DNS Extensions Yuji Kamite
Internet-Draft NTT Communications
Expires: April 15, 2005 Masaya Nakayama
The University of Tokyo
October 14, 2004
TKEY Secret Key Renewal Mode
draft-ietf-dnsext-tkey-renewal-mode-05
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
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which he or she is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with
RFC 3668.
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document defines a new mode in TKEY and proposes an atomic
method for changing secret keys used for TSIG periodically.
Originally, TKEY provides methods of setting up shared secrets other
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than manual exchange, but it cannot control timing of key renewal
very well though it can add or delete shared keys separately. This
proposal is a systematical key renewal procedure intended for
preventing signing DNS messages with old and non-safe keys
permanently.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Defined Words . . . . . . . . . . . . . . . . . . . . . . 3
1.2 New Format and Assigned Numbers . . . . . . . . . . . . . 4
1.3 Overview of Secret Key Renewal Mode . . . . . . . . . . . 4
2. Shared Secret Key Renewal . . . . . . . . . . . . . . . . . . 5
2.1 Key Usage Time Check . . . . . . . . . . . . . . . . . . . 5
2.2 Partial Revocation . . . . . . . . . . . . . . . . . . . . 6
2.3 Key Renewal Message Exchange . . . . . . . . . . . . . . . 7
2.3.1 Query for Key Renewal . . . . . . . . . . . . . . . . 7
2.3.2 Response for Key Renewal . . . . . . . . . . . . . . . 7
2.3.3 Attributes of Generated Key . . . . . . . . . . . . . 8
2.3.4 TKEY RR structure . . . . . . . . . . . . . . . . . . 8
2.4 Key Adoption . . . . . . . . . . . . . . . . . . . . . . . 10
2.4.1 Query for Key Adoption . . . . . . . . . . . . . . . . 10
2.4.2 Response for Key Adoption . . . . . . . . . . . . . . 10
2.5 Keying Schemes . . . . . . . . . . . . . . . . . . . . . . 11
2.5.1 DH Exchange for Key Renewal . . . . . . . . . . . . . 11
2.5.2 Server Assigned Keying for Key Renewal . . . . . . . . 12
2.5.3 Resolver Assigned Keying for Key Renewal . . . . . . . 13
2.6 Considerations about Non-compliant Hosts . . . . . . . . . 14
3. Secret Storage . . . . . . . . . . . . . . . . . . . . . . . . 15
4. Compulsory Key Revocation . . . . . . . . . . . . . . . . . . 15
4.1 Compulsory Key Revocation by Server . . . . . . . . . . . 15
4.2 Authentication Methods Considerations . . . . . . . . . . 15
5. Special Considerations for Two Servers' Case . . . . . . . . 16
5.1 To Cope with Collisions of Renewal Requests . . . . . . . 16
6. Key Name Considerations . . . . . . . . . . . . . . . . . . . 17
7. Example Usage of Secret Key Renewal Mode . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1 Normative References . . . . . . . . . . . . . . . . . . . 21
11.2 Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . . 23
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1. Introduction
TSIG [RFC2845] provides DNS message integrity and the
request/transaction authentication by means of message authentication
codes (MAC). TSIG is a practical solution in view of calculation
speed and availability. However, TSIG does not have exchanging
mechanism of shared secret keys between server and resolver, and
administrators might have to exchange secret keys manually. TKEY
[RFC2930] is introduced to solve such problem and it can exchange
secrets for TSIG via networks.
In various modes of TKEY, a server and a resolver can add or delete a
secret key be means of TKEY message exchange. However, the existing
TKEY does not care fully about the management of keys which became
too old, or dangerous after long time usage.
It is ideal that the number of secret which a pair of hosts share
should be limited only one, because having too many keys for the same
purpose might not only be a burden to resolvers for managing and
distinguishing according to servers to query, but also does not seem
to be safe in terms of storage and protection against attackers.
Moreover, perhaps holding old keys long time might give attackers
chances to compromise by scrupulous calculation.
Therefore, when a new shared secret is established by TKEY, the
previous old secret should be revoked immediately. To accomplish
this, DNS servers must support a protocol for key renewal. This
document specifies procedure to refresh secret keys between two hosts
which is defined within the framework of TKEY, and it is called "TKEY
Secret Key Renewal Mode".
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", "MAY" and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
1.1. Defined Words
* Inception Time: Beginning of the shared secret key lifetime. This
value is determined when the key is generated.
