INTERNET-DRAFT KINK M. Thomas
J. Vilhuber
January 21, 2003
Kerberized Internet Negotiation of Keys (KINK)
draft-ietf-kink-kink-05.txt
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Abstract
The Kerberized Internet Negotiation of Keys protocol (KINK) defines a
low-latency, computationally inexpensive, easily managed, and
cryptographically sound protocol to set up and maintain IPsec
security associations using Kerberos authentication. KINK reuses many
ISAKMP Quick Mode payloads to create, delete and maintain IPsec
security associations which should lead to substantial reuse of
existing IKE implementations.
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 RFC-2119.
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Table of Contents
Introduction .................................................... 2
Terminology ..................................................... 2
Protocol Overview ............................................... 4
Message Flows ................................................... 5
Standard KINK Message Flow ..................................... 5
GETTGT Message Flow ............................................ 5
CREATE Security Association .................................... 5
DELETE Security Association .................................... 7
STATUS Message Flow ............................................ 10
KINK Message Format ............................................. 10
KINK Payloads .................................................. 13
KINK Quick Mode Payload Profile ................................. 22
General Quick Mode Differences ................................. 22
Security Association Payloads .................................. 23
Proposal and Transform Payloads ................................ 23
Identification Payloads ........................................ 23
Nonce Payloads ................................................. 24
Notify Payloads ................................................ 24
Delete Payloads ................................................ 25
KE Payloads .................................................... 25
IPsec DOI Message Formats ....................................... 26
REPLY Message Considerations ................................... 26
ACK Message Considerations ..................................... 26
CREATE Message ................................................. 27
DELETE Message ................................................. 28
STATUS Message ................................................. 29
Key Derivation .................................................. 30
Transport Considerations ........................................ 30
Security Considerations ......................................... 31
Security Policy Database Considerations ........................ 31
IANA Considerations ............................................. 32
Forward Compatibility Considerations ............................ 32
New Versions of Quick Mode ..................................... 32
New DOI ........................................................ 33
Related Work .................................................... 33
Normative References ............................................ 34
Informative References .......................................... 35
Mailing List .................................................... 35
Author's Addresses .............................................. 35
Acknowledgments ................................................. 36
IPR Notice ...................................................... 36
Full Copyright .................................................. 36
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1. Introduction
KINK is designed to provide a secure, scalable mechanism for estab-
lishing 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 incur a low computational
cost, to have low latency, to have a small footprint, and to avoid or
minimize the use of public key operations. In particular, the proto-
col provides the capability to establish security associations in two
messages with minimal computational effort.
Kerberos [KERB] and [KERBEROS] provides an efficient mechanism for
trusted third party authentication for clients and servers. (Ker-
beros also provides an mechanisms for inter-realm authentication
natively and with [PKCROSS].) Clients obtain tickets from an online
authentication server (the Key Distribution Center or KDC). Tickets
are then used to construct credentials for authenticating the client
to the server. As a result of this authentication operation, the
client and the server will also share a secret. KINK uses this pro-
perty 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. Initial authentication
to the KDC may be performed using either symmetric keys or asymmetric
keys using [PKINIT]; however, subsequent requests for tickets, as
well as authenticated exchanges between client and server always
utilize symmetric cryptography. Therefore, public key operations (if
any) are limited and are amortized over the lifetime of the initial
authentication operation to the Kerberos KDC. For example, a client
may use a single public key exchange with the KDC to efficiently
establish multiple security associations with many other servers in
the extended 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) rather than O(n*m), where "n" is the number of clients
and "m" is the number of servers.
This document specifies the Kerberized Internet Negotiation of Keys
Protocol and the domain of interpretation (DOI) for establishing and
maintaining IPsec Security Associations [IPSEC]. No other domains of
interpretation are defined in this document.
2. Terminology
Ticket
A Kerberos term for a record that helps a client authenticate
itself to a server; it contains the client's identity, a session
key, a lifetime, and other information, all sealed using the
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server's secret key. The combination of a ticket and an authentica-
tor (which proves freshness and knowledge of the key within the
ticket) creates an authentication credential.
KDC
Key Distribution Center, a network service that supplies tickets
and temporary session keys; or an instance of that service or the
host on which it runs. The KDC services both initial ticket and
Ticket-Granting Ticket (TGT) requests. The initial ticket portion
is referred to as the Authentication Server (or service). The
Ticket-Granting Ticket portion is referred to as the Ticket-
Granting Server (or service).
Realm
A Kerberos administrative domain. A single KDC may be responsible
for one or more realms. A fully qualified principal name includes
a realm name along with a principal name unique within that realm.
TGT
A ticket granting ticket is a normal Kerberos ticket which the KDC
issues for the Kerberos service. The main purpose of a TGT is to
capture the results of initial authentication for subsequent ticket
granting requests, thus providing a single sign-on service.
User-User
Kerberos normally divides the world into clients and servers where
the server maintains a table of keys (keytab) which is used to
encrypt/decrypt service tickets. In situations where a principal
may not have a keytab (ex. a human/client principal rather than a
service principal), Kerberos provides the means of issuing what is
known as a User-User ticket. To produce the User-User ticket, the
KDC requires the ticket granting tickets from both client princi-
pals. Kerberos does not specify a means obtaining a client's
ticket granting ticket, and is thus application specific.
