wdiff draft-ietf-trans-gossip-01.txt draft-ietf-trans-gossip-02.txt

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

October 20, 2015

March 21, 2016

Gossiping in CT
draft-ietf-trans-gossip-01
draft-ietf-trans-gossip-02

Abstract

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

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

Status of This Memo

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provisions of BCP 78 and BCP 79.

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time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on April September 22, 2016.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Defining the problem . . . . . . . . . . . . . . . . . . . . 3 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Terminology and data flow . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Pre-Loaded vs Locally Added Anchors . . . . . . . . . . . 5
5. Who gossips with whom . . . . . . . . . . . . . . . . . . . . 6 5
6. What to gossip about and how . . . . . . . . . . . . . . . . 6
7. Data flow . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8. Gossip Mechanisms . . . . . . . . . . . . . . . . . . . . . . 6
7.1. 7
8.1. SCT Feedback . . . . . . . . . . . . . . . . . . . . . . 6
7.1.1. 7
8.1.1. SCT Feedback data format . . . . . . . . . . . . . . 8
8.1.2. HTTPS client to server . . . . . . . . . . . . . . . 7
7.1.2. 8
8.1.3. HTTPS server to auditors operation . . . . . . . . . . . . . . 9
7.1.3. SCT Feedback data format . 11
8.1.4. HTTPS server to auditors . . . . . . . . . . . . . . 10
7.2. 13
8.2. STH pollination . . . . . . . . . . . . . . . . . . . . . 10
7.2.1. 14
8.2.1. HTTPS Clients and Proof Fetching . . . . . . . . . . 12
7.2.2. 15
8.2.2. STH Pollination without Proof Fetching . . . . . . . 13
7.2.3. 17
8.2.3. Auditor and Monitor Action . . . . . . . . . . . . . 13
7.2.4. . . . . . . 17
8.2.4. STH Pollination data format . . . . . . . . . . . . . 13
7.3. 17
8.3. Trusted Auditor Stream . . . . . . . . . . . . . . . . . 14
7.3.1. 17
8.3.1. Trusted Auditor data format . . . . . . . . . . . . . 14
8. 18
9. 3-Method Ecosystem . . . . . . . . . . . . . . . . . . . . . 14
8.1. 19
9.1. SCT Feedback . . . . . . . . . . . . . . . . . . . . . . 15
8.2. 19
9.2. STH Pollination . . . . . . . . . . . . . . . . . . . . . 15
8.3. 20
9.3. Trusted Auditor Relationship . . . . . . . . . . . . . . 16
8.4. 21
9.4. Interaction . . . . . . . . . . . . . . . . . . . . . . . 17
9. 22
10. Security considerations . . . . . . . . . . . . . . . . . . . 17
9.1. 22
10.1. Attacks by actively malicious logs . . . . . . . . . . . 22
10.2. Dual-CA Compromise . . . . . . . . . . . . . . . . . . . 23
10.3. Censorship/Blocking considerations . . . . . . . . . . . 17
9.2. 23
10.4. Privacy considerations . . . . . . . . . . . . . . . . . 19
9.2.1. 25
10.4.1. Privacy and SCTs . . . . . . . . . . . . . . . . . . 19
9.2.2. 25
10.4.2. Privacy in SCT Feedback . . . . . . . . . . . . . . . 19
9.2.3. 25
10.4.3. Privacy for HTTPS clients performing STH Proof
Fetching . . . . . . . . . . . . . . . . . . . . . . 20
9.2.4. 26
10.4.4. Privacy in STH Pollination . . . . . . . . . . . . . 20
9.2.5. 26
10.4.5. Privacy in STH Interaction . . . . . . . . . . . . . 21
9.2.6. 27
10.4.6. Trusted Auditors for HTTPS Clients . . . . . . . . . 21
9.2.7. 28
10.4.7. HTTPS Clients as Auditors . . . . . . . . . . . . . . 22
10. 28

11. Policy Recommendations . . . . . . . . . . . . . . . . . . . 22
10.1. Mixing 29
11.1. Blocking Recommendations . . . . . . . . . . . . . . . . . 22
10.2. Blocking Recommendations 29
11.1.1. Frustrating blocking . . . . . . . . . . . . . . . . 24
10.2.1. Frustrating 29
11.1.2. Responding to possible blocking . . . . . . . . . . 29
11.2. Proof Fetching Recommendations . . . . . . 24
10.2.2. Responding to possible blocking . . . . . . . 31
11.3. Record Distribution Recommendations . . . 24

11. IANA considerations . . . . . . . 31
11.3.1. Mixing Algorithm . . . . . . . . . . . . . . 25
12. Contributors . . . . 32
11.3.2. Flushing Attacks . . . . . . . . . . . . . . . . . . 33
11.3.3. The Deletion Algorithm . . 25
13. ChangeLog . . . . . . . . . . . . . 34
12. IANA considerations . . . . . . . . . . . . . 25
13.1. Changes between ietf-00 and ietf-01 . . . . . . . . 45
13. Contributors . . 25
13.2. Changes between -01 and -02 . . . . . . . . . . . . . . 25
13.3. Changes between -00 and -01 . . . . . . . . 45
14. ChangeLog . . . . . . 25
14. . . . . . . . . . . . . . . . . . . . . 45
14.1. Changes between ietf-01 and ietf-02 . . . . . . . . . . 45
14.2. Changes between ietf-00 and ietf-01 . . . . . . . . . . 46
14.3. Changes between -01 and -02 . . . . . . . . . . . . . . 46
14.4. Changes between -00 and -01 . . . . . . . . . . . . . . 46
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 47
15.1. Normative References . . . . . . . . . . . . . . . . . . 47
15.2. Informative References . . 26 . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26 47

1. Introduction

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

[TODO:

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

One of the major challenges of any gossip protocol is limiting damage
to user privacy. The goal of CT gossip is to publish and distribute
information about the logs and their operations, but not to leak 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
should provide protective benefits for the system as a whole even if
their adoption is not universal.

2. Defining the problem

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

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

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

Disseminating information about a log poses a potential threat to the
privacy of end users. Some data of interest (e.g. SCTs) are is linkable
to specific log entries and thereby to specific sites, websites, which makes
sharing them with others privacy-sensitive. a privacy concern. Gossiping about this
data has to take privacy considerations into account in order not to leak
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 7.2). 8.2).

3. Overview

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

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

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

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

Sharing an STH is considered reasonably safe from a privacy
perspective as long as the same STH is shared by a large number of
other log clients. This "safety safety in numbers" numbers can be achieved by
requiring only
allowing gossiping of STHs only of issued in a certain "freshness" window of time, while
also refusing to gossip about STHs from logs with too high an STH
issuance frequency (see Section 7.2). 8.2).

4. Terminology and data flow

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

The following picture shows how certificates, SCTs

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 STHs flow
through 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 CT system piece of client software.

5. Who gossips with SCT whom

o HTTPS clients and servers (SCT Feedback and STH Pollination. It does
not show what goes in the Trusted Auditor Relationship stream.

+- Cert ---- +----------+
| Pollination)

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

o CT auditors (Trusted Auditor Relationship)

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

o HTTPS clients and CT auditors (Trusted Auditor Relationship)

6. What to gossip about and how

There are three separate gossip streams:

o SCT Feedback - transporting SCTs and certificate chains from HTTPS
clients to CT auditors 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
interact 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.

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

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

5. Who gossips with whom

o HTTPS clients and servers (SCT

8. Gossip Mechanisms

8.1. SCT Feedback and STH Pollination)

o HTTPS servers and CT auditors (SCT Feedback)

o CT auditors and monitors (Trusted Auditor Relationship)

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

o HTTPS

The goal of SCT Feedback is for clients to share SCTs and certificate
chains with CT auditors (Trusted Auditor Relationship)

6. What to gossip about and how

There are three separate gossip streams:

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

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

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

7. Gossip Mechanisms

7.1. SCT Feedback

The goal of SCT Feedback is for clients to share SCTs and certificate
chains with CT auditors and monitors while still preserving the privacy of the end
user. The sharing of SCTs contribute to the overall goal of
detecting misbehaving logs by providing auditors and
monitors with SCTs from many
vantage points, making it possible more likely to catch a higher number of violations violation of a log's
MMD and also catch logs 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 incentivises server operators to
take part in SCT Feedback.

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

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

HTTPS clients store SCTs and certificate chains they see, and later
send them to the originating HTTPS server by posting them to a well-
known URL (associated with that server), as described in
Section 7.1.1. 8.1.2. Note that clients will send the same SCTs and chains
to servers a server multiple times with the assumption that a potential man-
in-the-middle 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
and
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 7.1.2.

7.1.1. HTTPS client to server

When an 8.1.4.

