Network Working Group R. Moskowitz
Internet-Draft ICSAlabs, a Division of TruSecure
Expires: April 22, 2004 Corporation
P. Nikander (editor)
P. Jokela
Ericsson Research Nomadiclab
T. Henderson
The Boeing Company
October 23, 2003
Host Identity Protocol
draft-moskowitz-hip-08
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This memo specifies the details of the Host Identity Protocol (HIP).
The overall description of protocol and the underlying architectural
thinking is available in the separate HIP architecture specification.
The Host Identity Protocol is used to establish a rapid
authentication between two hosts and to provide continuity of
communications between those hosts independent of the networking
layer.
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The various forms of the Host Identity, Host Identity Tag (HIT) and
Local Scope Identifier (LSI), are covered in detail. It is described
how they are used to support authentication and the establishment of
keying material, which is then used by IPsec Encapsulated Security
payload (ESP) to establish a two-way secured communication channel
between the hosts. The basic state machine for HIP provides a HIP
compliant host with the resiliency to avoid many denial-of-service
(DoS) attacks. The basic HIP exchange for two public hosts shows the
actual packet flow. Other HIP exchanges, including those that work
across NATs are covered elsewhere.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 A new name space and identifiers . . . . . . . . . . . . . 5
1.2 The HIP protocol . . . . . . . . . . . . . . . . . . . . . 5
2. Conventions used in this document . . . . . . . . . . . . 7
3. Host Identifier (HI) and its representations . . . . . . . 8
3.1 Host Identity Tag (HIT) . . . . . . . . . . . . . . . . . 8
3.1.1 Generating a HIT from a HI . . . . . . . . . . . . . . . . 8
3.2 Local Scope Identifier (LSI) . . . . . . . . . . . . . . . 9
3.3 Security Parameter Index (SPI) . . . . . . . . . . . . . . 10
3.4 Difference between an LSI and the SPI . . . . . . . . . . 11
3.5 TCP and UDP pseudo-header computation . . . . . . . . . . 11
4. Host Identity Protocol . . . . . . . . . . . . . . . . . . 12
4.1 HIP base exchange . . . . . . . . . . . . . . . . . . . . 12
4.1.1 HIP Cookie Mechanism . . . . . . . . . . . . . . . . . . . 13
4.1.2 Authenticated Diffie-Hellman protocol . . . . . . . . . . 15
4.1.3 HIP Birthday . . . . . . . . . . . . . . . . . . . . . . . 16
4.2 Sending data on HIP packets . . . . . . . . . . . . . . . 16
4.3 Rekey . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.4 Bootstrap support . . . . . . . . . . . . . . . . . . . . 16
4.5 Certificate distribution . . . . . . . . . . . . . . . . . 17
5. HIP packet flow and state machine . . . . . . . . . . . . 18
5.1 HIP Scenarios . . . . . . . . . . . . . . . . . . . . . . 18
5.2 Refusing a HIP exchange . . . . . . . . . . . . . . . . . 19
5.3 Reboot and SA timeout restart of HIP . . . . . . . . . . . 19
5.4 HIP State Machine . . . . . . . . . . . . . . . . . . . . 20
5.4.1 HIP States . . . . . . . . . . . . . . . . . . . . . . . . 20
5.4.2 HIP State Processes . . . . . . . . . . . . . . . . . . . 20
5.4.3 Simplified HIP State Diagram . . . . . . . . . . . . . . . 22
6. Packet formats . . . . . . . . . . . . . . . . . . . . . . 24
6.1 Payload format . . . . . . . . . . . . . . . . . . . . . . 24
6.1.1 HIP Controls . . . . . . . . . . . . . . . . . . . . . . . 25
6.1.2 Checksum . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.2 HIP parameters . . . . . . . . . . . . . . . . . . . . . . 26
6.2.1 TLV format . . . . . . . . . . . . . . . . . . . . . . . . 27
6.2.2 Defining new parameters . . . . . . . . . . . . . . . . . 28
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6.2.3 SPI_LSI . . . . . . . . . . . . . . . . . . . . . . . . . 29
6.2.4 BIRTHDAY_COOKIE . . . . . . . . . . . . . . . . . . . . . 29
6.2.5 DIFFIE_HELLMAN . . . . . . . . . . . . . . . . . . . . . . 30
6.2.6 HIP_TRANSFORM . . . . . . . . . . . . . . . . . . . . . . 31
6.2.7 ESP_TRANSFORM . . . . . . . . . . . . . . . . . . . . . . 31
6.2.8 HOST_ID . . . . . . . . . . . . . . . . . . . . . . . . . 32
6.2.9 HOST_ID_FQDN . . . . . . . . . . . . . . . . . . . . . . . 33
6.2.10 CERT . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
6.2.11 HMAC . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
6.2.12 HIP_SIGNATURE . . . . . . . . . . . . . . . . . . . . . . 35
6.2.13 HIP_SIGNATURE_2 . . . . . . . . . . . . . . . . . . . . . 36
6.2.14 NES_INFO . . . . . . . . . . . . . . . . . . . . . . . . . 37
6.2.15 ENCRYPTED . . . . . . . . . . . . . . . . . . . . . . . . 38
7. HIP Packets . . . . . . . . . . . . . . . . . . . . . . . 40
7.1 I1 - the HIP initiator packet . . . . . . . . . . . . . . 40
7.2 R1 - the HIP responder packet . . . . . . . . . . . . . . 41
7.3 I2 - the second HIP initiator packet . . . . . . . . . . . 42
7.4 R2 - the second HIP responder packet . . . . . . . . . . . 43
7.5 NES - the HIP New SPI Packet . . . . . . . . . . . . . . . 43
7.6 BOS - the HIP Bootstrap Packet . . . . . . . . . . . . . . 44
7.7 CER - the HIP Certificate Packet . . . . . . . . . . . . . 45
7.8 PAYLOAD - the HIP Payload Packet . . . . . . . . . . . . . 45
8. Packet processing . . . . . . . . . . . . . . . . . . . . 47
8.1 Processing outgoing application data . . . . . . . . . . . 47
8.2 Processing incoming application data . . . . . . . . . . . 48
8.3 Initiation of a HIP exchange . . . . . . . . . . . . . . . 49
8.3.1 Sending multiple I1s in parallel . . . . . . . . . . . . . 50
8.3.2 Processing incoming ICMP Protocol Unreachable messages . . 50
8.4 Processing incoming I1 packets . . . . . . . . . . . . . . 50
8.4.1 R1 Management . . . . . . . . . . . . . . . . . . . . . . 51
8.5 Processing incoming R1 packets . . . . . . . . . . . . . . 51
8.6 Processing incoming I2 packets . . . . . . . . . . . . . . 54
8.7 Processing incoming R2 packets . . . . . . . . . . . . . . 56
8.8 Initiating rekeying . . . . . . . . . . . . . . . . . . . 57
8.9 Processing NES packets . . . . . . . . . . . . . . . . . . 58
8.9.1 Processing an initial NES packet . . . . . . . . . . . . . 59
8.9.2 Processing a reply NES packet . . . . . . . . . . . . . . 60
8.10 Processing BOS packets . . . . . . . . . . . . . . . . . . 60
8.11 Processing CER packets . . . . . . . . . . . . . . . . . . 60
8.12 Processing PAYLOAD packets . . . . . . . . . . . . . . . . 61
9. HIP KEYMAT . . . . . . . . . . . . . . . . . . . . . . . . 62
10. HIP Fragmentation Support . . . . . . . . . . . . . . . . 64
11. ESP with HIP . . . . . . . . . . . . . . . . . . . . . . . 65
11.1 Security Association Management . . . . . . . . . . . . . 65
11.2 Security Parameter Index (SPI) . . . . . . . . . . . . . . 65
11.3 Supported Transforms . . . . . . . . . . . . . . . . . . . 65
11.4 Sequence Number . . . . . . . . . . . . . . . . . . . . . 65
12. HIP Policies . . . . . . . . . . . . . . . . . . . . . . . 67
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13. Security Considerations . . . . . . . . . . . . . . . . . 68
14. IANA Considerations . . . . . . . . . . . . . . . . . . . 70
15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . 71
Normative references . . . . . . . . . . . . . . . . . . . 72
Informative references . . . . . . . . . . . . . . . . . . 74
Authors' Addresses . . . . . . . . . . . . . . . . . . . . 74
A. Backwards compatibility API issues . . . . . . . . . . . . 76
B. Probabilities of HIT collisions . . . . . . . . . . . . . 79
C. Probabilities in the cookie calculation . . . . . . . . . 80
D. Using responder cookies . . . . . . . . . . . . . . . . . 81
E. Running HIP over IPv4 UDP . . . . . . . . . . . . . . . . 84
Intellectual Property and Copyright Statements . . . . . . 85
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1. Introduction
The Host Identity Protocol (HIP) provides a rapid exchange of Host
Identities between two hosts. The exchange also establishes a pair
IPsec Security Associations (SA), to be used with IPsec Encapsulated
Security Payload (ESP) [15]. The HIP protocol is designed to be
resistant to Denial-of-Service (DoS) and Man-in-the-middle (MitM)
attacks, and when used to enable ESP, provides DoS and MitM
protection for upper layer protocols, such as TCP and UDP.
1.1 A new name space and identifiers
The Host Identity Protocol introduces a new namespace, the Host
Identity. The effects of this change are explained in the companion
document, the HIP architecture [17] specification.
There are three representations of the Host Identity, the full Host
Identifier (HI), the Host Identity Tag (HIT), and the Local Scope
Identifier (LSI). Three representations are used, as each meets a
different design goal of HIP, and none of them can be removed and
meet these goals. The HI represents directly the Identity. It is a
public key. Since there are different public key algorithms that can
be used with different key lengths, the HI is not good for using as a
packet identifier, or as a index into the various operational tables
needed to support HIP.
A hash of the HI, the Host Identity Tag (HIT), thus becomes the
operational representation. It is 128 bits long. It is used in the
HIP payloads, and it is intended be used to index the corresponding
state in the end hosts.
In many environments, 128 bits is still considered large. For
example, currently used IPv4 based applications are constrained with
32 bit API fields. Thus, the third representation, the 32 bit LSI,
is needed. The LSI provides a compression of the HIT with only a
local scope so that it can be carried efficiently in any application
level packet and used in API calls.
1.2 The HIP protocol
The base HIP exchange consists of four packets. The four-packet
design helps to make HIP DoS resilient. The protocol exchanges
Diffie-Hellman keys in the 2nd and 3rd packets, and authenticates the
parties in the 3rd and 4th packets. Additionally, it starts the
cookie exchange in the 2nd packet, completing it in the 3rd packet.
The exchange uses the Diffie-Hellman exchange to hide the Host
Identity of the Initiator in packet 3. The Responder's Host Identity
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is not protected. It should be noted, however, that both the
Initiator's and the Responder's HITs are transported as such (in
cleartext) in the packets, allowing an eavesdropper with a priori
knowledge about the parties to verify their identies.
Data packets start after the 4th packet. The 3rd and 4th HIP packets
may carry a data payload in the future. However, the details of this
are to be defined later as more implementation experience is gained.
Finally, HIP is designed as an end-to-end authentication and key
establishment protocol. It lacks much of the fine-grain policy
control found in IKE that allows IKE to support complex gateway
policies. Thus, HIP is not a complete replacement for IKE.
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2. 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 [3].
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3. Host Identifier (HI) and its representations
The structure of the Host Identifier (HI) is the public key of an
asymmetric key pair. Correspondingly, the host itself is entity that
holds the private key from the key pair. See the HIP architecture
specification [17] for more details about the difference between an
identity and the corresponding identifier.
DSA is the MUST implement public key algorithm for all HIP
implementations, other algorithms MAY be supported. DSA was chosen
as the default algorithm due to its small signature size.
A Host Identity Tag (HIT) is used in protocols to represent the Host
Identity. Another representation of the Host Identity, the Local
Scope Identifier (LSI), can also be used in protocols and APIs.
LSI's advantage over HIT is its size; its disadvantage is its local
scope.
3.1 Host Identity Tag (HIT)
The Host Identity Tag is a 128 bit value -- a hash of the Host
Identifier. There are two advantages of using a hash over the actual
Identity in protocols. Firstly, its fixed length makes for easier
protocol coding and also better manages the packet size cost of this
technology. Secondly, it presents a consistent format to the
protocol whatever underlying identity technology is used.
There are two types of HITs. HITs of the first type, called *type 1
HIT*, consist of an initial 2 bit prefix of 01, followed by 126 bits
of the SHA-1 hash of the public key. HITs of the second type consist
of an initial 2 bit prefix of 10, a Host Assigning Authority (HAA)
field, and only the last 64 bits come from a SHA-1 hash of the Host
Identity. This latter format for HIT is recommended for 'well known'
systems. It is possible to support a resolution mechanism for these
names in hierarchical directories, like the DNS. Another use of HAA
is in policy controls, see Section 12.
This document fully specifies only type 1 HITs. HITs that consists
of the HAA field and the hash are specified in [20].
Any conforming implementation MUST be able to deal with both types of
HITs. When handling other than type 1 HITs, the implementation is
RECOMMENDED to explicitly learn and record the binding between the
Host Identifier and the HIT, as it may not be able generate such HITs
from Host Identifiers.
3.1.1 Generating a HIT from a HI
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The 126 or 64 hash bits in a HIT MUST be generated by taking the
least significant 126 or 64 bits of the SHA-1 [18] hash of the Host
Identifier as it is represented in the Host Identity field in a HIP
payload packet.
For Identities that are DSA public keys, the HIT is formed as
follows:
1. The DSA public key is encoded as defined in RFC2536 [10] Section
2, taking the fields T, Q, P, G, and Y, concatenated. Thus, the
data to be hashed is 1 + 20 + 3 * 64 + 3 * 8 * T octets long,
where T is the size parameter as defined in RFC2536 [10]. The
size parameter T, affecting the field lengths, MUST be selected
as the minimum value that is long enough to accomodate P, G, and
Y. The fields MUST be encoded in network byte order, as defined
in RFC2536 [10].
2. A SHA-1 hash [18] is calculated over the encoded key.
3. The least significant 126 or 64 bits of the hash result are used
to create the HIT, as defined above.