Principal
Kerberos named entities are known as principals, Principals are
either client or service principals. A principal is an entity that
engages in a security relationship. A Kerberos principal name is
roughly equivalent to an X.509 distinguished name (it associates
the principal with an adminsitrative domain). Principals may be
client or servers. A server principal is generally distinguished
by a flag in a KDC principal database and by a keytab maintained by
the server.
DER
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ASN.1 Distinguished Encoding Rules; Kerberos version 5 uses this
encoding format of ASN.1.
Quick-Mode
IKE defines two phases: an authentication phase (phase 1, or Main
Mode) and a security association maintenance phase (phase 2, or
Quick Mode). KINK reuses IKE Quick Mode.
AP-REQ/AP-REP
Kerberos defines an standardized message format and transport for
contacting a KDC to perform initial authentication, and for grant-
ing subsequent service tickets. When a client needs to authenticate
to a server, Kerberos provides a standardized message format, but
leaves the transport as application specific. The messages which
perform this function are AP-REQ between the client and the server,
and AP-REP between the server and client if mutual authentication
is needed.
3. Protocol Overview
KINK is a command/response protocol which can create, delete and
maintain IPsec security associations. Each command or response con-
tains a common header along with a set of type-length-value payloads
which are constrained according to the type of command or response.
KINK itself is a stateless protocol in that each command or response
does not require storage of hard state for KINK itself. This is in
contrast to IKE's use of Main Mode to first establish an ISAKMP secu-
rity association followed by subsequent Quick Mode exchanges.
KINK uses Kerberos mechanisms to provide mutual authentication,
replay protection. For security association establishment. KINK pro-
vides privacy of the payloads which follow the Kerberos authentica-
tor. KINK's design 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 the means of
using Kerberos User-User mechanisms when there isn't 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 authenti-
cation.
KINK directly reuses [ISAKMP] Quick Mode payloads, with some minor
changes and omissions. In most cases, KINK exchanges are a single
command and its response. The lone exception is the CREATE command
which allows a final acknowledgment message when the respondent needs
a full three-way handshake. This is only needed when the optimistic
keying route is not taken, though it is expected that that will not
be the norm. KINK also provides rekeying and dead peer detection as
basic features.
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4. Message Flows
KINK message flows all follow the same pattern between the two peers:
a command, a response and a possible acknowledgment with CREATE's.
The actual Kerberos KDC traffic here is for illustrative purposes
only. In practice, when a principal obtains various tickets is a sub-
ject of Kerberos and local policy consideration. In these flows, we
assume that A and B both have TGT's from their KDC.
4.1. Standard KINK Message Flow
A B KDC
------ ------ ---
1 COMMAND------------------->
2 <------------------REPLY
3 [ ACK---------------------> ]
Figure 1: KINK Message Flow
4.2. GETTGT Message Flow
If the initiator determines that it will not be able to get a normal
service ticket for the respondent (eg, B is a client principal), it
MUST first fetch the TGT from the respondent in order to get a User-
User service ticket:
A B KDC
------ ------ ---
1 GETTGT+KRB_TGT_REQ------->
2 <-------REPLY+KRB_TGT_REP
3 TGS-REQ+TGT(B)------------------------------------->
4 <--------------------------------------------TGS-REP
Figure 2: GETTGT Message Flow
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4.3. CREATE Security Association
This flow instantiates a security association. The CREATE command
takes an "optimistic" approach where security associations are
initially created on the expectation that the respondent will chose
the initial proposed payload. The optimistic payload is defined as
the first transform of the first proposal of the first conjugate.
The initiator MUST checks to see if the optimistic payload was
selected by comparing all transforms and attributes which MUST be
identical from the initiator's optimistic proposal with the lone
exception of LIFE_KILOBYTES and LIFE_SECONDS. Both of these
attributes MAY be set to a lower value by the respondent and still
expect optimistic keying, but MUST NOT be set to a higher value which
MUST generate an error.
CREATE'ing a security association on an existing SPI is an error in
KINK and MUST be rejected with an ISAKMP notification of INVALID-SPI.
A B KDC
------ ------ ---
A creates initial inbound SA (B->A)
1 CREATE+ISAKMP------------>
B creates inbound SA to A (A->B). If B chooses A's optimistic
proposal, it creates the outbound SA as well (B->A).
2 <------------REPLY+ISAKMP
A creates outbound SA and modifies inbound SA if it first
proposal wasn't acceptable.
3 [ ACK--------------------> ]
[ B creates the outbound SA to A (B-A). ]
Figure 3: CREATE Message Flow
The security associations are instantiated as follows: In step one
host A creates an inbound security association in its security asso-
ciation database from B->A using the optimistic proposal in the
ISAKMP SA proposal. It is then ready to receive any messages from B.
A then sends the CREATE message to B. If B agrees to A's optimistic
proposal, B instantiates a security association in its database from
A->B. B then instantiates the security association from B->A. It then
sends a REPLY to A without a NONCE payload and without requesting an
ACK. If B does not choose the first proposal, it sends the actual
choice in the REPLY, a NONCE payload and requests that the REPLY be
acknowledged. Upon receipt of the REPLY, A modifies the inbound secu-
rity association as necessary, instantiates the security association
from A->B, If B requested an ACK, A now sends the ACK message. Upon
receipt of the ACK, B installs the final security association from
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B->A.
Note: if B adds a nonce, or does not choose the first proposal, it
MUST request an ACK so that it can install the final outbound secu-
rity association. The initiator MUST always generate an ACK if the
ACKREQ bit is set in the KINK header, even if it believes that the
respondent was in error.
