Network Working Group R. Moskowitz, Ed.
Internet-Draft HTT Consulting
Intended status: Standards Track R. Hummen
Expires: June 22, 2015 COMSYS, RWTH Aachen
December 19, 2014
HIP Diet EXchange (DEX)
draft-moskowitz-hip-dex-02
Abstract
This document specifies the Host Identity Protocol Diet EXchange (HIP
DEX), a variant of the Host Identity Protocol Version 2 (HIPv2). The
HIP DEX protocol design aims at reducing the overhead of the employed
cryptographic primitives by omitting public-key signatures and hash
functions. In doing so, the main goal is to still deliver similar
security properties to HIPv2.
The HIP DEX protocol is primarily designed for computation or memory-
constrained sensor/actuator devices. Like HIPv2, it is expected to
be used together with a suitable security protocol such as the
Encapsulated Security Payload (ESP) for the protection of upper layer
protocol data. In addition, HIP DEX can also be used as a keying
mechanism for security primitives at the MAC layer, e.g., for IEEE
802.15.4 networks.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 22, 2015.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. The HIP Diet EXchange (DEX) . . . . . . . . . . . . . . . 4
1.2. Memo Structure . . . . . . . . . . . . . . . . . . . . . 5
2. Terms and Definitions . . . . . . . . . . . . . . . . . . . . 5
2.1. Requirements Terminology . . . . . . . . . . . . . . . . 5
2.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Definitions . . . . . . . . . . . . . . . . . . . . . . . 6
3. Host Identity (HI) and its Structure . . . . . . . . . . . . 7
3.1. Host Identity Tag (HIT) . . . . . . . . . . . . . . . . . 8
3.2. Generating a HIT from an HI . . . . . . . . . . . . . . . 8
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Creating a HIP Association . . . . . . . . . . . . . . . 8
4.1.1. HIP Puzzle Mechanism . . . . . . . . . . . . . . . . 10
4.1.2. HIP State Machine . . . . . . . . . . . . . . . . . . 11
4.1.3. HIP DEX Security Associations . . . . . . . . . . . . 14
4.1.4. User Data Considerations . . . . . . . . . . . . . . 15
5. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Payload Format . . . . . . . . . . . . . . . . . . . . . 15
5.2. HIP Parameters . . . . . . . . . . . . . . . . . . . . . 15
5.2.1. HIT_SUITE_LIST . . . . . . . . . . . . . . . . . . . 16
5.2.2. DH_GROUP_LIST . . . . . . . . . . . . . . . . . . . . 16
5.2.3. HOST_ID . . . . . . . . . . . . . . . . . . . . . . . 16
5.2.4. HIP_CIPHER . . . . . . . . . . . . . . . . . . . . . 17
5.2.5. ENCRYPTED_KEY . . . . . . . . . . . . . . . . . . . . 17
5.3. HIP Packets . . . . . . . . . . . . . . . . . . . . . . . 17
5.3.1. I1 - the HIP Initiator Packet . . . . . . . . . . . . 18
5.3.2. R1 - the HIP Responder Packet . . . . . . . . . . . . 19
5.3.3. I2 - the Second HIP Initiator Packet . . . . . . . . 21
5.3.4. R2 - the Second HIP Responder Packet . . . . . . . . 22
5.4. ICMP Messages . . . . . . . . . . . . . . . . . . . . . . 23
6. Packet Processing . . . . . . . . . . . . . . . . . . . . . . 23
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6.1. Solving the Puzzle . . . . . . . . . . . . . . . . . . . 23
6.2. HIP_MAC Calculation and Verification . . . . . . . . . . 24
6.2.1. CMAC Calculation . . . . . . . . . . . . . . . . . . 24
6.3. HIP DEX KEYMAT Generation . . . . . . . . . . . . . . . . 25
6.4. Initiation of a HIP Diet EXchange . . . . . . . . . . . . 28
6.5. Processing Incoming I1 Packets . . . . . . . . . . . . . 28
6.6. Processing Incoming R1 Packets . . . . . . . . . . . . . 28
6.7. Processing Incoming I2 Packets . . . . . . . . . . . . . 28
6.8. Processing Incoming R2 Packets . . . . . . . . . . . . . 31
6.9. Processing UPDATE, NOTIFY, CLOSE, and CLOSE_ACK Packets . 32
6.10. Handling State Loss . . . . . . . . . . . . . . . . . . . 32
7. HIP Policies . . . . . . . . . . . . . . . . . . . . . . . . 32
8. Security Considerations . . . . . . . . . . . . . . . . . . . 33
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34
11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 34
11.1. Changes in draft-moskowitz-hip-rg-dex-06 . . . . . . . . 34
11.2. Changes in draft-moskowitz-hip-dex-00 . . . . . . . . . 34
11.3. Changes in draft-moskowitz-hip-dex-01 . . . . . . . . . 34
11.4. Changes in draft-moskowitz-hip-dex-02 . . . . . . . . . 35
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
12.1. Normative References . . . . . . . . . . . . . . . . . . 35
12.2. Informative References . . . . . . . . . . . . . . . . . 36
Appendix A. C code for ECDH point multiplication on an 8
bit processor . . . . . . . . . . . . . . . . . . . 38
Appendix B. Password-based two-factor authentication during
the HIP DEX handshake . . . . . . . . . . . . . . . 39
1. Introduction
This memo specifies the Host Identity Protocol Diet EXchange (HIP
DEX). HIP DEX builds on the Base EXchange (BEX) of the Host Identity
Protocol Version 2 (HIPv2) [I-D.ietf-hip-rfc5201-bis]. Notably, HIP
DEX preserves the protocol semantics as well as the general packet
structure of HIPv2. It is therefore recommended that
[I-D.ietf-hip-rfc5201-bis] is well-understood before reading this
document.
The main differences between HIP BEX and HIP DEX are:
1. Minimum collection of cryptographic primitives.
* Static Elliptic Curve Diffie-Hellman key pairs for encryption
of the session key.
* AES-CTR for symmetric encryption and AES-CMAC for MACing
functions.
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* A simple fold function for HIT generation.
2. Forfeit of Perfect Forward Secrecy with the dropping of ephemeral
Diffie-Hellman.
3. Forfeit of digital signatures with the removal of a hash
function. Reliance on ECDH derived key used in HIP_MAC to prove
ownership of the private key.
4. Diffie-Hellman derived key ONLY used to protect the HIP packets.
A separate secret exchange within the HIP packets creates the
session key(s).
5. Optional retransmission strategy tailored to handle the
potentially extensive processing time of the employed
cryptographic operations on sensor/actuator devices.
In this document, we focus on the protocol specifications related to
these differences. Where differences are not call out explicitly,
HIP DEX is the same as specified in [I-D.ietf-hip-rfc5201-bis].
1.1. The HIP Diet EXchange (DEX)
The HIP Diet EXchange is a two-party cryptographic protocol used to
establish a secure communication context between hosts. The first
party is called the Initiator and the second party the Responder.
The four-packet exchange helps to make HIP DoS resilient. The
protocol exchanges Static Elliptic Curve Diffie-Hellman keys in the
2nd and 3rd packets and transmits keying material for the session key
and authenticates the parties in the 3rd and 4th packets.
Additionally, the Responder starts a puzzle exchange in the 2nd
packet, with the Initiator completing it in the 3rd packet before the
Responder performs computationally expensive operations or stores any
state from the exchange. Hence, HIP DEX operationally is very
similar to HIP BEX. The model is also fairly equivalent to
802.11-2007 [IEEE.802-11.2007] Master Key and Pair-wise Transient
Key, but handled in a single exchange.
HIP DEX does not have the option to encrypt the Host Identity of the
Initiator in the 3rd packet. The Responder's Host Identity also is
not protected. Thus, contrary to HIPv2, there is no attempt at
anonymity.
Data packets start to flow after the 4th packet. The 3rd and 4th HIP
packets may carry a data payload in the future. However, the details
of this may be defined later.
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An existing HIP association can be updated with the update mechanism
defined [I-D.ietf-hip-rfc5201-bis]. Likewise, the association can be
torn down with the defined closing mechanism for HIPv2 if it is no
longer needed. HIP DEX thereby omits the HIP_SIGNATURE parameters of
the original HIPv2 specification.
Finally, HIP DEX is designed as an end-to-end authentication and key
establishment protocol. As such, it can be used in combination with
Encapsulated Security Payload (ESP) [I-D.ietf-hip-rfc5202-bis] as
well as with other end-to-end security protocols. In addition, HIP
DEX can also be used as a keying mechanism for security primitives at
the MAC layer, e.g., for IEEE 802.15.4 networks [IEEE.802-15-4.2011].
