HTT Consulting

Oak Park MI USA rgm@htt-consult.com

Chair of Communication and Distributed Systems, RWTH Aachen

Ahornstrasse 55 Aachen 52074 Germany hummen@comsys.rwth-aachen.de http://www.comsys.rwth-aachen.de/team/rene-hummen/

Internet HIP WG RFC Request for Comments I-D Internet-Draft HIP 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.

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) . HIP DEX preserves the protocol semantics as well as the general packet structure of HIPv2. Hence, it is recommended that is well-understood before reading this document. The main differences between HIP BEX and HIP DEX are: Minimum collection of cryptographic primitives to reduce the protocol overhead. Static Elliptic Curve Diffie-Hellman key pairs for peer authentication and encryption of the session key. AES-CTR for symmetric encryption and AES-CMAC for MACing function. A simple fold function for HIT generation. Forfeit of Perfect Forward Secrecy with the dropping of an ephemeral Diffie-Hellman key agreement. 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. Diffie-Hellman derived key ONLY used to protect the HIP packets. A separate secret exchange within the HIP packets creates the session key(s). Optional retransmission strategy tailored to handle the potentially extensive processing time of the employed cryptographic operations on computationally constrained devices. By eliminating the need for public-key signatures and the ephemeral DH key agreement, HIP DEX reduces the computation, energy, transmission, and memory requirements for public-key cryptography (see ) in the HIPv2 protocol design. Moreover, by dropping the cryptographic hash function, HIP DEX affords a more efficient protocol implementation than HIP BEX with respect to the corresponding computation and memory requirements. This makes HIP DEX especially suitable for constrained devices as defined in . 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 .

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 DoS resilient. The Initiator and the Responder exchange their static Elliptic Curve Diffie-Hellman (ECDH) keys in the 2nd and 3rd handshake packet. The parties then authenticate each other in the 3rd and 4th 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. This is to prevent overuse of the static ECDH-derived keying material. Moreover, the Responder starts a puzzle exchange in the 2nd packet and the Initiator completes this exchange in the 3rd 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 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 data payload in the future. However, the details of this may be defined later. An existing HIP association can be updated with the update mechanism defined in . 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) 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 . 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) that allows IKEv2 to support complex gateway policies. Thus, HIP DEX is not a replacement for IKEv2.

The rest of this memo is structured as follows. defines the central keywords, notation, and terms used throughout this document. defines the structure of the Host Identity and its various representations. gives an overview of the HIP Diet EXchange protocol. Sections and define the detailed packet formats and rules for packet processing. Finally, Sections , , and discuss policy, security, and IANA considerations, respectively.

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.

indicates that x is optional. indicates that x is encrypted. indicates that y is a parameter of X. indicates that x exists i times. signifies "Initiator to Responder" communication (requests). signifies "Responder to Initiator" communication (replies). signifies concatenation of information - e.g., X | Y is the concatenation of X and Y. denotes the partitioning of X into n K-bit segments and the iterative folding of these segments via XOR. I.e., X = x_1, x_2, ..., x_n, where x_i is of length K and the last segment x_n is padded to length K by appending 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. denotes the lowest order K bits of the result of the MAC function M on the input x.

The ECDH-based HIP handshake for establishing a new HIP association. 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. ). A shorthand for the HI in IPv6 format. It is generated by folding the HI (c.f. ). 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. The shared state between two peers after completion of the HIP DEX handshake. The host that initiates the HIP DEX handshake. This role is typically forgotten once the handshake is completed. The host that responds to the Initiator in the HIP DEX handshake. This role is typically forgotten once the handshake is completed. 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 HIP DEX and is not used for HIT generation. The natural output length of RHASH in bits. CMAC-based Key Derivation Function.

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 for more details on the difference between an identity and the corresponding identifier. HIP DEX implementations MUST support the Elliptic Curve Diffie-Hellman (ECDH) key exchange for generating the HI as defined in . 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: The HIT suite ID MUST only be a DEX HIT ID (see ). The DEX HIT is not generated via a cryptographic hash. Rather, it is a compression of the HI. 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.

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 ) 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).

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. and 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 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 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 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

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 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 .

