Internet DRAFT - draft-kent-trans-domain-validation-cert-checks
draft-kent-trans-domain-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 Domain Validation Certificates
draft-kent-trans-domain-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-DV] 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.
Kent & Andrews Expires June 24, 2016 [Page 1]
Internet-Draft Domain Validation Certificate Checks December 2015
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on June 24, 2016.
Copyright Notice
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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Syntactic Checks . . . . . . . . . . . . . . . . . . . . . . 3
2.1. DV Certificate Field Syntax Requirements . . . . . . . . 4
2.2. DV Certificate Extension Syntax Requirements . . . . . . 6
2.3. Certificate Public Key . . . . . . . . . . . . . . . . . 8
2.3.1. RSA Public Keys . . . . . . . . . . . . . . . . . . . 8
2.3.2. DSA Public Keys . . . . . . . . . . . . . . . . . . . 9
2.3.3. ECC Public Keys . . . . . . . . . . . . . . . . . . . 9
2.4. Certificate Signature . . . . . . . . . . . . . . . . . . 9
3. Semantic Verification of a DV Certificate . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Informative References . . . . . . . . . . . . . . . . . 10
6.2. Normative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
Kent & Andrews Expires June 24, 2016 [Page 2]
Internet-Draft Domain Validation Certificate Checks December 2015
1. Introduction
The following checks are extracted from the CA Browser Forum (CABF)
document "Baseline Requirements for the Issuance and Management of
Publicly-Trusted Certificates" version 1_2_3 [CABF-DV]. (If a new
version 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 DV 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 1
(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 certificates of CAs (that MUST begin the
certificate validation path) 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 are applied
to the EE certificate but not the subordinate CA certificate.
[CABF-DV] 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 1. 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]) to the SCT.
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 1.
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
Kent & Andrews Expires June 24, 2016 [Page 3]
Internet-Draft Domain Validation Certificate Checks December 2015
on the certificate fields. The following sections deal with
extensions, public keys, and signatures.
2.1. DV Certificate Field Syntax Requirements
1. Version number: The certificate MUST be an X.509 v3 certificate.
This requirement is derived from Appendix B of [CABF-DV], where
it is explicitly stated for Root and Subordinate CA
certificates. Since other portions of [CABF-DV] mandate support
for extensions and only v3 certificates can contain extensions
[RC5280], this requirement is inferred to apply to EE
certificates as well.
2. serialNumber: No requirements beyond those imposed by [RFC5280]
are mandated by [CABF-DV]. Section 9.6 of [CABF-DV] suggests
that a serial number contain at least 20 bits of entropy so the
minimum serialNumber length should be 20 bits.
3. signature: For any certificate issued after December 31, 2010,
the allowed digest algorithms are: SHA-1, SHA-256, SHA-384 or
SHA-512. If RSA is used to sign the certificate, the minimum
modulus size is 2048 bits. (No requirement is imposed on the
public exponent.) If DSA is used to sign the certificate, the
following pairs of values are permitted: L= 2048, N= 224 or L=
2048, N=256). If the certificate signature is based on ECC
(presumably ECDSA), the allowed curves are NIST P-256, P-384 and
P-521. To verify that a certificate employs an accepted digest
and signature algorithm, one examines the OID contained in this
field. OIDs defined in the following RFCs are applicable here:
[RFC4055], [RFC5480], and [RFC5758]. (This set of checks does
not apply to certificates issued before the date cited above.)
4. issuer: The Issuer name MUST contain the countryName attribute
and it MUST contain an ISO-3166-1 country code. This
requirement is derived from section 9.1.4 of [CABF-DV]. The
Issuer name MUST contain the organizationName attribute. This
requirement is derived from section 9.1.3 of [CABF-DV].
5. validity: An EE certificate issued after July 1, 2012 MUST not
contain a validity interval longer than 60 months. ([CABF-DV]
establishes criteria in Section 9.4.1 that describe the
circumstances under which EE certificates may be issued with
validity intervals between 39 and 60 months. Since these
criteria cannot be evaluated without external knowledge, this
RFC adopts the 60-month limit for syntactic checking.)