* Expiry Limit: Time limit of the key's validity. This value is
determined when a new key is generated. After Expiry Limit, server
and client (resolver) must not authenticate TSIG signed with the key.
Therefore, Renewal to the next key should be carried out before
Expiry Limit.
* Partial Revocation Time: Time when server judges the key is too old
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and must be updated. It must be between Inception Time and Expiry
Limit. This value is determined by server freely following its
security policy. e.g., If the time from Inception to Partial
Revocation is short, renewal will be carried out more often, which
might be safer.
* Revocation Time: Time when the key becomes invalid and can be
removed. This value is not determined in advance because it is the
actual time when revocation is completed.
* Adoption Time: Time when the new key is adopted as the next key
formally. After Adoption, the key is valid and server and client can
generate or verify TSIG making use of it. Adoption Time also means
the time when it becomes possible to remove the previous key, so
Revocation and Adoption are usually done at the same time.
Partial
Inception Revocation Revocation Expiry Limit
| | | |
|----------------|- - - - - - >>|- (revoked) -|
| | | |
previous key | | |
|- - - -|-------------------->> time
| | new key
Inception Adoption
1.2. New Format and Assigned Numbers
TSIG
ERROR = (PartialRevoke), TBD
TKEY
Mode = (server assignment for key renewal), TBD
Mode = (Diffie-Hellman exchange for key renewal), TBD
Mode = (resolver assignment for key renewal), TBD
Mode = (key adoption), TBD
1.3. Overview of Secret Key Renewal Mode
When a server receives a query from a client signed with a TSIG key,
It always checks if the present time is within the range of usage
duration it considers safe. If it is judged that the key is too old,
i.e., after Partial Revocation Time, the server comes to be in
Partial Revocation state about the key, and this key is called
partially revoked.
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In this state, if a client sends a normal query (e.g., question about
A RR) other than TKEY Renewal request with TSIG signed with the old
key, the server returns an error message to notify that the time to
renew has come. This is called "PartialRevoke" error message. It is
server's choice whether it returns PartialRevoke or not. If and only
if the server is ready for changing its own key, it decides to return
PartialRevoke.
The client which got this error is able to notice that it is
necessary to refresh the secret. To make a new shared secret, it
sends a TKEY Renewal request, in which several keying methods are
available. It can make use of TSIG authentication signed with the
partially revoked key mentioned above.
After new secret establishment, the client sends a TKEY Adoption
request for renewal confirmation. This can also be authenticated with
the partially revoked key. If this is admitted by the server, the new
key is formally adopted, and at the same time the corresponding old
secret is invalidated. Then the client can send the first query again
signed with the new key.
Key renewal procedure is executed based on two-phase commit
mechanism. The first phase is the TKEY Renewal request and its
response, which means preparatory confirmation for key update. The
second phase is Adoption request and its response. If the server gets
request and client receives the response successfully, they can
finish renewal process. If any error happens and renewal process
fails during these phases, client should roll back to the beginning
of the first phase, and send TKEY Renewal request again. This
rollback can be done until the Expiry Limit of the key.
2. Shared Secret Key Renewal
Suppose a server and a client agree to change their TSIG keys
periodically. Key renewal procedure is defined between two hosts.
2.1. Key Usage Time Check
Whenever a server receives a query with TSIG and can find a key that
is used for signing it, the server checks its Inception Time, Partial
Revocation Time and Expiry Limit (this information is usually
memorized by the server).
When the present time is before Inception Time, the server MUST NOT
verify TSIG with the key, and server acts the same way as when the
key used by the client is not recognized. It follows [RFC2845] 4.5.1.
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When the present time is equal to Inception Time, or between
Inception Time and Partial Revocation Time, the behavior of the
server is the same as when a valid key is found. It follows [RFC2845]
4.5.2 and 4.5.3.
When the present time is the same as the Partial Revocation Time, or
between the Partial Revocation Time and Expiry Limit, the server
comes to be in Partial Revocation state about the TSIG key and
behaves according to the next section.
When the present time is the same as the Expiry Time or after it, the
server MUST NOT verify TSIG with the key, and returns error messages
in the same way as when the key used by the client is not recognized.
It follows [RFC2845] 4.5.1.
2.2. Partial Revocation
In Partial Revocation state, we say the server has partially revoked
the key and the key has become a "partially revoked key".
If server has received a query signed with the partially revoked key
for TKEY Renewal request (See section 2.3.) or Key Adoption request
(See section 2.4.), then server does proper process following each
specification. If it is for TKEY key deletion request ([RFC2930]
4.2), server MAY process usual deletion operation defined therein.