The HIP DEX state machine has the same states as the HIPv2 state machine (see 4.4. in ). 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 . 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 ). 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 ) 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 ). 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 at Responder boot-up. 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, . 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 TimeoutIncrement 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 R2 | data or| | | | |recv R1, | | | EC timeout| | | | |send I2 +--|-----------------+ | receive I2,| | | | | | +-------------+ | send R2| | | | | +------>| ESTABLISHED |<----------+ | | | | | +-------------+ | | | | | | | | 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 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: Source HIT Destination HIT HIP_Encrypt Key HIP_MAC Key The HIP_Encrypt and HIP_MAC keys are extracted from the Diffie-Hellman derived key as described in . 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 ). The keys for the Pair-wise Key SA are derived based on the wrapped keying material exchanged in the ENCRYPTED_KEY parameter (see ) 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 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.

The User Data Considerations in Section 4.5. of 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. This 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 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 where possible. Still, HIP DEX further restricts and/or extends the following existing parameter types: DH_GROUP_LIST and HOST_ID are restricted to ECC-based suites. HIP_CIPHER is restricted to AES-128-CTR and NULL-ENCRYPT. HIT_SUITE_LIST is limited to the HIT suite ECDH/FOLD. RHASH and RHASH_len are redefined to CMAC for the PUZZLE, SOLUTION, and HIP_MAC parameters (see and ). In addition, HIP DEX introduces the following new parameter: TLV Type Length Data ENCRYPTED_KEY643variableEncrypted container for the session key exchange

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 . 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 11 Curve448 [RFC7748] CKDF 12

The ECDH groups 7 - 9 are defined in and . ECDH group 10 is covered in and Appendix D of . These curves, when used with HIP MUST have a co-factor of 1. The ECDH groups 11 and 12 are defined in . 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 . With HIP DEX, the Suite IDs are limited to:

Suite ID Value RESERVED 0 NULL-ENCRYPT 1 ([RFC2410]) AES-128-CTR 5 ([RFC3686])

Mandatory implementation: AES-128-CTR. Implementors SHOULD support NULL-ENCRYPT () 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. It is defined in Section 5.2.9 of . 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 profile:

Algorithm profiles Value ECDH 11 [RFC6090] (REQUIRED)

HIP DEX HIs are serialized equally to the ECC-based HIs in HIPv2 (see Section 5.2.9. of ). 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 .

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 . The following HIT Suite IDs are 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 8 0x80

Note that the HIP DEX HIT Suite ID 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 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 ). This random value MUST have a length of at least 64 bit. The puzzle value #I and the puzzle solution #J (see ) are used as the initialization vector (IV) for the encryption process. To this end, the IV is computed as FOLD(I | J, 128). 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 .

HIP DEX uses the same eight basic HIP packets as HIPv2 (see Section 5.3 of ). 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 . 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 . 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 for detailed information about the "HIP Opportunistic Mode". As a compression of the employed HIs, the Initiator's and the Responder's HITs both 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 a HIP DEX HIT. They do not include information about the specific DH group ID of the corresponding 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 . 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 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 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 ). 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-routablility 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. The DH_GROUP_LIST parameter contains the Responder's order of preference based on which it chose 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 . 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. 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 ) 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. Currently, the only transport format defined is IPsec ESP . 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, 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 . 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 . 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. Currently, the only transport format defined is the ESP transport format. The MAC is calculated over the whole HIP envelope, excluding any parameters after the HIP_MAC parameter as described in . 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, 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 MAC is calculated over the whole HIP envelope, excluding any parameters after the HIP_MAC, as described in . 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 . 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 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 otherwise. The processing of outgoing and incoming application data remains the same as in HIP BEX (see Sections 6.1 and 6.2 in ).