It is worth mentioning that the HIP DEX base protocol does not cover
all the fine-grained policy control found in Internet Key Exchange
Version 2 (IKEv2) [RFC5996] that allows IKEv2 to support complex
gateway policies. Thus, HIP DEX is not a replacement for IKEv2.
1.2. Memo Structure
The rest of this memo is structured as follows. Section 2 defines
the central keywords, notation, and terms used throughout the rest of
the document. Section 3 defines the structure of the Host Identity
and its various representations. Section 4 gives an overview of the
HIP Diet EXchange protocol. Sections 5 and 6 define the detailed
packet formats and rules for packet processing. Finally, Sections 7,
8, and 9 discuss policy, security, and IANA considerations,
respectively.
2. Terms and Definitions
2.1. Requirements Terminology
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 [RFC2119].
2.2. Notation
[x] indicates that x is optional.
{x} indicates that x is encrypted.
X(y) indicates that y is a parameter of X.
<x>i indicates that x exists i times.
--> signifies "Initiator to Responder" communication (requests).
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<-- signifies "Responder to Initiator" communication (replies).
| signifies concatenation of information-- e.g., X | Y is the
concatenation of X and Y.
FOLD (X, K) denotes the partitioning of X into n K-bit fragments
and the iterative folding of these fragments via XOR. The last
fragment thereby is padded to K bit by appending 0 bits, i.e., X =
x_1, x_2, ..., x_n, where length of x_i is K and x_n is padded to
length K via 0 bits. FOLD then is computed as FOLD(X, K) = t_n,
where t_i = t_i-1 XOR x_i and t_1 = x_1.
Ltrunc (M(x), K) denotes the lowest order K bits of the result of
the MAC function M on the input x.
2.3. Definitions
HIP Diet Exchange (DEX): The ECDH-based HIP handshake for
establishing a new HIP association.
Host Identity (HI): The static ECDH public key that represents the
identity of the host. In HIP DEX, a host proves ownership of the
private key belonging to its HI by creating a HIP_MAC with the
derived ECDH key (c.f. Section 3).
Host Identity Tag (HIT): A shorthand for the HI in IPv6 format. It
is generated by folding the HI (c.f. Section 3).
HIT Suite: A HIT Suite groups all algorithms that are required to
generate and use an HI and its HIT. In particular, these
algorithms are: 1) ECDH and 2) FOLD.
HIP association: The shared state between two peers after completion
of the DEX.
Initiator: The host that initiates the DEX. This role is typically
forgotten once the DEX is completed.
Responder: The host that responds to the Initiator in the DEX. This
role is typically forgotten once the DEX is completed.
Responder's HIT Hash Algorithm (RHASH): In HIP DEX, RHASH is
redefined as CMAC. Still, note that CMAC is a message
authentication code and not a cryptographic hash function. Thus,
a mapping from CMAC(x,y) to RHASH(z) must be defined where RHASH
is used. Moreover, RHASH has different security properties in HIT
DEX and is not used for HIT generation.
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Length of the Responder's HIT Hash Algorithm (RHASH_len): The
natural output length of RHASH in bits.
CKDF: CMAC-based Key Derivation Function.
3. Host Identity (HI) and its Structure
In this section, the properties of the Host Identity and Host
Identity Tag are discussed, and the exact format for them is defined.
In HIP, the public key of an asymmetric key pair is used as the Host
Identity (HI). Correspondingly, the host itself is defined as the
entity that holds the private key of the key pair. See the HIP
architecture specification [I-D.ietf-hip-rfc4423-bis] for more
details on the difference between an identity and the corresponding
identifier.
HIP DEX implementations MUST support the Elliptic Curve Diffie-
Hellman (ECDH) [RFC6090] key exchange for generating the HI as
defined in Section 5.2.3. No additional algorithms are supported at
this time.
A compressed encoding of the HI, the Host Identity Tag (HIT), is used
in the handshake to represent the Host Identity. The DEX Host
Identity Tag (HIT) is different from the BEX HIT in two ways:
o The HIT suite ID MUST only be a DEX HIT ID (see Section 5.2.1).
o The DEX HIT is not generated via a cryptographic hash. Rather, it
is a compression of the Host Identity.
Due to the latter property, an attacker may be able to find a
collision with a HIT that is in use. Hence, policy decisions such as
access control MUST NOT be based solely on the HIT. Instead, the HI
of a host SHOULD be considered.
Carrying HIs and HITs in the header of user data packets would
increase the overhead of packets. Thus, it is not expected that
these parameters are carried in every packet, but other methods are
used to map the data packets to the corresponding HIs. In some
cases, this allows to use HIP DEX without any additional headers in
the user data packets. For example, if ESP is used to protect data
traffic, the Security Parameter Index (SPI) carried in the ESP header
can be used to map the encrypted data packet to the correct HIP DEX
association.
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3.1. Host Identity Tag (HIT)
With HIP DEX, the Host Identity Tag is a 128-bit value - a
compression of the HI prepended with a specific prefix. There are
two advantages of using a hashed encoding over the actual variable-
sized Host Identity public key in protocols. First, the fixed length
of the HIT keeps packet sizes manageable and eases protocol coding.
Second, it presents a consistent format for the protocol, independent
of the underlying identity technology in use.
The structure of the HIT is based on RFC 4843-bis
[I-D.ietf-hip-rfc4843-bis], called Overlay Routable Cryptographic
Hash Identifiers (ORCHIDs), and consists of three parts: first, an
IANA assigned prefix to distinguish it from other IPv6 addresses.
Second, a four-bit encoding of the algorithms that were used for
generating the HI and the compressed representation of HI. Third, a
96-bit hashed representation of the Host Identity. In contrast to
HIPv2, HIP DEX employs HITs that are NOT generated by means of a
cryptographic hash. Instead, the HI is compressed to 96 bits as
defined in the following section.
3.2. Generating a HIT from an HI
The HIT does not follow the exact semantics of an ORCHID as there is
no hash function in HIP DEX. Still, its structure is strongly
aligned with the ORCHID design. The same IPv6 prefix used in BEX is
used for DEX. The DEX HIT suite (see Section 9) is used for the four
bits of the Orchid Generation Algorithm (OGA) field in the ORCHID.
The hash representation in an ORCHID is replaced with FOLD(HI,96).
4. Protocol Overview
This section gives a simplified overview of the HIP DEX protocol
operation and does not contain all the details of the packet formats
or the packet processing steps. Section 5 and Section 6 describe
these aspects in more detail and are normative in case of any
conflicts with this section. Importantly, the information given in
this section focuses on the differences between the HIPv2 and HIP DEX
protocol specifications.
4.1. Creating a HIP Association
By definition, the system initiating a HIP Diet EXchange is the
Initiator, and the peer is the Responder. This distinction is
typically forgotten once the base exchange completes, and either
party can become the Initiator in future communications.
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The HIP Diet EXchange serves to manage the establishment of state
between an Initiator and a Responder. The first packet, I1,
initiates the exchange, and the last three packets, R1, I2, and R2,
constitute an authenticated Diffie-Hellman [DIF76] key exchange for
the Master Key SA generation. This Master Key SA is used by the
Initiator and the Responder to wrap secret keying material in the I2
and R2 packets. Based on the exchanged keying material, the peers
then derive a Pair-wise Key SA if cryptographic keys are needed,
e.g., for an ESP-based protection of user data.
The Initiator first sends a trigger packet, I1, to the Responder.
The packet contains the HIT of the Initiator and possibly the HIT of
the Responder, if it is known. Moreover, the I1 packet initializes
the negotiation of the Diffie-Hellman group that is used for
generating the the Master Key SA. Therefore, the I1 packet contains
a list of Diffie Hellman Group IDs supported by the Initiator. Note
that in some cases it may be possible to replace this trigger packet
by some other form of a trigger, in which case the protocol starts
with the Responder sending the R1 packet. In such cases, another
mechanism to convey the Initiator's supported DH Groups (e.g., by
using a default group) must be specified.
The second packet, R1, starts the actual authenticated Diffie-Hellman
exchange. It contains a puzzle -- a cryptographic challenge that the
Initiator must solve before continuing the exchange. The level of
difficulty of the puzzle can be adjusted based on level of trust with
the Initiator, current load, or other factors. In addition, the R1
contains the Responder's Diffie-Hellman parameter and lists of
cryptographic algorithms supported by the Responder. Based on these
lists, the Initiator can continue, abort, or restart the base
exchange with a different selection of cryptographic algorithms.
In the I2 packet, the Initiator MUST display the solution to the
received puzzle. Without a correct solution, the I2 message is
discarded. The I2 also contains a key wrap parameter that carries a
secret keying material of the Initiator. This keying material is
only half the final session key. The packet is authenticated by the
sender (Initiator) via a MAC.