If server receives other types of query signed with the partially
revoked key, and both the corresponding MAC and signed TIME are
verified, then server begins returning answer whose TSIG error code
is "PartialRevoke" (See section 9.). Server MUST randomly but with
increasing frequency return PartialRevoke when in the Partial
Revocation state.
Server can decide when it actually sends PartialRevoke, checking if
it is appropriate time for renewal. Server MUST NOT return
PartialRevoke if this is apart long lived TSIG transaction (such as
AXFR) that started before the Partial Revocation Time.
If the client receives PartialRevoke and understands it, then it MUST
retry the query with the old key unless a new key has been adopted.
Client SHOULD start the process to renew the TSIG key. For key
renewal procedure, see details in Section 2.3 and 2.4.
PartialRevoke period (i.e., time while server returns PartialRevoke
randomely) SHOULD be small, say 2-5% of key lifetime. This is
server's choice.
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Server MUST keep track of clients ignoring PartialRevoke, thus
indicating ignorance of this TKEY mode.
PartialRevoke error messages have the role to inform clients of the
keys' partial revocation and urge them to send TKEY Renewal requests.
These error responses MUST be signed with those partial revoked keys
if the queries are signed with them. They are sent only when the
signing keys are found to be partially revoked. If the MAC of TSIG
cannot be verified with the partially revoked keys, servers MUST NOT
return PartialRevoke error with MAC, but MUST return another error
such as "BADSIG" without MAC (following [RFC2845] 4.5.3); in other
words, a server informs its key's partial revocation only when the
MAC in the received query is valid.
2.3. Key Renewal Message Exchange
2.3.1. Query for Key Renewal
If a client has received a PartialRevoke error and authenticated the
response based on TSIG MAC, it sends a TKEY query for Key Renewal (in
this document, we call it Renewal request, too.) to the server. The
request MUST be signed with TSIG or SIG(0) [RFC2931] for
authentication. If TSIG is selected, the client can sign it with the
partial revoked key.
Key Renewal can use one of several keying methods which is indicated
in "Mode" field of TKEY RR, and its message structure is dependent on
that method.
2.3.2. Response for Key Renewal
The server which has received Key Renewal request first tries to
verify TSIG or SIG(0) accompanying it. If the TSIG is signed and
verified with the partially revoked key, the request MUST be
authenticated.
After authentication, server must check existing old key's validity.
If the partially revoked key indicated in the request TKEY's OldName
and OldAlgorithm field (See section 2.3.4.) does not exist at the
server, "BADKEY" [RFC2845] is given in Error field for response. If
any other error happens, server returns appropriate error messages
following the specification described in section 2.5. If there are no
errors, server returns a Key Renewal answer. This answer MUST be
signed with TSIG or SIG(0) for authentication.
When this answer is successfully returned and no error is detected by
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client, a new shared secret can be established. The details of
concrete keying procedure are given in the section 2.5.
Note:
Sometimes Adoption message and new Renewal request will cross on
the wire. In this case the newly generated key Adoption message is
resent.
2.3.3. Attributes of Generated Key
As a result of this message exchange, client comes to know the newly
generated key's attributes such as key's name, Inception Time and
Expiry Limit. They are decided by the server and told to the client;
in particular, however, once the server has decided Expiry Limit and
returned a response, it should obey the decision as far as it can. In
other words, they SHOULD NOT change time values for checking Expiry
Limit in the future without any special reason, such as security
issue like "Emergency Compulsory Revocation" described in section 8.
On the other hand, Partial Revocation Time of this generated key is
not decided based on the request, and not informed to the client. The
server can determine any value as long as it is between Inception
Time and Expiry Limit. However, the period from Inception to Partial
Revocation SHOULD be fixed as the server side's configuration or be
set the same as the corresponding old key's one.
Note:
Even if client sends Key Renewal request though the key described
in OldName has not been partially revoked yet, server does renewal
processes. At the moment when the server accepts such requests
with valid authentication, it MUST forcibly consider the key is
already partially revoked, that is, the key's Partial Revocation
Time must be changed into the present time (i.e., the time when
the server receives the request).
2.3.4. TKEY RR structure
TKEY RR for Key Renewal message has the structure given below. In
principle, format and definition for each field follows [RFC2930].
Note that each keying scheme sometimes needs different interpretation
of RDATA field; for detail, see section 2.5.
