rfc4430
Network Working Group S. Sakane
Request for Comments: 4430 K. Kamada
Category: Standards Track Yokogawa Electric Corp.
M. Thomas
J. Vilhuber
Cisco Systems
March 2006
Kerberized Internet Negotiation of Keys (KINK)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes the Kerberized Internet Negotiation of Keys
(KINK) protocol. KINK defines a low-latency, computationally
inexpensive, easily managed, and cryptographically sound protocol to
establish and maintain security associations using the Kerberos
authentication system. KINK reuses the Quick Mode payloads of the
Internet Key Exchange (IKE), which should lead to substantial reuse
of existing IKE implementations.
Table of Contents
1. Introduction ....................................................3
1.1. Conventions Used in This Document ..........................3
2. Protocol Overview ...............................................4
3. Message Flows ...................................................4
3.1. GETTGT Message Flow ........................................5
3.2. CREATE Message Flow ........................................6
3.2.1. CREATE Key Derivation Considerations ................7
3.3. DELETE Message Flow ........................................8
3.4. STATUS Message Flow ........................................9
3.5. Reporting Errors ...........................................9
3.6. Rekeying Security Associations ............................10
3.7. Dead Peer Detection .......................................10
3.7.1. Coping with Dead User-to-User Peers ................12
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RFC 4430 KINK March 2006
4. KINK Message Format ............................................13
4.1. KINK Alignment Rules ......................................15
4.2. KINK Payloads .............................................16
4.2.1. KINK_AP_REQ Payload ................................17
4.2.2. KINK_AP_REP Payload ................................18
4.2.3. KINK_KRB_ERROR Payload .............................19
4.2.4. KINK_TGT_REQ Payload ...............................20
4.2.5. KINK_TGT_REP Payload ...............................21
4.2.6. KINK_ISAKMP Payload ................................21
4.2.7. KINK_ENCRYPT Payload ...............................22
4.2.8. KINK_ERROR Payload .................................23
5. Differences from IKE Quick Mode ................................25
5.1. Security Association Payloads .............................26
5.2. Proposal and Transform Payloads ...........................26
5.3. Identification Payloads ...................................26
5.4. Nonce Payloads ............................................26
5.5. Notify Payloads ...........................................27
5.6. Delete Payloads ...........................................28
5.7. KE Payloads ...............................................28
6. Message Construction and Constraints for IPsec DOI .............28
6.1. REPLY Message .............................................28
6.2. ACK Message ...............................................28
6.3. CREATE Message ............................................29
6.4. DELETE Message ............................................30
6.5. STATUS Message ............................................31
6.6. GETTGT Message ............................................32
7. ISAKMP Key Derivation ..........................................32
8. Key Usage Numbers for Kerberos Key Derivation ..................33
9. Transport Considerations .......................................33
10. Security Considerations .......................................34
11. IANA Considerations ...........................................35
12. Forward Compatibility Considerations ..........................35
12.1. New Versions of Quick Mode ...............................36
12.2. New DOI ..................................................36
13. Related Work ..................................................36
14. Acknowledgements ..............................................37
15. References ....................................................37
15.1. Normative References .....................................37
15.2. Informative References ...................................38
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RFC 4430 KINK March 2006
1. Introduction
KINK is designed to provide a secure, scalable mechanism for
establishing keys between communicating entities within a centrally
managed environment in which it is important to maintain consistent
security policy. The security goals of KINK are to provide privacy,
authentication, and replay protection of key management messages and
to avoid denial of service vulnerabilities whenever possible. The
performance goals of the protocol are to have a low computational
cost, low latency, and a small footprint. It is also to avoid or
minimize the use of public key operations. In particular, the
protocol provides the capability to establish IPsec security
associations (SAs) in two messages with minimal computational effort.
These requirements are described in RFC 3129 [REQ4KINK].
Kerberos [KERBEROS] provides an efficient authentication mechanism
for clients and servers using a trusted third-party model. Kerberos
also provides a mechanism for cross-realm authentication natively. A
client obtains a ticket from an online authentication server, the Key
Distribution Center (KDC). The ticket is then used to construct a
credential for authenticating the client to the server. As a result
of this authentication operation, the server will also share a secret
key with the client. KINK uses this property as the basis of
distributing keys for IPsec.
The central key management provided by Kerberos is efficient because
it limits computational cost and limits complexity versus IKE's
necessity of using public key cryptography [IKE]. Initial
authentication to the KDC may be performed using either symmetric
keys, or asymmetric keys using the Public Key Cryptography for
Initial Authentication in Kerberos [PKINIT]; however, subsequent
requests for tickets as well as authenticated exchanges between the
client and servers always utilize symmetric cryptography. Therefore,
public key operations (if any) are limited and are amortized over the
lifetime of the credentials acquired in the initial authentication
operation to the KDC. For example, a client may use a single public
key exchange with the KDC to efficiently establish multiple SAs with
many other servers in the realm of the KDC. Kerberos also scales
better than direct peer-to-peer keying when symmetric keys are used.
The reason is that since the keys are stored in the KDC, the number
of principal keys is O(n+m) rather than O(n*m), where "n" is the
number of clients and "m" is the number of servers.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Sakane, et al. Standards Track [Page 3]
RFC 4430 KINK March 2006
It is assumed that the readers are familiar with the terms and
concepts described in Kerberos Version 5 [KERBEROS], IPsec [IPSEC],
and IKE [IKE].
2. Protocol Overview
KINK is a command/response protocol that can create, delete, and
maintain IPsec SAs. Each command or response contains a common
header along with a set of type-length-value payloads. The type of a
command or a response constrains the payloads sent in the messages of
the exchange. KINK itself is a stateless protocol in that each
command or response does not require storage of hard state for KINK.
This is in contrast to IKE, which uses Main Mode to first establish
an Internet Security Association and Key Management Protocol (ISAKMP)
SA followed by subsequent Quick Mode exchanges.
KINK uses Kerberos mechanisms to provide mutual authentication and
replay protection. For establishing SAs, KINK provides
confidentiality for the payloads that follow the Kerberos AP-REQ
payload. The design of KINK mitigates denial of service attacks by
requiring authenticated exchanges before the use of any public key
operations and the installation of any state. KINK also provides a
means of using Kerberos User-to-User mechanisms when there is not a
key shared between the server and the KDC. This is typically, but
not limited to, the case with IPsec peers using PKINIT for initial
authentication.
KINK directly reuses Quick Mode payloads defined in section 5.5 of
[IKE], with some minor changes and omissions. In most cases, KINK
exchanges are a single command and its response. An optional third
message is required when creating SAs, only if the responder rejects
the first proposal from the initiator or wants to contribute the
keying materials. KINK also provides rekeying and dead peer
detection.
3. Message Flows
All KINK message flows follow the same pattern between the two peers:
a command, a response, and an optional acknowledgement in a CREATE
flow. A command is a GETTGT, CREATE, DELETE, or STATUS message; a
response is a REPLY message; and an acknowledgement is an ACK
message.
KINK uses Kerberos as the authentication mechanism; therefore, a KINK
host needs to get a service ticket for each peer before actual key
negotiations. This is basically a pure Kerberos exchange and the
actual KDC traffic here is for illustrative purposes only. In
practice, when a principal obtains various tickets is a subject of
Sakane, et al. Standards Track [Page 4]
RFC 4430 KINK March 2006
Kerberos and local policy consideration. As an exception, the GETTGT
message flow of KINK (described in section 3.1) is used when a User-
to-User authentication is required. In this flow, we assume that
both A and B have ticket-granting tickets (TGTs) from their KDCs.
After a service ticket is obtained, KINK uses the CREATE message flow
(section 3.2), DELETE message flow (section 3.3), and STATUS message
flow (section 3.4) to manage SAs. In these flows, we assume that A
has a service ticket for B.
3.1. GETTGT Message Flow
This flow is used to retrieve a TGT from the remote peer in User-to-
User authentication mode.
