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Host Identity Protocol T. Heer, Ed.
Internet-Draft K. Wehrle
Intended status: Experimental Distributed Systems Group, RWTH
Expires: September 1, 2009 Aachen University
M. Komu
HIIT
February 28, 2009
End-Host Authentication for HIP Middleboxes
draft-heer-hip-middle-auth-02
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Abstract
The Host Identity Protocol [RFC5201] is a signaling protocol for
secure communication, mobility, and multihoming that introduces a
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 allows
middleboxes to verify the liveness and freshness of a HIP association
and, thus, to 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 [RFC2119].
Notation
[x] indicates that x is optional.
{x} indicates that x is under signature.
Initiator is the host that initiates a HIP association
(cf. HIP base protocol).
Responder is the host that responds to the INITIATOR
(cf. HIP base protocol).
--> signifies "Initiator to Responder" communication.
<-- signifies "Responder to Initiator" communication.
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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 . . . . . . . . 15
3.1. Adversary model and Security Services . . . . . . . . . . 15
4. Security Services for the HIP Payload Channel . . . . . . . . 16
4.1. Access Control . . . . . . . . . . . . . . . . . . . . . . 17
4.2. Resource allocation . . . . . . . . . . . . . . . . . . . 17
5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
8. Normative References . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
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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 securely address and
authenticate their peers. HIP was designed to be middlebox-friendly
and to allow 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 are
designed to process HIP packets, and other middleboxes, which are
unaware of HIP. This document addresses only HIP-aware middleboxes
while the behavior of HIP in combination with HIP-unaware 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
provide Authentication, Authorization, and Accounting (AAA)-related
services 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 explicitly
register to the middlebox. HIP behavior for interacting and
registering to such middleboxes is specified in [RFC5203]. Thus, we
focus on middleboxes that build their state based on packets they
forward (path-coupled signaling).
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 [RFC5206] 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 allow 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
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mechanism defined in [RFC5201] to slow down resource-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 ensure the identity of an end-host because of a
possible replay attack against which the basic HIP protocol as
specified in [RFC5201] does not provide adequate protection.
To illustrate the need for additional security measures for HIP-aware
middleboxes, we briefly outline the replay attack: 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.
Assume that a middlebox M checks HIP HIs in order to restrict traffic
passing through the box. At some later point in time, Attacker A
collaborates with another attacker B. They replay the very same BEX
over a middlebox M on the communication path. Note that it is not
required that the middlebox M was on the communication path between
the peers when the BEX was recorded.
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
it cannot can distinguish the replayed BEX from a the genuine
handshake. As the attackers overheard the SPI numbers, they can
taverse the middlebox with "fake" ESP packets with valid SPI numbers,
and hence, send data across m without proper authentication. 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 replayed 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 AAA-related services, such as 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 this attack.
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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 and therefore should be avoided.
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) their signatures and echo back.
The challenge-response mechanism is similar to the ECHO_REQUEST/
ECHO_RESPONSE mechanism employed already by HIP end-hosts. It
assumes that the end-hosts exchange at least two HIP packets with
each other. The middlebox adds a CHALLENGE_REQUEST parameter to the
first HIP control packet. Similar to the ECHO_REQUEST parameter in
the original HIP protocol, this parameter contains an opaque data
field that must be echoed by its receiver. The receiver echoes the
opaque data field in a CHALLENGE_RESPONSE parameter. The
CHALLENGE_RESPONSE parameter must be covered by the packet signature,
thereby proving that the receiver is in possession of the private key
that corresponds to the 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 of which authentication is required left to middlebox
implementers.
2.1.1. CHALLENGE_REQUEST
Middleboxes MAY add CHALLENGE_REQUEST parameters to 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
(e.g., a nonce). The HIP packets that carry middlebox challenges may
contain multiple CHALLENGE_REQUEST parameters, since all middleboxes
on the path may add these parameters. Hence, the MBs should restrict
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the size of the variable data field in the CHALLENG_REQUEST
parameter. The total length of the packets SHOULD not exceed 1280
bytes to avoid IPv6 fragmentation [RFC2460].
The middleboxes add the CHALLENGE_REQUEST parameter to the
unprotected part of a HIP message. Thus it does not corrupt any HMAC
or public-key signatures that protect the HIP packet. However, the
middlebox MUST recompute the IP- and HIP header checksums as defined
in [RFC5201] 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 CHALLENGE_REQUEST parameters must copy the contents of each
parameter without modification to an CHALLENGE_RESPONSE parameter.
