Network Working Group P. Jokela
Internet-Draft Ericsson Research NomadicLab
Expires: April 27, 2006 R. Moskowitz
ICSAlabs, a Division of TruSecure
Corporation
P. Nikander
Ericsson Research NomadicLab
October 24, 2005
Using ESP transport format with HIP
draft-ietf-hip-esp-01
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo specifies an Encapsulated Security Payload (ESP) based
mechanism for transmission of user data packets, to be used with the
Host Identity Protocol (HIP).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions used in this document . . . . . . . . . . . . . . 5
3. Using ESP with HIP . . . . . . . . . . . . . . . . . . . . . . 6
3.1. ESP Packet Format . . . . . . . . . . . . . . . . . . . . 6
3.2. Conceptual ESP Packet Processing . . . . . . . . . . . . . 6
3.2.1. Semantics of the Security Parameter Index (SPI) . . . 7
3.3. Security Association Establishment and Maintenance . . . . 7
3.3.1. ESP Security Associations . . . . . . . . . . . . . . 7
3.3.2. Rekeying . . . . . . . . . . . . . . . . . . . . . . . 8
3.3.3. Security Association Management . . . . . . . . . . . 9
3.3.4. Security Parameter Index (SPI) . . . . . . . . . . . . 9
3.3.5. Supported Transforms . . . . . . . . . . . . . . . . . 9
3.3.6. Sequence Number . . . . . . . . . . . . . . . . . . . 9
3.3.7. Lifetimes and Timers . . . . . . . . . . . . . . . . . 10
4. The Protocol . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. ESP in HIP . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1.1. Setting up an ESP Security Association . . . . . . . . 11
4.1.2. Updating an Existing ESP SA . . . . . . . . . . . . . 12
5. Parameter and Packet Formats . . . . . . . . . . . . . . . . . 13
5.1. New Parameters . . . . . . . . . . . . . . . . . . . . . . 13
5.1.1. ESP_INFO . . . . . . . . . . . . . . . . . . . . . . . 13
5.1.2. ESP_TRANSFORM . . . . . . . . . . . . . . . . . . . . 14
5.1.3. NOTIFY Parameter . . . . . . . . . . . . . . . . . . . 15
5.2. HIP ESP Security Association Setup . . . . . . . . . . . . 16
5.2.1. Setup During Base Exchange . . . . . . . . . . . . . . 16
5.3. HIP ESP Rekeying . . . . . . . . . . . . . . . . . . . . . 17
5.3.1. Initializing Rekeying . . . . . . . . . . . . . . . . 17
5.3.2. Responding to the Rekeying Initialization . . . . . . 18
5.4. ICMP Messages . . . . . . . . . . . . . . . . . . . . . . 18
5.4.1. Unknown SPI . . . . . . . . . . . . . . . . . . . . . 18
6. Packet Processing . . . . . . . . . . . . . . . . . . . . . . 19
6.1. Processing Outgoing Application Data . . . . . . . . . . . 19
6.2. Processing Incoming Application Data . . . . . . . . . . . 19
6.3. HMAC and SIGNATURE Calculation and Verification . . . . . 20
6.4. Processing Incoming ESP SA Initialization (R1) . . . . . . 20
6.5. Processing Incoming Initialization Reply (I2) . . . . . . 21
6.6. Processing Incoming ESP SA Setup Finalization (R2) . . . . 21
6.7. Dropping HIP Associations . . . . . . . . . . . . . . . . 21
6.8. Initiating ESP SA Rekeying . . . . . . . . . . . . . . . . 21
6.9. Processing Incoming UPDATE Packets . . . . . . . . . . . . 23
6.9.1. Processing UPDATE Packet: No Outstanding Rekeying
Request . . . . . . . . . . . . . . . . . . . . . . . 23
6.10. Finalizing Rekeying . . . . . . . . . . . . . . . . . . . 24
6.11. Processing NOTIFY Packets . . . . . . . . . . . . . . . . 25
7. Keying Material . . . . . . . . . . . . . . . . . . . . . . . 26
8. Security Considerations . . . . . . . . . . . . . . . . . . . 27
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9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
10.1. Normative references . . . . . . . . . . . . . . . . . . . 29
10.2. Informative references . . . . . . . . . . . . . . . . . . 29
Appendix A. A Note on Implementation Options . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
Intellectual Property and Copyright Statements . . . . . . . . . . 32
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1. Introduction
In the Host Identity Protocol Architecture [7], hosts are identified
with public keys. The Host Identity Protocol [5] base exchange
allows any two HIP-supporting hosts to authenticate each other and to
create a HIP association between themselves. During the base
exchange, the hosts generate a piece of shared keying material using
an authenticated Diffie-Hellman exchange.
The HIP base exchange specification [5] does not describe any
transport formats, or methods for user data, to be used during the
actual communication; it only defines that it is mandatory to
implement the Encapsulated Security Payload (ESP) [4] based transport
format and method. This document specifies how ESP is used with HIP
to carry actual user data.
