| HIP WG | R. Moskowitz, Ed. |
| Internet-Draft | HTT Consulting |
| Intended status: Standards Track | R. Hummen |
| Expires: August 17, 2020 | Hirschmann Automation and Control |
| M. Komu | |
| Ericsson | |
| February 14, 2020 |
HIP Diet EXchange (DEX)
draft-ietf-hip-dex-13
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.
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. Unlike HIPv2, HIP DEX does not support Perfect Forward Secrecy (PFS), and MUST only be used on devices where PFS is prohibitively expensive. 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.
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 working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 17, 2020.
Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
This document 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) [RFC7401]. HIP DEX preserves the protocol semantics as well as the general packet structure of HIPv2. Hence, it is recommended that [RFC7401] is well-understood before reading this document.
The main differences between HIP BEX and HIP DEX are:
By eliminating the need for public-key signatures and the ephemeral DH key agreement, HIP DEX reduces the computational, energy, transmission, and memory requirements for public-key cryptography (see [LN08]) in the HIPv2 protocol design. This makes HIP DEX especially suitable for constrained devices as defined in [RFC7228].
This document focuses on the protocol specifications related to differences between HIP BEX and HIP DEX. Where differences are not called out explicitly, the protocol specification of HIP DEX is the same as defined in [RFC7401].
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 DEX Denial of Service (DoS) resilient. The Initiator and the Responder exchange their static Elliptic Curve Diffie-Hellman (ECDH) keys in the R1 and I2 handshake packet. The parties then authenticate each other in the I2 and R2 handshake packet based on the ECDH-derived keying material. The Initiator and the Responder additionally transmit keying material for the session key in these last two handshake packets (I2 and R2). This is to prevent overuse of the static ECDH-derived keying material. Moreover, the Responder starts a puzzle exchange in the R1 packet and the Initiator completes this exchange in the I2 packet before the Responder performs computationally expensive operations or stores any state from the exchange. Given this handshake structure, HIP DEX operationally is very similar to HIP BEX. Moreover, the employed 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 I2 packet. The Responder's Host Identity also is not protected. Thus, contrary to HIPv2, HIP DEX does not provide for end-point anonymity and any signaling (i.e., HOST_ID parameter contained with an ENCRYPTED parameter) that indicates such anonymity should be ignored.
As in [RFC7401], data packets start to flow after the R2 packet. The I2 and R2 packets may carry a data payload in the future. The details of this may be defined later.
An existing HIP association can be updated with the update mechanism defined in [RFC7401]. Likewise, the association can be torn down with the defined closing mechanism for HIPv2 if it is no longer needed. In doing so, HIP DEX 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) [RFC7402] 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) [RFC7296] that allows IKEv2 to support complex gateway policies. Thus, HIP DEX is not a replacement for IKEv2.
HIP DEX, by design, does not support Perfect Forward Secrecy (PFS). All current PFS approaches use Ephemeral DH, secured and identified in some manner (for example, with SIGMA or PAKE). There are classes of devices, like those based on the 8-bit 8051 microprocessor, where this is prohibitively expensive. Past experience with the 8051 based ZWAVE ZW0500 product has shown that EC25519 key pair generation exceeded 10 seconds with unacceptable battery consumption. The ECDH wide-multiply of the static-static keys took 8 - 9 seconds with measurable battery drain. For these class of devices, the possible risk of no PFS has to be weighed against the risk of theft of preshared keys alternatives. Also, it is often the case that HIP DEX is only performed during device join time, and, thus, no PFS is considered an acceptable compromise.
Also, it is often the case that HIP DEX is only performed during device join time, and thus no PFS is considered an acceptable compromised.
HIP DEX MUST only be used in communicating with such constrained devices (e.g., class 0 and 1 devices as defined in section 3 in [RFC7228]).
The rest of this memo is structured as follows. Section 2 defines the central keywords, notation, and terms used throughout this 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, 9, and 10 discuss policy, interoperability between HIPv2 vs DEX, security, and IANA considerations, respectively. Appendix A defines a two factor authentication scheme and Appendix B highlights some discussions with the IESG.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
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 packets to represent the HI. The DEX Host Identity Tag (HIT) is different from the BEX HIT in two ways:
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 use of 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. When other user data packet formats are used, the corresponding extensions need to define a replacement for the ESP_TRANSFORM [RFC7402] parameter along with associated semantics, but this procedure is outside the scope of this document.
With HIP DEX, the HIT 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 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 7343, 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 the HI. Third, a 96-bit hashed representation of the HI. 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.
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 HIPv2 is used for HIP DEX. The HIP DEX HIT suite (see Section 10) 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).
HIP DEX, by design lacks a cryptographic hash function. The generation of the HIT is one of the few places in the protocol where this presents a challenge. CMAC was first considered for this purpose, but to use CMAC for HIT generation would require using a static key, either ZERO or some published value. NIST does not consider CMAC an approved cryptograhic hash as:
Since collision-resistance is not possible with the tools at hand, any reasonable function (e.g. FOLD) that takes the full value of the HI into generating the HIT can be used, provided that collision detection is part of the HIP-DEX deployment design. This is achieved here through either an ACL or some other lookup process that externally binds the HIT and HI.
