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Public Notary Transparency Kent, S.
Internet Draft BBN Technologies
Expires: December 2015 June 2015
Intended Status: RFC -- Informational
Threat Analysis for Certificate Transparency
draft-ietf-trans-threat-analysis-00.txt
Abstract
This document describes a threat model for the Web PKI context in
which security mechanisms to detect mis-issuance of web site
certificates will be developed. The threat model covers both
syntactic and semantic mis-issuance, using a taxonomy of threats
starting with whether the mis-issuance was done by the CA
maliciously or not; then whether or not the certificate was logged;
and then whether the log(s) or monitor(s) are benign or malicious,
whether the certificate subject is self-monitoring and whether a
client is doing any checks.
Status of this Memo
This Internet-Draft is submitted to IETF 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-Drafts.
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
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This Internet-Draft will expire on December 31,2015.
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Table of Contents
1. Introduction....................................................... 3
2. Semantic mis-issuance.............................................. 4
2.1. Non-malicious Web PKI CA context ............................... 4
2.1.1. Certificate logged ......................................... 4
2.1.1.1. Benign log.............................................. 4
2.1.1.1.1. Self-monitoring Subject ............................. 5
2.1.1.1.2. Benign third party Monitor .......................... 5
2.1.1.2. Malicious or conspiring log............................. 5
2.1.1.2.1. Self-monitoring Subject ............................. 6
2.1.1.2.2. Benign third party Monitor .......................... 6
2.1.1.3. Malicious or conspiring third party Monitor............. 6
2.1.2. Certificate not logged ..................................... 6
2.1.2.1. Self-monitoring Subject................................. 7
2.1.2.2. Careful browser......................................... 7
2.2. Malicious Web PKI CA context ................................... 7
2.2.1. Certificate logged ......................................... 7
2.2.1.1. Benign log.............................................. 7
2.2.1.1.1. Self-monitoring Subject ............................. 7
2.2.1.1.2. Benign third party Monitor .......................... 8
2.2.1.1.3. Malicious or conspiring third party Monitor ......... 8
2.2.1.2. Malicious or conspiring log............................. 8
2.2.1.2.1. Malicious or conspiring third party Monitor ......... 8
2.2.2. Certificate not logged ..................................... 9
2.2.2.1. Self-monitoring Subject................................. 9
2.2.2.2. Careful browser......................................... 9
3. Syntactic mis-issuance............................................. 9
3.1. Non-malicious Web PKI CA context ............................... 9
3.1.1. Certificate logged ........................................ 10
3.1.1.1. Benign log............................................. 10
3.1.1.2. Malicious or conspiring log or third party Monitor..... 11
3.1.1.3. Self-monitoring Subject and Benign third party Monitor. 11
3.1.1.4. Careful browser........................................ 11
3.1.2. Certificate not logged .................................... 12
3.2. Malicious Web PKI CA context .................................. 12
3.2.1. Certificate logged ........................................ 12
3.2.1.1. Benign log............................................. 12
3.2.1.2. Malicious or conspiring log or third party Monitor..... 12
3.2.1.3. Self-monitoring Subject and Benign third party Monitor. 13
3.2.1.4. Careful browser........................................ 13
3.2.2. Certificate is not logged ................................. 13
4. Notes Applicable to Sections 2 and 3)............................. 13
5. Security Considerations........................................... 16
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6. IANA Considerations............................................... 16
7. Acknowledgments................................................... 16
8. References........................................................ 17
8.1. Normative References .......................................... 17
8.2. Informative References ........................................ 17
Author's Addresses................................................... 17
Copyright Statement.................................................. 17
Attack Model and Discussion of Detection and Mitigation Options
1. Introduction
Certificate mis-issuance may arise in one of several ways. The ways
by which CT enables a Subject (or others) to detect and redress mis-
issuance depends on the context and the entities involved in the
mis-issuance. This attack model applies to use of CT in the Web PKI
context. If CT is extended to apply to other contexts, each context
will require its own attack model, although most elements of the
model described here are likely to be applicable.
Certificates are issued by CAs. So the top level differentiation in
this analysis is whether the CA that mis-issued a certificate did so
maliciously or not. Next, for each scenario, the model considers
whether or not the certificate was logged. Scenarios are further
differentiated based on whether the logs and monitors are benign or
malicious and whether a certificate's Subject is self-monitoring or
is using a third party Monitoring service. Finally, the analysis
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considers whether a browser is performing checking relevant to CT.
