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Is this intentional? -- Found something which looks like a code comment -- if you have code sections in the document, please surround them with '' and '' lines. Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-20) exists of draft-ietf-hip-rfc4423-bis-12 -- Obsolete informational reference (is this intentional?): RFC 5996 (Obsoleted by RFC 7296) Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group R. Moskowitz, Ed. 3 Internet-Draft HTT Consulting 4 Intended status: Standards Track R. Hummen 5 Expires: January 21, 2016 COMSYS, RWTH Aachen 6 July 20, 2015 8 HIP Diet EXchange (DEX) 9 draft-moskowitz-hip-dex-04 11 Abstract 13 This document specifies the Host Identity Protocol Diet EXchange (HIP 14 DEX), a variant of the Host Identity Protocol Version 2 (HIPv2). The 15 HIP DEX protocol design aims at reducing the overhead of the employed 16 cryptographic primitives by omitting public-key signatures and hash 17 functions. In doing so, the main goal is to still deliver similar 18 security properties to HIPv2. 20 The HIP DEX protocol is primarily designed for computation or memory- 21 constrained sensor/actuator devices. Like HIPv2, it is expected to 22 be used together with a suitable security protocol such as the 23 Encapsulated Security Payload (ESP) for the protection of upper layer 24 protocol data. In addition, HIP DEX can also be used as a keying 25 mechanism for security primitives at the MAC layer, e.g., for IEEE 26 802.15.4 networks. 28 Status of This Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at http://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on January 21, 2016. 45 Copyright Notice 47 Copyright (c) 2015 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (http://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 63 1.1. The HIP Diet EXchange (DEX) . . . . . . . . . . . . . . . 4 64 1.2. Memo Structure . . . . . . . . . . . . . . . . . . . . . 5 65 2. Terms and Definitions . . . . . . . . . . . . . . . . . . . . 6 66 2.1. Requirements Terminology . . . . . . . . . . . . . . . . 6 67 2.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . 6 68 2.3. Definitions . . . . . . . . . . . . . . . . . . . . . . . 6 69 3. Host Identity (HI) and its Structure . . . . . . . . . . . . 7 70 3.1. Host Identity Tag (HIT) . . . . . . . . . . . . . . . . . 8 71 3.2. Generating a HIT from an HI . . . . . . . . . . . . . . . 8 72 4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 9 73 4.1. Creating a HIP Association . . . . . . . . . . . . . . . 9 74 4.1.1. HIP Puzzle Mechanism . . . . . . . . . . . . . . . . 10 75 4.1.2. HIP State Machine . . . . . . . . . . . . . . . . . . 11 76 4.1.3. HIP DEX Security Associations . . . . . . . . . . . . 15 77 4.1.4. User Data Considerations . . . . . . . . . . . . . . 16 78 5. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 16 79 5.1. Payload Format . . . . . . . . . . . . . . . . . . . . . 16 80 5.2. HIP Parameters . . . . . . . . . . . . . . . . . . . . . 16 81 5.2.1. DH_GROUP_LIST . . . . . . . . . . . . . . . . . . . . 17 82 5.2.2. HIP_CIPHER . . . . . . . . . . . . . . . . . . . . . 17 83 5.2.3. HOST_ID . . . . . . . . . . . . . . . . . . . . . . . 17 84 5.2.4. HIT_SUITE_LIST . . . . . . . . . . . . . . . . . . . 18 85 5.2.5. ENCRYPTED_KEY . . . . . . . . . . . . . . . . . . . . 18 86 5.3. HIP Packets . . . . . . . . . . . . . . . . . . . . . . . 19 87 5.3.1. I1 - the HIP Initiator Packet . . . . . . . . . . . . 20 88 5.3.2. R1 - the HIP Responder Packet . . . . . . . . . . . . 21 89 5.3.3. I2 - the Second HIP Initiator Packet . . . . . . . . 23 90 5.3.4. R2 - the Second HIP Responder Packet . . . . . . . . 24 91 5.4. ICMP Messages . . . . . . . . . . . . . . . . . . . . . . 25 92 6. Packet Processing . . . . . . . . . . . . . . . . . . . . . . 25 93 6.1. Solving the Puzzle . . . . . . . . . . . . . . . . . . . 25 94 6.2. HIP_MAC Calculation and Verification . . . . . . . . . . 26 95 6.2.1. CMAC Calculation . . . . . . . . . . . . . . . . . . 26 96 6.3. HIP DEX KEYMAT Generation . . . . . . . . . . . . . . . . 27 97 6.4. Initiation of a HIP Diet EXchange . . . . . . . . . . . . 30 98 6.5. Processing Incoming I1 Packets . . . . . . . . . . . . . 30 99 6.6. Processing Incoming R1 Packets . . . . . . . . . . . . . 31 100 6.7. Processing Incoming I2 Packets . . . . . . . . . . . . . 34 101 6.8. Processing Incoming R2 Packets . . . . . . . . . . . . . 37 102 6.9. Processing Incoming NOTIFY Packets . . . . . . . . . . . 38 103 6.10. Processing UPDATE, CLOSE, and CLOSE_ACK Packets . . . . . 39 104 6.11. Handling State Loss . . . . . . . . . . . . . . . . . . . 39 105 7. HIP Policies . . . . . . . . . . . . . . . . . . . . . . . . 39 106 8. Security Considerations . . . . . . . . . . . . . . . . . . . 39 107 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40 108 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 41 109 11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 41 110 11.1. Changes in draft-moskowitz-hip-rg-dex-06 . . . . . . . . 41 111 11.2. Changes in draft-moskowitz-hip-dex-00 . . . . . . . . . 41 112 11.3. Changes in draft-moskowitz-hip-dex-01 . . . . . . . . . 42 113 11.4. Changes in draft-moskowitz-hip-dex-02 . . . . . . . . . 42 114 11.5. Changes in draft-moskowitz-hip-dex-03 . . . . . . . . . 42 115 11.6. Changes in draft-moskowitz-hip-dex-04 . . . . . . . . . 43 116 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 43 117 12.1. Normative References . . . . . . . . . . . . . . . . . . 