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Certificate Transparency (CT) System Architecture

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Authors Stephen Kent , David Mandelberg , Karen Seo
Last updated 2016-08-05
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Public Notary Transparency                                       S. Kent
Internet Draft                                             D. Mandelberg
Intended status: Standards Track                                  K. Seo
Expires: July 2016                                      BBN Technologies
                                                          August 5, 2016

             Certificate Transparency (CT) System Architecture


   This document describes the architecture for Certificate Transparency
   (CT) focusing on the Web PKI context. It defines the goals of CT and
   the elements that comprise the CT system. It also describes the major
   features of these elements. Other documents, cited in the References,
   establish requirements for these CT system elements and describe
   their operation in greater detail.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on July 30, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors. All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1. Introduction...................................................2
      1.1. Requirements Language.....................................5
   2. Beneficiaries of CT............................................6
   3. The Elements of the CT Architecture............................7
      3.1. Logs.....................................................10
      3.2. CT-aware Certification Authorities (CAs).................11
      3.3. Monitors.................................................12
      3.4. CT-aware Subjects (TLS web servers)......................13
      3.5. CT-aware TLS clients (web browsers)......................14
      3.6. Auditors.................................................15
   4. Security Considerations.......................................15
   5. IANA Considerations...........................................16
   6. References....................................................16
      6.1. Normative References.....................................16
      6.2. Informative References...................................17
   7. Acknowledgments...............................................17

1. Introduction

   Certificate transparency (CT) is a set of mechanisms designed to
   deter, detect, and facilitate remediation of certificate mis-issuance
   (as defined below). CT deters mis-issuance by encouraging CAs to
   publish the certificates that they issue in a set of publically-
   accessible logs. Each log uses a Merkle tree design to ensure that it
   is an append-only database, and the log entries are digitally signed
   by the log operator. Monitoring of logs detects mis-issuance.
   Remediation of mis-issuance is effected via certificate revocation.

   In the context of CT, the term mis-issuance refers to violations of
   either semantic or syntactic constraints associated with certificates
   [draft-trans-threat-analysis]. The fundamental semantic constraint
   for a (Web PKI) certificate is that it was issued to an entity that
   is authorized to represent the Subject name in the certificate. If
   any Subject Alternative Names (SANs) are present in the certificate,
   the entity also must be authorized to represent them. (It is also
   assumed that the entity requested the certificate from the CA that

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   issued it.) Throughout the remainder of this document we refer to a
   semantically mis-issued certificate as "bogus."

   A certificate is characterized as syntactically mis-issued if it
   violates syntax constraints associated with the class of certificates
   that it purports to represent. Syntax constraints for certificates
   are established by certificate profiles, and typically are
   application-specific. For example, certificates used in the Web PKI
   environment might be characterized as domain validation (DV) or
   extended validation (EV) certificates.  Certificates issued for use
   by applications such as IPsec or S/MIME have different syntactic
   constraints from those issued in the Web PKI context. Throughout the
   remainder of this document we refer to a syntactically mis-issued
   certificate as "erroneous." From a security perspective, erroneous
   certificates are not perceived as being as significant a concern as
   bogus certificates.

   As noted above, CT deters mis-issuance by encouraging CAs to log the
   certificates that they issue. A CT log is a publicly auditable,
   append-only, database of issued certificates [6962-bis] based on a
   binary Merkle hash tree [Merkle]. Each CT log operates in a fashion
   that enables external entities (Auditors) to detect inconsistent
   behavior. As a result, logs need not be operated by trusted (third)
   parties. Some forms of log misbehavior require comparing information
   gleaned from multiple sources, e.g., using mechanisms such as the
   ones described in [Gossip]. If an Auditor detects misbehavior by the
   log, it will notify Monitors (described below) and Browser Vendors
   that it serves. In turn, the Monitors and Browser Vendors are
   expected to cease relying onlogs that repeatedlymisbehave in a
   fashion indicative of malice. (Ultimately, what constitutes malicious
   misbehavior will be determined by Monitors and Browser Vendors, and
   thus is outside the scope of this document.)