8.1.1. SCT Feedback data format

The data shared between HTTPS client connects to an clients and servers, as well as between
HTTPS server, servers and CT auditors, is a JSON array [RFC7159]. Each item
in the client receives array is a set of SCTs as part of JSON object with the TLS handshake. following content:

o x509_chain: An array of base64-encoded X.509 certificates. The client MUST discard
SCTs that are not signed by a log known to
first element is the client and SHOULD
store end-entity certificate, the remaining SCTs together with second certifies
the corresponding certificate
chain for later use in first and so on.

o sct_data: An array of objects consisting of the base64
representation of the binary SCT Feedback. 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 at least one element. It also
always contains a full chain from a leaf certificate to a self-signed
trust anchor.

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

8.1.2. HTTPS client to server

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

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

[TODO: define "same domain"]

Note that log known to the SCT client and SHOULD store also contains the
remaining SCTs received together with a locally constructed certificate chain
which is trusted (i.e. terminated in certificates. 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 NOT send be keyed by the same set of SCTs to exact domain name
the same server more
often than TBD.

[benl says: "sent client contacted. They MUST NOT be sent to any domain not
matching the server" only really counts if the server
presented a valid SCT in the handshake and original domain (e.g. if the certificate original domain is known
to be unrevoked (which will
sub.example.com they must not be hard for a MitM sent to sustain)]

[TODO: expand on rate/resource limiting motivation]

Refer sub.sub.example.com or to Section 10.1 for recommendations about strategies.

An SCT
example.com.) They MUST NOT be sent to any other HTTPS server than one serving Subject Alternate Names
specified in the domain to which certificate. In the certificate signed by case of certificates that
validate multiple domain names, the same SCT refers. is expected to be stored
multiple times.

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

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

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

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

The data sent in the POST is defined in Section 7.1.3. same HTTPS servers perform server, it again
receives a number set of sanity checks on SCTs from clients
before storing them:

1. if and calculates a bit-wise compare of certificate chain, and again
creates an SCT plus chain matches a pair already sct_feedback or similar object. If this object does not
exactly match an existing object in the store, this SCT and chain pair MAY be discarded

2. if then the SCT can't be verified client MUST
add this new object to be a valid SCT for the
accompanying leaf cert, issued by a known log, the SCT SHOULD be
discarded

3. if store, associated with the leaf cert is not for a exact domain for which the server
name contacted, as described above. An exact comparison is
authoritative, the SCT MUST be discarded

Check number 1 is for detecting duplicates and minimizing processing
and storage by the server. It's important needed to note
ensure that the check
should be on pairs attacks involving alternate chains are detected. An
example of SCT and chain such an attack is described in order to catch different
chains accompanied by [TODO double-CA-compromise
attack]. However, at least one optimization is safe and MAY be
performed: If the same SCT. This mis-matched certificate chain
information may be useful as a diagnostic tool for HTTPS server
operators.

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

Check number 3 is to help malfunctioning clients SCTs from leaking which
sites they visit and additionally
the two objects in the first (existing) object.

If the client does connect to prevent DoS attacks.

Note that an the same HTTPS server MAY choose to store a submitted SCT and the
accompanying certificate chain even when the SCT can't be verified
according subsequent
time, it MUST send to check number 2. One such case would be when a
certificate chain validation is performed and the chain ends server sct_feedback objects in a
trust anchor configured on the server. In this instance, the server
could also be configured to not bother with known-to-be-good (i.e.
administratively-vetted) leaf certificates, and only store unknown
leaf certificates
that chain are associated with that domain name. It is not necessary to a known trust anchor.
send an sct_feedback object constructed from the current TLS session.

The risk of
spamming and denial client MUST NOT send the same set of service can be mitigated by configuring SCTs to the same server with all known acceptable certificates (or certificate hashes)
applicable more
often than TBD.

[ TODO: expand on rate/resource limiting motivation ]

Refer to this server. This information may enable Section 11.3 for recommendations about strategies.

Because SCTs can be used as a tracking mechanism (see
Section 10.4.2), 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
server operator to detect attacks or unusual behavior of Certificate
Authorities even outside Client SHOULD store SCT Feedback
using a 'double-keying' approach, which isolates third party domains
by the Certificate Transparency ecosystem.

7.1.2. HTTPS server to auditors first party domain. This is described in XXX. 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 servers receiving SCTs from clients SHOULD share SCTs Client MAY treat SCT Feedback in the same manner it treats
other security mechanisms that can enable tracking (such as HSTS and
certificate chains with CT auditors by either serving them on
HPKP.)

[ XXX is currently https://www.torproject.org/projects/torbrowser/
design/#identifier-linkability How should it be references? Do we
need to copy this out into another document? An appendix? ]

If the
well-known URL:

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

or by HTTPS POSTing them client has configuration options for not sending cookies
to a set third parties, SCTs of preconfigured auditors. third parties MUST be treated as cookies
with respect to this setting. This
allows an prevents third party tracking
through the use of SCTs/certificates, which would bypass the cookie
policy.

SCTs and corresponding certificates are POSTed to the originating
HTTPS server to choose between an active push model or a
passive pull model.

The data received in a GET of at the well-known URL or URL:

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

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

HTTPS servers 8.1.1. This data
SHOULD share all SCTs and accompanying certificate
chains they see that pass the checks be sent in Section 7.1.1. If this is an
infeasible amount of data, the server may choose to expire
submissions according to an undefined policy. Suggestions already established TLS session. This makes it
hard for such a
policy can be found an attacker to disrupt SCT Feedback without also disturbing
ordinary secure browsing (https://). This is discussed more in
Section 10.1.

HTTPS servers MUST NOT share any other data 11.1.1.

Some clients have trust anchors or logs that they may learn from
the submission of SCT Feedback are locally added (e.g.
by HTTPS clients, like the HTTPS
client IP address an administrator or by the time of submission.

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

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

Auditors SHOULD regularly poll HTTPS servers at the well-known
collected-sct-feedback URL. The frequency of the polling and how to
determine which domains to poll is outside user themselves). These additions are
potentially privacy-sensitive because they can carry information
about the scope of specific configuration, computer, or user.

Certificates validated by locally added trust anchors will commonly
have no SCTs associated with them, so in this
document. However, the selection case no action is
needed with respect to CT Gossip. SCTs issued by locally added logs
MUST NOT be influenced by potential
HTTPS clients connecting directly to the auditor. For example, if reported via SCT Feedback.

If a
poll certificate is validated by SCTs that are issued by publicly
trusted logs, but chains to example.com occurs directly after a local trust anchor, the client submits an MAY
perfom SCT Feedback for
example.com, an adversary observing this SCT and certificate chain bundle. If it
does so, the auditor can trivially
conclude client MUST include the activity full chain of certificates
chaining to the client.

7.1.3. local trust anchor in the x509_chain array.
Perfoming SCT Feedback data format

The data shared between HTTPS clients and servers, as well as between
HTTPS servers in this scenario may be advantageous for the
broader internet and CT auditors/monitors, is a JSON object [RFC7159]
with the following content:

o sct_feedback: An array of objects consisting of

* x509_chain: An array of base64-encoded X.509 certificates. The
first element is ecosystem, but may also disclose information
about the end-entity certificate, client. If the second chains client elects to the first and so on.

* sct_data: An array of objects consisting of the base64
representation of the binary omit SCT data Feedback, it can
still choose to perform STH Pollination after fetching an inclusion
proof, as defined specified in
[RFC-6962-BIS] Section 3.3.

The 'x509_chain' element MUST contain at least 8.2.

We require the leaf certificate
and SHOULD contain client to send the full chain to a root accepted by all of (or nothing at all) for
two reasons. Firstly, it simplifies the
logs in operation on the set of logs issuing all server if
there are not two code paths. Secondly, omitting the SCTs chain does not
actually preserve user privacy. The Issuer field in the 'sct_data'
element.

Some clients have trust anchors that are locally added (e.g. by an
administrator or by certificate
describes the user themselves). A local trust anchors signing certificate. And if the certificate is
potentially privacy-sensitive since being
submitted at all, it may carry information about means the specific computer or user. If a certificate is covered by SCTs
issued by publicly trusted logs, but it chains to a privacy-sensitive
local trust anchor, logged, and has SCTs.
This means that the client SHOULD submit it as an "x509\_chain"
consisting only of Issuer can be queried and obtained from the leaf certificate.

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

[TBD: There was discussion about including a few field for
client->server reporting, which is log
so omitting the exact set and order of
certificates sent by parent from the client's submission does not actually
help user privacy.

