Host Identity Protocol T. Heer, Ed.
Internet-Draft K. Wehrle
Intended status: Experimental Distributed Systems Group, RWTH
Expires: January 8, 2009 Aachen University
M. Komu
HIIT
July 7, 2008
End-Host Authentication for HIP Middleboxes
draft-heer-hip-middle-auth-01
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
The Host Identity Protocol [RFC2119]is a signaling protocol for
secure communication, mobility, and multihoming by introducing a
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cryptographic namespace. This document specifies an extension for
HIP that enables middleboxes to unambiguously verify the identities
of hosts that communicate across them. This extension enables
middleboxes to verify the liveness and freshness of a HIP association
and, thus, enables reliable and secure access control in middleboxes.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
Notation
[x] indicates that x is optional.
{x} indicates that x is under signature.
Initiator is the host which initiates a HIP association
(cf. HIP base protocol).
Responder is the host which responds to the INITIATOR
(cf. HIP base protocol).
--> signifies "Initiator to Responder" communication.
<-- signifies "Responder to Initiator" communication.
Figure 1
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Authentication and Replay Attacks . . . . . . . . . . . . 5
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Signed Middlebox Nonces . . . . . . . . . . . . . . . . . 6
2.2. Identity Verification by Middleboxes . . . . . . . . . . . 8
2.3. Failure Signaling . . . . . . . . . . . . . . . . . . . . 13
2.4. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 13
2.5. HIP Parameters . . . . . . . . . . . . . . . . . . . . . . 13
3. Security Services for the HIP Control Channel . . . . . . . . 16
3.1. Adversary model and Security Services . . . . . . . . . . 16
4. Security Services for the HIP Payload Channel . . . . . . . . 17
4.1. Access Control . . . . . . . . . . . . . . . . . . . . . . 18
4.2. Resource allocation . . . . . . . . . . . . . . . . . . . 19
5. Security Considerations . . . . . . . . . . . . . . . . . . . 19
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
8. Normative References . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . . . . 22
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1. Introduction
The Host Identity Protocol (HIP) introduces a new cryptographic
namespace, based on public keys, in order to secure Internet
communication. This namespace allows hosts to authenticate their
peers. HIP was designed to be middlebox-friendly and allows
middleboxes to inspect HIP control traffic. Examples of such
middleboxes are firewalls and Network Address Translators (NATs).
In this context, one can distinguish HIP-aware middleboxes, which
were designed to process HIP packets, and other middleboxes, which
are not aware of the Host Identity Protocol. This document addresses
only HIP-aware middleboxes while the behavior of HIP in combination
with non-HIP-aware middleboxes is specified in
[I-D.ietf-hip-nat-traversal]. Moreover, the scope of this document
is restricted to middleboxes that use HIP in order to enforce access
control and, thus, need to authenticate the communicating peers that
send traffic over the middlebox. The class of middleboxes this
document focuses on does not require the end-host to establish an
explicit registration with the middlebox. HIP behavior for
interacting and registering to such middleboxes is specified in
[I-D.ietf-hip-registration]. Thus, we focus on middleboxes that
build their state based on packets they forward.
An example of such a middlebox is a firewall that only allows traffic
from certain hosts to traverse. We assume that access control is
performed based on Host Identities (HIs). Such an authenticating
middlebox needs to observe the HIP Base EXchange (BEX) or a HIP
mobility update [I-D.ietf-hip-mm] and check the Host Identifiers
(HIs) in the packets.
Along the lines of [I-D.irtf-hiprg-nat], an authentication solution
for middleboxes must have some vital properties. For one, the
middlebox must be able to unambiguously identify one or both of the
communicating peers. Additionally, the solution must not allow for
new attacks against the middlebox. This document specifies a HIP
extension that allows middleboxes to participate in the HIP handshake
and the HIP update process in order to enable these middleboxes to
reliably verify the identities of the communicating peers. To this
end, this HIP extension defines how middleboxes can interact with
end-hosts in order to verify their identities.
Verifying public-key (PK) signatures is costly in terms of CPU
cycles. Thus, in addition to authentication capabilities, it is also
necessary to provide middleboxes with a way of defending against
resource-exhaustion attacks that target PK signature verification.