The scenarios are organized as illustrated by the following outline:
Web PKI CA - malicious vs non-malicious
Certificate - logged vs not logged
Log - benign vs malicious
third party Monitor - benign vs malicious
Certificate's Subject - self-monitoring (or not)
Browser - careful (or not)
The term certificate mis-issuance is defined here to encompass
issuance of a syntactically incorrect certificate or issuance of a
certificate to an unauthorized party (semantically incorrect).
(Throughout the remainder of this document we refer to a
semantically incorrect certificate as ''bogus.'') This analysis
addresses semantic mis-issuance first, then syntactic mis-issuance.
The following sections examine each of these cases. As noted above,
the focus here is on the Web PKI context, although most of the
analysis is applicable to other PKI contexts.
2. Semantic mis-issuance
2.1. Non-malicious Web PKI CA context
In this section, we address the case where the CA has no intent to
issue a bogus certificate.
A CA may have mis-issued a certificate as a result of an error or, in
the case of a bogus certificate, because it was the victim of a
social engineering attack or an attack such as the one that affected
DigiNotar
[https://www.vasco.com/company/about_vasco/press_room/news_archive/20
11/news_diginotar_reports_security_incident.aspx]. In the case of an
error, the CA should have a record of the erroneous certificate and
be prepared to revoke this certificate once it has discovered and
confirmed the error. In the event of an attack, a CA may have no
record of a bogus certificate.
2.1.1. Certificate logged
2.1.1.1. Benign log
The log (or logs) is benign and thus is presumed to provide
consistent, accurate responses to requests from all clients.
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If a bogus (pre-)certificate has been submitted to one or more logs
prior to issuance to acquire an embedded SCT, or post-issuance to
acquire a standalone SCT, detection of mis-issuance is the
responsibility of a Monitor
2.1.1.1.1. Self-monitoring Subject
If a Subject is tracking the log(s) to which a certificate was
submitted, and is performing self-monitoring, then it will be able to
detect a bogus(pre-)certificate and request revocation, (If there are
many logs, it may not be feasible for a Subject to track all of
them.) In this case, the CA will make use of the log entry (supplied
by the Subject) to determine the serial number of the mis-issued
certificate, and revoke it (after investigation). (See Notes 1 and
2.)
2.1.1.1.2. Benign third party Monitor
If a benign third party monitor is checking the logs to which a
certificate was submitted and is protecting the targeted Subject, it
will detect a bogus certificate and will alert the Subject, (If there
are many logs, it may not be feasible for a Monitor to track all of
them.) The Subject, in turn, will ask the CA to revoke the bogus
certificate. In this case, the CA will make use of the log entry
(supplied by the Subject) to determine the serial number of the bogus
certificate, and revoke it (after investigation). (See Notes 1 and
2.)
2.1.1.2. Malicious or conspiring log
In this case, the bogus (pre-)certificate has been submitted to one
or more logs that are either simply malicious or are conspiring with
the attacker. It is assumed that the logs issue SCTs (in an attempt
to fool browsers and/or Monitors). In this context, a log probably
will suppress a bogus certificate log entry. (This case encompasses
the scenario in which a log creates an entry for the certificate but
reports it selectively.)
Note that a malicious log also could create and report entries for
bogus certificates that have not been issued by the indicated CA.
These could cause the Monitor to report non-existent semantic
problems to the Subject who would in turn report them to the
(apparently) issuing CA. This might cause the CA to do needless
investigative work or perhaps incorrectly revoke and re-issue the
Subject's certificate.
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2.1.1.2.1. Self-monitoring Subject
If a malicious or conspiring log suppresses a bogus certificate log
entry, a Subject performing self-monitoring will not detect the
bogus certificate. In this scenario, CT relies on a ''gossiping''
mechanism to detect this sort of log misbehavior, as a deterrent. It
is not clear if such a mechanism is viable if there are very large
numbers of self-monitoring Subjects.
2.1.1.2.2. Benign third party Monitor
Because a malicious or conspiring log will suppress a bogus
certificate log entry, a benign third party Monitor that is
protecting the targeted Subject also will not detect a bogus
certificate. In this scenario, CT relies on a ''gossiping'' mechanism
to detect this sort of log misbehavior, as a deterrent. However, a
Monitor (third party or self) must participate in the gossiping
mechanism in order to become aware of log misbehavior.
