DNSEXT Working Group Yuji Kamite
INTERNET-DRAFT NTT Communications
<draft-ietf-dnsext-tkey-renewal-mode-01.txt> Masaya Nakayama
Expires: Nov. 11, 2002 The University of Tokyo
May 11, 2002
TKEY Secret Key Renewal Mode
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other
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The list of current Internet-Drafts can be accessed at
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The list of Internet-Draft Shadow Directories can be accessed at
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Abstract
This document defines a new mode in TKEY [RFC2930] and proposes an
atomic method for changing secret keys used for TSIG [RFC2845]
periodically. Originally, TKEY provides methods of setting up shared
secrets other 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.
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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 Renewal Request . . . . . . . . . . . . . . . . . . . . . . . 6
2.3.1 Query for DH Exchange for Key Renewal . . . . . . . . . . 6
2.3.2 Response for DH Exchange for Key Renewal . . . . . . . . 8
2.4 Key Adoption . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4.1 Query for Key Adoption . . . . . . . . . . . . . . . . . 9
2.4.2 Response for Key Adoption . . . . . . . . . . . . . . . . 10
2.5 Considerations about Non-compliant Hosts . . . . . . . . . . 10
3 Secret Storage . . . . . . . . . . . . . . . . . . . . . . . . . 11
4 Compulsory Key Revocation by Server . . . . . . . . . . . . . . . 11
4.1 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5 Special Considerations for Two Servers' Case . . . . . . . . . . 12
5.1 To Cope with Collisions of Renewal Requests . . . . . . . . . 12
6 Key Name Considerations . . . . . . . . . . . . . . . . . . . . . 13
7 Example Usage of Secret Key Renewal Mode . . . . . . . . . . . . 13
8 Security Considerations . . . . . . . . . . . . . . . . . . . . . 16
9 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . 16
10 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . . 18
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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 availablity. 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 doesn't 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" in this
document are to be interpreted as described in [RFC 2119].
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
and must be updated. It must be between Inception Time and Expiry
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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), to be defined
TKEY
Mode = (DH exchange for key renewal), to be defined
For detailed format, see section 2.3.
Mode = (key adoption), to be defined
For detailed format, see section 2.4.
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.
In this state, whenever a client sends a normal query (e.g., question
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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.
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 this process, Diffie-Hellman
Exchange is used. After new secret establishment, the client sends a
TKEY Adoption request. 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 no
valid key is found, following [RFC2845].
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, required in
[RFC2845].
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
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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 no valid key is found, following [RFC2845].
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".
Server which has received queries signed with the partially revoked
key MUST NOT answer them except when they are "DH exchange for key
renewal" requests (See section 2.3.) or "key adoption" requests (See
section 2.4.). Instead, it returns TSIG error messages whose error
codes are "PartialRevoke" (See section 9.)
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 keys
used for queries' TSIG 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 should return
another error such as "BADSIG" without MAC; in other words, a server
informs its key's partial revocation only when the MAC in the
received query is valid.
2.3. Renewal Request
2.3.1. Query for DH Exchange 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 Diffie-Hellman
exchange 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. The client can use the partial
revoked key for TSIG.
The request message has the structure given below.
Question section's QNAME field is the same as the NAME filed of
TKEY written below.
In additional section, there is one KEY RR and one TKEY RR. KEY RR
is the client's Diffie-Hellman public key [RFC2539]. TKEY RR has
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the structure and values described below:
Field Type Comment
------- ------ -------
NAME domain used for a new key, see below
TYPE u_int16_t (defined in [RFC2930])
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 "DH exchange for key renewal"
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
Mode field of TKEY RR in the message stores the value of "DH
exchange for key renewal". See section 9.
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" is the domain name for a new secret as a result of this
key renewal message. It specifies the algorithm used with a newly
produced key.
"Inception" and "Expiration" are those requested for the keying
material, that is, requested usage period of a new key. "Key Data"
is used as a random. This is provided in the same way as [RFC2930]
4.1 in order to avoid always deriving the same keying material.