   A bogus certificate that has been logged will be detected by an
   entity (a Monitor) that observes the log and that has knowledge of
   all legitimate certificates issued to the set of certificate Subjects
   that it serves. If a Monitor detects a log entry for a certificate
   that is inconsistent with the reference data for a Subject, the
   Monitor notifies the Subject. (A Subject may perform self-
   monitoring.) Thus Monitors implement the mis-issuance detection
   aspect of CT.

   CAs are presumed to be deterred from logging mis-issued certificates,
   because of the implied reputational consequences. (The assumption is
   that a CA that is detected repeatedly mis-issuing certificates will,
   in time, be blacklisted by the Browser Vendors (who control the set
   of CAs that are accepted by Browsers).

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   Revocation of a bogus/erroneous certificate is the primary means of
   remedying mis-issuance. A browser vendor may distribute a "blacklist"
   of mis-issued certificates or a bad-CA-list of certificates of CAs
   that have mis-issued certificates. Browsers may then use such lists
   to reject certificates on the blacklist, or certificates issued by
   CAs whose certificates are on the bad-CA-list. This form of
   revocation, although not codified in IETF standards, is also a means
   of remediation for mis-issuance. Throughout the remainder of this
   document, references to certificate revocation as a remedy encompass
   these and analogous forms of revocation.

   Figure 1 provides a top-level view of these elements of CT and their

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   +-----+     +----+
   | Log |<--->| CA |<**********************
   |     |     +----+                      *
   |     |        ^                        *
   |     |        *    +++++++++++++++++++ * ++++++
   |     |        v    v                   *      +
   |     |     +---------+                 *      +
   |     |<--->| Subject |<*************   *      +
   |     |     +---------+             *   *      +
   |     |       ^  ^  ^               *   *      +
   |     |       *  +  ******          *   *      +
   |     |       v  v       *          *   *      +
   |     |     +---------+  *          *   *      +
   |     |<--->| Browser |  *          *   *      +
   |     |     +---------+  *          *   *      +
   |     |        ^    ^    *          *   *      +
   |     |        *    ++++ * ++++++++ * + * +++  +
   |     |        v         v          *   *   +  +
   |     |     +----------------+      *   *   +  +
   |     |<***>| Browser Vendor |<***  *   *   +  +
   |     |     +----------------+   *  *   *   +  +
   |     |                          v  v   v   +  +
   |     |                        +---------+  +  +
   |     |<---------------------->| Monitor |  +  +
   |     |                        +---------+  +  +
   |     |                           ^  ^      +  +
   |     |                           +  *  +++++  +
   |     |                           v  v  v      +
   |     |                        +---------+     +
   |     |<---------------------->| Auditor |<+++++
   +-----+                        +---------+

   <---> Interface defined by CT
   <***> Interface out of scope for CT
   <+++> Proposed in Experimental Gossip Design

                 Figure 1 Elements of the CT Architecture

1.1. Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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2. Beneficiaries of CT

   There are three classes of beneficiaries of CT: certificate Subjects,
   TLS Clients, and Certification Authorities (CAs). In the initial
   context of CT, the Web PKI, Subjects are web sites and TLS Clients
   are Browsers employing HTTPS to access these web sites. CAs are the
   issuers of certificates used in the Web PKI context.

   A certificate Subject benefits from CT because CT enables Monitors to
   detect certificates that have been mis-issued in the name of that
   Subject. A Subject learns of a bogus/erroneous certificate (issued in
   its name), via a Monitor, as noted above. (The Monitor function may
   be provided by the Subject itself, i.e., self-monitoring, or by a
   third party trusted by the Subject.) When a Subject is informed of
   certificate mis-issuance by a Monitor, the Subject is expected to
   request/demand revocation of the bogus/erroneous certificate by the
   issuing CA and/or by the browser vendors (if the CA refuses to revoke
   the certificate).