2.1.1.3. Malicious or conspiring third party Monitor
A third party Monitor that is conspiring with the entity that caused
the mis-issuance, or a Monitor that is simply malicious will not
notify the targeted Subject of a bogus certificate. This is true
irrespective of whether the Monitor checks the logs or whether the
logs are benign or malicious/conspiring.
Note that independent of any mis-issuance on the part of the CA, a
malicious Monitor could issue false warnings to a Subject that it
protects. These could cause the Subject to report non-existent
semantic problems to the issuing CA and cause the CA to do needless
investigative work or perhaps incorrectly revoke and re-issue the
Subject's certificate.
2.1.2. Certificate not logged
If the CA does not submit a pre-certificate to a log, whether a log
is benign or malicious/conspiring does not matter. The same is true
if a Subject is issued a certificate without an SCT and does not log
the certificate itself, to acquire an SCT. Also, since there is no
log entry in this scenario, there is no difference in outcome between
a benign and a malicious/conspiring third party Monitor. In both
cases, there will be no reporting of the problem to the Subject based
on examination of log entries.
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2.1.2.1. Self-monitoring Subject
A Subject performing self-monitoring will be able to detect the lack
of an embedded SCT in the certificate it received from the CA. The
Subject SHOULD notify the CA if the Subject believed that its
certificate was supposed to be logged. If the certificate was
supposed to be logged, but was not, the CA can use the certificate
supplied by the Subject to investigate and remedy the problem. (A
failure to log the certificate might be the result of an operations
error, or evidence of an attack.)
2.1.2.2. Careful browser
If a browser rejects certificates without SCTs and notifies the
Subject and/or the issuing CA when no SCT is provided, this form of
mis-issuance will be detected (see Note 3.) However, it is not clear
how such behavior by browsers can be deployed incrementally
throughout the Internet. Also, there is an obvious potential for
DDoS attacks if browsers can be tricked into contacting CAs and/or
Subjects based on this behavior. If, when an SCT is not provided,
clients do not reject certificates and do not notify the CA or the
Subject, this form of mis-issuance will not be detected unless the
Subject is self-monitoring (See 2.1.2.1 and Note 3.)
2.2. Malicious Web PKI CA context
In this section, we address the scenario in which the mis-issuance
is intentional, not due to error. The CA is not the victim but the
attacker.
2.2.1. Certificate logged
2.2.1.1. Benign log
A bogus (pre-)certificate may be submitted to one or more benign logs
prior to issuance, to acquire an embedded SCT, or post-issuance to
acquire a standalone SCT. The log (or logs) replies correctly to
requests.
2.2.1.1.1. Self-monitoring Subject
If a Subject is checking the logs to which a certificate was
submitted and is performing self-monitoring, it will be able to
detect the bogus certificate and will request revocation. (If there
are many logs, it may not be feasible for a Subject to track all of
them.) The CA may refuse to revoke, or may substantially delay
revoking, the bogus certificate. The CA could make excuses about
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inadequate proof that the certificate is bogus, or argue that it
cannot quickly revoke the certificate because of legal concerns,
etc. In this case, the CT mechanisms will have detected mis-
issuance, but the information logged by CT does not help remedy the
problem. (See Notes 2 and 4.)
2.2.1.1.2. Benign third party Monitor
If a benign third party monitor is checking the logs to which a
certificate was submitted and is protecting the targeted Subject, it
will detect the bogus certificate and will alert the Subject. (If
there are many logs, it may not be feasible for a Monitor to track
all of them.) The Subject will then ask the CA to revoke the bogus
certificate. As in 2.2.1.1.1, the CA may or may not revoke the
certificate.
2.2.1.1.3. Malicious or conspiring third party Monitor
If the third party Monitor that is "protecting" the targeted Subject
is malicious or is conspiring with the entity that caused the mis-
issuance, then it will not notify the targeted Subject irrespective
of whether the logs it checks are benign or malicious/conspiring.
2.2.1.2. Malicious or conspiring log
The bogus (pre-)certificate may have been submitted to one or more
logs that are conspiring with the attacker. These logs may or may
not issue SCTs, but will hide the log entries from some or all
Monitors. In this case Monitors (third party and self) cannot detect
issuance of a bogus certificate based on monitoring these logs.
