Network Working Group P. Hoffman
Internet-Draft VPN Consortium
Intended status: Standards Track J. Schlyter
Expires: July 29, 2011 Kirei AB
January 25, 2011
Using Secure DNS to Associate Certificates with Domain Names For TLS
draft-ietf-dane-protocol-03
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. Instead of trusting a certification 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 July 29, 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
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carefully, as they describe your rights and restrictions with respect
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.
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 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.
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 also only applies to PKIX [RFC5280]
certificates.
Certificate associations are made between a certificate or the hash
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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
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 in a very unusual
sense. That is, 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 one 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
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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
query on a domain name for the TLSA RR can return one or more records
of the type TLSA. The TLSA RRType is TBD.
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 -- Hash of an end-entity certificate
2 -- Full end-entity certificate in DER encoding
3 -- Hash of an certification authority's certificate
4 -- Full certification authority's certificate in DER encoding
o A one-octet value, called "hash type", specifying the type of hash
algorithm used for the certificate association. This value is
defined in a new IANA registry. When no hashing is used (that is,
in the certificate types where the full certificate is given), the
hash type MUST be 0. 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 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 through 4 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.
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2.1. Making Certificate Associations
Items received in TLSA resource records can be treated like trust
anchors by the TLS client. The trust anchor MUST NOT be loaded for
longer than the TTL on the TSLA record.
The TLS client determines whether or not the certificate offered by
the TLS server matches the certificate association in the TLSA
resource record. If the certificate from the TLS server matches, the
TLS client accepts the certificate association. Each certificate
type has a different method for determining matching.
For types 1 and 3, the hash used in the comparison is the hash type
from the TLSA data.
Types 1 (hash of an end-entity certificate) and 2 (full end-entity
certificate) are matched against the first certificate offered by the
TLS server. With these two types, the trust anchor is used only for
exact matching, not for chained validation. For type 1, the
certificate association is valid if the hash of the first certificate
offered by the TLS server matches the value from the resource record.
For type 2, the certificate association is valid if the certificate
in the TLSA data matches to the first certificate offered by TLS.
Type 3 (hash of certification authority's certificate) can be used in
one of two ways. 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 repositor, and the hash
of that trust anchor matches the value from the TLSA data, then the
certificate association is valid.
Type 4 (full certification authority's certificate) 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 trust anchor from the TLSA data.
[[ Need discussion of self-signed certificates being CA certificates.
Need to be sure that this discussion uses correct PKIX terminology
and is carefully explained. ]]
2.2. 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
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certificate itself, presented in hex. An example of a SHA-256 hash
(type 2) of an end-entity certificate (type 1) would be:
www.example.com. IN TLSA (
1 2 5c1502a6549c423be0a0aa9d9a16904de5ef0f5c98
c735fcca79f09230aa7141 )
An example of an unhashed (type 0) CA certificate (type 4) would be:
www.example.com. IN TLSA (
4 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.
2.3. 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:
www.example.com. IN TYPE65534 \# 34 ( 01025c1502a6549c423be0a0aa
9d9a16904de5ef0f5c98c735fcca79f09230aa7141 )
The wire format for the RDATA in the second example given above would
be:
www.example.com. IN TYPE65534 \# 715 0400308202c5308201ada003020...
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
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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 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.
4. IANA Considerations
4.1. 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.
4.2. 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 Ref.
-------------------------------------------------------------
0 Reserved [This]
1 Hash of an end-entity cert [This]
2 Full end-entity cert in DER encoding [This]
3 Hash of an CA's cert [This]
4 Full CA's cert in DER encoding [This]
5-254 Unassigned
Applications to the registry can request specific values that have
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yet to be assigned.
4.3. 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:
Value Short description Ref.
-----------------------------------------------------
0 No hash used [This]
1 SHA-1 NIST FIPS 180-2
2 SHA-256 NIST FIPS 180-2
3 SHA-384 NIST FIPS 180-2
4-254 Unassigned
Applications to the registry can request specific values that have
yet to be assigned.
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 TLSA
records.
A DNS administrator who goes rogue and changes both the A 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
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used for all DNS additions and changes for a particular host.
[[ Add discussion of the idea that TLSA makes things worse if an
intermediate CA is compromised. Need more from Stephen Farrell. ]]
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,
Phill Hallam-Baker, Simon Josefsson, Warren Kumari, Adam Langley,
Ilari Liusvaara, and Ondrej Sury.
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.
[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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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.
Authors' Addresses
Paul Hoffman
VPN Consortium
Email: paul.hoffman@vpnc.org
Jakob Schlyter
Kirei AB
Email: jakob@kirei.se
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