DNSSEC Working Group TKEY
INTERNET-DRAFT February 1998
Expires August 1998
Secret Key Establishment for DNS (TKEY RR)
------ --- ------------- --- --- ----- ---
Donald E. Eastlake 3rd
Status of This Document
This draft, file name draft-ietf-dnssec-tkey-00.txt, is intended to
be become a Proposed Standard RFC. Distribution of this document is
unlimited. Comments should be sent to the DNS security mailing list
<dns-security@tis.com> or to the author.
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
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``working draft'' or ``work in progress.''
To learn the current status of any Internet-Draft, please check the
1id-abstracts.txt listing contained in the Internet-Drafts Shadow
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Abstract
[draft-ietf-dnsind-tsig-*.txt] provides a means of authenticating and
securing Domain Name System (DNS) queries and responses using shared
secret keys via the TSIG resource record (RR). However, it provides
no mechanism for setting up such keys other than manual exchange.
This document describes a TKEY RR that can be used in a number of
different modes to establish shared secret keys between a DNS
resolver and server.
Acknowledgments
The substantial comments and ideas of the following persons (listed
in alphabetic order) have been incorporated herein and are gratefully
acknowledged:
Olafur Gudmundsson <ogud@tis.com>
Stuart Kwan <skwan@microsoft.com>
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Table of Contents
Status of This Document....................................1
Abstract...................................................2
Acknowledgments............................................2
Table of Contents..........................................3
1. Introduction............................................4
1.1 General Principles.....................................4
1.2 Overview of Contents...................................5
2. The TKEY Resource Record................................6
3. Exchange via Resolver Query.............................8
3.1 Query for Server Assigned Keying.......................9
3.2 Query for Diffie-Hellman Exchanged Keying..............9
3.3 Query for GSS-API Established.........................10
4. Spontaneous Server Inclusion...........................11
4.1 Spontaneous Server Assigned Keying....................11
4.2 Spontaneous Diffie-Hellman Keying.....................11
4.3 Spontaneous GSS-API Exchange..........................11
4.4 Spontaneous Key Deletion..............................12
5. TKEY Dynamic Update Requests...........................13
5.1 Exchange via TKEY 'Add'...............................13
5.2 TKEY Deletion.........................................13
6. Methods of Encryption..................................14
7. Security Considerations................................14
References................................................15
Author's Address..........................................16
Expiration and File Name..................................16
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1. Introduction
The Domain Name System (DNS) is a hierarchical, distributed, highly
available database used for mapping between domain names and
addresses, for email routing, and for other information [RFC 1034,
1035]. It has been extended to provide for public key security and
dynamic update [RFCs 2065, 2136, 2137].
[draft-ietf-dnsind-tsig-*.txt] provides a means of more efficiently
authenticating and securing DNS messages using shared secret keys via
the TSIG resource record (RR) but provides no mechanism for setting
up such keys other than manual exchange. This document describes a
TKEY RR that can be used in a number of different modes to establish
such shared secret keys between a DNS resolver and server.
1.1 General Principles
TKEY is a meta-RR that is not stored or cached in the DNS. It
supports a variety of modes for the establishment and deletion of
shared secret keys between DNS entities such as resolvers and
servers. The establishment of such a key requires that state be
maintained at both the resolver and the server and the allocation of
the resources to maintain such state may require mutual agreement. In
the absence of such agreement, servers are free to return errors for
any attempt to use TKEY and resolvers are free to ignore any TKEY RRs
they receive.
In all cases herein, the term "resolver" includes that part of a
server which makes full and partial zone transfer queries as well as
other queries.
Servers are not required to implement any particular mode or modes of
the defined modes of TKEY shared secret key establishment or deletion
and may return errors for any they do not support. Based on
experience, more modes may be added or some modes described herein
may be deprecated.
The means by which the shared secret keying material exchanged via
TKEY is actually used in any particular TSIG algorithm is algorithm
dependent and is defined in connection with that algorithm. [But
should probably define for HMAC-MD5 as an example in this draft...]
It should be noted that this keying material and TSIGs that use it
are associated with DNS hosts. They are not tied to zones. They may
be used to authenticate queries and responses but they do not provide
zone stored DNS data origin authentication [RFC 2065].