To be more specific, this document specifies a set of HIP protocol
extensions and their handling. Using these extensions, a pair of ESP
Security Associations (SAs) is created between the hosts during the
base exchange. The resulting ESP Security Associations use keys
drawn from the keying material (KEYMAT) generated during the base
exchange. After the HIP association and required ESP SAs have been
established between the hosts, the user data communication is
protected using ESP. In addition, this document specifies methods to
update an existing ESP Security Association.
It should be noted that representations of host identity are not
carried explicitly in the headers of user data packets. Instead, the
ESP Security Parameter Index (SPI) is used to indicate the right host
context. The SPIs are selected during the HIP ESP setup exchange.
For user data packets, ESP SPIs (in possible combination with IP
addresses) are used indirectly to identify the host context, thereby
avoiding any additional explicit protocol headers.
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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 [1].
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3. Using ESP with HIP
The HIP base exchange is used to set up a HIP association between two
hosts. The base exchange provides two-way host authentication and
key material generation, but it does not provide any means for
protecting data communication between the hosts. In this document we
specify the use of ESP for protecting user data traffic after the HIP
base exchange. Note that this use of ESP is intended only for host-
to-host traffic; security gateways are not supported.
To support ESP use, the HIP base exchange messages require some minor
additions to the parameters transported. In the R1 packet, the
responder adds the possible ESP transforms in a new ESP_TRANSFORM
parameter before sending it to the Initiator. The Initiator gets the
proposed transforms, selects one of those proposed transforms, and
adds it to the I2 packet in an ESP_TRANSFORM parameter. In this I2
packet, the Initiator also sends the SPI value that it wants to be
used for ESP traffic flowing from the Responder to the Initiator.
This information is carried using the new ESP_INFO parameter. When
finalizing the ESP SA setup, the Responder sends its SPI value to the
Initiator in the R2 packet, again using ESP_INFO.
3.1. ESP Packet Format
The ESP specification [4] defines the ESP packet format for IPsec.
The HIP ESP packet looks exactly the same as the IPsec ESP transport
format packet. The semantics, however, are a bit different and are
described in more detail in the next subsection.
3.2. Conceptual ESP Packet Processing
ESP packet processing can be implemented in different ways in HIP.
It is possible to implement it in a way that a standards compliant,
unmodified IPsec implementation [4] can be used.
When a standards compliant IPsec implementation is used, the packet
processing may take the following steps: For outgoing packets, the
implementation recalculates upper layer checksums using HITs and,
after that, changes the packet source and destination addresses to
corresponding IP addresses. The packet is sent to the IPsec ESP for
transport mode handling and from there the encrypted packet is sent
to the network. When an ESP packet is received, the packet is first
put to the IPsec ESP transport mode handling, and after decryption,
the source and destination IP addresses are replaced with HITs and
finally, upper layer checksums are recalculated.
An alternative way to implement the packet processing is the BEET
(Bound End-to-End Tunnel) [11] mode. In BEET mode, the ESP packet is
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formatted as a transport mode packet, but the semantics of the
connection are the same as for tunnel mode. The "outer" addresses of
the packet are the IP addresses and the "inner" addresses are the
HITs. For outgoing traffic, after the packet has been encrypted, the
packet's IP header is changed to a new one, containing IP addresses
instead of HITs and the packet is sent to the network. When ESP
packet is received, the SPI value, together with the integrity
protection, allow the packet to be securely associated with the right
HIT pair. The packet header is replaces with a new header,
containing HITs and the packet is decrypted.
3.2.1. Semantics of the 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 a pair of HITs.
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 destination IP
address, uniquely identifies the receiver HIT at any given point of
time. The same SPI value may be used by several hosts. A single <
DST, SPI > value may denote different hosts and contexts at different
points of time, depending on the host that 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 [4] 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 the above 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
signaled with ESP_INFO parameters.
3.3. Security Association Establishment and Maintenance
3.3.1. ESP Security Associations
In HIP, ESP Security Associations are setup between the HIP nodes
during the base exchange [5]. Existing ESP SAs can be updated later
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using UPDATE messages. The reason for updating the ESP SA later can
be e.g. need for rekeying the SA because of sequence number rollover.
Upon setting up a HIP association, each association is linked to two
ESP SAs, one for incoming packets and one for outgoing packets. The
Initiator's incoming SA corresponds with the Responder's outgoing
one, and vice versa. The Initiator defines the SPI for the former
association, as defined in Section 3.2.1. This SA is called SA-RI,
and the corresponding SPI is called SPI-RI. Respectively, the
Responder's incoming SA corresponds with the Initiator's outgoing SA
and is called SA-IR, with the SPI being called SPI-IR.
The Initiator creates SA-RI as a part of R1 processing, before
sending out the I2, as explained in Section 6.4. The keys are
derived from KEYMAT, as defined in Section 7. The Responder creates
SA-RI as a part of I2 processing, see Section 6.5.
The Responder creates SA-IR as a part of I2 processing, before
sending out R2; see Section 6.5. The Initiator creates SA-IR when
processing R2; see Section 6.6.
The initial session keys are drawn from the generated keying
material, KEYMAT, after the HIP keys have been drawn as specified in
[5].
When the HIP association is removed, the related ESP SAs MUST also be
removed.