Field Type Comment
------- ------ -------
NAME domain used for a new key, see below
TYPE u_int16_t (defined in [RFC2930])
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CLASS u_int16_t (defined in [RFC2930])
TTL u_int32_t (defined in [RFC2930])
RDLEN u_int16_t (defined in [RFC2930])
RDATA:
Algorithm: domain algorithm for a new key
Inception: u_int32_t about the keying material
Expiration: u_int32_t about the keying material
Mode: u_int16_t scheme for key agreement
see section 9.
Error: u_int16_t see description below
Key Size: u_int16_t see description below
Key Data: octet-stream
Other Size: u_int16_t (defined in [RFC2930])
size of other data
Other Data: newly defined: see description below
For "NAME" field, both non-root and root name are allowed. It may
be used for a new key's name in the same manner as [RFC2930] 2.1.
"Algorithm" specifies which algorithm is used for agreed keying
material, which is used for identification of the next key.
"Inception" and "Expiration" are used for the valid period of
keying material. The meanings differ somewhat according to whether
the message is request or answer, and its keying scheme.
"Key Data" has different meanings according to keying schemes.
"Mode" field stores the value in accordance with the keying method,
and see section 2.5. Servers and clients supporting TKEY Renewal
method MUST implement "Diffie-Hellman exchange for key renewal"
scheme. All other modes are OPTIONAL.
"Error" is an extended RCODE which includes "PartialRevoke" value
too. See section 9.
"Other Data" field has the structure given below. They describe
attributes of the key to be renewed.
in Other Data filed:
Field Type Comment
------- ------ -------
OldNAME domain name of the old key
OldAlgorithm domain algorithm of the old key
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"OldName" indicates the name of the previous key (usually,
this is partially revoked key's name that client noticed by
PartialRevoke answer from server), and "OldAlogirthm"
indicates its algorithm.
2.4. Key Adoption
2.4.1. Query for Key Adoption
After receiving a TKEY Renewal answer, the client gets the same
secret as the server. Then, it sends a TKEY Adoption request. The
request's question section's QNAME field is the same as the NAME
filed of TKEY written below. In additional section, there is one TKEY
RR that has the structure and values described below.
"NAME" field is the new key's name to be adopted which was already
generated by Renewal message exchange. "Algorithm" is its
algorithm. "Inception" means the key's Inception Time, and
"Expiration" means Expiry Limit.
"Mode" field is the value of "key adoption". See section 9.
"Other Data" field in Adoption has the same structure as that of
Renewal request message. "OldName" means the previous old key, and
"OldAlogirthm" means its algorithm.
Key Adoption request MUST be signed with TSIG or SIG(0) for
authentication. The client can sign TSIG with the previous key. Note
that until Adoption is finished, the new key is treated as invalid,
thus it cannot be used for authentication immediately.
2.4.2. Response for Key Adoption
The server which has received Adoption request, it verifies TSIG or
SIG(0) accompanying it. If the TSIG is signed with the partially
revoked key and can be verified, the message MUST be authenticated.
If the next new key indicated by the request TKEY's "NAME" is not
present at the server, BADNAME [RFC2845] is given in Error field and
the error message is returned.
If the next key exists but it has not been adopted formally yet, the
server confirms the previous key's existence indicated by the
"OldName" and "OldAlgorithm" field. If it succeeds, the server
executes Adoption of the next key and Revocation of the previous key.
Response message duplicates the request's TKEY RR with NOERROR,
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including "OldName" and "OldAlgorithm" that indicate the revoked key.
If the next key exists but it is already adopted, the server returns
a response message regardless of the substance of the request TKEY's
"OldName". In this response, Response TKEY RR has the same data as
the request's one except as to its "Other Data" that is changed into
null (i.e., "Other Size" is zero), which is intended for telling the
client that the previous key name was ignored, and the new key is
already available.
Client sometimes has to retry Adoption request. Suppose the client
sent request signed with the partially revoked key, but its response
did not return successfully (e.g., due to the drop of UDP packet).
Client will probably retry Adoption request; however, the request
will be refused in the form of TSIG "BADKEY" error because the
previous key was already revoked. In this case, client will
retransmit Adoption request signed with the next key, and expect a
response which has null "Other Data" for confirming the completion of
renewal.
2.5. Keying Schemes
In Renewal message exchanges, there are no limitations as to which
keying method is actually used. The specification of keying
algorithms is independent of the general procedure of Renewal that is
described in section 2.3.
