Network Working Group P. Hoffman
Internet-Draft VPN Consortium
Intended status: Standards Track J. Schlyter
Expires: March 19, 2011 Kirei AB
W. Kumari
A. Langley
Google
September 15, 2010
Using Secure DNS to Associate Certificates with Domain Names For TLS
draft-hoffman-keys-linkage-from-dns-02
Abstract
TLS and DTLS uses 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. Instead of trusting a certificate authority to have made
this association correctly, the user might instead trust the
authoritative DNS server for the domain name to make that
association. This document describes how to use secure DNS to
associate the TLS server's certificate with the 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 March 19, 2011.
Copyright Notice
Copyright (c) 2010 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
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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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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 one
of many places in the certificate, must trust the trust anchor upon
which the server's certificate is rooted, and must perform correct
validation on the certificate.
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".
Some people want a different way to authenticate the association of
the server's certificate with the intended domain name without
trusting the 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.
A certificate association is a cryptographic hash of a certificate
(sometimes called a "fingerprint"). That is, a hash is taken of the
certificate, and that hash is the certificate association. The type
of hash function used can be chosen by the DNS administrator.
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.
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
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retrieved based on a DNS query, the domain name in the query is by
definition associated with the certificate.
This document only relates to securely getting the DNS information
for the certificate association using DNSSEC; other secure DNS
mechanisms are out of scope. 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 associating certificates for
TLS and DTLS; 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].
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].
This document is being discussed on the "keyassure" mailing list; see
<https://www.ietf.org/mailman/listinfo/keyassure>.
2. Getting TLS Certificate Associations from the DNS with the CERT RR
The CERT RR [RFC4398] allows expansion by defining new certificate
types. This document describes two new Certificate Types, TLSFP and
TLSRQ. A query on a domain name for the CERT RR can return one or
more records of the type CERT, and zero or more of those CERT
responses can be of type TLSFP and TLSRQ.
2.1. The TLSFP Certificate Type
This section describes the TLSFP certificate type of the CERT RR.
The TLSFP certificate type is TBD1. The key tag and algorithm fields
are both set to zero.
The format of the TLSFP certificate is a binary record, which MUST be
in the order defined here, is:
o A one-octet value, called "hash type", specifying the type of hash
algorithm used for the certificate association. This value has
the same values as those of the DS RR, as defined in the IANA
registry titled "Delegation Signer (DS) Resource Record (RR) Type
Digest Algorithms"
(<http://www.iana.org/assignments/ds-rr-types>).
o A one-octet value, called "validation preference", specifying the
preferences for further validation of the certificate in TLS. A
certificate association that contains a validation preference
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whose value is 1 indicates that the TLS administrator believes
that it is sufficient to match the certificate association with
the hash of certificate received in the TLS negotiation, and that
no further certificate validation is necessary. A certificate
association that contains a validation preference whose value is 0
indicates that the TLS administrator believes that it is not
sufficient to match the certificate association with the hash of
certificate received in the TLS negotiation, and that normal
certificate validation is necessary. Note that the validation
preference is an advisory, and it is not mandatory for a TLS
client to follow the advice.
o A variable-length set of bytes containing the hash of the
associated certificate (that is, of the certificate itself, not
the TLS ASN.1Cert object).
An example of a fingerprint for a single certificate:
www.example.com. IN CERT
TLSFP 0 0 AQDne3GdTpxjwLCgMzvgpBiOSQthjg==
An example of a fingerprint of a certificate that is found by its
hash value:
e77b719d4e9c63c0b0a0333be0a4188e490b618e.www.example.com. IN CERT
TLSFP 0 0 AQDne3GdTpxjwLCgMzvgpBiOSQthjg==
2.2. The TLSRQ Certificate Type
This section describes the TLSRQ certificate type of the CERT RR. If
a domain has many certificates associated with it, the number of
TLSFP CERT RRs may become impractical. In this case, a TLSRQ
certificate type may be used.