If the initiator determines that it will not be able to get a normal
(non-User-to-User) service ticket for the responder, it can try a
User-to-User authentication. In this case, it first fetches a TGT
from the responder in order to get a User-to-User service ticket:
A B KDC
------ ------ ---
1 GETTGT+KINK_TGT_REQ------>
2 <-------REPLY+KINK_TGT_REP
3 TGS-REQ+TGT(B)------------------------------------>
4 <-------------------------------------------TGS-REP
Figure 1: GETTGT Message Flow
The initiator MAY support the following events as triggers to go to
the User-to-User path. Note that the two errors described below will
not be authenticated, and how to act on them depends on the policy.
o The local policy says that the responder requires a User-
to-User authentication.
o A KRB_AP_ERR_USER_TO_USER_REQUIRED error is returned from
the responder.
o A KDC_ERR_MUST_USE_USER2USER error is returned from the
KDC.
Sakane, et al. Standards Track [Page 5]
RFC 4430 KINK March 2006
3.2. CREATE Message Flow
This flow creates SAs. The CREATE command takes an "optimistic"
approach, where SAs are initially created on the expectation that the
responder will choose the initial proposed payload. The optimistic
proposal is placed in the first transform payload(s) of the first
proposal. The initiator MUST check to see if the optimistic proposal
was selected by comparing all transforms and attributes, which MUST
be identical to those in the initiator's optimistic proposal with the
exceptions of LIFE_KILOBYTES and LIFE_SECONDS. Each of these
attributes MAY be set to a lower value by the responder and still
expect optimistic keying, but MUST NOT be set to a higher value that
MUST generate a NO-PROPOSAL-CHOSEN error. The initiator MUST use the
shorter lifetime.
When a CREATE command contains an existing Security Parameter Index
(SPI), the responder MUST reject it and SHOULD return an ISAKMP
notification with INVALID-SPI.
When a key exchange (KE) payload is sent from the initiator but the
responder does not support it, the responder MUST reject it with an
ISAKMP notification of INVALID-PAYLOAD-TYPE containing a KE payload
type as its notification data. When the initiator receives this
error, it MAY retry without a KE payload (as another transaction) if
its policy allows that.
A B KDC
------ ------ ---
A creates an optimistic inbound SA (B->A) unless using a KE.
1 CREATE+ISAKMP------------>
B creates an inbound SA (A->B).
B creates an outbound SA (B->A) if optimistic and not using a KE.
2 <-------------REPLY+ISAKMP
A creates an outbound SA (A->B).
A replaces an inbound SA (B->A) if non-optimistic.
A creates an inbound SA (B->A) if using a KE.
3 [ ACK---------------------> ]
[ B creates an outbound SA (B->A). ]
Figure 2: CREATE Message Flow
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Creating SAs has two modes: 2-way handshake and 3-way handshake.
The initiator usually begins a negotiation expecting a 2-way
handshake. When the optimistic proposal is not chosen by the
responder, the negotiation is switched to a 3-way handshake. When
and only when the initiator uses a KE payload, 3-way handshake is
expected from the beginning.
A 2-way handshake is performed in the following steps:
1) The host A creates an inbound SA (B->A) in its SA database
using the optimistic proposal in the ISAKMP SA proposal. It is
then ready to receive any messages from B.
2) A then sends the CREATE message to B.
3) If B agrees to A's optimistic proposal, B creates an inbound SA
(A->B) and an outbound SA (B->A) in its database. If B does
not choose the first proposal or wants to add a Nonce payload,
switch to step 3 of the 3-way handshake described below.
4) B then sends a REPLY to A without a Nonce payload and without
requesting an ACK.
5) Upon receipt of the REPLY, A creates an outbound SA (A->B).
A 3-way handshake is performed in the following steps:
1) The host A sends the CREATE message to B without creating any
SA.
2) B chooses one proposal according to its policy.
3) B creates an inbound SA (A->B) and sends the actual choice in
the REPLY. It SHOULD send the optional Nonce payload (as it
does not increase message count and generally increases entropy
sources) and MUST request that the REPLY be acknowledged.
4) Upon receipt of the REPLY, A creates the inbound SA (B->A) (or
modifies it as necessary, if switched from 2-way), and the
outbound SA (A->B).
5) A now sends the ACK message.
6) Upon receipt of the ACK, B installs the final outbound SA
(B->A).
If B does not choose the first proposal, adds a nonce, or accepts the
KE exchange, then it MUST request an ACK (i.e., set the ACKREQ bit)
so that it can install the final outbound SA. The initiator MUST
always generate an ACK if the ACKREQ bit is set in the KINK header,
even if it believes that the responder was in error.
3.2.1. CREATE Key Derivation Considerations
The CREATE command's optimistic approach allows an SA to be created
in two messages rather than three. The implication of a two-message
exchange is that B will not contribute to the key since A must set up
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RFC 4430 KINK March 2006
the inbound SA before it receives any additional keying material from
B. This may be suspect under normal circumstances; however, KINK
takes advantage of the fact that the KDC provides a reliable source
of randomness which is used in key derivation. In many cases, this
will provide an adequate session key so that B will not require an
acknowledgement. Since B is always at liberty to contribute to the
keying material, this is strictly a trade-off between the key
strength versus the number of messages, which KINK implementations
may decide as a matter of policy.
3.3. DELETE Message Flow
The DELETE command deletes existing SAs. The domain of
interpretation (DOI)-specific payloads describe the actual SA to be
deleted. For the IPsec DOI, those payloads will include an ISAKMP
payload containing the list of the SPIs to be deleted.
A B KDC
------ ------ ---
A deletes outbound SA to B.
1 DELETE+ISAKMP------------>
B deletes inbound and outbound SA to A.
2 <-------------REPLY+ISAKMP
A deletes inbound SA to B.
Figure 3: DELETE Message Flow
The DELETE command takes a "pessimistic" approach, which does not
delete inbound SAs until it receives acknowledgement that the other
host has received the DELETE. The exception to the pessimistic
approach is if the initiator wants to immediately cease all activity
on an inbound SA. In this case, it MAY delete the inbound SA as well
in step 1, above.
The ISAKMP payload contains ISAKMP Delete payload(s) that indicate
the inbound SA(s) for the initiator of this flow. KINK does not
allow half-open SAs; thus, when the responder receives a DELETE
command, it MUST delete SAs of both directions, and MUST reply with
ISAKMP Delete payload(s) that indicate the inbound SA(s) for the
responder of this flow. If the responder cannot find an appropriate
SPI to be deleted, it MUST return an ISAKMP notification with
INVALID_SPI, which also serves to inform the initiator that it can
delete the inbound SA.
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RFC 4430 KINK March 2006
A race condition with the DELETE flow exists. Due to network
reordering, etc., packets in flight while the DELETE operation is
taking place may arrive after the diagrams above, which recommend
deleting the inbound SA. A KINK implementation SHOULD implement a
grace timer that SHOULD be set to a period of at least two times the
average round-trip time, or to a configurable value. A KINK
implementation MAY choose to set the grace period to zero at
appropriate times to delete an SA ungracefully. The behavior
described here is referred from the behavior of the TCP [RFC793]
flags FIN and RST.
3.4. STATUS Message Flow
This flow is used to send any information to a peer or to elicit any
information from a peer. An initiator may send a STATUS command to
the responder at any time, optionally with DOI-specific ISAKMP
payloads. In the case of the IPsec DOI, these are generally in the
form of ISAKMP Notification payloads. A STATUS command is also used
as a means of dead peer detection described in section 3.7.
A B KDC
------ ------ ---
1 STATUS[+ISAKMP]---------->
2 <-----------REPLY[+ISAKMP]
Figure 4: STATUS Message Flow
3.5. Reporting Errors
When the responder detects an error in a received command, it can
send a DOI-specific payload to indicate the error in a REPLY message.
There are three types of payloads that can indicate errors:
KINK_KRB_ERROR payloads for Kerberos errors, KINK_ERROR payloads for
KINK errors, and KINK_ISAKMP payloads for ISAKMP errors. Details are
described in sections 4.2.3, 4.2.8, and 4.2.6, respectively.
If the initiator detects an error in a received reply, there is no
means to report it back to the responder. The initiator SHOULD log
the event and MAY take a remedial action by reinitiating the initial
command.