4.3.1. CREATE Key Derivation Considerations
The CREATE command's optimistic approach allows a security associa-
tion 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 the inbound security association before it
receives any additional keying material from B. Under normal cir-
cumstances this may be suspect, 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 acknowledgment. Since B is
always at liberty to contribute to the keying material, this is
strictly a key strength versus number of messages tradeoff which KINK
implementations may decide as a matter of policy.
4.4. DELETE Security Association
The DELETE command deletes an existing security association. The DOI
specific payloads describe the actual security association to be
deleted. For the IPSEC DOI, those payloads will include an ISAKMP
payload contains the SPI to be deleted in each direction.
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 4: DELETE Message Flow
The DELETE command takes a "pessimistic approach" which does not
delete incoming security associations until it receives acknowledg-
ment that the other host has received the DELETE. The exception to
the pessimistic approach is if the initiator wants to immediately
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cease all activity on an incoming SA. In this case, it MAY delete the
incoming SA as well in step one. If the receiver cannot find an
appropriate SPI to delete, it MUST return an ISAKMP INVALID_SPI
notification which also serves to inform the initiator that it can
delete the incoming SA. For simplicity, KINK does not allow half open
security associations; thus upon receiving a DELETE, the responder
MUST delete its security associations, and MUST reply with ISAKMP
delete notification messages if the SPI is found.
A race condition with DELETE exists. Packets in flight while the
DELETE operation is taking place may, due to network reordering, etc,
arrive after the diagrams above recommend deleting the incoming secu-
rity association. A KINK implementation SHOULD implement a grace
timer which SHOULD be set to a period of at least two times the aver-
age round trip time, or to a configurable value. A KINK implementa-
tion MAY chose to set the grace period to zero at appropriate times
to ungracefully delete a security association. The behavior described
here loosely mimics the behavior of the TCP [RFC793] flags FIN and
RST.
4.4.1. Rekeying Security Associations
KINK requires the initiator of a security association to be responsi-
ble for rekeying a security association. The reason is twofold: the
first is to prevent needless duplication of security associations as
the result of collisions due to an initiator and respondent both try-
ing to renew an existing security association. The second reason is
due to the client/server nature of Kerberos exchanges which expects
the client to get and maintain tickets. While KINK requires that a
KINK host be able to get and maintain tickets, in practice it is
often advantageous for servers to wait for clients to initiate ses-
sions so that they do not need to maintain a large ticket cache.
There are no special semantics for rekeying security associations in
KINK. That is, in order to rekey an existing security association,
the initiator must CREATE a new security association followed by
either DELETE'ing the old security association or letting it time
out. When identical flow selectors are available on different secu-
rity associations, KINK implementations SHOULD choose the security
association 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 which rekeys existing security associ-
ations will try to rekey the security association ahead of a hard SA
expiration. We call this time the rekey time Trekey. In order to
avoid synchronization with similar implementations, KINK initiators
MUST randomly pick a rekeying time between Trekey and the SA expira-
tion time minus the amount of time it would take to go through a full
retransmission time cycle, Tretrans. Trk SHOULD be set at least twice
as high as Tretrans.
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4.4.2. 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 SADB information and reflect that epoch in each AP-REQ and
AP-REP message. When a KINK peer creates state for a given security
association, it MUST also record the principal's epoch as well. If it
discovers on a subsequent message that the principal's epoch has
changed, it MUST consider all security associations created by that
principal as invalid, and take some action such as tearing those SA's
down.
While a KINK peer SHOULD use feedback from routing (in the form of
ICMP messages) as a trigger to check whether the peer is still alive
or not, a KINK peer MUST NOT conclude the peers is dead simply based
on unprotected routing information (said ICMP messages).
If there is suspicion that a peer may be dead (based on any informa-
tion available to the KINK peer, including lack of IPsec traffic,
etc), the KINK STATUS message SHOULD be used to coerce an acknowledg-
ment out of the peer. Since nothing is negiotiated about dead peer
detection in KINK, each peer can decide its own metric for 'suspi-
cion' and also what time-outs to use before declaring a peer dead due
to lack of response to the STATUS message. This is desireable, and
does not break interoperability.
The STATUS message has a two-fold effect: First, it elicits a crypto-
graphically secured (and replay-protected) response from the peer,
which tells us whether the peer is reachable/alive or not. Further,
it carries the epoch number of the peer, so we know whether the peer
has rebooted and lost all state or not. This is crucial to the KINK
protocol: In IKE, if a peer reboots, we loose all cryptographic con-
text, 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 4.5). Peer B MAY assume that the sender is alive, and the
epoch in the STATUS message will indicate whether the peer A has lost
state or not. Peer B MUST acknowledge the STATUS message with a REPLY
message, as described in section 4.5.
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
failover versus number of messages sent by nodes with large numbers
of security associations.
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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
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: 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 SA's with the peer. Once
there is traffic, peer A may want to know if the traffic 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.
4.5. STATUS Message Flow
At any point, a sender may send status, normally in the form of DOI
specific payloads to its peer. In the case of the IPsec DOI, these
are generally in the form of ISAKMP Notification Payloads.
A B KDC
------ ------ ---
1 STATUS+ISAKMP------------>
2 <-------------REPLY+ISAKMP
Figure 5: STATUS Message Flow
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5. KINK Message Format
All values in KINK are formatted in network byte order: Most
Significant Byte first.