This end-host MUST send this parameter within the signed part of its
reply. Note that middleboxes MAY also add ECHO_REQUEST_UNSIGNED
parameter as specified in [RFC5201] 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
CHALLENGE_REQUEST and CHALLENGE_RESPONSE parameters by hiding
encrypted or otherwise protected information about previous
authentication steps in the opaque data field.
2.1.2. CHALLENGE_RESPONSE
When a middlebox injects an opaque blob of data with a
CHALLENGE_REQUEST parameter, it expects to receive the same data
without modification as part of a CHALLENGE_RESPONSE parameter in a
subsequent packet. The opaque data MUST be copied as it is from the
corresponding CHALLENGE_REQUEST parameter. In the case of multiple
CHALLENGE_REQUEST parameters, their order MUST be preserved by the
corresponding CHALLENGE_RESPONSE parameters.
The CHALLENGE_REQUEST and CHALLENGE_RESPONSE parameters MAY be used
for any purpose, in particular when a middlebox has to carry state
information in a HIP packet to receive it in the next response
packet. The CHALLENGE_RESPONSE MUST be covered by the HIP_SIGNATURE.
The CHALLENGE_RESPONSE parameter is non-critical. Depending on its
local policy, a middlebox can react differently on a missing
CHALLENGE_RESPONSE 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 has
to defend itself against resource-exhaustion attacks when verifying
signatures in HIP packets. The HIP base protocol [RFC5201] 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.
The puzzle mechanism for middleboxes utilizes the CHALLENGE_REQUEST
and CHALLENGE_RESPONSE parameters. The CHALLENGE_REQUEST parameter
contains fields for setting the difficulty and the expiration date of
the puzzle. In contrast to the PUZZLE parameter in the HIP base
specifications, there is no dedicated puzzle seed field. Instead,
the hash of the opaque data field in the CHALLENGE_REQUEST parameter
serves as puzzle seed. The hash is generated by applying the
responder's hash algorithm (RHASH) to the opaque data field. The
destination end-host of the HIP control packet MUST solve the puzzle
and provide the solution in the CHALLENGE_RESPONSE parameter. The
middlebox can set the puzzle difficulty by adjusting the K value in
the CHALLENGE_REQUEST packet. The semantics of this field equal the
semantics of the PUZZLE parameter. Setting K to 0 signifies that no
puzzle solution is required.
As a puzzle increases the delay and computational cost for
establishing or updating a HIP association, a middlebox SHOULD only
increase K 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 increase K for HIP control packets that
leave the interface. The middlebox chooses the difficultly of the
puzzle according to its load and local policies.
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 CHALLENGE_REQUEST parameters to R1 and I2 packets
in order to verify the identities of the participating end-hosts.
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Middleboxes can choose either to authenticate the Initiator, the
Responder, or both. Middleboxes MUST NOT add CHALLENGE_REQUEST
parameters to I1 messages because this would expose the Responder to
DoS attacks. Thus, middleboxes MUST let unauthenticated and minimal
I1 packets traverse. Minimal means that the I1 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, + CQ1 | Add CQ | R1
<----------------- | |<----------------------------
| |
I2, {CR1} | Verify CR1 | I2, {CR1} + CQ2
-----------------> | Add CQ2 |---------------------------->
| |
| |
R2, {CR2} | Verify CR2 | R2, {CR2}
<----------------- | |<-----------------------------
'-----------------'
CQ: Middlebox challenge reQuest
CR: Middlebox challenge Response
{}: Signature with sender's HI as key
Figure 1
2.2.2. Identity Verification During Mobility Updates
HIP rekeying, mobility and multihoming UPDATE mechanisms for non-
NATted environments are described in [RFC5206]. 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 challenges to mobility related
HIP control messages in the case where both end-hosts are single-
homed. The middlebox challenges can be applied both ways as the
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UPDATE process consists of three packets (U1, U2, U3) which all
traverse through the same middlebox as shown in Figure 2.
In cases, in which fewer packets are used for updating an
association, the following rule applies.
