Internet DRAFT - draft-kent-trans-extended-validation-cert-checks
draft-kent-trans-extended-validation-cert-checks
Public Notary Transparency S. Kent
Internet-Draft BBN Technologies
Intended status: Standards Track R. Andrews
Expires: June 24, 2016 Symantec
December 22, 2015
Syntactic and Semantic Checks for Extended Validation Certificates
draft-kent-trans-extended-validation-cert-checks-02
Abstract
Certificate Transparency (CT) [RFC6962-bis] is a system for publicly
logging the existence of X.509 certificates as they are issued or
observed. The logging mechanism allows anyone to audit certification
authority (CA) activity and detect the issuance of "suspect"
certificates. Detecting mis-issuance of certificates is a primary
goal of CT.
A certificate is considered to be mis-issued if it fails to meet
syntactic and/or semantic criteria associated with the type of
certificate being issued. Mis-issuance can be detected by CT log
servers, whose feedback to a CA could prompt the CA to not issue a
suspect certificate. (Preventing the mis-issuance of such a
certificate is preferable to issuing it and detecting it later.)
Compliant CT log servers could offer these checks to a CA submitting
a pre-certificate to be logged. These checks are intended to be used
in an environment in which CAs optionally assert the version of the
EV guidelines to which the submitted pre-certificate purportedly
conforms. Log servers would then perform the checks of supported
[CABF-EV] versions and include the CA's assertion and the log
server's result in its Signed Certificate Timestamp (SCT).
Monitors can also perform checks to detect suspect certificates on
behalf of certificate Subjects. Checks performed by a Monitor also
serve to double check log servers that claim to have checked a
certificate, to identify those that are not doing the checks
properly, e.g., because of errors, compromise, or conspiracy. This
provides Monitors and CT clients with additional information when
choosing which logs to use.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Syntactic Checks . . . . . . . . . . . . . . . . . . . . . . 3
2.1. EV Certificate Field Syntax Requirements . . . . . . . . 4
2.2. Certificate Extension Syntax Requirements . . . . . . . . 5
2.3. Certificate Public Key: same as for DV certificates . . . 6
2.4. Certificate Signature: same as for DV certificates . . . 6
3. Semantic Verification of an EV Certificate . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Informative References . . . . . . . . . . . . . . . . . 8
6.2. Normative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The following checks are extracted from the CA Browser Forum (CABF)
document "Guidelines for the Issuance and Management of Extended
Validation Certificates" version 1.5.2 [CABF-EV]. (If a new version
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of the CABF guidelines is created that alters any of the checks
described below, a new CCID value MUST be assigned.) These
requirements are used to define what constitutes mis-issuance of a
certificate in the context of certificate transparency (CT) for Web
PKI certificates. The CABF guidelines from which these checks are
derived include many aspects of CA operation that are outside of the
scope of CT-based detection of certificate mis-issuance, i.e., they
impose requirements that could not be verified by a Monitor examining
certificate logs. Hence this document was created to provide an
enumeration of EV certificate checks for the Web PKI CT context.
The checks enumerated below are to be applied to any certificate
submitted to a log with the Certificate Class ID (CCID) value of 2
(see Section X of [CT RFC]). Note that "root" CA certificates are
not subject to verification against these criteria. Each log
maintains a list of the root CAs for which it is willing to accept
SCT generation requests. This implies that the log operator has
already determined that these CAs, and their corresponding self-
signed certificates, are acceptable. A subordinate CA certificate
will be checked only if it is submitted as the target of an SCT. If
a subordinate CA certificate appears as part of a chain submitted for
SCT generation, but is not the last certificate (the End-Entity or EE
certificate) in that chain, the checks enumerated below should be
applied to the EE certificate but not the subordinate CA certificate.
[CABF-EV] describes both syntactic and semantic requirements for
certificate issuance. This document deals primarily with syntactic
checks, but also describes how semantic checks are to be performed.
A log MAY perform the syntactic checks enumerated below if a
certificate is submitted with a CCID value of 2. If a log performs
these syntactic checks, it adds the SSV value appropriate for the
outcome of the check (see Section Z of [CT-RFC]).
Monitors SHOULD perform both the syntactic and semantic checks
described below for all certificates that they protect, and which are
marked with a CCID value of 2.
2. Syntactic Checks
An X.509 certificate consists of a set of fields (all but two of
which are mandatory), a set of optional extensions, a public key and
a signature. This section defines the syntactic requirements imposed
on the certificate fields. The following sections deal with
extensions, public keys, and signatures.
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2.1. EV Certificate Field Syntax Requirements
[CABF-EV] establishes syntactic requirements for EV certificates.