The following pseudo-code illustrates the process. The symbol :=
denotes assignment; the symbol += denotes appending. The
pseudo-function encode_in_network_byte_order takes two parameters, an
integer (bignum) and a length in bytes, and returns the integer
encoded into a byte string of the given length.
buffer := encode_in_network_byte_order ( DSA.T , 1 )
buffer += encode_in_network_byte_order ( DSA.Q , 20 )
buffer += encode_in_network_byte_order ( DSA.P , 64 + 8 * T )
buffer += encode_in_network_byte_order ( DSA.G , 64 + 8 * T )
buffer += encode_in_network_byte_order ( DSA.Y , 64 + 8 * T )
digest := SHA-1 ( buffer )
hit_126 := concatenate ( 01 , low_order_bits ( digest, 126 ) )
hit_haa := concatenate ( 10 , HAA, low_order_bits ( digest, 64 ) )
3.2 Local Scope Identifier (LSI)
LSIs are 32-bit localized representations of a Host Identity. The
purpose of an LSI is to facilitate using Host Identities in existing
IPv4 based protocols and APIs. The owner of the Host Identity does
not set the LSI that other hosts use for it; each host selects the
LSIs that it uses for representing its partners.
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A *local LSI* is an LSI that a remote host has assigned to a host.
In some implementations, local LSIs may be assigned to some interface
as an IP address. A *remote LSI* is an LSI that the host has
assigned to represent a remote host (and that the remote host has
accepted).
The LSIs MUST be allocated from the 1.0.0.0/8 subnet. That makes it
easier to differentiate between LSIs and IPv4 addresses at the API
level. By default, the low order 24 bits of an LSI SHOULD be equal
with the low order 24 bits of the corresponding HIT. That allows
easier mapping between LSIs and HITs, and makes the LSI assigned to a
host to be a fixed one.
If a host is forming a remote LSI for a HIT whose low order 24 bits
are equal with another already existing remote LSI, the host MAY
select another LSI to represent that host. If the low order 24 bits
of a remote HIT are equal to the low order 24 bits of a local LSI,
the host MAY select a different LSI to represent the remote host. In
either case, the host SHOULD assign the low order 24 bits of the LSI
randomly. All hosts SHOULD be prepared to handle local LSIs whose
low order 24 bits do not match with any of their own HITs. Note that
any such mechanisms may be subject to implementation complications,
see Appendix A.
If the LSI assigned by a peer to represent a host is unacceptable,
the host MAY terminate the HIP four-way handshake and start anew.
It is possible that the HITs of two remote hosts have equal low order
24 bits. Since HITs are basically random, if a host is communicating
with 1000 other hosts, the risk of such collision is roughly 0.006%,
and for a host communicating with 10000 other hosts, the risk is
about 0.06%. However, given a population of 100000 hosts, each
communicating with 1000 other hosts, the probability that there were
no collisions at all is only about 2%. In other words, even though
collisions are fairly rare events for any given host, they will
happen, and there must be a way to handle them. However, this
specification does not currently specify any such way. A future
version of this specification is expected to include a definition;
see also the discussion in Appendix A.
3.3 Security Parameter Index (SPI)
SPIs are used in ESP to find the right security association for
received packets. The ESP SPIs have added significance when used
with HIP; they are a compressed representation of the HITs in every
packet. Thus, SPIs MAY be used by intermediary systems in providing
services like address mapping. Note that since the SPI has
significance at the receiver, only the < DST, SPI >, where DST is a
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destination IP address, uniquely identifies the receiver HIT at every
given point of time. The same SPI value may be used by several
hosts. A single < DST, SPI > value may denote different hosts at
different points of time, depending on which host is currently
reachable at the DST.
Each host selects for itself the SPI it wants to see in packets
received from its peer. This allows it to select different SPIs for
different peers. The SPI selection SHOULD be random; the rules of
Section 2.1 of the ESP specification [15] must be followed. A
different SPI SHOULD be used for each HIP exchange with a particular
host; this is to avoid a replay attack. Additionally, when a host
rekeys, the SPI MUST be changed. Furthermore, if a host changes over
to use a different IP address, it MAY change the SPI.
One method for SPI creation that meets these criteria would be to
concatenate the HIT with a 32 bit random or sequential number, hash
this (using SHA1), and then use the high order 32 bits as the SPI.
The selected SPI is communicated to the peer in the third (I2) and
fourth (R2) packets of the base HIP exchange. Changes in SPI are
signalled with NES packets.
3.4 Difference between an LSI and the SPI
There is a subtle difference between an LSI and a SPI.
The LSI is designed to be relatively long-lived. A system selects
the LSI it locally uses to represent its peer, and it SHOULD reuse a
previous LSI for a HIT during a subsequent HIP exchange. This could
be important in a timeout recovery situation. The LSI only appears
in the 3rd and 4th HIP packets. The LSI is used anywhere in system
processes where IP addresses have traditionally have been used, like
in TCBs, IPv4 API calls, and FTP PORT commands.
The SPI is short-lived. It changes with each HIP exchange and with a
HIP rekey. A system notifies its peer of the SPI to use in ESP
packets sent to it. Since the SPI is in all but the first two HIP
packets, it can be used in intermediary systems to assist in address
remapping.
3.5 TCP and UDP pseudo-header computation
When computing TCP and UDP checksums on sockets bound to HITs or
LSIs, the IPv6 pseudo-header format [8] is used. Additionally, the
HITs MUST be used in the place of the IPv6 addresses in the IPv6
pseudo-header. Note that the pseudo-header for actual HIP payloads
is computed differently; see Section 6.1.2.
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4. Host Identity Protocol
The Host Identity Protocol is IP protocol TBD. The HIP payload could
be carried in every datagram. However, since HIP datagrams are
relatively large (at least 40 bytes), and ESP already has all of the
functionality to maintain and protect state, the HIP payload is
'compressed' into an ESP payload after the HIP exchange. Thus in
practice, HIP packets only occur in datagrams to establish or change
HIP state.
For testing purposes, the protocol number 99 is currently used.
4.1 HIP base exchange
The base HIP exchange serves to manage the establishment of state
between an Initiator and a Responder. The Initiator first sends a
trigger packet, I1, to the Responder. The second packet, R1, starts
the actual exchange. It contains a puzzle, that is, a cryptographic
challenge that the Initiator must solve before continuing the
exchange. In its reply, I2, the Initiator must display the solution.
Without a solution the I2 message is simply discarded.
The last three packets of the exchange, R1, I2, and R2, constitute a
standard authenticated Diffie-Hellman key exchange. The base
exchange is illustrated below.
Initiator Responder
I1: trigger exchange
-------------------------->
select pre-computed R1
R1: puzzle, D-H, key, sig
<-------------------------
check sig remain stateless
solve puzzle
I2: solution, D-H, {key}, sig
-------------------------->
compute D-H check cookie
check puzzle
check sig
R2: sig
<--------------------------
check sig compute D-H
In this section we cover the overall design of the base exchange.
The details are the subject of the rest of this memo.
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4.1.1 HIP Cookie Mechanism
The purpose of the HIP cookie mechanism is to protect the Responder
from a number of denial-of-service threats. It allows the Responder
to delay state creation until receiving I2. Furthermore, the puzzle
included in the cookie allows the Responder to use a fairly cheap
calculation to check that the Initiator is "sincere" in the sense
that it has churned CPU cycles in solving the puzzle.
The Cookie mechanism has been explicitly designed to give space for
various implementation options. It allows a responder implementation
to completely delay session specific state creation until a valid I2
is received. In such a case a validly formatted I2 can be rejected
earliest only once the Responder has checked its validity by
computing one hash function. On the other hand, the design also
allows a responder implementation to keep state about received I1s,
and match the received I2s against the state, thereby allowing the
implementation to avoid the computational cost of the hash function.
The drawback of this latter approach is the requirement of creating
state. Finally, it also allows an implementation to use any
combination of the space-saving and computation-saving mechanisms.
One possible way for a Responder to remain stateless but drop most
spoofed I2s is to base the selection of the cookie on some function
over the Initiator's Host Identity. The idea is that the Responder
has a (perhaps varying) number of pre-calculated R1 packets, and it
selects one of these based on the information carried in I1. When
the Responder then later receives I2, it checks that the cookie in
the I2 matches with the cookie sent in the R1, thereby making it
impractical for the attacker to first exchange one I1/R1, and then
generate a large number of spoofed I2s that seemingly come from
different IP addresses or use different HITs. The method does not
protect from an attacker that uses fixed IP addresses and HITs,
though. Against such an attacker it is probably best to create a
piece of local state, and remember that the puzzle check has
previously failed. See Appendix D for one possible implementation.
Note, however, that the implementations MUST NOT use the exact
implementation given in the appendix, and SHOULD include sufficient
randomness to the algorithm so that algorithm complexity attacks
become impossible [22].
The Responder can set the difficulty for Initiator, based on its
concern of trust of the Initiator. The Responder SHOULD use
heuristics to determine when it is under a denial-of-service attack,
and set the difficulty value K appropriately.
The Responder starts the cookie exchange when it receives an I1. The
Responder supplies a random number I, and requires the Initiator to
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find a number J. To select a proper J, the Initiator must create the
concatenation of I, the HITs of the parties, and J, and take a SHA-1
hash over this concatenation. The lowest order K bits of the result
MUST be zeros.
To generate a proper number J, the Initiator will have to generate a
number of Js until one produces the hash target of zero. The
Initiator SHOULD give up after trying 2^(K+2) times, and start over
the exchange. (See Appendix C.) The Responder needs to re-create
the concatenation of I, the HITs, and the provided J, and compute the
hash once to prove that the Initiator did its assigned task.
To prevent pre-computation attacks, the Responder MUST select the
number I in such a way that the Initiator cannot guess it.
Furthermore, the construction MUST allow the Responder to verify that
the value was indeed selected by it and not by the Initiator. See
Appendix D for an example on how to implement this.
It is RECOMMENDED that the Responder generates a new cookie and a new
R1 once every few minutes. Furthermore, it is RECOMMENDED that the
Responder remembers an old cookie at least 60 seconds after it has
been deprecated. These time values allow a slower Initiator to solve
the cookie puzzle while limiting the usability that an old, solved
cookie has to an attacker.
NOTE: The protocol developers explicitly considered whether R1 should
include a timestamp in order to protect the Initiator from replay
attacks. The decision was NOT to include a timestamp.
In R1, the values I and K are sent in network byte order. Similarily,
in I2 the values I and J are sent in network byte order. The SHA-1
hash is created by concatenating, in network byte order, the
following data, in the following order:
64-bit random value I, in network byte order, as appearing in R1
and I2.
128-bit initiator HIT, in network byte order, as appearing in the
HIP Payload in R1 and I2.
128-bit responder HIT, in network byte order, as appearing in the
HIP Payload in R1 and I2.
64-bit random value J, in network byte order, as appearing in I2.
In order to be a valid response cookie, the K low-order bits of the
resulting SHA-1 digest must be zero.
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Notes:
The length of the data to be hashed is 48 bytes.
All the data in the hash input MUST be in network byte order.
The order of the initiator and responder HITs are different in the
R1 and I2 packets, see Section 6.1. Care must be taken to copy
the values in right order to the hash input.
Precomputation by the Responder Sets up the challenge difficulty K.
Generates a random number I.
Creates a signed R1 and caches it.
Responder Selects a suitable cached R1.
Sends I and K in a HIP Cookie in the R1.
Saves I and K for a Delta time.
Initiator Generates repeated attempts to solve the challenge until a
matching J is found:
Ltrunc( SHA-1( I | HIT-I | HIT-R | J ), K ) == 0
Sends I and J in HIP Cookie in a I2.
Responder Verifies that the received I is a saved one.
Finds the right K based on I.
Computes V := Ltrunc( SHA-1( I | HIT-I | HIT-R | J ), K )
Rejects if V != 0
Accept if V == 0
4.1.2 Authenticated Diffie-Hellman protocol
The packets R1, I2, and R2 implement a standard authenticated
Diffie-Hellman exchange. The Responder sents its public
Diffie-Hellman key and its public authentication key, i.e., its host
identity, in R1. The signature in R1 allows the Initiator to verify
that the R1 has been once generated by the Responder. However, since
it is precomputed and therefore does not cover all of the packet, it
does not protect from replay attacks.
When the Initiator receives an R1, it computes the Diffie-Hellman
session key. It creates a HIP security association using keying
material from the session key (see Section 9), and uses the security
association to encrypt its public authentication key, i.e., host
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identity. The resulting I2 contains the Initiator's Diffie-Hellman
key and its the encrypted public authentication key. The signature
in I2 covers all of the packet.
The Responder extracts the Initiator Diffie-Hellman public key from
the I2, computes the Diffie-Hellman session key, creates a
corresponding HIP security association, and decrypts the Initiator's
public authentication key. It can then verify the signature using
the authentication key.
The final message, R2, is needed to protect the Initiator from replay
attacks.
4.1.3 HIP Birthday
The HIP Birthday is a reboot count used to manage state
re-establishment when one peer rebooted or timed out its SA. The
Birthday is increased every time the system boots. The Birthday also
has to be increased in accordance with the system's SA timeout
parameter. If the system has open SAs, it MUST increase its
Birthday. This impacts a system's approach to precomputing R1
packets.
Birthday SHOULD be a counter. It MUST NOT be reset by the user and a
system is unlikely to need a birthday larger than 2^64. Date-time in
GMT MAY be used if a cross-boot counter is not possible, but it has a
potential problem if the system time is set back by the user.
4.2 Sending data on HIP packets
A future version of this document may define how to include ESP
protected data on various HIP packets. However, currently the HIP
header is a terminal header, and not followed by any other headers.
The OPTIONAL PAYLOAD packet (see Section 7.8) MAY be used to transfer
data.
4.3 Rekey
HIP includes a simple rekey mechanism, allowing the hosts to
introduce new keying material at any time by introducing a new
Diffie-Hellman public key; see Section 7.5. All conforming HIP
implementations MUST support rekeying.
4.4 Bootstrap support
This memo defines an OPTIONAL HIP based bootstrap mechanism, intended
for ad hoc like environments; see Section 7.6. There is little
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operational experience of the usability of this mechanism, and it may
be dropped or completely revised in some future protocol version.
4.5 Certificate distribution
HIP does not define how to use certificates. However, it does define
a simple certificate transport mechanisms that MAY be used to
implement certificate based security policies. The certificate
payload is defined in Section 6.2.10, and the certificate packet in
Section 7.7.
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5. HIP packet flow and state machine
A typical HIP packet flow is shown below.
I --> Directory: lookup of R
I <-- Directory: return R's addresses, and HI and/or HIT
I1 I --> R (Hi. Here is my I1, let's talk HIP)
R1 I <-- R (OK. Here is my R1, handle this HIP cookie)
I2 I --> R (Compute, compute, here is my counter I2)
R2 I <-- R (OK. Let's finish HIP with my R2)
I --> R (ESP protected data)
I <-- R (ESP protected data)
5.1 HIP Scenarios
The HIP protocol and state machine is designed to recover from one of
the parties crashing and losing its state. The following scenarios
describe the main use cases covered by the design.