This document defines how middleboxes can utilize the HIP puzzle
mechanism defined in [I-D.ietf-hip-base] to slow down resource-
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exhaustion attacks.
The presented authentication extension only targets the HIP control
channel. Additional security considerations and possible security
services for the HIP payload channel are discussed in Section 4.
1.1. Authentication and Replay Attacks
Middleboxes may need to verify the HIs in the HIP base exchange
messages to perform access control based on Host Identities.
However, passive verification of HIs in the messages is not
sufficient to verify the identity of an end-host because of replay
attacks. The basic HIP protocol as specified in [I-D.ietf-hip-base]
does not provide adequate protection against these attacks.
To illustrate the need for additional security requirements with HIP-
aware middleboxes, we briefly outline a possible replay attack
targeted at middleboxes. Assume that a middlebox M checks HIP HIs in
order to restrict traffic passing through the box. Further assume
that the legitimate owner of Host Identity Tag (HIT) X establishes a
HIP association with the legitimate owner of HIT Y at some point in
time and an attacker A overhears the base exchange and records it.
Note that it is not required that the middlebox M is on the
communication path between the peers at that time.
At some later point in time, Attacker A collaborates with another
attacker B. They replay the very same BEX with the middlebox M on the
communication path. The middlebox has no way to distinguish
legitimate hosts X and Y from the attackers A and B as it can only
overhear the BEX passively and does not participate in the
authentication process. As the attackers overheard the SPI numbers,
they can bypass the middlebox with "fake" ESP packets with valid ESP
numbers. Since the middleboxes do not know the integrity and
encryption keys for ESP, they cannot distinguish valid ESP packets
from fake ones. Hence, collaborating attackers can use any recorded
BEX to falsely authenticate to the middlebox and thus impersonate any
host. This is problematic in cases in which the middlebox needs to
know the identity of the peers that communicate across it. Examples
for such cases are access control, logging of activities, and
accounting for traffic volume or connection duration.
This attack scenario is not addressed by the current HIP
specifications. Therefore, this document specifies a HIP extension
that allows middleboxes to defend against it.
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2. Protocol Overview
This section gives an overview of the interaction between hosts and
authenticating middleboxes. This document describes a framework that
middleboxes can use to implement authentication of end-hosts and
leaves its further use to other documents and to middlebox
implementors.
2.1. Signed Middlebox Nonces
The described attack scenario shows the necessity for unambiguous
end-host identity verification by middleboxes. Relying on nonces
generated by the end-hosts is not possible because middleboxes cannot
verify the freshness of these nonces. Introducing time-stamps
restricts the attack to a certain time frame but requires global time
synchronization.
The following sections specify how HIP hosts can prove their identity
by performing a challenge-response protocol between the middlebox and
the end-hosts. As the challenge, the middlebox adds information
(e.g. nonces) to HIP control packets which the end-hosts sign with
public-key (PK) signatures and echo back.
The challenge-response mechanism is similar to the ECHO_REQUEST/
ECHO_RESPONSE mechanism used by HIP end-hosts to authenticate their
peers. It assumes that the end-hosts exchange at least two HIP
packets with each other. The middlebox adds an ECHO_REQUEST_M
parameter to the first HIP control packet that contains a nonce. The
peer host receives the first packet and processes it normally.
However, the peer will also include an ECHO_RESPONSE_M in the second
message which contains the nonce from the ECHO_REQUEST_M. Before
sending the second message, the peer also signs it to prove that it
is in the possession of the private key that corresponds its HI.
The middlebox can either verify the identity of the initiator, the
responder, or both peers, depending on the purpose of the middlebox.
The choice which authentication is required left to middlebox
implementers.
2.1.1. ECHO_REQUEST_M
Middleboxes MAY add ECHO_REQUEST_M parameters to the the R1, I2, and
to any UPDATE packet. This parameter contains an opaque data block
of variable size which the middlebox uses to carry arbitrary data.
The HIP packets allowed to carry middlebox nonces may contain
multiple ECHO_REQUEST_M parameters. As all middleboxes on the path
may add ECHO_REQUEST_M parameters, the length of the data field of
each parameter SHOULD not exceed a maximum of 32 bytes. The total
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length of the packets SHOULD not exceed 1280 bytes to avoid IPv6
fragmentation.