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 | MnVer | Length |
+---------------+---------------+---------------+---------------+
| Domain of Interpretation (DOI) |
+-------------------------------+-------------------------------+
| Transaction ID (XID) |
+---------------+---------------+-+-----------------------------+
| CksumLen | NextPayload |A| Reserved |
+---------------+---------------+-+-----------------------------+
| |
~ Cksum ~
| |
+-------------------------------+-------------------------------+
| |
~ A series of payloads ~
| |
+-------------------------------+-------------------------------+
Figure 6: Format of a KINK message
Fields:
o Type (1 octet) - The type of message of this packet
Type Value
----- -----
RESERVED 0
CREATE 1
DELETE 2
REPLY 3
GETTGT 4
ACK 5
STATUS 6
o MjVer (4 bits) - Major protocol version number. This MUST be set
to 1.
o MnVer (4 bits) - Minor protocol version number. This MUST be set
to 0.
o Length (2 octets) - Length of the message in octets. Note that it
is legal within KINK to omit the last bytes of padding in the last
payload in the overall length.
o DOI (4 octets) - The domain of interpretation. All DOI's must be
registered with the IANA in the "Assigned Numbers" RFC [STD-2].
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The IANA Assigned Number for the Internet IP Security DOI (IPSEC
DOI) is one (1). This field defines the context of all other sub-
payloads in this payloads. If other sub-payloads have a DOI field
(example: 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 ini-
tiator and replicated in subsequent messages in the transaction. A
transaction is defined as a command, a reply, and an optional ack-
nowledgment. Transaction ID's are used by the initiator to
discriminate between multiple outstanding requests to a respon-
dent. It is not used for replay protection because that func-
tionality is provided by Kerberos. The value of XID is chosen by
the initiator and MUST be unique with all outstanding transac-
tions. XID's MAY be constructed by using a monotonic counter, or
random number generator.
o CksumLen (2 octets) -- CksumLen is the length in octets of the
keyed hash of the message. A CksumLen of zero implies that the
message is unauthenticated. Note that as with payload padding, the
length here denotes the actual number of octets of the checksum
structure not including any padding required.
o NextPayload (1 octet) -- Indicates the type of the first payload
after the message header
o A (1 bit) -- ACK Request. Set to one if the responder requires an
explicit acknowledgment that a REPLY was received. An initiator
MUST NOT set this flag, nor should any other command other than
CREATE request an ACK and then only when the optimistic SA is not
chosen.
o Reserved (15 bits) -- Reserved and must be zero
o Cksum (variable) - Keyed checksum over the entire message. 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 hash algorithm
used is the same as specified in the etype for the Kerberos ses-
sion key in the Kerberos ticket. If the etype does not specify a
hash algorithm, the message MUST be rejected.
The format of the Cksum field is as follows:
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
+---------------+---------------+---------------+---------------+
| checksum (variable) ~ padding (variable) |
+---------------+---------------+---------------+---------------+
Figure 7: KINK Checksum
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To compute the checksum, the checksum field is zeroed out and the
appropriate algorithm is run over the entire message (as given by
the Length field in the KINK header), and placed in the Checksum
field. To verify the checksum, the checksum is saved, and the
checksum field is zeroed out. The checksum algorithm is run over
the message, and the result is compared with the saved version. If
they do not match, the message MUST be dropped.
The KINK header is followed immediately by a series of
Type/Length/Value fields, defined in the next section.
5.1. 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
padded 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 8: Format of a KINK payload
Fields:
o NextPayload (2 octets) - The type of the next payload
NextPayload Number
---- ------
KINK_DONE 0 (same as ISAKMP_NEXT_NONE)
KINK_AP_REQ KINK_ISAKMP_PAYLOAD_BASE+0
KINK_AP_REP KINK_ISAKMP_PAYLOAD_BASE+1
KINK_KRB_ERROR KINK_ISAKMP_PAYLOAD_BASE+2
KINK_TGT_REQ KINK_ISAKMP_PAYLOAD_BASE+3
KINK_TGT_REP KINK_ISAKMP_PAYLOAD_BASE+4
KINK_ISAKMP KINK_ISAKMP_PAYLOAD_BASE+5
KINK_ENCRYPT KINK_ISAKMP_PAYLOAD_BASE+6
KINK_ERROR KINK_ISAKMP_PAYLOAD_BASE+7
NextPayload type KINK_DONE denotes that the current payload is the
final payload in the message.
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Note: the payload types are taken from the ISAKMP registry for
payload types. See the IANA consideration section for the value of
KINK_ISAKMP_PAYLOAD_BASE.
o RESERVED (1 octet) - Unused, MUST be set to 0.
o 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.
5.1.1. KINK Padding Rules
KINK has the following rules regarding alignment and padding:
o All length fields MUST reflect the actual number of octets in the
structure; ie they do not account for padding bytes
o Between KINK payloads, checksums, headers or any other other vari-
able length data, the adjacent fields MUST be aligned on 4 octet
boundaries.
o Variable length fields MUST always start immediately after the
last octet of the previous field. Ie, they are not padded to a 4
octet boundary.
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5.1.2. KINK_AP_REQ Payload
The KINK_AP_REQ payload relays a Kerberos AP-REQ to the respondent.