No prior state between the two systems.
The system with data to send is the Initiator. The process
follows standard 4 packet base exchange, establishing the SAs.
The system with data to send has no state with the receiver, but
the receiver has a residual SA.
Initiator acts as in no prior state, sending I1 and getting R1.
When the Receiver gets an I2, the old SAs are 'discovered' and
deleted; the new SAs are established.
The system with data to send has an SA, but the receiver does not.
The receiver 'detects' the situation when it receives an ESP
packet that contains an unknown SPI. The receiver sends an R1
with a NULL initiator HIT. The sender gets the R1 with a later
birthdate, discards old SA, and continues the base exchange to
establish new SAs for sending data.
A peer determines that it needs to reset Sequence number or rekey.
It sends NES. Receiver sends NES response, establishes new SAs
for peers.
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5.2 Refusing a HIP exchange
A HIP aware host may choose not to accept a HIP exchange. If the
host's policy is to only be an initiator, it should begin its own HIP
exchange. A host MAY choose to have such a policy since only the
initiator HI is protected in the exchange. There is a risk of a race
condition if each host's policy is to only be an Initiator, at which
point the HIP exchange will fail.
If the host's policy does not permit it to enter into a HIP exchange
with the Initiator, it should send an ICMP 'Destination Unreachable,
Administratively Prohibited' message. A more complex HIP packet is
not used here as it actually opens up more potential DoS attacks than
a simple ICMP message.
5.3 Reboot and SA timeout restart of HIP
Simulating a loss of state is a potential DoS attack. The following
process has been crafted to manage state recovery without presenting
a DoS opportunity.
If a host reboots or times out, it has lost its HIP state. If the
system that lost state has a datagram to deliver to its peer, it
simply restarts the HIP exchange. The peer sends an R1 HIP packet,
but does not reset its state until it receives the I2 HIP packet.
The I2 packet MUST have a Birthday greater than the current SA's
Birthday. This is to handle DoS attacks that simulate a reboot of a
peer. Note that either the original Initiator or the Responder could
end up restarting the exchange, becoming the new Initiator.
If a system receives an ESP packet for an unknown SPI, the assumption
is that it has lost the state and its peer did not. In this case, the
system treats the ESP packet like an I1 packet and sends an R1
packet. The initiator HIT is typically NULL in the R1, since the
system usually does not know the peer's HIT any more.
The system receiving the R1 packet first checks to see if it has an
established and recently used SA with the party sending the R1. If
such an SA exists, the system checks the Birthday, and if the
Birthday is greater than the current SA's Birthday, it processes the
R1 packet, optionally queuing the payload packet(s) to be resent
later. The peer system processes the I2 in the normal manner, and
replies with an R2. This will re-establish state between the two
peers. Note that the process will result in new ESP SAs being used,
and therefore simply resending ESP packets is not sufficient.
Note that there is a potential DoS attack. If an attacker can
simulate a situation where a large number of peers apparently loose
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their state at the same time, and all send R1 packet at once to a
server, the server will choke on trying to solve all the puzzles at
the same time. However, such an attack would require that the
attacker has specific knowledge about the SAs being used, and an
ability to trigger R1s as the SAs are used.
5.4 HIP State Machine
The HIP protocol itself has very little state. In the HIP base
exchange, there is an Initiator and a Responder. Once the SAs are
established, this distinction is lost. If the HIP state needs to be
re-established, the controlling parameters are which peer still has
state and which has a datagram to send to its peer. The following
state machine attempts to capture these processes.
The state machine is presented in a single system view, representing
either an Initiator or a Responder. There is not a complete overlap
of processing logic here and in the packet definitions. Both are
needed to completely implement HIP.
Implementors must understand that the state machine, as described
here, is informational. Specific implementations are free to
implement the actual functions differently.
5.4.1 HIP States
E0 State machine start
E1 Initiating HIP
E2 Waiting to finish HIP
E3 HIP SA established
E4 HIP SA established, rekeying
E-FAILED HIP SA establishment failed
5.4.2 HIP State Processes
+---------+
| E0 | Start state
+---------+
Datagram to send, send I1 and go to E1
Receive I1, send R1 and stay at E0
Receive I2, process
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if successful, send R2 and go to E3
if fail, stay at E0
Receive ESP for unknown SA, send R1 and stay at E0
Receive ANYOTHER, drop and stay at E0
+---------+
| E1 | Initiating HIP
+---------+
Receive I1, send R1 and stay at E1
Receive I2, process
if successful, send R2 and go to E3
if fail, stay at E1
Receive R1, process
if successful, send I2 and go to E2
if fail, go to E-FAILED
Receive ANYOTHER, drop and stay at E1
Timeout, increment timeout counter
If counter is less than I1_RETRIES_MAX, send I1 and stay at E1
If counter is greater than I1_RETRIES_MAX, go to E-FAILED
+---------+
| E2 | Waiting to finish HIP
+---------+
Receive I1, send R1 and stay at E2
Receive I2, process
if successful, send R2 and go to E3
if fail, stay at E2
Receive R2, process
if successful, go to E3
if fail, go to E-FAILED
Receive ANYOTHER, drop and stay at E2
Timeout, increment timeout counter
If counter is less than I2_RETRIES_MAX, send I2 and stay at E2
If counter is greater than I2_RETRIES_MAX, go to E-FAILED
+---------+
| E3 | HIP SA established
+---------+
Receive I1, send R1 and stay at E3
Receive I2, process with Birthday check
if successful, send R2, drop old SA and cycle at E3
if fail, stay at E3
Receive R1, process with SA and Birthday check
if successful, send I2, prepare to drop old SA and cycle at E3
if fail, stay at E3
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Receive R2, drop and stay at E3
Receive ESP for SA, process and stay at E3
Receive NES, process
if successful, send NES and stay at E3
if failed, stay at E3
Need rekey,
send NES and go to E4
+---------+
| E4 | HIP SA established, rekey pending
+---------+
Receive I1, send R1 and stay at E4
Receive I2, process with Birthday check
if successful, send R2, drop old SA and go to E3
if fail, stay at E4
Receive R1, process with SA and Birthday check
if successful, send I2, prepare to drop old SA and go to E3
if fail, stay at E4
Receive R2, drop and stay at E4
Receive ESP for SA, process and stay at E4
Receive NES, process
if successful, replace SAs and go to E3
if failed, stay at E4
Timeout, increment timeout counter
If counter is less than NES_RETRIES_MAX, send NES and stay at E4
If counter is greater than NES_RETRIES_MAX, go to E-FAILED
5.4.3 Simplified HIP State Diagram
Receive packets cause a move to a new state.
+---------+
| E0 |>---+
+---------+ |
| ^ | |
| | | Dgm to |
+-+ | send |
I1 | | (note: ESP- means ESP with unknown SPI)
ESP- | |
v |
+---------+ |
| E1 |>---|----------+
+---------+ | |
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| | |
| R1 | |
| |I2 |I2
v | |
+---------+ | |
| E2 |>---|----------|-----+
| | | | |
+---------+ | | |
| | | |
| R2 | | |I2
| | | |
v | | |
+---------+<---+ | |
| | | |
| E3 |<--------------+ |
| |<--------------------+
| |<-------+
| |----+ |
+---------+ | |
| ^ ^ | |
| | | | |
+--+ | | |
ESP, | rekey| |R1,I2
NES, | | |
I1, |NES | |
I2, | | |
R1 | | |
| | |
+---------+ | |
| |<---+ |
| E4 |>-------+
+---------+
| ^
| |
+--+
ESP,
I1,
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6. Packet formats
6.1 Payload format
All HIP packets start with a fixed header.
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 Header | Payload Len | Type | VER. | RES. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Controls | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender's Host Identity Tag (HIT) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver's Host Identity Tag (HIT) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ HIP Parameters /
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The HIP header is logically an IPv6 extension header. However, this
document does not describe processing for Next Header values other
than decimal 59, IPPROTO_NONE, the IPV6 no next header value. Future
documents MAY do so. However, implementations MUST ignore trailing
data if a Next Header value is received that is not implemented.
The Header Length field contains the length of the HIP Header and the
length of HIP parameters in 8 bytes units, excluding the first 8
bytes. Since all HIP headers MUST contain the sender's and
receiver's HIT fields, the minimum value for this field is 4, and
conversely, the maximum length of the HIP Parameters field is
(255*8)-32 = 2008 bytes.
The Packet Type indicates the HIP packet type. The individual packet
types are defined in the relevant sections. If a HIP host receives a
HIP packet that contains an unknown packet type, it MUST drop the
packet.
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The HIP Version is four bits. The current version is 1. The version
number is expected to be incremented only if there are incompatible
changes to the protocol. Most extensions can be handled by defining
new packet types, new parameter types, or new controls.
The following four bits are reserved for future use. They MUST be
zero when sent, and they SHOULD be ignored when handling a received
packet.
The HIT fields are always 128 bits (16 bytes) long.
6.1.1 HIP Controls
The HIP control section transfers information about the structure of
the packet and capabilities of the host.
The following fields have been defined:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | | | | | | | | | | | |C|E|A|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
C - Certificate One or more certificate packets (CER) follows this
HIP packet (see Section 7.7).
E - ESP sequence numbers The ESP transform requires 64-bit sequence
numbers. See Section 11.4 for processing this control.
A - Anonymous If this is set, the sender's HI in this packet is
anonymous, i.e., one not listed in a directory. Anonymous HIs
SHOULD NOT be stored. This control is set in packets R1 and/or
I2. The peer receiving an anonymous HI may choose to refuse it by
silently dropping the exchange.
The rest of the fields are reserved for future use and MUST be set to
zero on sent packets and ignored on received packets.
6.1.2 Checksum
The checksum field is located at the same location within the header
as the checksum field in UDP packets, enabling hardware assisted
checksum generation and verification. Note that since the checksum
covers the source and destination addresses in the IP header, it must
be recomputed on HIP based NAT boxes.
If IPv6 is used to carry the HIP packet, the pseudo-header [8]
contains the source and destination IPv6 addresses, HIP packet length
in the pseudo-header length field, a zero field, and the HIP protocol
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number (TBD) in the Next Header field. The length field is in bytes
and can be calculated from the HIP header length field: (HIP Header
Length + 1) * 8.
In case of using IPv4, the IPv4 UDP pseudo header format [1] is used.
In the pseudo header, the source and destination addresses are those
used in the IP header, the zero field is obviously zero, the protocol
is the HIP protocol number (TBD), and the length is calculated as in
the IPv6 case.
6.2 HIP parameters
The HIP Parameters are used to carry the public key associated with
the sender's HIT, together with other related security information.
The HIP Parameters consists of ordered parameters, encoded in TLV
format.
The following parameter types are currently defined.
TLV Type Length Data
SPI_LSI 0 16 Remote's SPI, Remote's LSI.
BIRTHDAY_COOKIE 2/4 32 System Boot Counter plus
two 64-bit fields:
- Random #I
- K or random #J
DIFFIE_HELLMAN 6 variable public key
NES_INFO 10 Old SPI, New SPI and other
info needed for NES
HIP_TRANSFORM 16 variable HIP Encryption and Integrity
Transform
ESP_TRANSFORM 18 variable ESP Encryption and
Authentication Transform
ENCRYPTED 20 variable Encrypted part of I2 or CER
packets
HOST_ID 32 variable Host Identity
HOST_ID_FQDN 34 variable Host Identity with Fully
Qualified Domain Name
CERT 64 variable HI certificate
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HMAC 65500 24 HMAC based message
authentication code, with
key material from HIP_TRANSFORM
HIP_SIGNATURE2 65532 variable Signature of the R1 packet
HIP_SIGNATURE 65534 variable Signature of the packet
6.2.1 TLV format
The TLV encoded parameters are described in the following
subsections. The type-field value also describes the order of these
fields in the packet. The parameters MUST be included into the
packet so that the types form an increasing order. If the order does
not follow this rule, the packet is considered to be malformed and it
MUST be discarded.
All the TLV parameters have a length which is a multiple of 8 bytes.
When needed, padding MUST be added to the end of the parameter so
that the total length becomes a multiple of 8 bytes. This rule
ensures proper alignment of data. If padding is added, the Length
field MUST NOT include the padding. Any added padding bytes MUST be
set zero by the sender, but their content SHOULD NOT be checked on
the receiving end.
Consequently, the Length field indicates the length of the Contents
field (in bytes). The total length of the TLV parameter (including
Type, Length, Contents, and Padding) is related to the Length field
according to the following formula:
Total Length = 11 + Length - (Length + 3) % 8;
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 |C| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Contents /
/ +-+-+-+-+-+-+-+-+
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Type code for the parameter
C Critical. Zero if this parameter is critical, and
MUST be recognized by the recipient, one otherwise.
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The C bit is considered to be a part of the Type field.
Consequently, critical parameters are always even
and non-critical ones have an odd value.
Length Length of the parameter, in bytes.
Contents Parameter specific, defined by Type
Padding Padding, 0-7 bytes, added if needed
Critical parameters MUST be recognized by the recipient. If a
recipient encounters a critical parameter that it does not recognize,
it MUST NOT process the packet any further.
Non-critical parameters MAY be safely ignored. If a recipient
encounters a non-critical parameter that it does not recognize, it
SHOULD proceed as if the parameter was not present in the received
packet.
6.2.2 Defining new parameters
Future specifications may define new parameters as needed. When
defining new parameters, care must be taken to ensure that the
parameter type values are appropriate and leave suitable space for
other future extensions. One must remember that the parameters MUST
always be arranged in the increasing order by the type code, thereby
limiting the order of parameters.
The following rules must be followed when defining new parameters.
1. The low order bit C of the Type code is used to distinguish
between critical and non-critical parameters.
2. A new parameter may be critical only if an old recipient ignoring
it would cause security problems. In general, new parameters
SHOULD be defined as non-critical, and expect a reply from the
recipient.
3. If a system implements a new critical parameter, it MUST provide
the ability to configure the associated feature off, such that
the critical parameter is not sent at all. The configuration
option must be well documented. By default, sending of such a
new critical parameter SHOULD be off. In other words, the
management interface MUST allow vanilla standards only mode as a
default configuration setting, and MAY allow new critical
payloads to be configured on (and off).
4. The following type codes are reserved for future base protocol
extensions, and may be assigned only through an appropriate WG or
RFC action.