The middleboxes add the ECHO_REQUEST_M parameter to the unprotected
part of a HIP message. Thus it does not corrupt any HMAC or public-
key signatures. However, the middlebox MUST recompute the IP- and
HIP header checksums as defined in [I-D.ietf-hip-base] and the UDP
headers of UDP encapsulated HIP packets as defined in
[I-D.ietf-hip-nat-traversal].
An end-host that receives a HIP control packet containing one or
multiple ECHO_REQUEST_M parameters must copy the contents of each
parameter without modification to an ECHO_RESPONSE_M parameter. This
end-host MUST send this parameter within the signed part of its
reply. Note that middleboxes MAY also rewrite the
ECHO_REQUEST_UNSIGNED parameter as specified in [I-D.ietf-hip-base]
when the receiver of the parameter does not have to sign the contents
of the ECHO_REQUEST.
Middleboxes can delay state creation by utilizing the ECHO_RESPONSE_M
and ECHO_REQUEST_M parameter by hiding encrypted or otherwise
protected information about previous authentication steps in the
opaque blob.
2.1.2. ECHO_RESPONSE_M
When a middlebox injects an opaque blob of data via an ECHO_REQUEST_M
parameter, it expects to receive the same data without modification
as part of an ECHO_RESPONSE_M parameter in a subsequent packet. The
opaque data MUST be copied as it is from the corresponding
ECHO_REQUEST_M parameter. In the case of multiple ECHO_REQUEST_M
parameters, their order MUST be preserved by the corresponding
ECHO_RESPONSE_M parameters.
The ECHO_REQUEST_M and ECHO_RESPONSE_M parameters MAY be used for any
purpose, in particular when a middlebox needs to carry state
information in a HIP packet and receive it in a subsequent response
packet. The ECHO_RESPONSE_M MUST be covered by the HIP_SIGNATURE.
The ECHO_RESPONSE_M parameter is non-critical. Depending on its
local policy, a middlebox can react differently on a missing
ECHO_RESPONSE_M parameter. Possible actions range from degraded or
restricted service such as bandwidth limitation up to refusing
connections and reporting access violations.
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2.1.3. Middlebox Puzzles
As PK operations are costly in terms of CPU cycles, a middlebox needs
to defend itself against resource-exhaustion attacks. The HIP base
protocol [I-D.ietf-hip-base] specifies a puzzle mechanism to protect
the Responder from I2 floods that require numerous public-key
operations. However, middleboxes cannot utilize this mechanism as
there is no defense against a collaborative replay attack, which
involves a malicious Initiator and a malicious Responder. This
section specifies how middleboxes can utilize the puzzle mechanism to
add their own puzzles to R1, I2, and any UPDATE packets. This allows
middleboxes to shelter against Denial of Service (DoS) attacks on PK
verification.
To defend against such attacks, a middlebox adds a puzzle in a
PUZZLE_M parameter to I2, R2 and UPDATE packets. The destination
end-host of the HIP control packet must solve it.
As a puzzle increases the delay and computational cost for
establishing or updating a HIP association, a middlebox SHOULD only
add puzzles to packets when it is under attack. Moreover,
middleboxes SHOULD distinguish attack directions. If the majority of
the CPU load is caused by verifying HIP control messages that arrive
from a certain interface, middleboxes MAY add puzzles to HIP control
packets that leave the interface. The middlebox chooses the
difficultly of the puzzle according to its load and local policies.
Middleboxes MAY decide to use just the PUZZLE_M parameter instead of
using PUZZLE_M in combination with ECHO_REQUEST_M because the
PUZZLE_M parameter also contains an opaque data field that guarantees
the freshness of the signature. However, the opaque data field in
the PUZZLE_M and the corresponding SOLUTION_M parameter is restricted
to 6 bytes which may not be sufficient for all purposes.
2.2. Identity Verification by Middleboxes
This section describes how middleboxes can influence the BEX and the
HIP update process in order to verify the identity of the HIP end-
hosts.