If the server clock and the client clock are off by more than the
policy-determined clock skew limit (usually 5 minutes), the server
MUST return a KRB_AP_ERR_SKEW. The optional client's time in the
KRB-ERROR SHOULD be filled out. If the server protects the error by
adding the Cksum field and returning the correct client's time, the
Sakane, et al. Standards Track [Page 9]
RFC 4430 KINK March 2006
client SHOULD compute the difference (in seconds) between the two
clocks based upon the client and server time contained in the
KRB-ERROR message. The client SHOULD store this clock difference and
use it to adjust its clock in subsequent messages. If the error is
not protected, the client MUST NOT use the difference to adjust
subsequent messages, because doing so would allow an attacker to
construct authenticators that can be used to mount replay attacks.
3.6. Rekeying Security Associations
KINK expects the initiator of an SA to be responsible for rekeying
the SA for two reasons. The first reason is to prevent needless
duplication of SAs as the result of collisions due to an initiator
and responder both trying to renew an existing SA. The second reason
is due to the client/server nature of Kerberos exchanges, which
expects the client to get and maintain tickets. While KINK expects
that a KINK host is able to get and maintain tickets, in practice it
is often advantageous for servers to wait for clients to initiate
sessions so that they do not need to maintain a large ticket cache.
There are no special semantics for rekeying SAs in KINK. That is, in
order to rekey an existing SA, the initiator must CREATE a new SA
followed by either deleting the old SA with the DELETE flow or
letting it time out. When identical flow selectors are available on
different SAs, KINK implementations SHOULD choose the SA most
recently created. It should be noted that KINK avoids most of the
problems of [IKE] rekeying by having a reliable delete mechanism.
Normally, a KINK implementation that rekeys existing SAs will try to
rekey the SA ahead of an SA termination, which may include the hard
lifetime in time/bytecount or the overflow of the sequence number
counter. We call this time "soft lifetime". The soft lifetime MUST
be randomized to avoid synchronization with similar implementations.
In the case of the lifetime in time, one reasonable approach to
determine the soft lifetime is picking a random time between T-rekey
and T-retrans and subtracting it from the hard lifetime. Here,
T-rekey is the reasonable maximum rekeying margin, and T-retrans is
the amount of time it would take to go through a full retransmission
cycle. T-rekey SHOULD be at least twice as high as T-retrans.
3.7. Dead Peer Detection
In order to determine that a KINK peer has lost its security database
information, KINK peers MUST record the current epoch for which they
have valid SA information for a peer and reflect that epoch in each
AP-REQ and AP-REP message. When a KINK peer creates state for a
given SA, it MUST also record the principal's epoch. If it discovers
Sakane, et al. Standards Track [Page 10]
RFC 4430 KINK March 2006
on a subsequent message that the principal's epoch has changed, it
MUST consider all SAs created by that principal as invalid, and take
some action such as tearing those SAs down.
While a KINK peer SHOULD use feedback from routing (in the form of
ICMP messages) as a trigger to check whether or not the peer is still
alive, a KINK peer MUST NOT conclude the peer is dead simply based on
unprotected routing information (said ICMP messages).
If there is suspicion that a peer may be dead (based on any
information available to the KINK peer, including lack of IPsec
traffic, etc.), the KINK STATUS message SHOULD be used to coerce an
acknowledgement out of the peer. Since nothing is negotiated about
dead peer detection in KINK, each peer can decide its own metric for
"suspicion" and also what timeouts to use before declaring a peer
dead due to lack of response to the STATUS message. This is
desirable, and does not break interoperability.
The STATUS message has a twofold effect. First, it elicits a
cryptographically secured (and replay-protected) response from the
peer, which tells us whether or not the peer is reachable/alive.
Second, it carries the epoch number of the peer, so we know whether
or not the peer has rebooted and lost all state. This is crucial to
the KINK protocol: In IKE, if a peer reboots, we lose all
cryptographic context, and no cryptographically secure communication
is possible without renegotiating keys. In KINK, due to Kerberos
tickets, we can communicate securely with a peer, even if the peer
rebooted, as the shared cryptographic key used is carried in the
Kerberos ticket. Thus, active cryptographic communication is not an
indication that the peer has not rebooted and lost all state, and the
epoch is needed.
Assume a Peer A sending a STATUS and a peer B sending the REPLY (see
section 3.4). Peer B MAY assume that the sender is alive, and the
epoch in the STATUS message will indicate whether or not the peer A
has lost state. Peer B MUST acknowledge the STATUS message with a
REPLY message, as described in section 3.4.
The REPLY message will indicate to peer A that the peer is alive, and
the epoch in the REPLY will indicate whether peer B has lost its
state or not. If peer A does not receive a REPLY message from peer B
in a suitable timeout, peer A MAY send another STATUS message. It is
up to peer A to decide how aggressively to declare peer B dead. The
level of aggressiveness may depend on many factors such as rapid fail
over versus number of messages sent by nodes with large numbers of
SAs.
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RFC 4430 KINK March 2006
Note that peer B MUST NOT make any inferences about a lack of STATUS
message from peer A. Peer B MAY use a STATUS message from peer A as
an indication of A's aliveness, but peer B MUST NOT expect another
STATUS message at any time (i.e., dead peer detection is not periodic
keepalives).
Strategies for sending STATUS messages are the following: Peer A may
decide to send a STATUS message only after a prolonged period where
no traffic was sent in either direction over the IPsec SAs with the
peer. Once there is traffic, peer A may want to know if the traffic
is going into a black hole, and send a STATUS message.
Alternatively, peer A may use an idle timer to detect lack of traffic
with the peer, and send STATUS messages in the quiet phase to make
sure the peer is still alive for when traffic needs to finally be
sent.
3.7.1. Coping with Dead User-to-User Peers
When an initiator uses a User-to-User ticket and a responder has lost
its previous TGT, the usual dead peer detection (DPD) mechanism does
not work, because the responder cannot decrypt the ticket with its
new TGT. In this case, the following actions are taken.
o When the responder receives a KINK command with a User-to-User
ticket that cannot be decrypted with its TGT, it returns a
REPLY with a KINK_TGT_REP payload containing the TGT.
o When the initiator receives a KINK_TGT_REP, it retrieves a new
service ticket with the TGT and retries the command.
This does not directly define a method to detect a dead User-to-User
peer, but to recover from the situation that the responder does not
have an appropriate TGT to decrypt a service ticket sent from the
initiator. After recovery, they can exchange their epochs, and usual
DPD mechanism will detect a dead peer if it really has been dead.
The initiator MUST NOT think the peer has been dead on the receipt of
a KINK_TGT_REP because of two reasons. One is that the message is
not authenticated, and the other is that losing a TGT does not
necessarily mean losing the SA database information. The initiator
SHOULD NOT forget the previous service ticket until the new one is
successfully obtained in order to reduce the cost when a forged
KINK_TGT_REP is received.
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RFC 4430 KINK March 2006
4. KINK Message Format
All values in KINK are formatted in network byte order (most
significant byte first). The RESERVED fields MUST be set to zero (0)
when a packet is sent. The receiver MUST ignore these fields.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | MjVer |RESRVED| Length |
+---------------+---------------+---------------+---------------+
| Domain of Interpretation (DOI) |
+-------------------------------+-------------------------------+
| Transaction ID (XID) |
+---------------+-+-------------+-------------------------------+
| NextPayload |A| RESERVED2 | CksumLen |
+---------------+-+-------------+-------------------------------+
| |
~ A series of payloads ~
| |
+-------------------------------+-------------------------------+
| |
~ Cksum (variable) ~
| |
+-------------------------------+-------------------------------+
Figure 5: Format of a KINK Message
Fields:
o Type (1 octet) -- The type of this message.
Type Value
----- -----
RESERVED 0
CREATE 1
DELETE 2
REPLY 3
GETTGT 4
ACK 5
STATUS 6
RESERVED TO IANA 7 - 127
Private Use 128 - 255
o MjVer (4 bits) -- Major protocol version number. This MUST be
set to 1.
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RFC 4430 KINK March 2006
o RESRVED (4 bits) -- Reserved and MUST be zero when sent, MUST
be ignored when received.
o Length (2 octets) -- Length of the message in octets. It is
not forbidden in KINK that there are unnecessary data after
the message, but the Length field MUST represent the actual
length of the message.
o DOI (4 octets) -- The domain of interpretation. All DOIs must
be registered with the IANA in the ISAKMP Domain of
Interpretation section of the isakmp-registry [ISAKMP-REG].