Two modes of TKEY, the server assigned and resolver assigned modes,
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perform encryption which may effect their export status from some
countries. All other aspects of DNS security, including the SIG,
KEY, NXT, and TSIG RRs and all other defined modes of TKEY perform
authentication (signatures and signature verification) only.
1.2 Overview of Contents
Section 2 below specifies the TKEY resource record (RR) and provides
a high level description of its consiituent fields.
Section 3 discusses key exchange via queries for type TKEY. This is
applicable to the server assigned, Diffie-Hellman exchange, and GSS-
API establishment modes.
Section 4 discusses spontaneous inclusion of TKEY RRs in responses by
servers. This is application to key deletion and to server assigned
and Diffie-Hellman exchange key establishment.
Section 5 discusses use of dynamic update requests for type TKEY.
This supports optional key exchange via resolver update request,
which is applicable to key deletion and to the resolver assigned
mode.
Section 6 describes encryption methods for transmitting secret key
information.
Finally, Section 7 touches on some security considerations.
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2. The TKEY Resource Record
The TKEY resource record (RR) has the structure given below. Its RR
type code is 249.
Field Type Comment
----- ---- -------
NAME domain see description below
TTYPE u_int16_t TKEY
CLASS u_int16_t ignored, should be zero
TTL u_int32_t always zero
RDLEN u_int16_t size of RDATA
RDATA: Algorithhm: domain
Inception: u_int32_t
Expiration: u_int32_t Mode: u_int16_t
Error: u_int16_t
Key Size: u_int16_t
Key Data: octet-stream
Other Size: u_int16_t
Other Data: octet-stream undefined by this protocol
The Name field's meaning differs somewhat with mode and context as
explained in subsequent sections.
The TTL field SHOULD always be zero to be sure that older DNS
implementations do not cache TKEY RRs.
The algorithm name is a domain name with the same meaning as in
[draft-ietf-dnsind-tsig-*.txt]. The algorithm determines how the
secret keying material exchanged using the TKEY RR is actually used
to derive the algorithm specific key that is used.
The inception time and expiration time are in number of seconds since
the beginning of 1 January 1970 GMT ignoring leap seconds. In
messages from a DNS resolver to a server where these fields are
meaningful, they are the requested validity interval for the keying
material. In messages from a server to a resolver where these fields
are meaningful, they specify that validity interval.
The mode field specifies the general scheme for key agreement. Note
that implementation of TKEY as a whole and of any particular mode is
optional. The following values of the Mode octet are defined or
reserved:
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Value Description
----- -----------
0 - reserved
1 server/responder assignment
2 Diffie-Hellman exchange
3 GSS-API negotiation
4 resolver/querier assignment
5 key deletion
6-65534 - available for IANA assignment
65535 -reserved
The error code field is an extended RCODE. The following values are
defined:
Value Description
----- -----------
0 - no error
1-15 a DNS RCODE
16 BADSIG
17 BADKEY
18 BADTIME
19 BADMODE other - available for IANA assignment
65535 - reserved
The key data size field is an unsigned 16 bit integer in network
order which specifies the size of the key exchange data field in
octets. The meaning of the key exchange data depends on the mode.
The other data size and data fields are not used. The RDLEN field
MUST equal the length of the RDATA section through the end of other
data or the RR is to be considered malformed and rejected.
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3. Exchange via Resolver Query
The most common method for a resolver and a server to establish a
shared secret key for use in TSIG is through queries from the
resolver for type TKEY. Such queries MUST either be accompanied by
one or more TKEY RRs in the additional information section to
indicate the mode(s) in use and other information where required or
the resolver and server must have a prior agreement that supplies any
information that would otherwise have had to be conveyed by TKEY
RR(s) in the query.
For TKEY(s) appearing in a query, The TKEY RR name SHOULD be a domain
locally unique at the resolver, less than 128 octets long, and
meaningful to the resolver to distinguish keys and/or key agreement
sessions. (For resolvers not wishing to make this use of the name,
it may be specified as root to minimize length.) For TKEY(s)
appearing in a response to a query, it SHOULD be a globally unique
server assigned domain. If the TKEY in a response is the result of a
query containing a TKEY with a non-root name, that query TKEY name
SHOULD be incorporated as the prefix of the response TKEY name.
