< draft-hoffman-keys-linkage-from-dns-00.txt   draft-hoffman-keys-linkage-from-dns-01.txt >
Network Working Group P. Hoffman Network Working Group P. Hoffman
Internet-Draft VPN Consortium Internet-Draft VPN Consortium
Intended status: Standards Track J. Schlyter Intended status: Standards Track J. Schlyter
Expires: February 14, 2011 Kirei AB Expires: February 25, 2011 Kirei AB
W. Kumari W. Kumari
A. Langley A. Langley
Google Google
August 13, 2010 August 24, 2010
Using Secure DNS to Associate Keys with Domain Names For TLS Using Secure DNS to Associate Certificates with Domain Names For TLS
draft-hoffman-keys-linkage-from-dns-00 draft-hoffman-keys-linkage-from-dns-01
Abstract Abstract
TLS uses PKIX certificates for authenticating the server. Users want TLS and DTLS uses certificates for authenticating the server. Users
their applications to verify that the key in the certificate provided want their applications to verify that the certificate provided by
by the TLS server is in fact associated with the domain name they the TLS server is in fact associated with the domain name they
expect. Instead of trusting a certificate authority to have made expect. Instead of trusting a certificate authority to have made
this association correctly, the user might instead trust the this association correctly, the user might instead trust the
authoritative DNS server for the domain name to make that authoritative DNS server for the domain name to make that
association. This document describes how to use secure DNS to association. This document describes how to use secure DNS to
associate the key that appears in a TLS server's certificate with the associate the TLS server's certificate with the the intended domain
the intended domain name. name.
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 14, 2011. This Internet-Draft will expire on February 25, 2011.
Copyright Notice Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License. described in the Simplified BSD License.
1. Introduction 1. Introduction
The first response from the server in TLS [RFC5246] may contain a The first response from the server in TLS may contain a certificate.
PKIX certificate. In order for the TLS client to authenticate that In order for the TLS client to authenticate that it is talking to the
it is talking to the expected TLS server, the client must validate expected TLS server, the client must validate that this certificate
that the key in this certificate is associated with the domain name is associated with the domain name used by the client to get to the
used by the client to get to the server. Currently, the client must server. Currently, the client must extract the domain name from one
extract the domain name from one of many places in the PKIX of many places in the certificate, must trust the trust anchor upon
certificate, must trust the trust anchor upon which the server's PKIX which the server's certificate is rooted, and must perform correct
certificate is rooted, and must perform correct PKIX validation on validation on the certificate.
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 Some people want a different way to authenticate the association of
the key in the server's certificate with the intended domain name the server's certificate with the intended domain name without
without trusting the CA. Given that the DNS administrator for a trusting the CA. Given that the DNS administrator for a domain name
domain name is authorized to give identifying information about the is authorized to give identifying information about the zone, it
zone, it makes sense to allow that administrator to also make an makes sense to allow that administrator to also make an authoritative
authoritative binding between the domain name and a public key that binding between the domain name and a certificate that might be used
might be used by a host at that domain name. The easiest way to do by a host at that domain name. The easiest way to do this is to use
this is to use the DNS. the DNS.
A key association is a cryptographic hash of the public key in a PKIX A certificate association is a cryptographic hash of a certificate
certificate (sometimes called a "fingerprint"). That is, a hash is (sometimes called a "fingerprint"). That is, a hash is taken of the
taken of the DER-encoded subjectPublicKeyInfo field of the PKIX certificate, and that hash is the certificate association. The type
certificate as defined in [RFC5280], and that hash is the key of hash function used can be chosen by the DNS administrator.
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], DNSSEC, which is defined in RFCs 4033, 4034, and 4035 ([RFC4033],
[RFC4034], and [RFC4035]), uses cryptographic keys and digital [RFC4034], and [RFC4035]), uses cryptographic keys and digital
signatures to provide authentication of DNS data. Information signatures to provide authentication of DNS data. Information
retrieved from the DNS and that is validated using DNSSEC is thereby retrieved from the DNS and that is validated using DNSSEC is thereby
proved to be the authoritative data. proved to be the authoritative data.
This document defines a secure method to associate the key in the This document defines a secure method to associate the certificate
PKIX certificate that is obtained from the TLS server with a domain that is obtained from the TLS server with a domain name using DNS
name using DNS protected by DNSSEC. Because the key association was protected by DNSSEC. Because the certificate association was
retrieved based on a DNS query, the domain name in the query is by retrieved based on a DNS query, the domain name in the query is by
definition associated with the key. definition associated with the certificate.
This document only relates to securely getting the DNS information This document only relates to securely getting the DNS information
for the key association using DNSSEC; other secure DNS mechanisms are for the certificate association using DNSSEC; other secure DNS
out of scope. The DNSSEC signature MUST be validated on all mechanisms are out of scope. The DNSSEC signature MUST be validated
responses in order to assure the proof of origin of the data. on all responses in order to assure the proof of origin of the data.