2.2.1. Identity Verification During BEX
Middleboxes MAY add ECHO_REQUEST_M and PUZZLE_M parameters to R1 and
I2 packets in order to verify the identities of the participating
parties. Middleboxes can choose either to authenticate the
Initiator, the Responder, or both. Middleboxes MUST NOT add
ECHO_REQUEST_M or PUZZLE_M parameters to I1 messages because this
would expose the Responder to DoS attacks. Thus, middleboxes MUST
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let unauthenticated and minimal I1 packets traverse. Minimal means
that the packet MUST NOT contain more than the minimal set of
parameters specified by HIP standards or internet drafts. In
particular, the I1 packet MUST NOT contain any attached payload.
Figure 1 illustrates the authentication process during the BEX.
Middlebox authentication of a HIP base exchange.
Main path:
Initiator Middlebox Responder
.-----------------.
I1 | | I1
-----------------> | |---------------------------->
| |
R1, + EQ1, [PM1] | Add EQ1, PM1 | R1
<----------------- | |<----------------------------
| |
I2, {ER1, [SM1]} | Verify SM1, EQ1 | I2, {ER1, [SM1]} + EQ2, [PM2]
-----------------> | Add EQ2, PM2 |---------------------------->
| |
| |
R2, {ER2, [SM2]} | Verify SM2, ER2 | R2, {ER2, [SM2]}
<----------------- | |<-----------------------------
'-----------------'
EQ: Middlebox Echo reQuest
ER: Middlebox Echo Response
PM: Puzzle of the Middlebox
SM: Solution of Middlebox puzzle
Figure 2
2.2.2. Identity Verification During Mobility Updates
HIP rekeying, mobility and multihoming UPDATE mechanisms for non-
NATted environments are described in [I-D.ietf-hip-mm]. This section
describes how middleboxes process UPDATE messages in non-NATted
environments and leave NATted environments for future revisions of
the draft.
The middleboxes can apply middlebox nonces and puzzles to mobility
related HIP control messages in the case where both end-hosts are
single-homed. The middlebox nonces and puzzles can be applied both
ways as the UPDATE process consists of three packets (U1, U2, U3)
which all traverse through the same middlebox as shown in Figure 3
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I).
In cases in which fewer packets are used for updating an association
the following rule applies.
RESPONSE RULE:
A HIP host, receiving an ECHO_REQUEST_M MUST reply an ECHO_RESPONSE_M
in its next UPDATE packet. If no further UPDATE packets are
necessary to complete the update procedure, an additional UPDATE
packet containing the ECHO_RESPONSE_M MUST be sent.
Middlebox authentication of a HIP mobility update over different
paths.
Initiator Middlebox 1 Responder
.------.
U1 | | U1 + EQ1, [PM1]
-----------------------------> | | ---------------------------->
| |
U2, {ER1, [SM1]} + EQ2, [PM2] | | U2, {ER1, [SM1]}
<----------------------------- |OK | <----------------------------
| |
U3, {ER2, [SM2]} | | U3, {ER2, [SM2]}
-----------------------------> | OK| ---------------------------->
'------'
EQ: Middlebox Echo reQuest
ER: Middlebox Echo Response
PM: Puzzle of the Middlebox
SM: Solution of Middlebox puzzle
Figure 3
Middlebox 1 can verify the identity of the Responder by checking its
PK signature and the presence of the ECHO_RESPONSE_M in the U2
packet. If necessary, the middlebox MAY add an ECHO_REQUEST_M for
the Initiator of the update. The middlebox can verify the
Initiator's identity by verifying its signature and the
ECHO_RESPONSE_M in the U3 packet.
A middlebox that is not located on the path between preferred
locators of the HIP end-hosts does not receive the U1 message.
Therefore, it will not recognize any ER1 or SM1 in the second UPDATE
packet. Thus, if a middlebox encounters non-matching or missing
ECHO_RESPONSE_M parameters, the middlebox SHOULD ignore these.
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When receiving an UPDATE message with an ECHO_REQUEST_M, a HIP host
SHOULD send an UPDATE message containing the corresponding
ECHO_RESPONSE_M covered by a HIP_SIGNATURE parameter. Otherwise the
middlebox may refuse to make the communication path available to the
HIP host.
2.2.3. Identity Verification for Multihomed Mobility Updates
Multihomed hosts may use multiple communication paths during an HIP
mobility update. Depending on whether the middlebox is located on
the communication path between the preferred locators or not, the
middlebox forwards different packets and, thus, needs to interact
differently with the updates. Figure 4 I) and II) illustrates an
update with Middlebox 1 on the path between the Initiator's and the
Responder's preferred locators and with Middlebox 2 on an alternative
path. Middlebox 2 is not located on the path between the preferred
locators of the HIP end-hosts does not receive the U1 message.