Type TKEY queries SHOULD NOT be recursive and any accompanying TKEY
RRs should have a zero TTL.
For every mode defined below, the inception and expiration times in
the query TKEY are set to the time interval for which the resolver
wishes the requested key to be valid and they are set in a successful
response to the actual time interval during which the server will
consider the key valid. Future modes may be defined which ignore the
inception and expiration time fields.
It is recommended that the server require that queries be signed.
However, for currently defined modes, relatively little damage will
be done if an unsigned query of this sort is accepted and processed,
as described below for each mode. In addition, requiring that a TKEY
query be signed by a TSIG (if there exists an acceptable exchanged
key between the parties) or a SIG(0) may itself impose significant
computational requirements on the server, particularly in verifying
SIG(0) public key signatures.
Responses to TKEY queries SHOULD always have DNS transaction
signatures to protect the integrity of any keying data, error codes,
etc. This signature MUST use a previously established secret (TSIG)
or public (SIG(0)) key and MUST NOT use any key that the response to
be verified is itself providing.
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3.1 Query for Server Assigned Keying
In server assigned keying, the DNS server host generates the keying
material and it is sent to the resolver encrypted under a resolver
host key. See section 6 for description of encryption methods.
A resolver sends a query for type TKEY accompanied by a TKEY RR
specifying the "server assignment" mode and a resolver host KEY RR to
be used in encrypting the response, both in the additional
information section. The TKEY algorithm field is set to the signature
algorithm the resolver plans to use. Any "key data" provided in the
query TKEY may be either ignored by the server or strongly mixed with
server generated randomness [RFC 1750] to derive the keying material
to be used. The KEY that appears in the query SHOULD have a zero
TTL. It need not be accompanied by a SIG(KEY) and if the query is
signed by the resolver host and that signature is verified, then any
SIG(KEY) provided MAY be ignored for key exchange purposes. The KEY
RR in such a query SHOULD have a name that corresponds to the
resolver host but it is only essential that it be a public key for
which the resolver has the corresponding private key so it can
decrypt the response data.
Accepting and responding to an unsigned query of this sort may drain
some entropy from an entropy pool being maintained by the server and
used for secret key generation and so might enable an entropy
exhaustion attack. In addition, some significant amount of
computational resources may be used in the public key encryption of
response data. To protect against these effects, a server SHOULD
require such a query to be signed as described at the beginning of
Section 3 and MAY rate limit responses.
The server response contains a TKEY in its answer section with the
server assigned mode. If the error field is non-zero, the query
failed for the reason given. If the error field is zero, the KEY RR
provided in the query will be echoed back and the key data portion of
the response TKEY RR will be the server assigned keying data
encrypted under the public key in the KEY RR. The name of the TKEY
RR will be the server assigned name of the key and SHOULD be globally
unique.
3.2 Query for Diffie-Hellman Exchanged Keying
Diffie-Hellman (DH) key exchange is means whereby two parties can
derive some shared secret information without requiring any secrecy
of the messages they exchange [Schneier]. Provisions have been made
for the storage of DH public keys in the DNS [draft-ietf-dnssec-dhk-
*.txt].
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A client sends a query for type TKEY accompanied by a TKEY RR in the
additional information section specifying the "Diffie-Hellman" mode
and accompanied by a KEY RR specifying a client host Diffie-Hellman
key. The TKEY algorithm field is set to the signature algorithm the
resolver plans to use and any "key data" provided MUST be either
ignored by the server or treated as an error.
Accepting and responding to an unsigned query of this sort may use
significant computation at the server; however, if the server
requires that the request be signed, then if no shared secret is in
place to permit a TSIG to be used on the request, it would be
necessary to use a SIG(0) the verification of which would impose its
own computational load.
The server response contains a TKEY in its answer section with the
Diffie-Hellman mode. If the error field is non-zero, the query failed
for the reason given. If the error field is zero, the client host
supplied Diffie-Hellman KEY should be echoed back and a server host
Diffie-Hellman KEY RR will also be present.