"Error" is an extended RCODE which includes "PartialRevoke" value.
See section 9.
In DH exchange for key renewal mode, "Other Data" field has the
structure given below. They describe attributes of the partially
revoked key.
in Other Data filed:
Field Type Comment
------- ------ -------
OldNAME domain name of the old key
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OldAlgorithm domain algorithm of the old key
2.3.2. Response for DH Exchange for Key Renewal
The server which has received a "DH exchange for key renewal" TKEY
request, it tries to verify TSIG or SIG(0) accompanying it. If the
verified TSIG is signed with the partially revoked key, the request
MUST be authenticated and accepted.
If the partially revoked key indicated in the request TKEY's OldName
field does not really exist at the server, or incompatible DH key is
found in the request, "BADKEY" [RFC 2845] is given in Error Field.
"FORMERR" is given if the query included no DH KEY.
If there are no errors, the server processes a response according to
Diffie-Hellman exchanged keying. Details of response messages are
below:
In answer section, there is one TKEY RR. The TKEY RR shows the
newly produced key's attributes such as a key name and algorithm.
Its format is defined as a response of the previous key renewal
request, so all values are equal to 2.3.1 except TKEY NAME, TKEY
RDLEN, RDATA's Inception, Expiration, Key Size and Key Data as long
as no error has occurred.
"NAME" field specifies the name of newly produced key which the
client will have to use.
"Inception" field and "Expiration" field mean the period of the
produced key usage. "Inception" should be 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.
Once the server has decided Expiry Limit and returned a response,
it should obey them as long as it can. In other words, they SHOULD
NOT change time values for checking Expiry Limit in the future
without any special reason (e.g., a security issue such as
"Emergency Compulsory Revocation" described in section 8).
Note that the Partial Revocation Time about a new key is not
decided and informed based only on the client's request. The server
can decide any value as long as it is between Inception and Expiry
Limit. However, it is recommended that the period from Inception to
Partial Revocation should be fixed as the server side's
configuration or should be set the same as the corresponding old
key's one.
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TKEY Key Data is used as an additional nonce to avoid deriving the
same keying material for the same pair of DH key, which is the same
as [RFC2930] 4.1.
If the TKEY error field is zero, 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 like [RFC2930] 4.1.
At this moment, the server gets a new secret. However, it might
receive another "DH exchange for key renewal" TKEY request whose
OldName indicates the same partial revoked key. Mostly such messages
originate in client's resending or rollback. In this case, the server
processes Diffie-Hellman exchanged keying again and MUST replace the
stored secret with the newest produced secret. The secret key
produced before comes to be invalid and should be removed from
memory.
Even if client sends "DH exchange for key renewal" though the key
described in OldName has not been partially revoked yet, server must
do renewal processes. But at the moment when the servers 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).
2.4. 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. TKEY RR 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. "Algorithm" is its algorithm. "Incep-
tion" means the key's Inception Time, and "Expiration" means Expiry
Limit.
"Mode" field is the value of "key adoption", which is defined
newly. See section 9.
"Other Data" field in Adoption has the same structure as that of
"DH exchange for key renewal". "OldName" means the previous old
key, and "OldAlogirthm" means its algorithm.
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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 next 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
really present at the server, BADNAME [RFC 2845] is given in Error
Field and the error message is returned.
If the next key really exists and it has not been adopted formally
yet, the server confirms the previous key's existence indicated by
the "OldName" 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, 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 should
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. 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.
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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
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 should keep all secrets and attached information, e.g.
Inception, Expiry Limit etc. safe 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. Make sure that this should be protected strongly not to be
read by others. If possible, they should be stored in encrypted form.
4. 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.
4.1. Example
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
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Exchanged Keying" as described in [RFC 2930] 4.1. Once they get
shared secret, they keep using TSIG for usual 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 more easy to use considering calculation efficiency.