The semantics of the TLSRQ certificate type are that the requesting
party should send another query for the CERT RR that is formed by
prepending a label to the host name that is the hex of the SHA-1 hash
value taken over the certificate (not the TLS ASN.1Cert object). If
that certificate is a valid certificate that would have been returned
in a TLSFP certificate type, requesting it with this encoded hash
prepended to the host name will yield a TLSFP certificate type.
There is no security problem with using SHA-1 even if the SHA-1 hash
function continues to weaken because the hash is simply used to
differentiate the various certificates used by the server.
Note that the TLSRQ certificate type can only be used if the
requesting party knows the hash of the certificate that is being used
by the TLS server. This usually means that the request will be sent
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by the application acting as the TLS client after it has received the
TLS server's Certificate message that contains the server's
certificate. Such a request can slow down the TLS handshake
processing, but is required in the case where different hosts with
different certificates respond on the same domain name.
The typical scenario would look like the following:
1. The application client that is about to use TLS sends a CERT
query for www.example.com.
2. The name server responds with a CERT record that has a TLSRQ
certificate type.
3. The application client starts the TLS handshake, and receives a
Certificate message from the server.
4. The application client takes the SHA-1 hash of the certificate,
encodes that value as hex. For this example, assume that encoded
value is "e77b719d4e9c63c0b0a0333be0a4188e490b618e".
5. The application client sends a CERT query for
e77b719d4e9c63c0b0a0333be0a4188e490b618e.www.example.com. It
receives a response with a TLSFP certificate type.
6. The application uses the information in the TLSFP certificate
type and associates it with www.example.com.
The TLSRQ certificate type has a certificate body with a single octet
of 0. The TLSFP certificate type is TBD2.
For example:
www.example.com. IN CERT TLSRQ 0 0 AA=
3. Use of TLS Certificate Associations from the DNS in TLS
In order to use one or more TLS certificate associations obtained
from the DNS, an application MUST assure that the certificates were
obtained using DNS protected by DNSSEC. There may be other methods
to securely obtain certificates in DNS, but those methods are not
covered by this document.
If a certificate association contains a hash type that is not
understood by the TLS client, that certificate association MUST be
completely ignored.
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An application that requests TLS certificate associations using the
method described in the previous section obtains zero or more usable
certificate associations. If the application receives zero usable
certificate associations, it process TLS in the normal fashion.
If a match between the certificate association(s) and the server's
end entity certificate in TLS is not found, the TLS client MUST abort
the handshake with an "access_denied" error.
If the TLS server authenticates itself with a self-signed
certificate, it SHOULD be sure that the validation preference in the
CERT RR is set to 1. If the TLS server administrator believes that
there is information in its certificate that is relevant to the TLS
client other than the public key (such as a extended value (EV)
name), it SHOULD be sure that the validation preference in the CERT
RR is set to 0.
4. IANA Considerations
This document requests that IANA allocates two certificate types from
the CERT RR certificate type registry
(<http://www.iana.org/assignments/cert-rr-types>). The types are to
be allocated from the 'IETF Consensus' range.
Decimal type: TBD1
Type: TLSFP
Meaning: TLS certificate associations
Decimal type: TBD2
Type: TLSRQ
Meaning: Client should request TLS certificate associations
retrieved using hashes
5. Security Considerations
The security of the protocols described in this document relies on
the security of DNSSEC as used by the client requesting A and CERT
records.
A DNS administrator who goes rogue and changes both the A and CERT
records for a domain name can cause the user to go to an unauthorized
server that will appear authorized.
The SHA-1 hash used in the queries after the TLSRQ certificate type
is only used to differentiate certificates. If there is a collision
between the SHA-1 hashes of two certificates used by the servers that
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are at the host name, there is no problem because both of those
certificates will have the same association to the domain name.
6. 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,
Simon Josefsson, Phill Hallam-Baker, among others.