3.3.2. Rekeying
After the initial HIP base exchange and SA establishment, both hosts
are in the ESTABLISHED state. There are no longer Initiator and
Responder roles and the association is symmetric. In this
subsection, the party that initiates the rekey procedure is denoted
with I' and the peer with R'.
An existing HIP-created ESP SA may need updating during the lifetime
of the HIP association. This document specifies the rekeying of an
existing HIP-created ESP SA, using the UPDATE message. The ESP_INFO
parameter introduced above is used for this purpose.
I' initiates the ESP SA updating process when needed (see
Section 6.8). It creates an UPDATE packet with required information
and sends it to the peer node. The old SAs are still in use, local
policy permitting.
R', after receiving and processing the UPDATE (see Section 6.9),
generates new SAs: SA-I'R' and SA-R'I'. It does not take the new
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outgoing SA into use, but still uses the old one, so there
temporarily exists two SA pairs towards the same peer host. The SPI
for the new outgoing SA, SPI-R'I', is specified in the received
ESP_INFO parameter in the UPDATE packet. For the new incoming SA, R'
generates the new SPI value, SPI-I'R', and includes it in the
response UPDATE packet.
When I' receives a response UPDATE from R', it generates new SAs, as
described in Section 6.9: SA-I'R' and SA-R'I'. It starts using the
new outgoing SA immediately.
R' starts using the new outgoing SA when it receives traffic on the
new incoming SA. After this, R' can remove the old SAs. Similarly,
when the I' receives traffic from the new incoming SA, it can safely
remove the old SAs.
3.3.3. Security Association Management
An SA pair is indexed by the 2 SPIs and 2 HITs (both local and remote
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.
3.3.4. 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 updates the ESP SA, it provides a new inbound SPI to and
gets a new outbound SPI from its partner.
3.3.5. Supported Transforms
All HIP implementations MUST support AES [3] and HMAC-SHA-1-96 [2].
If the Initiator does not support any of the transforms offered by
the Responder, it should abandon the negotiation and inform the peer
with a NOTIFY message about a non-supported transform.
In addition to AES, all implementations MUST implement the ESP NULL
encryption algorithm. When the ESP NULL encryption is used, it MUST
be used together with SHA1 or MD5 authentication as specified in
Section 5.1.2
3.3.6. Sequence Number
The Sequence Number field is MANDATORY when ESP is used with HIP.
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Anti-replay protection MUST be used in an ESP SA established with
HIP. This means that each host MUST rekey before its sequence number
reaches 2^32, or if extended sequence numbers are used, 2^64.
In some instances, a 32-bit sequence number is inadequate. In the
ESP_TRANSFORM parameter, a peer MAY require that a 64-bit sequence
numbers be used. In this case the higher 32 bits are NOT included in
the ESP header, but are simply kept local to both peers. The 64-bit
sequence number is required in fast networks when there is a risk
that the sequence number will rollover too often. See [9].
3.3.7. Lifetimes and Timers
HIP does not negotiate any lifetimes. All ESP lifetimes are local
policy. The only lifetimes a HIP implementation MUST support are
sequence number rollover (for replay protection), and SHOULD support
timing out inactive ESP SAs. 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.
Each implementation SHOULD implement per-HIT configuration of the
inactivity timeout, allowing statically configured HIP associations
to stay alive for days, even when inactive.
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4. The Protocol
In this section, the protocol for setting up an ESP association to be
used with HIP association is described.
4.1. ESP in HIP
4.1.1. Setting up an ESP Security Association
Setting up an ESP Security Association between hosts using HIP
consists of three messages passed between the hosts. The parameters
are included in R1, I2, and R2 messages during base exchange.
Initiator Responder
I1
---------------------------------->
R1: ESP_TRANSFORM
<----------------------------------
I2: ESP_TRANSFORM, ESP_INFO
---------------------------------->
R2: ESP_INFO
<----------------------------------
Setting up an ESP Security Association between HIP hosts requires
three messages to exchange the information that is required during an
ESP communication.
The R1 message contains the ESP_TRANSFORM parameter, in which the
sending host defines the possible ESP transforms it is willing to use
for the ESP SA.
The I2 message contains the response to an ESP_TRANSFORM received in
the R1 message. The sender must select one of the proposed ESP
transforms from the ESP_TRANSFORM parameter in the R1 message and
include the selected one in the ESP_TRANSFORM parameter in the I2
packet. In addition to the transform, the host includes the ESP_INFO
parameter, containing the SPI value to be used by the peer host.
In the R2 message, the ESP SA setup is finalized. The packet
contains the SPI information required by the Initiator for the ESP
SA.
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4.1.2. Updating an Existing ESP SA
The update process is accomplished using two messages. The HIP
UPDATE message is used to update the parameters of an existing ESP
SA. The UPDATE mechanism and message is defined in [5] and the
additional parameters for updating an existing ESP SA are described
here.
The following picture shows a typical exchange when an existing ESP
SA is updated. Messages include SEQ and ACK parameters required by
the UPDATE mechanism.
