx_equals(bundle))
      bundle.num_reports_to_thirdparty++
    else {
      // This check ensures that a SCT Bundle is not considered reported
      // if it is submitted over a connection with the same SCTs. This
      // satisfies the constraint in Paragraph 5 of {{feedback-clisrv}}
      // Consider three submission scenarios:
      // Submitted SCTs      Connection SCTs    Considered Submitted
      // A, B                A, B               No - no new information
      // A                   A, B               Yes - B is a new SCT
      // A, B                A                  No - no new information
      if(connectionBundle.sct_list is NOT a subset of bundle.sct_list)
        bundle.num_reports_to_thirdparty++
    }
  }
}

   Instances of the SCTDomainEntry class are stored as part of a larger
   class that manages the entire SCT Cache, storing them in a hashmap
   keyed by domain.  This class also tracks the current size of the
   cache, and will trigger cache eviction.




























Nordberg, et al.         Expires January 9, 2017               [Page 47]

Internet-Draft               Gossiping in CT                   July 2016


//Suggestions:
#define CACHE_PRESSURE_SAFE                   .50
#define CACHE_PRESSURE_IMMINENT               .70
#define CACHE_PRESSURE_ALMOST_FULL            .85
#define CACHE_PRESSURE_FULL                   .95
#define WAIT_BETWEEN_IMMINENT_CACHE_EVICTION  5 minutes

class SCTStoreManager
{
  hashmap<String, SCTDomainEntry> all_sct_entries
  uint32                         current_cache_size
  datetime                       imminent_cache_pressure_check_performed

  float current_cache_percentage() {
    return current_cache_size / MAX_CACHE_SIZE;
  }

  static def update_cache_percentage() {
    // This function calculates the current size of the cache
    // and updates current_cache_size
    /* ... perform calculations ... */
    current_cache_size = /* new calculated value */

    // Perform locking to prevent multiple of these functions being
    // called concurrently or unnecessarily
    if(current_cache_percentage() > CACHE_PRESSURE_FULL) {
        cache_is_full()
    }

    else if(current_cache_percentage() > CACHE_PRESSURE_ALMOST_FULL) {
      cache_pressure_almost_full()
    }

    else if(current_cache_percentage() > CACHE_PRESSURE_IMMINENT) {
      // Do not repeatedly perform the imminent cache pressure operation
      if(now() - imminent_cache_pressure_check_performed >
          WAIT_BETWEEN_IMMINENT_CACHE_EVICTION) {
        cache_pressure_is_imminent()
      }
    }
  }
}

   The SCTStoreManager contains a function that will be called
   periodically in the background, iterating through all SCTDomainEntry
   objects and performing maintenance tasks.  It removes data for
   domains we have not contacted in a long time.  This function is not




Nordberg, et al.         Expires January 9, 2017               [Page 48]

Internet-Draft               Gossiping in CT                   July 2016


   intended to clear data if the cache is getting full, separate
   functions are used for that.

 // Suggestions:
 #define TIME_UNTIL_OLD_SUBMITTED_SCTDATA_ERASED     3 months
 #define TIME_UNTIL_OLD_UNSUBMITTED_SCTDATA_ERASED   6 months

 def clear_old_data()
 {
   foreach(domainEntry in all_sct_stores)
   {
     // Queue proof fetches
     if(proof_fetching_enabled) {
       foreach(sctBundle in domainEntry.observed_records) {
         if(!sctBundle.has_been_fully_resolved_to_sths()) {
           foreach(s in bundle.sct_list) {
             if(!s.has_been_resolved_to_sth && !s.proof_outstanding) {
               sct.proof_outstanding = True
               queue_inclusion_proof(sct, inclusion_proof_callback)
             }
           }
         }
       }
     }

     // Do not store data for domains who are not supporting SCT
     if(!operator_is_server
        && domainEntry.sct_feedback_failing_longterm())
     {
       // Note that reseting these variables every single time is
       // necessary to avoid a bug
       all_sct_stores[domainEntry].num_submissions_attempted      = 0
       all_sct_stores[domainEntry].num_submissions_succeeded      = 0
       delete all_sct_stores[domainEntry].observed_records
       all_sct_stores[domainEntry].observed_records               = NULL
     }

     // This check removes successfully submitted data for
     // old domains we have not dealt with in a long time
     if(domainEntry.num_submissions_succeeded > 0
        && now() - domainEntry.last_contact_for_domain
           > TIME_UNTIL_OLD_SUBMITTED_SCTDATA_ERASED)
     {
       all_sct_stores.remove(domainEntry)
     }

     // This check removes unsuccessfully submitted data for
     // old domains we have not dealt with in a very long time



Nordberg, et al.         Expires January 9, 2017               [Page 49]