The R2 packet acknowledges the receipt of the I2 packet and completes
the handshake. The R2 contains a key wrap parameter that carries the
rest of the keying material of the Responder. The packet is
authenticated by the sender (Responder) via a MAC.
The HIP DEX handshake is illustrated below. The terms "ENC(DH,x)"
and "ENC(DH,y)" refer to the random values x and y that are wrapped
based on the Master Key SA (indicated by ENC and DH). Note that x
and y each constitute half the final session key material. The
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packets also contain other parameters that are not shown in this
figure.
Initiator Responder
I1:
--------------------------------->
remain stateless
R1: puzzle, HI
<--------------------------------
solve puzzle
perform ECDH
encrypt x
I2: solution, HI, ENC(DH,x), mac
--------------------------------->
check puzzle
perform ECDH
check mac
decrypt x
encrypt y
R2: ENC(DH,y), mac
<---------------------------------
check mac
decrypt y
4.1.1. HIP Puzzle Mechanism
The purpose of the HIP puzzle mechanism is to protect the Responder
from a number of denial-of-service threats. It allows the Responder
to delay state creation until receiving the I2 packet. Furthermore,
the puzzle allows the Responder to use a fairly cheap calculation to
check that the Initiator is "sincere" in the sense that it has
churned enough CPU cycles in solving the puzzle.
The puzzle mechanism enables a Responder to immediately reject an I2
packet if it does not contain a valid puzzle solution. To verify the
puzzle solution, the Responder only has to compute a single CMAC
operation. After a successful puzzle verification, the Responder can
securely create session-specific state and perform CPU-intensive
operations such as a Diffie-Hellman key generation. By varying the
difficulty of the puzzle, the Responder can frustrate CPU or memory
targeted DoS attacks. Under normal network conditions, the puzzle
difficulty SHOULD be zero, thus effectively reverting the puzzle
mechanism to a cookie-based DoS protection mechanism. Without
setting the puzzle difficulty to zero under normal network
conditions, potentially scarce computation resources at the Initiator
would be churned unnecessarily.
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Conceptually, the puzzle mechanism in HIP DEX is the same as in
HIPv2. Hence, this document refers to Sections 4.1.1 and 4.1.2 in
[I-D.ietf-hip-rfc5201-bis] for more detailed information about the
employed mechanism. Notably, the only difference between the puzzle
mechanism in HIP DEX and HIPv2 is that HIP DEX uses CMAC instead of a
hash function for solving and verifying a puzzle. The implications
of this change on the puzzle implementation are discussed in
Section 6.1.
4.1.2. HIP State Machine
The HIP DEX state machine has the same states as the BEX state
machine (see 4.4. in [I-D.ietf-hip-rfc5201-bis]). However, HIP DEX
features an retransmission strategy with an optional packet receipt
for the I2. The goal of this packet receipt is reducing premature I2
retransmissions in sensor networks with low computation resources and
high packet loss [HUM13]. As a result, there are minor changes to
the transitioning steps between specific states. The following
section documents these differences in the HIP DEX state machine
compared to HIP BEX.
4.1.2.1. HIP DEX Retransmission Mechanism
For the retransmission of I1 and I2 packets, the Initiator adopts the
retransmission strategy of HIP BEX (see Section 4.4.3. in
[I-D.ietf-hip-rfc5201-bis]). This strategy is based on a timeout
value that is set to the worst-case anticipated round-trip time
(RTT). For each received I1 or I2, the Responder sends an R1 or R2,
respectively. This design trait enables the Responder to remain
stateless until the reception of the I2. The Initiator stops
retransmitting I1 or I2 packets after the reception of the
corresponding R1 or R2. If the Initiator did not receive an R1 after
I1_RETRIES_MAX tries, it stops I1 retransmissions. Likewise, it
stops retransmitting I2 packets after I2_RETRIES_MAX unsuccessful
tries.
The Responder SHOULD NOT perform operations related to the Diffie-
Hellman key exchange or the keying material wrapped in the
ENCRYPTED_KEY parameters for retransmitted I2 packets. Instead, it
SHOULD re-use the previously established state to re-create the R2.
The potentially high processing time of an I2 packet at the Responder
may cause retransmissions if the time required for I2 transmission
and processing exceeds the RTT-based retransmission timeout. Thus,
the Initiator should also take the processing time of I2 packets into
account. To this end, the Responder MAY optionally notify the
Initiator about the anticipated delay if the I2 incurs a considerable
processing overhead. The Responder MAY therefore send a NOTIFY
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packet to the Initiator before it commences the ECDH operation. The
NOTIFY packet serves as an acknowledgement for the I2 and consists of
a NOTIFICATION parameter with Notify Message Type I2_ACKNOWLEDGEMENT
(see Section 5.2.19. in [I-D.ietf-hip-rfc5201-bis]). The
NOTIFICATION parameter contains the anticipated remaining processing
time for the I2 packet in milliseconds as Notification Data . This
processing time can, e.g., be estimated by measuring the computation
time of the ECDH key derivation operation at Responder boot-up.
After the I2 processing has finished, the Responder sends the regular
R2.
When the Initiator receives the NOTIFY packet, it resets the I2
retransmission timer to the processing time indicated by the
Responder in the NOTIFICATION parameter. If the indicated processing
time is shorter than the RTT-based timeout, the Initiator MUST set
the retransmission timer to the RTT-based timeout. Additionally, the
Initiator MUST NOT set a higher retransmission timeout than allowed
by a local policy. Hence, I2 retransmissions are never triggered in
shorter succession than without this optional retransmission
extension. Moreover, there is a defined upper bound to which
unauthenticated NOTIFY messages can delay the handshake in case of
lost R2 packets.
4.1.2.2. HIP State Processes
HIP DEX clarifies or introduces the following new transitions.
System behavior in state I2-SENT, Table 1.
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| Receive NOTIFY, | Set I2 retransmission timer according to |
| process | NOTIFY payload and stay at I2-SENT |
| | |
| Timeout | Increment timeout counter |
| | |
| | If counter is less than I2_RETRIES_MAX, |
| | send I2, reset timer to RTT and stay at |
| | I2-SENT |
| | |
| | If counter is greater than I2_RETRIES_MAX, |
| | go to E-FAILED |
+---------------------+---------------------------------------------+
Table 1: I2-SENT - Waiting to finish the HIP Diet EXchange
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4.1.2.3. Simplified HIP State Diagram
The following diagram shows the major state transitions. Transitions
based on received packets implicitly assume that the packets are
successfully authenticated or processed.
+-+ +------------------------------+
I1 received, send R1 | | | |
| v v |
Datagram to send +--------------+ I2 received, send R2 |
Send I1 +--------------| UNASSOCIATED |--------------+ |
+-+ | +-+ +--------------+ | |
send | | | | | | |
I1 t | | | | | Alg. not supported, send I1 | |
msec v | v | v | |
+---------+ I2 received, send R2 | |
+---->| I1-SENT |-------------------------------------+ | |
| +---------+ | | |
| | +----------------------+ | | +-+receive |
| send I2+-+ | R1 received, | I2 received, send R2 | | | | |I2, |
| t msec | v v send I2 | v v v | v send R2 |
| +---------+ | +---------+ |
| +->| I2-SENT |------------+ | R2-SENT |<--+ |
| | +---------+ +---------+ | |
| | | | | |
| | | data| | |
| |receive | or| | |
| |R1, send | EC timeout| receive I2,| |
| |I2 |R2 received +--------------+ | send R2| |
| | +----------->| ESTABLISHED |<--------+ | |
| | +--------------+ | |
| | | | | receive I2, send R2 | |
| | recv+------------+ | +------------------------+ |
| | CLOSE,| | | |
| | send| No packet sent| | |
| | CLOSE_ACK| /received for | timeout | |
| | | UAL min, send | +---------+<-+ (UAL+MSL) | |
| | | CLOSE +--->| CLOSING |--+ retransmit | |
| | | +---------+ CLOSE | |
+--|------------|----------------------+| | | | | |
+------------|-----------------------+ | | +-----------------+ |
| | +-----------+ +-------------------|----+
| +-----------+ | receive CLOSE, CLOSE_ACK | |
| | | send CLOSE_ACK received or | |
| | | timeout | |
| | | (UAL+MSL) | |
| v v | |
| +--------+ receive I2, send R2 | |
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+-----------------------| CLOSED |----------------------------+ |
+--------+ /-------------------------+
^ | \-------/ timeout (UAL+2MSL),
| | move to UNASSOCIATED
+-+
CLOSE received, send CLOSE_ACK
4.1.3. HIP DEX Security Associations
HIP DEX establishes two Security Associations (SA), one for the
Diffie-Hellman derived key, or Master Key, and one for the session
key, or Pair-wise Key.