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 is skipped in HIP DEX. Ltrunc( CMAC( I, HIT-I | HIT-R | J ), K ) == 0 V := Ltrunc( CMAC( I, HIT-I | HIT-R | J ), K ) Apart from these modifications, the procedures defined in Section 6.3 of 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: In the HIP header, the Checksum field is set to zero. In the HIP header, the Header Length field value is calculated to the beginning of the HIP_MAC parameter. The parameter order is described in Section 5.2.1 of . The CMAC calculation and verification process is as follows: Packet sender: Create the HIP packet, without the HIP_MAC or any other parameter with greater Type value than the HIP_MAC parameter has. Calculate the Header Length field in the HIP header. Compute the CMAC using either HIP-gl or HIP-lg integrity key retrieved from KEYMAT as defined in . Add the HIP_MAC parameter to the packet and any parameter with greater Type value than the HIP_MAC's that may follow. Recalculate the Length field in the HIP header. Packet receiver: Verify the HIP header Length field. 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. Recalculate the HIP packet length in the HIP header and clear the Checksum field (set it to all zeros). Compute the CMAC using either HIP-gl or HIP-lg integrity key as defined in and verify it against the received CMAC. Set Checksum and Header Length fields in the HIP header to original values. Note that the Checksum and Length fields contain incorrect values after this step.

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 from the Diffie-Hellman derived key, Kij, 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. 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 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 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 the Master Key SA employs both the Extract and Expand phases, whereas the Pair-wise Key SA MAY need both the Extract and Expand phases if the key is longer than 128 bits. Otherwise, it only requires the Expand phase. The CKDF-Extract function is the following operation:

CKDF-Extract(I, IKM, info) -> PRK where I Random #I from the PUZZLE parameter IKM Input keying material, i.e., either the Diffie-Hellman derived key 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) 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, IKM | info)

The CKDF-Expand function is the following operation:

CKDF-Expand(PRK, info, L) -> OKM where 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)) 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 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: HIP-gl encryption key for HOST_g's outgoing HIP packets HIP-gl integrity (CMAC) key for HOST_g's outgoing HIP packets HIP-lg encryption key for HOST_l's outgoing HIP packets 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: 128 or 256 bits 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 . The I1 packet contents are specified in .

I1 packets in HIP DEX are handled almost identical to HIPv2 (see Section 6.7 of ). 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: The Responder MUST check that the Responder's HIT in the received I1 packet is either one of its own HITs or NULL. Otherwise, it must drop the packet. If the Responder is in ESTABLISHED state, the Responder MAY respond to this with an R1 packet, prepare to drop an existing HIP security association with the peer, and stay at ESTABLISHED state. If the Responder is in I1-SENT state, it MUST make a comparison between the sender's HIT and its own (i.e., the receiver's) HIT. If the sender's HIT is greater than its own HIT, it should drop the I1 packet and stay at I1-SENT. If the sender's HIT is smaller than its own HIT, it SHOULD send the R1 packet and stay at I1-SENT. The HIT comparison is performed as defined in . If the implementation chooses to respond to the I1 packet with an R1 packet, it creates a new R1 according to the format described in . It chooses the HI based on the destination HIT and the DH_GROUP_LIST in the I1 packet. If the implementation does not support the DH group required by the Initiator or if the destination HIT in the I1 packet does not match the required DH group, it selects the mutually preferred and supported DH group based on the DH_GROUP_LIST parameter in the I1 packet. The implementation includes the corresponding ECDH public key in the HOST_ID parameter. If no suitable DH Group ID was contained in the DH_GROUP_LIST in the I1 packet, it sends an R1 packet with any suitable ECDH public key. If the received Responder's HIT in the I1 packet is not NULL, the Responder's in the R1 packet HIT MUST match the destination HIT in the I1 packet. Otherwise, the Responder MUST select a HIT with the same HIT Suite as the Initiator's HIT. If this HIT Suite is not supported by the Responder, it SHOULD select a REQUIRED HIT Suite from Section 5.2.10 of , which is currently RSA/DSA/SHA-256. Other than that, selecting the HIT is a local policy matter. The Responder expresses its supported HIP transport formats in the TRANSPORT_FORMAT_LIST as described in Section 5.2.11 of . The Responder MUST provide at least one payload transport format type. The Responder sends the R1 packet to the source IP address of the I1 packet. 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 likewise apply to HIP DEX.