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.
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 handshake completes, and either party can become the Initiator in future communications.
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 [DH76] 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 ESP-based protection of user data.
The Initiator first sends a trigger packet, I1, to the Responder. This packet contains the HIT of the Initiator and 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 key 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 handshake 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 packet is discarded. The I2 also contains a key wrap parameter that carries secret keying material of the Initiator. This keying material is only half of 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 of the final session key material. The 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
Figure 1: High-level overview of the HIP Diet EXchange
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.
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 [RFC7401] for more detailed information about the employed mechanism. Notably, the only differences between the puzzle mechanism in HIP DEX and HIPv2 are that HIP DEX does not employ pre-computation of R1 packets and uses CMAC instead of a hash function for solving and verifying a puzzle. The implications of these changes on the puzzle implementation are discussed in Section 6.1.
The HIP DEX state machine has the same states as the HIPv2 state machine (see Section 4.4. in [RFC7401]). However, HIP DEX features a retransmission strategy with an optional reception acknowledgement for the I2 packet. The goal of this additional acknowledgement is to reduce premature I2 retransmissions in case of devices with low computation resources [HWZ13]. As a result, there are minor changes regarding the transitions in the HIP DEX state machine. The following section documents these differences compared to HIPv2.
For the retransmission of I1 and I2 packets, the Initiator adopts the retransmission strategy of HIPv2 (see Section 4.4.3. in [RFC7401]). This strategy is based on a timeout that is set to a value larger than the worst-case anticipated round-trip time (RTT). For each received I1 or I2 packet, the Responder sends an R1 or R2 packet, respectively. This design trait enables the Responder to remain stateless until the reception and successful processing of the I2 packet. 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 packet after I1_RETRIES_MAX tries, it stops I1 retransmissions. Likewise, it stops retransmitting the I2 packet after I2_RETRIES_MAX unsuccessful tries.
For repeatedly received I2 packets, the Responder SHOULD NOT perform operations related to the Diffie-Hellman key exchange or the keying material wrapped in the ENCRYPTED_KEY parameters. Instead, it SHOULD re-use the previously established state to re-create the corresponding R2 packet in order to prevent unnecessary computation overhead.
The potentially high processing time of an I2 packet at a (resource-constrained) Responder may cause premature 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 the I2 packet at the Responder into account for retransmission purposes. To this end, the Responder MAY notify the Initiator about the anticipated delay once the puzzle solution was successfully verified and if the remaining I2 packet processing incurs a high processing delay. The Responder MAY therefore send a NOTIFY packet (see Section 5.3.6. in [RFC7401]) to the Initiator before the Responder commences the ECDH operation. The NOTIFY packet serves as an acknowledgement for the I2 packet and consists of a NOTIFICATION parameter with Notify Message Type I2_ACKNOWLEDGEMENT (see Section 5.2.19. in [RFC7401]). The NOTIFICATION parameter contains the anticipated remaining processing time for the I2 packet in milliseconds as two-octet Notification Data. This processing time can, e.g., be estimated by measuring the computation time of the ECDH key derivation operation during the Responder start-up procedure. After the I2 processing has finished, the Responder sends the regular R2 packet.
When the Initiator receives the NOTIFY packet, it sets the I2 retransmission timeout to the I2 processing time indicated in the NOTIFICATION parameter plus half the RTT-based timeout value. In doing so, the Initiator MUST NOT set the retransmission timeout to a higher value than allowed by a local policy. This is to prevent unauthenticated NOTIFY packets from maliciously delaying the handshake beyond a well-defined upper bound in case of a lost R2 packet. At the same time, this extended retransmission timeout enables the Initiator to defer I2 retransmissions until the point in time when the Responder should have completed its I2 packet processing and the network should have delivered the R2 packet according to the employed worst-case estimates.
HIP DEX clarifies or introduces the following new transitions.
System behavior in state I2-SENT, Table 1.
| Trigger | Action |
|---|---|
| Receive NOTIFY, process | Set I2 retransmission timer to value in I2_ACKNOWLEDGEMENT Notification Data plus 1/2 RTT-based timeout value and stay at I2-SENT |
| Timeout | Increment trial counter |
| If counter is less than I2_RETRIES_MAX, send I2, reset timer to RTT-based timeout, and stay at I2-SENT | |
| If counter is greater than I2_RETRIES_MAX, go to E-FAILED |
The following diagram shows the major state transitions. Transitions based on received packets implicitly assume that the packets are successfully authenticated or processed.