Now this document specifies three algorithms in this section, but
other future documents can make extensions defining other methods.
2.5.1. DH Exchange for Key Renewal
This scheme is defined as an extended method of [RFC2930] 4.1. This
specification only describes the difference from it and special
notice; assume that all other points, such as keying material
computation, are the exactly same as the specification of [RFC2930]
4.1.
Query
In Renewal request for type TKEY with this mode, there is one TKEY
RR and one KEY RR in the additional information section. KEY RR is
the client's Diffie-Hellman public key [RFC2539].
QNAME in question section is the same as that of "NAME" field in
TKEY RR, i.e., it means the requested new key's name.
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TKEY "Mode" field stores the value of "DH exchange for key
renewal". See section 9.
TKEY "Inception" and "Expiration" are those requested for the
keying material, that is, requested usage period of a new key.
TKEY "Key Data" is used as a random, following [RFC2930] 4.1.
Response
The server which received this request first verifies the TSIG,
SIG(0) or DNSSEC lookup of KEY RR used. After authentication, the
old key's existence validity is checked, following section 2.3. If
any incompatible DH key is found in the request, "BADKEY"
[RFC2845] is given in Error field for response. "FORMERR" is given
if the query included no DH KEY.
If there are no errors, the server processes a response according
to Diffie-Hellman algorithm and returns the answer. In this
answer, there is one TKEY RR in answer section and KEY RR(s) in
additional section.
As long as no error has occurred, all values of TKEY are equal to
that of the request message except TKEY NAME, TKEY RDLEN, RDATA's
Inception, Expiration, Key Size and Key Data.
TKEY "NAME" field in the answer specifies the name of newly
produced key which the client MUST use.
TKEY "Inception" and "Expiration" mean the periods of the produced
key usage. "Inception" is set to be the time when the new key is
actually generated or the time before it, and it will be regarded
as Inception Time. "Expiration" is determined by the server, and
it will be regarded as Expiry Limit.
TKEY "Key Data" is used as an additional nonce, following
[RFC2930] 4.1.
The resolver supplied Diffie-Hellman KEY RR SHOULD be echoed in
the additional section and a server Diffie-Hellman KEY RR will
also be present in the answer section, following [RFC2930] 4.1.
2.5.2. Server Assigned Keying for Key Renewal
This scheme is defined as an extended method of [RFC2930] 4.4. This
specification only describes the difference from it and special
notice; assume that all other points, such as secret encrypting
method, are the exactly same as the specification of [RFC2930] 4.4.
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Query
In Renewal request for type TKEY with this mode, there is one TKEY
RR and one KEY RR in the additional information section. KEY RR is
used in encrypting the response.
QNAME in question section is the same as that of "NAME" field in
TKEY RR, i.e., it means the requested new key's name.
TKEY "Mode" field stores the value of "server assignment for key
renewal". See section 9.
TKEY "Inception" and "Expiration" are those requested for the
keying material, that is, requested usage period of a new key.
TKEY "Key Data" is provided following the specification of
[RFC2930] 4.4.
Response
The server which received this request first verifies the TSIG,
SIG(0) or DNSSEC lookup of KEY RR used. After authentication, the
old key's existence validity is checked, following section 2.3.
"FORMERR" is given if the query specified no encryption key.
If there are no errors, the server response contains one TKEY RR
in the answer section, and echoes the KEY RR provided in the query
in the additional information section.
TKEY "NAME" field in the answer specifies the name of newly
produced key which the client MUST use.
TKEY "Inception" and "Expiration" mean the periods of the produced
key usage. "Inception" is set to be the time when the new key is
actually generated or the time before it, and it will be regarded
as Inception Time. "Expiration" is determined by the server, and
it will be regarded as Expiry Limit.
TKEY "Key Data" is the assigned keying data encrypted under the
public key in the resolver provided KEY RR, which is the same as
[RFC2930] 4.4.
2.5.3. Resolver Assigned Keying for Key Renewal
This scheme is defined as an extended method of [RFC2930] 4.5. This
specification only describes the difference from it and special
notice; assume that all other points, such as secret encrypting
method, are the exactly same as the specification of [RFC2930] 4.5.
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Query
In Renewal request for type TKEY with this mode, there is one TKEY
RR and one KEY RR in the additional information section. TKEY RR
has the encrypted keying material and KEY RR is the server public
key used to encrypt the data.
QNAME in question section is the same as that of "NAME" field in
TKEY RR, i.e., it means the requested new key's name.