   A Subject also may benefit from the Monitor element of CT even if the
   Subject's legitimate certificate(s) has(have) not been logged. If the
   bogus/erroneous certificate is logged and if a Monitor has been
   provided with reference data from the Subject, then monitoring of
   logs for certificates issued in the Subject's name suffices to detect
   an instance of mis-issuance targeting the Subject. (If a CA operates
   a Monitor on behalf of its Subjects, then the CA has the requisite
   information to detect bogus/erroneous certificates in logs that it

   A TLS client (Browser) benefits from CT if the TLS client rejects a
   mis-issued certificate, i.e., treats the certificate as invalid. A
   TLS client is protected from accepting a mis-issued certificate if
   that certificate is revoked, and if the TLS client checks the
   revocation status of the certificate. (A TLS client also is protected
   if a browser vendor "blacklists" a certificate or a CA as noted
   above.) A TLS client also may benefit from CT if the client validates
   a Signed Certificate Timestamp (SCT) [6962-bis] associated with a
   certificate, and rejects the certificate if the SCT is invalid.

   CAs are also CT beneficiaries. If one CA issues a legitimate
   certificate to a Subject, and another CA issues a bogus certificate,
   the second certificate can be detected by a Monitor (if the bogus
   certificate has been logged). In this fashion the CA that issued the
   legitimate certificate benefits, since the bogus certificate is
   detected and, presumably revoked. Even the CA that issued the bogus
   certificate is a potential beneficiary. If the bogus certificate was
   issued as a result of an error or an (undetected) attack, CT can help

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   the CA become aware of the error or attack and act accordingly. This
   is presumed to be beneficial to the reputation of this CA.

3. The Elements of the CT Architecture

   There are six elements of the CT architecture: logs, CAs, Monitors,
   Subjects, TLS clients (especially browsers and browser vendors), and
   Auditors. CAs, Subjects, and TLS clients are pre-existing elements
   affected by CT if they choose to participate. Because not all CAs,
   Subjects, and TLS clients may choose to participate in CT, these
   elements are qualified as "CT-aware" to distinguish them from
   existing instances of these types of Web PKI elements. Logs,
   Monitors, and Auditors are new elements introduced by CT and thus
   they are intrinsically CT "aware". Figure 2 shows how all of these
   elements interact with the central CT element, the log. Figure 3
   shows how the pre-existing elements interact with one another under
   CT. Figure 4 shows the interactions of monitors and auditors that are
   not covered by Figure 2.

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   +-----+                                     +---------------+
   | Log |<-  add-chain or add-pre-chain  -----| CA or Subject |
   |     |--  SCT for the new entry  --------->|               |
   |     |<-  get-proof-by-hash  --------------|               |
   |     |--  inclusion proof for the entry  ->|               |
   |     |                                     +---------------+
   |     |                       +---------+
   |     |<-  get-sth [1]  ------| Monitor |
   |     |--  current STH  ----->|         |
   |     |<-  get-entries [1]  --|         |
   |     |--  log entries  ----->|         |
   |     |                       +---------+
   |     |                             +---------+
   |     |<-  get-proof-by-hash [2]  --| Browser |
   |     |--  inclusion proof [2]  --->|         |
   |     |                             +---------+
   |     |                        +----------------+
   |     |<-  get log metadata  --| Browser Vendor |
   |     |--  log metadata  ----->|                |
   |     |                        +----------------+
   |     |                               +-----------------+
   |     |                               |     Auditor     |
   |     |                               |+---------------+|
   |     |<-  get-sth [1]  --------------|| MMD checking  ||
   |     |--  current STH  ------------->||               ||
   |     |<-  get-entries [1]  ----------||               ||
   |     |--  log entries  ------------->||               ||
   |     |                               |+---------------+|
   |     |<-  get-sth  ------------------|| STH frequency ||
   |     |--  current STH  ------------->|| checking      ||
   |     |                               |+---------------+|
   |     |<-  get-sth [1]  --------------|| Append-only   ||
   |     |--  current STH  ------------->|| checking      ||
   |     |<-  get-entries [1]  ----------||               ||
   |     |--  log entries  ------------->||               ||
   |     |<-  get-sth-consistency [3]  --||               ||
   |     |--  consistency proof  ------->||               ||
   +-----+                               |+---------------+|

    [1] The get-sth operation is performed periodically, and get-entries
   is performed each time a new STH is available.
    [2] See Section 3.5 for privacy and performance caveats.
    [3] If the Auditor stores copies of all Log entries, then this
   operation is not needed.