4.1.3.1. Master Key SA
The Master Key SA is used to authenticate HIP packets and to encrypt
selected HIP parameters in HIP DEX packet exchanges. Since only
little data is protected by this SA, it can be long-lived with no
need for rekeying.
The Master Key SA contains the following elements:
o Source HIT
o Destination HIT
o HIP_Encrypt Key
o HIP_MAC Key
The HIP_Encrypt and HIP_MAC keys are extracted from the Diffie-
Hellman derived key as described in Section 6.3. Their length is
determined by the HIP_CIPHER.
4.1.3.2. Pair-wise Key SA
The Pair-wise Key SA is used to authenticate and to encrypt user
data. It is refreshed (or rekeyed) using an UPDATE packet exchange.
The Pair-wise Key SA elements are defined by the data transform (e.g.
ESP_TRANSFORM [I-D.ietf-hip-rfc5202-bis]).
The keys for the Pair-wise Key SA are derived based on the wrapped
keying material exchanged in the ENCRYPTED_KEY parameter (see
Section 5.2.5) of the I2 and R2 packets. Specifically, the exchanged
keying material of the two peers is concatenated. This concatenation
forms the input to a Key Derivation Function (KDF). If the data
transform does not specify its own KDF, the key derivation function
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defined in Section 6.3 is used. Even though this input is randomly
distributed, a KDF Extract phase may be needed to get the proper
length for the input to the KDF Expand phase.
4.1.4. User Data Considerations
The User Data Considerations in Section 4.5. of
[I-D.ietf-hip-rfc5201-bis] also apply to HIP DEX. There is only one
difference between HIPv2 and HIP DEX. Loss of state due to system
reboot may be a critical performance issue for constrained sensor/
actuator devices. Thus, implementors MAY choose to use non-volatile,
secure storage for HIP states in order for them to survive a system
reboot. This will limit state loss during reboots to only those
situations with an SA timeout.
5. Packet Formats
5.1. Payload Format
HIP DEX employs the same fixed HIP header and payload structure as
HIP BEX. As such, the specifications in Section 5.1 of
[I-D.ietf-hip-rfc5201-bis] also apply to HIP DEX.
5.2. HIP Parameters
The HIP parameters carry information that is necessary for
establishing and maintaining a HIP association. For example, the
peer's public keys as well as the signaling for negotiating ciphers
and payload handling are encapsulated in HIP parameters. Additional
information, meaningful for end-hosts or middleboxes, may also be
included in HIP parameters. The specification of the HIP parameters
and their mapping to HIP packets and packet types is flexible to
allow HIP extensions to define new parameters and new protocol
behavior.
In HIP packets, HIP parameters are ordered according to their numeric
type number and encoded in TLV format.
HIP DEX reuses the HIP parameters of HIP BEX defined in Section 5.2.
of [I-D.ietf-hip-rfc5201-bis] where possible. Still, HIP DEX further
restricts and/or extends the following existing parameter types:
o HIT_SUITE_LIST is limited to HIT suite ECDH/FOLD.
o DH_GROUP_LIST and HOST_ID are restricted to ECC-based suites.
o HIP_CIPHER is restricted to NULL-ENCRYPT and AES-128-CTR.
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o RHASH and RHASH_len are redefined to CMAC for PUZZLE, SOLUTION,
HIP_MAC (see Section 6.1 and Section 6.2).
In addition, HIP DEX introduces the following new parameter:
+------------------+------+----------+------------------------------+
| TLV | Type | Length | Data |
+------------------+------+----------+------------------------------+
| ENCRYPTED_KEY | 643 | variable | Encrypted container for key |
| | | | generation exchange |
+------------------+------+----------+------------------------------+
5.2.1. HIT_SUITE_LIST
The HIT_SUITE_LIST parameter contains a list of the supported HIT
suite IDs of the Responder. The HIT suites in DEX are limited to:
HIT suite ID
ECDH/FOLD 8
Since the HIT of the Initiator is a DEX HIT, the Responder MUST only
respond with a DEX HIT suite ID.
5.2.2. DH_GROUP_LIST
The DH_GROUP_LIST parameter contains the list of supported DH Group
IDs of a host. The following ECC curves are supported in HIP DEX:
Group KDF Value
NIST P-256 [RFC5903] CKDF 7
NIST P-384 [RFC5903] CKDF 8
NIST P-521 [RFC5903] CKDF 9
SECP160R1 [SECG] CKDF 10
The ECDH groups 7 - 9 are defined in [RFC5903] and [RFC6090]. ECDH
group 10 is covered in Appendix D of [I-D.ietf-hip-rfc5201-bis].
5.2.3. HOST_ID
The HI Algorithms in DEX are limited to:
Algorithm
profiles Values
ECDH 1
ECC-based Host Identities are serialized as described in
Section 5.2.9. of [I-D.ietf-hip-rfc5201-bis]. The supported curves
for the HI in HIP DEX are defined in Section 5.2.2.
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5.2.4. HIP_CIPHER
The HIP ciphers in DEX are limited to:
Suite ID Value
RESERVED 0
NULL-ENCRYPT 1 ([RFC2410])
AES-128-CTR 5 ([RFC3686])
Mandatory implementation: AES-128-CTR. NULL-ENCRYPTION [RFC2410] is
included for testing purposes.
5.2.5. ENCRYPTED_KEY
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Encrypted value /
/ /
/ +-------------------------------+
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 643
Length length in octets, excluding Type, Length, and
Padding
Encrypted The value is encrypted using an encryption algorithm
value as defined in the HIP_CIPHER parameter.
The ENCRYPTED_KEY parameter encapsulates a random value that is later
used in the session key creation process. The Puzzle value I (see
[I-D.ietf-hip-rfc5201-bis]) is used as the initialization vector (IV)
for the encryption process. The AES-CTR counter is a 16 bit value
that is initialized to zero with the first use.
Once this encryption process is completed, the "encrypted value" data
field is ready for inclusion in the Parameter. If necessary,
additional Padding for 8-byte alignment is then added according to
the rules of TLV Format in [I-D.ietf-hip-rfc5201-bis].
5.3. HIP Packets
DEX uses the same eight basic HIP packets as in BEX (see
[I-D.ietf-hip-rfc5201-bis]). Four are for the HIP handshake (I1, R1,
I2, and R2), one is for updating (UPDATE), one is for sending
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notifications (NOTIFY), and two are for closing a HIP association
(CLOSE and CLOSE_ACK). There are some differences regarding the
included HIP parameters in the exchange packets of BEX and DEX. This
section covers these differences for the DEX packets. Packets not
discussed here, follow the structure defined in
[I-D.ietf-hip-rfc5201-bis].
An important difference between packets in HIP BEX and HIP DEX is
that the DIFFIE_HELLMAN and the HIP_SIGNATURE parameters are not
included in DEX. Thus, the R1 packet is completely unprotected and
can be spoofed. As a result, negotiation parameters contained in the
R1 packet have to be re-included in later, protected packets in order
to detect and prevent potential downgrading attacks. Moreover, the
I2, R2, UPDATE, NOTIFY, CLOSE, and CLOSE_ACK packets are not covered
by a signature and purely rely on the HIP_MAC parameter for packet
authentication. The processing of these packets is changed
accordingly.
In the future, an OPTIONAL upper-layer payload MAY follow the HIP
header. The Next Header field in the header indicates if there is
additional data following the HIP header.
5.3.1. I1 - the HIP Initiator Packet
The HIP header values for the I1 packet:
Header:
Packet Type = 1
SRC HIT = Initiator's HIT
DST HIT = Responder's HIT, or NULL
IP ( HIP ( DH_GROUP_LIST ) )
Minimum size = 48 bytes
Valid control bits: none
The I1 packet contains the fixed HIP header and the Initiator's
DH_GROUP_LIST. The Initiator's HIT Suite ID MUST be of a DEX type.
The Initiator receives the Responder's HIT either from a DNS lookup
of the Responder's FQDN, from some other repository, or from a local
table. The Responder's HIT MUST be a DEX HIT. If the Initiator does
not know the Responder's HIT, it may attempt to use opportunistic
mode by using NULL (all zeros) as the Responder's HIT. See also "HIP
Opportunistic Mode" [I-D.ietf-hip-rfc5201-bis].
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The Initiator includes a DH_GROUP_LIST parameter in the I1 to inform
the Responder of its preferred DH Group IDs. Only ECDH groups may be
included in this list. The group ID corresponding to the current
Initiator HIT MUST be listed first in this DH_GROUP_LIST parameter to
notify the Responder about the DH group that must be used to
successfully complete the handshake.