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0 - 511
65024 - 65535
5. The following type codes are reserved for experimentation and
private use. Types SHOULD be selected in a random fashion from
this range, thereby reducing the probability of collisions. A
method employing genuine randomness (such as flipping a coin)
SHOULD be used.
32768 - 49141
6. All other parameter type codes MUST be registed by the IANA. See
Section 14.
6.2.3 SPI_LSI
The SPI_LSI parameter contains the SPI that the receiving host must
use when sending data to the sending host, and the LSI that the
receiving host must to represent itself when talkin to the sending
host.
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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 0
Length 12
Reserved Zero when sent, ignored when received
SPI Security Parameter Index
LSI Local Scope Identifier
6.2.4 BIRTHDAY_COOKIE
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 | Length |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Birthday, 8 bytes |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Random # I, 8 bytes |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| K or Random # J, 8 bytes |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 2 (in R1) or 4 (in I2)
Length 28
Reserved Zero when sent, ignored when received
Birthday System boot counter
Random # I random number
K K is the number of verified bits (in R1 packet)
Random # J random number (in I2 packet)
Birthday, Random #I, K, and Random #J are represented as 64-bit
integers, in network byte order.
6.2.5 DIFFIE_HELLMAN
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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group ID | Public Value /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 6
Length length in octets, excluding Type, Length, and padding
Group ID defines values for p and g
Public Value the sender's public Diffie-Hellman key
The following Group IDs have been defined:
Group Value
Reserved 0
OAKLEY well known group 1 1
OAKLEY well known group 2 2
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1536-bit MODP group 3
2048-bit MODP group 4
3072-bit MODP group 5
4096-bit MODP group 6
6144-bit MODP group 7
8192-bit MODP group 8
The MODP Diffie-Hellman groups are defined in [14]. The OAKLEY
groups are defined in [6]. The OAKLEY well known group 5 is the same
as the 1536-bit MODP group.
6.2.6 HIP_TRANSFORM
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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transform-ID #1 | Transform-ID #2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transform-ID #n | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 16
Length length in octets, excluding Type, Length, and padding
Transform-ID Defines the HIP Suite to be used
The Suite-IDs are identical to those defined in Section 6.2.7.
There MUST NOT be more than six (6) HIP Suite-IDs in one HIP
transform TLV. The limited number of transforms sets the maximum size
of HIP_TRANSFORM TLV. The HIP_TRANSFORM TLV MUST contain at least one
of the mandatory Suide-IDs.
Mandatory implementations: ENCR-3DES-CBC with HMAC-SHA1 and ENCR-NULL
with HMAC-SHA1.
6.2.7 ESP_TRANSFORM
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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Suite-ID #1 | Suite-ID #2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Suite-ID #n | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Type 18
Length length in octets, excluding Type, Length, and padding
Suite-ID Defines the ESP Suite to be used
The following Suite-IDs are defined ([16],[19]):
Suite-ID Value
RESERVED 0
ESP-AES-CBC with HMAC-SHA1 1
ESP-3DES-CBC with HMAC-SHA1 2
ESP-3DES-CBC with HMAC-MD5 3
ESP-BLOWFISH-CBC with HMAC-SHA1 4
ESP-NULL with HMAC-SHA1 5
ESP-NULL with HMAC-MD5 6
There MUST NOT be more than six (6) ESP Suite-IDs in one
ESP_TRANSFORM TLV. The limited number of Suite-IDs sets the maximum
size of ESP_TRANSFORM TLV. The ESP_TRANSFORM MUST contain at least
one of the mandatory Suite-IDs.
Mandatory implementations: ESP-3DES-CBC with HMAC-SHA1 and ESP-NULL
with HMAC-SHA1.
6.2.8 HOST_ID
When the host sends a Host Identity to a peer, it MAY send the
identity without any verification information or use certificates to
proof the HI. If certificates are sent, they are sent in a separate
HIP packet (CER).
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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Host Identity /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 32
Length length in octets, excluding Type, Length, and padding
Host Identity actual host identity
The Host Identity is represented in RFC2535 [9] format. The
algorithms used in RDATA format are the following:
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Algorithms Values
RESERVED 0
DSA 3 [RFC2536] (REQUIRED)
RSA 5 [RFC3110] (OPTIONAL)
6.2.9 HOST_ID_FQDN
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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HI Length | FQDN Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Host Identity /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | FDQN /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 34
Length length in octets, excluding Type, Length, and padding
HI length length of the Host Identity
FQDN length length of the FQDN
Host Identity actual host identity
FQDN Fully Qualified Domain Name, in the binary format.
The Host Identity is represented in RFC2535 [9] format; see above.
The format for the FQDN is defined in RFC1035 [2] Section 3.1.
If there is no FQDN, the HOST_ID TLV is sent instead.
6.2.10 CERT
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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cert count | Cert ID | Cert type | /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Certificate /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | Padding |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 64
Length length in octets, excluding Type, Length, and padding
Cert count total count of certificates that are sent, possibly
in several consequtive CER packets
Cert ID the order number for this certificate
Cert Type describes the type of the certificate
The receiver must know the total number (Cert count) of certificates
that it will receive from the sender, related to the R1 or I2. The
Cert ID identifies the particular certificate and its order in the
certificate chain. The numbering in Cert ID MUST go from 1 to Cert
count.
The following certificate types are defined:
Cert format Type number
X.509 v3 1
The encoding format for X.509v3 certificate is defined in [11].
6.2.11 HMAC
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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| HMAC |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 65500
Length 20
HMAC
160 low order bits of the HMAC computed over the HIP
packet, excluding the HMAC parameter and any
following HIP_SIGNATURE or HIP_SIGNATURE2
parameters. The checksum field MUST be set to zero
and the HIP header length in the HIP common header
MUST be calculated not to cover any excluded
parameters when the HMAC is calculated.
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HMAC calculation and verification process:
Packet sender:
1. Create the HIP packet, without the HMAC and possibly following
the HIP_SIGNATURE or HIP_SIGNATURE2 TLVs.
2. Calculate the length field in the HIP header.
3. Compute the HMAC.
4. Add the HMAC TLV to the packet.
5. Recalculate the length field in the HIP header.
Packet receiver:
1. Verify the HIP header length field.
2. Remove the HIP TLV, and if the packet contains any HIP_SIGNATURE
or HIP_SIGNATURE2 fields, remove them too, saving the contents if
they will be needed later.
3. Recalculate the HIP packet length in the HIP header and clear the
checksum field (set it to all zeros).
4. Compute the HMAC and verify it against the received HMAC.
6.2.12 HIP_SIGNATURE
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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SIG alg | Signature /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 65534 (2^16-2)
Length length in octets, excluding Type, Length, and padding
SIG alg Signature algorithm
Signature the signature is calculated over the HIP packet,
excluding the HIP_SIGNATURE TLV field, but including
the HMAC field, if present. The checksum field MUST
be set to zero and the HIP header length in the HIP
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common header MUST be calculated to the beginning of
the HIP_SIGNATURE TLV when the signature is
calculated.
Signature calculation and verification process:
Packet sender:
1. Create the HIP packet without the HIP_SIGNATURE TLV.
2. Calculate the length field in the HIP header.
3. Compute the signature.
4. Add the HIP_SIGNATURE TLV to the packet.
5. Recalculate the length field in the HIP header.
Packet receiver:
1. Verify the HIP header length field.
2. Save the contents of the HIP_SIGNATURE TLV and remove it from the
packet.
3. Recalculate the HIP packet length in the HIP header and clear the
checksum field (set it to all zeros).
4. Compute the signature and verify it against the received
signature.
The signature algorithms are defined in Section 6.2.8. The signature
in the Signature field is encoded using the proper method depending
on the signature algorithm (e.g. in case of DSA, according to [10]).
The verification can use either the HI received from a HIP packet,
the HI from a DNS query, if the FQDN has been received either in the
HOST_ID_FQDN or in the CER packet, or one received by some other
means.
6.2.13 HIP_SIGNATURE_2
The TLV structure is the same as in Section 6.2.12. The fields are:
Type 65532 (2^16-4)
Length length in octets, excluding Type, Length, and padding
SIG alg Signature algorithm
Signature the signature is calculated over the R1 packet,
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excluding the HIP_SIGNATURE_2 TLV field, but
including the HMAC field, if present. Initiator's
HIT and Checksum field MUST be set to zero and the
HIP packet length in the HIP header MUST be
calculated to the beginning of the HIP_SIGNATURE_2
TLV when the signature is calculated.
Zeroing the Initiator's HIT makes it possible to create R1 packets
beforehand to minimize the effects of possible DoS attacks.
Signature calculation and verification process follows the process in
Section 6.2.12. The only difference is that instead of the the
HIP_SIGNATURE TLV the HIP_SIGNATURE_2 TLV is used, and the
Initiator's HIT is cleared (set to all zeros) before computing the
signature.
6.2.14 NES_INFO
The NES payload is used to reset Security Associations. It introduces
a new SPI to be used when sending data to the sender of the NES
packet. The keys for the new Security Association will be drawn from
KEYMAT. If the packet contains a Diffie-Hellman parameter, the
KEYMAT is first recomputed before drawing the new keys.
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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Keymat Index | NES ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Old SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| New SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 10
Length 12
R One if the NES is a reply to another NES,
otherwise zero.
Keymat Index Index, in bytes, where to continue to draw ESP keys
from KEYMAT. If the packet includes a new
Diffie-Hellman key, the field MUST be zero. Note
that the length of this field limits the amount of
keying material that can be drawn from KEYMAT. If
that amount is exceeded, the NES packet MUST contain
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a new Diffie-Hellman key.
NES ID Packet identifier. Used to tie NES packets
into pairs. Initialized to zero and incremented
for each NES.
Old SPI Old SPI for data sent to the source address of
this packet
New SPI New SPI for data sent to the source address of
this packet
Note that the NES_INFO used to include the SPI used in reverse
direction, too. However, since NES packets are now always sent in
pairs, that is not needed any more. Any middleboxes between the
communicating hosts will learn the reverse mappings from the NES
reply.
6.2.15 ENCRYPTED
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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IV /
/ /
/ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
/ Encrypted data /
/ /
/ +-------------------------------+
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 20
Length length in octets, excluding Type, Length, and padding
Reserved zero when sent, ignored when received
IV Initialization vector, if needed, otherwise nonexisting.
The length of the IV is inferred from the HIP transform.
Encrypted The data is encrypted using an encryption algorithm as
data defined in HIP transform.
Padding Any Padding, if necessary, to make the TLV a multiple
of 8 bytes.
The encrypted data is in TLV format itself. Consequently, the first
fields in the contents are Type and Length, allowing the contents to
be easily parsed after decryption.
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If the encryption algorithm requires the length of the data to be
encrypted to be a multiple of the cipher algorithm block size,
thereby necessitating padding, and if the encryption algorithm does
not specify the padding contents, then an implementation MUST append
the TLV parameter that is to be encrypted with an additional padding,
so that the length of the resulting cleartext is a multiple of the
cipher block size length. Such a padding MUST be constructed as
specified in [15] Section 2.4. On the other hand, if the data to be
encrypted is already a multiple of the block size, or if the
encryption algorithm does specify padding as per [15] Section 2.4,
then such additional padding SHOULD NOT be added.
The Length field in the inside, to be encrypted TLV does not include
the padding. The Length field in the outside ENCRYPTED TLV is the
length of the data after encryption (including the Reserved field,
the IV field, and the output from the encryption process specified
for that suite, but not any additional external padding). Note that
the length of the cipher suite output may be smaller or larger than
the length of the data to be encrypted, since the encryption process
may compress the data or add additional padding to the data.
The ENCRYPTED payload may contain additional external padding, if the
result of encryption, the TLV header and the IV is not a multiple of
8 bytes. The contents of this external padding MUST follow the rules
given in Section Section 6.2.1.
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7. HIP Packets
There are eight basic HIP packets. Four are for the base HIP
exchange, one is for rekeying, one is a broadcast for use when there
is no IP addressing (e.g., before DHCP exchange), one is used to send
certificates, and one is for sending unencrypted data.
Packets consist of the fixed header as described in Section 6.1,
followed by the parameters. The parameter part, in turn, consists of
zero or more TLV coded parameters.
In addition to the base packets, other packets types will be defined
later in separate specifications. For example, support for mobility
and multi-homing is not included in this specification.
Packet representation uses the following operations:
() parameter
x{y} operation x on content y
<x>i x exists i times
[] optional parameter
x | y x or y
In the future, an OPTIONAL upper layer payload MAY follow the HIP
header. The payload proto field in the header indicates if there is
additional data following the HIP header. The HIP packet, however,
MUST NOT be fragmented. This limits the size of the possible
additional data in the packet.
7.1 I1 - the HIP initiator packet
The HIP header values for the I1 packet:
Header:
Packet Type = 1
SRC HIT = Initiator's HIT
DST HIT = Responder's HIT, or NULL
IP(HIP())
The I1 packet contains only the fixed HIP header.
Valid control bits: None
The Initiator gets the Responder's HIT either from a DNS lookup of
the Responder's FQDN, from some other repository, or from a local
table. If the Initiator does not know the Responder's HIT, it may
attempt opportunistic mode by using NULL (all zeros) as the
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Responder's HIT.
Since this packet is so easy to spoof even if it were signed, no
attempt is made to add to its generation or processing cost.
Implementation MUST be able to handle a storm of received I1 packets,
discarding those with common content that arrive within a small time
delta.
7.2 R1 - the HIP responder packet
The HIP header values for the R1 packet:
Header:
Packet Type = 2
SRC HIT = Responder's HIT
DST HIT = Initiator's HIT
IP ( HIP ( BIRTHDAY_COOKIE,
DIFFIE_HELLMAN,
HIP_TRANSFORM,
ESP_TRANSFORM,
( HOST_ID | HOST_ID_FQDN ),
HIP_SIGNATURE_2 ) )
Valid control bits: C, A
The R1 packet may be followed by one or more CER packets. In this
case, the C-bit in the control field MUST be set.
If the responder HI is an anonymous one, the A control MUST be set.
The initiator HIT MUST match the one received in I1. If the R1 is a
response to an ESP packet with an unknown SPI, the Initiator HIT
SHOULD be zero. If the Responder has multiple HIs, the responder HIT
used MUST match Initiator's request. If the Initiator used
opportunistic mode, the Responder may select freely among its HIs.
The Birthday is a reboot count used to manage state re-establishment
when one peer rebooted or timed out its SA.
The Cookie contains a random # I and the difficulty K. The
difficulty K is the number of bits that the Initiator must get zero
in the puzzle.