Internet-Draft               Gossiping in CT                   July 2016


     if(now() - domainEntry.last_contact_for_domain
        > TIME_UNTIL_OLD_UNSUBMITTED_SCTDATA_ERASED)
     {
       all_sct_stores.remove(domainEntry)
     }

 SCTStoreManager.update_cache_percentage()
 }

   Inclusion Proof Fetching is handled fairly independently

 // This function is a callback invoked after an inclusion proof
 // has been retrieved. It can exist on the SCT class or independently,
 // so long as it can modify the SCT class' members
 def inclusion_proof_callback(inclusion_proof, original_sct, error)
 {
   // Unlike the STH code, this counter must be incremented on the
   // callback as there is a race condition on using this counter in the
   // cache_* functions.
   original_sct.proof_attempts++
   original_sct.proof_outstanding = False
   if(!error) {
     original_sct.has_been_resolved_to_sth = True
     insert_to_sth_datastore(inclusion_proof.new_sth)
   } else {
     original_sct.proof_failure_count++
   }
 }

   If the cache is getting full, these three member functions of the
   SCTStoreManager class will be used.

// -----------------------------------------------------------------
// This function is called when the cache is not yet full, but is
// nearing it. It prioritizes deleting data that should be safe
// to delete (because it has been shared with the site or resolved
// to a STH)
def cache_pressure_is_imminent()
{
  bundlesToDelete = []
  foreach(domainEntry in all_sct_stores) {
    foreach(sctBundle in domainEntry.observed_records) {

      if(proof_fetching_enabled) {
        // First, queue proofs for anything not already queued.
        if(!sctBundle.has_been_fully_resolved_to_sths()) {
          foreach(sct in bundle.sct_list) {
            if(!sct.has_been_resolved_to_sth



Nordberg, et al.         Expires January 9, 2017               [Page 50]

Internet-Draft               Gossiping in CT                   July 2016


               && !sct.proof_outstanding) {
              sct.proof_outstanding = True
              queue_inclusion_proof(sct, inclusion_proof_callback)
            }
          }
        }

        // Second, consider deleting entries that have been fully
        // resolved.
        else {
          bundlesToDelete.append( Struct(domainEntry, sctBundle) )
        }
      }

      // Third, consider deleting entries that have been successfully
      // reported
      if(sctBundle.num_reports_to_thirdparty > 0) {
        bundlesToDelete.append( Struct(domainEntry, sctBundle) )
      }
    }
  }

  // Third, delete the eligible entries at random until the cache is
  // at a safe level
  uint recalculateIndex                = 0
  #define RECALCULATE_EVERY_N_OPERATIONS 50

  while(bundlesToDelete.length > 0 &&
        current_cache_percentage() > CACHE_PRESSURE_SAFE) {
    uint rndIndex = rand() % bundlesToDelete.length
    bundlesToDelete[rndIndex].domainEntry.observed_records.remove(bundlesToDelete[rndIndex].sctBundle)
    bundlesToDelete.removeAt(rndIndex)

    recalculateIndex++
    if(recalculateIndex % RECALCULATE_EVERY_N_OPERATIONS == 0) {
      update_cache_percentage()
    }
  }

  // Finally, tell the proof fetching engine to go faster
  if(proof_fetching_enabled) {
    // This function would speed up proof fetching until an
    // arbitrary time has passed. Perhaps until it has fetched
    // proofs for the number of items currently in its queue? Or
    // a percentage of them?
    proof_fetch_faster_please()
  }




Nordberg, et al.         Expires January 9, 2017               [Page 51]

Internet-Draft               Gossiping in CT                   July 2016


  update_cache_percentage();
}

// -----------------------------------------------------------------
// This function is called when the cache is almost full. It will
// evict entries at random, while attempting to save entries that
// appear to have proof fetching failures
def cache_pressure_almost_full()
{
  uint recalculateIndex                = 0
  uint savedRecords                    = 0
  #define RECALCULATE_EVERY_N_OPERATIONS 50

  while(all_sct_stores.length > savedRecords &&
        current_cache_percentage() > CACHE_PRESSURE_SAFE) {
    uint rndIndex1 = rand() % all_sct_stores.length
    uint rndIndex2 = rand() % all_sct_stores[rndIndex1].observed_records.length

    if(proof_fetching_enabled) {
      if(all_sct_stores[rndIndex1].observed_records[rndIndex2].max_proof_failures() >
         MIN_PROOF_FAILURES_CONSIDERED_SUSPICIOUS) {
        savedRecords++
        continue
      }
    }