+--+ +----------------------------+
recv I1, send R1 | | | |
| v v |
+--------------+ recv I2, send R2 |
+----------------| UNASSOCIATED |----------------+ |
datagram | +--+ +--------------+ | |
to send, | | | Alg. not supported, | |
send I1 | | | send I1 | |
. v | v | |
. +---------+ recv I2, send R2 | |
+---->| I1-SENT |--------------------------------------+ | |
| +---------+ +----------------------+ | | |
| | recv R1, | recv I2, send R2 | | | |
| v send I2 | v v v |
| +---------+----------+ +---------+ |
| +--->| I2-SENT |<-------------+ +------------| R2-SENT |<---+ |
| | +---------+ recv NOTIFY, | | +---------+ | |
| | | | | reset timer | | data or| | |
| |recv R1, | | +--------------+ | EC timeout| | |
| |send I2 +-|--------------------+ | receive I2,| |
| | | | +-------------+ | send R2| |
| | | +-------->| ESTABLISHED |<---------+ | |
| | | recv R2 +-------------+ | |
| | | | | | receive I2, send R2 | |
| | +------------+ | +-------------------------------+ |
| | | +-----------+ | |
| | | no packet sent/received| +---+ | |
| | | for UAL min, send CLOSE| | |timeout | |
| | | v v |(UAL+MSL) | |
| | | +---------+ |retransmit | |
+--|----------|------------------------| CLOSING |-+CLOSE | |
| | +---------+ | |
| | | | | | | |
+----------|-------------------------+ | | +----------------+ |
| | +-----------+ +------------------|--+
| | |recv CLOSE, recv CLOSE_ACK | |
| +-------------+ |send CLOSE_ACK or timeout | |
| recv CLOSE, | | (UAL+MSL) | |
| send CLOSE_ACK v v | |
| +--------+ receive I2, send R2 | |
+---------------------| CLOSED |------------------------------+ |
+--------+ |
^ | | |
recv CLOSE, send CLOSE_ACK| | | timeout (UAL+2MSL) |
+-+ +------------------------------------+
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.
The Master Key SA is used to authenticate HIP packets and to encrypt selected HIP parameters in the HIP DEX packet exchanges. Since only a small amount of data is protected by this SA, it can be long-lived with no need for rekeying. At the latest, the system MUST initiate rekeying when its incoming ESP sequence counter is going to overflow, and the system MUST NOT replace its keying material until the rekeying packet exchange successfully completes as described in Section 6.8 in [RFC7402].
The Master Key SA contains the following elements:
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.
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 [RFC7402]).
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 defined in Section 6.3 is used. Even though the concatenated input is randomly distributed, a KDF Extract phase may be needed to get the proper length for the input to the KDF Expand phase.
The User Data Considerations in Section 4.5. of [RFC7401] 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 resource-constrained devices. Thus, implementors MAY choose to use non-volatile, secure storage for HIP states in order for them to survive a system reboot as discussed in Section 6.11. Using non-volatile storage will limit state loss during reboots to only those situations with an SA timeout.
HIP DEX employs the same fixed HIP header and payload structure as HIPv2. As such, the specifications in Section 5.1 of [RFC7401] also apply to HIP DEX.
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 HIPv2 defined in Section 5.2. of [RFC7401] where possible. Still, HIP DEX further restricts and/or extends the following existing parameter types:
In addition, HIP DEX introduces the following new parameter:
| TLV | Type | Length | Data |
|---|---|---|---|
| ENCRYPTED_KEY | TBD1 (suggested value 643) | variable | Encrypted container for the session key exchange |
| I_NONCE | TBD6 (suggested value 644) | variable | Nonce from Initator for Master Key |
The DH_GROUP_LIST parameter contains the list of supported DH Group IDs of a host. It is defined in Section 5.2.6 of [RFC7401]. With HIP DEX, the DH Group IDs are restricted to:
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
Curve25519 [RFC7748] CKDF 12
Curve448 [RFC7748] CKDF 13
The ECDH groups with values 7 - 9 are defined in [RFC5903] and [RFC6090]. ECDH group 10 is covered in [SECG] and Appendix D of [RFC7401]. These curves, when used with HIP MUST have a co-factor of 1.
The ECDH groups with values 12 and 13 are defined in [RFC7748]. These curves have cofactors of 8 and 4 (respectively).
The HIP_CIPHER parameter contains the list of supported cipher algorithms to be used for encrypting the contents of the ENCRYPTED and ENCRYPTED_KEY parameters. The HIP_CIPHER parameter is defined in Section 5.2.8 of [RFC7401]. With HIP DEX, the Suite IDs are limited to:
Suite ID Value
RESERVED 0
NULL-ENCRYPT 1 ([RFC2410])
AES-128-CTR TBD4 (suggested: 5) ([RFC3686])
Mandatory implementation: AES-128-CTR. Implementors SHOULD support NULL-ENCRYPT ([RFC2410]) for testing/debugging purposes but MUST NOT offer or accept this value unless explicitly configured for testing/debugging of HIP.
The HOST_ID parameter conveys the Host Identity (HI) along with optional information about a host. The HOST_ID parameter is defined in Section 5.2.9 of [RFC7401].
