Internet Engineering Task Force P. Hallam-Baker
Internet-Draft Comodo Group Inc.
Intended status: Standards Track R. Stradling
Expires: March 10, 2012 Comodo CA Ltd.
B. Laurie
Google Inc.
Sept 7, 2011
DNS Certification Authority Authorization (CAA) Resource Record
draft-ietf-pkix-caa-02
Abstract
The Certification Authority Authorization (CAA) DNS Resource Record
allows a DNS domain name holder to specify the certificate signing
certificate(s) authorized to issue certificates for that domain. CAA
resource records allow a public Certification Authority to implement
additional controls to reduce the risk of unintended certificate mis-
issue.
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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This Internet-Draft will expire on March 10, 2012.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Defined Terms . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. The CAA RR type . . . . . . . . . . . . . . . . . . . . . 5
3. Certification Authority Processing . . . . . . . . . . . . . . 6
3.1. Canonical Domain Name . . . . . . . . . . . . . . . . . . 7
3.2. Use of DNS Security . . . . . . . . . . . . . . . . . . . 7
3.3. Archive . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1.1. Canonical Presentation Format . . . . . . . . . . . . 9
4.2. CAA issue Property . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5.1. Mis-Issue by Authorized Certification Authority . . . . . 10
5.2. Suppression or spoofing of CAA records . . . . . . . . . . 10
5.2.1. Applications . . . . . . . . . . . . . . . . . . . . . 10
5.2.2. Certification Authorities . . . . . . . . . . . . . . 10
5.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 11
5.4. Abuse of the Critical Flag . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6.1. Registration of the CAA Resource Record Type . . . . . . . 11
6.2. Certification Authority Authorization Properties . . . . . 11
7. Normative References . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Definitions
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].
1.2. Defined Terms
The following terms are used in this document:
Authorization Entry An authorization assertion that grants or denies
a specific set of permissions to a specific group of entities.
Canonical Domain Name A Domain Name that is not an alias.
Canonical Domain Name Value The value of a Canonical Domain Name.
The value resulting from applying alias transformations to a
Domain Name that is not canonical.
Certificate An X.509 Certificate, as specified in RFC 5280
[RFC5280].
Certification Policy (CP) Specifies the criteria that a
Certification Authority undertakes to meet in its issue of
certificates.
Certification Practices Statement (CPS) Specifies the means by which
the criteria of the Certification Policy are met. In most cases
this will be the document against which the operations of the
Certification Authority are audited.
Certification Authority (CA) An entity that issues Certificates in
accordance with a specified Certification Policy.
Domain The set of resources associated with a DNS Domain Name.
Domain Name A DNS Domain name as specified in RFC 1035 [RFC1035] and
revisions.
Domain Name System (DNS) The Internet naming system specified in RFC
1035 [RFC1035] and revisions.
DNS Security (DNSSEC) Extensions to the DNS that provide
authentication services as specified in RFC 4033 [RFC4033] and
revisions.
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Extended Issuer Authorization Set The most specific Issuer
Authorization Set that is active for a domain. This is either the
Issuer Authorization Set for the domain itself, or if that is
empty, the Issuer Authorization Set for the corresponding Public
Delegation Point.
Issuer Authorization Set The set of Authorization Entries for a
domain name that are flagged for use by Issuers. Analogous to an
Access Control List but with no ordering specified.
Public Delegation Point A Domain Name that is obtained from a public
DNS registry.
Public Key Infrastructure X.509 (PKIX) Standards and specifications
issued by the IETF that apply the X.509 [X.509] certificate
standards specified by the ITU to Internet applications as
specified in RFC 5280 [RFC5280] and related documents.
Resource Record (RR) A set of attributes bound to a Domain Name.
Relying Party A party that makes use of an application whose
operation depends on use of a Certificate for making a security
decision.
Relying Application An application whose operation depends on use of
a Certificate for making a security decision.
2. Introduction
The Certification Authority Authorization (CAA) DNS Resource Record
allows a DNS domain name holder to specify the Certification
Authorities authorized to issue certificates for that domain.
Publication of CAA resource records allow a public Certification
Authority (CA) to implement additional controls to reduce the risk of
unintended certificate mis-issue.