The Diffie-Hellman value is ephemeral, but can be reused over a
number of connections. In fact, as a defense against I1 storms, an
implementation MAY use the same Diffie-Hellman value for a period of
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time, for example, 15 minutes. By using a small number of different
Cookies for a given Diffie-Hellman value, the R1 packets can be
pre-computed and delivered as quickly as I1 packets arrive. A
scavenger process should clean up unused DHs and Cookies.
The HIP_TRANSFORM contains the encryption and integrity algorithms
supported by the Responder to protect the HI exchange, in the order
of preference. All implementations MUST support the 3DES [7] with
HMAC-SHA-1-96 [4].
The ESP_TRANSFORM contains the ESP modes supported by the Responder,
in the order of preference. All implementations MUST support 3DES
[7] with HMAC-SHA-1-96 [4].
The signature is calculated over the whole HIP envelope, after
setting the initiator HIT and header checksum temporarily to zero.
This allows the Responder to use precomputed R1s. The Initiator
SHOULD validate this signature. It SHOULD check that the responder
HI received matches with the one expected, if any.
7.3 I2 - the second HIP initiator packet
The HIP header values for the I2 packet:
Header:
Type = 3
SRC HIT = Initiator's HIT
DST HIT = Responder's HIT
IP ( HIP ( SPI_LSI,
BIRTHDAY_COOKIE,
DIFFIE_HELLMAN,
HIP_TRANSFORM,
ESP_TRANSFORM,
ENCRYPTED { HOST_ID | HOST_ID_FQDN },
HIP_SIGNATURE ) )
Valid control bits: C, E, A
The HITs used MUST match the ones used previously.
If the initiator HI is an anonymous one, the A control MUST be set.
The Birthday is a reboot count used to manage state re-establishment
when one peer rebooted or timed out its SA.
The Cookie contains the random # I from R1 and the computed # J. The
low order K bits of the SHA-1(I | ... | J) MUST be zero.
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The Diffie-Hellman value is ephemeral. If precomputed, a scavenger
process should clean up unused DHs.
The HIP_TRANSFORM contains the encryption and integrity used to
protect the HI exchange selected by the Initiator. All
implementations MUST support the 3DES transform [7].
The Initiator's HI is encrypted using the HIP_TRANSFORM encryption
algorithm. The keying material is derived from the Diffie-Hellman
exchanged as defined in Section 9.
The ESP_TRANSFORM contains the ESP mode selected by the Initiator.
All implementations MUST support 3DES [7] with HMAC-SHA-1-96 [4].
The signature is calculated over whole HIP envelope. The Responder
MUST validate this signature. It MAY use either the HI in the packet
or the HI acquired by some other means.
7.4 R2 - the second HIP responder packet
The HIP header values for the R2 packet:
Header:
Packet Type = 4
SRC HIT = Responder's HIT
DST HIT = Initiator's HIT
IP ( HIP ( SPI_LSI, HMAC, HIP_SIGNATURE ) )
Valid control bits: E
The HMAC and signature are calculated over whole HIP envelope. The
Initiator MUST validate both the HMAC and the signature.
7.5 NES - the HIP New SPI Packet
The NES packet is MANDATORY.
The NES serves three functions. Firstly, it provides the peer system
with a new SPI to use when sending packets. Secondly, it optionally
provides a new Diffie-Hellman key to produce new keying material.
Thirdly, it provides any intermediate system with the mapping of the
old SPI to the new one. This is important to systems like NATs [21]
that use SPIs to maintain address translation state.
The NES packet is a HIP packet with NES_INFO and optional
DIFFIE_HELLMAN and in the HIP payload. The NES_INFO parameter
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contains the old and new SPI values. It also contains a NES ID and
HMAC to provide DoS and replay protection. Each system must have a
NES ID counter, initialized to zero and incremented on each NES.
The HIP header values for the NES packet:
Header:
Packet Type = 5
SRC HIT = Sender's HIT
DST HIT = Recipients's HIT
IP ( HIP ( [ DIFFIE_HELLMAN, ] NES_INFO , HMAC, HIP_SIGNATURE ) )
Valid control bits: None
During the life of an SA established by HIP, one of the hosts may
need to reset the Sequence Number to one (to prevent wrapping) and
rekey. The reason for rekeying might be an approaching sequence
number wrap in ESP, or a local policy on use of a key. Rekeying ends
the current SAs and starts new ones on both peers.
NES packets are always used in pairs, one in both directions, with
identical NES IDs. In the case both parties decide to rekey at the
same time, the result is four NES packets, two in both directions.
Intermediate systems that use the SPI will have to inspect HIP
packets for a NES packet. The packet is signed for the benefit of
the intermediate systems. Since intermediate systems may need the
new SPI values, the contents of this packet cannot be encrypted.
Processing NES signatures is a potential DoS attack against
intermediate systems.
7.6 BOS - the HIP Bootstrap Packet
The BOS packet is OPTIONAL.
In some situations, an Initiator may not be able to learn of a
Responder's information from DNS or another repository. Some examples
of this are DHCP and NetBios servers. Thus, a packet is needed to
provide information that would otherwise be gleaned from a
repository. This HIP packet is either self-signed in applications
like SoHo, or from a trust anchor in large private or public
deployments. This packet MAY be broadcasted in IPv4 or multicasted
to the all hosts multicast group in IPv6. The packet MUST NOT be
sent more often than once in every second. Implementations MAY
ignore received BOS packets.
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The HIP header values for the BOS packet:
Header:
Packet Type = 7
SRC HIT = Announcer's HIT
DST HIT = NULL
IP ( HIP ( ( HOST_ID | HOST_ID_FQDN ), HIP_SIGNATURE ) )
The BOS packet may be followed by a CER packet if the HI is signed.
In this case, the C-bit in the control field MUST be set. If the BOS
packet is broadcasted or multicasted, the following CER packet(s)
MUST be broadcasted or multicasted to the same multicast group and
scope, respectively.
Valid control bits: C, A
7.7 CER - the HIP Certificate Packet
The CER packet is OPTIONAL.
The Optional CER packets over the Announcer's HI by a higher level
authority known to the Recipient is an alternative method for the
Recipient to trust the Announcer's HI (over DNSSEC or PKI).
The HIP header values for CER packet:
Header:
Packet Type = 8
SRC HIT = Announcer's HIT
DST HIT = Recipients's HIT
IP ( HIP ( { <CERT>i }, HIP_SIGNATURE ) ) or
IP ( HIP ( ENCRYPTED { <CERT>i }, HIP_SIGNATURE ) )
Valid control bits: None
Certificates in the CER packet MAY be encrypted. The encryption
algorithm is provided in the HIP transform of the previous (R1 or I2)
packet.
7.8 PAYLOAD - the HIP Payload Packet
The PAYLOAD packet is OPTIONAL.
The HIP header values for the PAYLOAD packet:
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Header:
Packet Type = 64
SRC HIT = Senders's HIT
DST HIT = Recipients's HIT
IP ( HIP ( ), payload )
Valid control bits: None
Payload Proto field in the Header MUST be set to correspond the
correct protocol number of the payload.
The PAYLOAD packet is used to carry a non-ESP protected data. By
using the HIP header we ensure interoperability with NAT and other
middle boxes.
Processing rules of the PAYLOAD packet are the following:
Receiving: If there is an existing HIP security association with the
given HITs, and the IP addresses match the IP addresses associated
with the HITs, pass the packet to the upper layer, tagged with
metadata indicating that the packet was NOT integrity or
confidentiality protected.
Sending: If the IPsec SPD defines BYPASS for a given destination
HIT, send it with the PAYLOAD packet. Otherwise use ESP as
specified in the SPD.
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8. Packet processing
Each host is assumed to have a separate HIP protocol implementation
that manages the host's HIP associations and handles requests for new
ones. Each HIP association is governed by a state machine, with
states defined above in Section 5.4. The HIP implementation can
simultaneously maintain HIP associations with more than one host.
Furthermore, the HIP implementation may have more than one active HIP
association with another host; in this case, HIP associations are
distinguished by their respective HITs and IPsec SPIs. It is not
possible to have more than one HIP associations between any given
pair of HITs. Consequently, the only way for two hosts to have more
than one parallel associations is to use different HITs, at least in
one end.
The processing of packets depends on the state of the HIP
association(s) with respect to the authenticated or apparent
originator of the packet. A HIP implementation determines whether it
has an active association with the originator of the packet based on
the HITs or the SPI of the packet.
8.1 Processing outgoing application data
In a HIP host, an application can send application level data using
HITs or LSIs as source and destination identifiers. The HITs and LSIs
may be specified via a backwards compatible API (see Appendix A) or a
completely new API. However, whenever there is such outgoing data,
the stack has to protect the data with ESP, and send the resulting
datagram using appropriate source and destination IP addresses.
Here, we specify the processing rules only for the base case where
both hosts have only single usable IP addresses; the multi-address
multi-homing case will be specified separately.
If the IPv4 backward compatible APIs and therefore LSIs are
supported, it is assumed that the LSIs will be converted into proper
HITs somewhere in the stack. The exact location of the conversion is
an implementation specific issue and not discussed here. The
following conceptual algorithm discusses only HITs, with the
assumption that the LSI-to-HIT conversion takes place somewhere.
The following steps define the conceptual processing rules for
outgoing datagrams destinated to a HIT.
1. If the datagram has a specified source address, it MUST be a HIT.
If it is not, the implementation MAY replace the source address
with a HIT. Otherwise it MUST drop the packet.
2. If the datagram has an unspecified source address, the
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implementation must choose a suitable source HIT for the
datagram. In selecting a proper local HIT, the implementation
SHOULD consult the table of currently active HIP sessions, and
preferably select a HIT that already has an active session with
the target HIT.
3. If there no active HIP session with the given < source,
destination > HIT pair, one must be created by running the base
exchange. The implementation SHOULD queue at least one packet
per a HIP session to be formed, and it MAY queue more than one.
4. Once there is an active HIP session for the given < source,
destination > HIT pair, the outgoing datagram is protected using
the associated ESP security association. In a typical
implementation, this will result in an transport mode ESP
datagram that still has HITs in the place of IP addresses.
5. The HITs in the datagram are replaced with suitable IP addresses.
For IPv6, the rules defined in [12] SHOULD be followed. Note
that this HIT-to-IP-address conversion step MAY also be performed
at some other point in the stack, e.g., before ESP processing.
However, care must be taken to make sure that the right ESP SA is
employed.
8.2 Processing incoming application data
Incoming HIP datagrams arrive as ESP protected packets. In the usual
case the receiving host has a corresponding ESP security association,
identified by the SPI and destination IP address in the packet.
However, if the host has crashed or otherwise lost its HIP state, it
may not have such an SA.
The following steps define the conceptual processing rules for
incoming ESP protected datagrams targeted to an ESP security
association created with HIP.
1. Detect the proper IPsec SA, using the destination IP address and
the SPI. If the resulting SA is a non-HIP ESP SA, process the
packet according to standard IPsec rules. If there are no SAs
identified with the SPI, the host MAY send an R1 with a NULL
Initiator SPI. Sending such R1s MUST be rate limited.
2. If a proper HIP ESP SA is found, the packet is processed normally
by ESP, as if the packet were a transport mode packet. The
packet may be dropped by ESP, as usual. In a typical
implementation, the result of succesful ESP decryption and
verification is a datagram with the original IP addresses as
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source and destination.
3. The IP addresses in the datagram are replaced with the HITs
associated with the ESP SA. Note that this IP-address-to-HIT
conversion step MAY also be performed at some other point in the
stack, e.g., before ESP processing.
4. The datagram is delivered to the upper layer. Demultiplexing the
datagram the right upper layer socket is based on the HITs (or
LSIs).
8.3 Initiation of a HIP exchange
An implementation may originate a HIP exchange to another host based
on a local policy decision, usually triggered by an application
datagram, in much the same way that an IPsec IKE key exchange can
dynamically create a Security Association. Alternatively, a system
may initiate a HIP exchange if it has rebooted or timed out, or
otherwise lost its HIP state, as described in Section 5.3.
The implementation prepares an I1 packet and sends it to the IP
address that corresponds to the peer host. The IP address of the
peer host may be obtained via conventional mechanisms, such as DNS
lookup. The I1 contents are specified in Section 7.1. The selection
of which host identity to use, if a host has more than one to choose
from, is typically also be a policy decision.
The following steps define the conceptual processing rules for
initiating a HIP exchange:
1. The Initiator gets the Responder's HIT and one or more addresses
either from a DNS lookup of the responder's FQDN, from some other
repository, or from a local table. If the initiator does not know
the responder's HIT, it may attempt opportunistic mode by using
NULL (all zeros) as the responder's HIT.
2. The Initiator sends an I1 to one of the Responder's addresses.
The selection of which address to use is a local policy decision.
3. Upon sending an I1, the sender shall transition to state E1,
start a timer whose timeout value should be larger than the
worst-case anticipated RTT, and shall increment a timeout counter
associated with the I1.
4. Upon timeout, the sender SHOULD retransmit the I1 and restart the
timer, up to a maximum of I1_RETRIES_MAX tries.
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8.3.1 Sending multiple I1s in parallel
For the sake of minimizing the session establishment latency, an
implementation MAY send the same I1 to more than one of the
Responder's addresses. However, it MUST NOT send to more than three
(3) addresses in parallel. Furthermore, upon timeout, the
implementation MUST refrain from sending the same I1 packet to
multiple addresses. These limitations are placed order to avoid
congestion of the network, and potential DoS attacks that might
happen, e.g., because someone claims to have hundreds or thousands of
addresses.
As the Responder is not guaranteed to distinguish the duplicate I1's
it receives at several of its addresses (because it avoids to store
states when it answers back an R1), the Initiator may receive several
duplicate R1's.
The Initiator SHOULD then select the destination address using the
source address of the first received R1 as a source address for the
next I2, and discards subsequent R1's. This strategy seems to be
quite good in terms of RTT.
8.3.2 Processing incoming ICMP Protocol Unreachable messages
A host may receive an ICMP Destination Protocol Unreachable message
as a response to sending an HIP I1 packet. Such a packet may be an
indication that the peer does not support HIP, or it may be an
attempt to launch an attack by making the Initiator to believe that
the Responder does not support HIP.
When a system receives an ICMP Destination Protocol Unreachable
message while it is waiting for an R1, it MUST NOT terminate the
wait. It MAY continue as if it had not received the ICMP message,
and send a few more I1s. Alternatively, it MAY take the ICMP message
as a hint that the peer most probably does not support HIP, and
return to state E0 earlier than otherwise. However, at minimum, it
MUST continue waiting for an R1 for a reasonable time before
returning to E0.