H1 H2
UPDATE: SEQ, ESP_INFO [, DIFFIE_HELLMAN]
----------------------------------------------------->
UPDATE: SEQ, ACK, ESP_INFO [, DIFFIE_HELLMAN]
<-----------------------------------------------------
UPDATE: ACK
----------------------------------------------------->
The host willing to update the ESP SA creates and sends an UPDATE
message. The message contains the ESP_INFO parameter, containing the
old SPI value that was used, the new SPI value to be used, and the
index value for the keying material, giving the point from where the
next keys will be drawn. If new keying material must be generated,
the UPDATE message will also contain the DIFFIE_HELLMAN parameter,
defined in [5].
The host receiving the UPDATE message requesting update of an
existing ESP SA, MUST reply with an UPDATE message. In the reply
message, the host sends the ESP_INFO parameter containing the
corresponding values: old SPI, new SPI, and the keying material
index. If the incoming UPDATE contained a DIFFIE_HELLMAN parameter,
the reply packet MUST also contain a DIFFIE_HELLMAN parameter.
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5. Parameter and Packet Formats
In this section, new and modified HIP parameters are presented, as
well as modified HIP packets.
5.1. New Parameters
Two new HIP parameters are defined for setting up ESP transport
format associations in HIP communication and for rekeying existing
ones. Also, the NOTIFY parameter, described in [5], has two new
error parameters.
Parameter Type Length Data
ESP_INFO 65 12 Remote's old SPI,
new SPI and other info
ESP_TRANSFORM 4095 variable ESP Encryption and
Authentication Transform(s)
5.1.1. ESP_INFO
During the establishment and update of an ESP SA, the SPI value of
both hosts must be transmitted between the hosts. Additional
information that is required when the hosts are drawing keys from the
generated keying material is the index value into the KEYMAT from
where the keys are drawn. The ESP_INFO parameter is used to transmit
this information between the hosts.
During the initial ESP SA setup, the hosts send the SPI value that
they want the peer to use when sending ESP data to them. The value
is set in the New SPI field of the ESP_INFO parameter. In the
initial setup, an old value for the SPI does not exist, thus the Old
SPI value field is set to zero. The Old SPI field value may also be
zero when additional SAs are set up between HIP hosts, e.g. in case
of multihomed HIP hosts [12]. However, such use is beyond the scope
of this specification.
The Keymat index value points to the place in the KEYMAT from where
the keying material for the ESP SAs is drawn. The Keymat index value
is zero only when the ESP_INFO is sent during a rekeying process and
new keying material is generated.
During the life of an SA established by HIP, one of the hosts may
need to reset the Sequence Number to one 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.
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During the rekeying process, the ESP_INFO parameter is used to
transmit the changed SPI values and the keying material index.
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 | Keymat Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Old SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| New SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 65
Length 12
Keymat Index Index, in bytes, where to continue to draw ESP keys
from KEYMAT. If the packet includes a new
Diffie-Hellman key and the ESP_INFO is sent in an
UPDATE packet, the field MUST be zero. If the
ESP_INFO is included in base exchange messages, the
Keymat Index must have the index value of the point
from where the ESP SA keys are drawn. 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 packet MUST contain
a new Diffie-Hellman key.
Old SPI Old SPI for data sent to address(es) associated
with this SA. If this is an initial SA setup, the
Old SPI value is zero.
New SPI New SPI for data sent to address(es) associated
with this SA.
5.1.2. ESP_TRANSFORM
The ESP_TRANSFORM parameter is used during ESP SA establishment. The
first party sends a selection of transfrom families in the
ESP_TRANSFORM parameter and the peer must select one of the proposed
values and include it in the response ESP_TRANSFORM parameter.
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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 |E| Suite-ID #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Suite-ID #2 | Suite-ID #3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Suite-ID #n | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 4095
Length length in octets, excluding Type, Length, and
padding
E One if the ESP transform requires 64-bit
sequence numbers
(see
Section 3.3.6
Reserved zero when sent, ignored when received
Suite-ID defines the ESP Suite to be used
The following Suite-IDs are defined ([6],[8]):
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 parameter. The limited number of Suite-IDs sets the
maximum size of ESP_TRANSFORM parameter. The ESP_TRANSFORM MUST
contain at least one of the mandatory Suite-IDs.
Mandatory implementations: ESP-AES-CBC with HMAC-SHA1 and ESP-NULL
with HMAC-SHA1.
5.1.3. NOTIFY Parameter
The HIP base specification defines a set of NOTIFY error types. The
following error types are required for describing errors in ESP
Transform crypto suites during negotiation.
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NOTIFY PARAMETER - ERROR TYPES Value
------------------------------ -----
NO_ESP_PROPOSAL_CHOSEN 18
None of the proposed ESP Transform crypto suites was
acceptable.
INVALID_ESP_TRANSFORM_CHOSEN 19
The ESP Transform crypto suite does not correspond to
one offered by the responder.
5.2. HIP ESP Security Association Setup
The ESP Security Association is set up during the base exchange. The
following subsections define the ESP SA setup procedure both using
base exchange messages (R1, I2, R2) and using UPDATE messages.