HIP DEX uses the public portion of a host's static ECDH key-pair as the HI. Correspondingly, HIP DEX limits the HI algorithms to the following new profile:
Algorithm profiles Value
ECDH TBD5 (suggested: 11) [RFC6090] (REQUIRED)
For hosts that implement ECDH as the algorithm, the following curves are required:
Group Value
NIST P-256 1 [RFC5903]
NIST P-384 2 [RFC5903]
NIST P-521 3 [RFC5903]
SECP160R1 4 [SECG]
Curve25519 5 [RFC7748]
Curve448 6 [RFC7748]
HIP DEX HIs are serialized equally to the ECC-based HIs in HIPv2 (see Section 5.2.9. of [RFC7401]). The Group ID of the HIP DEX HI is encoded in the "ECC curve" field of the HOST_ID parameter. The supported DH Group IDs are defined in Section 5.2.1.
The HIT_SUITE_LIST parameter contains a list of the supported HIT suite IDs of the Responder. Based on the HIT_SUITE_LIST, the Initiator can determine which source HIT Suite IDs are supported by the Responder. The HIT_SUITE_LIST parameter is defined in Section 5.2.10 of [RFC7401].
The following new HIT Suite ID is defined for HIP DEX, and the relationship between the four-bit ID value used in the OGA ID field and the eight-bit encoding within the HIT_SUITE_LIST ID field is clarified:
HIT Suite Four-bit ID Eight-bit encoding
ECDH/FOLD TBD2 (suggestion: 4) TBD3 (suggestion: 0x40)
Note that the dedicated HIP DEX HIT Suite ID in the OGA ID field allows the peers to distinguish a HIP DEX handshake from a HIPv2 handshake. The Responder MUST respond with a HIP DEX HIT suite ID when the HIT of the Initiator is a HIP DEX HIT.
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 TBD1 (suggested value 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 (see Section 6.3). This random value MUST have a length of at least 64 bits. The puzzle value #I and the puzzle solution #J (see Section 4.1.2 in [RFC7401]) are used as the initialization vector (IV) for the encryption process. To this end, the IV is computed as FOLD(I | J, 128). Moreover, a 16 bit counter value, which is initialized to zero on first use, is appended to the IV value in order to guarantee that a non-repeating nonce is fed to the encryption algorithm defined by the HIP_CIPHER.
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 [RFC7401].
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Initiator Nonce /
/ /
/ +-------------------------------+
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type TBD6 (suggested value 644)
Length length in octets, excluding Type, Length, and
Padding
Initiator Nonce provided by the Initiator for use in the
Nonce Master Key
The I_NONCE parameter encapsulates a random value that is later used in the Master key creation process (see Section 6.3). This random value MUST have a length of 2 x RHASH_len. This parameter is sent to the Responder in I2 which echos it back to the Initiator in R2.
If necessary, additional Padding for 8-byte alignment is added according to the rules of TLV Format in [RFC7401].
HIP DEX uses the same eight basic HIP packets as HIPv2 (see Section 5.3 of [RFC7401]). Four of them are for the HIP handshake (I1, R1, I2, and R2), one is for updating an association (UPDATE), one is for sending notifications (NOTIFY), and two are for closing the association (CLOSE and CLOSE_ACK). There are some differences regarding the HIP parameters that are included in the handshake packets concerning HIP BEX and HIP DEX. This section covers these differences for the DEX packets. Packets not discussed here, follow the structure defined in [RFC7401].
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 HIP 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.
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 ) )
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 HIP DEX type as defined in Section 5.2.4.
Regarding the Responder's HIT, the Initiator may receive this HIT either from a DNS lookup of the Responder's FQDN, from some other repository, or from a local table. The Responder's HIT also MUST be of a HIP DEX type. 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 Section 4.1.8 of [RFC7401] for detailed information about the "HIP Opportunistic Mode".
As the Initiator's and the Responder's HITs are compressions of the employed HIs, they determine the DH Group ID that must be used in order to successfully conclude the triggered handshake. HITs, however, only include the OGA ID identifying the HI algorithm. They do not include information about the specific group ID of the HI. To inform the Responder about its employed and its otherwise supported DH Group IDs, the Initiator therefore includes the DH_GROUP_LIST parameter in the I1 packet. This parameter MUST include the DH group ID that corresponds to the currently employed Initiator HIT as the first list element. With HIP DEX, the DH_GROUP_LIST parameter MUST only include ECDH groups defined in Section 5.2.1.
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.
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 ])
Valid control bits: A
If the Responder's HI is an anonymous one, the A control bit 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 Section 4.1.8 of [RFC7401] for detailed information about the "HIP Opportunistic Mode".