Both parties can calculate the same shared secret quantity from the
pair of Diffie-Hellman keys used [Schneier] provided they use the
same modulus. If the server host does not have an appropriate
Diffie-Hellman key to use for the exchange, it should return the
BADKEY error.
3.3 Query for GSS-API Established
This is described in a separate document [draft-skwan-gss-tsig-*.txt]
which should be seen for the full description. Basically, when an
acceptable symmetric key is not yet in place, the resolver can send a
query for type TKEY with a TKEY specifying the GSS-API mode in the
additional information section and a GSS-API token in the key data
portion. The server responds with a TKEY specifying the GSS-API mode
and a GSS-API token in the key data portion. The resolver and server
feed these tokens to their local GSS implementation and interate
until an error is encountered or a key (GSS-API session) is
established. A similar exchange can be used to delete a GSS-API
session.
Any issues of possible encryption of the GSS-API token data being
transmitted are handled by the GSS-API level. In addition, the GSS-
API level provides authentication so that this mode of TKEY query and
response MAY be, but do not need to be, signed with TSIG or SIG(0).
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4. Spontaneous Server Inclusion
A DNS server may include TKEY RRs spontaneously as additional
information in responses. This SHOULD only be done if the server
knows the querier understands TKEY and has this option implemented.
The TKEY should have a zero TTL. This technique can be used to
establish a server assigned key, a Diffie-Hellman exchange key, for
GSS-API exchange, and to delete a key. A disadvantage of this
technique is that there is no way for the server to get any immediate
error or success indication back and, in the case of UDP, no way to
even know if the DNS response reached the resolver.
4.1 Spontaneous Server Assigned Keying
A server can include in the additional information section of a
response a server assignment mode TKEY with encrypted keying material
in its key data section along with a KEY RR specifying the client
public key used for the encryption. Such a response SHOULD be signed
but the KEY RR need not be signed by a SIG(KEY). A server should
only do this if there is sufficient room in a query and it has reason
to believe the resolver will understand such additional data. The
KEY RR used MUST be one for which the resolver host has the
corresponding private key or it will not be able to decrypt the
keying material.
4.2 Spontaneous Diffie-Hellman Keying
A server can include in the additional information section of a
response a Diffie-Hellman exchange mode TKEY along with two KEY RRs
specifying the client and server host public keys used for the
exchange. Such a response SHOULD be signed but the KEY RRs need not
be signed by a SIG(KEY). A server should only do this if there is
sufficient room in a query and it has reason to believe the resolver
host will understand such additional data.
4.3 Spontaneous GSS-API Exchange
A server can spontaneously include in the additional information
section of a response, a GSS-API mode TKEY. The information in the
key data section of such a TKEY is a GSS-API token which SHOULD be
feed by the resolver to its local GSS-API implementation. If such a
response is signed, the signature must verify before processing the
data. To the extent that GSS-API provides its own security, such a
response MAY NOT need to be signed. To the extent that GSS-API
Donald E. Eastlake, 3rd [Page 11]
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handled duplicated messages, such a spontaneous TKEY can be sent
repeatedly, until, perhaps, a response via a GSS-API mode TKEY query
is received.
4.4 Spontaneous Key Deletion
A server can hint to a client that it has deleted a key by
spontaneously including a TKEY RR in the additional information
section of a response with the key deletion mode. Such a response
SHOULD be signed. If authenticated, it deletes all keys with the
given name whose effective time period overlaps the inception to
expiration period given in the TKEY. Failure by a client to receive
or properly process such additional information in a response would
simply mean that the client might use a key that the server had
discarded and get back an error indication.
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5. TKEY Dynamic Update Requests
If a DNS server supports dynamic update [RFC 2136], then dynamic
update request can be used to exchange resolver assigned keys as
described in section 5.1 below and to delete previously exchanged
keys from a server as described in section 5.2 below.
5.1 Exchange via TKEY 'Add'
Optionally, a server can accept resolver assigned keys. The keying
material must be encrypted under a server host key for protection in
transmission as described in Section 6.