The AP-REQ MUST request mutual authentication. The service that the
KINK peer SHOULD request is "kink/fqdn@REALM" where "kink" is the
KINK IPsec service, "fqdn" is the fully qualified domain name of the
service host, and REALM is the Kerberos realm of the service. The
exception to this rule is when User-User service is requested in
which case the service name MUST be the service returned in the
GetTGT response 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 |
+---------------+---------------+---------------+---------------+
| EPOCH |
+---------------------------------------------------------------+
| |
~ KRB_AP_REQ ~
| |
+---------------------------------------------------------------+
Figure 9: KINK_AP_REQ Payload
Fields:
o EPOCH - the absolute time at which the creator of the AP-REQ has
valid security database (SADB) information. Typically this is when
the KINK keying daemon started if it does not retain SADB informa-
tion across different restarts. The format of this fields is net-
work order encoding of the standard posix four octet time stamp.
o KRB_AP_REQ - The value field of this payload contains a raw Ker-
beros KRB_AP_REQ.
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5.1.3. 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 |
+---------------------------------------------------------------+
| |
~ KRB_AP_REP ~
| |
+---------------------------------------------------------------+
Figure 10: KINK_AP_REP Payload
Fields:
o EPOCH - the absolute time at which the creator of the AP-REP has
valid security database (SADB) information. Typically this is when
the KINK keying daemon started if it does not retain SADB informa-
tion across different restarts. The format of this fields is net-
work order encoding of the standard posix four octet time stamp.
o KRB_AP_REP - The value field of this payload contains a raw Ker-
beros KRB_AP_REP.
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5.1.4. KINK_KRB_ERROR Payload
The KINK_KRB_ERROR payload relays Kerberos type errors back to the
initiator. The receiver MUST be prepared to receive any valid
[KERBEROS] error type, but the sender SHOULD send only the following
errors:
KRB5KRB_AP_ERR_BAD_INTEGRITY
KRB5KRB_AP_ERR_TKT_EXPIRED
KRB5KRB_AP_ERR_SKEW
KRB5KRB_AP_ERR_NOKEY
KRB5KRB_AP_ERR_BADKEYVER
KINK implementations MUST make use of keyed Kerberos errors when the
appropriate service key is available as specified in [KRBREVS]. In
particular, clock skew errors MUST be integrity protected. For
unauthenticated Kerberos errors, the receiver 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 11: KINK_KRB_ERROR Payload
Fields:
o KRB_ERROR - The value field of this payload contains a raw
Kerberos KRB_ERROR.
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5.1.5. 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 |
+---------------+---------------+---------------+---------------+
| RealmNameLen | RealmName (variable) ~
+---------------+---------------+---------------+---------------+
| |
~ RealmName(variable) ~
| |
+---------------------------------------------------------------+
Figure 12: KINK_TGT_REQ Payload
Fields:
o RealmNameLen - The length of the realm name that follows
o RealmName - The realm name that the responder should return a TGT
for. The responder MUST return a ticket for the principal
krbtgt/REALM/@REALM to the initiator so that a User-User service
ticket can be obtained by the initiator.
o RESERVED - reserved and must be zero
If the responder is unable to get a TGT for the domain, it must
reply with a KRB_ERROR payload type.
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5.1.6. KINK_TGT_REP Payload
The value field of this payload contains the TGT requested in a
previous KINK_TGT_REP 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 |
+---------------+---------------+---------------+---------------+
| PrincNameLen | PrincName (variable) ~
+---------------+---------------+---------------+---------------+
| |
~ PrincName(variable) +---------------+
| ~ padding |
+---------------------------------------------------------------+
| TGTlength | TGT (variable) |
+-------------------------------+---------------+---------------+
| ~
~ TGT (variable) +---------------+
| ~ padding |
+---------------------------------------------------------------+
Figure 13: KINK_TGT_REQ Payload
Fields:
o PrincNameLen - The length of the principal name that immediately
follows
o PrincName - The client principal that the initiator should request
a User to User service ticket for.
o TGTlength - The length of TGT that immediately follows
o TGT - the DER encoded TGT of the responder
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5.1.7. 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 14: KINK_ISAKMP Payload
Fields:
o InnerNextPload - First payload type of the inner series of ISAKMP
payloads.
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.
o RESERVED - reserved and must be zero
The KINK_ISAKMP payload encapsulates the IKE Quick Mode (phase
two) 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 SA's may be created within a single IKE mes-
sage, KINK explicitly requires that a new ISAKMP header be used
for each discrete SA operation. In other words, a KINK sender MUST
NOT send multiple quick mode transactions within a single
KINK_ISAKMP payload.
The purpose of the Quick Mode version is to allow backward compa-
tibility 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.
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5.1.8. KINK_ENCRYPT Payload
The KINK_ENCRYPT payload encapsulates other payloads and is encrypted
using the encryption algorithm specified by the etype of the session
key. This payload MUST be the final payload in the message. KINK
encrypt payloads 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| RESERVED |
+---------------+---------------+---------------+---------------+
| Payload (variable) |
+---------------+---------------+---------------+---------------+
Figure 15: KINK_ENCRYPT Payload
Fields:
o InnerNextPload (variable) - First payload type of the inner series
of encrypted KINK payloads.
o RESERVED - reserved and must be zero
Note: the coverage of the encrypted data begins at InnerNextPload
so that first payload's type is kept confidential. Thus, the
number of encrypted octets is PayloadLength - 4.