If any troubles should happen such as client host's long suspension
against expectation, the server will do compulsory revocation.
After recovery if the client sends a key Renewal request to refresh
the old key, it will fail because the key is removed from the
server. So, the client will send "Query for Diffie-Hellman
Exchanged Keying" with SIG(0) to make a new shared key again.
5. Special Considerations for Two servers' Case
This section refers to the case where both two hosts are DNS servers
which can act as full resolvers as well. If one server (called Server
A) comes to want to refresh a shared key (called "Key A-B"), it will
await a TKEY Renewal request from the other server (called Server B).
But perhaps Server A will have to send queries with TSIG immediately
to Server B to resolve some queries if it is asked by other clients.
At this time, Server A is allowed to send a Renewal request to Server
B, if Server A finds the key to use now is too old and should be
renewed. To provide this function, both servers should be able to
receive and process key renewal request from each other if they agree
to refresh their shared secret keys by DH exchange for key renewal
procedures.
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 "DH exchange for key renewal" to
Server A. But 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 "DH exchange for 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
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not happen so often because Renewal requests are usually invoked when
hosts want to send queries, but it is possible.
When one of two servers try 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. But
it is recommended that some serial number or key generation time
should 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 according to
servers to 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 about key
"00.client.example.com.server.example.com" was set as follows:
Inception Time is at 6:00, Expiry Limit is at 11:00.
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(2) At Server, a time value about renewal timing has been set too:
Partial Revocation Time is at 8:00.
(3) Suppose the present time is 7:00. If Client sends a query
signed with key "00.client.example.com.server.example.com" to ask
the IP address of "www.somedomain.com", finally it will get a
proper answer from Server with valid TSIG (No Error).
(4) At 9:00. Client sends a query signed with key
"00.client.example.com.server.example.com" to ask the IP address of
"www.otherdomain.com". But it will not get a proper answer from
Server. The response does not have any IP address information about
"www.otherdomain.com". Instead, it includes valid TSIG MAC signed
with "00.client.example.com.server.example.com", and its Error Code
indicates PartialRevoke.
(5) At 9:01. 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 which means 8:55)
Expiration = (value which means 14:00)
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 8:55)
Expiration = (value meaning 14:00)
Mode = (DH exchange for key renewal)
OldName = 00.client.example.com.server.example.com.
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OldAlgorithm = hmac-md5-sig-alg.reg.int.
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
11:00.
(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 9:02. 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 which means 8:55)
Expiration = (value which means 14:00)
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 will retry to send an original question about
"www.otherdomain.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 11:00. 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].
[RFC 2104] 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. This TKEY secret key renewal mode provides the
method of periodical key refreshment.
TKEY Secret Key Renewal Mode forbids clients to send queries as long
as they do not change old keys. This means that when keys become old,
clients must spend rather lots of time to get answers they wanted
originally because at first they must send key Renewal requests. 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 15:00, Partial Revocation Time at 12:00 and
Inception Time at 10:00. 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. But, 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 using Diffie-Hellman Exchanged Keying.
9. IANA Considerations
IANA needs to allocate a value for "DH exchange 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.
Kamite, et. al. [Page 16]
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10. References
[RFC2104]
H. Krawczyk, M.Bellare, R. Canetti, "Keyed-Hashing for Message
Authentication", RFC2104, February 1997.
[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.
Kamite, et. al. [Page 17]
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Authors' Addresses
Yuji Kamite
NTT Communications Corporation
Tokyo Opera City Tower 21F,
20-2, 3-chome, Nishi-Shinjuku, Shinjuku-ku,
Tokyo, 163-1421, Japan.
EMail: y.kamite@ntt.com
Masaya Nakayama
The University of Tokyo
Information Technology Center,
2-11-16 Yayoi, Bunkyo-ku,
Tokyo, 113-8658, Japan.
EMail: nakayama@nc.u-tokyo.ac.jp
Kamite, et. al. [Page 18]