This document only relates to securely getting keys for TLS; other This document only relates to securely associating certificates for
security protocols are handled in other documents. For example, keys TLS and DTLS; other security protocols are handled in other
for IPsec are covered in [RFC4025] and keys for SSH are covered in documents. For example, keys for IPsec are covered in [RFC4025] and
[RFC4255]. keys for SSH are covered in [RFC4255].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
2. Getting TLS Key Associations from the DNS This document is being discussed on the "keyassure" mailing list; see
<https://www.ietf.org/mailman/listinfo/keyassure>.
This section describes three equivalent methods for encoding TLS
associations: a new certificate type of 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 the appendix.
2.1. The TLSFP Certificate Type of the CERT RR 2. Getting TLS Certificate Associations from the DNS with the CERT RR
This section describes the TLSFP Certificate Type of the CERT RR.
The CERT RR [RFC4398] allows expansion by defining new certificate The CERT RR [RFC4398] allows expansion by defining new certificate
types. The new TLSFP certificate type is defined here. A query on a 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 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 type CERT, and zero or more of those CERT responses can be of type
TLSFP. TLSFP.
The format of the TLSFP certificate type is binary. In the record, o The TLSFP certificate type is TBD.
all integers consist of two bytes in network byte order. The 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 specifying the type of hash algorithm used for the key
association.
o A variable-length set of bytes containing the hash 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 the certificate type because a request for a
CERT RR can yield multiple results. If there is a later improvement
to 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. A query on 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 o The key tag and algorithm fields are both set to zero.
made to this RR later. The initial version number is 42.
o An integer specifying how many port numbers are listed. If this The format of the TLSFP certificate is a binary record, which MUST be
value is zero (0), the key association is valid for any port. in the order defined here, is:
o An optional unordered set of two-byte integers, ranging from 1 to o A one-octet value, called "hash type", specifying the type of hash
65535, specifying the TCP/UDP ports for which the key association algorithm used for the certificate association. This value has
is valid. the same values as those of the DS RR, as defined in [RFC4034] and
its successors.
o An integer specifying the type of hash algorithm used for the key o A one-octet value, called "validation preference", specifying the
association. preferences for further validation of the certificate in TLS. A
certificate association that contains a validation preference
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 o A variable-length set of bytes containing the hash of the
associated key. associated certificate.
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 is chosen. ]]
2.3. Using a TXT Resource Record with a _TLSFP Label Prefix
A request for a TXT RR whose domain is the label _tlsfp prepended to
a domain name can be used to get the KEY associated with the domain
name. A query of this can return one or more records of the type
TXT.
The format of the TXT response is ASCII text. The record, which MUST
be 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 is 42.
o Zero or more instances of "port=" followed by an TCP/UDP port for
which the key association is valid (expressed as an integer),
followed by ";". If a port is not specified, the key association
is valid 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 the set of bytes containing the hash of the
associated key; the bytes are encoded as lower-case hexadecimal.
For example: For example:
_tlsfp.www.example.com. IN TXT "ver=42; port=443; type=1; www.example.com. IN CERT TLSFP 0 0 ( AQGa+FZd8sAeS03ca8xDigDQ
hash=20960a7a55a70693c11b192eddc5983c38e72ffc1ff21ef6dc84 ePgJnQvgMe/kKyf8rzluiQ== )
58b7769d917d
2.4. Key Association Hash Algorithms
The initial list of key association hash algorithms is:
o 0 - reserved
o 1 - SHA2-256 [RFC4634]
o 2 - SHA2-384 [RFC4634] 3. Use of TLS Certificate Associations from the DNS in TLS
o 3 - SHA2-512 [RFC4634] In order to use one or more TLS certificate associations obtained
Defining other key association hash types requires IETF consensus as from the DNS, an application MUST assure that the certificates were
defined in [RFC2434]. 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.
For interoperability reasons, as few hash algorithm as possible An application that requests TLS certificate associations using the
should be reserved. The only reason to reserve additional types is method described in the previous section obtains zero or more
to increase security. certificate associations. If the application receives zero
certificate associations, it process TLS in the normal fashion.
3. Use of TLS Key Associations from the DNS in TLS If a certificate association contains a hash type that is not
understood by the TLS client, that certificate association MUST be
completely ignored.
In order to use one or more TLS key associations obtained from the If a match between the certificate association(s) and the server's
DNS, an application MUST assure that the keys were obtained using DNS end entity certificate in TLS is not found, the TLS client MUST abort
protected by DNSSEC. There may be other methods to securely obtain the handshake with an "access_denied" error.
keys in DNS, but those methods are not covered by this document.