8.1.3. HTTPS server to the client. This is
additional diagnostic information that a operation

HTTPS servers can be configured (or omit configuration), resulting
in, broadly, two modes of operation. In the simpler mode, the server could use to
check it's deployment... but is pretty much useless
will only track leaf certificates and SCTs applicable to CT or gossip.
Right now we're not including this, but we're polling those leaf
certificates. In the more complex mode, the server
operators to see if they would welcome this data.]

7.2. STH pollination will confirm the
client's chain validation and store the certificate chain. The goal of sharing Signed Tree Heads (STHs) through pollination
latter mode requires more configuration, but is necessary to share STHs between HTTPS clients, CT auditors, and monitors in
while still preserving the privacy prevent
denial of service (DoS) attacks on the end user. The sharing of
STHs contribute to server's storage space.

In the overall goal simple mode of detecting misbehaving logs by
providing CT auditors and monitors with SCTs from many vantage
points, making it possible to detect logs that are presenting
inconsistent views. operation, upon recieving a submission at the
sct-feedback well-known URL, an HTTPS servers supporting server will perform a set of
operations, checking on each sct_feedback object before storing it:

1. the protocol act as STH pools. HTTPS
clients and CT auditors server MAY modify the sct_feedback object, and monitors discard
all items in the possession of STHs should
pollinate STH pools by sending STHs to them, and retrieving new STHs
to send to other STH pools. CT auditors and monitors should perform
their auditing and monitoring duties by retrieving STHs from pools.

STH Pollination x509_chain array except the first item (which is carried out by sending STHs to HTTPS servers
supporting the protocol, and retrieving new STHs. In
the case end-entity certificate)

2. if a bit-wise compare of
HTTPS clients, STHs the sct_feedback object matches one
already in the store, this sct_feedback object SHOULD be sent in an already established TLS
session. This makes it hard
discarded

3. if the leaf cert is not for an attacker to disrupt STH gossiping
without also disturbing ordinary secure browsing (https://). This a domain for which the server is
discussed more
authoritative, the SCT MUST be discarded

4. if an SCT in Section 10.2.1.

HTPS clients send STHs to HTTPS servers by POSTing them the sct_data array can't be verified to be a valid
SCT for the well- accompanying leaf cert, and issued by a known URL:

https://<domain>/.well-known/ct/v1/sth-pollination log,
the individual SCT SHOULD be discarded

The data sent modification in the POST step number 1 is defined 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 Section 7.2.4.

The response contains zero or more STHs an end-entity
certificate with an existing SCT. By discarding all but the end-
entity certificate, we prevent a simple HTTPS server from storing
this data. Note that operation in this mode will not prevent the same format,
attack described in Section 7.2.4.

An HTTPS client may acquire STHs by several methods:

o 10.2. Skipping this step requires
additional configuration as described below.

The check in replies to pollination POSTs;

o asking logs that it recognises step 2 is for detecting duplicates and minimizing
processing and storage by the current STH, either
directly (v2/get-sth) or indirectly (for example over DNS)

o resolving server. As on the client, an SCT and certificate exact
comparison is needed to an STH via an inclusion proof

o resolving ensure that attacks involving alternate
chains are detected. Again, at least one STH to another via a consistency proof

HTTPS clients (who have STHs), CT auditors, and monitors SHOULD
pollinate STH pools with STHs. Which STHs to send and how often
pollination should happen optimization is regarded as undefined policy with safe and
MAY be performed. If the
exception certificate chain exactly matches an
existing certificate chain, the server may store the union of privacy concerns explained in the next section.
Suggestions for
SCTs from the policy may be found two objects in Section 10.1.

An HTTPS client could be tracked by giving it a unique or rare STH.
To address the first (existing) object. It should
do this concern, we place restrictions after completing the validity check on different
components of the system SCTs.

The check in step 3 is to ensure an STH will not be rare.

o HTTPS help malfunctioning clients sliently ignore STHs from logs with an STH issuance
frequency of more than one STH per hour. Logs use exposing
which sites they visit. It additionally helps prevent DoS attacks on
the STH
Frequency Count metadata to express this ([RFC-6962-BIS] sections
3.6 and 5.1).

o HTTPS clients silently ignore STHs server.

[ TBD: Thinking about building this, how does the SCT Feedback app
know which are not fresh.

An STH is considered fresh iff its timestamp is less than 14 days sites it's authoritative for? ]

The check in step 4 is to prevent DoS attacks where an adversary
fills up the past. Given store prior to attacking a maximum STH issuance rate of one per hour, client (thus preventing the
client's feedback from being recorded), or 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 attack where an
adversary simply attempts to be personally identifiable data, and STHs outside
this window not personally identifiable.

A log may cease operation, in which case there fill up server's storage space.

The more advanced server configuration will soon be no STH
within the validity window. Clients SHOULD perform all three methods
of gossip about a log that has ceased operation - it is possible the
log was still compromised and gossip can detect that. STH
Pollination is the one mechanism where a client must know about a log
shutdown. A client who does [TODO double-
CA-compromise] attack. In this configuration the server will not know about a log shutdown MUST NOT
attempt any heuristic
modify the sct_feedback object prior to detect performing checks 2, 3, and
4.

To prevent a shutdown. Instead the malicious client MUST
be informed about the shutdown from a verifiable source (e.g. a
software update). The client SHOULD be provided filling the final STH issued
by server's data store,
the log and HTTPS Server SHOULD resolve SCTs and STHs to this final STH. If perform an
SCT additional check:

1. if the x509_chain consists of an invalid certificate chain, or STH cannot be resolved to
the final STH... XXX?

When multiplied culminating trust anchor is not recognized by the number of logs from which a client accepts
STHs, this number of unique STHs grow and server, 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
server SHOULD modify the sct_feedback object, discarding all
items in Section 9.2.5.

7.2.1. the x509_chain array except the first item

The HTTPS Clients and Proof Fetching

There are two types of proofs a client server may retrieve.

An HTTPS client will retrieve SCTs from an HTTPS server, and must
obtain an inclusion proof to an STH in order choose to verify omit checks 4 or 5. This will place
the promise
made server at risk of having its data store filled up by the SCT.

An HTTPS client may receive SCT bundled with an inclusion proof 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
historical STH via an unspecified future mechanism. Because this
historical STH is considered personally identifiable
trusted root. This information per
above, may enable an HTTPS server operator
to detect attacks or unusual behavior of Certificate Authorities even
outside the client must obtain a consistency proof Certificate Transparency ecosystem.

8.1.4. HTTPS server to a more recent
STH.

If a client requested either proof directly auditors

HTTPS servers receiving SCTs from a log or auditor, it
would reveal clients SHOULD share SCTs and
certificate chains with CT auditors by either serving them on the client's browsing habits
well-known URL:

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

or by HTTPS POSTing them to a third party. To
mitigate this risk, set of preconfigured auditors. This
allows an HTTPS client MUST retrieve the proof server to choose between an active push model or a
passive pull model.

The data received in a
manner that disguises GET of the client.

Depending on well-known URL or sent in the client's DNS provider, DNS may provide an
appropriate intermediate layer POST
is defined in Section 8.1.1.

HTTPS servers SHOULD share all sct_feedback objects they see that obfuscates the linkability
between
pass the user checks in Section 8.1.3. If this is an infeasible amount of
data, the client and the request for inclusion (while
at the same time providing a caching layer for oft-requested
inclusion proofs.)

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

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

7.2.2. STH Pollination without Proof Fetching

An HTTPS client MAY participate
undefined policy. Suggestions for such a policy can be found in STH Pollination without fetching
proofs. In this situation, the client receives STHs
Section 11.3.

HTTPS servers MUST NOT share any other data that they may learn from a server,
applies
the same validation logic to them (signed submission of SCT Feedback by a known log,
within a validity window) and will later pass them to a HTTPS server.

When operating in this fashion, clients, like the HTTPS
client is promoting gossip
for Certificate Transparency, but derives no direct benefit itself. 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 comparison, one mode,
the x509_chain array will contain a client who resolves SCTs full certificate chain. This
chain may terminate in a trust anchor the auditor may recognize, or historical STHs to
recent STHs and pollinates them is assured that if
it was attacked,
there may not. (One scenario where this could occur is a probability that if the ecosystem will detect client
submitted a chain terminiating in a locally added trust anchor, and respond to
the attack (by distrusting server kept this chain.) In the log).

7.2.3. Auditor and Monitor Action

Auditors and Monitors participate in STH pollination by retrieving
STHs from HTTPS servers. They verify that other mode, the STH is valid by
checking the signature, and requesting x509_chain array
will consist of only a consistency proof from single element, which is the
STH to end-entity
certificate.

Auditors SHOULD provide the most recent STH.