Therefore, it will not recognize any ER1 or SM1 in the second UPDATE
packet. Thus, if a middlebox encounters non-matching or missing
ECHO_RESPONSE_M parameters, the middlebox SHOULD ignore these.
Complying to the RESPONSE RULE stated in Section Section 2.2.2, the
RESPONDER generates an additional fourth update packet on receiving
the ECHO_REQUEST_M. The update process for a middlebox on the
preferred communication path (Middlebox 1) and a middlebox off the
preferred communication path (Middlebox 2) is depicted in Figure 4.
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Middlebox authentication of a HIP mobility update over different
paths.
I) Main path:
Initiator Middlebox 1 Responder
.------.
U1 | | U1 + EQ1, [PM1]
-----------------------------> | | ---------------------------->
| |
U2, {ER1, [SM1]} + EQ2, [PM2] | | U2, {ER1, [SM1]}
<----------------------------- |OK | <----------------------------
| |
U3, {ER2, [SM2]} | | U3, {ER2, [SM2]}
-----------------------------> | OK| ---------------------------->
'------'
II) Alternative path:
Initiator Middlebox 2 Responder
U1 (bypasses Middlebox 2)
-------------------------------------------------------------------->
.------.
U2, {ER1, [SM1]} + EQ3, [PM3] | | U2, {ER1, [SM1]}
<----------------------------- | wrong| <----------------------------
| |
U3', {ER3, [SM3]} | | U3', {ER3, [SM3]} + EQ4, PM4
-----------------------------> |OK | ----------------------------->
| |
U4, {ER4, [SM4]} | | U4, {ER1, [SM1]}
<----------------------------- | OK| <----------------------------
'------'
EQ: Middlebox Echo reQuest
ER: Middlebox Echo Response
PM: Puzzle of the Middlebox
SM: Solution of Middlebox puzzle
Figure 4
2.2.4. Identity Signaling During Updates
As middleboxes need to be able to rapidly verify and forward HIP
packets, they need to be supplied with all information necessary to
do so. If end-host hand over communication to a new communication
path, middleboxes need to be able to learn the Host Identifiers (HIs)
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from the UPDATE packets. Therefore, HIP end-hosts MUST include the
HOST_ID parameter in all UPDATE packets that use combinations of
locators that have not been used before. Additionally, UPDATE
packets that contain ECHO_REQUEST or ECHO_RESPONSE parameters MUST
contain the HOST_ID parameter. Moreover, all packets that contain an
ECHO_RESPONSE_M parameter MUST contain the HOST_ID parameter.
2.2.5. Closing of Connections
At the time being, identity verification during the closing of a HIP
association is not supported. Hence, the middlebox MUST preserve the
state until it expires according to local policies. An appropriate
mechanism for middleboxes to verify CLOSE messages by middleboxes
will be provided in future versions of this document.
2.3. Failure Signaling
Middleboxes SHOULD inform the sender of a BEX or update message if it
does not satisfy the requirements of the middlebox. Reasons for non-
satisfactory packets are missing HOST_ID, ECHO_RESPONSE_M, and
SOLUTION_M parameters. Options for expressing such shortcomings are
ICMP packets if no HIP association is established and HIP_NOTIFY
packets in case of an already established HIP association. Defining
this signaling mechanism is future work.
2.4. Fragmentation
Analogously to the specification in [I-D.ietf-hip-base], HIP aware
middleboxes SHOULD support IP-level fragmentation and reassembly for
IPv6 and MUST support IP-level fragmentation and reassembly for IPv4.
However, when adding ECHO_REQUEST_M and PUZZLE_M parameters, a
middlebox SHOULD keep the total packet size below 1280 bytes to avoid
packet fragmentation in IPv6.
2.5. HIP Parameters
This HIP extension specifies four new HIP parameters that allow
middleboxes to authenticate HIP end-hosts and to protect against DoS
attacks.