5.2.1. Setup During Base Exchange
5.2.1.1. Modifications in R1
The ESP_TRANSFORM contains the ESP modes supported by the sender, in
the order of preference. All implementations MUST support AES [3]
with HMAC-SHA-1-96 [2].
The following figure shows the resulting R1 packet layout.
The HIP parameters for the R1 packet:
IP ( HIP ( [ R1_COUNTER, ]
PUZZLE,
DIFFIE_HELLMAN,
HIP_TRANSFORM,
ESP_TRANSFORM,
HOST_ID,
[ ECHO_REQUEST, ]
HIP_SIGNATURE_2 )
[, ECHO_REQUEST ])
5.2.1.2. Modifications in I2
The ESP_INFO contains the sender's SPI for this association as well
as the keymat index from where the ESP SA keys will be drawn. The
Old SPI value is set to zero.
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The ESP_TRANSFORM contains the ESP mode selected by the sender of R1.
All implementations MUST support AES [3] with HMAC-SHA-1-96 [2].
The following figure shows the resulting I2 packet layout.
The HIP parameters for the I2 packet:
IP ( HIP ( ESP_INFO,
[R1_COUNTER,]
SOLUTION,
DIFFIE_HELLMAN,
HIP_TRANSFORM,
ESP_TRANSFORM,
ENCRYPTED { HOST_ID },
[ ECHO_RESPONSE ,]
HMAC,
HIP_SIGNATURE
[, ECHO_RESPONSE] ) )
5.2.1.3. Modifications in R2
The R2 contains an ESP_INFO parameter, which has the SPI value of the
sender of the R2 for this association. The ESP_INFO also has the
keymat index value specifying where the ESP SA keys are drawn.
The following figure shows the resulting R2 packet layout.
The HIP parameters for the R2 packet:
IP ( HIP ( ESP_INFO, HMAC_2, HIP_SIGNATURE ) )
5.3. HIP ESP Rekeying
In this section, the procedure for rekeying an existing ESP SA is
presented.
5.3.1. Initializing Rekeying
When HIP is used with ESP, the UPDATE packet is used to initiate
rekeying. The UPDATE packet MUST carry an ESP_INFO and MAY carry a
DIFFIE_HELLMAN parameter.
Intermediate systems that use the SPI will have to inspect HIP
packets for those that carry rekeying information. The packet is
signed for the benefit of the intermediate systems. Since
intermediate systems may need the new SPI values, the contents cannot
be encrypted.
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The following figure shows the contents of a rekeying initialization
UPDATE packet.
The HIP parameters for the UPDATE packet initiating rekeying:
IP ( HIP ( ESP_INFO,
SEQ,
[DIFFIE_HELLMAN, ]
HMAC,
HIP_SIGNATURE ) )
5.3.2. Responding to the Rekeying Initialization
The UPDATE ACK is used to acknowledge the received UPDATE rekeying
initialization. The acknowledgement UPDATE packet MUST carry an
ESP_INFO and MAY carry a DIFFIE_HELLMAN parameter.
Intermediate systems that use the SPI will have to inspect HIP
packets for packets carrying rekeying information. The packet is
signed for the benefit of the intermediate systems. Since
intermediate systems may need the new SPI values, the contents cannot
be encrypted.
The following figure shows the contents of a rekeying acknowledgement
UPDATE packet.
The HIP parameters for the UPDATE packet:
IP ( HIP ( ESP_INFO,
SEQ,
ACK,
[ DIFFIE_HELLMAN, ]
HMAC,
HIP_SIGNATURE ) )
5.4. ICMP Messages
The ICMP message handling is mainly described in the HIP base
specification [5]. In this section, we describe the actions related
to ESP security associations.
5.4.1. Unknown SPI
If a HIP implementation receives an ESP packet that has an
unrecognized SPI number, it MAY respond (subject to rate limiting the
responses) with an ICMP packet with type "Parameter Problem", with
the Pointer pointing to the the beginning of SPI field in the ESP
header.
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6. Packet Processing
Packet processing is mainly defined in the HIP base specification
[5]. This section describes the changes and new requirements for
packet handling when the ESP transport format is used. Note that all
HIP packets (currently protocol 99) MUST bypass ESP processing.
6.1. Processing Outgoing Application Data
Outgoing application data handling is specified in the HIP base
specification [5]. When ESP transport format is used, and there is
an active HIP session for the given < source, destination > HIT pair,
the outgoing datagram is protected using the ESP security
association. In a typical implementation, this will result in a
BEET-mode ESP packet being sent. BEET-mode [11] was introduced above
in Section 3.2.
1. Detect the proper ESP SA using the HITs in the packet header or
other information associated with the packet
2. Process the packet normally, as if the SA was a transport mode
SA.
3. Ensure that the outgoing ESP protected packet has proper IP
header format depending on the used IP address family, and proper
IP addresses in its IP header, e.g., by replacing HITs left by
the ESP processing. Note that this placement of proper IP
addresses MAY also be performed at some other point in the stack,
e.g., before ESP processing.
6.2. Processing Incoming Application Data
Incoming HIP user data packets 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 basic incoming data handling is specified in the HIP base
specification. Additional steps are required when ESP is used for
protecting the data traffic. 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 ESP SA using 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 ICMP packet as defined in Section 5.4. How to handle
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lost state is an implementation issue.