After retrieving following URL accepting HTTPS POSTing of
SCT feedback data:

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

[ TBD: Should that be .well-known? Depends on whether auditors will
operate in their own URL name space or not. ]

Auditors SHOULD regularly poll HTTPS servers at the consistency proof well-known
collected-sct-feedback URL. The frequency of the polling and how to
determine which domains to poll is outside the most recent STH, they
SHOULD pollinate scope of this new STH among participating
document. However, the selection MUST NOT be influenced by potential
HTTPS Servers. In
this way, as STHs "age out" and are no longer fresh, their "lineage"
continues clients connecting directly to be tracked in the system.

7.2.4. 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 data format pollination

The data sent from goal of sharing Signed Tree Heads (STHs) through pollination is
to share STHs between HTTPS clients and CT monitors and 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 is a JSON object [RFC7159] with supporting the following content:

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

[XXX An STH is considered fresh iff TBD.]

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

The most natural trusted auditor arrangement arguably is a web
browser that is "logged CT auditors in to" a provider the possession of various internet
services. Another equivalent arrangement is a trusted party like a
corporation to which an employee is connected through a VPN or STHs can pollinate STH
pools by sending STHs to them, and retrieving new STHs to send to
other similar means. A third might be individuals or smaller groups STH pools. CT auditors can improve the value of people running their own services. In such a setting, auditing
by retrieving
proofs from that third party could be considered reasonable STHs from a
privacy perspective. The HTTPS client does its own auditing and
might additionally share SCTs and pools.

HTPS clients send STHs with the trusted party to
contribute HTTPS servers by POSTing them to herd immunity. Here, the ordinary [RFC-6962-BIS]
protocol well-
known URL:

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

The data sent in the POST is sufficient defined in Section 8.2.4. This data
SHOULD be sent in an already established TLS session. This makes it
hard for the client an attacker to do the auditing while SCT
Feedback and disrupt STH Pollination can be used gossiping without also disturbing
ordinary secure browsing (https://). This is discussed more in whole
Section 11.1.1.

The response contains zero or more STHs in parts the same format, 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 recognises for the
gossip part.

Another well established trusted party arrangement on the internet
today is the relation between internet users current STH, either
directly (v2/get-sth) or indirectly (for example over DNS)

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

o resolving already know one STH to another via a great deal about which sites their users
visit. As mentioned in Section XXX, in order for consistency proof

HTTPS clients (that have STHs) and CT auditors SHOULD pollinate STH
pools with STHs. Which STHs to be
able to retrieve proofs in a privacy preserving manner, logs could
expose a DNS interface in addition to send and how often pollination should
happen is regarded as undefined policy with the ordinary HTTPS interface.
An informal writeup of such a protocol can be found at XXX.

7.3.1. Trusted Auditor data format

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

8. 3-Method Ecosystem

The use exception of three distinct methods privacy
concerns explained below. Suggestions for monitoring logs the policy may seem
excessive, but each represents a needed component be found in the CT
ecosystem. To understand why, the drawbacks of each component must
Section 11.3.

An HTTPS client could be outlined. In tracked by giving it a unique or rare STH.
To address this discussion concern, we assume that an attacker knows
which mechanisms an HTTPS client and HTTPS server implement.

8.1. SCT Feedback

SCT Feedback requires the cooperation place restrictions on different
components of the system to ensure an STH will not be rare.

o HTTPS clients and more
importantly HTTPS servers. Although SCT Feedback does require a
significant amount silently ignore STHs from logs with an STH issuance
frequency of server-side logic to respond to more than one STH per hour. Logs use the
corresponding APIs, STH
Frequency Count metadata to express this functionality does not require
customization, so it may be pre-provides ([RFC-6962-BIS-09]
sections 3.6 and work out of 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 box.
However, to take full advantage past. Given a maximum STH issuance rate of the system, one per hour, an HTTPS server would
wish
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 perform some configuration be personally identifiable data, and STHs
outside this window to optimize its operation:

o Minimize its disk commitment be personally identifiable.

When multiplied by whitelisting known SCTs and
certificate chains

o Maximize its chance the number of detecting a misissued certificate by
configuring logs from which a trust store client accepts
STHs, this number of CAs

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

These configuration needs, unique STHs grow and the simple fact that it would require
some deployment of software, means negative privacy
implications grow with it. It's important that some percentage of this is taken into
account when logs are chosen for default settings in HTTPS
servers will not deploy SCT Feedback.

If SCT Feedback was the only mechanism clients.
This concern is discussed upon in Section 10.4.5.

A log may cease operation, in which case there will soon be no STH
within the ecosystem, any server
that did not implement the feature, would open itself and its users
to attack without any possibility validity window. Clients SHOULD perform all three methods
of detection.

If SCT Feedback was not deployed, users who wished to have gossip about a log that has ceased operation since it is possible
the
strongest measure of privacy protection (by disabling STH Pollination
Proof Fetching log was still compromised and forgoing a Trusted Auditor) could be attacked
without risk of detection.

8.2. STH Pollination gossip can detect that. STH
Pollination requires is the cooperation of HTTPS clients, HTTPS
servers, and logs.

For 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 fully participate in STH Pollination, and have this
mechanism detect attacks against it, a shutdown. Instead the client must have MUST
be informed about the shutdown from a way to
safely perform Proof Fetching in verifiable source (e.g. a privacy preserving manner.
software update). The client may pollinate SHOULD be provided the final STH issued
by the log and SHOULD resolve SCTs and STHs it receives without performing Proof
Fetching, but we do not consider this option in to this section.

HTTPS Servers must deploy software (although, as in the case with final STH. If an
SCT
Feedback this logic can or STH cannot be pre-provided) and commit some configurable
amount of disk space resolved to the endeavor.

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

Unlike SCT Feedback, should follow
the STH Pollination mechanism is not hampered if
only a minority of HTTPS servers deploy it. However, it makes an
assumption that an requirements and recommendations set forth in Section 11.1.2.

8.2.1. HTTPS client performs anonymized Clients and Proof Fetching
(such as the DNS mechanism discussed). However, any manner that is
anonymous for some (such as clients who use shared DNS services such
as

There are two types of proofs a large ISP), client may not be anonymous for others.

For instance, DNS leaks a considerable amount of information
(including what data is already present retrieve; inclusion proofs
and consistency proofs.

An HTTPS client will retrieve SCTs from an HTTPS server, and must
obtain an inclusion proof to an STH in order to verify the cache) in plaintext
over promise
made by the network. For this reason, some percentage of SCT.

An HTTPS clients client may choose also receive an SCT bundled with an inclusion
proof to not enable the Proof Fetching component of a historical STH
pollination. (Although they can still request and send STHs among
participating HTTPS servers, as mentioned earlier via an unspecified future mechanism.
Because 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 historical STH Pollination was not deployed, HTTPS Clients visiting HTTPS
Servers who did not deploy SCT Feedback could be attacked without
risk of detection.

8.3. Trusted Auditor Relationship

The Trusted Auditor Relationship is expected to be considered personally identifiable
information per above, the rarest gossip
mechanism, as an HTTPS Client is providing an unadulterated report of
its browsing history to client must obtain a third party. While there are valid and
common reasons consistency proof to
a more recent STH.

A client SHOULD perform proof fetching. A client MUST NOT perform
proof fetching for doing so, there is no appropriate way 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 enter
into a locally added
trust anchor.

[ TBD: Linus doesn't like this relationship without retrieving informed consent because we're mandating behavior that
is not necessarily safe. Is it unsafe? Not sure.]

If a client requested either proof directly from a log or auditor, it
would reveal the
user.

However, the Trusted Auditor Relationship mechanism still provides
value client's browsing habits to a class of third party. To
mitigate this risk, an HTTPS Clients. For example, web crawlers have no
concept of a "user" and no expectation of privacy. Organizations
already performing network monitoring for anomalies or attacks can
run their own Trusted Auditor for client MUST retrieve the same purpose with marginal
increase proof 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
manner that decision later without consequence. A Trusted Auditor
connection can be made more confidential than disguises the client.

Depending on the client's DNS (through provider, DNS may provide an
appropriate intermediate layer that obfuscates the use of
TLS), and can even be made (somewhat) anonymous through linkability
between the use user of
anonymity services such as Tor. (Note that this does ignore the de-
anonymization possibilities available from viewing a user's browsing
history.)

If client and the Trusted Auditor relationship was request for inclusion (while
at the only same time providing a caching layer for oft-requested
inclusion proofs.)

[ TODO: Add a reference to Google's DNS mechanism deployed,
users who do not enable it (the majority) would be able more proper than
http://www.certificate-transparency.org/august-2015-newsletter ]

Anonymity networks such as Tor also present a mechanism for a client
to be
attacked without risk of detection.

If anonymously retrieve a proof from an auditor or log.