2.5.1. ECHO_REQUEST_M
A middlebox MAY apply ECHO_REQUEST_M parameter to R1, I2, and UPDATE
packets. The structure of the ECHO_REQUEST_M parameter is depicted
in the following figure.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque data (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 65332
Length Variable
Opaque data Opaque data, should be interpreted only by the
middlebox that adds ECHO_REQUEST_M and receives
the corresponding ECHO_RESPONSE_M.
2.5.2. ECHO_RESPONSE_M
The ECHO_RESPONSE_M is the reply to the ECHO_REQUEST_M parameter.
The receiver of an ECHO_RESPONSE_M parameter SHOULD reply with n
ECHO_RESPONSE_M. Otherwise, the middlebox that added the parameter
MAY decide to degrade or deny its service. The contents of the
ECHO_REQUEST_M parameter must be copied to the ECHO_RESPONSE_M
parameter without any modification. The ECHO_RESPONSE_M parameter is
non-critical and covered by the SIGNATURE. The structure of the
ECHO_RESPONSE_M parameter is depicted below:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque data (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 962
Length Variable
Opaque data Opaque data, should be interpreted only by the
middlebox that adds adds ECHO_REQUEST_M and
receives the corresponding ECHO_RESPONSE_M.
2.5.3. PUZZLE_M
A middlebox MAY add a PUZZLE_M parameter to R1, I2, and UPDATE
packets. A HIP packet may contain multiple PUZZLE_M parameters as
multiple middleboxes may be located on a communication path. These
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puzzles serve as defense against DoS attacks. Hosts that receive a
PUZZLE_M parameter SHOULD reply with a SOLUTION_M parameter in the
subsequent I2, R2, or UPDATE packet. With the exception of an
extended opaque field, the syntax and semantics of the puzzle are
defined in [I-D.ietf-hip-base]. The extended opaque data field helps
middleboxes to recognize their puzzles and solutions, respectively,
when a packet contains more than one puzzle.
A middlebox MUST preserve the order of PUZZLE_M parameters in a
packet and attach its own PUZZLE_M parameter after all other PUZZLE_M
parameters. Preserving the order of PUZZLE_M parameters may speed up
the middlebox recognition of the puzzles.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| K, 1 byte | Lifetime | Opaque, 6 bytes /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Random # I, 8 bytes |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 65334
Length 16
K K is the number of verified bits
Lifetime Puzzle lifetime 2^(value-32) seconds
Opaque Data set by the middlebox, indexing the middlebox
Random #I Random number
2.5.4. SOLUTION_M
The SOLUTION_M parameter contains the solution for the corresponding
PUZZLE_M parameter. End-hosts that receive a PUZZLE_M parameter
SHOULD solve the puzzle according to the specification in
[I-D.ietf-hip-base] and send the resulting solution in the SOLUTION_M
parameter. Exclusion of a solution MAY result in degraded or denied
service by the middlebox that added the PUZZLE_M parameter. The
format and meaning of the fields in the SOLUTION_M parameter resemble
the specifications of the SOLUTION parameter in [I-D.ietf-hip-base].
The reader is advised to refer to that document for further details.
The extended opaque data field helps middleboxes to recognize their
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puzzles and the resulting solutions, respectively, when a packet
contains multiple puzzles.
The relative order of SOLUTION_M parameters in a HIP control packet
MUST match the order of the PUZZLE_M parameters in the previously
received packet. Preserving the order of PUZZLE_M for the
corresponding SOLUTION_M parameters may help middleboxes to recognize
the puzzles and solutions relevant to them.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| K, 1 byte | Reserved | Opaque, 6 bytes /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Random # I, 8 bytes |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Puzzle solution #J, 8 bytes |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 322
Length 20
K K is the number of verified bits
Reserved Zero when sent, ignored when received
Opaque Copied unmodified from the received PUZZLE
parameter
Random #I Random number
Puzzle solution Random number
3. Security Services for the HIP Control Channel
In this section, we define the attacker model that the security
analysis in the later sections will be based on.
3.1. Adversary model and Security Services
For discussing the security properties of the proposed HIP extension
we first define an attacker model. We assume a Dolev-Yao threat
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model in which an adversary can eavesdrop on all traffic regardless
of its source and destination. The adversary can inject arbitrary
packets with any source and destination addresses. Consequently, an
adversary can also replay previously eavesdropped messages. However,
the adversary cannot subvert the cryptographic ciphers and hash
function, nor can it take over one of the communicating nodes.