R1 packets in HIP DEX are handled identically to HIPv2 (see Section 6.8 in ) 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: A system receiving an R1 MUST first check to see if it has sent an I1 packet to the originator of the R1 packet (i.e., it has a HIP association that is in state I1-SENT and that is associated with the HITs in the R1). Unless the I1 packet was sent in opportunistic mode (see Section 4.1.8 of ), the IP addresses in the received R1 packet SHOULD be ignored by the R1 processing and, when looking up the right HIP association, the received R1 packet SHOULD be matched against the associations using only the HITs. If a match exists, the system should process the R1 packet as described below. Otherwise, if the system is in any state other than I1-SENT or I2-SENT with respect to the HITs included in the R1 packet, it SHOULD silently drop the R1 packet and remain in the current state. If the HIP association state is I1-SENT or I2-SENT, the received Initiator's HIT MUST correspond to the HIT used in the original I1 packet. Also, the Responder's HIT MUST correspond to the one used in the I1 packet, unless this packet contained a NULL HIT. If the HIP association state is I1-SENT, and multiple valid R1 packets are present, the system MUST select from among the R1 packets with the largest R1 generation counter. The system MUST check that the Initiator's HIT Suite is contained in the HIT_SUITE_LIST parameter in the R1 packet (i.e., the Initiator's HIT Suite is supported by the Responder). If the HIT Suite is supported by the Responder, the system proceeds normally. Otherwise, the system MAY stay in state I1-SENT and restart the HIP DEX handshake by sending a new I1 packet with an Initiator HIT that is supported by the Responder and hence is contained in the HIT_SUITE_LIST in the R1 packet. The system MAY abort the handshake if no suitable source HIT is available. The system SHOULD wait for an acceptable time span to allow further R1 packets with higher R1 generation counters or different HIT and HIT Suites to arrive before restarting or aborting the HIP DEX handshake. The system MUST check that the DH Group ID in the HOST_ID parameter in the R1 matches the first DH Group ID in the Responder's DH_GROUP_LIST in the R1 packet, and also that this Group ID corresponds to a value that was included in the Initiator's DH_GROUP_LIST in the I1 packet. If the DH Group ID of the HOST_ID parameter does not express the Responder's best choice, the Initiator can conclude that the DH_GROUP_LIST in the I1 or R1 packet was adversely modified. In such a case, the Initiator MAY send a new I1 packet; however, it SHOULD NOT change its preference in the DH_GROUP_LIST in the new I1 packet. Alternatively, the Initiator MAY abort the HIP DEX handshake. Moreover, if the DH Group ID indicated in the HOST_ID parameter does not match the DH Group ID of the HI employed by the Initiator, the system SHOULD wait for an acceptable time span to allow further R1 packets with different DH Group IDs to arrive before restarting or aborting the HIP DEX handshake. When restarting the handshake, the Initiator MUST consult local policies (see ) regarding the use of another, mutually supported DH group for the subsequent handshake with the Responder. If the HIP association state is I2-SENT, the system MAY re-enter state I1-SENT and process the received R1 packet if it has a larger R1 generation counter than the R1 packet responded to previously. The R1 packet may have the A-bit set - in this case, the system MAY choose to refuse it by dropping the R1 packet and returning to state UNASSOCIATED. The system SHOULD consider dropping the R1 packet only if it used a NULL HIT in the I1 packet. If the A-bit is set, the Responder's HIT is anonymous and SHOULD NOT be stored permanently. The system SHOULD attempt to validate the HIT against the received Host Identity by using the received Host Identity to construct a HIT and verify that it matches the Sender's HIT. The system MUST store the received R1 generation counter for future reference. The system attempts to solve the puzzle in the R1 packet. The system MUST terminate the search after exceeding the remaining lifetime of the puzzle. If the puzzle is not successfully solved, the implementation MAY either resend the I1 packet within the retry bounds or abandon the HIP base exchange. The system computes standard Diffie-Hellman keying material according to the public value and Group ID provided in the HOST_ID parameter. The Diffie-Hellman keying material Kij is used for key extraction as specified in . The system selects the HIP_CIPHER ID from the choices presented in the R1 packet and uses the selected values subsequently when generating and using encryption keys, and when sending the I2 packet. If the proposed alternatives are not acceptable to the system, it may either resend an I1 packet within the retry bounds or abandon the HIP base exchange. The system chooses one suitable transport format from the TRANSPORT_FORMAT_LIST and includes the respective transport format parameter in the subsequent I2 packet. The system initializes the remaining variables in the associated state, including Update ID counters. The system prepares and sends an I2 packet as described in . The system SHOULD start a timer whose timeout value SHOULD be larger than the worst-case anticipated RTT, and MUST increment a trial counter associated with the I2 packet. The sender SHOULD retransmit the I2 packet upon a timeout and restart the timer, up to a maximum of I2_RETRIES_MAX tries. If the system is in state I1-SENT, it SHALL transition to state I2-SENT. If the system is in any other state, it remains in the current state. 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 HIPv2 processing rules has been adapted 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 also apply to HIP DEX.