The R1 packet generation counter is used to determine the currently valid generation of puzzles. The value is increased periodically, 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 [RFC7401]). Responders SHOULD set K to zero by default and only increase the puzzle difficulty to protect against a DoS attack targeting the HIP DEX handshake. A puzzle difficulty of zero effectively turns the puzzle mechanism into a return-routability test and is strongly encouraged during normal operation in order to conserve energy resources as well as to prevent unnecessary handshake delay in case of a resource-constrained Initiator. Please also refer to Section 7 for further recommendations on choosing puzzle difficulty.
The DH_GROUP_LIST parameter contains the Responder's order of preference based on the Responder's choice the ECDH key contained in the HOST_ID parameter (see below). This allows the Initiator to determine whether its own DH_GROUP_LIST in the I1 packet was manipulated by an attacker. There is a further risk that the Responder's DH_GROUP_LIST was manipulated by an attacker, as the R1 packet cannot be authenticated in HI DEX. Thus, this parameter is repeated in the R2 packet to allow for a final, cryptographically secured validation.
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 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 with 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.
The HOST_ID parameter depends on the received DH_GROUP_LIST parameter and the Responder HIT in the I1 packet. Specifically, if the I1 contains a Responder HIT, the Responder verifies that this HIT matches the required DH group based on the received DH_GROUP_LIST parameter included in the I1. In case of a positive result, the Responder selects the corresponding HOST_ID for inclusion in the R1 packet. Likewise, if the Responder HIT in the I1 packet is NULL (i.e., during an opportunistic handshake), the Responder chooses its HOST_ID according to the Initiator's employed DH group as indicated in the received DH_GROUP_LIST parameter and sets the source HIT in the R1 packet accordingly. If the Responder however does not support the DH group required by the Initiator or if the Responder HIT in the I1 packet does not match the required DH group, the Responder selects the mutually preferred and supported DH group based on the DH_GROUP_LIST parameter in the I1 packet. The Responder then includes the corresponding ECDH key in the HOST_ID parameter. This parameter also indicates the selected DH group. Moreover, the Responder sets the source HIT in the R2 packet based on the destination HIT from the I1 packet. Based on the deviating DH group ID in the HOST_ID parameter, the Initiator then SHOULD abort the current handshake and initiate a new handshake with the mutually supported DH group as far as local policies (see Section 7) permit.
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 different format types are DEFAULT, ESP and ESP-TCP as explained in Section 3.1 in [RFC6261].
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.
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,
I_NONCE,
HIP_MAC
[<, ECHO_RESPONSE_UNSIGNED>i )] )
Valid control bits: A
The HITs MUST match the ones used in the R1 packet.
If the Initiator's HI is an anonymous one, the A control bit MUST be set.
If present in the R1 packet, the Initiator MUST include an unmodified copy of the R1_COUNTER parameter into the I2 packet.
The Solution contains the Random #I from the R1 packet and the computed #J value. 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 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 parameter 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 parameter(s) contain 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 A.
The TRANSPORT_FORMAT_LIST parameter 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 packet. The different format types are DEFAULT, ESP and ESP-TCP as explained in Section 3.1 in [RFC6261].
The I_NONCE parameter contains the nonce, supplied by the Initiator for the Master Key generation as shown in Section 6.3. This is echoed back to the Initiator in the R2 packet.
The MAC is calculated over the whole HIP envelope, excluding any parameters after the HIP_MAC parameter as described in Section 6.2. The Responder MUST validate the HIP_MAC parameter.
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,
I_NONCE,
HIP_MAC)
Valid control bits: none
The HITs used MUST match the ones used in the I2 packet.
The Responder repeats the DH_GROUP_LIST, HIP_CIPHER, HIT_SUITE_LIST, and TRANSPORT_FORMAT_LIST parameters in the R2 packet. These parameters MUST be the same as included in the R1 packet. The parameter are re-included here because the R2 packet 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 parameter 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 I_NONCE parameter contains the nonce, supplied by the Initiator for the Master Key generation as shown in Section 6.3. The Responder is echoing the value back to the Initiator to show it used the Initiator provided nonce.
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 parameter.
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 [RFC7401] also apply to HIP DEX.
Due to the adopted protocol semantics and the inherited general packet structure, the packet processing in HIP DEX only differs from HIPv2 in very few places. Here, we focus on these differences and refer to Section 6 in [RFC7401] otherwise.
The processing of outgoing and incoming application data remains the same as in HIP BEX (see Sections 6.1 and 6.2 in [RFC7401]).
It should be noted that many of the packet processing rules are denoted here with "SHOULD" but may be updated to "MUST" when further implementation experience provides better guidance. Please refer also to Appendix B for argumentation about the SHOULDs.
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 Section 6.3 of [RFC7401] is skipped in HIP DEX.
Apart from these modifications, the procedures defined in Section 6.3 of [RFC7401] also apply for HIP DEX.
The following subsections define the actions for processing the HIP_MAC parameter.
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:
The parameter order is described in Section 5.2.1 of [RFC7401].