TKEY "Mode" field stores the value of "resolver assignment for key
renewal". See section 9.
TKEY "Inception" and "Expiration" are those requested for the
keying material, that is, requested usage period of a new key.
TKEY "Key Data" is the encrypted keying material.
Response
The server which received this request first verifies the TSIG,
SIG(0) or DNSSEC lookup of KEY RR used. After authentication, the
old key's existence validity is checked, following section 2.3.
"FORMERR" is given if the server does not have the corresponding
private key for the KEY RR that was shown sin the request.
If there are no errors, the server returns a response. The
response contains a TKEY RR in the answer section to tell the
shared key's name and its usage time values.
TKEY "NAME" field in the answer specifies the name of newly
produced key which the client MUST use.
TKEY "Inception" and "Expiration" mean the periods of the produced
key usage. "Inception" is set to be the time when the new key is
actually generated or the time before it, and it will be regarded
as Inception Time. "Expiration" is determined by the server, and
it will be regarded as Expiry Limit.
2.6. Considerations about Non-compliant Hosts
Key Renewal requests and responses must be exchanged between hosts
which can understand them and do proper processes. PartialRevoke
error messages will be only ignored if they should be returned to
non-compliant hosts.
Note that server does not inform actively the necessity of renewal to
clients, but inform it as responses invoked by client's query.
Server needs not care whether the PartialRevoke errors has reached
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client or not. If client has not received yet because of any reasons
such as packet drops, it will resend the queries, and finally will be
able to get PartialRevoke information.
3. Secret Storage
Every server keeps all secrets and attached information, e.g.,
Inception Time, Expiry Limit, etc. safely to be able to recover from
unexpected stop. To accomplish this, formally adopted keys SHOULD be
memorized not only on memory, but also be stored in the form of some
files. It will become more secure if they are stored in ecrypted
form.
4. Compulsory Key Revocation
4.1. Compulsory Key Revocation by Server
There is a rare but possible case that although servers have already
partially revoked keys, clients do not try to send any Renewal
requests. If this state continues, in the future it will become the
time of Expiry Limit. After Expiry Limit, the keys will be expired
and completely removed, so this is called Compulsory Key Revocation
by server.
If Expiry Limit is too distant from the Partial Revocation Time, then
even though very long time passes, clients will be able to refresh
secrets only if they add TSIG signed with those old partially revoked
keys into requests, which is not safe.
On the other hand, if Expiry Limit is too close to Partial Revocation
Time, perhaps clients might not be able to notice their keys' Partial
Revocation by getting "PartialRevoke" errors.
Therefore, servers should set proper Expiry Limit to their keys,
considering both their keys' safety, and enough time for clients to
send requests and process renewal.
4.2. Authentication Methods Considerations
It might be ideal to provide both SIG(0) and TSIG as authentication
methods. For example:
A client and a server start SIG(0) authentication at first, to
establish TSIG shared keys by means of "Query for Diffie-Hellman
Exchanged Keying" as described in [RFC2930] 4.1. Once they get
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shared secret, they keep using TSIG for queries and responses.
After a while the server returns a "ParitalRevoke" error and they
begin a key renewal process. Both TSIG signed with partially
revoked keys and SIG(0) are okay for authentication, but TSIG would
be easier to use considering calculation efficiency.
Suppose now client is halted for long time with some reason.
Because server does not execute any renewal process, it will
finally do Compulsory Revocation. Even if client restarts and sends
a key Renewal request, it will fail because old key is already
deleted at server.
At this moment, however, if client also uses SIG(0) as another
authentication method, it can make a new shared key again and
recover successfully by sending "Query for Diffie-Hellman Exchanged
Keying" with SIG(0).
5. Special Considerations for Two servers' Case
This section refers to the case where both hosts are DNS servers
which can act as full resolvers as well and using one shared key
only. If one server (called Server A) wants to refresh a shared key
(called "Key A-B"), it will await a TKEY Renewal request from the
other server (called Server B). However, perhaps Server A wants to
refresh the key right now.
In this case, Server A is allowed to send a Renewal request to Server
B, if Server A knows the Key A-B is too old and wants to renew it
immediately.
Note that the initiative in key renewal belongs to Server A because
it can notice the Partial Revocation Time and decide key renewal. If
Server B has information about Partial Revocation Time as well, it
can also decide for itself to send Renewal request to Server A.
However, it is not essential for both two servers have information
about key renewal timing.
