Network Working Group P. Hoffman
Internet-Draft VPN Consortium
Intended status: Standards Track J. Schlyter
Expires: September 13, 2011 Kirei AB
March 12, 2011
Using Secure DNS to Associate Certificates with Domain Names For TLS
draft-ietf-dane-protocol-06
Abstract
TLS and DTLS use certificates for authenticating the server. Users
want their applications to verify that the certificate provided by
the TLS server is in fact associated with the domain name they
expect. DNSSEC provides a mechanism for a zone operator to sign DNS
information directly. This way, bindings of keys to domains are
asserted not by external entities, but by the entities that operate
the DNS. This document describes how to use secure DNS to associate
the TLS server's certificate with the intended domain name.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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 September 13, 2011.
Copyright Notice
Copyright (c) 2011 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
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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
1.1. Certificate Associations . . . . . . . . . . . . . . . . . 3
1.2. Securing Certificate Associations . . . . . . . . . . . . 4
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Getting TLS Certificate Associations from the DNS . . . . . . 4
2.1. Requested Domain Name . . . . . . . . . . . . . . . . . . 5
2.2. Format of the Resource Record . . . . . . . . . . . . . . 5
2.3. Making Certificate Associations . . . . . . . . . . . . . 6
2.3.1. Format of Certificates Used to Identify End
Entities . . . . . . . . . . . . . . . . . . . . . . . 7
2.4. Presentation Format . . . . . . . . . . . . . . . . . . . 8
2.5. Wire Format . . . . . . . . . . . . . . . . . . . . . . . 8
3. Use of TLS Certificate Associations in TLS . . . . . . . . . . 9
4. Mandatory-to-Implement Algorithms . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5.1. TLSA RRtype . . . . . . . . . . . . . . . . . . . . . . . 10
5.2. TLSA Certificate Types . . . . . . . . . . . . . . . . . . 10
5.3. TLSA Hash Types . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
The first response from the server in TLS may contain a certificate.
In order for the TLS client to authenticate that it is talking to the
expected TLS server, the client must validate that this certificate
is associated with the domain name used by the client to get to the
server. Currently, the client must extract the domain name from the
certificate, must trust a trust anchor upon which the server's
certificate is rooted, and must successfully validate the
certificate.
Some people want a different way to authenticate the association of
the server's certificate with the intended domain name without
trusting a CA. Given that the DNS administrator for a domain name is
authorized to give identifying information about the zone, it makes
sense to allow that administrator to also make an authoritative
binding between the domain name and a certificate that might be used
by a host at that domain name. The easiest way to do this is to use
the DNS.
This document applies to both TLS [RFC5246] and DTLS [4347bis]. In
order to make the document more readable, it mostly only talks about
"TLS", but in all cases, it means "TLS or DTLS". This document only
relates to securely associating certificates for TLS and DTLS with
host names; other security protocols are handled in other documents.
For example, keys for IPsec are covered in [RFC4025] and keys for SSH
are covered in [RFC4255].
1.1. Certificate Associations
In this document, a certificate association is based on a
cryptographic hash of a certificate (sometimes called a
"fingerprint") or on the certificate itself. For a fingerprint, a
hash is taken of the binary, DER-encoded certificate, and that hash
is the certificate association; the type of hash function used can be
chosen by the DNS administrator. When using the certificate itself
in the certificate association, the entire certificate in the normal
format is used. This document only applies to PKIX [RFC5280]
certificates.
Certificate associations are made between a certificate or the hash
of a certificate and a domain name. Server software that is running
TLS that is found at that domain name would use a certificate that
has a certificate association given in the DNS, as described in this
document. A DNS query can return multiple certificate associations,
such as in the case of different server software on a single host
using different certificates (even if they are normally accessed with
different host names), or in the case that a server is changing from
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one certificate to another.
1.2. Securing Certificate Associations
This document defines a secure method to associate the certificate
that is obtained from the TLS server with a domain name using DNS
protected by DNSSEC. Because the certificate association was
retrieved based on a DNS query, the domain name in the query is by
definition associated with the certificate.
DNSSEC, which is defined in RFCs 4033, 4034, and 4035 ([RFC4033],
[RFC4034], and [RFC4035]), uses cryptographic keys and digital
signatures to provide authentication of DNS data. Information
retrieved from the DNS and that is validated using DNSSEC is thereby
proved to be the authoritative data. The DNSSEC signature MUST be
validated on all responses in order to assure the proof of origin of
the data.
This document only relates to securely getting the DNS information
for the certificate association using DNSSEC; other secure DNS
mechanisms are out of scope.
1.3. Terminology
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].
