wdiff draft-hoffman-keys-linkage-from-dns-00.txt draft-hoffman-keys-linkage-from-dns-01.txt

Network Working Group P. Hoffman
Internet-Draft VPN Consortium
Intended status: Standards Track J. Schlyter
Expires: February 14, 25, 2011 Kirei AB
W. Kumari
A. Langley
Google
August 13, 24, 2010

Using Secure DNS to Associate Keys Certificates with Domain Names For TLS
draft-hoffman-keys-linkage-from-dns-00
draft-hoffman-keys-linkage-from-dns-01

Abstract

TLS and DTLS uses PKIX certificates for authenticating the server. Users
want their applications to verify that the key in 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 key that appears in a 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
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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 February 14, 25, 2011.

This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License.

1. Introduction

The first response from the server in TLS [RFC5246] may contain a
PKIX certificate.
In order for the TLS client to authenticate that it is talking to the
expected TLS server, the client must validate that the key in 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 PKIX certificate, must trust the trust anchor upon
which the server's PKIX certificate is rooted, and must perform correct PKIX
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 key in 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 public key certificate that might be used
by a host at that domain name. The easiest way to do this is to use
the DNS.

A key certificate association is a cryptographic hash of the public key in a PKIX certificate
(sometimes called a "fingerprint"). That is, a hash is taken of the DER-encoded subjectPublicKeyInfo field of the PKIX
certificate as defined in [RFC5280],
certificate, and that hash is the key 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 key in the
PKIX certificate
that is obtained from the TLS server with a domain name using DNS
protected by DNSSEC. Because the key certificate association was
retrieved based on a DNS query, the domain name in the query is by
definition associated with the key. certificate.

This document only relates to securely getting the DNS information
for the key 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 getting keys associating certificates for TLS;
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 Key Certificate Associations from the DNS

This section describes three equivalent methods for encoding TLS
associations: a new certificate type of with the existing CERT RR (defined
in [RFC4398]), a new resource record (RR) called "TLSFP" and a TXT RR
that can be emitted when the query has "_tlsfp" as the leftmost
label.

EXTREMELY IMPORTANT NOTE: Only one of these methods describe in this
document should be selected for the final protocol. We have listed
them in our approximate order of preference, but look forward to
discussion. When that decision is made, the two methods not used
will be moved to

This section describes the appendix.

2.1. The TLSFP Certificate Type of the CERT RR RR.
The CERT RR [RFC4398] allows expansion by defining new certificate
types. The new TLSFP certificate type is defined here. 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.

o The TLSFP certificate type is TBD.

o The key tag and algorithm fields are both set to zero.

The format of the TLSFP certificate type is binary. In the record,
all integers consist of two bytes in network byte order. The a binary record, which MUST be
in the order defined here, is:

o An integer specifying how many port numbers are listed. If this
value is zero (0), the key association is valid for any port.

o An optional unordered set of two-byte integers, ranging from 1 to
65535, specifying the TCP/UDP ports for which the key association
is valid.

o An integer A one-octet value, called "hash type", specifying the type of hash
algorithm used for the key certificate association.

o A variable-length set of bytes containing This value has
the hash same values as those of the
associated key.

For example:

www.example.com. IN CERT TLSFP 0 0 ( AQG7ASCWCnpVpwaT
wRsZLt3FmDw45y/8H/Ie9tyEWLd2nZF9 )

Note that, unlike the following two format proposals, no version
number is needed for DS RR, as defined in [RFC4034] and
its successors.

o A one-octet value, called "validation preference", specifying the certificate type because a request
preferences for a
CERT RR can yield multiple results. If there is a later improvement
to further validation of the TLSFP certificate type, it could be sent along with a TLSFP
certificate type in a response.

2.2. The TLSFP Resource Record

The new RR TLSFP resource record is defined here. TLS. A query on
certificate association that contains a
domain name for the TLSFP type can return one or more records of the
type TLSFP.

The format of the TLSFP response is binary. In the record, all
integers consist of two bytes in network byte order. The record,
which MUST be in the order defined here, is:

o The version number. This is useful if non-critical changes are
made to this RR later. The initial version number is 42.

o An integer specifying how many port numbers are listed. If this validation preference
whose value is zero (0), 1 indicates that the key association TLS administrator believes
that it is valid for any port.

o An optional unordered set of two-byte integers, ranging from 1 sufficient to
65535, specifying the TCP/UDP ports for which match the key certificate association
is valid.

o An integer specifying the type of hash algorithm used for the key
association.

o A variable-length set of bytes containing with
the hash of certificate received in the
associated key.