Even when using a privacy-preserving layer between the Trusted Auditor relationship was not deployed, crawlers client and
organizations 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 build it themselves for their own needs. By
standardizing it, users who wish know that SCTs or STHs recieved around the same time are more
likely to opt-in (for instance those
unwilling come from a particular client. This could potentially go
so far as correlation of activity at different times to participate fully in STH Pollination) can have an
interoperable standard they can 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 choose and change their
trusted auditor.

8.4. Interaction

The interactions obfuscate these patterns. A suggestion of the mechanisms is thus outlined:

HTTPS Clients
such a policy can be attacked without risk of detection if they do
not participate found in any Section 11.2.

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 the three mechanisms.

HTTPS an attack. Clients are afforded should follow the greatest chance of detecting an attack requirements and
recommendations set forth in Section 11.1.2 when they either participate 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 with without Proof Fetching
or have a Trusted Auditor relationship. Participating in SCT
Feedback enables a

An HTTPS Client to assist client MAY participate in detecting STH Pollination without fetching
proofs. In this situation, the exact
target of an attack, although they do not gain any direct benefit client receives STHs from it.

HTTPS Servers that omit SCT Feedback may never learn about targeted
attacks against them, even if a server,
applies the attack occurred and same validation logic to them (signed by a known log,
within the log
distrusted. They do gain some herd immunity, enabling validity window) and will later pass them to detect
attacks, through their clients participating in STH Pollination or a
Trusted Auditor Relationship. an HTTPS
server.

When operating in this fashion, the HTTPS Servers omit SCT feedback, 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 allow was attacked,
there is a portion of their
users probability that the ecosystem will detect and respond to be attacked without detection;
the vulnerable users are
those who do not attack (by distrusting the log).

8.2.3. Auditor Action

CT auditors participate in STH Pollination with Proof Fetching
and pollination by retrieving STHs from
HTTPS servers. They verify that not have a Trusted Auditor relationship.

9. Security considerations

9.1. Censorship/Blocking considerations

We assume a network attacker who the STH is able to fully control valid by checking the
client's internet connection for some period of time - including
selectively blocking requests to certain hosts
signature, and truncating TLS
connections based on information observed or guessed about client
behavior. In order to successfully detect log misbehavior, requesting a consistency proof from 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 STH to auditors in SCT Feedback (server push
mechanism)

3. Servers making SCTs available the
most recent STH.

After retrieving the consistency proof to auditors (auditor pull
mechanism)

4. Clients fetching proofs in the most recent STH, they
SHOULD pollinate this new STH Pollination

5. Clients sending among participating HTTPS Servers. In
this way, as STHs "age out" and are no longer fresh, their "lineage"
continues to servers be tracked in the system.

8.2.4. STH Pollination

6. Servers sending STHs to data format

The data sent from HTTPS clients in STH Pollination

7. Clients sending SCTs to Trusted Auditors

If a party cannot connect and CT auditors 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 HTTPS servers is
a connection
was not blocked. In this situation, JSON object [RFC7159] with the party can factor this into
strategies suggested following content:

o sths - an array of 0 or more fresh SignedTreeHead's as defined in
[RFC-6962-BIS-09] Section 10.1 3.6.1.

8.3. Trusted Auditor Stream

HTTPS clients MAY send SCTs and in Section 10.2.2.

The connections cert chains, as well as STHs,
directly to auditors. If sent, this data MAY include data that allow positive affirmation
reflects locally added logs or trust anchors. Note that there are 1, 2, 4, 5,
privacy implications in doing so, these are outlined in
Section 10.4.1 and
7.

More insidious Section 10.4.6.

The most natural trusted auditor arrangement arguably is blocking the connections a web
browser that do not allow positive
confirmation: 3 and 6. An attacker may truncate is "logged in to" a or drop provider of various internet
services. Another equivalent arrangement is a response
from trusted party like a server
corporation to which an employee is connected through a client, 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 the server believes it has
shared data 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 recipient, when it has not. However, in both
scenatios (3 and 6), trusted party to
contribute to herd immunity. Here, the server cannot distinguish ordinary [RFC-6962-BIS-09]
protocol is sufficient for 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 do the
fact that these connections auditing while SCT
Feedback and STH Pollination can be undetectably blocked does not
actually alter the threat model of servers responding to these
requests. The choice of algorithm to release data is crucial to
protect against these attacks, strategies are suggested used in
Section 10.1.

Handling censorship and network blocking (which is indistinguishable
from network error) whole or in parts for the
gossip part.

Another well established trusted party arrangement on the internet
today is relegated to the implementation policy chosen
by clients. Suggestions for client behavior relation between internet users and their providers of
DNS resolver services. DNS resolvers are specified 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 10.2.

9.2. Privacy considerations

CT Gossip deals with 8.2.1, in order for HTTPS Clients which are trying clients to share
indicators that correspond
be able to their browsing history. The most
sensitive relationships retrieve proofs in a privacy preserving manner, logs could
expose a DNS interface in addition to the CT ecosystem are the relationships
between HTTPS clients and ordinary HTTPS servers. Client-server relationships interface.
An informal writeup of such a protocol can be aggregated into found at XXX.

8.3.1. Trusted Auditor data format

Trusted Auditors expose a network graph with potentially serious
implications for correlative de-anonymisation of clients and
relationship-mapping or clustering of servers or of clients.

There are, however, certain clients that do not require privacy
protection. Examples of these clients are web crawlers or robots but
even in this case, REST API at the method fixed URI:

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

Submissions are made by which these clients crawl sending an HTTPS POST request, with the web
may body
of the POST in fact be considered sensitive information. In general, it is
better to err on a JSON object. Upon successful receipt the side Trusted
Auditor returns 200 OK.

The JSON object consists of safety, two top-level keys: 'sct_feedback' and not assume a client
'sths'. The 'sct_feedback' value is okay
with giving up its privacy.

9.2.1. Privacy and SCTs

An SCT contains information that links it to a particular web site.
Because the client-server relationship is sensitive, gossip between
clients and servers about unrelated SCTs an array of JSON objects as
defined in Section 8.1.1. The 'sths' value is risky. Therefore, a
client with an SCT array of STHs as
defined in Section 8.2.4.

Example:

{
'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 given server should transmit that
information needed component in only two channels: to a server associated with the SCT
itself; and to a trusted CT auditor, if one exists.

9.2.2. Privacy in SCT Feedback

SCTs introduce yet another mechanism for HTTPS servers to store state
on
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, client and potentially track users. HTTPS server implement.

9.1. SCT Feedback

SCT Feedback requires the cooperation of HTTPS clients which
allow users and more
importantly HTTPS servers. Although SCT Feedback does require a
significant amount of server-side logic to respond to clear history or cookies associated with an origin
MUST clear stored SCTs associated with the origin as well.

Auditors should treat all SCTs as sensitive data. SCTs received
directly from an HTTPS client are especially sensitive, because
corresponding APIs, this functionality does not require
customization, so it may be pre-provided and work out of the
auditor is a trusted by box.
However, to take full advantage of the client system, an HTTPS server would
wish to not reveal their associations
with servers. Auditors MUST NOT share such SCTs in any way,
including sending them perform some configuration to an external log, without first mixing them
with multiple other SCTs learned through submissions from multiple
other clients. Suggestions for mixing optimize its operation:

o Minimize its disk commitment by maintaining a list of known SCTs are presented in
Section 10.1.

There is
and certificate chains (or hashes thereof)

o Maximize its chance of detecting a possible fingerprinting attack where misissued certificate by
configuring a log issues trust store of CAs

o Establish a unique
SCT "push" mechanism for targeted log client(s). A colluding log and HTTPS server
operator could therefore be a threat POSTing SCTs to CT auditors

These configuration needs, and the privacy simple fact that it would require
some deployment of software, means that some percentage of an HTTPS
client. Given all the other opportunities for HTTPS
servers to
fingerprint clients - TLS session tickets, HPKP and HSTS headers,
HTTP Cookies, etc. - this will not deploy SCT Feedback.

It is acceptable.

The fingerprinting attack described above would be mitigated by a
requirement worthwhile to note that logs MUST use an attacker may be able to prevent
detection of an attack on a deterministic signature scheme when
signing SCTs ([RFC-6962-BIS] Section 2.1.4). A log signing using RSA webserver (in all cases) if SCT Feedback
is not required to use a deterministic signature scheme.

Since logs are allowed to issue a new implemented. This attack is detailed in Section 10.1).

If SCT for a certificate already
present Feedback was the only mechanism in the log, mandating deterministic signatures does ecosystem, any server
that did not stop
this fingerprinting attack altogether. It does make implement the attack
harder feature would open itself and its users to pull off
attack without being detected though.

There any possibility of detection.