Since this packet is so easy to spoof even if it were protected, no
attempt is made to add to its generation or processing cost. As a
result, the DH_GROUP_LIST in the I1 packet is not protected.
Implementations MUST be able to handle a storm of received I1
packets, discarding those with common content that arrive within a
small time delta.
5.3.2. R1 - the HIP Responder Packet
The HIP header values for the R1 packet:
Header:
Packet Type = 2
SRC HIT = Responder's HIT
DST HIT = Initiator's HIT
IP ( HIP ( [ R1_COUNTER, ]
PUZZLE,
DH_GROUP_LIST,
HIP_CIPHER,
HOST_ID,
HIT_SUITE_LIST,
TRANSPORT_FORMAT_LIST,
[ <, ECHO_REQUEST_UNSIGNED >i ])
Minimum size = 120 bytes
Valid control bits: A
If the Responder's HI is an anonymous one, the A control MUST be set.
The Initiator's HIT MUST match the one received in the I1 packet if
the R1 is a response to an I1. If the Responder has multiple HIs,
the Responder's HIT MUST match the Initiator's request. If the
Initiator used opportunistic mode, the Responder may select among its
HIs as described below. See also "HIP Opportunistic Mode"
[I-D.ietf-hip-rfc5201-bis].
The R1 packet generation counter is used to determine the currently
valid generation of puzzles. The value is increased periodically,
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and it is RECOMMENDED that it is increased at least as often as
solutions to old puzzles are no longer accepted.
The Puzzle contains a Random value #I and the puzzle difficulty K.
The difficulty K indicates the number of lower-order bits, in the
puzzle CMAC result, that MUST be zeros (see
[I-D.ietf-hip-rfc5201-bis]). Responders SHOULD set K to zero by
default and only increase the puzzle difficulty to protect against a
DoS attack targeting the DEX handshake. A puzzle difficulty of zero
effectively disables the puzzle mechanism and is strongly encouraged
to conserve energy resources as well as to prevent unnecessary
handshake delay in case of a resource-constrained Initiator during
normal operation.
The HIP_CIPHER contains the encryption algorithms supported by the
Responder to protect the key exchange, in the order of preference.
All implementations MUST support the AES-CTR [RFC3686].
The used Initiator HIT already determines the DH group that must be
used to successfully conclude the handshake. However, while the
Initiator HIT conveys that ECDH keys must be used for the HOST_ID, it
does not provide information about the employed DH group. Still, the
Responder can determine the DH Group ID based on the DH_GROUP_LIST
parameter in the I1. If the I1 contains a Responder HIT, the
Responder verifies that its Responder HIT matches the required DH
group of the Initiator. The Responder then selects the corresponding
HOST_ID for inclusion in the R1. If the Responder HIT is NULL (i.e.,
during an opportunistic handshake) the Responder chooses its HOST_ID
according to the Initiator's preference expressed in the
DH_GROUP_LIST parameter in the I1. The Responder sends back its own
preference based on which it chose the HOST_ID as DH_GROUP_LIST.
This allows the Initiator to determine whether its own DH_GROUP_LIST
in the I1 was manipulated by an attacker. There is a further risk
that the Responder's DH_GROUP_LIST was manipulated by an attacker, as
R1 cannot be authenticated in DEX as it can in BEX. Thus, it is
repeated in R2 allowing for a final check at that point.
The HIT_SUITE_LIST parameter is an ordered list of the Responder's
supported and preferred HIT Suites. It enables a Responder to notify
the Initiator about other available HIT suites than the one used in
the current handshake. Based on the received HIT_SUITE_LIST, the
Initiator MAY decide to abort the current handshake and initiate a
new handshake based on a different mutually supported HIT suite.
This mechanism can, e.g., be used to move from an initial HIP DEX
handshake to a HIP BEX handshake for peers supporting both protocol
variants.
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The TRANSPORT_FORMAT_LIST parameter is an ordered list of the
Responder's supported and preferred transport format types. The list
allows the Initiator and the Responder to agree on a common type for
payload protection.
The ECHO_REQUEST_UNSIGNED parameters contain data that the sender
wants to receive unmodified in the corresponding response packet in
the ECHO_RESPONSE_UNSIGNED parameter. The R1 packet may contain zero
or more ECHO_REQUEST_UNSIGNED parameters.
5.3.3. I2 - the Second HIP Initiator Packet
The HIP header values for the I2 packet:
Header:
Type = 3
SRC HIT = Initiator's HIT
DST HIT = Responder's HIT
IP ( HIP ( [R1_COUNTER,]
SOLUTION,
HIP_CIPHER,
ENCRYPTED_KEY,
HOST_ID,
TRANSPORT_FORMAT_LIST,
HIP_MAC,
[<, ECHO_RESPONSE_UNSIGNED>i )] )
Minimum size = 180 bytes
Valid control bits: A
The HITs used MUST match the ones used in the R1.
If the Initiator's HI is an anonymous one, the A control bit MUST be
set.
If present in the I1 packet, the Initiator MUST include an unmodified
copy of the R1_COUNTER parameter received in the corresponding R1
packet into the I2 packet.
The Solution contains the Random #I from R1 and the computed #J. The
low-order #K bits of the RHASH(I | ... | J) MUST be zero.
The HIP_CIPHER contains the single encryption transform selected by
the Initiator, that it uses to encrypt the ENCRYPTED and
ENCRYPTED_KEY parameters. The chosen cipher MUST correspond to one
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of the ciphers offered by the Responder in the R1. All
implementations MUST support the AES-CTR transform [RFC3686].
The HOST_ID parameter contains the Initiator HI corresponding to the
Initiator HIT.
The ENCRYPTED_KEY contains an Initiator generated random value that
MUST be uniformly distributed. This random value is encrypted with
the Master Key SA using the HIP_CIPHER encryption algorithm.
The ECHO_RESPONSE_UNSIGNED contains the unmodified Opaque data copied
from the corresponding echo request parameter(s). This parameter can
also be used for two-factor password authentication as shown in
Appendix B.
The TRANSPORT_FORMAT_LIST contains the single transport format type
selected by the Initiator. The chosen type MUST correspond to one of
the types offered by the Responder in the R1. Currently, the only
transport format defined is the ESP transport format
[I-D.ietf-hip-rfc5202-bis].
The MAC is calculated over the whole HIP envelope, excluding any
parameters after the HIP_MAC, as described in Section 6.2. The
Responder MUST validate the HIP_MAC.
5.3.4. R2 - the Second HIP Responder Packet
The HIP header values for the R2 packet:
Header:
Packet Type = 4
SRC HIT = Responder's HIT
DST HIT = Initiator's HIT
IP ( HIP ( DH_GROUP_LIST,
HIP_CIPHER,
ENCRYPTED_KEY,
HIT_SUITE_LIST,
TRANSPORT_FORMAT_LIST,
HIP_MAC)
Minimum size = 108 bytes
Valid control bits: none
The HITs used MUST match the ones used in the I2.
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The Responder repeats the DH_GROUP_LIST, HIP_CIPHER, HIT_SUITE_LIST,
and TRANSPORT_FORMAT_LIST parameters in R2. These parameters MUST be
the same as included in R1. The parameter are re-included here
because R2 is MACed and thus cannot be altered by an attacker. For
verification purposes, the Initiator re-evaluates the selected suites
and compares the results against the chosen ones. If the re-
evaluated suites do not match the chosen ones, the Initiator acts
based on its local policy.
The ENCRYPTED_KEY contains an Responder generated random value that
MUST be uniformly distributed. This random value is encrypted with
the Master Key SA using the HIP_CIPHER encryption algorithm.
The MAC is calculated over the whole HIP envelope, excluding any
parameters after the HIP_MAC, as described in Section 6.2. The
Initiator MUST validate the HIP_MAC.
5.4. ICMP Messages
When a HIP implementation detects a problem with an incoming packet,
and it either cannot determine the identity of the sender of the
packet or does not have any existing HIP association with the sender
of the packet, it MAY respond with an ICMP packet. Any such reply
MUST be rate-limited as described in [RFC4443]. In most cases, the
ICMP packet has the Parameter Problem type (12 for ICMPv4, 4 for
ICMPv6), with the Pointer field pointing to the field that caused the
ICMP message to be generated. The problem cases specified in
Section 5.4. of [I-D.ietf-hip-rfc5201-bis] also apply to HIP DEX.
6. Packet Processing
Due to the adopted protocol semantics and the inherited general
packet structure, packet processing in HIP DEX only differs from HIP
BEX in very few places. Here, we focus on these differences and
refer to Section 6 in [I-D.ietf-hip-rfc5201-bis] otherwise.
The processing of outgoing and incoming application data remains the
same as in HIP BEX (see Sections 6.1 and 6.2 in
[I-D.ietf-hip-rfc5201-bis]).