The resolver sends an update request adding a TKEY RR that specifies
the keying data with a KEY RR in the additional information section
specifying the server host public key used to encrypt the data. The
name of the key and the keying data are completely controlled by the
sending resolver so a globally unique key name SHOULD be used. The
server SHOULD require that this request be signed with a TSIG, if
there already exists an appropriate shared secret, or a SIG(0) by the
resolver host. The KEY RR used MUST be one for which the server has
the corresponding private key or it will not be able to decrypt the
keying material.
5.2 TKEY Deletion
Keys established via TKEY can be treated as soft state. Since DNS
transactions are originated by the resolver, the resolver can simply
toss keys, although it may have to go through another key exchange if
it later needs one. Similarly, the server can discard keys although
that will result in an error on receiving a query with a TSIG using
the discarded key.
The key expiration provided in the TKEY and the ability of each party
to discard keys may be adequate but servers that support dynamic
update [RFC 2136] may optionally implement key deletion whereby the
server discards a key on receipt from a resolver of a delete request
for a TKEY with the key's name. The mode and most fields of the TKEY
being "deleted" are ignored. But, to allow for the possibility of
multiple keys with the same name but different time periods, the only
key deleted are those whose time period overlaps with that specified
by the inception and expiration in the TKEY being "deleted".
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6. Methods of Encryption
For the server assigned and resolver assigned key exchange, the
keying material is sent within the keying data field of a TKEY RR
encrypted under the private key corresponding to the public key in an
accompanying KEY RR [RFC 2065]. The secret keying material being send
will generally be fairly short, usually less than 256 bits, because
that is adequate for strong protection with modern keyed hash or
symmetric algorithms.
If the KEY RR specifies the RSA algorithm, then the keying material
is encrypted as per the description of RSA encryption in PKCS-1.
(Note, the secret keying material being sent is directly RSA
encrypted, it is not "enveloped" under some other symetric
algorithm.) In the unlikely event that the keying material will not
fit within one RSA modulus of the chosen public key, additional RSA
encryption blocks are included. The length of each block is clear
from the public RSA key specified and the PKCS-1 padding makes it
clear what part of the encyrpted data is actually keying material and
what part is formatting or the required at least eight bytes of
random [RFC 1750] padding.
7. Security Considerations
To avoid different interpretations of the inception and expiration
times in TKEY RRs, resovlers and servers exchanging them must have
the same idea of what time it is.
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References
PKCS-1 - RSA Encryption Standard (An RSA Laboratories Technical
Note).
RFC 1034 - P. Mockapetris, "Domain Names - Concepts and Facilities",
STD 13, November 1987.
RFC 1035 - P. Mockapetris, "Domain Names - Implementation and
Specifications", STD 13, November 1987.
RFC 1750 - D. Eastlake, S. Crocker & J. Schiller, "Randomness
Recommendations for Security", December 1994.
RFC 2065 - D. Eastlake, C. Kaufman, "Domain Name System Security
Extensions", 01/03/1997.
RFC 2136 - P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", 04/21/1997.
RFC 2137 - Donald Eastlake, "Secure Domain Name System Dynamic
Update", 04/21/1997.
draft-ietf-dnsind-tsig-*.txt - P. Vixie, O. Gudmundsson, D.
Eastlake, "Secret Key Transaction Signatures for DNS (TSIG)".
draft-ietf-dnssec-dhk-*.txt - D. Eastlake
draft-skwan-gss-tsig-*.txt - S. Kwan, P. Garg, R. Viswanathan, "GSS
Algorithm for TSIG (GSS-TSIG)"
[Schneier] - Bruce Schneier, "Applied Cryptography: Protocols,
Algorithms, and Source Code in C", 1996, John Wiley and Sons
Donald E. Eastlake, 3rd [Page 15]
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Author's Address
Donald E. Eastlake 3rd
CyberCash, Inc.
318 Acton Street
Carlisle, MA 01741 USA
Telephone: +1 978 287 4877
+1 703 620-4200 (main office, Reston, VA)
FAX: +1 978 371 7148
email: dee@cybercash.com
Expiration and File Name
This draft expires August 1998.
Its file name is draft-ietf-dnssec-tkey-00.txt.
Donald E. Eastlake, 3rd [Page 16]