For example:

www.example.com. IN TLSFP 42 1 443 1 20960a7a55a706
93c11b192eddc5983c38e72ffc1ff21ef6dc8458b7769d917d

[[ This will need a proper RRTYPE definition. That will be added
later if this option TLS negotiation, and that
no further certificate validation is chosen. ]]

2.3. Using a TXT Resource Record with a _TLSFP Label Prefix necessary. A request for certificate
association that contains a TXT RR validation preference whose domain value is 0
indicates that the label _tlsfp prepended to
a domain name can be used TLS administrator believes that it is not
sufficient to get match the KEY associated certificate association with the domain
name. A query of this can return one or more records of the type
TXT.

The format hash of the TXT response is ASCII text. The record, which MUST
be
certificate received in the order defined here, is:

o One instance of "ver=", the version number, followed by ";",
followed by ";". This is useful if non-critical changes are made
to this RR later. The initial version number TLS negotiation, and that normal
certificate validation is 42.

o Zero or more instances of "port=" followed by an TCP/UDP port for
which necessary. Note that the key association validation
preference is valid (expressed as an integer),
followed by ";". If a port advisory, and it is not specified, the key association
is valid mandatory for all ports.

o The type of hash algorithm used for key association, specified as
"type=nn;" where "nn" is an integer defined below.

o "hash=" followed by a TLS
client to follow the advice.

o A variable-length set of bytes containing the hash of the
associated key; the bytes are encoded as lower-case hexadecimal. certificate.

For example:

_tlsfp.www.example.com.

www.example.com. IN TXT "ver=42; port=443; type=1;
hash=20960a7a55a70693c11b192eddc5983c38e72ffc1ff21ef6dc84
58b7769d917d

2.4. Key Association Hash Algorithms

The initial list of key association hash algorithms is:

o CERT TLSFP 0 - reserved

o 1 - SHA2-256 [RFC4634]

o 2 - SHA2-384 [RFC4634]

o 3 - SHA2-512 [RFC4634]
Defining other key association hash types requires IETF consensus as
defined in [RFC2434].

For interoperability reasons, as few hash algorithm as possible
should be reserved. The only reason to reserve additional types is
to increase security. 0 ( AQGa+FZd8sAeS03ca8xDigDQ
ePgJnQvgMe/kKyf8rzluiQ== )

3. Use of TLS Key Certificate Associations from the DNS in TLS

In order to use one or more TLS key certificate associations obtained
from the DNS, an application MUST assure that the keys certificates were
obtained using DNS protected by DNSSEC. There may be other methods
to securely obtain
keys certificates in DNS, but those methods are not
covered by this document.

An application that requests TLS keys certificate associations using the
method described in the previous section obtains zero or more key
certificate associations. If the application receives zero key
certificate associations, it process TLS in the normal fashion.

If one or more key associations are received from
the DNS:

o If the PKIX certificate given by the TLS server is signed by a CA
trusted by the client, the application compares each key certificate association with the the hash of the key from the certificate,
using the same contains a hash function type that is given in not
understood by the key TLS client, that certificate association
type. MUST be
completely ignored.

If a match is found, the TLS handshake continues as normal,
including the TLS client doing all PKIX validation checks.

o If between the PKIX certificate given by association(s) and the server's
end entity certificate in TLS server is not signed by a
CA trusted by found, the client, TLS client MUST abort
the application compares each key
association handshake with an "access_denied" error.

If the TLS server authenticates itself with a self-signed
certificate, it SHOULD be sure that the hash of the key from validation preference in the certificate,
using
CERT RR is set to 1. If the same hash function TLS server administrator believes that
there is given information in the key association
type. If a match its certificate that is found, relevant to the TLS handshake continues using
client other than the public key from the certificate, but with no PKIX validations checks
being performed.

In either of the above cases, if (such as a match between the key
association(s) is not found, extended value (EV)
name), it SHOULD be sure that the TLS client MUST abort validation preference in the handshake
with an "access_denied" error. CERT
RR is set to 0.

4. IANA Considerations

[[ TBD. Will include the registration for

This document requests that IANA allocates one certificate type from
the TLSFP CERT RR if that certificate type registry. The type is to be allocated
from the style chosen, as well as a new registry for hash algorithm types,
depending on what style is decided on. ]] 'IETF Consensus' range.

Decimal type: TBD

Type: TLSFP

Meaning: TLS Certificate Associations

5. Security Considerations

[[ TBD. This section will need to describe, at least, the "attack"
where a DNS administrator goes rogue and changes both the A and TLSFP CERT
records for a domain name. Also will discuss the need for secure
DNS. ]]

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, and Jeff Hodges,
Simon Josefsson, 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.

[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.

[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.

[RFC4634] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and HMAC-SHA)", RFC 4634, July 2006.

[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.

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
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 fingerprints. hashes of keys. Briefly: TLS requires that
the server gives a PKIX 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 After 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".

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 information 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 chosen, not clear.
Further, the other 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 listed
earlier will go into this appendix. 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.

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