Even in the face of this strong attacker, the proposed HIP extension
enables middleboxes to verify the identity of the communicating HIP
peers. It ensures that both peers are involved in the communication
and that the HIP BEX or update packets are fresh, i.e. not replayed.
It enables the middlebox to verify the source and destination (in
terms of HIs) of the HIP association and the integrity of RSA and DSA
signed HIP packets.
4. Security Services for the HIP Payload Channel
The presented extension for HIP authentication by middleboxes only
covers the HIP control channel, i.e., the HIP control messages.
Depending on the binding between the HIP control and payload channel,
certain security properties for the payload channel can be derived
from the strong cryptographic authentication of the end-hosts.
Assuming that there is a secure binding between packets belonging to
a payload stream and the control stream, the same security properties
as in Section 3 apply to the payload stream.
ESP [I-D.ietf-hip-esp] is currently the default payload encapsulation
format for HIP. A limitation of ESP is that does not provide a
secure binding between the HIP control channel and the ESP traffic on
a per-packet basis, the achievable level of security for the payload
channel is lower.
This section discusses security properties of an ESP payload channel
bound to a HIP control channel. Depending on the assumed adversary
model, certain security services are possible. We briefly describe
two application scenarios and how they benefit from the resulting
security services. For the payload channel, HIP in combination with
the middlebox authentication scheme offers the following security
services:
Attribute binding: Middleboxes can extract certain payload channel
attributes (e.g. locators and SPIs) from the control channel.
These attributes can be used to enforce certain restrictions on
the payload channel, e.g., to exhibit the same attributes as the
control channel. The attributes can either be stated explicitly
in the HIP control packets or can be derived from the IP or UDP
packets carrying the HIP control messages.
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Host involvement: Middleboxes can verify whether a certain host was
involved in the establishment of a HIP association and thus,in the
establishment of the payload channel.
Based on these security services we construct two use cases that
illustrate the use of HIP authentication by middleboxes: access
control and resource allocation as described in the following
sections.
4.1. Access Control
Middleboxes can manage resources based on HIs. As an example, let us
assume that a middlebox only forwards HIP payload packets after a
successful HIP BEX or HIP update. The middlebox uses the parameters
in the control channel (specifically IP addresses and SPIs) to filter
the payload traffic. The middlebox only forwards traffic from and to
specific authenticated hosts and drops other traffic.
The feasibility of subverting the function of the middlebox depends
on the assumed adversary model.
4.1.1. Adversary model and Security Services
If we assume a Dolev-Yao threat model, attribute binding is not
helpful to aid packet filtering for access control. An attacker can
send packets from any IP address and can read packets destined to any
IP address. Without per packet verification by the middlebox, such
an attacker can inject arbitrary forged packets into the HIP payload
channel and make them traverse the middlebox. The attacker can also
read the packets from the HIP payload channel, and hence, communicate
across the middlebox. However, the injected packets are disclosed by
inconsistencies in the ESP sequence numbers, which makes the attack
visible to the middlebox as well as the HIP end hosts. Moreover,
attackers can only inject packets into an already established HIP
payload channel. Opening a new payload channel and replaying a
closing of the channel are not possible.
An attacker that is not able to send IP packets from an arbitrary
source address and receive IP packets addressed to any destination,
cannot use the ESP channel to send fake ESP packets when the
middleboxes bind HIs and SPI numbers to addresses. By fixing the set
of source and destination IP addresses, the opportunity to
successfully inject packets into the payload channel is limited to
hosts that can send packets from the same source address as the
legitimate HIP hosts. Moreover, an attacker can only receive
injected packets if it is on the communication path towards the
legitimate HIP peer. Attackers cannot open new HIP payload channels
and thus have no influence on the bound payload stream parameters.
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Finally, attackers cannot close HIP associations of legimitate peers.
4.2. Resource allocation
When using HIs to limit the resources (e.g. bandwidth) allocated for
a certain host, the HIs can be used to authenticate the hosts in a
similar fashion to the access control illustrated above. Regarding
authentication, both use cases share the same strengths and
weaknesses. However, the implications for the targeted scenarios
differ. Therefore, we restrict the following discussion to these
differences.
