Public Notary Transparency S. Kent
Internet Draft D. Mandelberg
Intended status: Standards Track K. Seo
Expires: April 2016 BBN Technologies
October 2, 2015
Certificate Transparency (CT) System Architecture
draft-kent-trans-architecture-00.txt
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Abstract
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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 CT. It also describes the critical
features of these elements. Other documents describe in detail the
operation of these elements.
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. Certification Authorities (CAs)..........................11
3.3. Monitors.................................................12
3.4. Subjects (TLS web servers)...............................14
3.5. TLS clients (web browsers)...............................15
3.6. Auditors.................................................16
3.6.1. Checking MMD, STH Frequency Count and Append-only
property...................................................16
3.6.2. Checking for Consistency of Log Views...............17
4. Security Considerations.......................................18
5. IANA Considerations...........................................18
6. References....................................................18
6.1. Normative References.....................................18
6.2. Informative References...................................19
7. Acknowledgments...............................................19
Appendix A. Log Checking Algorithms (Normative)..................20
A.1. Append-only Check........................................20
A.2. Inclusion Proof Verification.............................21
A.3. Verifying consistency between two STHs...................21
A.4. Verifying log root hash using log entries................22
Appendix B. SCT Transmission (Normative).........................24
Appendix C. Matching an SCT to a Certificate.....................26
1. Introduction
Certificate transparency (CT) is a set of mechanisms designed to
deter, detect, and facilitate remediation of certificate mis-
issuance. CT deters mis-issuance by encouraging CAs to publish the
certificates that they issue in a publically-accessible log. The 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.
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The term mis-issuance refers to violations of either semantic or
syntactic constraints associated with certificates. 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, in addition to all Subject Alternative names (SANs), if
any are present. (It is also assumed that the entity requested the
certificate from the CA that 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."
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 [cite 6962-bis] based on
a binary Merkle hash tree [Merkle]. Each CT log operates in a fashion
that enables anyone to detect inconsistent behavior, thus logs need
not be operated by trusted (third) parties. (Detection of
inconsistent behavior by a log is the function of a CT Auditor. 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 and Subjects that have registered with it.) A
(semantically) mis-issued certificate that has been logged can be
detected by any entity that monitors the log and that has knowledge
of all legitimate certificates issued to the named certificate
Subject. Thus CAs are 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 may be
shunned by the community.)
CT enables detection of mis-issuance via the Monitor function [cite
Monitor]. A CT Monitor examines all entries from a set of logs and
compares these entries to reference data for a set of one or more
Subjects. The reference data consists, at a minimum, of a list of
Subject and Subject Alternative Names and the pubic key information
associated with each, supplied by the Subject. If a Monitor detects a
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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. In the Web PKI context, a Subject is a web
site. Monitors implement the mis-issuance detection aspect of CT.
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 for which
the issuing CA is 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.
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+-----+ +----+
| Log |<--->| CA |<*********************
| | +----+ *
| | ^ *
| | * *
| | v *
| | +---------+ *
| |<--->| Subject |<************* *
| | +---------+ * *
| | ^ ^ * *
| | * ******* * *
| | v * * *
| | +---------+ * * *
| |<--->| Browser | * * *
| | +---------+ * * *
| | ^ * * *
| | * * * *
| | v v * *
| | +----------------+ * *
| |<***>| Browser Vendor |<*** * *
| | +----------------+ * * *
| | v v v
| | +---------+
| |<---------------------->| Monitor |
| | +---------+
| | ^
| | *
| | v
| | +---------+
| |<---------------------->| Auditor |
+-----+ +---------+
Legend:
<---> Interface defined by CT
<***> Interface out of scope for CT
Figure 1 Elements of the CT Architecture
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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,
relying parties (RPs), and Certification Authorities (CAs). In the
initial context of CT, the Web PKI, Subjects are web sites and RPs
are browsers employing HTTPS to access these web sites. CAs are
issuers of certificates used in the Web PKI context.
A certificate Subject benefits from CT because CT helps (Monitors)
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 CT 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.
A Subject also may benefit from the Monitor function of CT even if
the Subject's legitimate certificate(s) has(have) not been logged.
Monitoring of logs for certificates issued in the Subject's name
suffices to detect an instance of mis-issuance targeting the Subject,
if the bogus/erroneous certificate is logged.
A TLS client (e.g., a 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 CT Monitoring (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
the CA become aware of the error or attack and act accordingly. This
is presumed to be beneficial to the reputation of this CA.