8.4 Processing incoming I1 packets
An implementation SHOULD reply to an I1 with an R1 packet, unless the
implementation is unable or unwilling to setup a HIP association. If
the implementation is unable to setup a HIP association, the host
SHOULD send an ICMP Destination Protocol Unreachable,
Administratively Prohibited, message to the I1 source address. If
the implementation is unwilling to setup a HIP association, the host
MAY ignore the I1. This latter case may occur during a DoS attack
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such as an I1 flood.
The implementation MUST be able to handle a storm of received I1
packets, discarding those with common content that arrive within a
small time delta.
A spoofed I1 can result in an R1 attack on a system. An R1 sender
MUST have a mechanism to rate limit R1s to an address.
Under no circumstances does the HIP state machine transition upon
sending an R1.
The following steps define the conceptual processing rules for
responding to an I1 packet:
1. The responder MUST check that the responder HIT in the received
I1 is either one of its own HITs, or NULL.
2. If the implementation chooses to respond to the I1 with and R1
packet, it creates a new R1 or selects a precomputed R1 according
to the format described in Section 7.2.
3. The R1 MUST contain the received responder HIT, unless the
received HIT is NULL, in which case the Responder may freely
select among its HITs.
4. The HIP control bits MUST be ignored in the received I1. The
received initiator HIT MUST be copied over to the Initiator HIT
field in the R1.
5. The responder sends the R1 to the source IP address of the I1
packet.
8.4.1 R1 Management
All compliant implementations MUST produce R1 packets. An R1 packet
MAY be precomputed. An R1 packet MAY be reused for time Delta T,
which is implementation dependent. R1 information MUST not be
discarded until Delta S after T. Time S is the delay needed for the
last I2 to arrive back to the responder.
An implementation MAY keep state about received I1s and match the
received I2s against the state, as discussed in Section 4.1.1.
8.5 Processing incoming R1 packets
A system receiving an R1 MUST first check to see if it has sent an I1
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to the originator of the R1 (i.e., it is in state E1). If so, it
SHOULD process the R1 as described below, send an I2, and go to state
E2, setting a timer to protect the I2. If the system is in state E0
or state E2 with respect to that host, it SHOULD silently drop the
R1.
If the system is in state E3/E4, it SHOULD process with a Security
Association and Birthday check as described in Section 5.3, before
further processing. In this last case, if the R1 is successfully
processed, the system sends an I2, sets a retransmit timer to protect
the I2, prepares to replace its old Security Associations with the
newly generated ones upon receiving a matching R2, and goes to state
E3. Note that if the system was in state E4, it stops the rekey
attempt and goes to state E3.
The following steps define the conceptual processing rules for
responding to an R1 packet:
1. A system receiving an R1 MUST first check to see if it has sent
an I1 to the originator of the R1 (i.e., it has a HIP
association that is in state E1 and that is associated with the
HITs in the R1). If so, it should process the R1 as described
below.
2. If the received R1 has a NULL Initiator HIT, the system should
look for a HIP association in state E3/E4 that is associated
with the received Responder HIT. If it finds one, it MUST
perform the Security Association and Birthday checks as
described in Section 5.3. If the SA and Birthday checks
succeed, it should process the R1 as described below.
3. Otherwise, if the system is in state E0 or state E2 with respect
to the HITs included in the R1, it SHOULD silently drop the R1
and remain in E0 or E2.
4. If the HIP association state is E1, the received Initiator's HIT
MUST correspond to the HIT used in the original, I1 and the
Responder's HIT MUST correspond to the one used, unless the I1
contained a NULL HIT. If the HIP association state is E3/E4,
the received Initiator's HIT MUST either be NULL or correspond
to the one stored in the HIP association, and the Responder's
HIT MUST correspond to the one stored in the association.
5. The system SHOULD validate the R1 signature before applying
further packet processing, according to Section 6.2.13.
6. The R1 packet may have the C bit set -- in this case, the system
should anticipate the receipt of HIP CER packets that contain
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the host identity corresponding to the responder's HIT.
7. The R1 packet may have the A bit set -- in this case, the system
MAY choose to refuse it by dropping the R1 and returning to
state E0. The system SHOULD consider dropping the R1 only if it
used a NULL HIT in I1. If the A bit is set, the Responder's HIT
is anonymous and should not be stored.
8. The system SHOULD attempt to validate the HIT against the
received Host Identity.
9. The system MUST store the received Birthday count for future
reference.
10. The attempts to solve the cookie puzzle in R1. The system MUST
terminate the search after a number of tries, the minimum of the
degree of difficulty specified by the K value or an
implementation- or policy-defined maximum retry count. It is
RECOMMENDED that the system does not try more than 2^(K+2)
times. If the cookie puzzle is not successfully solved, the
implementation may either resend I1 within the retry bounds or
abandon the HIP exchange.
11. The system computes standard Diffie-Hellman keying material
according to the public value and Group ID provided in the
DIFFIE_HELLMAN parameter. The Diffie-Hellman keying material
Kij is used for key extraction as specified in Section 9. If
the received Diffie-Hellman Group ID is not supported, the
implementation may either resend I1 within the retry bounds or
abandon the HIP exchange.
12. The system selects the HIP transform and ESP transform from the
choices presented in the R1 packet and uses the selected values
subsequently when generating and using encryption keys, and when
sending the I2. If the proposed alternatives are not acceptable
to the system, it may either resend I1 within the retry bounds
or abandon the HIP exchange.
13. The system prepares and creates an incoming IPsec ESP security
association. It may also prepare a security association for
outgoing traffic, but since it does not have the correct SPI
value yet, it cannot activate it.
14. The system initialized the remaining variables in the associated
state, including NES ID counters.
15. The system prepares and sends an I2, as described in Section
7.3.
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16. The system SHOULD start a timer whose timeout value should be
larger than the worst-case anticipated RTT, and MUST increment a
timeout counter associated with the I2. The sender SHOULD
retransmit the I2 upon a timeout and restart the timer, up to a
maximum of I2_RETRIES_MAX tries.
17. If the system is in state E1, it shall transition to state E2.
If the system is in state E3, it remains so. If the system is in
state E4, it stops the rekey attempt and goes to E3.
18. If the system is in state E3/E4, the system prepares to drop its
old outgoing Security Association and replace the incoming
Security Association with the newly generated ones upon
receiving a matching R2.
8.6 Processing incoming I2 packets
Upon receipt of an I2, the system MAY perform initial checks to
determine whether the I2 corresponds to a recent R1 that has been
sent out, if the Responder keeps such state. For example, the sender
could check whether the I2 is from an address or HIT that has
recently received an R1 from it. If the I2 is considered to be
suspect, it MAY be silently discarded by the system.
Otherwise, the HIP implementation SHOULD process the I2. This
includes validation of the cookie puzzle solution, generating the
Diffie-Hellman key, decrypting the Initiator's Host Identity,
verifying the signature, creating state, and finally sending an R2.
The following steps define the conceptual processing rules for
responding to an I2 packet:
1. The system MAY perform checks to verify that the I2 corresponds
to a recently sent R1. Such checks are implementation
dependent. See Appendix D for a description of an example
implementation.
2. The system MUST check that the Responder's HIT corresponds to
one of its own HITs.
3. The system MUST validate the solution to the cookie puzzle by
computing the SHA-1 hash described in Section 7.3.
4. The I2 MUST have a single value in each of the HIP_TRANSFORM and
ESP_TRANSFORM parameters, which MUST each match one of the
values offered to the Initiator in the R1 packet.
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5. The system must derive Diffie-Hellman keying material Kij based
on the public value and Group ID in the DIFFIE_HELLMAN
parameter. This key is used to derive the HIP and ESP
association keys, as described in Section 9. If the
Diffie-Hellman Group ID is unsupported, the I2 packet is
silently dropped.
6. The encrypted HOST_ID or HOST_ID_FQDN is decrypted by the
Initiator encryption key defined in Section 9. If the decrypted
data is not an HOST_ID or HOST_ID_FQDN parameter, the I2 packet
is silently dropped.
7. The implementation SHOULD also verify that the Initiator's HIT
in the I2 corresponds to the Host Identity sent in the I2.
8. The system MUST verify the HIP_SIGNATURE according to Section
6.2.12 and Section 7.3.
9. If the system is in state E3 with respect to the HITs, the
system MUST perform the birthday cookie check as defined in
Section 5.3.
10. If the checks above are valid, then the system proceeds with
further I2 processing; otherwise, it discards the I2 and remains
in the same state.
11. The I2 packet may have the C bit set -- in this case, the system
should anticipate the receipt of HIP CER packets that contain
the host identity corresponding to the responder's HIT.
12. The I2 packet may have the A bit set -- in this case, the system
MAY choose to refuse it by dropping the I2 and returning to
state E0. If the A bit is set, the Initiator's HIT is anonymous
and should not be stored.
13. The I2 packet may have the E bit set. If so, the HIP
implementation MUST treat the ESP sequence numbers as 64 bit
quantities as described in Section 11.4.
14. The Birthday count is extracted from the BIRTHDAY_COOKIE
parameter and stored for future use.
15. The SPI_LSI field is parsed to obtain the SPI that will be used
for the Security Association outbound from the Responder and
inbound to the Initiator.
16. If the system supports LSIs, it should check that the received
LSI is an acceptable representation of its own identity, and
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create an appropriate state. If the LSI is not acceptable, the
behaviour is currently undefined; see Appendix A.
17. The system prepares and creates both incoming and outgoing ESP
security associations.
18. The system initialized the remaining variables in the associated
state, including NES ID counters.
19. Upon successful processing of an I2 in states E0, E1, and E2, an
R2 is sent and the state machine transitions to state E3.
20. Upon successful processing of an I2 in state E3/E4 (which
includes a successful Birthday check), the old Security
Association is dropped and a new one is installed, an R2 is
sent, and the state machine transitions to E3, dropping any
possibly ongoing rekeying attempt.
8.7 Processing incoming R2 packets
An R2 received in states E0, E1, E3 or E4 results in the R2 being
dropped and the state machine staying in the same state. If an R2 is
received in state E2, it SHOULD be processed.
The following steps define the conceptual processing rules for
responding to an I2 packet:
1. The system MUST verify that the HITs in use correspond to the
HITs that were received in R1.
2. The system MUST verify the HMAC according to the procedures in
Section 6.2.11.
3. The system MUST verify the HIP signature according to the
procedures in Section 6.2.12.
4. If any of the checks above fail, there is a high probability of
an ongoing man-in-the-middle or other security attack. The
system SHOULD act accordingly, based on its local policy.
5. The R2 packet may have the E bit set. If so, the HIP
implementation MUST treat the ESP sequence numbers as 64 bit
quantities as described in Section 11.4.
6. If the system is already in state E3 or E4, it drops the old
outbound ESP Security association. If the system is in state E4,
it stops the rekey attempt.
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7. The SPI_LSI field is parsed to obtain the SPI that will be used
for the ESP Security Association inbound to the Responder. The
system uses this SPI to create or activate the outgoing ESP
security association used to send packets to the peer.
8. If the system supports LSIs, it should check that the received
LSI is an acceptable representation of its own identity, and
create an appropriate state. If the LSI is not acceptable, the
behaviour is currently undefined; see Appendix A.
9. Upon successful processing of the R2, the state machine moves to
state E3.
8.8 Initiating rekeying
A system may initiate the rekey procedure at any time. It MUST
initiate a rekey if its incoming ESP sequence counter is about to
overflow.
The following steps define the conceptual processing rules for
initiating a rekey:
1. The system SHOULD generate a new Diffie-Hellman public key.
2. The system MUST increment its outgoing NES ID counter.
3. The system creates a NES packet, which SHOULD contain a
Diffie-Hellman parameter. If the NES packet contains the
Diffie-Hellman parameter, the Keymat Index in the NES_INFO
parameter MUST be zero.
4. The system sends the NES packet and transitions to state E4.
5. The system SHOULD start a timer whose timeout value should be
larger than the worst-case anticipated RTT, and MUST increment a
timeout counter associated with NES. The sender SHOULD retransmit
the NES upon a timeout and restart the timer, up to a maximum of
NES_RETRIES_MAX tries.
6. The system MUST NOT delete its existing SAs, but continue using
them if its policy still allows. The NES procedure SHOULD be
initiated prior enough in order to make sure that the SA replay
counters do not overflow.
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8.9 Processing NES packets
When a system receives a NES packet, its processing depends on
whether the packet is a reply to a previously sent NES packet or the
NES is a new packet.
The following steps define the conceptual processing rules responding
handling a received NES packet:
1. If the system is in state E3 and the NES has the R-bit set, the
packet is silently dropped.
2. If the NES ID in the received NES is smaller than the stored
incoming NES ID, the packet MUST BE dropped. Note that if the
NES ID is equal to the stored value, the packet can be expected
to be a retransmission, and acted accordingly. However, the
HMAC verification (next step) MUST NOT be skipped. (A
byte-by-byte comparison of the received and a store packet would
be OK, though.)
3. The system MUST verify the HMAC in the NES packet. If the
verification fails, the packet MUST be dropped.
4. If the received NES contains a Diffie-Hellman parameter, the
received Keymat Index MUST zero. If this test fails, the packet
SHOULD be dropped and the system SHOULD log an error message.
5. If the received NES does not contain a Diffie-Hellman parameter,
the received Keymat Index MUST be larger or equal to the index
of the next byte to be drawn from the current KEYMAT. If this
test fails, the packet SHOULD be dropped and the system SHOULD
log an error message.
6. The system MAY verify the SIGNATURE in the NES packet. If the
verification fails, the packet SHOULD be dropped and an error
message logged.
7. The system MUST record the NES ID in the received packet, for
replay protection.
8. If the system is in state E3, and the NES does not have the
R-bit set, the packet is processed as specified in Section
8.9.1.
9. If the system is in state E4, and the NES has the R-bit set, the
packet is processed as specified in Section 8.9.2.
10. If the system is in state E4, and the NES doesn't have the R-bit
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set, there are apparently two NES packets crossing each other in
the network. Consequently, the R-bit-less packet is handled as
specified in Section 8.9.1. However, in order not to
unnecessarily spend cycles in Diffie-Hellman computations, there
are minor differences to the case of receiving such a packet in
state E3.
8.9.1 Processing an initial NES packet
When a system receives an initial NES packet, i.e. one that does not
have the R-bit set, it prepares new incoming and outgoing SAs, but
does not change the outgoing SA yet. Once it has the new SAs in
place, it sends a reply NES. The contents of the reply NES depend on
whether the system was in state E3 or E4 upon receiving the initial
NES packet.