                     Figure 2 Interactions with a Log

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   +---------+                                               +---------+
   | Browser |--  log metadata[1]  ------------------------->| Browser |
   | Vendor  |--  revocation information[1]  --------------->|         |
   |         |                                               |         |
   |         |                 +---------+                   |         |
   |         |  / request   \--| Subject |                   |         |
   |         |  | to        |  |         |                   |         |
   |         |  | blacklist |  |         |                   |         |
   |         |  | a CA or   |  |         |                   |         |
   |         |<-\ EE cert   /  |         |                   |         |
   +---------+                 |         |                   |         |
                               |         |                   |         |
   +----+                      |         |                   |         |
   | CA |  / certificate \-----|         |                   |         |
   |    |<-\ request     /     |         |                   |         |
   |    |--  certificate[2]  ->|         |                   |         |
   |    |                      |         |                   |         |
   |    |  / request       \---|         |                   |         |
   |    |  | revocation of |   |         |                   |         |
   |    |<-\ a certificate /   |         |                   |         |
   +----+                      |         |                   |         |
                               |         |  / TLS        \---|         |
                               |         |<-\ connection /   |         |
                               |         |--  certificate  ->|         |
                               |         |--  SCT[3]  ------>|         |
                               |         |<-  HTTPS  ------->|         |
                               +---------+                   +---------+

    [1] Not subject to standardization.
    [2] Optionally including SCTs in an extension.
    [3] Optional, via an OCSP response or in a TLS extension.

               Figure 3 Interfaces of Pre-existing Elements

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   +---------+                                             +---------+
   | Monitor |<-  establish a business relationship [1]  ->| Subject |
   |         |<-  list of protected subject names  --------|         |
   |         |  / per protected subject name, a  \---------|         |
   |         |<-\ list of acceptable public keys /         |         |
   |         |                                             +---------+
   |         |
   |         |                                        +----+
   |         |--  notification of mis-issuance  --+-->| CA |
   |         |                                    |   +----+
   |         |                                    |
   |         |                                    |   +----------------+
   |         |                                    +-->| Browser Vendor |
   |         |                                        +----------------+
   |         |
   |         |                                            +---------+
   |         |<-  notification of log mis-behavior [2]  --| Auditor |
   +---------+                                            +---------+

    [1] In the case of a self-monitor, the business relationship is
   trivial - the Subject and Monitor are the same organization.
    [2] An entity performing the Monitor function MAY also choose to
   implement some of the Auditor functions. In that case the
   Monitor/Auditor interface is trivial. If the Auditor is separate, we
   note that there is no interface defined at the time of this writing.

                  Figure 4 Monitor and Auditor Interfaces

3.1. Logs

   Logs are the central elements of the CT architecture. Logging of
   certificates enables Monitors to detect mis-issuance and,
   subsequently, to trigger Subjects to issue revocation requests to CAs
   and/or browser vendors and to notify CAs and browser vendors
   directly. Logging also deters mis-issuance, as noted above. The
   interfaces to a log are defined in [6962-bis], as are the details of
   how a log operates.

   Briefly, a certificate chain (that must be verifiable under a trust
   anchor acceptable to the log) is submitted to a log by a CA, Subject
   or other party. The log creates an entry for the terminal certificate
   in the chain, and returns this Signed Certificate Timestamp (SCT) to
   the submitter. The SCT can be conveyed to a browser in one of three
   ways: it can be incorporated into a certificate by the CA that issues
   it, as described later. (A CA also may submit a so-called "pre-
   certificate" to a log, to acquire an SCT for inclusion in the
   certificate, prior to signing the certificate.) It also can be

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   conveyed explicitly in the TLS handshake or in OSCP data generated by
   a CA. The SCT is a token that can be verified by browsers to
   establish, to first order, that a certificate has been logged. See
   [6962-bis] for additional details of SCTs.