The format of the encryption payload uses the normal [KERBEROS]
semantics of prepending a crypto-specific initialization vector
and padding the entire message out to the crypto-specific number
of bytes. For example, with DES-CBC, the initialization vector
will be 8 octets long, and the entire message will be padded to an
8 octet boundary. Note that KINK Encrypt payload MUST NOT include
a checksum since this is redundant with the message integrity
checksum in the KINK header.
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5.1.9. 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 which 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 16: KINK_ERROR Payload
ErrorCode Number 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
KINK_INVMIN 4 - Invalid Minor Version
KINK_INTERR 5 - An unrecoverable internal error
KINK_BADQMVERS 6 - Unsupported Quick Mode Version
RESERVED 7 - 8191
Private Use 8192 - 16383
6. KINK Quick Mode Payload Profile
KINK directly uses ISAKMP payloads to negotiate security associa-
tions. In particular, KINK uses IKE phase II payload types (aka Quick
Mode). In general, there should be very few changes necessary to an
IKE implementation to establish the security associations, and unless
there is a note to the contrary in the memo, all capabilities and
requirements in [IKE] MUST be supported. IKE Phase I payloads MUST
NOT be sent.
Unlike IKE, KINK defines specific commands for creation, deletion,
and status of security associations, mainly to facilitate predictable
SA creation/deletion (see section 4.3 and 4.4). As such, KINK places
certain restrictions on what payloads may be sent with which com-
mands, 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 II payload is
not mentioned here, it means that there are no differences in its
use.
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6.1. General Quick Mode Differences
o The Security Association Payload header for IP is defined in
[IPDOI] section 4.6.1. For this memo, the Domain of Interpreta-
tion 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 it defines an optmis-
tic proposal for CREATE commands to allow SA creation to com-
plete in two messages.
o IKE Quick Mode (phase 2) uses the hash algorithm used in main
mode (phase 1) to generate the keying material. KINK MUST use
the hashing algorithm specified in the session ticket's etype.
o KINK does not use the HASH payload at all.
o KINK allows the NONCE payload Nr to be optional to facilitate
optimistic keying.
6.2. Security Association Payloads
KINK supports the following security association 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 for these attributes.
6.3. 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.
6.4. Identification Payloads
The Identification payload carries information that is used to iden-
tify the traffic that is to be protected using the keys exchanges in
this memo. KINK restricts the ID types to the following values:
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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
6.5. Nonce Payloads
The Nonce payload contains random data that MUST be used in key
generation by the initiating KINK peer, and MAY be used by the
responding KINK peer. See section 8 for the discussion of their use
in key generation.
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6.6. Notify Payloads
Notification information 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.
For example, a secure front end or security gateway may use the
Notify message to synchronize SA communication. The table below
lists the Notification messages and their corresponding values that
are supported in KINK.
NOTIFY MESSAGES - ERROR TYPES
Errors Value
INVALID-PAYLOAD-TYPE 1
SITUATION-NOT-SUPPORTED 3
INVALID-MAJOR-VERSION 5
INVALID-MINOR-VERSION 6
INVALID-EXCHANGE-TYPE 7
INVALID-FLAGS 8
INVALID-MESSAGE-ID 9
INVALID-PROTOCOL-ID 10
INVALID-SPI 11
INVALID-TRANSFORM-ID 12
ATTRIBUTES-NOT-SUPPORTED 13
NO-PROPOSAL-CHOSEN 14
BAD-PROPOSAL-SYNTAX 15
PAYLOAD-MALFORMED 16
INVALID-KEY-INFORMATION 17
INVALID-ID-INFORMATION 18
ADDRESS-NOTIFICATION 26
NOTIFY-SA-LIFETIME 27
UNEQUAL-PAYLOAD-LENGTHS 30
RESERVED (Future Use) 31 - 8191
Private Use 8192 - 16383
NOTIFY MESSAGES - STATUS TYPES
Status Value
CONNECTED 16384
RESERVED (Future Use) 16385 - 24575
DOI-specific codes 24576 - 32767
Private Use 32768 - 40959
RESERVED (Future Use) 40960 - 65535
6.7. Delete Payloads
KINK directly uses ISAKMP delete payloads with no changes.
6.8. KE Payloads
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IKE requires that perfect forward secrecy be supported through the
use of the KE payload. However, Kerberos in general does not provide
PFS so it is somewhat questionable whether a system which is heavily
relying on Kerberos benefits from PFS. KINK retains the ability to
use PFS, but relaxes the requirement from must implement to SHOULD
implement.
7. IPsec DOI Message Formats
KINK messages are either commands, replies, or acknowledgments. A
command is sent by an initiator to the respondent. A reply is sent
by the respondent to the initiator. If the respondent desires confir-
mation of the reply, it sets the ACKREQ bit in the message header.
The ACKREQ bit MUST NOT be set by the respondent except in the lone
case of a CREATE message for which one of the security associations
did not use the optimistic payload. In that case, the ACKREQ bit MUST
be set. All commands, responses and acknowledgments 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 Kerberos mechanisms. In addition, commands and responses
MUST use a cryptographic hash over the entire message if the two
peers share a symmetric key via a ticket exchange.
7.1. REPLY Message Considerations
The REPLY message is a generic reply which MUST contain either a
KINK_AP_REP, a KRB-ERROR, or KINK_ERROR payload. REPLY's MAY contain
additional DOI specific payloads such as ISAKMP payloads which are
defined in the following sections. The checksum in the KRB-ERROR
message is not used, since the KINK header already contains a check-
sum field.