The following steps define the conceptual processing rules responding
handling a received initial NES packet:
1. If the system is in state E3, it consults its policy to see if it
needs to generate a new Diffie-Hellman key, and generates a new
key if needed. If the system is in state E4, it may already have
generated a new Diffie-Hellman key, and SHOULD use it.
2. If either the received NES contains a new Diffie-Hellman key, the
system has a new Diffie-Hellman key from the previous step, or
both, the system generates new KEYMAT. If there is only one new
Diffie-Hellman key, the other old key is used.
3. If the system generated new KEYMAT in the previous step, it sets
Keymat Index to zero, independent on whether the received NES
included a Diffie-Hellman key or not.
4. The system draws keys for new incoming and outgoing ESP SAs,
starting from the Keymat Index, and prepares new incoming and
outgoing ESP SAs. The SPI for the outgoing SA is the new SPI
value from the received NES. The SPI for the incoming SA is
locally generated, and SHOULD be random. The system MUST NOT
start using the new outgoing SA before it receives traffic on the
new incoming SA.
5. The system prepares a reply NES packet, with the R-bit one,
Keymat Index being the one used above, NES ID being equal to the
one in the received NES, old SPI being the current incoming SPI,
and new SPI being the newly generated incoming SPI. If the
system generated a new Diffie-Hellman key above, the new key is
included in the packet in the Diffie-Hellman payload. Note that
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if the system is in state E4, the new Diffie-Hellman key was
probably generated and sent already earlier, in which case it
MUST NOT be included into the reply packet.
8.9.2 Processing a reply NES packet
When a system receives a reply NES packet, i.e. one that has the
R-bit set, it starts to use the new outgoing SA. It must also
complete its new incoming SA.
The following steps define the conceptual processing rules responding
handling a received reply NES packet:
1. If either the received NES contains a new Diffie-Hellman key, the
system has a new Diffie-Hellman key from initiating rekey, or
both, the system generates new KEYMAT. If there is only one new
Diffie-Hellman key, the old key is used as the other key.
2. If the system generated new KEYMAT in the previous step, it sets
Keymat Index to zero, independent on whether the received NES
included a Diffie-Hellman key or not.
3. The system draws keys for new incoming and outgoing ESP SAs,
starting from the Keymat Index, and prepares new incoming and
outgoing ESP SAs. The SPI for the outgoing SA is the new SPI
value from the NES. The SPI for the incoming SA was generated
when rekey was initiated.
4. The system starts to send to the new outgoing SA. It should
start receiving data on the new incoming SA as soon as the peer
receives data on the new SA.
5. If the system has no data to send for 500ms, it SHOULD send an
ESP packet anyway. The purpose of this packet is to acknowledge
the other party that the NES reply came through, and to allow the
other party to switch over to the new outgoing SA. It is
RECOMMENDED that the system sends an empty ESP packet, i.e., one
where the Next Header field is IPPROTO_NONE (decimal 59).
8.10 Processing BOS packets
Processing BOS packets is OPTIONAL, and currently undefined.
8.11 Processing CER packets
Processing CER packets is OPTIONAL, and currently undefined.
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8.12 Processing PAYLOAD packets
Processing PAYLOAD packets is OPTIONAL, and currently undefined.
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9. HIP KEYMAT
HIP keying material is derived from the Diffie-Hellman Kij produced
during the base HIP exchange. The Initiator has Kij during the
creation of the I2 packet, and the Responder has Kij once it receives
the I2 packet. This is why I2 can already contain encrypted
information.
The KEYMAT is derived by feeding Kij and the HITs into the following
operation; the | operation denotes concatenation.
KEYMAT = K1 | K2 | K3 | ...
where
K1 = SHA-1( Kij | sort(HIT-I | HIT-R) | 0x01 )
K2 = SHA-1( Kij | K1 | 0x02 )
K3 = SHA-1( Kij | K2 | 0x03 )
...
K255 = SHA-1( Kij | K254 | 0xff )
K256 = SHA-1( Kij | K255 | 0x00 )
etc.
Sort(HIT-I | HIT-R) is defined as the numeric network byte order
comparison of the HITs, with lower HIT preceding higher HIT,
resulting in the concatenation of the HITs in the said order. The
initial keys are drawn sequentially in the following order:
Initiator HIP encryption key
Initiator HIP integrity (HMAC) key
Responder HIP encryption key (currently unused)
Responder HIP integrity (HMAC) key
Initiator ESP encryption key
Initiator ESP authentication key
Responder ESP encryption key
Responder ESP authentication key
The number of bits drawn for a given algorithm is the "natural" size
of the keys. For the mandatory algorithms, the following sizes
apply:
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3DES 192 bits
SHA-1 160 bits
NULL 0 bits
Subsequent rekeys without Diffie-Hellman just require drawing out
more sets of ESP keys. In the situation where Kij is the result of a
HIP rekey exchange with Diffie-Hellman, there is only the need from
one set of ESP keys, without the HIP keys. These are then the only
keys taken from the KEYMAT.
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10. HIP Fragmentation Support
A HIP implementation must support IP fragmentation / reassembly.
Fragment reassembly MUST be implemented in both IPv4 and IPv6, but
fragment generation MUST be implemented only in IPv4 (IPv4 stacks and
networks will usually do this by default) and SHOULD be implemented
in IPv6. In the IPv6 world, the minimum MTU is larger, 1280 bytes,
than in the IPv4 world. The larger MTU size is usually sufficient for
most HIP packets, and therefore fragment generation may not be
needed. If a host expects to send HIP packets that are larger than
the minimum IPv6 MTU, it MUST implement fragment generation even for
IPv6.
In the IPv4 world, HIP packets may encounter low MTUs along their
routed path. Since HIP does not provide a mechanism to use multiple
IP datagrams for a single HIP packet, support of path MTU discovery
does not bring any value to HIP in the IPv4 world. HIP aware NAT
systems MUST perform any IPv4 reassembly/fragmentation.
All HIP implementations MUST employ a reassembly algorithm that is
sufficiently resistant against DoS attacks.
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11. ESP with HIP
XXX: Since HIP is designed for host usage, not for gateways, only ESP
transport mode is supported with HIP. The SA is not bound to an IP
address; all internal control of the SA is by the HIT and LSI. XXX
BEET mode.
Since HIP does not negotiate any lifetimes, all lifetimes are local
policy. The only lifetimes a HIP implementation MUST support are
sequence number rollover (for replay protection), and SA timeout. An
SA times out if no packets are received using that SA. The default
timeout value is 15 minutes. Implementations MAY support lifetimes
for the various ESP transforms.
11.1 Security Association Management
An SA pair is indexed by the 2 SPIs and 2 HITs (both HITs since a
system can have more than one HIT). An inactivity timer is
recommended for all SAs. If the state dictates the deletion of an
SA, a timer is set to allow for any late arriving packets.
11.2 Security Parameter Index (SPI)
The SPIs in ESP provide a simple compression of the HIP data from all
packets after the HIP exchange. This does require a per HIT- pair
Security Association (and SPI), and a decrease of policy granularity
over other Key Management Protocols like IKE.
When a host rekeys, it gets a new SPI from its partner.
11.3 Supported Transforms
All HIP implementations MUST support 3DES [7] and HMAC-SHA-1-96 [4].
If the Initiator does not support any of the transforms offered by
the Responder in the R1 HIP packet, it MUST use 3DES and
HMAC-SHA-1-96 and state so in the I2 HIP packet.
In addition to 3DES, all implementations MUST implement the ESP NULL
encryption and authentication algorithms. These algoritms are
provided mainly for debugging purposes, and SHOULD NOT be used in
production environments. The default configuration in
implementations MUST be to reject NULL encryption or authentication.
11.4 Sequence Number
The Sequence Number field is MANDATORY in ESP. Anti-replay
protection MUST be used in an ESP SA established with HIP.
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This means that each host MUST rekey before its sequence number
reaches 2^32, or if extended sequence numbers are used, 2^64. Note
that in HIP rekeying, unlike IKE rekeying, only one Diffie-Hellman
key can be changed, that of the rekeying host. However, if one host
rekeys, the other host SHOULD rekey as well.
In some instances, a 32 bit sequence number is inadequate. In either
the I2 or R2 packets, a peer MAY require that a 64 bit sequence
number be used. In this case the higher 32 bits are NOT included in
the ESP header, but are simply kept local to both peers. 64 bit
sequence numbers must only be used for ciphers that will not be open
to cryptoanalysis as a result. AES is one such cipher.
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12. HIP Policies
There are a number of variables that will influence the HIP exchanges
that each host must support. All HIP implementations MUST support
more than one simultaneous HIs, at least one of which SHOULD be
reserved for anonymous usage. Although anonymous HIs will be rarely
used as responder HIs, they will be common for Initiators. Support
for more than two HIs is RECOMMENDED.
Many Initiators would want to use a different HI for different
Responders. The implementations SHOULD provide for an ACL of
initiator HIT to responder HIT. This ACL SHOULD also include
preferred transform and local lifetimes. For HITs with HAAs,
wildcarding SHOULD be supported. Thus if a Community of Interest,
like Banking, gets an RAA, a single ACL could be used. A global
wildcard would represent the general policy to be used. Policy
selection would be from most specific to most general.
The value of K used in the HIP R1 packet can also vary by policy. K
should never be greater than 20, but for trusted partners it could be
as low as 0.
Responders would need a similar ACL, representing which hosts they
accept HIP exchanges, and the preferred transform and local
lifetimes. Wildcarding SHOULD be supported for this ACL also.
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13. Security Considerations
HIP is designed to provide secure authentication of hosts and to
provide a fast key exchange for IPsec ESP. HIP also attempts to
limit the exposure of the host to various denial-of-service and man-
in-the-middle attacks. In so doing, HIP itself is subject to its own
DoS and MitM attacks that potentially could be more damaging to a
host's ability to conduct business as usual.
HIP enabled ESP is IP address independent. This might seem to make
it easier for an attacker, but ESP with replay protection is already
as well protected as possible, and the removal of the IP address as a
check should not increase the exposure of ESP to DoS attacks.
Furthermore, this is in line with the forthcoming revision of ESP.
Denial-of-service attacks take advantage of the cost of start of
state for a protocol on the Responder compared to the 'cheapness' on
the Initiator. HIP makes no attempt to increase the cost of the
start of state on the Initiator, but makes an effort to reduce the
cost to the Responder. This is done by having the Responder start
the 3-way cookie exchange instead of the Initiator, making the HIP
protocol 4 packets long. In doing this, packet 2 becomes a 'stock'
packet that the Responder MAY use many times. The duration of use is
a paranoia versus throughput concern. Using the same Diffie- Hellman
values and random puzzle I has some risk. This risk needs to be
balanced against a potential storm of HIP I1 packets.
This shifting of the start of state cost to the Initiator in creating
the I2 HIP packet, presents another DoS attack. The attacker spoofs
the I1 HIP packet and the Responder sends out the R1 HIP packet.
This could conceivably tie up the 'initiator' with evaluating the R1
HIP packet, and creating the I2 HIP packet. The defense against this
attack is to simply ignore any R1 packet where a corresponding I1 or
ESP data was not sent.
A second form of DoS attack arrives in the I2 HIP packet. Once the
attacking Initiator has solved the cookie challenge, it can send
packets with spoofed IP source addresses with either invalid
encrypted HIP payload component or a bad HIP signature. This would
take resources in the Responder's part to reach the point to discover
that the I2 packet cannot be completely processed. The defense
against this attack is after N bad I2 packets, the Responder would
discard any I2s that contain the given Initiator HIT. Thus will shut
down the attack. The attacker would have to request another R1 and
use that to launch a new attack. The Responder could up the value of
K while under attack. On the downside, valid I2s might get dropped
too.
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A third form of DoS attack is emulating the restart of state after a
reboot of one of the partners. To protect from such an attack, a
system Birthday is included in the R1 and I2 packets to prove loss of
state to a peer. The inclusion of the Birthday creates a very
deterministic process for state restart. Any other action is a DoS
attack.
A fourth form of DoS attack is emulating the end of state. HIP has
no end of state packet. It relies on a local policy timer to end
state.
Man-in-the-middle attacks are difficult to defend against, without
third-party authentication. A skillful MitM could easily handle all
parts of HIP; but HIP indirectly provides the following protection
from a MitM attack. If the Responder's HI is retrieved from a signed
DNS zone, a certificate, or through some other secure means, the
Initiator can use this to validate the R1 HIP packet.
Likewise, if the Initiator's HI is in a secure DNS zone, a trusted
certificate, or otherwise securely available, the Responder can
retrieve it after it gets the I2 HIP packet and validate that.
However, since an Initiator may choose to use an anonymous HI, it
knowingly risks a MitM attack. The Responder may choose not to
accept a HIP exchange with an anonymous Initiator.
Since not all hosts will ever support HIP, ICMP 'Destination Protocol
Unreachable' are to be expected and present a DoS attack. Against an
Initiator, the attack would look like the Responder does not support
HIP, but shortly after receiving the ICMP message, the Initiator
would receive a valid R1 HIP packet. Thus to protect from this
attack, an Initiator should not react to an ICMP message until a
reasonable delta time to get the real Responder's R1 HIP packet. A
similar attack against the Responder is more involved. First an ICMP
message is expected if the I1 was a DoS attack and the real owner of
the spoofed IP address does not support HIP. The Responder SHOULD
NOT act on this ICMP message to remove the minimal state from the R1
HIP packet (if it has one), but wait for either a valid I2 HIP packet
or the natural timeout of the R1 HIP packet. This is to allow for a
sophisticated attacker that is trying to break up the HIP exchange.
Likewise, the Initiator should ignore any ICMP message while waiting
for an R2 HIP packet, deleting state only after a natural timeout.
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14. IANA Considerations
IANA has assigned IP Protocol number TBD to HIP.
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15. Acknowledgments
The drive to create HIP came to being after attending the MALLOC
meeting at IETF 43. Baiju Patel and Hilarie Orman really gave the
original author, Bob Moskowitz, the assist to get HIP beyond 5
paragraphs of ideas. It has matured considerably since the early
drafts thanks to extensive input from IETFers. Most importantly, its
design goals are articulated and are different from other efforts in
this direction. Particular mention goes to the members of the
NameSpace Research Group of the IRTF. Noel Chiappa provided the
framework for LSIs and Keith Moore the impetus to provide
resolvability. Steve Deering provided encouragement to keep working,
as a solid proposal can act as a proof of ideas for a research group.
Many others contributed; extensive security tips were provided by
Steve Bellovin. Rob Austein kept the DNS parts on track. Paul
Kocher taught Bob Moskowitz how to make the cookie exchange expensive
for the Initiator to respond, but easy for the Responder to validate.