    // If proof fetching is not enabled we need some other logic
    else {
      if(sctBundle.num_reports_to_thirdparty == 0) {
        savedRecords++
        continue
      }
    }

    all_sct_stores[rndIndex1].observed_records.removeAt(rndIndex2)
    if(all_sct_stores[rndIndex1].observed_records.length == 0) {
      all_sct_stores.removeAt(rndIndex1)
    }

    recalculateIndex++
    if(recalculateIndex % RECALCULATE_EVERY_N_OPERATIONS == 0) {
      update_cache_percentage()
    }
  }

  update_cache_percentage();
}




Nordberg, et al.         Expires January 9, 2017               [Page 52]

Internet-Draft               Gossiping in CT                   July 2016


// -----------------------------------------------------------------
// This function is called when the cache is full, and will evict
// cache entries at random
def cache_is_full()
{
  uint recalculateIndex                = 0
  #define RECALCULATE_EVERY_N_OPERATIONS 50

  while(all_sct_stores.length > 0 &&
        current_cache_percentage() > CACHE_PRESSURE_SAFE) {
    uint rndIndex1 = rand() % all_sct_stores.length
    uint rndIndex2 = rand() % all_sct_stores[rndIndex1].observed_records.length

    all_sct_stores[rndIndex1].observed_records.removeAt(rndIndex2)
    if(all_sct_stores[rndIndex1].observed_records.length == 0) {
      all_sct_stores.removeAt(rndIndex1)
    }


    recalculateIndex++
    if(recalculateIndex % RECALCULATE_EVERY_N_OPERATIONS == 0) {
      update_cache_percentage()
    }
  }

  update_cache_percentage();
}

12.  IANA considerations

   [ TBD ]

13.  Contributors

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

14.  ChangeLog

14.1.  Changes between ietf-02 and ietf-03

   o  TBD's resolved.

   o  References added.

   o  Pseduocode changed to work for both clients and servers.




Nordberg, et al.         Expires January 9, 2017               [Page 53]

Internet-Draft               Gossiping in CT                   July 2016


14.2.  Changes between ietf-01 and ietf-02

   o  Requiring full certificate chain in SCT Feedback.

   o  Clarifications on what clients store for and send in SCT Feedback
      added.

   o  SCT Feedback server operation updated to protect against DoS
      attacks on servers.

   o  Pre-Loaded vs Locally Added Anchors explained.

   o  Base for well-known URL's changed.

   o  Remove all mentions of monitors - gossip deals with auditors.

   o  New sections added: Trusted Auditor protocol, attacks by actively
      malicious log, the Dual-CA compromise attack, policy
      recommendations,

14.3.  Changes between ietf-00 and ietf-01

   o  Improve language 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.

14.4.  Changes between -01 and -02

   o  STH Pollination defined.

   o  Trusted Auditor Relationship defined.

   o  Overview section rewritten.

   o  Data flow picture added.




Nordberg, et al.         Expires January 9, 2017               [Page 54]

Internet-Draft               Gossiping in CT                   July 2016


   o  Section on privacy considerations expanded.

14.5.  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.

   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.

15.  References

15.1.  Normative References

   [RFC-6962-BIS-09]
              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.

15.2.  Informative References

   [double-keying]
              Perry, M., Clark, E., and S. Murdoch, "Cross-Origin
              Identifier Unlinkability", May 2015,
              <https://www.torproject.org/projects/torbrowser/
              design/#identifier-linkability>.

   [draft-ct-over-dns]
              Laurie, B., Phaneuf, P., and A. Eijdenberg, "Certificate
              Transparency over DNS", February 2016,
              <https://github.com/google/certificate-transparency-
              rfcs/blob/master/dns/draft-ct-over-dns.md>.



Nordberg, et al.         Expires January 9, 2017               [Page 55]

Internet-Draft               Gossiping in CT                   July 2016


   [draft-ietf-trans-threat-analysis-03]
              Kent, S., "Attack Model and Threat for Certificate
              Transparency", October 2015,
              <https://datatracker.ietf.org/doc/draft-ietf-trans-threat-
              analysis/>.

   [dual-ca-compromise-attack]
              Gillmor, D., "can CT defend against dual CA compromise?",
              n.d., <https://www.ietf.org/mail-
              archive/web/trans/current/msg01984.html>.

   [gossip-mixing]
              Ritter, T., "A Bit on Certificate Transparency Gossip",
              June 2016, <https://ritter.vg/blog-
              a_bit_on_certificate_transparency_gossip.html>.

   [trickle]  Serjantov, A., Dingledine, R., and . Paul Syverson, "From
              a Trickle to a Flood: Active Attacks on Several Mix
              Types", October 2002,
              <http://freehaven.net/doc/batching-taxonomy/taxonomy.pdf>.

Authors' Addresses

   Linus Nordberg
   NORDUnet

   Email: linus@nordu.net


   Daniel Kahn Gillmor
   ACLU

   Email: dkg@fifthhorseman.net


   Tom Ritter

   Email: tom@ritter.vg













Nordberg, et al.         Expires January 9, 2017               [Page 56]