The processing of I2 packets follows similar rules as HIPv2 (see Section 6.9 of ). 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: 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 for a description of an example implementation. The system MUST check that the Responder's HIT corresponds to one of its own HITs and MUST drop the packet otherwise. The system MUST further check that the Initiator's HIT Suite is supported. The Responder SHOULD silently drop I2 packets with unsupported Initiator HITs. If the system's state machine is in the R2-SENT state, the system MUST check to see 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 reset the R2-SENT timer. The system SHOULD re-use the previously established state to re-create the corresponding R2 packet in order to prevent unnecessary computation overhead. 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 ). 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. 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 retransmitting I1 packets (provided that the I2 packet passes the additional checks below). If the system's state machine is in any state other 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. The system MUST validate the solution to the puzzle as described in . 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. The system MUST derive Diffie-Hellman keying material Kij based on the public value and Group ID in the HOST_ID parameter. This keying material is used to derive the keys of the Master Key SA as described in . If the Diffie-Hellman Group ID is unsupported, the I2 packet is silently dropped. If the processing time for the derivation of the Diffie-Hellman keying material Kij is likely to cause premature I2 retransmissions by the Initiator, the system MAY send a NOTIFY packet before starting the key derivation process. The NOTIFY packet contains a NOTIFICATION parameter with Notify Message Type I2_ACKNOWLEDGEMENT. The NOTIFICATION parameter indicates the anticipated remaining processing time for the I2 packet in milliseconds as two-octet Notification Data. 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 be able to make this verification.) The system MUST process the TRANSPORT_FORMAT_LIST parameter. Other documents specifying transport formats (e.g., ) contain specifications for handling any specific transport selected. The system MUST verify the HIP_MAC according to the procedures in . 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. 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. The system MUST decrypt 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 ). The system initializes the remaining variables in the associated state, including Update ID counters. Upon successful processing of an I2 packet when the system's state machine is in state UNASSOCIATED, I1-SENT, I2-SENT, or R2-SENT, an R2 packet is sent as described in and the system's state machine transitions to state R2-SENT. 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 as described in , and the system's state machine transitions to R2-SENT. 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 a maximal amount of retransmissions of I2 packets), the state machine transitions to ESTABLISHED. 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. The considerations about malformed I2 packets in Sections 6.9.1 of also apply to HIP DEX.

R2 packets in HIP DEX are handled identically to HIPv2 (see Section 6.10 of ) 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: If the system is in any other state than I2-SENT, the R2 packet is silently dropped. 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 I2-SENT state. The system MUST verify the HIP_MAC according to the procedures in . The system MUST re-evaluate the DH_GROUP_LIST, HIP_CIPHER, HIT_SUITE_LIST, and TRANSPORT_FORMAT_LIST parameters in the R2 packet and compare the results against the chosen suites. 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. The system MUST decrypt 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 ). Upon successful processing of the R2 packet, the state machine transitions to state ESTABLISHED. 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 may be 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: 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 I2-SENT state. If this check fails, the NOTIFY packet SHOULD be dropped silently. If the NOTIFY packet contains a NOTIFICATION parameter with Notify Message Type I2_ACKNOWLEDGEMENT, the system SHOULD set the I2 retransmission timer to the I2 processing time indicated in the NOTIFICATION parameter plus half the RTT-based timeout value. The system MUST NOT set the retransmission timeout to a higher value than allowed by a local policy. Moreover, the system SHOULD reset the I2 retransmission timer to the RTT-based timeout value after the next I2 retransmission.