A note on terminology: Some people have said that this protocol is a
form of "certificate exclusion". This is true, but only in the sense
that a DNS reply that contains two of the certificate types defined
here inherently excludes every other possible certificate in the
universe (other than those found with a pre-image attack against on
of those two). The certificate type defined here is better thought
of as "enumeration" of a small number of certificate associations,
not "exclusion" of a near-infinite number of other certificates.
Some of the terminology in this draft may not match with the
terminology used in RFC 5280. This will be fixed in future versions
of this draft, with help from the PKIX community. In specific, we
need to say (in a PKIX-appropriate way) that when we say "valid up
to" and "chains to", full RFC 5280 path processing including
revocation status checking is intended.
2. Getting TLS Certificate Associations from the DNS
This document defines a new DNS resource record type, "TLSA". A
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query on a prepared domain name for the TLSA RR can return one or
more records of the type TLSA. The TLSA RRType is TBD.
2.1. Requested Domain Name
Domain names are prepared for requests in the following manner.
1. The decimal representation of the port number on which a TLS-
based service is assumed to exist is prepended with an underscore
character ("_") to become the left-most label in the prepared
domain name.
2. The protocol name of the transport on which a TLS-based service
is assumed to exist is prepended with an underscore character
("_") to become the second left-most label in the prepared domain
name. The transport names defined for this protocol are "tcp",
"udp" and "sctp".
3. The domain name is appended to the result of step 2 to complete
the prepared domain name.
For example, to request a TLSA resource record for an HTTP server
running TLS on port 443 at "www.example.com", you would use
"_443._tcp.www.example.com" in the request. To request a TLSA
resource record for an SMTP server running the STARTTLS protocol on
port 25 at "mail.example.com", you would use
"_25._tcp.mail.example.com".
2.2. Format of the Resource Record
The format of the data in the resource record is a binary record with
three values, which MUST be in the order defined here:
o A one-octet value, called "certificate type", specifying the
provided association that will be used to match the target
certificate. This will be an IANA registry in order to make it
easier to add additional certificate types in the future. The
types defined in this document are:
1 -- A certificate that identifies an end entity
2 -- A certification authority's certificate
Both types are structured using the RFC 5280 formatting rules and
use the DER encoding. As described later in this document, type 1
certificates do not need to correctly use all PKIX semantics.
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o A one-octet value, called "reference type", specifying how the
certificate association is presented. This value is defined in a
new IANA registry. The types defined in this document are:
0 -- Full certificate
1 -- SHA-256 hash of the certificate
2 -- SHA-512 hash of the certificate
Using the same hash algorithm as is used in the signature in the
certificate will make it more likely that the TLS client will
understand this TLSA data.
o The "certificate for association". This is the bytes containing
the full certificate or the hash of the associated certificate
(that is, the certificate or the hash of the certificate itself,
not of the TLS ASN.1Cert object).
Certificate types 1 and 2 explicitly only apply to PKIX-formatted
certificates. If TLS allows other formats later, or if extensions to
this protocol are made that accept other formats for certificates,
those certificates will need certificate types.
2.3. Making Certificate Associations
The two certificate types for TLS have very different semantics. A
TLS client conforming to this protocol receiving a certificate for
association of type 1 MUST compare it, using the specified hash type,
with the end entity certificate received in TLS. A TLS client
conforming to this protocol receiving a certificate for association
of type 2 MUST treat it as a trust anchor for that domain name.
Certificate type 1 (a certificate that identifies an end entity) is
matched against the first certificate offered by the TLS server. The
certificate for association is used only for exact matching, not for
chained validation. With reference type 0, the certificate
association is valid if the certificate in the TLSA data matches to
the first certificate offered by TLS. With reference types other
than 0, the certificate association is valid if the hash of the first
certificate offered by the TLS server matches the value from the TLSA
data.
Certificate type 2 (certification authority's certificate) can be
used in one of two ways. With reference type 0, the certificate in
the TLSA resource record is used in chaining from the end entity
given in TLS. The certificate association is valid if the first
certificate in the certificate bundle can be validly chained to the
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trust anchor from the TLSA data. With reference types other than 0,
if the hash of any certificate past the first in the certificate
bundle from TLS matches the trust anchor from the TLSA data, and the
chain in the certificate bundle is valid up to that TLSA trust
anchor, then the certificate association is valid. Alternately, if
the first certificate offered chains to an existing trust anchor in
the TLS client's trust anchor repository, and the hash of that trust
anchor matches the value from the TLSA data, then the certificate
association is valid.