2. If the SPI matches with an active HIP-based ESP SA, the IP
addresses in the datagram are replaced with the HITs associated
with the SPI. Note that this IP-address-to-HIT conversion step
MAY also be performed at some other point in the stack, e.g.,
after ESP processing. Note also that if the incoming packet has
IPv4 addresses, the packet must be converted to IPv6 format
before replacing the addresses with HITs (such that the transport
checksum will pass if there are no errors).
3. The transformed packet is next 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 successful ESP decryption and verification is a
datagram with the associated HITs as source and destination.
4. The datagram is delivered to the upper layer. Demultiplexing the
datagram to the right upper layer socket is performed as usual,
except that the HITs are used in place of IP addresses during the
demultiplexing.
6.3. HMAC and SIGNATURE Calculation and Verification
The new HIP parameters described in this document, ESP_INFO and
ESP_TRANSFORM, must be protected using HMAC and signature
calculations. In a typical implementation, they are included in R1,
I2, R2, and UPDATE packet HMAC and SIGNATURE calculations as
described in [5].
6.4. Processing Incoming ESP SA Initialization (R1)
The ESP SA setup is initialized in the R1 message. The receiving
host (Initiator) select one of the ESP transforms from the presented
values. If no suitable value is found, the negotiation is
terminated. The selected values are subsequently used when
generating and using encryption keys, and when sending the reply
packet. If the proposed alternatives are not acceptable to the
system, it may abandon the ESP SA establishment negotiation, or it
may resend the I1 message within the retry bounds.
After selecting the ESP transform, and performing other R1
processing, the system prepares and creates an incoming 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.
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6.5. Processing Incoming Initialization Reply (I2)
The following steps are required to process the incoming ESP SA
initialization replies in I2. The steps below assume that the I2 has
been accepted for processing (e.g., has not been dropped due to HIT
comparisons as described in [5]).
o The ESP_TRANSFORM parameter is verified and it MUST contain a
single value in the parameter and it MUST match one of the values
offered in the initialization packet.
o The ESP_INFO New SPI field is parsed to obtain the SPI that will
be used for the Security Association outbound from the Responder
and inbound to the Initiator. For this initial ESP SA
establishment, the Old SPI value MUST be zero. The Keymat Index
field MUST contain the index value to the KEYMAT from where the
ESP SA keys are drawn.
o The system prepares and creates both incoming and outgoing ESP
security associations.
o Upon successful processing of the initialization reply message,
the possible old Security Associations (as left over from an
earlier incarnation of the HIP association) are dropped and the
new ones are installed, and a finalizing packet, R2, is sent.
Possible ongoing rekeying attempts are dropped.
6.6. Processing Incoming ESP SA Setup Finalization (R2)
Before the ESP SA can be finalized, the ESP_INFO New SPI field is
parsed to obtain the SPI that will be used for the ESP Security
Association inbound to the sender of the finalization message R2.
The system uses this SPI to create or activate the outgoing ESP
security association used for sending packets to the peer.
6.7. Dropping HIP Associations
When the system drops a HIP association, as described in the HIP base
specification, the associated ESP SAs MUST also be dropped.
6.8. Initiating ESP SA Rekeying
During ESP SA rekeying, the hosts draw new keys from the existing
keying material, or a new keying material is generated from where the
new keys are drawn.
A system may initiate the SA rekeying procedure at any time. It MUST
initiate a rekey if its incoming ESP sequence counter is about to
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overflow. The system MUST NOT replace its keying material until the
rekeying packet exchange successfully completes.
Optionally, a system may include a new Diffie-Hellman key for use in
new KEYMAT generation. New KEYMAT generation occurs prior to drawing
the new keys.
The rekeying procedure uses the UPDATE mechanism defined in [5].
Because each peer must update its half of the security association
pair (including new SPI creation), the rekeying process requires that
each side both send and receive an UPDATE. A system will then rekey
the ESP SA when it has sent parameters to the peer and has received
both an ACK of the relevant UPDATE message and corresponding peer's
parameters. It may be that the ACK and the required HIP parameters
arrive in different UPDATE messages. This is always true if a system
does not initiate ESP SA update but responds to an update request
from the peer, but may also occur if two systems initiate update
nearly simultaneously. In such a case, if the system has an
outstanding update request, it saves the one parameter and waits for
the other before completing rekeying.
The following steps define the processing rules for initiating an ESP
SA update:
1. The system decides whether to continue to use the existing KEYMAT
or to generate new KEYMAT. In the latter case, the system MUST
generate a new Diffie-Hellman public key.
2. The system creates an UPDATE packet, which contains the ESP_INFO
parameter. In addition, the host may include the optional
DIFFIE_HELLMAN parameter. If the UDPATE contains the
DIFFIE_HELLMAN parameter, the Keymat Index in the ESP_INFO
parameter MUST be zero, and the Diffie-Hellman group ID must be
unchanged from that used in the initial handshake. If the UPDATE
does not contain DIFFIE_HELLMAN, the ESP_INFO Keymat Index MUST
be greater or equal to the index of the next byte to be drawn
from the current KEYMAT.