The Audit function of CT is intended to detect logs that conspire to
suppress log entries, based on consistency checking of logs and use
of a ''gossip'' mechanism. If a Monitor learns of malfeasant log
operation, it SHOULD alert the Subjects that it is protecting. The
Monitor SHOULD also avoid using such a log. However, unless a gossip
mechanism proves effective in detecting such misbehavior,, CT cannot
be relied upon to detect this form of mis-issuance. (See Note 5
below.)
2.2.1.2.1. Malicious or conspiring third party Monitor
A conspiring third party Monitor will not notify the targeted
Subject of any mis-issuance or of any malfeasant log behavior that
it detects.
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2.2.2. Certificate not logged
Because the CA is presumed malicious, it may choose to not submit a
(pre-)certificate to a log. This means there is no SCT for the
certificate.
When a CA does not (pre-)submit a certificate to a log, whether a log
is benign or malicious/conspiring does not matter. Also, since there
is no log entry, there is no difference in behavior between a benign
and a malicious/conspiring third party Monitor. Neither will report a
problem to the Subject.
2.2.2.1. Self-monitoring Subject
A Subject performing self-monitoring will be able to detect the lack
of SCT and notify the CA about the bogus certificate and request
revocation. The CA may refuse to revoke, or may substantially delay
revoking, the bogus certificate. It could make excuses about
inadequate proof that the certificate is bogus, or argue that it
cannot quickly revoke the certificate because of local, legal
concerns, etc. In this case, the CT mechanisms have detected mis-
issuance, but the information logged by CT does not help remedy the
problem. (See Notes 2 and 4.)
2.2.2.2. Careful browser
If clients reject certificates without SCTs and notify the Subject
and/or the issuing CA when no SCT is provided, this form of mis-
issuance will be detected (see Note 3.) If, when an SCT is not
provided, clients do not reject certificates and do not notify the
CA or the Subject, this form of mis-issuance will succeed unless the
Subject is self-monitoring (See 2.2.2.1 and Note 3.) However, it is
not clear how such behavior by browsers can be deployed
incrementally throughout the Internet. Also, there is an obvious
potential for DDoS attacks if browsers can be tricked into
contacting CAs and/or Subjects based on this behavior.
3. Syntactic mis-issuance
3.1. Non-malicious Web PKI CA context
This section analyzes the scenario in which the CA has no intent to
issue a syntactically incorrect certificate. Throughout the
remainder of this document we refer to a syntactically incorrect
certificate as ''erroneous''.
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3.1.1. Certificate logged
3.1.1.1. Benign log
If a (pre-)certificate is submitted to a benign log, syntactic mis-
issuance can (optionally) be detected, and noted. This will happen
only if the log performs syntactic checks in general, and if the log
is capable of performing the checks applicable to the submitted
(pre-)certificate. (A (pre-)certificate SHOULD be logged even if it
fails syntactic validation; logging takes precedence over detection
of syntactic mis-issuance.) If syntactic validation fails, this will
be noted in the SCT returned to the submitter.
. If the (pre-)certificate is submitted by the non-malicious issuing
CA, and if the CA has a record of the certificate content, then
the CA SHOULD remedy the syntactic problem and re-submit the
(pre-)certificate to a log or logs. If this is a pre-certificate
submitted prior to issuance, syntactic checking by a log helps
avoid issuance of a malformed certificate. If the CA does not
have a record of the certificate contents, then presumably it
was a bogus certificate and the CA SHOULD revoke it.
. If a certificate is submitted by its Subject, and it is deemed
erroneous, then the Subject SHOULD contact the issuing CA and
request a new certificate. If the Subject is a legitimate
subscriber of the CA, then the CA will either have a record of
the certificate content or can obtain a copy of the certificate
from the Subject. The CA will remedy the syntactic problem and
either re-submit a corrected (pre-)certificate to a log and send
it to the Subject or the Subject will re-submit it to a log.
Here too syntactic checking by a log enables a Subject to be
informed that its certificate is malformed and thus may hasten
issuance of a replacement certificate.
. If a (pre-)certificate is submitted by a third party, that party
might contact the Subject or the issuing CA, but because the
party is not the Subject of the certificate it is not clear how
the CA will respond.
Bottom line: Syntactic mis-issuance of a certificate can be avoided
by a CA if it makes use of logs that are capable of performing these
checks for the types of certificates that are submitted, and if the
CA acts on the feedback it receives. If a CA uses a log that does not
perform such checks, or if the CA requests checking relative to
criteria not supported by the log, then syntactic mis-issuance will
not be detected or avoided by this mechanism. Similarly, syntactic
mis-issuance can be remedied if a Subject submits a certificate to a
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log that performs syntactic checks, and if the Subject asks the
issuing CA to fix problems detected by the log. (The issuer is
presumed to be willing to re-issue the certificate, correcting any
problems, because the issuing CA is not malicious.)