If SCT Feedback is another similar fingerprinting attack where an HTTPS server
tracks a client not deployed by using a variation of cert chains. The risk for
this webserver, malicious logs will
be able to attack is accepted on all users of the same grounds as webserver (who do not have a
Trusted Auditor relationship) with impunity. Additionally, users who
wish to have the unique SCT attack
described above. [XXX any mitigations possible here?]

9.2.3. Privacy for HTTPS clients performing strongest measure of privacy protection (by
disabling STH Pollination Proof Fetching

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

The actual fully participate in STH Pollination, and have this
mechanism by which Proof Fetching is done carries
considerable privacy concerns. Although out of scope for detect attacks against it, the
document, DNS is client must have a mechanism currently discussed. DNS leaks data way to
safely perform Proof Fetching in
plaintext over the network (including what sites the user is visiting
and what sites they have previously visited) - thus it a privacy preserving manner. (The
client may pollinate STHs it receives without performing Proof
Fetching, but we do not be
suitable for some.

9.2.4. Privacy consider this option in STH Pollination

An STH linked to an this section.)

HTTPS client may indicate the following about
that client:

o that the client gossips;

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

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

o which Servers must deploy software (although, as in the case with SCT
Feedback this logic can be pre-provided) 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 commit some configurable
amount of disk space to the
fingerprinting attack described in Section 9.2.2, but can operate
cross-origin. If a log endeavor.

Logs (or HTTPS Server cooperating with a log)
provides a unique STH third party) must provide access to clients to query
proofs in a client, the targeted client will be privacy preserving manner, most likely through DNS.

Unlike SCT Feedback, the
only client pollinating that STH cross-origin.

It is mitigated partially because the log Pollination mechanism is limited in the number not hampered if
only a minority of
STHs HTTPS servers deploy it. However, it can issue. It must 'save' one of its STHs each MMD to
perform the attack.

9.2.5. Privacy in STH Interaction

An makes an
assumption that an HTTPS client may pollinate any STH within performs Proof Fetching (such as the last 14 days. An
HTTPS Client may also pollinate an STH for
DNS mechanism discussed). Unfortunately, any log manner 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
anonymous for some (such as clients who use shared DNS services such
as a server large ISP), may 'prime' a
client and not be able to reliably detect the client at a later time.

It's clear that a single site can track anonymous for others.

For instance, DNS requests expose a user any way they wish, but
this attack works cross-origin and considerable amount of sensitive
information (including what data 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 already present in the M logs
Carol trusts, chosen to be older and less common, but still cache) in
plaintext over the
validity window. Carol visits B and chooses to release network. For this reason, some percentage of the
STHs she has stored, according
HTTPS clients may choose to some policy.

Modeling a representation for how common older STHs are in not enable the pools Proof Fetching component
of clients, STH Pollination. (Although they can still request and examining that with a given policy of how to choose
which of those STHs to send to B, 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 should would be possible to calculate
statistics about how unique Carol looks when talking able to B and how
useful/accurate such a tracking mechanism is.

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

9.2.6. Trusted Auditors for detection.

If STH Pollination was not deployed, HTTPS Clients

Some visiting HTTPS clients may choose to use a trusted auditor. This trust
relationship leaks a large amount
Servers who did not deploy SCT Feedback could be attacked without
risk of information from the client to
the auditor. In particular, it will identify the web sites that the
client has visited detection.

9.3. Trusted Auditor Relationship

The Trusted Auditor Relationship is expected to be the auditor. Some clients may already share
this information rarest gossip
mechanism, as an HTTPS Client is providing an unadulterated report of
its browsing history to a third party, party. While there are valid and
common reasons for example, when using a server
to synchronize browser history across devices in a server-visible
way, or when doing DNS lookups through 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 trusted DNS resolver. class of HTTPS Clients. For
clients with such example, web crawlers have no
concept of a relationship "user" and no expectation of privacy. Organizations
already established, sending SCTs to
a trusted auditor performing network auditing for anomalies or attacks can run by
their own Trusted Auditor for the same organization does not appear to
leak any additional information to the trusted third party.

Clients who wish to contact an auditor without associating their
identities purpose with their SCTs may wish to use an anonymizing network
like Tor to submit SCT Feedback marginal increase
in privacy concerns.

The ability to the auditor. Auditors SHOULD
accept SCT Feedback change one's Trusted Auditor is a form of Trust
Agility that arrives over such anonymizing networks.

Clients sending feedback allows a user to an auditor may prefer choose who to reduce the
temporal granularity of the history leakage trust, and be able to
revise that decision later without consequence. A Trusted Auditor
connection can be made more confidential than DNS (through the auditor by caching use of
TLS), and delaying their SCT Feedback reports. This elaborated upon in XXX
Mixing. This strategy is only as effective as can even be made (somewhat) anonymous through the granularity use of the
timestamps embedded in the SCTs and STHs.

9.2.7. HTTPS Clients as Auditors

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

o an Auditing HTTPS Client potentially leaks their history to the
logs Tor. (Note that they query. Querying the log through a cache or a proxy
with many other users may avoid this leakage, but may leak
information to the cache or proxy, in does ignore the same way that an non-
Auditing HTTPS Client leaks information to de-
anonymization possibilities available from viewing a trusted auditor.

o an effective user's browsing
history.)

If the Trusted Auditor needs a strategy about what to do in relationship was the
event that only mechanism deployed,
users who do not enable it discovers misbehavior from a log. Misbehavior from
a log involves the log being unable (the majority) would be able to provide either (a) a
consistency proof between two valid STHs or (b) an inclusion proof be
attacked without risk of detection.

If the Trusted Auditor relationship was not deployed, crawlers and
organizations would build it themselves for a certificate their own needs. By
standardizing it, users who wish to an 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 time after of the log's MMD has
elapsed from three mechanisms.

HTTPS Clients are afforded the issuance greatest chance of the SCT. The log's inability to
provide detecting an attack
when they either proof will not be externally cryptographically-
verifiable, as it may be indistinguishable from participate in both SCT Feedback and STH Pollination
with Proof Fetching or if they have a network error.

10. Policy Recommendations

This section Trusted Auditor relationship.
(Participating in SCT Feedback is intended required to prevent a malicious log
from refusing to ever resolve an SCT to an STH, as suggestions put forward in
Section 10.1). Additionally, participating in SCT Feedback enables
an HTTPS Client to implementors assist in detecting the exact target of an attack.

HTTPS
Clients, HTTPS Servers, and Auditors. It is not a requirement for
technique Servers that omit SCT Feedback enable malicious logs to carry
out attacks without risk of implementation, so long as privacy considerations
established above detection. If these servers are obeyed.

10.1. Mixing Recommendations

In several components of targeted
specifically, even if the CT Gossip ecosystem, the recommendation attack is made detected, without SCT Feedback
they may never learn that data from multiple sources be ingested, mixed, provided
to a third party, stored for an indeterminate period of time, and
eventually deleted. The instances they were specifically targeted. HTTPS
servers without SCT Feedback do gain some measure of these recommendations herd immunity,
but only because their clients participate in this
draft are:

o When STH Pollination (with
Proof Fetching) or have a client receives SCTs during Trusted Auditor Relationship.

When HTTPS Servers omit SCT Feedback, feedback, it should store allows their users to be
attacked without detection by a malicious log; the SCTs and Certificates for some amount of time, provide some vulnerable users
are those who do not have a Trusted Auditor relationship.

10. Security considerations

10.1. Attacks by actively malicious logs

One of
them back 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 server at some point, and eventually remove them
from its store

o When first attack, the log can present a client receives STHs during STH Pollination, it should
store them for some amount of time, mix them with other STHs,
release some split view of them them the log for
all time. The only way to detect this attack is to various servers at some point, resolve some each view
of them the log to new STHs, the two most recent STHs and eventually remove them from
its store

o When then force the log to
present a server receives SCTs during SCT Feedback, consistency proof. (Which 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 cannot.) This attack can be
detected by CT auditors participating in STH Pollination, it should
store them for some period of time, mix them with other STHs,
provide some as long as
they are explicitly built to handle the situation of them a log
continuously presenting a split view.

In the second attack, the log can sign an SCT, and refuse to connecting clients, may resolve them ever
include the certificate that the SCT refers 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, in the tree.

(Alternately, it should store them for some period can include it in a branch of time, mix them
with SCTs received from other clients, the tree and act upon them at some
period of time

Each of these instances have specific requirements issue an
STH, but then abandon that branch.) Whenever someone requests an
inclusion proof for user privacy,
and each have options that may not be invoked. As one example, SCT (or a
HTTPS client should not mix SCTs consistency proof from server A that STH),
the log would respond with SCTs from server
B an error, and release server B's SCTs to Server A. As another example, a
HTTPS server client may choose to resolve several STHs 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

XXX describes an attack possible by an adversary who compromises two
Certificate Authorites and a Log. This attack is difficult to defend
against in the CT ecosystem, and XXX describes a single more
current STH via proof fetching, but it 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 must compare the chain built by clients to the chain supplied by
the log. If the chains differ significantly, the auditor can raise a
concern.