Bill Sommerfeld supplied the Birthday concept to simplify reboot
management. Rodney Thayer and Hugh Daniels provide extensive
feedback. In the early times of this draft, John Gilmore kept Bob
Moskowitz challenged to provide something of value.
During the later stages of this document, when the editing baton was
transfered to Pekka Nikander, the input from the early implementors
were invaluable. Without having actual implementations, this
document would not be on the level it is now.
In the usual IETF fashion, a large number of people have contributed
to the actual text or ideas. The list of these people include Jeff
Ahrenholz, Francis Dupont, Derek Fawcus, George Gross, Andrew
McGregor, Julien Laganier, Miika Komu, Mika Kousa, Jan Melen, Henrik
Petander, Michael Richardson, Tim Shepard, and Jukka Ylitalo. Our
apologies to anyone who's name is missing.
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Normative references
[1] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
1980.
[2] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within ESP
and AH", RFC 2404, November 1998.
[5] Maughan, D., Schneider, M. and M. Schertler, "Internet Security
Association and Key Management Protocol (ISAKMP)", RFC 2408,
November 1998.
[6] Orman, H., "The OAKLEY Key Determination Protocol", RFC 2412,
November 1998.
[7] Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher Algorithms",
RFC 2451, November 1998.
[8] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[9] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[10] Eastlake, D., "DSA KEYs and SIGs in the Domain Name System
(DNS)", RFC 2536, March 1999.
[11] Housley, R., Polk, W., Ford, W. and D. Solo, "Internet X.509
Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 3280, April 2002.
[12] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
[13] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
Addressing Architecture", RFC 3513, April 2003.
[14] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)", RFC
3526, May 2003.
[15] Kent, S., "IP Encapsulating Security Payload (ESP)",
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draft-ietf-ipsec-esp-v3-06 (work in progress), July 2003.
[16] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
draft-ietf-ipsec-ikev2-10 (work in progress), August 2003.
[17] Moskowitz, R., "Host Identity Protocol Architecture",
draft-moskowitz-hip-arch-03 (work in progress), May 2003.
[18] NIST, "FIPS PUB 180-1: Secure Hash Standard", April 1995.
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Informative references
[19] Bellovin, S. and W. Aiello, "Just Fast Keying (JFK)",
draft-ietf-ipsec-jfk-04 (work in progress), July 2002.
[20] Moskowitz, R. and P. Nikander, "Using Domain Name System (DNS)
with Host Identity Protocol (HIP)", draft-nikander-hip-dns-00
(to be issued) (work in progress), June 2003.
[21] Nikander, P., "SPI assisted NAT traversal (SPINAT) with Host
Identity Protocol (HIP)", draft-nikander-hip-nat-00 (to be
issued) (work in progress), June 2003.
[22] Crosby, SA. and DS. Wallach, "Denial of Service via Algorithmic
Complexity Attacks", in Proceedings of Usenix Security
Symposium 2003, Washington, DC., August 2003.
Authors' Addresses
Robert Moskowitz
ICSAlabs, a Division of TruSecure Corporation
1000 Bent Creek Blvd, Suite 200
Mechanicsburg, PA
USA
EMail: rgm@icsalabs.com
Pekka Nikander
Ericsson Research Nomadiclab
JORVAS FIN-02420
FINLAND
Phone: +358 9 299 1
EMail: pekka.nikander@nomadiclab.com
Petri Jokela
Ericsson Research Nomadiclab
JORVAS FIN-02420
FINLAND
Phone: +358 9 299 1
EMail: petri.jokela@nomadiclab.com
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Thomas R. Henderson
The Boeing Company
P.O. Box 3707
Seattle, WA
USA
EMail: thomas.r.henderson@boeing.com
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Appendix A. Backwards compatibility API issues
Tom Henderson has several times expressed the thought that that the
LSI could be completely local and does not need to be exchanged.
Applications could continue to use IP addresses in socket calls, and
kernel does whatever NATting (including application NATting) is
required. It was pointed out that this approach was going to be
prone to some kinds of data flows escaping the HIP protection, unless
the local housekeeping in an implementation was especially good.
Example: FTP opens control connection to IP address. One or both
parties move. FTP later opens data connection to the old IP address.
Kernel must identify that the application really means to connect to
the host that was previously at that IP address -- but obviously if
the old address is reused by another host, this becomes difficult.
Related to this, the discussion also opened up the question of DNS
resolution. Should the HIT/LSI be returned to applications as a
(spoofed) address in the resolution process, allowing apps to use the
socket API with HIT or LSI values instead of an IP address? While
this seems to be the original intention of LSIs, there are a couple
of difficulties especially in the IPv4 case:
How does kernel know whether value being passed in a socket call
is an IP address or an LSI? The fact that a name resolver library
gave an application an LSI is no guarantee that the application
will use that information in its socket call. It may also have
cached some IP address from before or received an IP address as
side information. This difficulty is now relieved as the LSIs are
constrained to the well-known private subnet space.
Handing an LSI may confuse legacy applications that assume that
what is being passed to them is an IP address. Good examples of
this are diagnostic tools such as dig and ping. The conclusion is
that HIP should most not be used with diagnostic applications.
What does kernel do with an LSI that it cannot map to an address
based on information that it has locally cached?
It seems that some modification to the resolver library (to
explicitly convey HIP information rather than spoofing IP addresses),
as well as modifications to socket API to explicitly let the kernel
know that the application is HIP aware, are the cleanest long-term
solution, but what to do about legacy applications?? -- still
partially an open issue. The HUT team has been considering these
problems.
In summary, there seems to be two schools of thought, and their
approaches can be summarized as follows:
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1. Have the resolver return an LSI in place of an actual IP address,
and have applications use the LSI instead of the IP address in
socket calls. By making LSI fall into an unassigned IP address
space, the kernel can distinguish between calls made with an LSI
and calls made with an IP address. This is the approach
currently recommended in this specification.
Now some complications with this approach are the following:
1. One probably does not want to invoke a HIP exchange just
because an application did a DNS lookup on a hostname;
applications use resolver for other reasons than just prior
to initiating a flow. Since LSIs are a subset of the HIT
bits, we could avoid having to do the HIP exchange to find
the LSIs, if it weren't for the possibility of collision.
So, this approach seems to be pointing us towards doing HIP
exchanges prior to the application's initial socket calls.
Note that this would also break the opportunity to piggyback
data on the I2/R2.
2. Some applications might actually want IP addresses
explicitly, such as diagnostic applcations.
3. Some applications likely will obtain addresses out-of-band,
so even in this scenario, the kernel may be faced with a case
where it would like to use HIP from a policy standpoint, but
the invoking application called connect() with an IP address.
One way around might be to have a separate resolver call that
returns an LSI, or enhance the data structure returned by
resolver to include LSI in addition to IP address, but this then
throws the burden on applications to be HIP-aware.
2. Have the LSI be kept in kernel land, with the kernel doing the
right housekeeping. Unless the application is HIP aware and can
explicitly signal that it wants to use HIP (such as with a
setsockopt() flag), the use of HIP is coordinated by a policy
table that temporarily traps packets going to particular IP
addresses until a HIP exchange completes and the security
association is set up. In this case, the kernel does the right
checksum munging and handles readdressing, in a manner
transparent to the applications, which just think that they have
connected to an IP address as before. This essentially means
that the is responsible for managing the association between IP
address (as perceived by an application) and actual destination
identity. This is similar to the way that opportunistic IPsec
works today.
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This solution has the following drawbacks:
1. One is architecturally forced back into the mode of using IP
addresses to indirectly identify hosts, at least initially.
The IP stack has to maintain tables that say things like "if
a socket call comes in for address X, do a HIP exchange to
that address." (note that this may require an opportunistic
exchange) This constrains the kinds of mobility that the
system can handle.
2. One can construct scenarios for which the kernel and
applications are out of sync with respect to IP addresses,
and flows may escape their intended IPsec envelope. Example:
I open FTP control connection to address X. That host moves
to address Y. Some other host takes up address X. I next
receive a connect() to address X. Which host does that
process want to connect to?
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Appendix B. Probabilities of HIT collisions
The birthday paradox sets a bound for the expectation of collisions.
It is based on the square root of the number of values. A 64-bit
hash, then, would put the chances of a collision at 50-50 with 2^32
hosts (4 billion). A 1% chance of collision would occur in a
population of 640M and a .001% collision chance in a 20M population.
A 128 bit hash will have the same .001% collision chance in a 9x10^16
population.
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Appendix C. Probabilities in the cookie calculation
A question: Is it guaranteed that the Initiator is able to solve the
puzzle in this way when the K value is large?
Answer: No, it is not guaranteed. But it is not guaranteed even in
the old mechanism, since the Initiator may start far away from J and
arrive to J after far too many steps. If we wanted to make sure that
the Initiator finds a value, we would need to give some hint of a
suitable J, and I don't think we want to do that.
In general, if we model the hash function with a random function, the
probability that one iteration gives are result with K zero bits is
2^-K. Thus, the probablity that one iteration does *not* give K zero
bits is (1 - 2^-K). Consequently, the probablity that 2^K iterations
does not give K zero bits is (1 - 2^-K)^(2^K).
Since my calculus starts to be rusty, I made a small experiment and
found out that
lim (1 - 2^-k)^(2^k) = 0.36788
k->inf
lim (1 - 2^-k)^(2^(k+1)) = 0.13534
k->inf
lim (1 - 2^-k)^(2^(k+2)) = 0.01832
k->inf
lim (1 - 2^-k)^(2^(k+3)) = 0.000335
k->inf
Thus, if hash functions were random functions, we would need about
2^(K+3) iterations to make sure that the probability of a failure is
less than 1% (actually less than 0.04%). Now, since my perhaps
flawed understanding of hash functions is that they are "flatter"
than random functions, 2^(K+3) is probably an overkill. OTOH, the
currently suggested 2^K is clearly too little. The draft has been
changed to read 2^(K+2).
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Appendix D. Using responder cookies
As mentioned in Section 4.1.1, the Responder may delay state creation
and still reject most spoofed I2s by using a number of pre-calculated
R1s and a local selection function. This appendix defines one
possible implementation in detail. The purpose of this appendix is
to give the implementators an idea on how to implement the mechanism.
The method described in this appendix SHOULD NOT be used in any real
implementation. If the implementation is based on this appendix, it
SHOULD contain some local modification that makes an attacker's task
harder.
The basic idea is to create a cheap, varying local mapping function
f:
f( IP-I, IP-R, HIT-I, HIT-R ) -> cookie-index
That is, given the Initiator's and Responder's IP addresses and
HITs, the function returns an index to a cookie. When processing an
I1, the cookie is embedded in an pre-computed R1, and the Responder
simply sends that particular R1 to the Initiator. When processing an
I2, the cookie may still be embedded in the R1, or the R1 may be
depracated (and replaced with a new one), but the cookie is still
there. If the received cookie does not match with the R1 or saved
cookie, the I2 is simply dropped. That prevents the Initiator from
generating spoofed I2s with a probability that depends on the number
of pre-computed R1s.
As a concrete example, let us assume that the Responder has an array
of R1s. Each slot in the array contains a timestamp, an R1, and an
old cookie that was sent in the previous R1 that occupied that
particular slot. The Responder replaces one R1 in the array every
few minutes, thereby replacing all the R1s gradually.
To create a varying mapping function, the Responder generates a
random number every few minutes. The octets in the IP addresses and
HITs are XORed together, and finally the result is XORed with the
random number. Using pseudo-code, the function looks like the
following.
Pre-computation:
r1 := random number
Index computation:
index := r1 XOR hit_r[0] XOR hit_r[1] XOR ... XOR hit_r[15]
index := index XOR hit_i[0] XOR hit_i[1] XOR ... XOR hit_i[15]
index := index XOR ip_r[0] XOR ip_r[1] XOR ... XOR ip_r[15]
index := index XOR ip_i[0] XOR ip_i[1] XOR ... XOR ip_i[15]
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The index gives the slot used in the array.
It is possible that an Initiator receives an I1, and while it is
computing I2, the Responder deprecates an R1 and/or chooses a new
random number for the mapping function. Therefore the Responder must
remember the cookies used in deprecated R1s and the previous random
number.
To check an received I2, the Responder can use a simple algorithm,
expressed in pseudo-code as follows.
If I2.hit_r does not match my_hits, drop the packet.
index := compute_index(current_random_number, I2)
If current_cookie[index] == I2.cookie, go to cookie check.
If previous_cookie[index] == I2.cookie, go to cookie check.
index := compute_index(previous_random_number, I2)
If current_cookie[index] == I2.cookie, go to cookie check.
If previous_cookie[index] == I2.cookie, go to cookie check.
Drop packet.
cookie_check:
V := Ltrunc( SHA-1( I2.I, I2.hit_i, I2.hit_r, I2.J ), K )
if V != 0, drop the packet.
Whenever the Responder receives an I2 that fails on the index check,
it can simply drop the packet on the floor and forget about it. New
I2s with the same or other spoofed parameters will get dropped with a
reasonable probability and minimal effort.
If a Responder receives an I2 that passes the index check but fails
on the puzzle check, it should create a state indicating this. After
two or three failures the Responder should cease checking the puzzle
but drop the packets directly. This saves the Responder from the
SHA-1 calculations. Such block should not last long, however, or
there would be a danger that a legitimite Initiator could be blocked
from getting connections.
A key for the success of the defined scheme is that the mapping
function must be considerably cheaper than computing SHA-1. It also
must detect any changes in the IP addresses, and preferably most
changes in the HITs. Checking the HITs is not that essential,
though, since HITs are included in the cookie computation, too.
The effectivity of the method can be varied by varying the size of
the array containing pre-computed R1s. If the array is large, the
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probability that an I2 with a spoofed IP address or HIT happens to
map to the same slot is fairly slow. However, a large array means
that each R1 has a fairly long life time, thereby allowing an
attacker to utilize one solved puzzle for a longer time.
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Appendix E. Running HIP over IPv4 UDP
In the IPv4 world, with the deployed NAT devices, it may make sense
to run HIP over UDP. When running HIP over UDP, the following packet
structure is used. The structure is followed by the HITs, as usual.
Both the Source and Destionation port MUST be 272.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+\
| Source port | Destination port | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ >UDP
| Length | Checksum | /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<
| HIP Controls | HIP pkt Type | Ver. | Res. | >HIP
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/
It is currently undefined how the actual data transfer, using ESP, is
handled. Plain ESP may not go through all NAT devices.
It is currently FORBIDDEN to use this packet format with IPv6.
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