   All clients that interact with a log require access to metadata
   associated with each log upon which they rely. This metadata includes
   the URL and public key for the log, the list of trust anchors
   accepted by the log, the hash and signature algorithms employed, etc.
   Log metadata is made available to log clients via out of band means
   that are generally outside the scope of the CT specifications. In the
   Web PKI context, CT assumes that browser vendors will make the
   necessary log metadata available to browsers via the same mechanisms
   used to convey trust anchor (and vendor-managed revocation data). Log
   metadata provided via this channel is not mutable by log operators
   (since it is part of browser configuration data), with one exception.
   When a log ceases operation, it publishes its final STH, enabling
   clients to verify previous log entries and to detect any
   (unauthorized) additions to the log. See [6962-bis] for additional

   An open question is how other log clients receive the metadata they
   require to interact with the log in a predictable fashion. For
   example, a log may elect to check the syntax of certificates relative
   to [RFC5280], or it may skip some of all of the checks specified
   there. Absent a way to determine what checks a log will perform on
   submitted certificates, a CA (or other submitter) has no way to know
   whether a submitted certificate will be accepted by a given log.
   Similarly, a Monitor needs to acquire log metadata so that the
   Monitor can locate the log and verify the signatures on log entries.

3.2. CT-aware Certification Authorities (CAs)

   A (CT-aware) CA interacts with a log to submit a certificate (or a
   pre-certificate) to create a log entry. (Most logged certificates are
   expected to be end-entity certificates, each associated with the web
   site that it represents. However, it also is possible to log a CA
   certificate under certain circumstances. See Section 3.2.3 of [6962-
   bis].) The pre-certificate capability is offered to facilitate rapid
   deployment of CT. It has the advantage that web sites need not make
   any software changes to acquire one or more SCTs, because the SCTs
   are embedded in the certificate itself. There is, however, a downside
   of embedding SCTs in certificates. If a log that provided an SCT is
   compromised or otherwise becomes unacceptable to browsers and
   Monitors, the certificate associated with that SCT will have to be
   re-issued with a replacement SCT. Thus, in the long term, other
   options for conveying an SCT, i.e., via the TLS handshake or in an

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   OCSP response (perhaps "stapled" into the handshake [RFC6961]), are
   preferred [TLS-Server].

   A CA also may submit a "name-redacted" pre-certificate to a log. A
   name-redacted pre-certificate includes one or more "?" labels in lieu
   of DNS name components. See Section 4.2 of [6962-bis] for more
   details. Name-redaction is a feature of CT designed to enable an
   organization to request a CA to log its certificates without
   revealing all of the DNS name components in the certificate that will
   be matched to the log entry. This is an attractive feature for
   organizations that want to benefit from CT without revealing internal
   server names as a side effect of logging. An end-entity certificate
   that is to be treated as logged via this mechanism contains a
   critical (X.509v3) extension that indicates which labels have been
   redacted in the log entry. This extension is needed to enable TLS
   clients and Monitors to match a received certificate against the
   corresponding log entry in an unambiguous fashion. See Section <TBD>
   of [CA-Subject] for more details.

   The CT architecture does not mandate a specific number of SCTs that
   should be associated with a certificate. Browser vendors might
   establish requirements for the minimum number of associated SCTs in
   different contexts, but such requirements are outside the scope of
   the CT architecture.

   [CA-Subject] describes the requirements imposed on CT-enabled CAs.

3.3. Monitors

   The primary role of a Monitor is to observe a set of logs, looking
   for log entries of interest. A Subject may act as a self-monitor, or
   may make use of the services of a third-party Monitor, as noted

   In the self-monitoring context, log entries of interest are ones that
   contain a Subject or Subject Alternative Name (SAN) associated with
   the Subject's web site(s). (Name-constrained CA certificates and
   wildcard certificates also have to be examined to detect certificates
   that would match the end-entity certificates associated with a
   Subject's web sites.) Whenever a certificate of interest is detected,
   the Subject compares it with the public key information associated
   with its certificate(s). If there is a mismatch, this indicates that
   this logged certificate was mis-issued. The Subject contacts the CA
   that issued the certificate (using the Issuer name in the

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   certificate) and requests revocation of the mis-issued certificate,
   to resolve the problem. (The means by which a Subject determines how
   to contact a CA based on the issuer name is outside the scope of the
   CT architecture.) The means by which a Subject determines which set
   of logs to watch also is outside the scope of the CT architecture. It
   is anticipated that there will be a small number of logs that are
   widely used, and that the metadata for these logs will be available
   from browser vendors.