6. subject: A certificate MAY contain a NULL Subject name. If it
contains a non-null Subject name:
Kent & Andrews Expires June 24, 2016 [Page 4]
Internet-Draft Domain Validation Certificate Checks December 2015
A. it MAY contain a commonName attribute. If this attribute is
present, it MUST contain a single IP address or Fully-
Qualified Domain Name that is one of the values contained in
the Certificate's subjectAltName extension. This
requirement is derived from section 9.2.2 of [CABF-DV].
Thus verification of this attribute requires comparing
values in this attribute against the content of the
subjectAltName extension, which MUST be present (see below).
B. it MAY contain an organizationalUnitName attribute. This
requirement is derived from section 9.2.6 of [CABF-DV].
C. if the name does not contain an organizationName attribute,
then the streetAddress attribute MUST NOT be present. If
the organizationName attribute is present, the streetAddress
attribute MAY be present. This requirement is derived from
section 9.2.4b of [CABF-DV].
D. if the name does not contain an organizationName attribute,
then the localityName attribute MUST NOT be present. If the
organizationName attribute is present, the localityName
attribute MAY be present. This requirement is derived from
section 9.2.4c of [CABF-DV].
E. if the name does not contain an organizationName attribute,
then the stateOrProvinceName attribute MUST NOT be present.
If the organizationName attribute is present, and the
localityName is absent, then the stateOrProvinceName
attribute MUST be present. If the organizationName
attribute is present, and the localityName is present, then
the stateOrProvinceName attribute MAY be present. This
requirement is derived from section 9.2.4d of [CABF-DV].
F. if the name does not contain an organizationName attribute,
then the postalCode attribute MUST NOT be present. If the
name contains an organizationName attribute, then the
postalCode attribute MAY be present. This requirement is
derived from section 9.2.4e of [CABF-DV].
G. if the name contains an organizationName attribute, then the
countryName attribute MUST be present. If the name does not
contain an organizationName attribute, then the countryName
attribute MAY be present. This requirement is derived from
section 9.2.5 of [CABF-DV].
H. The Subject MAY contain other attributes as specified in
Appendix A of [RFC5280]. These attributes MUST NOT contain
Kent & Andrews Expires June 24, 2016 [Page 5]
Internet-Draft Domain Validation Certificate Checks December 2015
metadata such as '.', '-', or ' ' (i.e. space) characters.
This requirement is derived from section 9.2.8 of [CABF-DV].
7. subjectPublicKeyInfo: If this field contains an RSA public key
the minimum modulus size is 2048 bits. (No requirement is
imposed on the public exponent.) If it carries a DSA key, the
following pairs of values are permitted: L= 2048, N= 224 or L=
2048, N=256. If the field conveys an ECC (presumably ECDSA)
public key, the allowed curves are NIST P-256, P-384 and P-521.
To verify that a certificate employs an accepted digest and
signature algorithm, one examines the OID contained in this
field. OIDs defined in the following RFCs are applicable here:
[RFC4055], [RFC5480], and [RFC5758].
8. issuerUniqueId: This is an optional field (a BIT STRING) in a v3
certificate. [CABF-DV] imposes no requirements on this field,
so no constraints beyond those in [RFC5280] are applicable.
9. subjectUniqueId: This is an optional field (a BIT STRING) in a
v3 certificate. [CABF-DV] imposes no requirements on this
field, so no constraints beyond those in [RFC5280] are
applicable.
10. signatureAlgorithm: This field MUST match the signature field
contained within the certificate (see # 3 above).
11. signatureValue: This field is verified using the public key
extracted from the certificate of the Issuer of this
certificate, and the algorithms specified in the preceding
field.
2.2. DV Certificate Extension Syntax Requirements
An X.509 v3 certificate may contain extensions. [CABF-DV] mandates
the presence of several extensions, and imposes requirements on their
content.
1. The certificate MUST contain the subjectAltName extension, and
that extension MUST contain at least one entry. Each entry MUST
be either a dNSName containing a Fully-Qualified Domain Name
(FQDN) or an iPAddress. Wildcard FQDNs are permitted. No other
entry types are permitted. This requirement is derived from
section 9.2.1 of [CABF-DV].