5.1. To Cope with Collisions of Renewal Requests
At least one of two hosts which use Key Renewal must know their key
renewal information such as Partial Revocation Time. It is okay that
both hosts have it.
Provided that both two servers know key renewal timing information,
there is possibility for them to begin partial revocation and sending
Renewal requests to each other at the same time. Such collisions will
not happen so often because Renewal requests are usually invoked when
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hosts want to send queries, but it is possible.
When one of two servers tries to send Renewal requests, it MUST
protect old secrets that it has partially revoked and prevent it from
being refreshed by any requests from the other server (i.e., it must
lock the old secret during the process of renewal). While the server
is sending Renewal requests and waiting responses, it ignores the
other server's Renewal requests.
Therefore, servers might fail to change secrets by means of their own
requests to others. After failure they will try to resend, but they
should wait for random delays by the next retries. If they get any
Renewal requests from others while they are waiting, their shared
keys may be refreshed, then they do not need to send any Renewal
requests now for themselves.
6. Key Name Considerations
Since both servers and clients have only to distinguish new secrets
and old ones, keys' names do not need to be specified strictly.
However, it is recommended that some serial number or key generation
time be added to the name and that the names of keys between the same
pair of hosts should have some common labels among their keys. For
example, suppose A.example.com. and B.example.com. share the key
"<serial number>.A.example.com.B.example.com." such as
"10010.A.example.com.B.example.com.". After key renewal, they change
their secret and name into "10011.A.example.com.B.example.com."
Servers and clients must be able to use keys properly for each query.
Because TSIG secret keys themselves do not have any particular IDs to
be distinguished and would be identified by their names and
algorithm, it must be understood correctly what keys are refreshed.
7. Example Usage of Secret Key Renewal Mode
This is an example of Renewal mode usage where a Server,
server.example.com, and a Client, client.exmple.com have an initial
shared secret key named "00.client.example.com.server.example.com".
(1) The time values for key
"00.client.example.com.server.example.com" was set as follows:
Inception Time is at 1:00, Expiry Limit is at 21:00.
(2) At Server, renewal time has been set: Partial Revocation Time
is at 20:00.
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(3) Suppose the present time is 19:55. If Client sends a query
signed with key "00.client.example.com.server.example.com" to ask
the IP address of "www.example.com", finally it will get a proper
answer from Server with valid TSIG (NOERROR).
(4) At 20:05. Client sends a query to ask the IP address of
"www2.example.com". It is signed with key
"00.client.example.com.server.example.com". Server returns an
answer for the IP address. However, server has begun retuning
PartialRevoke Error randomely. This answer includes valid TSIG MAC
signed with "00.client.example.com.server.example.com", and its
Error Code indicates PartialRevoke. Client understands that the
current key is partially revoked.
(5) At 20:06. Client sends a Renewal request to Server. This
request is signed with key
"00.client.example.com.server.example.com". It includes data such
as:
Question Section:
QNAME = 01.client.example.com. (Client can set this freely)
TYPE = TKEY
Additional Section:
01.client.example.com. TKEY
Algorithm = hmac-md5-sig-alg.reg.int.
Inception = (value meaning 20:00)
Expiration = (value meaning next day's 16:00)
Mode = (DH exchange for key renewal)
OldName = 00.client.example.com.server.example.com.
OldAlgorithm = hmac-md5-sig-alg.reg.int.
Additional Section also contains a KEY RR for DH and a TSIG RR.
(6) As soon as Server receives this request, it verifies TSIG. It
is signed with the partially revoked key
"00.client.example.com.server.example.com". and Server accepts the
request. It creates a new key by Diffie-Hellman calculation and
returns an answer which includes data such as:
Answer Section:
01.client.example.com.server.example.com. TKEY
Algorithm = hmac-md5-sig-alg.reg.int.
Inception = (value meaning 20:00)
Expiration = (value meaning next day's 16:00)
Mode = (DH exchange for key renewal)
OldName = 00.client.example.com.server.example.com.
OldAlgorithm = hmac-md5-sig-alg.reg.int.
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Answer Section also contains KEY RRs for DH.
Additional Section also contains a TSIG RR.
This response is signed with key
"00.client.example.com.server.example.com" without error.
At the same time, Server decides to set the Partial Revocation Time
of this new key "01.client.example.com.server.example.com." as next
day's 15:00.
(7) Client gets the response and checks TSIG MAC, and calculates
Diffie-Hellman. It will get a new key, and it has been named
"01.client.example.com.server.example.com" by Server.