7. References
7.1. 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.
[RFC4398] Josefsson, S., "Storing Certificates in the Domain Name
System (DNS)", RFC 4398, March 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
7.2. 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
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Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
January 2006.
Appendix A. Ideas Considered But Not Necessarily Chosen
This appendix will list some of the ideas that have been kicked
around in this space and give a few paragraphs why they weren't
chosen for the current version this proposal. The following is a
placeholder for the list that will be filled out more in future
versions of this document:
o A flag that indicates that the certificate with the associated key
must be signed by a trusted CA. Briefly: this was not added
because it is up to the TLS server to decide which type of
certificate it wants to serve up. Serving a self-signed
certificate would effectively disable traditional PKIX validation,
whereas serving a certificate signed by a trusted CA would require
both validation by DNSSEC and the trusted CA.
o A flag that indicates that all connections to this server need to
be TLS secured. Briefly: this is a good idea but it is not
related to the key of the certificate given in TLS, and thus
should be indicated in a different method.
o Giving keys instead of hashes of keys. Briefly: TLS requires that
the server gives a certificate, and some systems use the metadata
from a CA-signed certificate for display, so there is value to
always looking in the certificate.
o Hashes of keys vs. hashes of certificates. Briefly: we have
changed our minds (at least once) on this. Our original thinking
was that there are many reasons why someone might change their
certificate while leaving the public key alone, and those changes
should not have to force them to change the DNS record because
they do not actually change what the TLS client cares about; thus,
use hashes of keys. Our new thinking is that there are
certificate semantics that we want to pass (namely, should the
client actually do the certificate validation), and attaching
those semantics to keys is confusing; thus, use hashes of
certificates.
o List TLS/DTLS ports or services for which the certificate is
associated. Briefly: we had this in an earlier version of this
document but got rid of it when it was pointed out that this is an
edge case, and most servers differentiate these services by domain
names such as "mail.example.com" and "www.example.com".
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o Different ways of encoding this information in the DNS. Briefly:
we considered a new RR type and coming up with an encoding of the
TXT RR type, but didn't see any significant advantage of them over
using the CERT RR, and there were disadvantages. A disadvantage
of a new RR type is getting DNS servers and clients to recognize
it; a disadvantage of coming up with a new TXT format is that
doing so prevents wildcards. There is a lot more to discuss here,
but the authors are now happy with a new sub-type for the CERT RR.
o Having the hash be over the TLS certificate structure instead of
just the end-entity certificate. Briefly: the TLS certificate
structure currently allows a chain of PKIX certificates, and the
semantics of what is being associated in a chain is not clear.
Further, the structure might be changed in the future (such as to
allow a group of web-of-trust OpenPGP certificates), and the
semantics of what is being associated would become even less
clear.
Appendix B. Changes between -00 and -01
Change the association from being a hash of the key of a PKIX
certificate to being a hash of a certificate (PKIX or other). This,
of course, makes large changes throughout the document.
Expanded the document to cover DTLS as well.
Added a pointer to the keyassure mailing list.
Removed the proposals for two alternate formats (the TLSFP Resource
Record and the TXT record encoding). Added a bit to Appendix A about
this.
Got rid of the specification for ports within a single domain name.
Made the hash type one octet and used the DS registry instead of
defining our own.
Added "Necessarily" chosen in the title of Appendix A to show that we
might (continue to) change our minds after discussion.
Added Simon Josefsson to the acknowledgements.
Appendix C. Changes between -01 and -02
Added the TLSRQ certificate type and its semantics.
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Pointed to the IANA registry for DS hash types.
Added Phill Hallam-Baker to the acknowledgements.
Authors' Addresses
Paul Hoffman
VPN Consortium
Email: paul.hoffman@vpnc.org
Jakob Schlyter
Kirei AB
Email: jakob@kirei.se
Warren Kumari
Google
Email: warren@kumari.net
Adam Langley
Google
Email: agl@google.com
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