[ What does 'differ significantly' mean? We should provide guidance.
I _think_ the correct algorithm to raise a concern is:

If one chain is not a subset of the other AND If the root
certificates of the chains are different THEN It's suspicious.

Justification: - 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.)

What did I miss? ]

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
scenatios (3 and 6), the server cannot distinguish the client as a
cooperating member of the CT ecosystem or as an attacker performing a
sybil attack, aiming to flush the server's data store. Therefore the
fact that these connections can be undetectably blocked does not
actually alter the threat model of servers responding to these
requests. The choice of algorithm to release data is crucial to
protect against these attacks; strategies are suggested in
Section 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. Privacy considerations

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

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

10.4.1. Privacy and SCTs

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

10.4.2. 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
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 unqiue 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. [XXX any mitigations possible
here?]

10.4.3. Privacy for HTTPS clients performing STH Proof Fetching

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

[ TBD how regularly? This has operational implications for log
operators ]

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) an may not be
suitable for some.

10.4.4. Privacy in STH Pollination

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

o that the client gossips;

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

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

o which software and software version is being used.

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 10.4.2, 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.

10.4.5. 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.

10.4.6. 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
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.

10.4.7. 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.

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-Alives or SPDY.

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

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 cirsumstances 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 recieve an error.
These situations are:

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

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

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

5. The client has attempted to share a specific piece of data with a
Trusted Auditor - but recieves 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, a client SHOULD make
every effort to send this SCT via SCT Feedback. 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, so if
implemented, it should connect using public key pinning and not
considered an item delivered until it recieves 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 occured all
at once, in a flurry of activity, a log would know that SCTs or STHs
recieved 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
patterns could be recognizable as a single user, or could reveal what
sites are commonly visited together in the aggregate.

[ TBD: What other recommendations do we want to make here? We can
talk more about the inadequecies of DNS... The first paragraph is 80%
identical between here and above ]

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
previously been considered in the case of Mix Networks and Remailers,
including papers such as "From a Trickle to a Flood: Active Attacks
on Several Mix Types", [Y], and [Z].

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 recieve 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
cryptographyically 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.

11.3.2. 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.

Servers are most vulnerable to flushing attacks, as they release
records to anonymous connections. An attacker can perform a Sybil
attack - connecting to the server hundreds or thousands of times in
an attempt to trigger repeated release of a record, and then
deletion. For this reason, servers must be especially aggressive
about retaining data for a longer period of time.

Clients are vulnerable to flushing attacks targetting STHs, as these
can be given to any cooperating server and an attacker can generally
induce connections to random servers using javascript. It would be
more difficult to perform a flushing attack against SCTs, as the
target server must be authenticated (and an attacker impersonating an
authentic server presents a recursive problem for the attacker).
Nonetheless, flushing SCTs should not be ruled impossible. A Trusted
Auditor may also be vulnerable to flushing attacks if it does not
perform auditing operations itself.

Flushing attacks are defended against using non-determinism and dummy
messages. The goal is to ensure that an adversary does not know for
certain if the data in question has been released or not, and if it
has been deleted or not.

[ TBD: At present, we do not have any support for dummy messages. Do
we want to define a dummy message that clients and servers alike know
to ignore? Will HTTP Compression leak the presence of >1 dummy
messages?
Is it sufficient to define a dummy message as _anything_ with an
invalid siganture? This would negatively impact SCT Feedback servers
that log all things just in case they're interesting. ]

11.3.3. The Deletion Algorithm

No entity in CT Gossip is required to delete SCTs or STHs at any
time, except to respect user's wishes such as private browsing mode
or clearing history. However, requiring infinite storage space is
not a desirable characteristic in a protocol, so deletion is
expected.

While deletion of SCTs and STHs 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 an SCT or STH until it has been successfully
resolved to a new STH via a proof.

When it is time to delete a record, it is important that the decision
to do so not be done deterministicly. Introducing non-determinism in
the decision is absolutely necessary to prevent an adversary from
knowing with certainty that the record has been successfully flushed
from a target. Therefore, we speak of making a record 'eligible for
deletion' and then being processed by the 'deletion algorithm'.
Making a record eligible for deletion simply means that it will have
the deletion algorithm run. The deletion algorithm will use a
probability based system and a secure random number generator to
determine if the record will be deleted.

Although the deletion algorithm is specifically designed to be non-
deterministic, if the record has been resolved via proof to a new STH
the record may be safely deleted, as long as the new STH is retained.

The actual deletion algorithm may be [STATISTICS HERE]. [ Something
as simple as 'Pick an integer securely between 1 and 10. If it's
greater than 7, delete the record.' Or something more complicated. ]

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

11.3.3.1. Experimental Algorithms

More complex algorithms could be inserted at any step. Three
examples are illustrated:

SCTs are not eligible to be submitted to an Auditor of Last Resort.
Therefore, it is more important that they be resolved to STHs and
reported via SCT feedback. If fetching an inclusion proof regularly
fails for a particular SCT, one can require it be reported more times
than normal via SCT Feedback before becoming eligible for deletion.

Before an item is made eligible for deletion by a client, the client
could aim to make it difficult for a point-in-time attacker to flush
the pool by not making an item eligible for deletion until the client
has moved networks (as seen by either the local IP address, or a
report-back providing the client with its observed public IP
address). The HTTPS client could also require reporting over a
timespan, e.g. it must be reported at least N time, M weeks apart.
This strategy could be employed always, or only when the client has
disabled proof fetching and the Auditor of Last Resort, as those two
mechanisms (when used together) will enable a client to report most
attacks.

11.3.3.2. Concrete Recommendations

The recommendations for behavior are: - If proof fetching is enabled,
do not delete an SCT until it has had a proof resolving it to an STH.
- If proof fetching continually fails for an SCT, do not make the
item eligible for deletion of the SCT until it has been released,
multiple times, via SCT Feedback. - If proof fetching continually
fails for an STH, do not make the item eligible for deletion until it
has been queued for release to an Auditor of Last Resort. - Do not
dequeue entries to an Auditor of Last Resort if reporting fails.
Instead keep the items queued until they have been successfully sent.
- Use a probability based system, with a cryptographically secure
random number generator, to determine if an item should be deleted.
- Select items from the datastores by selecting random indexes into
the datastore. Use a cryptographically secure random number
generator.

[ TBD: More? ]

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

11.3.3.2.1. STH Data Structures

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

class STH
{
uint32 proof_attempts
uint32 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. One thing to note
about this pseudocode is that it aggressively removes STHs once they
have been resolved to a newer STH (if proof fetching is configured).
The only STHs in the store are ones that have never been resolved to
a newer STH, either because proof fetching does not occur, has
failed, or because the STH is considered too new to request a proof
for. It seems less likely that servers will perform proof fetching.
Therefore it would be recommended that the various constants in use
be increased considerably to ensure STHs are pollinated more
aggressively.

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 under no obligation called to do
so.

These requirements should be met, but possibly delete the general problem of
aggregating multiple pieces of data, choosing when and how many to
release, given STH
// from the data store
def delete_maybe(STH s) {
//Perform statistical test and when see if I should delete this bundle
}
// This function is called to remove (certainly) delete the given STH
// from the data store
def delete_now(STH s) {
this.sth_list.remove(s)
}

// When it is shared. This problem has been
previously been considered in time to perform STH Pollination, the case HTTPS Client
// calls this function to get a selection of Mix Networks and Remailers,
including papers such STHs to send as [X], [Y], and [Z].

Certain common recommendations
// 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
if(r not in indexes
&& now() - this.sth_list[i].timestamp < ONE_WEEK)
indexes.insert(r)
}

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

We also suggest a function that can be made:

o When choosing how many times to release data before expiring it
from a cache, use a random number chosen from a distribution,
rather than called periodically in the
background, iterating through the STH store, performing a fixed number. cleaning
operation and queuing consistency proofs. This prevents an adversary from
knowing with certainty that function can live as
a member functions of the STHStore class.