The IANA Assigned Number for the Internet IP Security DOI
[IPDOI] is one (1). This field defines the context of all
sub-payloads in this message. If sub-payloads have a DOI
field (e.g., Security Association Payload), then the DOI in
that sub-payload MUST be checked against the DOI in this
header, and the values MUST be the same.
o XID (4 octets) -- The transaction ID. A KINK transaction is
bound together by a transaction ID, which is created by the
command initiator and replicated in subsequent messages in the
transaction. A transaction is defined as a command, a reply,
and an optional acknowledgement. Transaction IDs are used by
the initiator to discriminate between multiple outstanding
requests to a responder. It is not used for replay protection
because that functionality is provided by Kerberos. The value
of XID is chosen by the initiator and MUST be unique with all
outstanding transactions. XIDs MAY be constructed by using a
monotonic counter or random number generator.
o NextPayload (1 octet) -- Indicates the type of the first
payload after the message header.
o A, or ACKREQ (1 bit) -- ACK Request. Set to one if the
responder requires an explicit acknowledgement that a REPLY
was received. An initiator MUST NOT set this flag, nor should
a responder except for a REPLY to a CREATE when the optimistic
proposal is chosen.
o RESERVED2 (7 bits) -- Reserved and MUST be zero on send, MUST
be ignored by a receiver.
o CksumLen (2 octets) -- CksumLen is the length in octets of the
cryptographic checksum of the message. A CksumLen of zero
implies that the message is unauthenticated.
Sakane, et al. Standards Track [Page 14]
RFC 4430 KINK March 2006
o Cksum (variable) -- Kerberos keyed checksum over the entire
message excluding the Cksum field itself. When any padding
bytes are required between the last payload and the Cksum
field, they MUST be included in the calculation. This field
MUST always be present whenever a key is available via an
AP-REQ or AP-REP payload. The key used MUST be the session
key in the ticket. When a key is not available, this field is
not present, and the CksumLen field is set to zero. The
content of this field is the output of the Kerberos 5 get_mic
function [KCRYPTO]. The get_mic function used is specified by
a checksum type, which is a "required checksum mechanism" of
the etype for the Kerberos session key in the Kerberos ticket.
If the checksum type is not a keyed algorithm, the message
MUST be rejected.
To compute the checksum, the CksumLen field is zeroed out and
the Length field is filled with the total packet length
without the checksum. Then, the packet is passed to the
get_mic function and its output is appended to the packet.
Any KINK padding after the Cksum field is not allowed, except
the Kerberos internal one, which may be included in the output
of the get_mic function. Finally, the CksumLen field is
filled with the checksum length and the Length field is filled
with the total packet length including the checksum.
To verify the checksum, a length-without-checksum is
calculated from the value of Length field, subtracting the
CksumLen. The Length field is filled with the length-
without-checksum value and the CksumLen field is zeroed out.
Then, the packet without checksum (offset from 0 to length-
without-checksum minus 1 of the received packet) and the
checksum (offset from length-without-checksum to the last) are
passed to the verify_mic function. If verification fails, the
message MUST be dropped.
The KINK header is followed immediately by a series of
Type/Length/Value fields, defined in section 4.2.
4.1. KINK Alignment Rules
KINK has the following rules regarding alignment and padding:
o All length fields MUST reflect the actual number of octets in
the structure; i.e., they do not account for padding bytes
required by KINK alignments.
o KINK headers, payloads, and the Cksum field MUST be aligned on
4-octet boundaries.
Sakane, et al. Standards Track [Page 15]
RFC 4430 KINK March 2006
o Variable length fields (except the Cksum field) MUST always
start immediately after the last octet of the previous field.
That is, they are not aligned to 4-octet boundaries.
4.2. KINK Payloads
Immediately following the header, there is a list of
Type/Length/Value (TLV) payloads. There can be any number of
payloads following the header. Each payload MUST begin with a
payload header. Each payload header is built on the generic payload
header. Any data immediately follows the generic header. Payloads
are all implicitly aligned to 4-octet boundaries, though the payload
length field MUST accurately reflect the actual number of octets in
the payload.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Next Payload | RESERVED | Payload Length |
+---------------+---------------+---------------+---------------+
| value (variable) |
+---------------+---------------+---------------+---------------+
Figure 6: Format of a KINK Payload
Fields:
o Next Payload (1 octet) -- The type of the next payload.
NextPayload Value
---- -----
KINK_DONE 0
KINK_AP_REQ 1
KINK_AP_REP 2
KINK_KRB_ERROR 3
KINK_TGT_REQ 4
KINK_TGT_REP 5
KINK_ISAKMP 6
KINK_ENCRYPT 7
KINK_ERROR 8
RESERVED TO IANA 9 - 127
Private Use 128 - 255
Next Payload type KINK_DONE denotes that the current payload
is the final payload in the message.
o RESERVED (1 octet) -- Reserved and MUST be set to zero by a
sender, MUST be ignored by a receiver.
Sakane, et al. Standards Track [Page 16]
RFC 4430 KINK March 2006
o Payload Length (2 octets) -- The length of this payload,
including the type and length fields.
o Value (variable) -- This value of this field depends on the
type.
4.2.1. KINK_AP_REQ Payload
The KINK_AP_REQ payload relays a Kerberos AP-REQ to the responder.
The AP-REQ MUST request mutual authentication.
This document does not specify how to generate the principal name.
That is, complete principal names may be stored in local policy,
Fully Qualified Domain Names (FQDNs) may be converted to principal
names, IP addresses may be converted to principal names by secure
name services, etc., but see the first paragraph of the Security
Considerations section.
If the peer's principal name for the KINK service is generated from
an FQDN, the principal name, which the initiator starts from, will be
"kink/fqdn@REALM"; where "kink" is a literal string for the KINK
IPsec service, "fqdn" is the fully qualified domain name of the
service host, and "REALM" is the Kerberos realm of the service. A
principal name is case sensitive, and "fqdn" part MUST be lowercase
as described in [KERBEROS].
The value field of this payload has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Next Payload | RESERVED | Payload Length |
+---------------+---------------+---------------+---------------+
| EPOCH |
+---------------------------------------------------------------+
| |
~ AP-REQ ~
| |
+---------------------------------------------------------------+
Figure 7: KINK_AP_REQ Payload
Fields:
o Next Payload, RESERVED, Payload Length -- Defined in the
beginning of this section.
Sakane, et al. Standards Track [Page 17]
RFC 4430 KINK March 2006
o EPOCH -- The absolute time at which the creator of the AP-REQ
has valid SA information. Typically, this is when the KINK
keying daemon started if it does not retain SA information
across restarts. The value in this field is the least
significant 4 octets of so-called POSIX time, which is the
elapsed seconds (but without counting leap seconds) from
1970-01-01T00:00:00 UTC. For example, 2038-01-19T03:14:07 UTC
is represented as 0x7fffffff.
o AP-REQ -- The value field of this payload contains a raw
Kerberos AP-REQ.
4.2.2. KINK_AP_REP Payload
The KINK_AP_REP payload relays a Kerberos AP-REP to the initiator.
The AP-REP MUST be checked for freshness as described in [KERBEROS].
The value field of this payload has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Next Payload | RESERVED | Payload Length |
+---------------+---------------+---------------+---------------+
| EPOCH |
+---------------------------------------------------------------+
| |
~ AP-REP ~
| |
+---------------------------------------------------------------+
Figure 8: KINK_AP_REP Payload
Fields:
o Next Payload, RESERVED, Payload Length -- Defined in the
beginning of this section.
o EPOCH -- The absolute time at which the creator of the AP-REP
has valid SA information. Typically, this is when the KINK
keying daemon started if it does not retain SA information
across restarts. The value in this field is the least
significant 4 octets of so-called POSIX time, which is the
elapsed seconds (but without counting leap seconds) from
1970-01-01T00:00:00 UTC. For example, 2038-01-19T03:14:07 UTC
is represented as 0x7fffffff.
Sakane, et al. Standards Track [Page 18]
RFC 4430 KINK March 2006
o AP-REP -- The value field of this payload contains a raw
Kerberos AP-REP.