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3. The Elements of the CT Architecture
There are six elements of the CT architecture: logs, CAs, Monitors,
Subjects, TLS clients (and vendors of the client's software), and
Auditors. (CAs, Subjects, and TLS clients are pre-existing elements
affected by CT; logs, Monitors, and Auditors are new elements
introduced by CT.) Figure 2 shows how all of these elements interact
with the central 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 Vendor |-- log metadata[1] ------------------>| Browser |
| |-- revocation information[1] -------->| |
+----------------+ | |
| |
+----+ +---------+ | |
| CA | / certificate \-----| Subject | | |
| |<-\ request / | | | |
| |-- certificate[2] ->| | | |
+----+ | | | |
| | | |
| | / 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 revocation requests to CAs and/or browser
vendors. 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 interested party. The log creates an entry for the chain,
hashing it with information from other log submissions. The log
returns a Signed Certificate Timestamp (SCT) to the submitter. The
SCT can be conveyed to RPs in one of three ways: it can be
incorporated into a certificate by the CA that issues it; it can be
conveyed via the TLS handshake between an RP and a web site; or it
can be embedded into an OCSP response sent to an RP. (Only the issuer
of a certificate can submit a so-called "pre-certificate" to a log,
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to acquire an SCT for inclusion into the certificate, prior to
signing the certificate.) The SCT is a token that can be verified by
RPs (and Monitors) to establish, to first order, that a certificate
has been logged. See [6962-bis] for additional details.
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 RPs via out of band means that are
outside the scope of the CT specifications. In the Web PKI context,
CT assumes that browser vendors will make log metadata available to
browsers via the same mechanisms used to convey trust anchor (and
vendor-managed revocation data). Thus log metadata 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 details.
3.2. Certification Authorities (CAs)
A 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 not acceptable to RPs and Monitors,
the certificate associated with that SCT may have to be re-issued
with a replacement SCT. Thus, in the long term, the options of
conveying an SCT via the TLS handshake or in an OCSP response
(perhaps "stapled" into the handshake [RFC6961], are preferred.
However, transmission of an SCT via the TLS handshake requires
changes to web site software to acquire and insert SCTs. Transmission
via an OCSP response requires that either RPs fetch such responses
(which appears not to be the norm), or that a web site passes the
OCSP data via the TLS handshake (and that the OCSP signer be prepared
to generate this modified form of response).
A CA 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. Name-redaction is a feature of CT designed to
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enable an organization to log 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 MUST contain 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 3.2.2 of [6962-bis]
for more details.
The CT architecture does not mandate a specific number of SCTs that
should be associated with a certificate. TLS clients and Monitors
might establish requirements for the minimum number of associated
SCTs in different contexts, but such requirements are outside the
scope of the CT architecture.
After an SCT has been returned, it is RECOMMENDED that a CA verify
that a certificate (or pre-certificate) that it has submitted has in
fact been logged. To perform this verification, the CA waits for an
interval dictated by the Maximum Merge Delay (MMD) associated with
the log, and then requests both a Signed Tree Head (STH) and an
inclusion proof. The CA SHOULD then verify the data returned by the
log, as described in Sections 3.6, 4.3 and 4.5 of [6962-bis].
<we plan to insert much of Rob's text on redacted certificates here,
since that text specifies CA behavior for CT.>
3.3. Monitors
The primary role of a Monitor is to watch 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.
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 the Subject's 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 certificate), and requests revocation of the mis-issued
certificate, to resolve the problem. (The means by which a Subject
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determines how to contact a CA based on the issuer name is outside
the scope of this specification.) The means by which a Subject
determines which set of logs to watch is outside the scope of the CT
specifications. 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 (see Section 3.5 below).
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.
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
certificate. It is RECOMMENDED that third-party Monitors make public
the set of logs that they watch, and the set of third-party Auditors
they rely upon, to help clients decide when choosing a third-party
Monitor.
A Monitor (self or third-party) that is "watching" a log periodically
queries the log to determine if there is a new STH, using the get-sth
interface (see Section 4.3 of [6962-bis]). When a new STH is
detected, the Monitor then uses the get-entries interface to the log
(see Section 4.7 of [6962-bis] to retrieve all new log entries
(relative to the previous STH acquired by the Monitor). (This command
requires the Monitor to indicate the start and end entries, by index,
data that is provided by get-STH.) The Monitor examines each log
entry to determine if it is of interest, as per the definition above.