3. The system sends the UPDATE packet. For reliability, the
underlying UPDATE retransmission mechanism SHOULD be used.
4. The system MUST NOT delete its existing SAs, but continue using
them if its policy still allows. The rekeying procedure SHOULD
be initiated early enough to make sure that the SA replay
counters do not overflow.
5. In case a protocol error occurs and the peer system acknowledges
the UPDATE but does not itself send an ESP_INFO, the system may
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not finalize the outstanding ESP SA update request. To guard
against this, a system MAY re-initiate the ESP SA update
procedure after some time waiting for the peer to respond, or it
MAY decide to abort the ESP SA after waiting for an
implementation-dependent time. The system MUST NOT keep an
oustanding ESP SA update request for an indefinite time.
To simplify the state machine, a host MUST NOT generate new UPDATEs
while it has an outstanding ESP SA update request, unless it is
restarting the update process.
6.9. Processing Incoming UPDATE Packets
When a system receives an UPDATE packet, it must be processed if the
following conditions hold:
1. A corresponding HIP association must exist. This is usually
ensured by the underlying UPDATE mechanism.
2. The state of the HIP association is ESTABLISHED or R2-SENT.
If the above conditions hold, the following steps define the
conceptual processing rules for handling the received UPDATE packet:
1. If the received UPDATE contains a DIFFIE_HELLMAN parameter, the
received Keymat Index MUST be zero and the Group ID must match
the Group ID in use on the association. If this test fails, the
packet SHOULD be dropped and the system SHOULD log an error
message.
2. If there is no outstanding rekeying request, the packet
processing continues as specified in Section 6.9.1.
3. If there is an outstanding rekeying request, the UPDATE MUST be
acknowledged, the received ESP_INFO (and possibly DIFFIE_HELLMAN)
parameters must be saved, and the packet processing continues as
specified in Section 6.10.
6.9.1. Processing UPDATE Packet: No Outstanding Rekeying Request
The following steps define the conceptual processing rules for
handling a received UPDATE packet with ESP_INFO parameter:
1. The system consults its policy to see if it needs to generate a
new Diffie-Hellman key, and generates a new key (with same Group
ID) if needed. The system records any newly generated or
received Diffie-Hellman keys, for use in KEYMAT generation upon
finalizing the ESP SA update.
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2. If the system generated new Diffie-Hellman key in the previous
step, or it received a DIFFIE_HELLMAN parameter, it sets ESP_INFO
Keymat Index to zero. Otherwise, the ESP_INFO Keymat Index MUST
be greater or equal to the index of the next byte to be drawn
from the current KEYMAT. In this case, it is RECOMMENDED that
the host use the Keymat Index requested by the peer in the
received ESP_INFO.
3. The system creates an UPDATE packet, which contains an ESP_INFO
parameter, and the optional DIFFIE_HELLMAN parameter.
4. The system sends the UPDATE packet and stores any received
ESP_INFO, and DIFFIE_HELLMAN parameters. At this point, it only
needs to receive an acknowledgement for the sent UPDATE to finish
ESP SA update. In the usual case, the acknowledgement is handled
by the underlying UPDATE mechanism.
6.10. Finalizing Rekeying
A system finalizes rekeying when it has both received the
corresponding UPDATE acknowledgement packet from the peer and it has
successfully received the peer's UPDATE. The following steps are
taken:
1. If the received UPDATE messages contains a new Diffie-Hellman
key, the system has a new Diffie-Hellman key from initiating ESP
SA update, or both, the system generates new KEYMAT. If there is
only one new Diffie-Hellman key, the old existing 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 UPDATE
included a Diffie-Hellman key or not. If the system did not
generate new KEYMAT, it uses the greater Keymat Index of the two
(sent and received) ESP_INFO parameters.
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 received in an ESP_INFO parameter. The SPI for the
incoming SA was generated when the ESP_INFO was sent to the peer.
The order of the keys retrieved from the KEYMAT during rekeying
process is similar to that described in Section 7. Note, that
only IPsec ESP keys are retrieved during rekeying process, not
the HIP keys.
4. The system cancels any timers protecting the UPDATE.
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5. The system starts to send to the new outgoing SA and prepares to
start receiving data on the new incoming SA.
6.11. Processing NOTIFY Packets
The processing of NOTIFY packets is described in the HIP base
specification.
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7. Keying Material
The keying material is generated as described in the HIP base
specification. During the base exchange, the initial keys are drawn
from the generated material. After the HIP association keys have
been drawn, the ESP keys are drawn in the following order:
SA-gl ESP encryption key for HOST_g's outgoing traffic
SA-gl ESP authentication key for HOST_g's outgoing traffic
SA-lg ESP encryption key for HOST_l's outgoing traffic
SA-lg ESP authentication key for HOST_l's outgoing traffic
HOST_g denotes the host with the greater HIT value, and HOST_l the
host with the lower HIT value. When HIT values are compared, they
are interpreted as positive (unsigned) 128-bit integers in network
byte order.