4.2.3. KINK_KRB_ERROR Payload
The KINK_KRB_ERROR payload relays Kerberos type errors back to the
initiator. The initiator MUST be prepared to receive any valid
Kerberos error type [KERBEROS].
KINK implementations SHOULD make use of a KINK Cksum field when
returning KINK_KRB_ERROR and the appropriate service key is
available. Especially in the case of clock skew errors, protecting
the error at the server creates a better user experience because it
does not require clocks to be synchronized. However, many Kerberos
implementations do not make it easy to obtain the session key in
order to protect error packets. For unauthenticated Kerberos errors,
the initiator MAY choose to act on them, but SHOULD take precautions
against make-work kinds of attacks.
Note that KINK does not make use of the text or e_data field of the
Kerberos error message, though a compliant KINK implementation MUST
be prepared to receive them and MAY log them.
The value field of this payload has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Next Payload | RESERVED | Payload Length |
+---------------+---------------+---------------+---------------+
| |
~ KRB-ERROR ~
| |
+---------------------------------------------------------------+
Figure 9: KINK_KRB_ERROR Payload
Fields:
o Next Payload, RESERVED, Payload Length -- Defined in the
beginning of this section.
o KRB-ERROR -- The value field of this payload contains a raw
Kerberos KRB-ERROR.
Sakane, et al. Standards Track [Page 19]
RFC 4430 KINK March 2006
4.2.4. KINK_TGT_REQ Payload
The KINK_TGT_REQ payload provides a means to get a TGT from the peer
in order to obtain a User-to-User service ticket from the KDC.
The value field of this payload has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Next Payload | RESERVED | Payload Length |
+---------------+---------------+---------------+---------------+
| |
~ PrincName (variable) ~
| |
+---------------------------------------------------------------+
Figure 10: KINK_TGT_REQ Payload
Fields:
o Next Payload, RESERVED, Payload Length -- Defined in the
beginning of this section.
o PrincName -- The name of the principal that the initiator
wants to communicate with. It is assumed that the initiator
knows the responder's principal name (including the realm
name) in the same way as the non-User-to-User case. The TGT
returned MUST NOT be an inter-realm TGT and its cname and
crealm MUST match the requested principal name, so that the
initiator can rendezvous with the responder at the responder's
realm.
PrincName values are octet string representations of a
principal and realm name formatted just like the octet string
used in the "NAME" component of Generic Security Service
Application Program Interface (GSS-API) [RFC2743] exported
name token for the Kerberos V5 GSS-API mechanism [RFC1964].
See RFC 1964, section 2.1.3.
If the responder is not the requested principal and is unable to get
a TGT for the name, it MAY return a KRB_AP_ERR_NOT_US. If the
administrative policy prohibits returning a TGT, it MAY return a
KINK_U2UDENIED.
Sakane, et al. Standards Track [Page 20]
RFC 4430 KINK March 2006
4.2.5. KINK_TGT_REP Payload
The value field of this payload contains the TGT requested in a
previous KINK_TGT_REQ payload of a GETTGT command.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Next Payload | RESERVED | Payload Length |
+---------------+---------------+---------------+---------------+
| |
~ TGT (variable) ~
| |
+---------------------------------------------------------------+
Figure 11: KINK_TGT_REP Payload
Fields:
o Next Payload, RESERVED, Payload Length -- Defined in the
beginning of this section.
o TGT -- The Distinguished Encoding Rules (DER)-encoded TGT of
the responder.
4.2.6. KINK_ISAKMP Payload
The value field of this payload has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Next Payload | RESERVED | Payload Length |
+---------------+-------+-------+---------------+---------------+
| InnerNextPload| QMMaj | QMMin | RESERVED |
+---------------+-------+-------+---------------+---------------+
| Quick Mode Payloads (variable) |
+---------------+---------------+---------------+---------------+
Figure 12: KINK_ISAKMP Payload
Fields:
o Next Payload, RESERVED, Payload Length -- Defined in the
beginning of this section.
o InnerNextPload -- First payload type of the inner series of
ISAKMP payloads.
Sakane, et al. Standards Track [Page 21]
RFC 4430 KINK March 2006
o QMMaj -- The major version of the inner payloads. MUST be set
to 1.
o QMMin -- The minor version of the inner payloads. MUST be set
to 0.
The KINK_ISAKMP payload encapsulates the IKE Quick Mode (phase 2)
payloads to take the appropriate action dependent on the KINK
command. There may be any number of KINK_ISAKMP payloads within a
single KINK message. While [IKE] is somewhat fuzzy about whether
multiple different SAs may be created within a single IKE message,
KINK explicitly requires that a new ISAKMP header be used for each
discrete SA operation. In other words, a KINK implementation MUST
NOT send multiple Quick Mode transactions within a single KINK_ISAKMP
payload.
The purpose of the Quick Mode version is to allow backward
compatibility with IKE and ISAKMP if there are subsequent revisions.
At the present time, the Quick Mode major and minor versions are set
to one and zero (1.0), respectively. These versions do not
correspond to the ISAKMP version in the ISAKMP header. A compliant
KINK implementation MUST support receipt of 1.0 payloads. It MAY
support subsequent versions (both sending and receiving), and SHOULD
provide a means to resort back to Quick Mode version 1.0 if the KINK
peer is unable to process future versions. A compliant KINK
implementation MUST NOT mix Quick Mode versions in any given
transaction.
4.2.7. KINK_ENCRYPT Payload
The KINK_ENCRYPT payload encapsulates other KINK payloads and is
encrypted using the session key and the algorithm specified by its
etype. This payload MUST be the final one in the outer payload chain
of the message. The KINK_ENCRYPT payload MUST be encrypted before
the final KINK checksum is applied.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Next Payload | RESERVED | Payload Length |
+---------------+---------------+---------------+---------------+
| InnerNextPload| RESERVED2 |
+---------------+---------------+---------------+---------------+
| Payload (variable) |
+---------------+---------------+---------------+---------------+
Figure 13: KINK_ENCRYPT Payload
Sakane, et al. Standards Track [Page 22]
RFC 4430 KINK March 2006
Fields:
o Next Payload, RESERVED, Payload Length -- Defined in the
beginning of this section. This payload is the last one in a
message, and accordingly, the Next Payload field must be
KINK_DONE (0).
o InnerNextPload -- First payload type of the inner series of
encrypted KINK payloads.
o RESERVED2 -- Reserved and MUST be zero when sent, MUST be
ignored when received.
The coverage of the encrypted data begins at InnerNextPload so that
the first payload's type is kept confidential. Thus, the number of
encrypted octets is PayloadLength - 4.
The format of the encryption payload follows the normal Kerberos
semantics. Its content is the output of an encrypt function defined
in the Encryption Algorithm Profile section of [KCRYPTO]. Parameters
such as encrypt function itself, specific-key, and initial state are
defined with the etype. The encrypt function may have padding in
itself and there may be some garbage data at the end of the decrypted
plaintext. A KINK implementation MUST be prepared to ignore such
padding after the last sub-payload inside the KINK_ENCRYPT payload.
Note that each encrypt function has its own integrity protection
mechanism. It is redundant with the checksum in the KINK header, but
this is unavoidable because it is not always possible to remove the
integrity protection part from the encrypt function.
4.2.8. KINK_ERROR Payload
The KINK_ERROR payload type provides a protocol-level mechanism of
returning an error condition. This payload should not be used for
either Kerberos-generated errors or DOI-specific errors that have
their own payloads defined. The error code is in network order.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Next Payload | RESERVED | Payload Length |
+---------------+---------------+---------------+---------------+
| ErrorCode |
+---------------+---------------+---------------+---------------+
Figure 14: KINK_ERROR Payload
Sakane, et al. Standards Track [Page 23]
RFC 4430 KINK March 2006
Fields:
o Next Payload, RESERVED, Payload Length -- Defined in the
beginning of this section.
o ErrorCode -- One of the following values in the network byte
order:
ErrorCode Value Purpose
--------- ----- -------------------
KINK_OK 0 No error detected
KINK_PROTOERR 1 The message was malformed
KINK_INVDOI 2 Invalid DOI
KINK_INVMAJ 3 Invalid Major Version
RESERVED 4
KINK_INTERR 5 An unrecoverable internal error
KINK_BADQMVERS 6 Unsupported Quick Mode Version
KINK_U2UDENIED 7 Returning a TGT is prohibited
RESERVED TO IANA 8 - 8191
Private Use 8192 - 16383
RESERVED 16384 -
The responder MUST NOT return KINK_OK. When received, the initiator
MAY act as if the specific KINK_ERROR payload were not present. If
the initiator supports multiple Quick Mode versions or DOIs,
KINK_BADQMVERS or KINK_INVDOI is received, and the Cksum is verified,
then it MAY retry with another version or DOI. A responder SHOULD
return a KINK error with KINK_INVMAJ, when it receives an unsupported
KINK version number in the header. When KINK_U2UDENIED is received,
the initiator MAY retry with the non-User-to-User mode (if it has not
yet been tried).