(This procedure applies to wildcard certificate log entries as well
as to certificates with fully-specified DNS names.)
If a Monitor encounters a log entry for a name-redacted certificate
(Section 3.2.2 in [6962-bis]) it MUST evaluate whether that
certificate is of interest. To do so, the Monitor compares the non-
redacted part of the name in the log entry against the list of names
of interest to this Monitor. The redacted name, is transformed into a
wildcard name by substituting "*" for "?" name components. The
resulting name is then compared to the list of names of interest to
the Monitor. If a match is found, the Monitor then compares the list
of public keys for the name. If the public key in the log entry does
not match any in this list, the Subject associated with the specified
name is notified.
A Monitor MAY retain its own copies of log entries, but it is not
required to do so. Local caching of log entries would be useful for a
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third party log that acquires a new client, since the Monitor could
examine the older entries for certificates that are now of interest.
For a self-Monitor, maintaining a cache of old log entries may not be
useful and may represent a storage burden.
Note that the Monitor function, as described above, does not try to
detect mis-behavior by a log. That is an Auditor function, which is
described below. A Monitor MAY incorporate some or all of the Auditor
functions; it MAY make use of third-party Auditors, or it may eschew
responsibility for auditing. A third-party Monitor SHOULD make known
to its clients which, if any, Auditor functions it offers to its
clients. The means by which Subjects determine the set of functions
provided by a third-party Monitor is not defined by this document; it
will be described in a Monitor API specification [cite Monitor].
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.
3.4. Subjects (TLS web servers)
A Subject (e.g., a web site operator) MAY submit its certificate(s)
to a log, and acquire an SCT for each certificate it submits, using
the add-chain log interface (see Section 4.1 of [6962-bis]). There
are three reasons for a Subject 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 (including via
OCSP). Appendix B describes the requirements imposed on Subjects for
delivery of SCTs to CT-enabled TLS clients.
When a Subject has acquired an SCT, it SHOULD perform the same checks
described for a CA (see Section 3.2 above), to verify that the log
has created an entry for each submitted certificate.
It is RECOMMENDED that every Subject either perform self-monitoring,
or become a client of a third-party Monitor (see Section 3.3 above).
When a Subject becomes aware of a mis-issued certificate (based on
the Monitor function), the Subject confirms that the log entry
conflicts with one of its certificates. (In this context, a conflict
arises if the name in a Subject's certificate matches or is
encompassed by the name in the log entry, and the certificate was not
issued to the Subject.) If a conflict is detected, the Subject
contacts the CA that issued the certificate and requests that it be
revoked, using whatever mechanisms the CA provides for such requests.
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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.
3.5. 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. If the CA refuses to revoke the certificate,
and it is acting "improperly", then the Subject could notify browser
vendors who could blacklist the CA or the certificate in question,
effecting revocation via other means. Thus a TLS client that
processes certificate revocation status data, e.g., CRLs, OCSP
responses, can be protected from bogus certificates that have been
logged, detected, and revoked. Appendix B describes the requirements
imposed on a CT-enabled browser to signal its capability and to
accept SCTs conveyed via any of the three methods defined there.
Appendix C describes the process a CT-enabled browser uses to match
an SCT to a certificate if the SCT is not embedded in the
certificate.
If a TLS client required that a 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, will 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. (A typical TLS client, i.e., a browser, would use the
log metadata provided by the browser vendor to contact one of more
logs, and to verify signed data from each log.) This requires
acquisition of additional data from each log, i.e., an inclusion
proof (see Section 4.5 of [6962-bis]). Requesting an inclusion proof
for a certificate discloses to a log that the RP is interested in the
certificate in question. For a browser, this would disclose which web
sites a user was visiting (if the web sites provided SCTs), a
potential privacy concern for many users. Also, the data acquisition
and processing may pose an unacceptable burden for some TLS clients,
(e.g., browsers), and thus may not be performed in realtime anyway.
Thus a TLS client is NOT REQUIRED to reject a certificate when no
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associated SCT is available. Nonetheless, if an SCT is provided with
a certificate, its signature SHOULD be verified and the SCT data
compared to the certificate in question, if doing so would not impose
an undue burden on the TLS client. (Such checks MAY be performed in
realtime, or may be deferred. If the checks are deferred and they
fail, the client will know that the supplied SCT was bogus. The
client SHOULD retain this knowledge and reject a certificate
associated with a bogus SCT.) If the signature check fails or the SCT
does not correspond to the certificate in question, the certificate
is suspect and SHOULD be treated as invalid by the TLS client.