The four HIP keys are only drawn from KEYMAT during a HIP I1->R2
exchange. Subsequent rekeys using UPDATE will only draw the four ESP
keys from KEYMAT. Section 6.9 describes the rules for reusing or
regenerating KEYMAT based on the rekeying.
The number of bits drawn for a given algorithm is the "natural" size
of the keys. For the mandatory algorithms, the following sizes
apply:
AES 128 bits
SHA-1 160 bits
NULL 0 bits
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8. Security Considerations
In this document the usage of ESP [4] between HIP hosts to protect
data traffic is introduced. The Security Considerations for ESP are
discussed in the ESP specification.
There are different ways to establish an ESP Security Association
between two nodes. This can be done, e.g. using IKE [10]. This
document specifies how Host Identity Protocol is used to establish
ESP Security Associations.
The following issues are new, or changed from the standard ESP usage:
o Initial keying material generation
o Updating the keying material
The initial keying material is generated using the Host Identity
Protocol [5] using Diffie-Hellman procedure. This document extends
the usage of UDPATE packet, defined in the base specification, to
modify existing ESP SAs. The hosts may rekey, i.e. force the
generation of new keying material using Diffie-Hellman procedure.
The initial setup of ESP SA between the hosts is done during the base
ecxhange and the message exchange is protected with using methods
provided by base exchange. Changing of connection parameters means
basically that the old ESP SA is removed and a new one is generated
once the UPDATE message exchange has been completed. The message
exchange is protected using the HIP association keys. Both HMAC and
signing of packets is used.
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9. IANA Considerations
This document defines additional parameters for the Host Identity
Protocol [5]. These parameters are defined in Section 5.1.1 and
Section 5.1.2 with the following numbers:
o ESP_INFO is 65.
o ESP_TRANSFORM is 4095.
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10. References
10.1. Normative references
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within ESP
and AH", RFC 2404, November 1998.
[3] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
Algorithm and Its Use with IPsec", RFC 3602, September 2003.
[4] Kent, S., "IP Encapsulating Security Payload (ESP)",
draft-ietf-ipsec-esp-v3-10 (work in progress), March 2005.
[5] Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-03
(work in progress), June 2005.
[6] Schiller, J., "Cryptographic Algorithms for use in the Internet
Key Exchange Version 2", draft-ietf-ipsec-ikev2-algorithms-05
(work in progress), April 2004.
[7] Moskowitz, R. and P. Nikander, "Host Identity Protocol
Architecture", draft-ietf-hip-arch-03 (work in progress),
August 2005.
[8] Schneier, B., "Applied Cryptography Second Edition: protocols
algorithms and source in code in C", 1996.
10.2. Informative references
[9] Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", draft-ietf-ipsec-rfc2401bis-06 (work in progress),
April 2005.
[10] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[11] Nikander, P., "A Bound End-to-End Tunnel (BEET) mode for ESP",
draft-nikander-esp-beet-mode-03 (work in progress), June 2005.
[12] Nikander, P., "End-Host Mobility and Multihoming with the Host
Identity Protocol", draft-ietf-hip-mm-02 (work in progress),
July 2005.
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Appendix A. A Note on Implementation Options
It is possible to implement this specification in multiple different
ways. As noted above, one possible way of implementing is to rewrite
IP headers below IPsec. In such an implementation, IPsec is used as
if it was processing IPv6 transport mode packets, with the IPv6
header containing HITs instead of IP addresses in the source and
destionation address fields. In outgoing packets, after IPsec
processing, the HITs are replaced with actual IP addresses, based on
the HITs and the SPI. In incoming packets, before IPsec processing,
the IP addresses are replaced with HITs, based on the SPI in the
incoming packet. In such an implementation, all IPsec policies are
based on HITs and the upper layers only see packets with HITs in the
place of IP addresses. Consequently, support of HIP does not
conflict with other use of IPsec as long as the SPI spaces are kept
separate.
Another way for implementing is to use the proposed BEET mode (A
Bound End-to-End mode for ESP) [11]. The BEET mode provides some
features from both IPsec tunnel and transport modes. The HIP uses
HITs as the "inner" addresses and IP addresses as "outer" addresses
like IP addresses are used in the tunnel mode. Instead of tunneling
packets between hosts, a conversion between inner and outer addresses
is made at end-hosts and the inner address is never sent in the wire
after the initial HIP negotiation. BEET provides IPsec transport
mode syntax (no inner headers) with limited tunnel mode semantics
(fixed logical inner addresses - the HITs - and changeable outer IP
addresses).
Compared to the option of implementing the required address rewrites
outside of IPsec, BEET has one implementation level benefit. The
BEET-way of implementing the address rewriting keeps all the
configuration information in one place, at the SADB. On the other
hand, when address rewriting is implemented separately, the
implementation must make sure that the information in the SADB and
the separate address rewriting DB are kept in synchrony. As a
result, the BEET mode based way of implementing is RECOMMENDED over
the separate implementation.
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Authors' Addresses
Petri Jokela
Ericsson Research NomadicLab
JORVAS FIN-02420
FINLAND
Phone: +358 9 299 1
Email: petri.jokela@nomadiclab.com
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
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