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

if(now() - sth.timestamp > ONE_WEEK) {
//STH is too old, we must remove it has
if(proof_fetching_enabled
&& auditor_of_last_resort_enabled
&& (sth.proof_failure_count / sth.proof_attempts)
> MIN_PROOF_FAILURE_RATIO_CONSIDERED_SUSPICIOUS) {
queue_sth_for_auditor_of_last_resort(sth)
delete_maybe(sth)
} else {
delete_now(sth)
}
}

else if(proof_fetching_enabled
&& now() - sth.timestamp > TWO_DAYS
&& now() - sth.timestamp > LOG_MMD) {
sth.proof_attempts++
queue_consistency_proof(sth, consistency_proof_callback)
}
}
}

11.3.3.2.2. STH Deletion Procedure

The STH Deletion Procedure is run after successfully flushed submitting a cache
list of STHs to a potentially incriminating piece of data.

o [TODO Enumerating third party during pollination. The following
pseudocode would be included in the problems of different types STHStore class, and called with
the result of mixes vs
Cottrell Mix]

o [TODO Integrating get_pollination_selection(), after the IP address into STHs have been
(successfully) sent to the third party.

// 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()
def after_submit_to_thirdparty(STH[] sth_list)
{
foreach(sth in sth_list)
{
sth.num_reports_to_thirdparty++

if(proof_fetching_enabled) {
if(now() - sth.timestamp > LOG_MMD) {
sth.proof_attempts++
queue_consistency_proof(sth, consistency_proof_callback)
}

if(auditor_of_last_resort_enabled
&& sth.proof_failure_count >
MIN_PROOF_ATTEMPTS_CONSIDERED_SUSPICIOUS
&& (sth.proof_failure_count / sth.proof_attempts) >
MIN_PROOF_FAILURE_RATIO_CONSIDERED_SUSPICIOUS) {
queue_sth_for_auditor_of_last_resort(sth)
}
}
else { //proof fetching not enabled
if(sth.num_reports_to_thirdparty
> MIN_STH_REPORTS_TO_THIRDPARTY) {
delete_maybe(sth)
}
}
}
}

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

11.3.3.2.3. SCT Data Structures

TBD TBD This section is not well abstracted to be used for both
servers and clients. TKTK
The SCT class contains data pertaining specifically to the algorithm for releasing
data]

o [TODO Prefer aggregating multiple piece of SCT
itself.

class SCT
{
uint32 proof_attempts
uint32 proof_failure_count
bool has_been_resolved_to_sth
byte[] data into a single STH
when possible]

o [TODO
}

The importance SCT bundle will contain the trusted certificate chain the HTTPS
client built (chaining to a trusted root certificate.) It also
contains the list of Flushing Attacks, and tying in network
connection, associated SCTs, the exact domain it is
applicable to, and time interval]

10.2. Blocking Recommendations

10.2.1. Frustrating blocking

When making gossip connections metadata pertaining to HTTPS Servers or Trusted Auditors, how often it is desirable has been
reported to minimize the plaintext metadata third party.

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)
return false
}
return true
}

def max_proof_failure_count() {
uint32 max = 0
foreach(s in this.sct_list) {
if(s.proof_failure_count > max)
max = proof_failure_count
}
return max
}
}
We suppose a large data structure is used, such as a hashmap, indexed
by the connection domain name. For each domain, the structure will contain a
data structure that can be used to identify holds the connection SCTBundles seen for that domain, as
well as encapsulating some logic relating to SCT Feedback for that
particular domain.

class SCTStore
{
string domain
datetime last_contact_for_domain
uint32 num_submissions_attempted
uint32 num_submissions_succeeded
SCTBundle[] observed_records

// This function is called after recieving an SCTBundle.
// For Clients, this is after a gossip successful connection
and therefore be of interest to block. Additionally, introducing
some randomness into client behavior may be important - we assume a
// HTTPS Server, calling this function with an SCTBundle
// constructed from that the adversary certificate chain and SCTs
// For Servers, this is able after receiving SCT Feedback
def insert(SCTBundle b) {
if(operator_is_server) {
if(!passes_validity_checks(b))
return
}
foreach(e in this.observed_records) {
if(e.equals(b))
return
else if(e.approx_equals(b)) {
e.insert_scts(b.sct_list)
return
}
}
this.observed_records.insert(b)
}

// When it is time to inspect the behavior of perform SCT Feedback, the HTTPS
client Client
// calls this function to get a selection of SCTBundles to send
// as feedback
def get_gossip_selection() {
if(len(observed_records) > MAC_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 delete_maybe(SCTBundle b) {
//Perform statistical test and understand how it makes gossip connections.

As an example, see if I should delete this bundle
}

def delete_now(SCTBundle b) {
this.observed_records.remove(b)
}

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

We also suggest a client, after establishing a TLS connection (and
receiving an SCT, but not making it's own HTTPS request yet),
immediately opens a second TLS connection for function that can be called periodically in the purpose of gossip
background, iterating through all SCTStore objects in the large
hashmap (here called 'all_sct_stores') and removing old data.

def clear_old_data()
{
foreach(storeEntry in all_sct_stores)
{
if(storeEntry.num_submissions_succeeded == 0
&& storeEntry.num_submissions_attempted
> MIN_SCT_ATTEMPTS_FOR_DOMAIN_TO_BE_IGNORED)
{
all_sct_stores.remove(storeEntry)
}
else if(storeEntry.num_submissions_succeeded > 0
&& now() - storeEntry.last_contact_for_domain
> TIME_UNTIL_OLD_SCTDATA_ERASED)
{
all_sct_stores.remove(storeEntry)
}
}
}

11.3.3.2.4. SCT Deletion Procedure

The SCT Deletion procedure is more complicated than the adversary can reliably block this second connection respective
STH procedure. This is because servers may elect not to block
gossip without affecting normal browsing. For participate
in SCT Feedback, and this reason it is
recommended must be accounted for by being more
conservative in sending SCT reports to run them.

The following pseudocode would be included in the gossip protocols over an existing connection
to SCTStore class, and
called with the server, making use result of connection multiplexing such as HTTP
Keep-Alives or SPDY.

Truncation is get_gossip_selection() after the SCT
Feedback has been sent (successfully) to the server. We 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 note
that the first
response - truncation will allow experimental algorithm from above is included in the
pseudocode as an attacker to block gossip
reliably.

10.2.2. Responding illustration.

// This function is called after successfully providing SCT Feedback
// to possible blocking

[Not sure here. Maybe this section will get folded up into a server. It is passed the
above. Or maybe it relates feedback sent to the escape valve. -tjr]

11. server, which
// is the output of get_gossip_selection()
def after_submit_to_thirdparty(SCTBundle[] submittedBundles)
{
foreach(bundle in submittedBundles)
{
bundle.num_reports_to_thirdparty++

if(proof_fetching_enabled) {
if(!bundle.has_been_fully_resolved_to_sths()) {
foreach(s in bundle.sct_list) {
if(!s.has_been_resolved_to_sth) {
s.proof_attempts++
queue_inclusion_proof(sct, inclusion_proof_callback)
}
}
}
else {
if(run_ct_gossip_experiment_one) {
if(bundle.num_reports_to_thirdparty
> MIN_SCT_REPORTS_TO_THIRDPARTY
&& bundle.num_reports_to_thirdparty * 1.5
> bundle.max_proof_failure_count()) {
maybe_delete(bundle)
}
}
else { //Do not run experiment
if(bundle.num_reports_to_thirdparty
> MIN_SCT_REPORTS_TO_THIRDPARTY) {
maybe_delete(bundle)
}
}
}
}
else {//proof fetching not enabled
if(bundle.num_reports_to_thirdparty
> (MIN_SCT_REPORTS_TO_THIRDPARTY
* NO_PROOF_FETCHING_REPORT_INCREASE_FACTOR)) {
maybe_delete(bundle)
}
}
}
}

// This function is a callback invoked after an inclusion proof
// has been retrieved
def inclusion_proof_callback(inclusion_proof, original_sct, error)
{
if(!error) {
original_sct.has_been_resolved_to_sth = True
insert_to_sth_datastore(inclusion_proof.new_sth)
} else {
original_sct.proof_failure_count++
}
}

12. IANA considerations

[TBD]

12.

[ 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.

13.

14. ChangeLog

13.1.

14.1. 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 adutitors.

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

14.2. Changes between ietf-00 and ietf-01

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

o STH Pollination Proof Fetching defined and indicated as optional.

o 3-Method Ecosystem section added.

o Cases with Logs ceasing operation handled.

o Text on tracking via STH Interaction added.

o Section with some early recommendations for mixing added.

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

13.2.

14.3. Changes between -01 and -02

o STH Pollination defined.

o Trusted Auditor Relationship defined.

o Overview section rewritten.

o Data flow picture added.

o Section on privacy considerations expanded.

13.3.

14.4. 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.

14.

15. References

15.1. Normative References

[RFC-6962-BIS]

[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

[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/>.

Authors' Addresses

Linus Nordberg
NORDUnet

Email: linus@nordu.net

Daniel Kahn Gillmor
ACLU

Email: dkg@fifthhorseman.net

Tom Ritter

Email: tom@ritter.vg