A TLS client that is a browser MAY 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.
The details of such discrimination are outside the scope of this
specification. However, such discrimination MUST NOT cause the
certificate to be treated as revoked/invalid, until such time as an
incremental deployment strategy (that is backwards compatible) is
defined and approved by the IETF.
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. 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., so-called gossiping [Gossip]. There is as yet no
standard for gossiping and thus the last check is NOT required of
Auditors at this time.
3.6.1. Checking MMD, STH Frequency Count and Append-only property
Checking that a log is behaving correctly with regard to MMD, STH
Frequency Count and Append-only property SHOULD be performed using
the algorithm described in Appendix A:
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1. The MMD for a log is the maximum time that may elapse between the
time that an SCT is issued and a log entry is created. When an
Auditor executes the algorithm in Appendix A, Step 7 enables it to
detect when the MMD has been exceeded for the certificate append
that triggered the new STH. The Auditor's polling period SHOULD be
chosen to be small relative to the MMD in order to maximize the
chance of successful detection of an MMD violation.
2. To prevent the use of an STH to identify an individual log client,
a log MUST NOT generate an STH more frequently than is declared in
the log metadata. To verify that a log is not violating this
guarantee, when an Auditor executes the algorithm in Appendix A,
Step 5 enables it to determine how long it has been since the STH
changed and to detect if this period is shorter than the minimum
required. The Auditor's polling period SHOULD be chosen to be more
frequent than the minimum frequency in order to maximize the
chance of successful detection of too frequent generation of STHs.
3. In order to verify the append-only property, an Auditor executes
the algorithm as described in Appendix A.1.
3.6.2. Checking for Consistency of Log Views
In order for an Auditor to verify that a log provides a consistent
view to all query sources, the Auditor needs to see the results of
queries to the log from a broad range of requesters. In principle
this could be accomplished using a gossip protocol that has the
following constraints:
1. TLS clients are not expected to interact directly with a Log for
performance and privacy reasons (see Section 3.5).
2. TLS clients generally do not communicate directly with one another
(with a few exceptions). As such, a gossip protocol would be
easier to deploy if it does not rely on direct communication among
TLS clients.
3. If TLS clients have to acquire and distribute CT information about
the web sites they visit, this should not overburden the browsers,
Subject web sites, or Logs.
4. There needs to be a mechanism for Auditor(s) to inform Monitors
(and maybe browser vendors) about mis-behaving logs. The Auditors
could be standalone entities selected by Monitors and browsers,
(more properly, browser vendors), as a way to obtain information
about misbehaving logs. Alternatively, these parties could operate
their own Auditors.
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5. Browser vendors need to be able to update the blacklists of mis-
issued certificates and the bad-log-lists used by their browsers.
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
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
<TBD>
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-08 (work in progress), July 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.
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[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.
6.2. Informative References
[draft-trans-threat-analysis] Kent, S., "Attack Model for Certificate
Transparency," draft-ietf-trans-threat-analysis-01 (work in
progress), June 2015.
[Gossip] Nordberg, L., Gillmore, and Ritter, T., "Gossiping in CT,"
draft-ietf-trans-gossip-00 (work in progress), August 2015.
[Auditor] ?? work in progress.
[Monitor] ?? work in progress.
7. Acknowledgments
Some of the text included in this document (including the algorithms
described in Appendices A and B), was produced by B. Laurie, A.
Langley, E. Messeri, and R. Stradling in earlier versions of [6962-
bis]. It has been extracted and edited for use here.
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Appendix A. Log Checking Algorithms (Normative)
This appendix specifies nominal algorithms for use in performing
various checks based on log data. An Auditor, Monitor, or TLS client,
performing a specified check MUST implement an algorithm equivalent
to the one described here, i.e., an algorithm that yields the same
results when supplied with the same inputs. These algorithms were
developed by Ben Laurie, et al., and initially included in the
document that has now become the log specification [6962-bis].
A.1. Append-only Check
This is a check performed by an Auditor to verify that a log is
operating in a fashion consistent with the "append-only" requirement
(see Section 3.6 above).
1. Fetch the current STH (see Section 4.3 of [6962-bis]).
2. Verify the STH signature.
3. Fetch all the entries in the tree corresponding to the STH (see
Section 4.7 of [6962-bis]).