Many of these requirements are the same as for DV certificates. The
syntactic checks for DV certificates appear in [RFC-DV]. To avoid
possible inconsistency between that RFC and this one, when the
syntactic check for an EV certificate is the same as for a DV
certificate, the phrase "same as for DV certificates" is inserted.
1. Version number: same as for DV certificates
2. certificate serial number: same as for DV certificates (Note
that this is not the Subject Registration Number attribute
discussed below.)
3. signature: same as for DV certificates
4. issuer: same as for DV certificates
5. validity: The maximum validity interval is 27 months.
6. subject: A certificate MAY contain a NULL Subject name. If it
contains a non-null Subject name:
A. It MUST contain the organizationName attribute. This
requirement is derived from Section 9.2.1 of [CABF-EV].
B. It MAY contain a commonName attribute. If this attribute is
present, it MUST contain a Fully-Qualified Domain Name (not
a wildcard name). This requirement is derived from
Section 9.2.3 of [CABF-EV].
C. It MUST contain the businessCategory attribute, and the
value of that attribute must match one of the four allowed
values. This requirement is derived from Section 9.2.4 of
[CABF-EV].
D. It MUST contain the jurisdictionCountryName attribute as
specified in [CABF-EV]. This requirement is derived from
Section 9.2.5 of [CABF-EV]. (Note that this attribute is
NOT defined in [RFC5280].)
E. It MAY contain the jurisdictionLocalityName as specified in
[CABF-EV]. This requirement is derived from Section 9.2.5
of [CABF-EV]. (Note that this attribute is NOT defined in
[RFC5280].)
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F. It MAY contain the jurisdictionStateOrProvinceName as
specified in [CABF-EV]. This requirement is derived from
Section 9.2.5 of [CABF-EV]. (Note that this attribute is
NOT defined in [RFC5280].)
G. It MUST contain the Subject Registration Number (also known
as Subject:serialNumber) attribute. This requirement is
derived from Section 9.2.6 of [CABF-EV].
H. The countryName, stateOrProvinceName and localityName MUST
be present and populated with values consistent with the
syntax defined in [RFC5280]. This requirement is derived
from Section 9.2.7 of [CABF-EV].
I. The localityName, streetAddress, and postalCode attributes
MAY be present and if present, MUST be populated with values
consistent with the syntax defined in [RFC5280]. This
requirement is derived from Section 9.2.7 of [CABF-EV].
J. Other Subject attributes, as specified in Appendix A of
[RFC5280], MAY appear. These attributes MUST NOT contain
metadata such as '.', '-', or ' ' (i.e. space) characters.
This requirement is derived from section 9.2.8 of [CABF-EV].
7. subjectPublicKeyInfo: same as for DV certificates.
8. issuerUniqueId: same as for DV certificates.
9. subjectUniqueId: same as for DV certificates.
10. signatureAlgorithm: same as for DV certificates.
11. signatureValue: same as for DV certificates.
2.2. Certificate Extension Syntax Requirements
An X.509 v3 certificate may contain extensions. [CABF-EV] mandates
the presence of several extensions, and imposes requirements on their
content.
1. The subjectAltName extension: same requirements as for DV
certificates, except Wildcard FQDNs are not permitted. This
requirement is derived from Section 9.2.2 of [CABF-EV].
2. A certificate issued to a Subscriber MUST include the
certificatePolicies extension. It MAY or MAY NOT be marked
CRITICAL. It MUST contain one or more policy identifiers
associated with an extended validation policy of the Issuer.
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This requirement is derived from Section 9.3.2 and 9.3.5 of
[CABF-EV]. There are two commonly cited references for EV OIDs:
http://en.wikipedia.org/wiki/Extended_Validation_Certificate and
https://code.google.com/p/chromium/codesearch#chromium/src/net/
cert/ev_root_ca_metadata.cc&sq=package:chromium
A log or Monitor checking a certificate that purports to be an EV
certificate SHOULD use these references to verify that it
contains an appropriate policy OID.
This extension MUST contain certificatePolicies:policyIdentifier,
certificatePolicies:policyQualifiers:policyQualifierId (id-qt 1)
and certificatePolicies:policyQualifiers:qualifier:cPSuri (the
URL for the CA's CPS). This requirement is derived from
Section 9.7 paragraph (3) of [CABF-EV].
3. basicConstraints: same as for DV certificates.
4. The cRLDistributionPoints extension MUST be present in a CA
certificate. It MUST NOT be marked critical and it MUST contain
an HTTP URL. This extension MUST be present in a Subscriber
certificate if the certificate does not specify an OCSP responder
location in the authorityInformationAccess extension.
5. keyUsage extension: same as for DV certificates.
6. authorityInformationAccess extension: same as for DV certificates
7. extendedKeyUsage extension: same as for DV certificates.