In general, the responder MAY choose to return these errors in reply
to unauthenticated commands, but SHOULD take care to avoid being
involved in denial of service attacks. Similarly, the initiator MAY
choose to act on unauthenticated errors, but SHOULD take care to
avoid denial of service attacks.
Sakane, et al. Standards Track [Page 24]
RFC 4430 KINK March 2006
5. Differences from IKE Quick Mode
KINK directly uses ISAKMP payloads to negotiate SAs. In particular,
KINK uses IKE phase 2 payload types (aka Quick Mode). In general,
there should be very few changes necessary to an IKE implementation
to establish the SAs, and unless there is a note to the contrary in
the memo, all capabilities and requirements in [IKE] MUST be
supported. IKE phase 1 payloads MUST NOT be sent.
Unlike IKE, KINK defines specific commands for creation, deletion,
and status of SAs, mainly to facilitate predictable SA
creation/deletion (see sections 3.2 and 3.3). As such, KINK places
certain restrictions on what payloads may be sent with which
commands, and some additional restrictions and semantics of some of
the payloads. Implementors should refer to [IKE] and [ISAKMP] for
the actual format and semantics. If a particular IKE phase 2 payload
is not mentioned here, it means that there are no differences in its
use.
o The Security Association Payload header for IP is defined in
section 4.6.1 of [IPDOI]. For this memo, the Domain of
Interpretation MUST be set to 1 (IPsec) and the Situation
bitmap MUST be set to 1 (SIT_IDENTITY_ONLY). All other fields
are omitted (because SIT_IDENTITY_ONLY is set).
o KINK also expands the semantics of IKE in that it defines an
optimistic proposal for CREATE commands to allow SA creation to
complete in two messages.
o IKE Quick Mode (phase 2) uses the hash algorithm used in main
mode (phase 1) to generate the keying material. For this
purpose, KINK MUST use a pseudo-random function determined by
the etype of the session key.
o KINK does not use the HASH payload at all.
o KINK allows the Nonce payload Nr to be optional to facilitate
optimistic keying.
Sakane, et al. Standards Track [Page 25]
RFC 4430 KINK March 2006
5.1. Security Association Payloads
KINK supports the following SA attributes from [IPDOI]:
class value type
-------------------------------------------------
SA Life Type 1 B
SA Life Duration 2 V
Encapsulation Mode 4 B
Authentication Algorithm 5 B
Key Length 6 B
Key Rounds 7 B
Refer to [IPDOI] for the actual definitions of these attributes.
5.2. Proposal and Transform Payloads
KINK directly uses the Proposal and Transform payloads with no
differences. KINK, however, places additional relevance to the first
proposal and first transform of each conjugate for optimistic keying.
5.3. Identification Payloads
The Identification payload carries information that is used to
identify the traffic that is to be protected by the SA that will be
established. KINK restricts the ID types, which are defined in
section 4.6.2.1 of [IPDOI], to the following values:
ID Type Value
------- -----
ID_IPV4_ADDR 1
ID_IPV4_ADDR_SUBNET 4
ID_IPV6_ADDR 5
ID_IPV6_ADDR_SUBNET 6
ID_IPV4_ADDR_RANGE 7
ID_IPV6_ADDR_RANGE 8
5.4. Nonce Payloads
The Nonce payload contains random data that MUST be used in key
generation. It MUST be sent by the initiating KINK peer, and MAY be
sent by the responding KINK peer. See section 7 for the discussion
of its use in key generation.
Sakane, et al. Standards Track [Page 26]
RFC 4430 KINK March 2006
5.5. Notify Payloads
Notify payloads are used to transmit several informational data, such
as error conditions and state transitions to a peer. For example,
notification information transmit can be error messages specifying
why an SA could not be established. It can also be status data that
a process managing an SA database wishes to communicate with a peer
process.
Types in the range 0 - 16383 are intended for reporting errors
[ISAKMP]. An implementation receiving a type in this range that it
does not recognize in a response MUST assume that the corresponding
request has failed entirely. Unrecognized error types in a request
and status types in a request or response MUST be ignored, and they
SHOULD be logged. Notify payloads with status types MAY be added to
any message and MUST be ignored if not recognized. They are intended
to indicate capabilities, and as part of SA negotiation are used to
negotiate non-cryptographic parameters.
The table below lists the Notification messages and their
corresponding values. PAYLOAD-MALFORMED denotes some error types
defined by [ISAKMP]. Hence INVALID-PROTOCOL-ID, for example, is not
used in this document. INVALID-MAJOR-VERSION and INVALID-MINOR-
VERSION are not used because KINK_BADQMVERS is used to tell the
initiator that the version of IKE is not supported.
NOTIFY MESSAGES - ERROR TYPES Value
----------------------------- -----
INVALID-PAYLOAD-TYPE 1
Sent if the ISAKMP payload type is not recognized. It is also
sent when the KE payload is not supported by the responder.
Notification Data MUST contains the one-octet payload type.
INVALID-SPI 11
Sent if the responder has an SPI indicated by the initiator in
case of CREATE flow, or if the responder does not have an SPI
indicated by the initiator in case of DELETE flow.
NO-PROPOSAL-CHOSEN 14
Sent if none of the proposals in the SA payload was
acceptable.
Sakane, et al. Standards Track [Page 27]
RFC 4430 KINK March 2006
PAYLOAD-MALFORMED 16
Sent if the KINK_ISAKMP payload received was invalid because
some type, length, or value was out of range. It is also sent
when the request was rejected for reason that was not matched
with other error types.
5.6. Delete Payloads
KINK directly uses ISAKMP Delete payloads with no changes.
5.7. KE Payloads
IKE requires that perfect forward secrecy (PFS) be supported through
the use of the KE payload. KINK retains the ability to use PFS, but
relaxes the requirement from must implement to SHOULD implement. The
reasons are described in the Security Considerations section.
6. Message Construction and Constraints for IPsec DOI
All commands, responses, and acknowledgements are bound together by
the XID field of the message header. The XID is normally a
monotonically incrementing field, and is used by the initiator to
differentiate between outstanding requests to a responder. The XID
field does not provide replay protection as that functionality is
provided by the Kerberos mechanisms. In addition, commands and
responses MUST use a cryptographic checksum over the entire message
if the two peers share a key via a ticket exchange.
In all cases in this section, if a message contains a KINK_AP_REQ or
KINK_AP_REP payload, other KINK payloads MAY be encapsulated in a
KINK_ENCRYPT payload.
6.1. REPLY Message
The REPLY message is a generic reply that MUST contain either a
KINK_AP_REP, a KINK_KRB_ERROR, or a KINK_ERROR payload. REPLY
messages MAY contain additional DOI-specific payloads such as ISAKMP
payloads that are defined in the following sections.
6.2. ACK Message
ACKs are sent only when the ACKREQ bit is set in a REPLY message. An
ACK message MUST contain an AP-REQ payload and no other payload.
Sakane, et al. Standards Track [Page 28]
RFC 4430 KINK March 2006
6.3. CREATE Message
This message initiates an establishment of new security
association(s). The CREATE message must contain an AP-REQ payload
and any DOI-specific payloads.