3.1.1.2. Malicious or conspiring log or third party Monitor
A log or Monitor that is conspiring with the attacker or is
independently malicious, will either not perform syntactic checks,
even though it claims to do so, or simply not report errors. The log
entry and the SCT for an erroneous certificate will assert that the
certificate syntax was verified.
As with detection of semantic mis-issuance, a ''gossip'' mechanism
could reveal mis-behavior by logs or Monitors with respect to
syntactic checking. For example, if for a given certificate, some
logs (or Monitors) are reporting syntactic errors and some which
claim to do syntactic checking, are not reporting these errors, this
is indicative of misbehavior by these logs and/or Monitors.
Note that a malicious log (or Monitor) could report syntactic errors
for a syntactically valid certificate. This could result in
reporting of non-existent syntactic problems to the issuing CA, which
might cause the CA to do needless investigative work or perhaps
incorrectly revoke and re-issue the Subject's certificate.
3.1.1.3. Self-monitoring Subject and Benign third party Monitor
If a Subject or benign Monitor performs syntactic checks, it will
detect the erroneous certificate and the issuing CA will be notified
(by the Subject). If the Subject is a legitimate subscriber of the
CA, then the CA will either have a record of the certificate content
or can obtain a copy of the certificate from the Subject. The CA
SHOULD revoke the erroneous certificate (after investigation) and
remedy the syntactic problem. The CA SHOULD either re-submit the
(pre-)certificate to one or more logs and then send the result to the
Subject, or send the certificate to the Subject, who will re-submit
it to one or more logs.
3.1.1.4. Careful browser
If TLS clients reject erroneous certificates and notify the Subject
and/or the issuing CA, then syntactic mis-issuance will be detected
(see Note 3.) Unfortunately, experience suggests that many browsers
do not perform thorough syntactic checks on certificates, and so it
seems unlikely that browsers will be a reliable way to detect
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erroneous certificates. This argues for syntactic checking by other
elements of the CT system, e.g., logs and/or Monitors.
3.1.2. Certificate not logged
If a CA does not submit a certificate to a log, there can be no
syntactic checking by the log. Detection of syntactic errors will
depend on Subjects or Monitors performing the requisite checks.
3.2. Malicious Web PKI CA context
This section analyzes the scenario in which the CA's issuance of a
syntactically incorrect certificate is intentional, not due to error.
The CA is not the victim but the attacker.
3.2.1. Certificate logged
3.2.1.1. Benign log
Because the CA is presumed to be malicious, the CA may cause the log
to not perform checks, in one of several ways. (See [DOMVAL] and
[EXTVAL] for more details on validation checks and CCIDs).
1. The CA may assert that the certificate is being issued w/o
regard to any guidelines (the ''no guidelines'' reserved CCID).
2. The CA may assert a CCID that has not been registered, and thus
no log will be able to perform a check.
3. The CA may check to see which CCIDs a log declares it can
check, and chose a registered CCID that is not checked by the log
in question. In this fashion the CA can prevent the log from
performing checks, and the SCT and log entry will not contain an
indication of a failed check.
4. The CA may submit a (pre-) certificate to a log that is known
to not perform any syntactic checks, and thus avoid syntactic
checking.
3.2.1.2. Malicious or conspiring log or third party Monitor
A malicious or conspiring log or third party Monitor will either not
perform syntactic checks or not report any problems that it
discovers. (See 3.1.1.2 for further problems).
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3.2.1.3. Self-monitoring Subject and Benign third party Monitor
Irrespective of whether syntactic checks are performed by a log, a
malicious CA will acquire an embedded SCT, or post-issuance will
acquire a standalone SCT. If Subjects or Monitors perform syntactic
checks that detect the syntactic mis-issuance and report the problem
to the CA, a malicious/conspiring CA may do nothing or may delay
action to remedy the problem.
3.2.1.4. Careful browser
As noted above (3.1.1.4) many browsers fail to perform thorough
syntax checks on certificates. Such browsers would benefit from
having such checks performed by a log and reported in the SCT.
(Remember, in this scenario, the log is benign.) However, if a
browser does not discriminate against certificates that do not
contain SCTs (or that are not accompanied by an SCT in the TLS
handshake), only minimal benefits would accrue to them from syntax
checks perform by logs.