4.2.1. Adversary Model and Security Services
When assuming an Dolev-Yao threat model, an attacker is able to use
resources allocated for the payload channel of another host by
injecting packets into this channel. However, the attacker cannot
open a new payload channel with another host nor can it close an
existing one. When binding the IP addresses of the HIP payload
channel to the IP addresses used in the HIP control channel and
assuming an attacker that cannot receive IP packets addressed to the
IP address of an authenticated host, the attacker cannot utilize the
resources allocated to authenticated host. However, the attacker can
still inject packets and waste resources, yet without having any
benefit other than causing disturbance to the other host.
Specifically, it cannot increase the share of resources allocated to
itself. Hence, this measure takes incentive from selfish users that
try to benefit by mounting a DoS attack. Defense against purely
malicious attackers that aim at creating disturbance without
immediate benefit is difficult to achieve.
5. Security Considerations
This HIP extension specifies how HIP-aware middleboxes interact with
the handshake and mobility-signaling of the Host Identity Protocol.
Its scope is restricted to the authentication of end-hosts and does
not include the issue of authenticating ESP traffic at the middlebox.
Providing middleboxes with a way of adding puzzles to the HIP control
packets may cause both HIP peers, including the Responder, to spend
CPU time on solving these puzzles. Thus, it is advised that HIP
implementations for servers employ mechanisms to prevent middlebox
puzzles from being used as DoS attacks. Under high CPU load, servers
can rate limit or assign lower priority to packets containing
middlebox puzzles.
If multiple middleboxes add ECHO_REQUEST_M parameters to a HIP
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control packet, the remaining space in the packet might not be
sufficient for further parameters to be added. Moreover, as the
ECHO_REQUEST_M must be echoed within an ECHO_RESPONSE_M, the space in
the subsequent packet may not be sufficient to include all
ECHO_RESPONSE_M parameters. Thus, middleboxes SHOULD keep the size
of the nonces small.
6. IANA Considerations
This document specifies four new HIP parameter types. The
preliminary parameter type numbers are 322, 962, 65332, and 65334.
7. Acknowledgments
Thanks to Shaohui Li, and Janne Lindqvist for the fruitful
discussions on this topic. Many thanks to Stefan Goetz, Ari Keranen,
Samu Varjonen, Rene Hummen, and Kate Harrison for commenting and
helping to improve the quality of this document.
8. Normative References
[I-D.ietf-hip-base]
Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
"Host Identity Protocol", draft-ietf-hip-base-10 (work in
progress), October 2007.
[I-D.ietf-hip-esp]
Jokela, P., "Using ESP transport format with HIP",
draft-ietf-hip-esp-06 (work in progress), June 2007.
[I-D.ietf-hip-mm]
Henderson, T., "End-Host Mobility and Multihoming with the
Host Identity Protocol", draft-ietf-hip-mm-05 (work in
progress), March 2007.
[I-D.ietf-hip-nat-traversal]
Schmitt, V., "HIP Extensions for the Traversal of Network
Address Translators", draft-ietf-hip-nat-traversal-02
(work in progress), July 2007.
[I-D.ietf-hip-registration]
Laganier, J., "Host Identity Protocol (HIP) Registration
Extension", draft-ietf-hip-registration-02 (work in
progress), June 2006.
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[I-D.irtf-hiprg-nat]
Stiemerling, M., "NAT and Firewall Traversal Issues of
Host Identity Protocol (HIP) Communication",
draft-irtf-hiprg-nat-04 (work in progress), March 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Authors' Addresses
Tobias Heer (editor)
Distributed Systems Group, RWTH Aachen University
Ahornstrasse 55
Aachen 52062
Germany
Phone: +49 241 80 214 36
Email: heer@cs.rwth-aachen.de
URI: http://ds.cs.rwth-aachen.de/members/heer
Klaus Wehrle
Distributed Systems Group, RWTH Aachen University
Ahornstrasse 55
Aachen 52062
Germany
Phone: +49 241 80 214 30
Email: wehrle@cs.rwth-aachen.de
URI: http://ds.cs.rwth-aachen.de/members/klaus
Miika Komu
Helsinki Institute for Information Technology
Metsanneidonkuja 4
Espoo
Finland
Phone: +358503841531
Fax: +35896949768
Email: miika@iki.fi
URI: http://www.hiit.fi/
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