The CMAC calculation and verification process is as follows:
Packet sender:
Packet receiver:
The HIP DEX KEYMAT process is used to derive the keys for the Master Key SA as well as for the Pair-wise Key SA. The keys for the Master Key SA are based on the Diffie-Hellman derived key, Kij, which is produced during the HIP DEX handshake. The Initiator generates Kij during the creation of the I2 packet and the Responder generates Kij once it receives the I2 packet. This is why the I2 packet can already contain authenticated and/or encrypted information.
The keys derived for the Pair-wise Key SA are not used during the HIP DEX handshake. Instead, these keys are made available as payload protection keys (e.g., for IPsec). 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 to derive the keys of the Pair-wise Key SA based on the concatenation of the random values that are contained in the exchanged ENCRYPTED_KEY parameters.
The HIP DEX KEYMAT process is based on the is the Hash-based Key Derivation Function (HKDF) defined in [RFC5869] and consists of two components, CKDF-Extract and CKDF-Expand. The CKDF-Extract function compresses a non-uniformly distributed key, such as the output of a Diffie-Hellman key derivation, to extract the key entropy into a fixed length output. The CKDF-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 the Master Key SA employs always both the Extract and Expand phases. The Pair-wise Key SA needs only the Extract phase when the key is smaller or equal to 128 bits, but otherwise requires also the Expand phase.
The CKDF-Extract function is the following operation:
CKDF-Extract(I, IKM, info) -> PRK
Inputs:
I Random #I, provided by the Responder, from the PUZZLE
parameter
IKM Input keying material
the Diffie-Hellman derived key, concatenated with the
random I_NONCE value for the Master Key SA
the Diffie-Hellman derived key, concatenated with the
random values of the ENCRYPTED_KEY parameters in
the same order as the HITs with sort(HIT-I | HIT-R)
for the Pair-wise Key SA
info sort(HIT-I | HIT-R) | "CKDF-Extract"
where "CKDF-Extract" is an octet string
Output:
PRK a pseudorandom key (of RHASH_len/8 octets)
The pseudorandom key PRK is calculated as follows:
PRK = CMAC(I, IKM | info)
The CKDF-Expand function is the following operation:
CKDF-Expand(PRK, info, L) -> OKM
Inputs:
PRK a pseudorandom key of at least RHASH_len/8 octets
(either the output from the extract step or the
concatenation of the random values of the
ENCRYPTED_KEY parameters in the same order as the
HITs with sort(HIT-I | HIT-R) in case of no extract)
info sort(HIT-I | HIT-R) | "CKDF-Expand"
where "CKDF-Expand" is an octet string
L length of output keying material in octets
(<= 255*RHASH_len/8)
Output:
OKM output keying material (of L octets)
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 for the Master Key SA are drawn sequentially in the order that is determined by the numeric comparison of the two HITs, with the 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:
The number of bits drawn for a given algorithm is the "natural" size of the keys regarding the algorithm defined in the HIP_CIPHER. For the mandatory algorithms, the following size applies:
If other key sizes are used, they must be treated as different encryption algorithms and defined separately.
The initiation of a HIP DEX handshake proceeds as described in Section 6.6 of [RFC7401]. The I1 packet contents are specified in Section 5.3.1.
I1 packets in HIP DEX are handled almost identical to HIPv2 (see Section 6.7 of [RFC7401]). The main differences are that the Responder SHOULD select a HIP DEX HIT Suite 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 at the Responder. Hence, the R1 pre-computation SHOULD be omitted in HIP DEX.
Correspondingly, the modified conceptual processing rules for responding to an I1 packet are as follows:
Note that only steps 4 and 5 have been changed with regard to the processing rules of HIPv2. The considerations about R1 management (except pre-computation) and malformed I1 packets in Sections 6.7.1 and 6.7.2 of [RFC7401] likewise apply to HIP DEX.
R1 packets in HIP DEX are handled identically to HIPv2 (see Section 6.8 in [RFC7401]) with the following exceptions: HIP DEX uses ECDH public keys as HIs and does not employ signatures.
The modified conceptual processing rules for responding to an R1 packet are as follows:
Note that step 4 from the original processing rules of HIPv2 (signature verification) has been removed in the above processing rules for HIP DEX. Moreover, step 7 of the original processing rules has been adapted in step 6 above to account for the fact that HIP DEX uses ECDH public keys as HIs. The considerations about malformed R1 packets in Sections 6.8.1 of [RFC7401] also apply to HIP DEX.
The processing of I2 packets follows similar rules as HIPv2 (see Section 6.9 of [RFC7401]). The main differences to HIPv2 are that HIP DEX introduces a new session key exchange via the ENCRYPTED_KEY parameter as well as an I2 reception acknowledgement for retransmission purposes. Moreover, with HIP DEX the Initiator is responsible for triggering retransmissions, whereas the Responder merely replies to received I2 packets.