If a TLS clients accepts certificates that do not appear to have
been syntactically checked by a log (as indicated by the SCT), a
malicious CA need not worry about failing a log-based check.
Similarly, if there is no requirement for a TLS client to reject a
certificate that was logged by an operator that does not perform
syntactic checks, the fourth approach noted in 3.2.1.1 will succeed
as well. If a client were configured to know which versions of
certificate types are applicable to its use of a certificate, the
second and third strategies noted above could be thwarted.
3.2.2. Certificate is not logged
Since certificates are not logged in this scenario, the Monitor
function cannot detect the issuance of an erroneous certificate
(based on examination of logs). Thus there is no difference between a
benign or a malicious/conspiring log or a benign or
conspiring/malicious Monitor. A self-Monitoring Subject also will not
detect the error based on examination of log entries. (A Subject MAY
detect a syntax error by examining the certificate returned to the
Subject.) However, even if errors are detected and reported to the
CA, a malicious/conspiring CA may do nothing to fix the problem or
may delay action.
4. Notes Applicable to Sections 2 and 3
1. If a CA submits a bogus certificate to one or more logs, but
these logs are not tracked by a Monitor that is protecting the
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targeted Subject, CT will not mitigate this type of mis-issuance
attack. It is not clear whether every Monitor MUST offer to track
every Subject that requests protection. Absent such a guarantee, how
do Subjects know which set of Monitors will provide ''sufficient''
coverage? If a Subject acts as its own Monitor, this problem is
solved for that Subject. It also is not clear how a Monitor becomes
aware of all (relevant?) logs, including newly created logs. The
means by which Monitors become aware of new logs MUST accommodate
self-monitoring by a potentially very large number of web site
operators.
2. A CA being presented with evidence of a bogus certificate, in the
form of a log entry, will need to examine its records to determine
if it has knowledge of the certificate in question. It also will
likely require the targeted Subject to provide assurances that it is
the authorized entity representing the Subject name (subjectAltname)
in question. Thus a Subject should not expect immediate revocation
of a contested certificate. The time frame in which a CA will
respond to a revocation request usually is described in the CPS for
the CA. Other certificate fields and extensions may be of interest
for forensic purposes, but are not required to effect revocation nor
to verify that the certificate to be revoked is bogus, based on
applicable criteria. The SCT and log entry, because each contains a
timestamp from a third party, is probably valuable for forensic
purposes (assuming a non-conspiring log operator).
3. If a TLS client were to reject a certificate that lacks an
embedded SCT, or is not accompanied by an SCT transported via the
TLS handshake, this behavior needs to be defined in a way that is
compatible with incremental deployment. Issuing a warning to a
(human) user is probably insufficient, based on experience with
warnings displayed for expired certificates, lack of certificate
revocation status information, and similar errors that violate RFC
5280 path validation rules. Unless a mechanism is defined that
accommodates incremental deployment of this capability, attackers
probably will avoid submitting bogus certificates to (non-
conspiring) logs as a means of evading detection.
4. A targeted Subject might request the parent of a malicious CA to
revoke the certificate of the non-cooperative CA. However, a request
of this sort may be rejected, e.g., because of the potential for
significant collateral damage. A browser might be configured to
reject all certificates issued by the malicious CA, e.g., using a CA
hot list distributed by a browser vendor. However, if the malicious
CA has a sufficient number of legitimate clients, treating all of
them as bogus still represents serious collateral damage. If this
specification were to require that a browser can be configured to
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reject a specific, bogus certificate identified by a Monitor, then
the bogus certificate could be rejected in that fashion. This
mitigation strategy calls for communication between Monitors and
browsers, or between Monitors and browser vendors. Such
communication has not been specified, i.e., there are no standard
ways to configure a browser to reject individual bogus certificates
based on information provided by an external entity such as a
Monitor. Moreover, the same or another malicious CA could issue new
bogus certificates for the targeted Subject, which would have to be
detected and rejected in this (as yet unspecified) fashion. Thus,
for now, CT does not seem to provide a way to mitigate this form of
attack, even though it provides a basis for detecting such attacks.