CREATE KINK Header
KINK_AP_REQ
[KINK_ENCRYPT]
KINK_ISAKMP payloads
SA Payload
Proposal Payloads
Transform Payloads
Nonce Payload (Ni)
[KE]
[IDci, IDcr]
[Notification Payloads]
Replies are of the following forms:
REPLY KINK Header
KINK_AP_REP
[KINK_ENCRYPT]
KINK_ISAKMP payloads
SA Payload
Proposal Payloads
Transform Payload
[Nonce Payload (Nr)]
[KE]
[IDci, IDcr]
[Notification Payloads]
Note that there MUST be at least a single proposal payload and a
single transform payload in REPLY messages. There will be multiple
proposal payloads only when an SA bundle is negotiated. Also: unlike
IKE, the Nonce payload Nr is not required, and if it exists, an
acknowledgement must be requested to indicate that the initiator's
outgoing SAs must be modified. If any of the first proposals are not
chosen by the recipient, it SHOULD include the Nonce payload.
KINK, like IKE, allows the creation of many SAs in one create
command. If any of the optimistic proposals are not chosen by the
responder, it MUST request an ACK.
If an IPsec DOI-specific error is encountered, the responder must
reply with a Notify payload describing the error:
Sakane, et al. Standards Track [Page 29]
RFC 4430 KINK March 2006
REPLY KINK Header
KINK_AP_REP
[KINK_ENCRYPT]
[KINK_ERROR]
KINK_ISAKMP payloads
[Notification Payloads]
If the responder finds a Kerberos error for which it can produce a
valid authenticator, the REPLY takes the following form:
REPLY KINK Header
KINK_AP_REP
[KINK_ENCRYPT]
KINK_KRB_ERROR
Finally, if the responder finds a Kerberos or KINK type of error for
which it cannot create an AP-REP, it MUST reply with a lone
KINK_KRB_ERROR or KINK_ERROR payload:
REPLY KINK Header
[KINK_KRB_ERROR]
[KINK_ERROR]
6.4. DELETE Message
This message indicates that the sending peer has deleted or will
shortly delete Security Association(s) with the other peer.
DELETE KINK Header
KINK_AP_REQ
[KINK_ENCRYPT]
KINK_ISAKMP payloads
Delete Payloads
[Notification Payloads]
There are three forms of replies for a DELETE. The normal form is:
REPLY KINK Header
KINK_AP_REP
[KINK_ENCRYPT]
[KINK_ERROR]
KINK_ISAKMP payloads
Delete Payloads
[Notification Payloads]
If an IPsec DOI-specific error is encountered, the responder must
reply with a Notify payload describing the error:
Sakane, et al. Standards Track [Page 30]
RFC 4430 KINK March 2006
REPLY KINK Header
KINK_AP_REP
[KINK_ENCRYPT]
[KINK_ERROR]
KINK_ISAKMP payloads
[Notification Payloads]
If the responder finds a Kerberos error for which it can produce a
valid authenticator, the REPLY takes the following form:
REPLY KINK Header
KINK_AP_REP
[KINK_ENCRYPT]
KINK_KRB_ERROR
If the responder finds a KINK or Kerberos type of error, it MUST
reply with a lone KINK_KRB_ERROR or KINK_ERROR payload:
REPLY KINK Header
[KINK_KRB_ERROR]
[KINK_ERROR]
6.5. STATUS Message
The STATUS command is used in two ways:
1) As a means to relay an ISAKMP Notification message.
2) As a means of probing a peer whether its epoch has changed for
dead peer detection.
STATUS contains the following payloads:
KINK Header
KINK_AP_REQ
[[KINK_ENCRYPT]
KINK_ISAKMP payload
[Notification Payloads]]
There are three forms of replies for a STATUS. The normal form is:
REPLY KINK Header
KINK_AP_REP
[[KINK_ENCRYPT]
[KINK_ERROR]
KINK_ISAKMP payload
[Notification Payloads]]
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If the responder finds a Kerberos error for which it can produce a
valid authenticator, the REPLY takes the following form:
REPLY KINK Header
KINK_AP_REP
[KINK_ENCRYPT]
KINK_KRB_ERROR
If the responder finds a KINK or Kerberos type of error, it MUST
reply with a lone KINK_KRB_ERROR or KINK_ERROR payload:
REPLY KINK Header
[KINK_KRB_ERROR]
[KINK_ERROR]
6.6. GETTGT Message
A GETTGT command is only used to carry a Kerberos TGT and is not
related to SA management; therefore, it contains only KINK_TGT_REQ
payload and does not contain any DOI-specific payload.
There are two forms of replies for a GETTGT. In the normal form,
where the responder is allowed to return its TGT, the REPLY contains
KINK_TGT_REP payload. If the responder is not allowed to return its
TGT, it MUST reply with a KINK_ERROR payload.
7. ISAKMP Key Derivation
KINK uses the same key derivation mechanisms defined in section 5.5
of [IKE], which is:
KEYMAT = prf(SKEYID_d, [g(qm)^xy |] protocol | SPI | Ni_b [| Nr_b])
The following differences apply:
o prf is the pseudo-random function corresponding to the session
key's etype. They are defined in [KCRYPTO].
o SKEYID_d is the session key in the Kerberos service ticket
from the AP-REQ. Note that subkeys are not used in KINK and
MUST be ignored if received.
o Both Ni_b and Nr_b are the part of the Nonce payloads (Ni and
Nr, respectively) as described in section 3.2 of [IKE]. Nr_b
is optional, which means that Nr_b is treated as if a zero
length value was supplied when the responder's nonce (Nr) does
not exist. When Nr exists, Nr_b MUST be included in the
calculation.
Sakane, et al. Standards Track [Page 32]
RFC 4430 KINK March 2006
Note that g(qm)^xy refers to the keying material generated when KE
payloads are supplied using Diffie-Hellman key agreement. This is
explained in section 5.5 of [IKE].
The rest of the key derivation (e.g., how to expand KEYMAT) follows
IKE. How to use derived keying materials is up to each service
(e.g., section 4.5.2 of [IPSEC]).
8. Key Usage Numbers for Kerberos Key Derivation
Kerberos encrypt/decrypt functions and get_mic/verify_mic functions
require "key usage numbers". They are used to generate specific keys
for cryptographic operations so that different keys are used for
different purposes/objects. KINK uses two usage numbers, listed
below.
Purpose Usage number
------- ------------
KINK_ENCRYPT payload (for encryption) 39
Cksum field (for checksum) 40
9. Transport Considerations
KINK uses UDP on port 910 to transport its messages. There is one
timer T which SHOULD take into consideration round-trip
considerations and MUST implement a truncated exponential back-off
mechanism. The state machine is simple: any message that expects a
response MUST retransmit the request using timer T. Since Kerberos
requires that messages be retransmitted with new times for replay
protection, the message MUST be re-created each time including the
checksum of the message. Both commands and replies with the ACKREQ
bit set are kept on retransmit timers. When a KINK initiator
receives a REPLY with the ACKREQ bit set, it MUST retain the ability
to regenerate the ACK message for the transaction for a minimum of
its full retransmission timeout cycle or until it notices that
packets have arrived on the newly constructed SA, whichever comes
first.
When a KINK peer retransmits a message, it MUST create a new Kerberos
authenticator for the AP-REQ so that the peer can differentiate
between replays and dropped packets. This results in a potential
race condition when a retransmission occurs before an in-flight reply
is received/processed. To counter this race condition, the
retransmitting party SHOULD keep a list of valid authenticators that
are outstanding for any particular transaction.
Sakane, et al. Standards Track [Page 33]
RFC 4430 KINK March 2006
When a KINK peer retransmits a command, it MUST use the same ticket
within the retransmissions. This is to avoid race conditions on
using different keys, which result in different KEYMATs between an
initiator and a responder. For this reason, (1) an initiator MUST
obtain a ticket whose lifetime is greater than the initiator's
maximum transaction time including timeouts, or (2) it MUST continue
to use the same ticket within a set of retransmissions, and iff it
receives an error (most likely KRB_AP_ERR_TKT_EXPIRED) from the
responder, it starts a new transaction with a new ticket.
10. Security Considerations
The principal names are the identities of the KINK services, but the
traffic protected by SAs are identified by DOI-specific selectors (IP
addresses, port numbers, etc.). This may lead to a breakaway of
SA-protected data from authentication. For example, if two different
hosts claim that they have the same IP address, it may be impossible
to predict which principal's key protects the data. Thus, an
implementation must take care for the binding between principal names
and the SA selectors.