8. nameConstraints extension: same as for DV certificates
9. Other extensions defined in [RFC5280]: same as for DV
certificates.
2.3. Certificate Public Key: same as for DV certificates
2.4. Certificate Signature: same as for DV certificates
3. Semantic Verification of an EV Certificate
The fundamental semantic check that a Monitor MUST perform is to
detect bogus certificates on behalf of its clients. A client of a
Monitor provides the Monitor with a set of certificates that have
been issued to the client. (Note that a client may have multiple
certificates issued to its name, and thus there is not a one-to-one
mapping between names and public keys.) These certificates MUST be
acquired in a secure fashion, not using certificate discovery
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protocols or relying on databases operated by a CA or RA. Armed with
this information, a Monitor can examine every log entry to determine
if it contains the same Subject or subjectAltName as that of a
client. If a log entry matches either of these names, and if it
contains a public key distinct from the set of keys provided by the
Subject, this is evidence of mis-issuance. (Note that a Monitor
cannot rely on a log operated by a CA, to detect mis-issuance by that
CA.) If a Monitor identifies what appears to be a bogus certificate,
it notifies the client. The means by which notification is effected
is not specified.
[CABF-EV] imposes a number of requirements on certificate issuance
that cannot be verified without access to reference information for
the certificate Subject, information about the CA hierarchy, or
information about internal procedures of the CA. Monitors are not
presumed to be able to perform such checks. Examples of such checks
appear in Sections 7, 8, 9.1, and 9.2.4 of [CABF-EV].
Additional semantic checks SHOULD be performed by a Monitor, if it
has access to the requisite information. These are enumerated below.
1. A certificate issued to a subordinate CA that is not controlled
by a Root CA MUST NOT contain the anyPolicy policy identifier.
This requirement is derived from section 9.3.4 (1) of [CABF-EV].
Verification of this requirement requires knowledge of CA
organizational relationships and thus may not be available to all
Monitors.
2. A certificate issued to a subordinate CA that is not controlled
by the issuing CA MUST include one or more policy identifiers
defined by the issuing CA that explicitly identify the EV
Policies that are implemented by the Subordinate CA. This
requirement is derived from section 9.3.4 (1) of [CABF-EV]. If
the extension contains any of the OIDs noted explicitly above, it
is acceptable. Verification of this requirement requires
knowledge of CA organizational relationships and thus may not be
available to all Monitors.
3. The Subject's Jurisdiction of Incorporation, Registration, or
Place of Business MUST not be in any country with which the laws
of the CA's jurisdiction prohibit doing business. This
suggestion is derived from Section 11.12.2 (1) (B) of [CABF-EV].
4. The Subject's organizationName attribute MUST contain the
Subject's full legal organization name as listed in the official
records of the Incorporating or Registration Agency in the
Subject's Jurisdiction of Incorporation or Registration, although
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abbreviations are permitted. This requirement is derived from
Section 9.2.1 of [CABF-EV].
5. The Domain Names in the subjectAltName extension MUST be owned or
controlled by the Subject, or MUST have been owned or controlled
by the Subject at the time of certificate issuance. This
requirement is derived from Section 9.2.2 of [CABF-EV].
6. The Domain Name in the Subject Common Name field, if present,
MUST be owned or controlled by the Subject, MUST have been owned
or controlled by the Subject at the time of certificate issuance.
This requirement is derived from Section 9.2.2 of [CABF-EV].
7. The Subject Jurisdiction of Incorporation or Registration Fields
MUST not contain information that is not relevant to the level of
the Incorporating Agency or Registration Agency. This
requirement is derived from Section 9.2.5 of [CABF-EV].
8. The Subject Physical Address of Place of Business Fields MUST
contain the address of a physical location of the Subject's Place
of Business. This requirement is derived from Section 9.2.7 of
[CABF-EV].
4. IANA Considerations
TBD
5. Security Considerations
TBD
6. References
6.1. Informative References
[CABF-EV] CA/Browser Forum, "Guidelines For The Issuance And
Management Of Extended Validation Certificates, Version
1.5.2", October 2014, <https://cabforum.org/wp-
content/uploads/EV-V1_5_2Libre.pdf>.
6.2. Normative References
[I-D.ietf-trans-rfc6962-bis]
Laurie, B., Langley, A., Kasper, E., Messeri, E., and R.
Stradling, "Certificate Transparency", draft-ietf-trans-
rfc6962-bis-11 (work in progress), November 2015.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
Authors' Addresses
Stephen Kent
BBN Technologies
10 Moulton St.
Cambridge, MA 02138
US
Email: kent@bbn.com
Rick Andrews
Symantec
350 Ellis Street
Mountain View, CA 94043
US
Email: Rick_Andrews@symantec.com
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