5. The combination of a malicious CA and one or more conspiring logs
motivates the definition of an audit function, to detect conspiring
logs. If a Monitor protecting s Subject does not see bogus
certificates, it cannot alert the Subject. If one or more SCTs are
present in a certificate, or passed via the TLS handshake, a client
has no way to know that the logged certificate is not visible to
Monitors. Only if Monitors and clients reject certificates that
contain SCTs from conspiring logs (based on info from an audit) will
CT be able to deter use of such logs. Thus the means by which a
Monitor performing an audit function detects such logs, and informs
TLS clients must be specified for this to be effective.
Absent a ''gossip'' mechanism that enables Monitors to verify that
data from logs are reported in a consistent fashion, CT cannot claim
to provide protection against logs that are malicious or may
conspire with, or are victims of, attackers effecting certificate
mis-issuance. Developing such a mechanism is not easy. The mechanism
SHOULD protect the privacy of users (with respect to which web sites
they visit). It needs to scale to accommodate a potentially large
number of self-monitoring Subjects and a vast number of browsers (if
browsers are part of the mechanism). Even when a gossip mechanism is
defined, it will be necessary to describe how the CT system will
deal with a mis-behaving or compromised log. For example, will there
be a mechanism to alert all TLS clients to reject SCTs issued by
such a log? Absent a description of a mitigation strategy to deal
with mis-behaving or compromised logs, CT cannot ensure detection of
mis-issuance in a wide range of scenarios.
Monitors play a critical role in detecting semantic certificate mis-
issuance, for Subjects that have requested monitoring of their
certificates. A monitor (including a Subject performing self-
monitoring) examines logs for certificates associated with one or
more Subjects. It must obtain a list of valid certificates for the
Subject being monitored, in a secure manner, to use as a reference.
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It also must be able to identify and track a potentially large
number of logs on behalf of its Subjects. This may be a daunting
task for Subjects that elect to perform self-monitoring.
Note: A Monitor must not rely on a CA or RA database for this
information or use certificate discovery protocols; this information
must be acquired by the Monitor based on reference certificates
provided by a Subject. If a Monitor were to rely on a CA or RA
database (for the CA that issued a targeted certificate), the
Monitor would not detect mis-issuance due to malfeasance on the part
of that CA or the RA, or due to compromise of the CA or the RA. If
a CA or RA database is used, it would support detection of mis-
issuance by an unauthorized CA. A Monitor must not rely on
certificate discovery mechanisms to build the list of valid
certificates since such mechanisms might result in bogus
certificates being added to the list.
As noted above, Monitors represent another target for adversaries
who wish to effect certificate mis-issuance. If a Monitor is
compromised by, or conspires with, an attacker, it will fail to
alert a Subject to a bogus certificate targeting that Subject, as
noted above. It is RECOMMENDED that a Subject request certificate
monitoring from multiple sources to guard against such failures.
Operation of a Monitor by a Subject, on its own behalf, avoids
dependence on third party Monitors. However, the burden of Monitor
operation may be viewed as too great for many web sites, and thus
this mode of operation ought not be assumed to be universal when
evaluating protection against Monitor compromise.
5. Security Considerations
A threat model is, by definition, a security-centric document. Unlike
a protocol description, a threat model does not create security
problems nor does it purport to address security problems. This model
postulates a set of threats (i.e., motivated, capable adversaries)
and examines classes of attacks that these threats are capable of
effecting, based on the motivations ascribed to the threats.
6. IANA Considerations
None.
7. Acknowledgments
The author would like to thank David Mandelberg and Karen Seo for
help with the editing and formatting, and other members of the TRANS
working group for reviewing this document.
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," BCP 14, RFC 2119, March 1997.
8.2. Informative References
[TRANS] Laurie, B., Langley, A., Kasper, E., Messeri, E.,
Stradling, R., "Certificate Transparency," draft-ietf-
trans-rfc6962-bis-07 (March 9, 2015), work in progress.
[DOMVAL] Kent, S., ''Syntactic and Semantic Checks for Domain
Validation Certificates,'' draft-kent-trans-domain-
validation-cert-checks-00, (December 2014), work in
progress.
[EXTVAL] Kent, S., ''Syntactic and Semantic Checks for Extended
Validation Certificates,'' draft-kent-trans-extended-
validation-cert-checks-00 (December 2014), work in
progress.
Author's Addresses
Stephen Kent
BBN Technologies
10 Moulton Street
Cambridge MA 02138
USA
Phone: +1 (617) 873-3988
Email: skent@bbn.com
Copyright Statement
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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(http://trustee.ietf.org/license-info) in effect on the date of
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