Network Working Group M. Pala
Internet-Draft CableLabs
Intended status: Experimental November 13, 2017
Expires: May 17, 2018
OCSP over DNS (ODIN)
draft-pala-odin-03
Abstract
With the increase number of protocols and applications that rely on
digital certificates to authenticate either the communication channel
(TLS) or the data itself (PKIX), the need for providing an efficient
revocation system is paramount. Although the Online Certificate
Status Protocol (OCSP) allows for efficient lookup of the revocation
status of a certificate, the distribution of this information via
HTTP (or very rarely) HTTPS is not particularly efficient for high
volume websites without incurring in high distribution costs (e.g.,
CDN).
In particular, this specification defines how to distribute OCSP
responses over DNS and how to define OCSP-over-DNS URLs in
certificates. The use of the DNS system to distribute such
information is meant to lower the costs of providing revocation
services (by leveraging the distributed nature of DNS cache) and
increase the availability of revocation information (by providing an
additional access method for revocation information retrieval).
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 May 17, 2018.
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Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Overview of existing solutions . . . . . . . . . . . . . . . 3
4. Scope Statement . . . . . . . . . . . . . . . . . . . . . . . 3
5. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 3
6. The OCSP Resource Record (OCSPRR) . . . . . . . . . . . . . . 4
6.1. The OCSP RDATA Wire Format . . . . . . . . . . . . . . . 4
6.2. The OCSP RRType . . . . . . . . . . . . . . . . . . . . . 4
6.3. Time Validity . . . . . . . . . . . . . . . . . . . . . . 5
7. Specifying DNS URLs for OCSP RR . . . . . . . . . . . . . . . 5
7.1. URL definition . . . . . . . . . . . . . . . . . . . . . 5
7.2. DNS URL Processing . . . . . . . . . . . . . . . . . . . 6
7.3. OCSPRR URI Examples . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
9. Security Considerations . . . . . . . . . . . . . . . . . . . 7
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
11. Normative References . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Requirements notation
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 [RFC2119].
2. Introduction
With the increasing number of highly available and highly utilized
websites that require secure communications to protect the flow of
information from the server to the client and the raising number of
devices (IoT) that require strong authentication capabilities, the
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need for a low-cost efficient approach to revocation information
availability is crucial. The OCSP-over-DNS approach allows clients
to determine the revocation status of digital certificates by
optimizing the delivery mechanism for revocation information
distribution to the client. This transport protocol can be used in
lieu of or in addition to other PKIX endorsed transport mechanisms
such as HTTP. This specification addresses the problem of providing
a highly-available distributed system for OCSP responses [RFC6960].
This document defines the DNS records to be used for OCSP data
publication and the definition of additional URLs for the
AuthorityInfoAccess (AIA) extension in certificates.
3. Overview of existing solutions
Currently there are three main options to retrieve the revocation
information associated with a digital certificates:
o by retrieving the freshest CRL
o by querying an OCSP responder for a freshly computed response
o by retrieving a pre-signed OCSP response from a web site
(typically a content distribution network or CDN)
o by verifying pre-computed OCSP responses embedded (stapled) during
the TLS negotiation (only in the TLS case, though)
All of these methods are based on the ability from the application to
extract URLs out of the CRL (CrlDistributionPoint) or of the OCSP
responder (AuthorityInfoAccess) from the certificate and query
(almost uniquely via HTTP/HTTPS, although supported protocols might
include LDAP and FTP) the corresponding server to retrieve the
required data.
4. Scope Statement
This document focuses only on the definition of the required options
for providing OCSP responses over DNS as an alternative transport
protocol. The reliability and accessibility of DNS records (e.g.,
issues related to TCP vs. UDP DNS responses) are out of the scope of
this document.
5. Protocol Overview
In order to validate a certificate using OCSP-over-DNS, the client
should check the certificate for a DNS-based OCSP URI ("dns://") and
then retrieve the OCSP response from the DNS. After this point, all
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procedures are to be performed according to the OCSP protocol as
defined in [RFC5019]. In particular, clients using OCSP-over-DNS,
SHOULD:
1. Lookup the OCSP URI provided in the AIA of the certificate to be
checked. The format of the URI comprises the id-ad-ocsp
identifier and a base URL where the scheme (``dns://'') is used.
The format of the full URI is discussed in Section 7.
2. Retrieve the DNS record carrying the required OCSP response.
6. The OCSP Resource Record (OCSPRR)
The OCSP DNS resource record (RR) is used to distribute a
certificate's revocation status to clients. The contents of the OCSP
RR record are described in Section 6.1.
The type value for the OCSP RR type is defined in Section 6.2.
The OCSP RR is class independent.
The OCSP RR Time to Live (TTL) should not exceed the validity period
of the OCSP response that is contained in the record.
6.1. The OCSP RDATA Wire Format
The RDATA for an OCSP RR consists of a single field which carries the
DER encoded OCSP response for the identified certificate.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| /
+ OCSP Response Data /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The OCSP response should contain only one response that refers to the
certificate which contains that URL. Following this schema, the OCSP
DNS URIs within the AIA extension SHOULD be unique for each
certificate issued by a single CA.
6.2. The OCSP RRType
This document uses a new DNS RR type, OCSP, whose value (TBD) was
allocated by IANA from the Resource Record (RR) TYPEs subregistry of
the Domain Name System (DNS) Parameters registry.
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6.3. Time Validity
The time validity should reflect the frequency of updates in
revocation information (i.e., the TTL should not be set to expire
after the OCSP response expiration). In practice, as an operational
matter, operators SHOULD ensure that the records are published in a
way that the TTL is low enough that they expire from caches before
the OCSP response expiration.