2. A certificate issued to a CA MUST include the certificatePolicies
extension. It MAY or MAY NOT be marked CRITICAL. The
policyQualifiers field MAY be present, and the policyQualifierId
and/or the cPSuri fields may be populated, using the syntax
Kent & Andrews Expires June 24, 2016 [Page 6]
Internet-Draft Domain Validation Certificate Checks December 2015
specified in [RFC5280]. This requirement is derived from
Appendix B, Section 3.A of [CABF-DV].
A. If this extension contains the OID 2.23.140.1.2.1, then the
Subject field MUST NOT contain an organizationName,
streetAddress, localityName, stateOrProvinceName, or
postalCode attribute. This requirement is derived from
section 9.3.1 of [CABF-DV].
B. If this extension contains the OID 2.23.140.1.2.2, then the
Subject field MUST contain organizationName, localityName,
and countryName attributes. This requirement is derived from
section 9.3.1 of [CABF-DV]. ([CABF-DV] also states that the
stateOrProvinceName attribute MUST be present, "if
applicable". Since the applicability of this attribute
cannot be readily determined, this Appendix views the
presence of this attribute as optional.)
3. The basicConstraints extension MUST be present, marked CRITICAL
and the cA flag MUST be set TRUE in a CA certificate. This
requirement is derived from Appendix B Section 2.D of [CABF-DV].
The presence of this extension is optional for an EE certificate.
If the extension is present in an EE certificate it MUST have the
cA flag set to FALSE. (If a certificate does not contain this
extension it is presumed to be an EE certificate and MUST be
processed as such with regard to all other verification checks.)
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 is optional for EE certificates, but
if present the same syntactic constraints apply. This
requirement is derived from Appendix B, Sections 2.B and 3.B of
[CABF-DV].
5. The keyUsage extension MUST be present in a CA certificate and it
MUST be marked critical. The keyCertSign and cRLSign bits MUST
be set. The digitalSignature bit MAY be set as well. The
keyUsage extension MAY be present in an EE certificate. If it is
present in an EE certificate, the keyCertSign and cRLSign bits
MUST NOT be set. These requirements are derived from Appendix B,
Section 2.E of [CABF-DV].
6. The authorityInformationAccess extension MAY be present and, if
present, MUST NOT be marked CRITICAL and MUST contain
accessMethod 1.3.6.1.5.5.7.48.1 and MAY specify accessMethod
1.3.6.1.5.5.7.48.2. This requirement is derived from Appendix B,
Sections 2.C and 3.C of [CABF-DV].
Kent & Andrews Expires June 24, 2016 [Page 7]
Internet-Draft Domain Validation Certificate Checks December 2015
7. The extKeyUsage extension MAY be present in a CA certificate. If
present, it need not be marked CRITICAL. If the extension is
present in a CA certificate, and if the certificate contains the
nameConstraints extension, then the value id-kp-serverAuth MUST
be present. This requirement is derived from Section 9.7 and
Appendix B, Section 2.G of [CABF-DV]. The extKeyUsage extension
MUST be present in an EE certificate. Either the value id-kp-
serverAuth or id-kp-clientAuth or both values MUST be present.
id-kp-emailProtection MAY be present. This requirement is
derived from Appendix B, Section 3.F of [CABF-DV].
8. The nameConstraints extension MAY appear in CA certificates and
need not be marked CRITICAL (contrary to [RFC5280]). If the
certificate also contains the extKeyUsage extension and that
extension contains the value id-kp-serverAuth, then that
extension MUST NOT contain the anyExtendedKeyUsage value in the
KeyPurposeId. Moreover, the nameConstraints extension MUST
impose constraints on dNSName, iPAddress and DirectoryName name
types. Both the permittedSubtrees and excludedSubtrees fields
MAY be employed. This requirement is derived from Section 9.7
and Appendix B, Section 2.F of [CABF-DV].
9. Other extensions defined in [RFC5280] MAY be present and MUST be
marked with respect to criticality as specified therein.