   A third-party Monitor watches for certificates of interest to its
   clients. Each client of a third party Monitor supplies the Monitor
   with a list of Subject names and SANs associated with the client's
   web site(s), and public key information associated with each name.
   (As a special case, if a CA offers a Monitor service to its clients,
   then the CA/Monitor already has this information.) The Monitor
   watches a set of logs looking for entries that match the client
   certificates of interest. If it detects an apparent mis-issued
   certificate, the Monitor contacts the client and forwards the log
   entry, along with log metadata. The client (Subject) then follows the
   procedure noted above to request revocation of the mis-issued

   Note that a Monitor does not try to detect mis-behavior by a log.
   That is the responsibility of an Auditor. [Monitor-Auditor] defines
   the requirements for a Monitor (self of third-party) and discusses
   additional operational details.

   Note also that CT does not include any mechanisms designed to detect
   misbehavior by a Monitor. A self-Monitor does not require such
   mechanisms; Subjects who elect to rely upon third-party Monitors
   would benefit from such mechanisms. See [Monitor-Auditor] for the
   requirements imposed on Monitors by CT and for a more detailed
   description of how a Monitor operates.

3.4. CT-aware Subjects (TLS web servers)

   A (CT-aware) Subject (e.g., a web site operator) can submit its
   certificate(s) to a log, and acquire an SCT for each certificate it
   submits (see Section 4.1 of [6962-bis]). There are three reasons why
   a Subject may choose to log its own certificate(s): (1) its CA did
   not embed an SCT in the certificate(s) it issued to the Subject, (2)
   the Subject wants to acquire SCTs from additional logs, or (3) the
   Subject wants the flexibility offered by conveying SCTs (from logs of
   its choosing) in the TLS handshake. [CA-Subject] describes the
   requirements imposed on Subjects for delivery of SCTs to CT-aware TLS

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   Every Subject should either perform self-monitoring, or become a
   client of a third-party Monitor so that bogus certificates issued in
   the name of the Subject will be detected. When a Subject is notified
   of a bogus certificate issued in its name, the Subject contacts the
   CA that issued the certificate and requests that it be revoked, using
   whatever mechanisms the CA provides for such requests. The Subject
   may also contact browser vendors and ask that they put the
   certificate on a blacklist of mis-issued certificates or put the CA's
   certificate on a bad-CA-list, if the CA refuses to revoke the bogus
   certificate. [CA-Subject] describe the requirements established for
   for CT-aware Subjects.

3.5. CT-aware TLS clients (web browsers)

   As noted in Section 2, a TLS client can benefit from CT even without
   actively participating. A Monitor will detect a mis-issued, logged
   certificate and notify the affected Subject. The Subject will, in
   turn attempt to trigger revocation by the CA that mis-issued the
   certificate in question, ultimately asking browser vendors to
   blacklist the certificate (or the CA) if revocation is not effected.
   Thus a TLS client that processes certificate revocation status data,
   e.g., CRLs, OCSP responses, will be protected from bogus certificates
   that have been logged, detected, and revoked.

   If a TLS client required that every certificate it accepted was
   accompanied by an SCT, the client could have some confidence that the
   certificate had been logged. This would increase confidence that the
   certificate, if it were mis-issued, would have been revoked. However,
   there are two problems with mandating that every TLS client reject
   (treat as invalid) any certificate that is not accompanied by an SCT.
   First, such behavior does not accommodate incremental deployment of
   CT. Second, the mere presence of an SCT is not a guarantee that the
   certificate has been logged.

   To have high confidence that a certificate has been logged, a TLS
   client would have to verify that a log entry exists for the
   certificate. This requires acquisition of an inclusion proof from the
   log (see Section 4.5 of [6962-bis]). Requesting an inclusion proof
   directly from a log for a certificate discloses to a log that the TLS
   client is interested in the certificate in question. For a browser,
   this would disclose which web sites a user was visiting, a potential
   privacy concern for many users. Also, the data acquisition and
   processing might pose an unacceptable burden for some TLS clients,
   (e.g., browsers), and might not be performed in realtime anyway. Thus
   CT-aware TLS clients are not expected to fetch an inclusion proof in
   realtime, e.g., during TLS connection establishment. Such clients
   also are not expected to reject a certificate that has no associated

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   SCT, because there is no plan for incremental deployment of CT that
   accommodates such rejection in a backwards compatible fashion.
   Nonetheless, if an SCT is provided with a certificate, a CT-aware TLS
   client could verify the signature and the SCT data for the
   certificate in question. If performing these checks would not impose
   an undue burden on the TLS client, the checks would help detect
   errors in SCTs and provided feedback to log operators (via Subjects).