(8) At 20:07. Client sends an Adoption request to Server. This
request is signed with the previous key
"00.client.example.com.server.example.com". It includes:
Question Section:
QNAME = 01.client.example.com.server.example.com.
TYPE = TKEY
Additional Section:
01.client.example.com.server.example.com. TKEY
Algorithm = hmac-md5-sig-alg.reg.int.
Inception = (value meaning 20:00)
Expiration = (value meaning next day's 16:00)
Mode = (key adoption)
OldName = 00.client.example.com.server.example.com.
OldAlgorithm = hmac-md5-sig-alg.reg.int.
Additional Section also contains a TSIG RR.
(9) Server verifies the query's TSIG. It is signed with the
previous key and authenticated. It returns a response whose TKEY RR
is the same as the request's one. The response is signed with key
"00.client.example.com.server.example.com.". As soon as the
response is sent, Server revokes and removes the previous key. At
the same time, key "01.client.example.com.server.example.com." is
validated.
(10) Client acknowledges the success of Adoption by receiving the
response. Then, it retries to send an original question about
"www2.example.com". It is signed with the adopted key
"01.client.example.com.server.example.com", so Server authenticates
it and returns an answer.
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(11) This key is used until next day's 15:00. After that, it will
be partially revoked again.
8. Security Considerations
This document considers about how to refresh shared secret. Secret
changed by this method is used at servers in support of TSIG
[RFC2845].
[RFC2104] says that current attacks to HMAC do not indicate a
specific recommended frequency for key changes but periodic key
refreshment is a fundamental security practice that helps against
potential weaknesses of the function and keys, and limits the damage
of an exposed key. TKEY Secret Key Renewal provides the method of
periodical key refreshment.
In TKEY Secret Key Renewal, clients need to send two requests
(Renewal and Adoption) and spend time to finish their key renewal
processes. Thus the usage period of secrets should be considered
carefully based on both TKEY processing performance and security.
This document specifies the procedure of periodical key renewal, but
actually there is possibility for servers to have no choice other
than revoking their secret keys immediately especially when the keys
are found to be compromised by attackers. This is called "Emergency
Compulsory Revocation". For example, suppose the original Expiry
Limit was set at 21:00, Partial Revocation Time at 20:00 and
Inception Time at 1:00. if at 11:00 the key is found to be
compromised, the server sets Expiry Limit forcibly to be 11:00 or
before it.
Consequently, once Compulsory Revocation (See section 4.) is carried
out, normal renewal process described in this document cannot be done
any more as far as the key is concerned. However, after such
accidents happened, the two hosts are able to establish secret keys
and begin renewal procedure only if they have other (non-compromised)
shared TSIG keys or safe SIG(0) keys for the authentication of
initial secret establishment such as Diffie-Hellman Exchanged Keying.
9. IANA Considerations
IANA needs to allocate a value for "DH exchange for key renewal",
"server assignment for key renewal", "resolver assignment for key
renewal" and "key adoption" in the mode filed of TKEY. It also needs
to allocate a value for "PartialRevoke" from the extended RCODE
space.
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10. Acknowledgements
The authors would like to thank Olafur Gudmundsson, whose helpful
input and comments contributed greatly to this document.
11. References
11.1. Normative References
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[RFC2539]
D. Eastlake 3rd, "Storage of Diffie-Hellman Keys in the Domain Name
System (DNS)", RFC 2539, March 1999.
[RFC2845]
Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)", RFC 2845,
May 2000.
[RFC2930]
D. Eastlake 3rd, ``Secret Key Establishment for DNS (TKEY RR)'',
RFC 2930, September 2000.
[RFC2931]
D. Eastlake 3rd, "DNS Request and Transaction Signatures (SIG(0)s
)", RFC 2931, September 2000.
11.2. Informative References
[RFC2104]
H. Krawczyk, M.Bellare, R. Canetti, "Keyed-Hashing for Message
Authentication", RFC2104, February 1997.
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Authors' Addresses
Yuji Kamite
NTT Communications Corporation
Tokyo Opera City Tower
3-20-2 Nishi Shinjuku, Shinjuku-ku, Tokyo
163-1421, Japan
EMail: y.kamite@ntt.com
Masaya Nakayama
Information Technology Center, The University of Tokyo
2-11-16 Yayoi, Bunkyo-ku, Tokyo
113-8658, Japan
EMail: nakayama@nc.u-tokyo.ac.jp
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