The modified HIP DEX conceptual processing rules for responding to an I2 packet are:
Note that steps 11 (encrypted HOST_ID) and 15 (signature verification) from the original processing rules of HIPv2 have been removed in the above processing rules for HIP DEX. Moreover, step 10 of the HIPv2 processing rules has been adapted to account for optional extension of the retransmission mechanism. Step 16 has been added to the processing rules in this document. The considerations about malformed I2 packets in Sections 6.9.1 of [RFC7401] also apply to HIP DEX.
R2 packets in HIP DEX are handled identically to HIPv2 (see Section 6.10 of [RFC7401]) with the following exceptions: HIP DEX introduces a new session key exchange via the ENCRYPTED_KEY parameter and does not employ signatures.
The modified conceptual processing rules for responding to an R2 packet are as follows:
Note that step 4 (signature verification) from the original processing rules of HIPv2 has been replaced with a negotiation re-evaluation in the above processing rules for HIP DEX. Moreover, step 6 has been added to the processing rules.
Processing of NOTIFY packets is OPTIONAL. If processed, any errors in a received NOTIFICATION parameter SHOULD be logged. Received errors MUST be considered only as informational, and the receiver SHOULD NOT change its HIP state purely based on the received NOTIFY packet.
If a NOTIFY packet is received in state I2-SENT, this packet is an I2 reception acknowledgement of the optional retransmission mechanism extension and SHOULD be processed. The following steps define the conceptual processing rules for such incoming NOTIFY packets in state I2-SENT:
UPDATE, CLOSE, and CLOSE_ACK packets are handled similarly in HIP DEX as in HIPv2 (see Sections 6.11, 6.12, 6.14, and 6.15 of [RFC7401]). The only difference is the that the HIP_SIGNATURE is never present and, therefore, is not required to be processed by the receiving party.
[RFC7402] specifies the rekeying of an existing HIP SA using the UPDATE message. This rekeying procedure can also be used with HIP DEX. However, where rekeying involves a new Diffie-Hellman key exchange, HIP DEX peers MUST establish a new HIP association in order to create a new Pair-wise Key SA due to the use of static ECDH key-pairs with HIP DEX.
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. Such storing of the R1 generation counter values MUST be configured by explicit HITs.
There are a number of variables that will influence the HIP exchanges that each host must support. The value of puzzle difficulty K used in the HIP R1 must be chosen with care. Values for the K that are too high will exclude clients with weak CPUs because these devices cannot solve the puzzle within a reasonable amount of time. The K value should only be raised if a Responder is under high load, i.e., it cannot process all incoming HIP handshakes any more.
If a Responder is not under high load, K SHOULD be 0.
All HIP DEX implementations SHOULD provide for an Access Control List (ACL), representing for which hosts they accept HIP diet exchanges, and the preferred transport format and local lifetimes. Wildcarding SHOULD be supported for such ACLs.
HIP DEX and HIPv2 both use the same protocol number and packet formats. Hence, an implementation that either supports HIP DEX or HIPv2 has to be able to detect the dialect that the peer is speaking. This section outlines how a HIP DEX implementation can achieve such detection for the two relevant cases where:
In the first case, the HIP DEX implementation (Initiator) inspects the Responder's HIT prior to sending the I1 packet. If the OGA ID field of this HIT does not indicate the HIP DEX HIT Suite ID, the HIP DEX implementation cancels the handshake. If the Responder is unknown prior to sending the I1 packet (i.e., opportunistic mode), the HIP DEX implementation performs the above check on reception of the R1 packet and cancels the handshake in case of a negative result. In both failure scenarios, the implementation should report an error to the user via appropriate means.
In the second case, the HIP DEX implementation (Responder) inspects the Initiator's HIT on reception of an I1 packet. If the OGA ID field of this HIT does not indicate the HIP DEX HIT Suite ID, the HIP DEX implementation cancels the handshake and sends an ICMP packet with type Parameter Problem, with the Pointer pointing to the source HIT, to the Initiator. As an off-path adversary could also send such an ICMP packet with the aim to prevent the HIP DEX handshake from completing, the Initiator SHOULD NOT react to an ICMP message before retransmission counter reaches I1_RETRIES_MAX in its state machine (see Table 3 in [RFC7401]).
HIP DEX closely resembles HIPv2. As such, the security considerations discussed in Section 8 of [RFC7401] similarly apply to HIP DEX. HIP DEX, however, replaces the SIGMA-based authenticated Diffie-Hellman key exchange of HIPv2 with an exchange of random keying material that is encrypted with a Diffie-Hellman derived key. Both the Initiator and Responder contribute to this keying material. As a result, the following additional security considerations apply to HIP DEX:
The optional retransmission extension of HIP DEX is based on a NOTIFY packet that the Responder can use to inform the Initiator about the reception of an I2 packet. The Responder, however, cannot protect the authenticity of this packet as it did not yet set up the Master Key SA. Hence, an eavesdropping adversary may send spoofed reception acknowledgements for an overheard I2 packet and signal an arbitrary I2 processing time to the Initiator. The adversary can, e.g., indicate a lower I2 processing time than actually required by the Responder in order to cause premature retransmissions. To protect against this attack, the Initiator SHOULD set the NOTIFY-based timeout value to the maximum indicated packet processing time in case of conflicting NOTIFY packets. This allows the legitimate Responder to extend the retransmission timeout to the intended length. The adversary, however, can still arbitrarily delay the protocol handshake beyond the Responder's actual I2 processing time. To limit the extend of such a maliciously induced handshake delay, this specification additionally requires the Initiator not to set the NOTIFY-based timeout value higher than allowed by a local policy.