2.3. Certificate Public Key
2.3.1. RSA Public Keys
1. If a subordinate CA certificate contains an RSA public key, and
the certificate has a validity period beginning on or before 31
Dec 2010 and ending on or before 31 Dec 2013, that key MUST have
a minimum modulus size of 1024 bits. If a subordinate CA
certificate contains an RSA public key, and the certificate has a
validity period beginning after 31 Dec 2010 or ending after 31
Dec 2013, that key MUST have a minimum modulus size of 2048 bits.
This requirement is derived from Appendix A (2) of [CABF-DV].
2. If an EE certificate contains an RSA public key, and the
certificate has a validity period ending on or before 31 Dec
2013, that key MUST have a minimum modulus size of 1024 bits. If
an EE certificate contains an RSA public key, and the certificate
has a validity period ending after 31 Dec 2013, that key MUST
have a minimum modulus size of 2048 bits. This requirement is
derived from Appendix A (3) of [CABF-DV].
Kent & Andrews Expires June 24, 2016 [Page 8]
Internet-Draft Domain Validation Certificate Checks December 2015
3. The value of the public exponent of an RSA public key MUST be an
odd number equal to 3 or more. This requirement is derived from
Appendix A (4) of [CABF-DV].
2.3.2. DSA Public Keys
1. If a certificate contains a DSA public key, the minimum modulus
and divisor size (in bits) MUST be L= 2048, N= 224 or L= 2048, N=
256. This requirement is derived from Appendix A (2) and (3) of
[CABF-DV].
2. If a certificate contains a DSA public key, the public key MUST
include all domain parameters. This requirement is derived from
Appendix A (4) of [CABF-DV].
2.3.3. ECC Public Keys
1. If a certificate contains an ECC public key, that key MUST employ
one of these curves: NIST P-256, P-384, or P-521. This
requirement is derived from Appendix A (2) and (3) of [CABF-DV].
2.4. Certificate Signature
The certificate's signatureAlgorithm MUST be SHA-1, SHA-256, SHA-384
or SHA-512. This requirement is derived from Appendix A (2) and (3)
of [CABF-DV].
3. Semantic Verification of a DV 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
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 other than the one(s) provided by the Subject,
this is evidence of mis-issuance. A Monitor SHOULD track activity in
all logs that are considered trustworthy by its clients. There is no
mechanism defined that allows a Monitor to know what logs belong to
this set. Thus it is RECOMMENDED that each Monitor make known the
set of logs that it tracks, and each client is advised to select a
Monitor that satisfies the client's criteria in this regard. If a
Monitor identifies what appears to be a bogus certificate, it
Kent & Andrews Expires June 24, 2016 [Page 9]
Internet-Draft Domain Validation Certificate Checks December 2015
notifies the client. The means by which notification is effected is
not specified.
[CABF-DV] 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 7.1, 9.1.3, 9.1.4, 9.2.4a, 9.2.6, 9.4.1 and 9.5 of
[CABF-DV].
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 an affiliate
of a "root" CA MUST NOT contain the anyPolicy policy identifier.
This requirement is derived from section 9.3.3 of [CABF-DV].
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 an affiliate of
a "root" CA MAY include one or more explicit policy identifiers
(either 2.23.140.1.2.1 or 2.23.140.1.2.2 or policy identifiers
defined by the CA in its CP and/or CPS). It also MAY include the
anyPolicy OID. This requirement is derived from section 9.3.3 of
[CABF-DV]. 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.
4. IANA Considerations
TBD
5. Security Considerations
TBD
6. References
6.1. Informative References
[CABF-DV] CA/Browser Forum, "Baseline Requirements for the Issuance
and Management of Publicly-Trusted Certificates, v.1.2.3",
October 2014, <https://cabforum.org/wp-content/uploads/
BRv1.2.3.pdf>.
Kent & Andrews Expires June 24, 2016 [Page 10]
Internet-Draft Domain Validation Certificate Checks December 2015
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.
[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
Kent & Andrews Expires June 24, 2016 [Page 11]