   A TLS client that is a browser might discriminate against a
   certificate presented for a web site if the certificate is not
   accompanied by an SCT, e.g., providing an indication of this via the
   user interface. See [browser-vendor] for the requirements established
   for CT-aware browsers and browser vendors.

3.6. Auditors

   Auditors perform checks intended to detect mis-behavior by logs.
   There are four log behavior properties that Auditors check:

   1. The Maximum Merge Delay (MMD)

   2. The STH Frequency Count

   3. The append-only property

   4. The consistency of the log view presented to all query sources

   The first three of these checks are easily performed using existing
   log interfaces and log metadata (see [6962-bis]). For example, an
   Auditor could submit a certificate to a log and request an STH after
   the indicated MMD, to verify that the log is achieving its advertised
   MMD. The last check is more difficult to perform because it requires
   a way to share log responses among a set of CT elements, perhaps
   including browsers, web sites, Monitors, and Auditors, e.g., using
   so-called gossiping [Gossip]. There is as yet no standard for
   gossiping and thus the last check is NOT part of Auditor requirements
   at this time. See [Monitor-Auditor] for additional details of Auditor

4. Security Considerations

   CT is a system created to improve security for X.509 public key
   certificates, especially in the Web PKI context. An attack analysis
   [draft-trans-threat-analysis] examines the types of attacks that can
   be mounted against CT, to effect mis-issuance, and how CT addresses
   (or fails to address) each type of attack. That analysis is based on
   the architecture described in this document, and thus readers of this

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   document are referred to that one for a thorough discussion of the
   security aspects of CT. Briefly, CT logs represent a viable means of
   deterring semantic mis-issuance of certificates. Monitors are an
   effective way to detect semantic mis-issuance of logged certificates.
   The CT architecture enables certificate Subjects to request
   revocation of mis-issued certificates, thus remedying such mis-
   issuance. Residual vulnerabilities exist with regard to some forms of
   log and Monitor misbehavior, because the architecture does not
   include normative means of detecting such behavior.  The current
   design also does not ensure the ability of Monitors to detect
   syntactic mis-issuance of certificates. This is because provisions
   for asserting the type of certificate being issued, for inclusion in
   an SCT, have not been standardized.

5. IANA Considerations


6. References

6.1. Normative References

   [Merkle] Merkle, R. C. (1988). "A Digital Signature Based on a
             Conventional Encryption Function." Advances in Cryptology -
             CRYPTO '87. Lecture Notes in Computer Science 293. p. 369

   [6962-bis] Laurie, B., Langley, A., Kasper, E., Messeri, E., and R.
             Stradling, "Certificate Transparency," draft-ietf-trans-
             rfc6962-bis-10 (work in progress), October 2015.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
             (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
             Extensions: Extension Definitions", RFC 6066, January 2011.

   [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
             Galperin, S., and C. Adams, "X.509 Internet Public Key
             Infrastructure Online Certificate Status Protocol - OCSP",
             RFC 6960, June 2013.

   [RFC6961] Pettersen, Y., "The Transport Layer Security (TLS) Multiple
             Certificate Status Request Extension," RFC 6961, June 2013.

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6.2. Informative References

   [draft-trans-threat-analysis] Kent, S., "Attack Model and Threat for
             Certificate Transparency," draft-ietf-trans-threat-
             analysis-03 (work in progress), October 2015.

   [Gossip] Nordberg, L., Gillmor, D., and Ritter, T., "Gossiping in
             CT," draft-ietf-trans-gossip-01 (work in progress), October

   [Monitor-Auditor] <TBD>

   [CA-Subject] <TBD>

   [browser-vendor] <TBD>

7. Acknowledgments


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   Authors' Addresses

   Stephen Kent
   BBN Technologies
   10 Moulton St.
   Cambridge, MA  02138


   David Mandelberg
   BBN Technologies
   10 Moulton St.
   Cambridge, MA  02138


   Karen Seo
   BBN Technologies
   10 Moulton St.
   Cambridge, MA  02138


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