6.1. Solving the Puzzle
The procedures for solving and verifying a puzzle in HIP DEX are
strongly based on the corresponding procedures in HIPv2. The only
exceptions are that HIP DEX does not use pre-computation of R1
packets and that RHASH is set to CMAC. As a result, the pre-
computation step in in Section 6.3 of [I-D.ietf-hip-rfc5201-bis] is
skipped in HIP DEX.
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Moreover, the Initiator solves a puzzle by computing:
Ltrunc( CMAC( I, HIT-I | HIT-R | J ), K ) == 0
Similarly, the Responder verifies a puzzle by computing:
V := Ltrunc( CMAC( I, HIT-I | HIT-R | J ), K )
Apart from these modifications, the procedures defined in Section 6.3
of [I-D.ietf-hip-rfc5201-bis] also apply for HIP DEX.
6.2. HIP_MAC Calculation and Verification
The following subsections define the actions for processing the
HIP_MAC parameter.
6.2.1. CMAC Calculation
The HIP_MAC calculation uses RHASH, i.e., CMAC, as the underlying
cryptographic function. The scope of the calculation for HIP_MAC is:
CMAC: { HIP header | [ Parameters ] }
where Parameters include all HIP parameters of the packet that is
being calculated with Type values ranging from 1 to (HIP_MAC's Type
value - 1) and exclude parameters with Type values greater or equal
to HIP_MAC's Type value.
During HIP_MAC calculation, the following applies:
o In the HIP header, the Checksum field is set to zero.
o In the HIP header, the Header Length field value is calculated to
the beginning of the HIP_MAC parameter.
Parameter order is described in [I-D.ietf-hip-rfc5201-bis].
The CMAC calculation and verification process is as follows:
Packet sender:
1. Create the HIP packet, without the HIP_MAC or any other parameter
with greater Type value than the HIP_MAC parameter has.
2. Calculate the Header Length field in the HIP header.
3. Compute the CMAC using either HIP-gl or HIP-lg integrity key
retrieved from KEYMAT as defined in Section 6.3.
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4. Add the HIP_MAC parameter to the packet and any parameter with
greater Type value than the HIP_MAC's that may follow.
5. Recalculate the Length field in the HIP header.
Packet receiver:
1. Verify the HIP header Length field.
2. Remove the HIP_MAC parameter, as well as all other parameters
that follow it with greater Type value, saving the contents if
they will be needed later.
3. Recalculate the HIP packet length in the HIP header and clear the
Checksum field (set it to all zeros).
4. Compute the CMAC using either HIP-gl or HIP-lg integrity key as
defined in Section 6.3 and verify it against the received CMAC.
5. Set Checksum and Header Length fields in the HIP header to
original values.
6.3. HIP DEX KEYMAT Generation
The HIP DEX KEYMAT process is used to derive the keys for Master Key
SA as well as for the Pair-wise Key SA. The keys for the Master Key
SA are based from the Diffie-Hellman derived key, Kij, produced
during the HIP Diet EXchange. The Initiator generates Kij during the
creation of the I2 packet and the Responder generates Kij once it
receives the I2 packet. Hence, I2, R2, UPDATE, CLOSE, and CLOSE_ACK
packets can contain authenticated and/or encrypted information.
The keys of the Pair-wise Key SA are not directly used in the HIP DEX
handshake. Instead, these keys are made available as payload
protection keys. Some payload protection mechanisms have their own
Key Derivation Function, and if so this mechanism SHOULD be used.
Otherwise, the HIP DEX KEYMAT process MUST be used.
The HIP DEX KEYMAT process consists of two components, CKDF-Extract
and CKDF-Expand. The Extract function COMPRESSES a non-uniformly
distributed key, as is the output of a Diffie-Hellman key derivation,
to EXTRACT the key entropy into a fixed length output. The Expand
function takes either the output of the Extract function or directly
uses a uniformly distributed key and EXPANDS the length of the key,
repeatedly distributing the key entropy, to produce the keys needed.
The key derivation for Master Key SA employs both the Extract and
Expand phases, while the Pair-wise Key SA MAY need the Extract and
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Expand phases if the key is longer than 128 bits. Otherwise, it only
needs the Expand phase.
The CKDF-Extract function is the following operation, where | denotes
the concatenation.
CKDF-Extract(I, BigK, info) -> PRK
where
I I from the PUZZLE parameter
BigK Diffie-Hellman derived key or concatenation of the
keying material exchanged in the ENCRYPTED_KEY
parameters in the same order as the HITs for the info
input (i.e., if HIT-I < HIT-R, then the keying
material of the Initiator precedes the keying
material of the Responder)
info sort(HIT-I | HIT-R) | "CKDF-Extract"
PRK a pseudorandom key (of RHASH_len/8 octets)
| denotes the concatenation
The pseudorandom key PRK is calculated as follows:
PRK = CMAC(I, BigK | info)
The CKDF-Expand function is the following operation:
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CKDF-Expand(PRK, info, L) -> OKM
PRK a pseudorandom key of at least RHASH_len/8 octets
(usually, the output from the extract step or
concatenation of the keying material exchanged
in the ENCRYPTED_KEY parameters in the same order
as the HITs for the info input, i.e., if
HIT-I < HIT-R, then the keying material of the
Initiator precedes the keying material of the
Responder)
info sort(HIT-I | HIT-R) | "CKDF-Expand"
L length of output keying material in octets
(<= 255*RHASH_len/8)
| denotes the concatenation
The output keying material OKM is calculated as follows:
N = ceil(L/RHASH_len/8)
T = T(1) | T(2) | T(3) | ... | T(N)
OKM = first L octets of T
where
T(0) = empty string (zero length)
T(1) = CMAC(PRK, T(0) | info | 0x01)
T(2) = CMAC(PRK, T(1) | info | 0x02)
T(3) = CMAC(PRK, T(2) | info | 0x03)
...
(where the constant concatenated to the end of each T(n) is a
single octet.)
sort(HIT-I | HIT-R) is defined as the network byte order
concatenation of the two HITs, with the smaller HIT preceding the
larger HIT, resulting from the numeric comparison of the two HITs
interpreted as positive (unsigned) 128-bit integers in network byte
order.
The initial keys are drawn sequentially in the order that is
determined by the numeric comparison of the two HITs, with comparison
method described in the previous paragraph. HOST_g denotes the host
with the greater HIT value, and HOST_l the host with the lower HIT
value.
The drawing order for initial keys:
1. HIP-gl encryption key for HOST_g's outgoing HIP packets
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2. HIP-gl integrity (CMAC) key for HOST_g's outgoing HIP packets
3. HIP-lg encryption key for HOST_l's outgoing HIP packets
4. HIP-lg integrity (CMAC) key for HOST_l's outgoing HIP packets
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 or 256 bits
If other key sizes are used, they must be treated as different
encryption algorithms and defined separately.
6.4. Initiation of a HIP Diet EXchange
The initiation of a HIP DEX handshake proceeds as described in
Section 6.6. of [I-D.ietf-hip-rfc5201-bis].
6.5. Processing Incoming I1 Packets
I1 packets in HIP DEX are handled identically to HIP BEX (see
Section 6.7. in [I-D.ietf-hip-rfc5201-bis]). The only differences
are that the Responder SHOULD select a DEX HIT in the R1 response.
Moreover, as R1 packets are neither covered by a signature nor incur
the overhead of generating an ephemeral Diffie-Hellman key-pair, pre-
computation of an R1 is only marginally beneficial, but would incur
additional memory resources. Hence, the R1 pre-computation is
omitted in HIP DEX.
6.6. Processing Incoming R1 Packets
R1 packets in HIP DEX are handled identically to HIP BEX with the
following differences (see Section 6.8. in
[I-D.ietf-hip-rfc5201-bis]). Only step 4 is omitted in HIP DEX as
there is no HIP_SIGNATURE in the R1 packet.
6.7. Processing Incoming I2 Packets
Upon receipt of an I2 packet, the system MAY perform initial checks
to determine whether the I2 packet corresponds to a recent R1 packet
that has been sent out, if the Responder keeps such state. For
example, the sender could check whether the I2 packet is from an
address or HIT for which the Responder has recently received an I1.
To this end, the R1 packet may have had Opaque data included that was
echoed back in the I2 packet. If the I2 packet is considered to be
suspect, it MAY be silently discarded by the system.
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Otherwise, the HIP implementation SHOULD process the I2 packet. This
includes validation of the puzzle solution, generating the Diffie-
Hellman key, verifying the MAC, extracting the ENCRYPTED_KEY,
creating state, and finally sending an R2 packet.