UPDATE, CLOSE, and CLOSE_ACK packets are handled similarly in HIP DEX as in HIP BEX (see Sections 6.11, 6.12, 6.14, and 6.15 of ). The only difference is the that the HIP_SIGNATURE is never present and, therefore, is not required to be processed by the receiving party.

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.

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 #K used in the HIP R1 must be chosen with care. Values of #K that are too high will exclude clients with weak CPUs because these devices cannot solve the puzzle within a reasonable amount of time. #K 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.

HIP DEX closely resembles HIPv2. As such, the security considerations discussed in Section 8 of 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 by 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 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 the Responder. Since the Initiator is expected to be a sensor or an actuator device, there is a natural concern about the quality of its random number generator. HIP DEX lacks the Perfect Forward Secrecy (PFS) property of HIPv2. Consequently, if an HI is compromised, all HIP connections protected with that HI are compromised. The puzzle mechanism using CMAC may need further study regarding the level of difficulty. The HIP DEX HIT generation may present new attack opportunities. The R1 packet is unauthenticated and offers an adversary a new attack vector against the Initiator. This is mitigated by only processing a received R1 packet when the Initiator has previously sent a corresponding I1 packet. Moreover, the Responder repeats the DH_GROUP_LIST, HIP_CIPHER, HIT_SUITE_LIST, and TRANSPORT_FORMAT_LIST parameters in the R2 packet in order to enable the Initiator to verify that these parameters have not been modified by an attacker in the unprotected R1 packet. 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.

The following changes to the "Host Identity Protocol (HIP) Parameters" registries have been made: This document defines the new HIT Suite "ECDH/FOLD" (see ). This document defines the new HIP parameter "ENCRYPTED_KEY" with type number 643 (see ). This document defines the new HIP Cipher ID "AES-128-CTR" (see ). This document defines the new HI Algorithm "ECDH" (see ). This document specifies a new algorithm-specific subregistry named "ECDH Curve Label". The values for this subregistry are defined in .

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.

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.

Added the new ECDH groups of Curve2519 and Curve448 from RFC 7748.

The Internet Draft was adopted by the HIP WG.

A major change in the ENCRYPT parameter to use AES-CTR rather than AES-CBC.

Draft name change. HIPRG ended in IRTF, HIP DEX is now individual submission. Added the change section. Added a Definitions section. Changed I2 and R2 packets to reflect use of AES-CTR for ENCRYPTED_KEY parameter. Cleaned up KEYMAT Generation text. Added Appendix with C code for the ECDH shared secret generation on an 8 bit processor.

Numerous editorial changes. New retransmission strategy. New HIT generation mechanism. Modified layout of ENCRYPTED_KEY parameter. Clarify to use puzzle difficulty of zero under normal network conditions. Align inclusion directive of R1_COUNTER with HIPv2 (from SHOULD to MUST). Align inclusion of TRANSPORT_FORMAT_LIST with HIPv2 (added to R1 and I2). HIP_CIPHER, HIT_SUITE_LIST, and TRANSPORT_FORMAT_LIST must now be echoed in R2 packet. Added new author.

Introduced formal definition of FOLD function. Clarified use of CMAC for puzzle computation in section "Solving the Puzzle". Several editorial changes.

Addressed HI crypto agility. Clarified purpose of secret exchanged via ENCRYPTED_KEY parameter. Extended the IV in the ENCRYPTED_KEY parameter. Introduced forward-references to HIP DEX KEYMAT process and improved KEYMAT section. Replaced Appendix A on "C code for ECC point multiplication" with short discussion in introduction. Updated references. Further editorial changes.

Improved retransmission extension. Updated and strongly revised packet processing rules. Updated security considerations. Updated IANA considerations. Move the HI Algorithm for ECDH to a value of 11. Many editorial changes.

NC State University NC State University Chair of Communication and Distributed Systems, RWTH Aachen Chair of Communication and Distributed Systems, RWTH Aachen Chair of Communication and Distributed Systems, RWTH Aachen Chair of Communication and Distributed Systems, RWTH Aachen Chair of Communication and Distributed Systems, RWTH Aachen SECG

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.