RESPONSE RULE:
A HIP host, receiving a CHALLENGE_REQUEST MUST reply with a
CHALLENGE_RESPONSE in its next UPDATE packet. If no further UPDATE
packets are necessary to complete the update procedure, an additional
UPDATE packet containing the CHALLENGE_RESPONSE MUST be sent.
Initiator Middlebox Responder
.------.
U1 | | U1 + CQ1
-----------------------------> | | --------------------------->
| |
U2, {CR1} + CQ2 | | U2, {CR1}
<----------------------------- |OK | <---------------------------
| |
U3, {CR2} | | U3, {CR2}
-----------------------------> | OK| --------------------------->
'------'
CQ: Middlebox challenge reQuest
CR: Middlebox challenge Response
{}: Signature with sender's HI as key
Middlebox authentication of a HIP mobility update over a single path.
Figure 2
Middlebox 1 in Figure 2 can verify the identity of the Responder by
checking its PK signature and the presence of the CHALLENGE_RESPONSE
in the U2 packet. If necessary, the middlebox MAY add an
CHALLENGE_REQUEST for the Initiator of the update. The middlebox can
verify the Initiator's identity by verifying its signature and the
CHALLENGE_RESPONSE in the U3 packet.
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 of the hosts or
not, the middlebox forwards different packets and, thus, needs to
interact differently with the updates. Figure 3 I) and II)
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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
CHALLENGE_RESPONSE (CR1) in the second UPDATE packet. Thus, if a
middlebox encounters non-matching or missing CHALLENGE_RESPONSE
parameter in an initial update packet, the middlebox SHOULD ignore
it.
Complying to the RESPONSE RULE stated in Section Section 2.2.2, the
RESPONDER generates an additional fourth update packet on receiving
the CHALLENGE_REQUEST. 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 3.
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I) Main path:
Initiator Middlebox 1 Responder
.------.
U1 | | U1 + CQ1
----------------------------> | | --------------------------->
| |
U2, {CR1} + CQ2 | | U2, {CR1}
<---------------------------- |OK | <---------------------------
| |
U3, {CR2} | | U3, {CR2}
----------------------------> | OK| --------------------------->
'------'
II) Alternative path:
Initiator Middlebox 2 Responder
U1 (bypasses Middlebox 2)
------------------------------------------------------------------>
.------.
U2, {CR1} + CQ3 | | U2, {CR1}
<---------------------------- | wrong| <---------------------------
| |
U3', {CR3} | | U3', {CR3} + CQ4
----------------------------> |OK | ---------------------------->
| |
U4, {CR4} | | U4, {CR4}
<---------------------------- | OK| <---------------------------
'------'
CQ: Middlebox challenge reQuest
CR: Middlebox challenge Response
{}: Signature with sender's HI as key
Middlebox authentication of a HIP mobility update over different
paths.
Figure 3
2.2.4. Identity Signaling During Updates
As middleboxes have to verify rapidly and forward HIP packets, they
need to be supplied with all information necessary to do so. If end-
hosts hand over communication to a new communication path,
middleboxes need to be able to learn their Host Identifiers (HIs)
from the UPDATE packets. Therefore, all packets that contain a
CHALLENGE_RESPONSE parameter MUST contain the HOST_ID parameter.
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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 packet or update packet
if it does not satisfy the requirements of the middlebox. Reasons
for non-satisfactory packets are missing HOST_ID or
CHALLENGE_RESPONSE parameters. Other reasons may be middlebox
policies regarding, for example, insufficient client capabilities or
or insufficient credentials delivered in a HIP CERT parameter
[I-D.varjonen-hip-cert]. 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 [RFC5201], 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 CHALLENGE_REQUEST 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. CHALLENGE_REQUEST
A middlebox MAY append the CHALLENGE_REQUEST parameter to R1, I2, and
UPDATE packets. The structure of the CHALLENGE_REQUEST parameter is
depicted in the following figure. The semantics of the K and
Lifetime fields is identical to the fields defined in the PUZZLE
parameter in [RFC5201]. The opaque data field serves as nonce and
puzzle seed value. To generate the seed corresponding to the 8-byte
value I in [RFC5201], the receiver of the puzzle applies RHASH to the
opaque data field and truncates the result to 8-byte length. Note
that the opaque data field must provide enough randomness to serve as
puzzle seed.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| K, 1 byte | Lifetime | /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
/ /
/ Opaque, (variable length) /
/ +-+-+-+-+-+-+-+-+-+-|
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 65334
Length Variable
K K is the number of verified bits
Lifetime Challenge lifetime 2^(value-32) seconds
Opaque Opaque data that serves as nonce and as basis for the
puzzle. The puzzle value I is generated by hashing the
opaque data field with the hash function RHASH and
truncating it to 8-byte length.