The following steps define the conceptual processing rules for
responding to an I2 packet:
1. The system MAY perform checks to verify that the I2 packet
corresponds to a recently sent R1 packet. Such checks are
implementation dependent. See Appendix A in
[I-D.ietf-hip-rfc5201-bis] for a description of an example
implementation.
2. The system MUST check that the Responder's HIT corresponds to
one of its own HITs and MUST drop the packet otherwise.
3. The system MUST further check that the Initiator's HIT Suite is
supported. The Responder SHOULD silently drop I2 packets with
unsupported Initiator HITs.
4. If the system's state machine is in the R2-SENT state, the
system MUST check if the newly received I2 packet is similar to
the one that triggered moving to R2-SENT. If so, it MUST
retransmit a previously sent R2 packet and the state machine
stays in R2-SENT.
5. If the system's state machine is in the I2-SENT state, the
system MUST make a comparison between its local and sender's
HITs (similarly as in Section 6.3). If the local HIT is smaller
than the sender's HIT, it should drop the I2 packet, use the
peer Diffie-Hellman key, ENCRYPTED_KEY keying material and nonce
#I from the R1 packet received earlier, and get the local
Diffie-Hellman key, ENCRYPTED_KEY keying material, and nonce #J
from the I2 packet sent to the peer earlier. Otherwise, the
system should process the received I2 packet and drop any
previously derived Diffie-Hellman keying material Kij and
ENCRYPTED_KEY keying material it might have generated upon
sending the I2 packet previously. The peer Diffie-Hellman key,
ENCRYPTED_KEY, and the nonce #J are taken from the just arrived
I2 packet. The local Diffie-Hellman key, ENCRYPTED_KEY keying
material, and the nonce I are the ones that were sent earlier in
the R1 packet.
6. If the system's state machine is in the I1-SENT state, and the
HITs in the I2 packet match those used in the previously sent I1
packet, the system uses this received I2 packet as the basis for
the HIP association it was trying to form, and stops
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retransmitting I1 packets (provided that the I2 packet passes
the additional checks below).
7. If the system's state machine is in any other state than R2-
SENT, the system SHOULD check that the echoed R1 generation
counter in the I2 packet is within the acceptable range if the
counter is included. Implementations MUST accept puzzles from
the current generation and MAY accept puzzles from earlier
generations. If the generation counter in the newly received I2
packet is outside the accepted range, the I2 packet is stale
(and perhaps replayed) and SHOULD be dropped.
8. The system MUST validate the solution to the puzzle as described
in Section 6.
9. The I2 packet MUST have a single value in the HIP_CIPHER
parameter, which MUST match one of the values offered to the
Initiator in the R1 packet.
10. The system must derive Diffie-Hellman keying material Kij based
on the public value and Group ID in the HOST_ID parameter. This
key is used to derive the keys of the Master Key SA as described
in Section 6.3. If the Diffie-Hellman Group ID is unsupported,
the I2 packet is silently dropped.
11. The implementation SHOULD also verify that the Initiator's HIT
in the I2 packet corresponds to the Host Identity sent in the I2
packet. (Note: some middleboxes may not able to make this
verification.)
12. The system MUST process the TRANSPORT_FORMAT_LIST parameter.
Other documents specifying transport formats (e.g.
[I-D.ietf-hip-rfc5202-bis]) contain specifications for handling
any specific transport selected.
13. The system MUST verify the HIP_MAC according to the procedures
in Section 5.2.12.
14. If the checks above are valid, then the system proceeds with
further I2 processing; otherwise, it discards the I2 and its
state machine remains in the same state.
15. The I2 packet may have the A bit set -- in this case, the system
MAY choose to refuse it by dropping the I2 and the state machine
returns to state UNASSOCIATED. If the A bit is set, the
Initiator's HIT is anonymous and should not be stored
permanently.
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16. The system MUST extract the keying material from the
ENCRYPTED_KEY parameter. This keying material is a partial
input to the key derivation process for the Pair-wise Key SA
(see Section 6.3).
17. The system initializes the remaining variables in the associated
state, including Update ID counters.
18. Upon successful processing of an I2 message when the system's
state machine is in state UNASSOCIATED, I1-SENT, I2-SENT, or R2-
SENT, an R2 packet is sent and the system's state machine
transitions to state R2-SENT.
19. Upon successful processing of an I2 packet when the system's
state machine is in state ESTABLISHED, the old HIP association
is dropped and a new one is installed, an R2 packet is sent, and
the system's state machine transitions to R2-SENT.
20. Upon the system's state machine transitioning to R2-SENT, the
system starts a timer. The state machine transitions to
ESTABLISHED if some data has been received on the incoming HIP
association, or an UPDATE packet has been received (or some
other packet that indicates that the peer system's state machine
has moved to ESTABLISHED). If the timer expires (allowing for
maximal amount of retransmissions of I2 packets), the state
machine transitions to ESTABLISHED.
6.8. Processing Incoming R2 Packets
An R2 packet received in states UNASSOCIATED, I1-SENT, or ESTABLISHED
results in the R2 packet being dropped and the state machine staying
in the same state. If an R2 packet is received in state I2-SENT, it
MUST be processed.
The following steps define the conceptual processing rules for an
incoming R2 packet:
1. If the system is in any other state than I2-SENT, the R2 packet
is silently dropped.
2. The system MUST verify that the HITs in use correspond to the
HITs that were received in the R1 packet that caused the
transition to the I1-SENT state.
3. The system MUST verify the HIP_MAC according to the procedures in
Section 6.2.
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4. The system MUST re-evaluate the DH_GROUP_LIST, HIP_CIPHER,
HIT_SUITE_LIST, and TRANSPORT_FORMAT_LIST parameters in the R2
and compare the results against the chosen suites.
5. If any of the checks above fail, there is a high probability of
an ongoing man-in-the-middle or other security attack. The
system SHOULD act accordingly, based on its local policy.
6. The system MUST extract the keying material from the
ENCRYPTED_KEY parameter. This keying material is a partial input
to the key derivation process for the Pair-wise Key SA (see
Section 6.3).
7. Upon successful processing of the R2 packet, the state machine
transitions to state ESTABLISHED.
6.9. Processing UPDATE, NOTIFY, CLOSE, and CLOSE_ACK Packets
UPDATE, NOTIFY, CLOSE, and CLOSE_ACK packets are handled similarly in
HIP DEX as in HIP BEX (see Sections 6.11. - 6.15. in
[I-D.ietf-hip-rfc5201-bis]). The only difference is the that the
HIP_SIGNATURE is never present and, therefore, is not needed be
processed.
6.10. Handling State Loss
Implementors MAY choose to use non-volatile, secure storage for HIP
states in order for them to survive a system reboot. If no secure
storage capabilities are available, the system SHOULD delete the
corresponding HIP state, including the keying material. If the
implementation does drop the state (as RECOMMENDED), it MUST also
drop the peer's R1 generation counter value, unless a local policy
explicitly defines that the value of that particular host is stored.
An implementation MUST NOT store a peer's R1 generation counters by
default, but storing R1 generation counter values, if done, MUST be
configured by explicit HITs.
7. HIP Policies
There are a number of variables that will influence the HIP exchanges
that each host must support. All HIP DEX implementations SHOULD
provide for an ACL of Initiator's HI to Responder's HI. This ACL
SHOULD also include preferred transform and local lifetimes.
Wildcards SHOULD also be supported for this ACL.
The value of the puzzle difficulty #K used in the HIP R1 must be
chosen with care. Too high numbers of #K will exclude clients with
weak CPUs because these devices cannot solve the puzzle within
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reasonable time. #K SHOULD only be raised if a Responder is under
high load, i.e., it can no longer process all incoming HIP
handshakes. Otherwise, the responder SHOULD set #K to 0.
8. Security Considerations
HIP DEX replaces the SIGMA-based authenticated Diffie-Hellman key
exchange of HIPv2 with an exchange of random keying material that is
encrypted by a Diffie-Hellman derived key. Both the Initiator and
Responder contribute to this keying material.
o The strength of the keys for the Pair-wise Key SA is based on the
quality of the random keying material generated by the Initiator
and Responder. Since the Initiator is expected to be a sensor/
actuator device, there is a natural concern about the quality of
its random number generator.
o DEX lacks Perfect Forward Secrecy (PFS). If the Initiator's HI is
compromised, ALL HIP connections protected with that HI are
compromised.
o The puzzle mechanism using CMAC may need further study that it
does present the desired level of difficulty.
o The DEX HIT generation MAY present new attack opportunities;
further study is needed.
The R1 packet is unprotected and offers an attacker new resource
attacks against the Initiator. This is mitigated by only processing
a received R1 when the Initiator has previously sent a corresponding
I1. Moreover, the Responder repeats the DH_GROUP_LIST, HIP_CIPHER,
HIT_SUITE_LIST, and TRANSPORT_FORMAT_LIST parameters in R2 in order
to verify that these parameters have not been modified by an attacker
in the R1 packet.