Section 5.3.1 mentions that implementations need to be able to handle storms of I1 packets. Contrary to HIPv2, R1 packets cannot be pre-computed in HIP DEX and also the state machine does not include an "R1_SENT" state (that would enable caching of R1 packets). Therefore, an implementation has to cache information (e.g., at least the HITs) from incoming I1 packets and rate control the incoming I1 packets to avoid unnecessary packet processing during I1 packet storms.
The SECP160R1 curve is included more for historical reasons than a demostrated need for a lightweight curve for constrained systems. Until it is deprecated from [RFC7401], it is included, but implementors should carefully weigh any advantages actual or perceived over EC25519. Actual implementations of EC25519 on an 8501 show that it can be used on very constrained hardware. The over-the-air and storage costs for EC25519 are also closely comparable with SECP160R1.
Thus the security risk of the weak SECP160R1 must be shown to be of value over any slight costs of using EC25519 instead.
With the curves specified here, there is a straightforward key extraction attack, which is a very serious problem the use of static keys by HIP-DEX. Thus it is MANDATORY to validate the peer's Public Key.
For the curve SECP160R1, peers MUST validate each other's public value Q by ensuring that the point is a valid point on the elliptic curve. This process consists of three steps: (1) verify that Q is not the point at infinity (O), (2) verify that for Q = (x, y) both integers x and y are in the correct interval, and (3) ensure that (x, y) is a correct solution to the elliptic curve equation. For this curve, implementors do not need to verify membership in the correct subgroup.
With the NIST curves, there are no internal length markers, so each number representation occupies as many octets as implied by the curve parameters. For P-256, this means that each of X and Y use 32 octets, padded on the left by zeros if necessary. For P-384, they take 48 octets each. For P-521, they take 66 octets each.
For Curve25519 and Curve448, the contents of the public value are the byte string inputs and outputs of the corresponding functions defined in [RFC7748]: 32 bytes for EC25519 and 56 bytes for EC448.
The following changes to the "Host Identity Protocol (HIP) Parameters" registries have been made:
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. Special thanks to Mohit Sethi in helping with the draft during IESG process.
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.
The draft was then renamed from draft-moskowitz-hip-dex to draft-ietf-hip-dex.
to match
[RFC7401].
HIP DEX allows identifying 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.
During IESG review, a concern was raised on the number of SHOULDS in this document. Here is an analysis of the 57 SHOULDS in HIP DEX.
5.2.2. HIP_CIPHER (same as 7401)
6.5. Processing Incoming I1 Packets
3. (same as 7401)
5. (same as 7401)
6.6. Processing Incoming R1 Packets (same as 7401)
6.7. Processing Incoming I2 Packets
3. (same as 7401)
7. (same as 7401)
11. (same as 7401)
6.8. Processing Incoming R2 Packets
5. (same as 7401)
6.9. Processing Incoming NOTIFY Packets
1st para (same as 7401)
6.11. Handling State Loss (same as 7401)
7. HIP Policies (1st and 3rd same as 7401)
46 of SHOULDS are also in [RFC7401]. Here are the sections with SHOULDS that match up with [RFC7401]:
Many of the other 11 SHOULDS are due to the nature of constrained devices and in most cases the text points this out:
In Section 4.1.1, this is clearly adjusting for how the puzzle could actually be an attack against a constrained device. Same situation in Section 5.3.2.
Section 6, clearly states that:
thus the SHOULD here is informative of future guidance.
The SHOULD in Section 6.3, clearly reflects new work with the new Sponge Function KDFs:
The keys derived for the Pair-wise Key SA are not used during the HIP DEX handshake. Instead, these keys are made available as payload protection keys (e.g., for IPsec). 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 to derive the keys of the Pair-wise Key SA based on the concatenation of the random values that are contained in the exchanged ENCRYPTED_KEY parameters.
In Section 6.5, the reason why this is a SHOULD should be clear to any implementer. That is the HIT Suite list in I1 is a desire on what suite to use. The Responder may have 'different ideas' about the Suite to use (like what it supports). Thus it is best that the Responder selects a DEX HIT, but there are good reasons, in an implementation not to do so. The implementer should know this and will deal with it appropriately.
The SHOULDs in Section 6.7 and Section 6.9 are there for considerations for constrained systems. Some constrained systems need this approach, others may not.
The 2nd SHOULD in Section 7 follows the same as above. ACLs and constrained systems tend to go together.
Similarly in Section 8 the SHOULD is again is highlighting constrained system processing considerations.