The server MUST return a KRB_AP_ERR_SKEW if the server clock and the
client clock are off by more than the policy-determined clock skew
limit (usually 5 minutes). The optional client's time in the KRB-
ERROR MUST be filled out, and the client MUST 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.
7.2. ACK Message Considerations
ACK's are sent only when the ACKREQ bit is set in a REPLY message.
ACK's MUST NOT contain any payloads beside a lone AP-REQ header. If
the initiator detects an error in the AP-REP or any other KINK or
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Kerberos error, it SHOULD take remedial action by reinitiating the
initial command with the appropriate error to instruct the KINK
receiver how to correct its original problem.
7.3. CREATE
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 payload
SA Payload[s]
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
SA Payload[s]
Proposal Payload
Transform Payload
[ Nonce Payload (Nr)]
[IDci, IDcr]
[Notification Payloads]
Note that there MUST be at least a single proposal payload and a
single transform payload in REPLY messages. Also: unlike IKE, the
Nonce Payload Nr is not required, and its absence means that the
optimistic mode SA's installed by the initiator are valid. If any of
the first proposals are not chosen by the recipient, it MUST include
the nonce payload as well to indicate that the initiator's outgoing
SA's must be modified.
KINK, like IKE allows the creation of many security associations in
one create command. If any of the optimistic creation mode proposals
is not chosen by the respondent, it MUST request an ACK.
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If an IPspec DOI specific error is encountered, the respondent must
reply with a Notify payload describing the error:
REPLY KINK Header
KINK_AP_REP
[KINK_ENCRYPT]
[KINK_ERROR]
KINK_ISAKMP
[Notification Payloads]
If the respondent 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 respondent finds a Kerberos or KINK type of error it
which it cannot create a AP-REP for, MUST reply with a lone
KINK_KRB_ERROR or KINK_ERROR payload:
REPLY KINK Header
[KINK_KRB_ERROR]
[KINK_ERROR]
7.4. DELETE
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_ERROR payload ]
KINK_ISAKMP payload
Delete Payload[s]
[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 payload ]
KINK_ISAKMP payload
Delete Payload[s]
[Notification Payloads]
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If an IPsec DOI specific error is encountered, the respondent must
reply with a Notify payload describing the error:
REPLY KINK Header
KINK_AP_REP payload
[ KINK_ENCRYPT payload ]
[ KINK_ERROR payload ]
KINK_ISAKMP payload
[Notification Payloads]
If the respondent 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 respondent finds a KINK or Kerberos type of error it MUST
reply with a lone KINK_KRB_ERROR or KINK_ERROR payload:
REPLY KINK Header
[KRB_ERROR]
[KINK_KRB_ERROR]
7.5. STATUS
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 payload
[ [KINK_ENCRYPT]
[ KINK_ERROR payload ]
KINK_ISAKMP payload
[Notification Payloads] ]
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There are two forms of replies for a STATUS. The normal form is:
REPLY KINK Header
KINK_AP_REP
[ [KINK_ENCRYPT]
[KINK_ERROR]
KINK_ISAKMP
[Notification Payloads] ]
If the respondent 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 respondent finds a KINK or Kerberos type of error it MUST
reply with a lone KINK_KRB_ERROR or KINK_ERROR payload:
REPLY
KINK Header
[KRB_ERROR]
[KINK_KRB_ERROR]
8. Key Derivation
KINK uses the same key derivation mechanisms that [IKE] uses in sec-
tion 5.5, which is:
KEYMAT = prf(SKEYID_d, [g(qm)^xy |] protocol | SPI | Ni_b [| Nr_b])
The following differences apply:
o SKEYID_d is the session key in the Kerberos service ticket from
the AP-REQ.
o Nr_b is optional
By optional, it is meant that the equivalent of a zero length
nonce was supplied.
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].
9. Transport Considerations
KINK uses UDP on port [XXX -- TBA by IANA] to transport its messages.
There is one timer T which SHOULD take into consideration round trip
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considerations and MUST implement a truncated exponential backoff
mechanism. The state machine is simple: any message which 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 recreated 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 regen-
erate the ACK message for the transaction for a minimum of its a 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 retransmit-
ting party SHOULD keep a list of valid authenticators which are out-
standing for any particular transaction.
10. Security Considerations
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 II payloads
to create and delete security associations, the security considera-
tions which pertain to IKE Phase I may be safely ignored.
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
keys 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 itself does not provide for perfect forward secrecy which
makes KINK's reliance on the IKE ability to do PFS somewhat suspect
from an overall system's standpoint. In isolation KINK itself should
be secure from offline analysis from compromised principal
passphrases if PFS is used, but the existence of other Kerberized
service which do not provide PFS makes this a less than optimal
situation on the whole.
10.1. Security Policy Database Considerations
KINK leaves the population of the IPsec security policy database out
of scope. There are, however, considerations which should be pointed
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out. First, even though when and when not to initiate a user to user
flow is left to the discretion of the KINK implementation, a Kerberos
client which initially authenticated using a symmetric key SHOULD NOT
use a user-user flow if the respondent is also in the same realm.
Likewise, a KINK initiator which authenticated in a public key realm
SHOULD use a user-user flow if the respondent is in the same realm.
At a minimum the security policy database for a KINK implementation
SHOULD contain a logical record of the KDC to contact, principal name
for the respondent, and whether the KINK implementation should use a
direct AP-REQ/AP-REP flow, or a User-User flow to CREATE/DELETE the
security association.