Conformance with a published CAA record is a necessary but not
sufficient condition for issue of a certificate. Before issuing a
certificate, a PKIX CA is required to validate the request according
to the policies set out in its Certificate Policy Statement. In the
case of a public CA that validates certificate requests as a third
party, the certificate will be typically issued under a public root
certificate embedded in one or more relevant Relying Applications.
Criteria for inclusion of embedded root certificates in applications
are outside the scope of this document but typically require the CA
to publish a Certificate Practices Statement (CPS) that specifies how
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the requirements of the Certificate Policy (CP) are achieved and
provide an annual audit statement of their performance against their
CPS performed by an independent third party auditor.
It is the intention of the authors to propose the CAA record defined
in this document as the basis for CA validation requirements to be
proposed in organizations that publish validation requirements.
CAA records only describe the current state of Certification
Authority certificate issue authority. Since a certificate is
typically valid for at least a year, it is possible that a
certificate that is not conformant with the CAA records currently
published was conformant with the CAA records published at the time
that it was issued. Thus Relying Applications MUST NOT use failure
to conform to currently published CAA records specifying issue
authorization as a certificate validity criteria.
2.1. The CAA RR type
A CAA RR publishes a CAA property entry that corresponds to the
specified domain name. Multiple property entries MAY be associated
with the same domain name by publishing multiple CAA RRs at that
domain name. Each property entry MAY be tagged with one or more of
the following flag values:
Critical If set, indicates that the corresponding property entry tag
MUST be understood if the semantics of the CAA record are to be
correctly understood by the specified audience.
Issuers MUST NOT issue certificates for a domain if the Extended
Issuer Authorization Set contains unknown property entry tags that
have both the Issuer and Critical bits set.
Issuer Specifies that the corresponding Property Entry is to be used
by Issuers and forms part of the Issuer Authorization Set for the
domain.
The following properties are defined:
issue <Domain Name> [; <tag=value> ]* The policy property entry
declares an authorization entry granting authorization to issue
under the specified Certificate Policy to the holder of the
specified domain.
The following example informs CAs that certificates must not be
issued except by the holder of the domain name 'ca.example.net' or an
authorized agent thereof. Since the policy is published at the
Public Delegation Point, the policy applies to all subordinate
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domains under example.com.
$ORIGIN example.com
. CAA 1 issue "ca.example.net"
A certificate issuer MAY specify additional parameters that allow
customers to specify additional parameters governing certificate
issue. For example, the Certification Policy under which the
certificate is to be issued or the authentication process to be used.
$ORIGIN example.com
. CAA 1 issue "ca.example.net; account=230123"
The syntax and semantics of such parameters is left to site policy
and is outside the scope of this document.
Future versions of this specification MAY use the critical flag to
introduce new semantics that MUST be understood for correct
processing of the record, preventing Certification Authorities that
do not recognize the record from issuing certificates.
In the following example, the property 'tbs' is flagged as critical.
The example.net CA is not authorized to issue under either policy
unless the processing rules for the 'tbs' property tag are
understood.
$ORIGIN example.com
. CAA 1 issue "ca.example.net; policy=ev"
. CAA 129 tbs "Unknown"
Note that the above restrictions only apply to issue of certificates.
Since the validity of an end entity certificate is typically a year
or more it is quite possible that the CAA records published at a
domain will change between the issue of the certificate and
verification by a relying party.
3. Certification Authority Processing
Before issue of a certificate, a compliant CA MUST check for
publication of a relevant CAA Resource Record(s) and if such
record(s) are published, that the certificate requested is consistent
with them. If the certificate requested is not consistent with the
relevant CAA RRs, the CA MUST NOT issue the certificate.
The Issuer Authorization Set for a domain name consists of the set of
all CAA Authorization Entries declared for the canonical form of the
specified domain.
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The Extended Issuer Authorization Set for a domain name consists of
the Issuer Authorization Set for that domain name if it is non-empty.
Otherwise the Extended Issuer Authorization Set for a domain name
consists of the Issuer Authorization Set for the corresponding Public
Delegation Point for that domain name.
If the Extended Issuer Authorization Set for a domain name is not
empty, a Certification Authority MUST NOT issue a certificate unless
it conforms to at least one authorization entry in the Extended
Issuer Authorization Set.