The end entity certificate from TLS, regardless of whether it was
matched with a TLSA type 1 certificate or chained to a TLSA type 2 CA
certificate, must have at least one identifier in the subject or
subjectAltName field of the matched certificates matches the expected
identifier for the TLS server. Further, the TLS session that is to
be set up MUST be for the specific port number and transport name
that was given in the TLSA query. The matching or chaining MUST be
done within the life of the TTL on the TSLA record.
2.3.1. Format of Certificates Used to Identify End Entities
When presented with a type 1 certificate, the TLS client MUST NOT
verify the correct PKIX semantics for the keyCertSign bit of the
keyUsage extension, nor of the the basicConstraints extension. This
is because PKIX (RFC 5280) makes it clear that all self-signed
certificates are CA certificates and cannot be end entity
certificates. The last paragraph of section 3.2 of RFC 5280 says:
"This specification covers two classes of certificates: CA
certificates and end entity certificates. CA certificates may be
further divided into three classes: cross-certificates, self-issued
certificates, and self-signed certificates. ... Self-issued
certificates are CA certificates in which the issuer and subject are
the same entity. ... Self-signed certificates are self-issued
certificates where the digital signature may be verified by the
public key bound into the certificate. Self-signed certificates are
used to convey a public key for use to begin certification paths.
End entity certificates are issued to subjects that are not
authorized to issue certificates."
This means that a self-signed certificate (one where the subject and
issuer are the same, and the public key in the certificate can be
used to directly evaluate the signature on the certificate) must
follow all the PKIX semantics rules for CAs, and probably need to
follow all the policy rules as well. This is clearly not what people
who want a simple way to associate their public signing key with
their domain name in an end entity certificate that can be used in
TLS.
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Because of these PKIX requirements on end entity certificates, the
processing rules for TLSA are very different for certificates that
identify end entities directly and CA certificates that can be used
to validate PKIX end entity certificates. The rules here allow self-
signed certificates offered as type 1 certificates to not follow all
the PKIX semantics rules.
2.4. Presentation Format
The RDATA of the presentation format of the TLSA resource record
consists of two numbers (certificate and hash type) followed by the
bytes containing the certificate or the hash of the associated
certificate itself, presented in hex. An example of a SHA-256 hash
(type 1) of an end entity certificate (type 1) would be:
_443._tcp.www.example.com. IN TLSA (
1 1 5c1502a6549c423be0a0aa9d9a16904de5ef0f5c98
c735fcca79f09230aa7141 )
An example of an unhashed CA certificate (type 2) would be:
_443._tcp.www.example.com. IN TLSA (
2 0 308202c5308201ada00302010202090... )
Because the length of hashes and certificates can be quite long,
presentation format explicitly allows line breaks and white space in
the hex values; those characters are removed when converting to the
wire format.
2.5. Wire Format
The wire format is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cert type | Hash type | /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
/ /
/ Certificate for association /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The wire format for the RDATA in the first example given above would
be:
_443._tcp.www.example.com. IN TYPE65534 \# 34 ( 01015c1502a6549c42
3be0a0aa9d9a16904de5ef0f5c98c735fcca79f09230aa7141 )
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The wire format for the RDATA in the second example given above would
be:
_443._tcp.www.example.com. IN TYPE65534 \# 715 0200308202c5308201a...
Note that in the preceding examples, "TYPE65534" is given as an
example. That RR Type is in the IANA "private use" range; the real
RR Type for TLSA will be issued by IANA, as described in the IANA
Considerations section below.
3. Use of TLS Certificate Associations in TLS
In order to use one or more TLS certificate associations described in
this document obtained from the DNS, an application MUST assure that
the certificates were obtained using DNS protected by DNSSEC. TLSA
records must only be trusted if they were obtained from a trusted
source. This could be a localhost DNS resolver answer with the AD
bit set, an inline validating resolver library primed with the proper
trust anchors, or obtained from a remote nameserver to which one has
a secured channel of communication.
If a certificate association contains a hash type that is not
understood by the TLS client, that certificate association MUST be
marked as unusable.
An application that requests TLS certificate associations using the
method described in this document obtains zero or more usable
certificate associations. If the application receives zero usable
certificate associations, it processes TLS in the normal fashion.
If a match between one of the certificate association(s) and the
server's end entity certificate in TLS is found, the TLS client
continues the TLS handshake. If no match between the usable
certificate association(s) and the server's end entity certificate in
TLS is found, the TLS client MUST abort the handshake with an
"access_denied" error.
4. Mandatory-to-Implement Algorithms
DNS systems conforming to this specification MUST be able to create
TLSA records containing certificate types 1 and 2. DNS systems
conforming to this specification MUST be able to create TLSA records
using hash type 0 (no hash used) and hash type 1 (SHA-256), and
SHOULD be able to create TLSA records using hash type 2 (SHA-512).