4. Confirm that the tree constructed from the fetched entries
produces the same hash as that in the STH.
5. Fetch the current STH again; repeat until the STH changes.
6. Verify the STH signature.
7. Fetch all the new entries in the tree corresponding to the STH. If
they remain unavailable for a period beyond the MMD for this log
then this should be viewed as misbehavior on the part of the log.
8. Either:
1. Verify that the updated list of all entries generates a tree
with the same hash as the new STH.
Or, if the Auditor is not keeping a local cache of all entries
from this log:
1. Fetch a consistency proof for the new STH with the previous
STH (see Section 4.4 of [6962-bis]).
2. Verify the consistency proof.
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3. Verify that the new entries generate the corresponding
elements in the consistency proof.
9. Go to Step 5.
A.2. Inclusion Proof Verification
This algorithm is performed by a log client that has received an
"audit_path" and "leaf_index" and wishes to verify inclusion of an
input "hash" for an STH with a given "tree_size" and "root_hash". It
demonstrates that the "hash" was included in the "root_hash".
1. Set "fn" to "leaf_index" and "sn" to "tree_size - 1".
2. Set "r" to "hash".
3. For each value "p" in the "audit_path" array:
If "LSB(fn)" is set, or if "fn" is equal to "sn", then:
1. Set "r" to "HASH(0x01 || p || r)"
2. If "LSB(fn)" is not set, then right-shift both "fn" and "sn"
equally until either "LSB(fn)" is set or "fn" is "0".
Otherwise:
Set "r" to "HASH(0x01 || r || p)"
Finally, right-shift both "fn" and "sn" one time.
4. Compare "r" against the "root_hash". If they are equal, then the
log has proven the inclusion of "hash".
A.3. Verifying consistency between two STHs
This algorithm is used by an Auditor to establish that two STHs
represent valid states for a log, consistent with the tree sizes
indicated. The algorithm assumes that the Auditor has acquired an STH
"first_hash" for tree size "first", an STH "second_hash" for tree
size "second" where "0 < first < second", and has received a
"consistency" array that they wish to use to verify both hashes.
1. If "first" is an exact power of 2, then prepend "first_hash" to
the "consistency" array.
2. Set "fn" to "first - 1" and "sn" to "second - 1".
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3. If "LSB(fn)" is set, then right-shift both "fn" and "sn" equally
until "LSB(fn)" is not set.
4. Set both "fr" and "sr" to the first value in the "consistency"
array.
5. For each subsequent value "c" in the "consistency" array:
If "LSB(fn)" is set, or if "fn" is equal to "sn", then:
1. Set "fr" to "HASH(0x01 || c || fr)"
Set "sr" to "HASH(0x01 || c || sr)"
2. If "LSB(fn)" is not set, then right-shift both "fn" and "sn"
equally until either "LSB(fn)" is set or "fn" is "0".
Otherwise:
Set "sr" to "HASH(0x01 || sr || c)"
Finally, right-shift both "fn" and "sn" one time.
6. After completing iterating through the "consistency" array as
described above, verify that the "fr" calculated is equal to the
"first_hash" supplied and that the "sr" calculated is equal to the
"second_hash" supplied.
A.4. Verifying log root hash using log entries
This algorithm is used by any log client to verify that an STH (of
"tree_size") for a log is consistent with a complete list of leaf
input "entries" from "0" up to "tree_size - 1".
1. Set "stack" to an empty stack.
2. For each "i" from "0" up to "tree_size - 1":
1. Push "HASH(0x00 || entries[i])" to "stack".
2. Set "merge_count" to the lowest value ("0" included) such
"LSB(i >> merge_count)" is not set. In other words, set
"merge_count" to the number of consecutive "1"s found starting
at the least significant bit of "i".
3. Repeat "merge_count" times:
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1. Pop "right" from "stack".
2. Pop "left" from "stack".
3. Push "HASH(0x01 || left || right)" to "stack".
3. If there is more than one element in the "stack", repeat the same
merge procedure (Step 2.3 above) until only a single element
remains.
4. The remaining element in "stack" is the Merkle Tree hash for the
given "tree_size" and should be compared by equality against the
supplied "root_hash".
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Appendix B. SCT Transmission (Normative)
A TLS-enabled web server that supports CT MUST convey SCT data
corresponding to at least one certificate in the chain via the TLS
handshake. Three mechanisms are defined for conveying the required
SCT data and Compliant TLS clients MUST implement all three
mechanisms.