3.1. Canonical Domain Name
The DNS defines the CNAME and DNAME mechanisms for specifying domain
name aliases. The canonical name of a DNS name is the name that
results from performing all DNS alias operations.
A Certification Authority MUST perform CNAME and DNAME processing as
defined in the DNS specifications 1035 [RFC1035] to resolve CAA
records.
3.2. Use of DNS Security
Use of DNSSEC to authenticate CAA RRs is strongly recommended but not
required. A CA MUST NOT issue certificates if doing so would
conflict with the corresponding extended issuer authorization set
whether the corresponding DNS records are signed or not.
Use of DNSSEC allows a CA to acquire and archive a non-repudiable
proof that they were authorized to issue certificates for the domain.
3.3. Archive
A compliant CA SHOULD maintain an archive of the DNS transactions
used to verify CAA eligibility.
In particular a CA SHOULD ensure that where DNSSEC data is available
that the corresponding signature and NSEC/NSEC3 records are preserved
so as to enable later compliance audits.
4. Mechanism
4.1. Syntax
A CAA RR contains a single property entry consisting of a tag value
pair. Each tag represents a property of the CAA record. The value
of a CAA property is that specified in the corresponding value field.
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A domain name MAY have multiple CAA RRs associated with it and a
given property MAY be specified more than once.
The CAA data field contains one property entry. A property entry
consists of the following data fields:
+0-1-2-3-4-5-6-7-|0-1-2-3-4-5-6-7-|
| Flags | Tag Length = n |
+----------------+----------------+...+---------------+
| Tag char 0 | Tag Char 1 |...| Tag Char n-1 |
+----------------+----------------+...+---------------+
+----------------+----------------+.....+---------------+
| Data byte 0 | Data byte 1 |.....| Data byte m-1 |
+----------------+----------------+.....+---------------+
Where n is the length specified in the tag length field and m is the
remaining octets in the data field (m = d - n - 2) where d is the
length of the data section.
The data fields are defined as follows:
Flags One octet containing the following fields:
Bit 0: Critical Flag If the value is set (1), the critical flag
is asserted and the property MUST be understood if the CAA
record is to be correctly processed.
A Certification Authority MUST NOT issue certificates for any
Domain that contains a CAA critical property for an unknown or
unsupported property type that has the issuer bit set.
Bit 7: Issuer Use If set, the property entry contains an
Authorization Entry that forms part of the Issuer Application
Authorization Set for the corresponding domain.
Note that according to the conventions set out in RFC 1035
[RFC1035] Bit 0 is the Most Significant Bit and Bit 7 is the Least
Significant. Thus the flags value 1 means that bit 7 is set while
a value of 128 means that bit 0 is set according to this
convention.
Tag Length A single octet containing an unsigned integer specifying
the tag length in octets. The tag length MUST be at least 1 and
SHOULD be no more than 15.
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Tag The property identifier, a sequence of ASCII characters.
Tag values MAY contain ASCII characters a through z and the
numbers 0 through 9. Tag values MUST NOT contain any other
characters. Matching of tag values is case insensitive.
Value A sequence of octets representing the property value.
Property values are encoded as binary values and MAY employ sub-
formats.
The length of the value field is specified implicitly as the
remaining length of the enclosing Resource Record data field.
4.1.1. Canonical Presentation Format
The canonical presentation format of the CAA record is as follows:
CAA <flags> <tag> <data>
Where:
flags Is an unsigned integer between 0 and 15.
tag Is a non-zero sequence of ASCII letter and numbers in lower
case.
data Is the ascii text Encoding of the value field
4.2. CAA issue Property
[TBS]
5. Security Considerations
CAA Records provide an accountability control. They are intended to
deter rather than prevent undesired behavior.
While a Certification Authority can choose to ignore published CAA
records, doing so increases the both the probability that they will
mis-issue a certificate and the consequences of doing so. Once it is
known that a CA observes CAA records, malicious registration requests
will disproportionately target the negligent CAs that do not, and so
the mis-issue rate amongst the negligent CAs will increase. Since
the CA could clearly have avoided the mis-issue by performing CAA
processing, the likelihood of sanctions against the negligent CA is
increased. Failure to observe CAA issue restrictions provides an
objective criteria for excluding issuers from embedded roots of
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trust.