Sending errors without cryptographic protection must be handled very
carefully. There is a trade-off between wanting to be helpful in
diagnosing a problem and wanting to avoid being a dupe in a denial of
service attack.
KINK cobbles together and reuses many parts of both Kerberos and IKE,
the latter which in turn is cobbled together from many other memos.
As such, KINK inherits many of the weaknesses and considerations of
each of its components. However, KINK uses only IKE phase 2 payloads
to create and delete SAs; the security considerations which pertain
to IKE phase 1 may be safely ignored. However, being able to ignore
IKE's authentication phase necessarily means that KINK inherits all
of the security considerations of Kerberos authentication as outlined
in [KERBEROS]. For one, a KDC, like an Authentication,
Authorization, and Accounting (AAA) server, is a point of attack and
all that implies. Much has been written about various shortcomings
and mitigations of Kerberos, and they should be evaluated for any
deployment.
KINK's use of Kerberos presents a couple of considerations. First,
KINK explicitly expects that the KDC will provide adequate entropy
when it generates session keys. Second, Kerberos is used as a user
authentication protocol with the possibility of dictionary attacks on
user passwords. This memo does not describe a particular method to
avoid these pitfalls, but recommends that suitable randomly generated
Sakane, et al. Standards Track [Page 34]
RFC 4430 KINK March 2006
keys should be used for the service principals such as using the
-randomkey option with MIT's "kadmin addprinc" command as well as for
clients when that is practical.
Kerberos does not currently provide perfect forward secrecy in
general. KINK with the KE payload can provide PFS for a service key
from a Kerberos key, but the KE is not mandatory because of the
computational cost. This is a trade-off and operators can choose the
PFS over the cost, and vice versa. KINK itself should be secure from
offline analysis from compromised principal passphrases if PFS is
used, but from an overall system's standpoint, the existence of other
Kerberized services that do not provide PFS makes this a less than
optimal situation.
11. IANA Considerations
The IANA has assigned a well-known port number for KINK.
The IANA has created a new registry for KINK parameters, and has
registered the following identifiers.
KINK Message Types (section 4)
KINK Next Payload Types (section 4.2)
KINK Error Codes (section 4.2.8)
Changes and additions to this registry follow the policies described
below. Their meanings are described in [BCP26].
o Using the numbers in the "Private Use" range is Private Use.
o Assignment from the "RESERVED TO IANA" range needs Standards
Action, or non-standards-track RFCs with Expert Review.
(Though the full specification may be a public and permanent
document of a standards body other than IETF, an RFC referring
it is needed.)
o Other change requires Standards Action.
12. Forward Compatibility Considerations
KINK can accommodate future versions of Quick Mode through the use of
the version field in the ISAKMP payload as well as new domains of
interpretation. In this memo, the only supported Quick Mode version
is 1.0, which corresponds to [IKE]. Likewise, the only DOI supported
is the IPsec domain of interpretation [IPDOI]. New Quick Mode
versions and DOIs MUST be described in subsequent memos.
Sakane, et al. Standards Track [Page 35]
RFC 4430 KINK March 2006
KINK implementations MUST reject ISAKMP versions that are greater
than the highest currently supported version with a KINK_BADQMVERS
error type. A KINK implementation that receives a KINK_BADQMVERS
message SHOULD be capable of reverting back to version 1.0.
12.1. New Versions of Quick Mode
The IPsec working group is defining the next-generation IKE protocol
[IKEv2], which does not use Quick Mode, but it is similar to the one
in IKEv1. The difference between the two is summarized in Appendix A
of [IKEv2]. Each of them must be considered in order to use IKEv2
with KINK.
12.2. New DOI
The KINK message header contains a field called "Domain of
Interpretation (DOI)" to allow other domains of interpretation to use
KINK as a secure transport mechanism for keying.
As one example of a new DOI, the MSEC working group defined the Group
Domain of Interpretation [GDOI], which defines a few new messages,
which look like ISAKMP messages, but are not defined in ISAKMP.
In order to carry GDOI messages in KINK, the DOI field in the KINK
header would indicate that GDOI is being used, instead of IPSEC-DOI,
and the KINK_ISAKMP payload would contain the payloads defined in the
GDOI document rather than the payloads used by [IKE] Quick Mode. The
version number in the KINK_ISAKMP header is related to the DOI in the
KINK header, so a maj.min version 1.0 under DOI GDOI is different
from a maj.min version 1.0 under DOI IPSEC-DOI.
13. Related Work
The IPsec working group has defined a number of protocols that
provide the ability to create and maintain cryptographically secure
SAs at layer three (i.e., the IP layer). This effort has produced
two distinct protocols:
o a mechanism for encrypting and authenticating IP datagram
payloads that assumes a shared secret between the sender and
receiver
o a mechanism for IPsec peers to perform mutual authentication
and exchange keying material
The IPsec working group has defined a peer-to-peer authentication and
keying mechanism, IKE (RFC 2409). One of the drawbacks of a peer-
to-peer protocol is that each peer must know and implement a site's
Sakane, et al. Standards Track [Page 36]
RFC 4430 KINK March 2006
security policy, which in practice can be quite complex. In
addition, the peer-to-peer nature of IKE requires the use of Diffie-
Hellman (DH) to establish a shared secret. DH, unfortunately, is
computationally quite expensive and prone to denial of service
attacks. IKE also relies on X.509 certificates to realize scalable
authentication of peers. Digital signatures are also computationally
expensive, and certificate-based trust models are difficult to deploy
in practice. While IKE does allow for a pre-shared key, key
distribution is required between all peers -- an O(n^2) problem --
which is problematic for large deployments.
14. Acknowledgements
Many have contributed to the KINK effort, including our working group
chairs Derek Atkins and Jonathan Trostle. The original inspiration
came from CableLab's PacketCable effort, which defined a simplified
version of Kerberized IPsec, including Sasha Medvinsky, Mike Froh,
and Matt Hur and David McGrew. The inspiration for wholly reusing
IKE phase 2 is the result of Tero Kivinen's document suggesting
grafting Kerberos authentication onto Quick Mode.
15. References
15.1. Normative References
[BCP26] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
2434, October 1998.
[IKE] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[IPDOI] Piper, D., "The Internet IP Security Domain of
Interpretation for ISAKMP", RFC 2407, November 1998.
[IPSEC] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[ISAKMP] Maughan, D., Schertler, M., Schneider, M., and J.
Turner, "Internet Security Association and Key
Management Protocol (ISAKMP)", RFC 2408, November 1998.
[ISAKMP-REG] IANA, "Internet Security Association and Key Management
Protocol (ISAKMP) Identifiers",
<http://www.iana.org/assignments/isakmp-registry>.
[KCRYPTO] Raeburn, K., "Encryption and Checksum Specifications
for Kerberos 5", RFC 3961, February 2005.
Sakane, et al. Standards Track [Page 37]
RFC 4430 KINK March 2006
[KERBEROS] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC
4120, July 2005.
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
15.2. Informative References
[GDOI] Baugher, M., Weis, B., Hardjono, T., and H. Harney,
"The Group Domain of Interpretation", RFC 3547, July
2003.
[IKEv2] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[PKINIT] Zhu, L. and B. Tung, "Public Key Cryptography for
Initial Authentication in Kerberos", Work in Progress,
February 2006.
[REQ4KINK] Thomas, M., "Requirements for Kerberized Internet
Negotiation of Keys", RFC 3129, June 2001.
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
Sakane, et al. Standards Track [Page 38]
RFC 4430 KINK March 2006
Authors' Addresses
Shoichi Sakane
Yokogawa Electric Corporation
2-9-32 Nakacho, Musashino-shi,
Tokyo 180-8750 Japan
EMail: Shouichi.Sakane@jp.yokogawa.com
Ken'ichi Kamada
Yokogawa Electric Corporation
2-9-32 Nakacho, Musashino-shi,
Tokyo 180-8750 Japan
EMail: Ken-ichi.Kamada@jp.yokogawa.com
Michael Thomas
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
EMail: mat@cisco.com
Jan Vilhuber
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
EMail: vilhuber@cisco.com
Sakane, et al. Standards Track [Page 39]
RFC 4430 KINK March 2006
Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
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Sakane, et al. Standards Track [Page 40]
ERRATA