1. A TLS extension (Section 7.4.1.4 of [RFC5246]) with type
"signed_certificate_timestamp" may be used. This mechanism allows
TLS servers to participate in CT without the cooperation of CAs,
unlike the other two mechanisms. It also allows SCTs to be
updated by the server as needed.
2. An Online Certificate Status Protocol (OCSP) [RFC6960] response
extension may be employed, where the OCSP response is provided in
the "certificate_status" TLS extension (Section 8 of [RFC6066]),
also known as OCSP stapling. This mechanism is already widely
(but not universally) implemented. It also allows SCTs to be
updated by the sever as needed.
3. An X509v3 certificate extension may be employed. This mechanism
allows the use of unmodified TLS servers. However, the included
SCTs cannot be changed without re-issuing the certificate. Thus a
web cannot readily update the SCT data, e.g., to add SCTs from
additional logs. If the SCT embedded in the certificate was issued
by a log that is no longer trusted by TLS clients, the server will
have to acquire a new certificate.
It is RECOMMENDED that TLS servers send SCTs from multiple logs, in
case one or more logs are not acceptable to the TLS clients that
visit the server. (A log might become unacceptable if, for example,
it has been identified as misbehaving by Auditors, or as the result
of a compromise of the log.) Multiple SCTs are represented in an SCT
list as follows:
opaque SerializedSCT<1..2^16-1>;
struct {
SerializedSCT sct_list <1..2^16-1>;
} SignedCertificateTimestampList;
Here, "SerializedSCT" is an opaque byte string that contains the
serialized SCT structure. This encoding ensures that TLS clients can
decode each SCT individually (i.e., if there is a version upgrade,
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out-of-date clients can still parse old SCTs while skipping over new
SCTs whose versions they don't understand).
As noted in (1) above, one or more SCTs can be sent during the TLS
handshake using a TLS extension with type
"signed_certificate_timestamp".
TLS clients that support this extension SHOULD send a ClientHello
extension with the appropriate type and empty "extension_data".
TLS servers MUST send SCTs in this TLS extension only to a TLS client
that has indicated support for the extension in the ClientHello. The
SCTs are sent by setting the "extension_data" to a
"SignedCertificateTimestampList".
Session resumption uses the original session information: TLS clients
SHOULD include the extension type in the ClientHello, but if the
session is resumed, the TLS server is not required to process it or
include the extension in the ServerHello.
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Appendix C. Matching an SCT to a Certificate
When a TLS client receives an SCT via one of the mechanisms described
in Appendix B above, the client needs to match the SCT to a
certificate in the certificate chain. For an SCT embedded in a
certificate, the matching is trivial: the SCT belongs to the
certificate in which it is embedded. In either of the other cases,
the client uses the following algorithm (or an equivalent algorithm
that produces the same results in all cases).
For each certificate in the certificate chain, starting with the
trust anchor and proceeding down to the TLS server's end entity
certificate:
1. Copy the certificate's tbsCertificate field.
2. If the tbsCertificate copy contains a redacted labels extension:
1. For each DNS-ID in the tbsCertificate copy:
1. Determine the number of labels to redact, X. For the
Nth DNS-ID, the number of labels to redact is either the
Nth INTEGER in the redacted labels extension (if that
extension has N or more INTEGERS) or the last INTEGER in
the extension (if the extension has fewer than N
INTEGERS).
2. For each of the leftmost X labels in the DNS-ID: if
the label is not the wildcard label ("*"), replace the
label with "?".
2. If the tbsCertificate copy contains a CN-ID (which MUST match
the first DNS-ID), change this CN-ID to be equal to the first
DNS-ID as (potentially) modified above.
3. In the tbsCertificate copy, remove the SCT list extension (if it
is present).
4. Compare the (potentially) modified tbsCertificate copy against the
tbs_certificate field in the SCT's signed_entry. If they are
bytewise equal, then this is the certificate that the SCT matches,
and this algorithm is finished.
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Authors' Addresses
Stephen Kent
BBN Technologies
10 Moulton St.
Cambridge, MA 02138
US
Email: kent@bbn.com
David Mandelberg
BBN Technologies
10 Moulton St.
Cambridge, MA 02138
US
Email: david@mandelberg.org
Karen Seo
BBN Technologies
10 Moulton St.
Cambridge, MA 02138
US
Email: kseo@bbn.com
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