TLS clients conforming to this specification MUST be able to
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correctly interpret TLSA records containing certificate types 1 and
2. TLS clients conforming to this specification MUST be able to
compare a certificate for association with a certificate from TLS
using hash type 0 (no hash used) and hash type 1 (SHA-256), and
SHOULD be able to make such comparisons with hash type 2 (SHA-512).
At the time this is written, it is expected that there will be a new
family of hash algorithms called SHA-3 within the next few years. It
is expected that some of the SHA-3 algorithms will be mandatory
and/or recommended for TLSA records after the algorithms are fully
defined. At that time, this specification will be updated.
5. IANA Considerations
5.1. TLSA RRtype
This document uses a new DNS RRType, TLSA, whose value is TBD. A
separate request for the RRType will be submitted to the expert
reviewer, and future versions of this document will have that value
instead of TBD.
5.2. TLSA Certificate Types
This document creates a new registry, "Certificate Types for TLSA
Resource Records". The registry policy is "RFC Required". The
initial entries in the registry are:
Value Short description Reference
----------------------------------------------------------
0 Reserved [This]
1 Certificate to identify an end entity [This]
2 CA's certificate [This]
3-254 Unassigned
255 Private use
Applications to the registry can request specific values that have
yet to be assigned.
5.3. TLSA Hash Types
This document creates a new registry, "Hash Types for TLSA Resource
Records". The registry policy is "Specification Required". The
initial entries in the registry are:
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Value Short description Reference
---------------------------------------------
0 No hash used [This]
1 SHA-256 NIST FIPS 180-3
2 SHA-512 NIST FIPS 180-3
3-254 Unassigned
255 Private use
Applications to the registry can request specific values that have
yet to be assigned.
6. Security Considerations
The security of the protocols described in this document relies on
the security of DNSSEC as used by the client requesting A/AAAA and
TLSA records.
A DNS administrator who goes rogue and changes both the A/AAAA and
TLSA records for a domain name can cause the user to go to an
unauthorized server that will appear authorized, unless the client
performs certificate validation and rejects the certificate. That
administrator could probably get a certificate issued anyway, so this
is not an additional threat.
The values in the TLSA data will be normally entered in the DNS
through the same system used to enter A/AAAA records, and other DNS
information for the host name. If the authentication for changes to
the host information is weak, an attacker can easily change any of
this information. Given that the TLSA data is not easily human-
readable, an attacker might change those records and A/AAAA records
and not have the change be noticed if changes to a zone are only
monitored visually.
If the authentication mechanism for adding or changing TLSA data in a
zone is weaker than the authentication mechanism for changing the
A/AAAA records, a man-in-the-middle who can redirect traffic to their
site may be able to impersonate the attacked host in TLS if they can
use the weaker authentication mechanism. A better design for
authenticating DNS would be to have the same level of authentication
used for all DNS additions and changes for a particular host.
SSL proxies can sometimes act as a man-in-the-middle for TLS clients.
In these scenarios, the clients add a new trust anchor whose private
key is kept on the SSL proxy; the proxy intercepts TLS requests,
creates a new TLS session with the intended host, and sets up a TLS
session with the client using a certificate that chains to the trust
anchor installed in the client by the proxy. In such environments,
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the TLSA protocol will prevent the SSL proxy from functioning as
expected because the TLS client will get a certificate association
from the DNS that will not match the certificate that the SSL proxy
uses with the client. The client, seeing the proxy's new certificate
for the supposed destination will not set up a TLS session.
7. Acknowledgements
Many of the ideas in this document have been discussed over many
years. More recently, the ideas have been discussed by the authors
and others in a more focused fashion. In particular, some of the
ideas here originated with Paul Vixie, Dan Kaminsky, Jeff Hodges,
Phill Hallam-Baker, Simon Josefsson, Warren Kumari, Adam Langley, Ben
Laurie, Ilari Liusvaara, Scott Schmit, and Ondrej Sury.
8. References
8.1. Normative References
[4347bis] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security version 1.2", draft-ietf-tls-rfc4347-bis (work in
progress), July 2010.
[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.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[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.
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8.2. Informative References
[RFC4025] Richardson, M., "A Method for Storing IPsec Keying
Material in DNS", RFC 4025, March 2005.
[RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely
Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
January 2006.
Authors' Addresses
Paul Hoffman
VPN Consortium
Email: paul.hoffman@vpnc.org
Jakob Schlyter
Kirei AB
Email: jakob@kirei.se
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