Random #I Random number
2.5.2. CHALLENGE_RESPONSE
The CHALLENGE_RESPONSE parameter is the response to the
CHALLENGE_REQUEST parameter. The receiver of a CHALLENGE_REQUEST
parameter SHOULD reply with a CHALLENGE_RESPONSE. Otherwise, the
middlebox that added the CHALLENGE_REQUEST parameter MAY decide to
degrade or deny its service. The contents of the CHALLENGE_REQUEST
parameter must be copied to the CHALLENGE_RESPONSE parameter without
any modification. If the puzzle difficulty in the CHALLENGE_REQUEST
parameter is set to any other value except 0, an appropriate puzzle
solution (adhering to the SOLUTION specifications in [RFC5201]) must
be provided in the CHALLENGE_RESPONSE parameter. The
CHALLENGE_RESPONSE parameter is non-critical and covered by the
SIGNATURE. The structure of the CHALLENGE_RESPONSE parameter is
depicted below:
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| K, 1 byte | Lifetime | /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
/ Puzzle solution #J, 8 bytes /
/ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
/ Opaque, (variable length) /
/ +-+-+-+-+-+-+-+-+-+-|
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 322
Length Variable
K K is the number of verified bits
Opaque Copied unmodified from the received
CHALLENGE_REQUEST parameter
Puzzle solution Random number
3. Security Services for the HIP Control Channel
In this section, we define the adversary 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
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 compromise 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.
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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 [RFC5202] is currently the default payload encapsulation format
for HIP. A limitation of ESP is that it does not provide a secure
binding between the HIP control channel and the ESP traffic on a per-
packet basis. Hence, the achievable level of security for the
payload channel is lower compared to the HIP control channel.
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.
Host involvement: Middleboxes can verify whether a certain host is
involved in the establishment of a HIP association and, thus,
involved 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.
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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 forged 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.
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
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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. Also, 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 is
unable to 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 and out of scope of this document.
5. Security Considerations
This HIP extension specifies how HIP-aware middleboxes interact with
the handshake and mobility-signaling of the Host Identity Protocol.
The scope is restricted to the authentication of end-hosts and
excludes the issue of stronger authentication of 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 CHALLENGE_REQUEST parameters to a HIP
control packet, the remaining space in the packet might not be
sufficient for further CHALLENGE_REQUEST parameters to be added.
Moreover, as the CHALLENGE_REQUEST must be echoed within a
CHALLENGE_RESPONSE, the space in the subsequent packet may not be
sufficient to include all CHALLENGE_RESPONSE parameters. Thus,
middleboxes SHOULD keep the size of the nonces small.
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6. IANA Considerations
This document specifies two new HIP parameter types. The preliminary
parameter type numbers are 322 and 65334.
7. Acknowledgments
Thanks to Shaohui Li, and Janne Lindqvist for the fruitful
discussions on this topic. Many thanks to Julien Laganier, 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-nat-traversal]
Komu, M., Henderson, T., Matthews, P., Tschofenig, H., and
A. Keraenen, "Basic HIP Extensions for Traversal of
Network Address Translators",
draft-ietf-hip-nat-traversal-05 (work in progress),
October 2008.
[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.
[I-D.varjonen-hip-cert]
Heer, T. and S. Varjonen, "HIP Certificates",
draft-varjonen-hip-cert-01 (work in progress), July 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC5201] Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
"Host Identity Protocol", RFC 5201, April 2008.
[RFC5202] Jokela, P., Moskowitz, R., and P. Nikander, "Using the
Encapsulating Security Payload (ESP) Transport Format with
the Host Identity Protocol (HIP)", RFC 5202, April 2008.
[RFC5203] Laganier, J., Koponen, T., and L. Eggert, "Host Identity
Protocol (HIP) Registration Extension", RFC 5203,
April 2008.
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[RFC5206] Nikander, P., Henderson, T., Vogt, C., and J. Arkko, "End-
Host Mobility and Multihoming with the Host Identity
Protocol", RFC 5206, April 2008.
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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