In contrast, a Certification Authority that processes CAA records
correctly can reasonably claim that any residual mis-issue event
could have been avoided had the Domain Name holder published
appropriate CAA records.
5.1. Mis-Issue by Authorized Certification Authority
Use of CAA records does not provide protection against mis-issue by
an authorized Certification Authority.
Domain name holders SHOULD ensure that the CAs they authorize to
issue certificates for their domains employ appropriate controls to
ensure that certificates are only issued to authorized parties within
their organization.
Such controls are most appropriately determined by the domain name
holder and the authorized CA(s) directly and are thus out of scope of
this document.
5.2. Suppression or spoofing of CAA records
Suppression of the CAA record or insertion of a bogus CAA record
could enable an attacker to obtain a certificate from a CA that was
not authorized to issue for that domain name.
5.2.1. Applications
Applications performing CAA checking SHOULD mitigate the risk of
suppresion or spoofing of CAA records by means of DNSSEC validation
where present. In cases where DNSSEC validation is not available,
CAA checking is of limited security value.
5.2.2. Certification Authorities
Since a certificate issued by a CA can be valid for several years,
the consequences of a spoofing or suppression attack are much greater
for Certification Authorities and so additional countermeasures are
justified.
A CA MUST mitigate this risk by employing DNSSEC verification
whenever possible and rejecting certificate requests in any case
where it is not possible to verify the non-existence or contents of a
relevant CAA record.
In cases where DNSSEC is not deployed in a corresponding domain, a CA
SHOULD attempt to mitigate this risk by employing appropriate DNS
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security controls. For example all portions of the DNS lookup
process SHOULD be performed against the authoritative name server.
Cached data MUST NOT be relied on but MAY be used to support
additional anti-spoofing or anti-suppression controls.
5.3. Denial of Service
Introduction of a malformed or malicious CAA RR could in theory
enable a Denial of Service attack.
This specific threat is not considered to add significantly to the
risk of running an insecure DNS service.
5.4. Abuse of the Critical Flag
A Certification Authority could make use of the critical flag to
trick customers into publishing records which prevent competing
Certification Authorities from issuing certificates even though the
customer intends to authorize multiple providers.
In practice, such an attack would be of minimal effect since any
competent competitor that found itself unable to issue certificates
due to lack of support for a property marked critical is going to
investigate the cause and report the reason to the customer who was
deceived. It is thus unlikely that the attack would succeed and the
attempt might lay the perpetrator open to civil or criminal
sanctions.
6. IANA Considerations
6.1. Registration of the CAA Resource Record Type
IANA has assigned Resource Record Type 257 for the CAA Resource
Record Type and added the line depicted below to the registry named
Resource Record (RR) TYPEs and QTYPEs as defined in BCP 42 RFC 5395
[RFC5395] and located at
http://www.iana.org/assignments/dns-parameters.
Value and meaning Reference
----------- --------------------------------------------- ---------
CAA 257 Certification Authority Restriction [RFC-THIS]
6.2. Certification Authority Authorization Properties
IANA has created the Certification Authority Authorization Properties
registry with the following initial values:
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Meaning Reference
----------- ----------------------------------------------- ---------
issue Authorization Entry by Domain [RFC-THIS]
auth Reserved
path Reserved
policy Reserved
Addition of tag identifiers requires a public specification and
expert review as set out in RFC5395 [RFC5395]
7. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4055] Schaad, J., Kaliski, B., and R. Housley, "Additional
Algorithms and Identifiers for RSA Cryptography for use in
the Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile", RFC 4055,
June 2005.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5395] Eastlake, D., "Domain Name System (DNS) IANA
Considerations", RFC 5395, November 2008.
[X.509] International Telecommunication Union, "ITU-T
Recommendation X.509 (11/2008): Information technology -
Open systems interconnection - The Directory: Public-key
and attribute certificate frameworks", ITU-T
Recommendation X.509, November 2008.
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Authors' Addresses
Phillip Hallam-Baker
Comodo Group Inc.
Email: philliph@comodo.com
Rob Stradling
Comodo CA Ltd.
Email: rob.stradling@comodo.com
Ben Laurie
Google Inc.
Email: benl@google.com
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