7. Specifying DNS URLs for OCSP RR
The Authority Information Access extension, as defined in [RFC5280],
provides information about the certificate in which the extension
appears. In order to specify the availability of OCSP responses over
DNS, Certification Authorities should use the OCSP accessMethod OID
(id-ad-ocsp) and use "dns" as the transport.
Please note that, when using this accessMethod, the use of the
dnsathority in the specified URI is discouraged as this might reduce
the benefits coming from the caching infrastructure of DNS and,
possibly, overload the referred DNS server.
7.1. URL definition
A DNS URL [RFC3986] begins with the protocol prefix "dns" and is
defined by the following grammar, following the ABNF notation defined
in [RFC5234].
dnsurl = scheme COLON SLASH SLASH [target]
[QUESTION [ TYPE=rr_type ]
; target: is the dns entry for
; the lookup operation.
; rr_type: is the type of record
; to be retrieved. If not specified,
; the default type is OCSPRR
scheme = "dns"
SLASH = %x2F ; forward slash ("/")
COLON = %x3A ; colon (":")
QUESTION = %x3F ; question mark ("?")
TYPE = "type" ; the keyword ("type")
Although this specification does not mandate for any specific format
for the <target> component of the DNS URL, some examples are provided
in Section 7.3 with the intent to illustrate, not define, the format.
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7.2. DNS URL Processing
In order to process the OCSP DNS URLs in a certificate, clients have
to extract the <target> and, if provided, the <type> of record from
the URL. After that, client MUST query for the specified record.
When the ``OCSPRR'' record type is used, the returned value MUST
contain the DER encoded OCSP response related to the certificate that
the client is going to validate.
7.3. OCSPRR URI Examples
When using the issuing CA's DNS sub-domain in the DNS URL, the hex
(or decimal) representation of the certificate's serialNumber MAY be
used as the hostname of the DNS URL. When combined with the specific
sub-domain of the issuing CA this provides a unique entry that can be
easily queried. For example, given that the sub-domain of the
issuing CA is "ca1.example.com", the resulting URL in the issued
certificate can be constructed as follows:
dns://04A3E45534A1B5.ca1.example.com?type=OCSPRR
Because the serialNumber of a certificate is guaranteed to be unique
within a (single) CA, different Certification Authorities MUST use
different sub-domains when using this publication algorithm to avoid
collisions across different CAs.
However, in some environments, the serial number that will be used in
the certificate to be issued can not be pre-fetched and embedded in
the AIA's DNS URL entry. In this case, the use of a monotonically
increasing or random integer number can be used instead.
In any case, it is important to notice that since the DNS entry is to
be used "AS IS" by the relying party that wants to fetch the OCSP
response by using the DNS URL, other techniques (e.g., the use of
prefixes for different issuing CAs combined with high-resolution
clock entries and small random or monotonic integer suffixes) can be
implemented independently by different Certificate Service Providers.
8. IANA Considerations
This document uses a new DNS RR type, OCSPRR, whose value (TBD) MUST
be allocated by IANA from the Resource Record (RR) TYPEs subregistry
of the Domain Name System (DNS) Parameters registry.
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9. Security Considerations
Several security considerations need to be explicitly considered for
the system administrators and application developers to understand
the weaknesses of the overall architecture. It is important to
highlight, however, that the following considerations are inherently
derived from the nature of the DNS infrastructure and that deployment
of the DNSSEC protocol might provide an efficient protection against
them.
By lacking the ability to authenticate the originating server
directly, the DNS (not DNSSEC) protocol (both in TCP and UDP mode) is
vulnerable to attacks where false responses are provided. Although
all the information stored in the OCSP RR is signed, the data
returned to the client could potentially be altered (e.g., by
providing an empty or old response). This type of attack can lead to
the application's inability to retrieve the revocation information,
thus this approach is vulnerable to Denial of Service (DoS), Man-in-
the-middle (MITM), and Reply Attacks.
As mentioned earlier, the deployment of DNSSEC can help in mitigating
the described family of attacks by providing a mean for the client
(or its resolver) to verify signatures of the DNS records themselves
via the DNS keys. This said, the use of DNS (instead of DNSSEC) is
equivalent, from a security considerations point of view, to today's
deployment best practices for OCSP where pre-computed responses are
delivered by CDNs via HTTP (not HTTPS). Therefore, the provisioning
of OCSP responses via DNS does not lower or alter the security
considerations that apply to the use of OCSP. Last but not least,
because of the availability (in most cases) of independent DNS
servers that an application can query, the use of multiple requests
to different DNS servers (for the same DNS record) might be
implemented as a mitigating measure in case an attack is suspected or
detected.
10. Acknowledgments
The authors would like to thank everybody who provided insightful
comments and helped in the definition of the deployment
considerations. In particular, the authors would like to thank Scott
A. Rea for his support. We also would like to thank DigiCert and
the initial discussion and support for the initial idea. Last but
not least, the authors would like to thank all the people that
expressed interest in implementing support for this proposal.
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11. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC4501] Josefsson, S., "Domain Name System Uniform Resource
Identifiers", RFC 4501, DOI 10.17487/RFC4501, May 2006,
<https://www.rfc-editor.org/info/rfc4501>.
[RFC5019] Deacon, A. and R. Hurst, "The Lightweight Online
Certificate Status Protocol (OCSP) Profile for High-Volume
Environments", RFC 5019, DOI 10.17487/RFC5019, September
2007, <https://www.rfc-editor.org/info/rfc5019>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[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, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<https://www.rfc-editor.org/info/rfc6960>.
Author's Address
Massimiliano Pala
CableLabs
858 Coal Creek Cir
Louisville, CO 80027
US
Email: m.pala@cablelabs.com
URI: http://www.linkedin.com/in/mpala
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