9. IANA Considerations
HIP DEX introduces the following new HIP HIT suite:
+-------+-------------+----------------------+----------------------+
| Index | Hash | Signature algorithm | Description |
| | function | family | |
+-------+-------------+----------------------+----------------------+
| 5 | FOLD | ECDH | ECDH HI folded to 96 |
| | | | bits |
+-------+-------------+----------------------+----------------------+
Table 2: HIT Suites
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In addition, this document specified a new HIP Parameter Type defined
in Section 5.2.5.
Moreover, a new HIP Cipher ID is defined in Section 5.2.4.
10. Acknowledgments
The drive to put HIP on a cryptographic 'Diet' came out of a number
of discussions with sensor vendors at IEEE 802.15 meetings. David
McGrew was very helpful in crafting this document.
11. Changelog
This section summarizes the changes made from draft-moskowitz-hip-rg-
dex-05, which was the first stable version of the draft. Note that
the draft was renamed after draft-moskowitz-hip-rg-dex-06.
11.1. Changes in draft-moskowitz-hip-rg-dex-06
o A major change in the ENCRYPT parameter to use AES-CTR rather than
AES-CBC.
11.2. Changes in draft-moskowitz-hip-dex-00
o Draft name change. HIPRG ended in IRTF, HIP DEX is now individual
submission.
o Added the change section.
o Added a Definitions section.
o Changed I2 and R2 packets to reflect use of AES-CTR for
ENCRYPTED_KEY parameter.
o Cleaned up KEYMAT Generation text.
o Added Appendix with C code for the ECDH shared secret generation
on an 8 bit processor.
11.3. Changes in draft-moskowitz-hip-dex-01
o Numerous editorial changes.
o New retransmission strategy.
o New HIT generation mechanism.
o Modified layout of ENCRYPTED_KEY parameter.
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o Clarify to use puzzle difficulty of zero under normal network
conditions.
o Align inclusion directive of R1_COUNTER with HIPv2 (from SHOULD to
MUST).
o Align inclusion of TRANSPORT_FORMAT_LIST with HIPv2 (added to R1
and I2).
o HIP_CIPHER, HIT_SUITE_LIST, and TRANSPORT_FORMAT_LIST must now be
echoed in R2 packet.
o Added new author.
11.4. Changes in draft-moskowitz-hip-dex-02
o Introduced formal definition of FOLD function.
o Clarified use of CMAC for puzzle computation in section "Solving
the Puzzle".
o Several editorial changes.
12. References
12.1. Normative References
[I-D.ietf-hip-rfc4843-bis]
Laganier, J. and F. Dupont, "An IPv6 Prefix for Overlay
Routable Cryptographic Hash Identifiers Version 2
(ORCHIDv2)", draft-ietf-hip-rfc4843-bis-05 (work in
progress), December 2013.
[I-D.ietf-hip-rfc5201-bis]
Moskowitz, R., Heer, T., Jokela, P., and T. Henderson,
"Host Identity Protocol Version 2 (HIPv2)", draft-ietf-
hip-rfc5201-bis-14 (work in progress), October 2013.
[I-D.ietf-hip-rfc5202-bis]
Jokela, P., Moskowitz, R., and J. Melen, "Using the
Encapsulating Security Payload (ESP) Transport Format with
the Host Identity Protocol (HIP)", draft-ietf-hip-
rfc5202-bis-05 (work in progress), November 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC2410] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and
Its Use With IPsec", RFC 2410, November 1998.
[RFC3686] Housley, R., "Using Advanced Encryption Standard (AES)
Counter Mode With IPsec Encapsulating Security Payload
(ESP)", RFC 3686, January 2004.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
12.2. Informative References
[DIF76] Diffie, W. and M. Hellman, "New Directions in
Cryptography", IEEE Transactions on Information Theory
vol. IT-22, number 6, pages 644-654, Nov 1976.
[HUM13] Hummen, R., Wirtz, H., Ziegeldorf, J., Hiller, J., and K.
Wehrle, "Tailoring End-to-End IP Security Protocols to the
Internet of Things", in Proceedings of IEEE International
Conference on Network Protocols (ICNP 2013), October 2013.
[I-D.ietf-hip-rfc4423-bis]
Moskowitz, R. and M. Komu, "Host Identity Protocol
Architecture", draft-ietf-hip-rfc4423-bis-07 (work in
progress), December 2013.
[IEEE.802-11.2007]
"Information technology - Telecommunications and
information exchange between systems - Local and
metropolitan area networks - Specific requirements - Part
11: Wireless LAN Medium Access Control (MAC) and Physical
Layer (PHY) Specifications", IEEE Standard 802.11, June
2007, <http://standards.ieee.org/getieee802/
download/802.11-2007.pdf>.
[IEEE.802-15-4.2011]
"Information technology - Telecommunications and
information exchange between systems - Local and
metropolitan area networks - Specific requirements - Part
15.4: Wireless Medium Access Control (MAC) and Physical
Layer (PHY) Specifications for Low-Rate Wireless Personal
Area Networks (WPANs)", IEEE Standard 802.15.4, September
2011, <http://standards.ieee.org/getieee802/
download/802.15.4-2011.pdf>.
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[RFC5903] Fu, D. and J. Solinas, "Elliptic Curve Groups modulo a
Prime (ECP Groups) for IKE and IKEv2", RFC 5903, June
2010.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
5996, September 2010.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090, February 2011.
[SECG] SECG, "Recommended Elliptic Curve Domain Parameters", SEC
2 , 2000, <http://www.secg.org/>.
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Appendix A. C code for ECDH point multiplication on an 8 bit processor
The following code illustrates the process to generate the shared
secret key from ECDH exchange.
// The following code is taken from the
// Relic library (http://code.google.com/p/relic-toolkit/)
// This code includes the function for point multiplication over
// prime fields
/**
* The following function multiplies a prime elliptic point by an
* integer using the binary method.
*
* @param[out] r - the result.
* @param[in] p - the point to multiply.
* @param[in] k - the integer.
*/
/**
* ep_t is library specific representation for an elliptic curve
* point
* bn_t is library specific representation for a multiple precision
* integer structure
*/
void ep_mul_basic(ep_t r, ep_t p, bn_t k) {
int i, l;
ep_t t;
ep_null(t);
if (bn_is_zero(k)) {
//If k is 0 then assign the prime elliptic curve point to a point
// at the infinity
ep_set_infty(r);
return;
}
TRY {
//ep_new allocates memory for the point and may throw ERR_NO_MEMORY
// incase of insufficient memory
ep_new(t);
//bn_bits returns the number of bits of a multiple precision
// integer l = bn_bits(k);
//bn_test_bit returns 0 if the bit at the given location is 0 and
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// returns non zero otherwise
if (bn_test_bit(k, l - 1)) {
ep_copy(t, p);
} else {
ep_set_infty(t);
}
for (i = l - 2; i >= 0; i--) {
//ep_dbl doubles a prime elliptic curve point
ep_dbl(t, t);
if (bn_test_bit(k, i)) {
//ep_add adds two prime elliptic curve points
ep_add(t, t, p);
}
}
ep_copy(r, t);
//ep_norm converts a point to affine coordinates
ep_norm(r, r);
}
CATCH_ANY {
THROW(ERR_CAUGHT);
}
FINALLY {
ep_free(t);
}
}
Appendix B. Password-based two-factor authentication during the HIP DEX
handshake
HIP DEX allows to identify authorized connections based on a two-
factor authentication mechanism. With two-factor authentication,
devices that are authorized to communicate with each other are
required to be pre-provisioned with a shared (group) key. The
Initiator uses this pre-provisioned key to encrypt the
ECHO_RESPONSE_UNSIGNED in the I2 packet. Upon reception of the I2,
the Responder verifies that its challenge in the
ECHO_REQUEST_UNSIGNED parameter in the R1 packet has been encrypted
with the correct key. If verified successfully, the Responder
proceeds with the handshake. Otherwise, it silently drops the I2
packet.
Note that there is no explicit signaling in the HIP DEX handshake for
this behavior. Thus, knowledge of two-factor authentication must be
configured externally prior to the handshake.
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Authors' Addresses
Robert Moskowitz (editor)
HTT Consulting
Oak Park, MI
USA
EMail: rgm@htt-consult.com
Rene Hummen
Chair of Communication and Distributed Systems, RWTH Aachen
Ahornstrasse 55
Aachen 52074
Germany
EMail: hummen@comsys.rwth-aachen.de
URI: http://www.comsys.rwth-aachen.de/team/rene-hummen/
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