An application that requests TLS keys using the method described in If the TLS server authenticates itself with a self-signed
the previous section obtains zero or more key associations. If the certificate, it SHOULD be sure that the validation preference in the
application receives zero key associations, it process TLS in the CERT RR is set to 1. If the TLS server administrator believes that
normal fashion. If one or more key associations are received from there is information in its certificate that is relevant to the TLS
the DNS: 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.
o If the PKIX certificate given by the TLS server is signed by a CA 4. IANA Considerations
trusted by the client, the application compares each key
association with the the hash of the key from the certificate,
using the same hash function that is given in the key association
type. If a match is found, the TLS handshake continues as normal,
including the TLS client doing all PKIX validation checks.
o If the PKIX certificate given by the TLS server is not signed by a This document requests that IANA allocates one certificate type from
CA trusted by the client, the application compares each key the CERT RR certificate type registry. The type is to be allocated
association with the the hash of the key from the certificate, from the 'IETF Consensus' range.
using the same hash function that is given in the key association
type. If a match is found, the TLS handshake continues using the
key from the certificate, but with no PKIX validations checks
being performed.
In either of the above cases, if a match between the key Decimal type: TBD
association(s) is not found, the TLS client MUST abort the handshake
with an "access_denied" error.
4. IANA Considerations Type: TLSFP
[[ TBD. Will include the registration for the TLSFP RR if that is Meaning: TLS Certificate Associations
the style chosen, as well as a new registry for hash algorithm types,
depending on what style is decided on. ]]
5. Security Considerations 5. Security Considerations
[[ TBD. This section will need to describe, at least, the "attack" [[ TBD. This section will need to describe, at least, the "attack"
where a DNS administrator goes rogue and changes both the A and TLSFP where a DNS administrator goes rogue and changes both the A and CERT
records for a domain name. Also will discuss the need for secure records for a domain name. Also will discuss the need for secure
DNS. ]] DNS. ]]
6. Acknowledgements 6. Acknowledgements
Many of the ideas in this document have been discussed over many Many of the ideas in this document have been discussed over many
years. More recently, the ideas have been discussed by the authors years. More recently, the ideas have been discussed by the authors
and others in a more focused fashion. In particular, some of the and others in a more focused fashion. In particular, some of the
ideas here originated with Paul Vixie, Dan Kaminsky, and Jeff Hodges, ideas here originated with Paul Vixie, Dan Kaminsky, Jeff Hodges,
among others. Simon Josefsson, among others.
7. References 7. References
7.1. Normative 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 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. 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. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005. RFC 4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions", Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005. RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005. Extensions", RFC 4035, March 2005.
[RFC4398] Josefsson, S., "Storing Certificates in the Domain Name [RFC4398] Josefsson, S., "Storing Certificates in the Domain Name
System (DNS)", RFC 4398, March 2006. 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 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008. (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 7.2. Informative References
[RFC4025] Richardson, M., "A Method for Storing IPsec Keying [RFC4025] Richardson, M., "A Method for Storing IPsec Keying
Material in DNS", RFC 4025, March 2005. Material in DNS", RFC 4025, March 2005.
[RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely [RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely
Publish Secure Shell (SSH) Key Fingerprints", RFC 4255, Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
January 2006. January 2006.
Appendix A. Ideas Considered But Not Chosen Appendix A. Ideas Considered But Not Necessarily Chosen
This appendix will list some of the ideas that have been kicked 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 around in this space and give a few paragraphs why they weren't
chosen for this proposal. The following is a placeholder for the chosen for the current version this proposal. The following is a
list that will be filled out more in future versions of this placeholder for the list that will be filled out more in future
document: versions of this document:
o A flag that indicates that the certificate with the associated key o A flag that indicates that the certificate with the associated key
must be signed by a trusted CA. Briefly: this was not added 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 because it is up to the TLS server to decide which type of
certificate it wants to serve up. Serving a self-signed certificate it wants to serve up. Serving a self-signed
certificate would effectively disable traditional PKIX validation, certificate would effectively disable traditional PKIX validation,
whereas serving a certificate signed by a trusted CA would require whereas serving a certificate signed by a trusted CA would require
both validation by DNSSEC and the trusted CA. both validation by DNSSEC and the trusted CA.
o A flag that indicates that all connections to this server need to 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 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 related to the key of the certificate given in TLS, and thus
should be indicated in a different method. should be indicated in a different method.
o Giving keys instead of fingerprints. Briefly: TLS requires that o Giving keys instead of hashes of keys. Briefly: TLS requires that
the server gives a PKIX certificate, and some systems use the the server gives a certificate, and some systems use the metadata
metadata from a CA-signed certificate for display, so there is from a CA-signed certificate for display, so there is value to
value to always looking in the certificate. always looking in the certificate.
o After a format for the information is chosen, the other two listed o Hashes of keys vs. hashes of certificates. Briefly: we have
earlier will go into this appendix. 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 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.
Authors' Addresses Authors' Addresses
Paul Hoffman Paul Hoffman
VPN Consortium VPN Consortium
Email: paul.hoffman@vpnc.org Email: paul.hoffman@vpnc.org
Jakob Schlyter Jakob Schlyter
Kirei AB Kirei AB
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