That said, there is considerable room for improvement on how a KINK
initiator could auto-discover how a respondent in a different realm
initially authenticated. This is left as an implementation detail as
well as the subject of possible future standardization efforts which
are outside of the scope of the KINK working group.
11. IANA Considerations
KINK requires that a new well known system port for UDP be created.
Since KINK uses standard message types from [IKE], KINK does not
require any new registries. Any new DOI's, ISAKMP types, etc for
future versions of KINK MUST use the registries defined for [IKE].
In addition, the ISAKMP payload types currently don't have a IANA
registry, but needs one. KINK defines its payload constants as a
sequential set of integers from KINK_ISAKMP_PAYLOAD_BASE to
KINK_ISAKMP_PAYLOAD_BASE+7.
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 suported is
the IPsec domain of interpretation [IPDOI]. New Quick Mode versions
and DOI's MUST be described in subsequent memos.
KINK implementations MUST reject ISAKMP versions which are greater
than the highest currently supported version with a KINK_BADQMVERS
error type. A KINK implementation which receives a KINK_BADQMVERS
message SHOULD be capable of reverting back to version 1.0.
The following sections describe how different quick-modes and dif-
ferent DOI's can be used within the KINK framework.
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12.1. New Versions of Quick Mode
The ipsec working group is defining the next generation IKE protocol
(IKEv2) which uses a slightly different quick mode from the one in
IKE v1. While the format of IKEv2 is not yet finalized, it does serve
as an example.
The only difference between the two is the format of the payloads
that contain the IPsec traffic selectors. Formerly, these were over-
loaded into the ID payloads, and now they are carried in slightly
more powerful TS (Traffic Selector) payloads.
Were KINK to replace the IKEv2 'CREATE_CHILD_SA' for the current
scheme, we would replace the contents of the KINK_ISAKMP payload
(which currently contains a simplified version of the IKEv1 Quick-
mode payloads) with the set of new payloads. Since the IKEv2
CREATE_CHILD_SA exchange is still part of the IPsec DOI (see A.2),
only the QMMaj version number in the KINK_ISAKMP header would be
bumped to a new (higher) version number to indicate the new expected
format of the contents of the KINK_ISAKMP payload. No other changes
would be needed.
KINK, therefore, merely acts as a transport mechanism to a Quick-mode
exchange.
12.2. New DOI
The KINK message header contains a field called "Domain of Interpre-
tation (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 is currently
defining the GDOI (Group Domain of Interpretation), 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 draft 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 pro-
vide the ability to create and maintain cryptographically secure
security associations at layer three (ie, the IP layer). This effort
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has produced two distinct protocols:
o a mechanism for encrypting and authenticating IP datagram
payloads which 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
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 computation-
ally 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 cer-
tificate based trust models are difficult to deploy in practice.
While IKE does allow for pre-shared symmetric keys, key distribution
is required between all peers -- an O(n2) problem -- which is prob-
lematic for large deployments.
14. Normative References
[KERBEROS]
J. Kohl, C. Neuman. The Kerberos Network Authentication Service
(V5). Request for Comments 1510.
[IPSEC]
S. Kent, R. Atkinson. Security Architecture for the Internet
Protocol. Request for Comments 2401.
[IKE]D. Harkins, D. Carrel. The Internet Key Exchange (IKE).
Request for Comments 2409.
[ISAKMP]
Maughhan, D., Schertler, M., Schneider, M., and J. Turner,
"Internet Security Association and Key Management Protocol
(ISAKMP)", RFC 2408, November 1998.
[IPDOI]
Piper, D., "The Internet IP Security Domain Of Interpretation
for ISAKMP", RFC 2407, November 1998.
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15. Informative References
[RFC2412]
Orman, H., "The OAKLEY Key Determination Protocol", RFC 2412,
November 1998.
[RFC793]
Postel, J., "Transmission Control Protocol", RFC 793, Sep-01-
1981
[KERB]B.C. Neuman, Theodore Ts'o. Kerberos: An Authentication Service
for Computer Networks, IEEE Communications, 32(9):33-38. Sep-
tember 1994.
[PKINIT]
B. Tung, C. Neuman, M. Hur, A. Medvinsky, S.Medvinsky, J. Wray,
J. Trostle. Public Key Cryptography for Initial Authentication
in Kerberos. draft-ietf-cat-kerberos-pk-init-11.txt
[PKCROSS]
M.Hur, B. Tung, T. Ryutov, C. Neuman, G. Tsudik, A. Medvinsky,
B. Sommerfeld. Public Key Cryptography for Cross-Realm Authen-
tication in Kerberos. draft-ietf-cat-kerberos-pk-cross-06.txt
16. Mailing List
Please send comments to the KINK mailing list (ietf-kink@vpnc.org).
You can subscribe by sending mail to ietf-kink-request@vpnc.org with
a line in the body of the mail with the word SUBSCRIBE in it.
17. Author's Addresses
Michael Thomas
Jan Vilhuber
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
E-mail: {mat,vilhuber}@cisco.com
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18. Acknowledgments
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 simplifed
version of Kerberized IPsec, including Sasha Medvinsky, Mike Froh,
and Matt Hur and David McGrew. The inspiration for wholly reusing IKE
Phase II is the result of the Tero Kivinen's draft suggesting graft-
ing Kerberos authentication onto quick mode.
19. IPR Notice
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Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
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The limited permissions granted above are perpetual and will not be
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Thomas, Vilhuber [Page 38]