43 118 12.2. Informative References . . . . . . . . . . . . . . . . . 44 119 Appendix A. Password-based two-factor authentication during 120 the HIP DEX handshake . . . . . . . . . . . . . . . 46 121 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 46 123 1. Introduction 125 This document specifies the Host Identity Protocol Diet EXchange (HIP 126 DEX). HIP DEX builds on the Base EXchange (BEX) of the Host Identity 127 Protocol Version 2 (HIPv2) [RFC7401]. HIP DEX preserves the protocol 128 semantics as well as the general packet structure of HIPv2. Hence, 129 it is recommended that [RFC7401] is well-understood before reading 130 this document. 132 The main differences between HIP BEX and HIP DEX are: 134 1. Minimum collection of cryptographic primitives to reduce the 135 protocol overhead. 137 * Static Elliptic Curve Diffie-Hellman key pairs for peer 138 authentication and encryption of the session key. 140 * AES-CTR for symmetric encryption and AES-CMAC for MACing 141 function. 143 * A simple fold function for HIT generation. 145 2. Forfeit of Perfect Forward Secrecy with the dropping of an 146 ephemeral Diffie-Hellman key agreement. 148 3. Forfeit of digital signatures with the removal of a hash 149 function. Reliance on ECDH derived key used in HIP_MAC to prove 150 ownership of the private key. 152 4. Diffie-Hellman derived key ONLY used to protect the HIP packets. 153 A separate secret exchange within the HIP packets creates the 154 session key(s). 156 5. Optional retransmission strategy tailored to handle the 157 potentially extensive processing time of the employed 158 cryptographic operations on computationally constrained devices. 160 By eliminating the need for public-key signatures and the ephemeral 161 DH key agreement, HIP DEX reduces the computation, energy, 162 transmission, and memory requirements for public-key cryptography 163 (see [LN08]) in the HIPv2 protocol design. Moreover, by dropping the 164 cryptographic hash function, HIP DEX affords a more efficient 165 protocol implementation than HIP BEX with respect to the 166 corresponding computation and memory requirements. This makes HIP 167 DEX especially suitable for constrained devices as defined in 168 [RFC7228]. 170 This document focuses on the protocol specifications related to 171 differences between HIP BEX and HIP DEX. Where differences are not 172 called out explicitly, the protocol specification of HIP DEX is the 173 same as defined in [RFC7401]. 175 1.1. The HIP Diet EXchange (DEX) 177 The HIP Diet EXchange is a two-party cryptographic protocol used to 178 establish a secure communication context between hosts. The first 179 party is called the Initiator and the second party the Responder. 180 The four-packet exchange helps to make HIP DEX DoS resilient. The 181 Initiator and the Responder exchange their static Elliptic Curve 182 Diffie-Hellman (ECDH) keys in the 2nd and 3rd handshake packet. The 183 parties then authenticate each other in the 3rd and 4th handshake 184 packet based on the ECDH-derived keying material. The Initiator and 185 the Responder additionally transmit keying material for the session 186 key in these last two handshake packets. This is to prevent overuse 187 of the static ECDH-derived keying material. Moreover, the Responder 188 starts a puzzle exchange in the 2nd packet and the Initiator 189 completes this exchange in the 3rd packet before the Responder 190 performs computationally expensive operations or stores any state 191 from the exchange. Given this handshake structure, HIP DEX 192 operationally is very similar to HIP BEX. Moreover, the employed 193 model is also fairly equivalent to 802.11-2007 [IEEE.802-11.2007] 194 Master Key and Pair-wise Transient Key, but handled in a single 195 exchange. 197 HIP DEX does not have the option to encrypt the Host Identity of the 198 Initiator in the 3rd packet. The Responder's Host Identity also is 199 not protected. Thus, contrary to HIPv2, there is no attempt at 200 anonymity. 202 Data packets start to flow after the 4th packet. The 3rd and 4th HIP 203 packets may carry data payload in the future. However, the details 204 of this may be defined later. 206 An existing HIP association can be updated with the update mechanism 207 defined in [RFC7401]. Likewise, the association can be torn down 208 with the defined closing mechanism for HIPv2 if it is no longer 209 needed. HIP DEX thereby omits the HIP_SIGNATURE parameters of the 210 original HIPv2 specification. 212 Finally, HIP DEX is designed as an end-to-end authentication and key 213 establishment protocol. As such, it can be used in combination with 214 Encapsulated Security Payload (ESP) [RFC7402] as well as with other 215 end-to-end security protocols. In addition, HIP DEX can also be used 216 as a keying mechanism for security primitives at the MAC layer, e.g., 217 for IEEE 802.15.4 networks [IEEE.802-15-4.2011]. It is worth 218 mentioning that the HIP DEX base protocol does not cover all the 219 fine-grained policy control found in Internet Key Exchange Version 2 220 (IKEv2) [RFC5996] that allows IKEv2 to support complex gateway 221 policies. Thus, HIP DEX is not a replacement for IKEv2. 223 1.2. Memo Structure 225 The rest of this memo is structured as follows. Section 2 defines 226 the central keywords, notation, and terms used throughout this 227 document. Section 3 defines the structure of the Host Identity and 228 its various representations. Section 4 gives an overview of the HIP 229 Diet EXchange protocol. Sections 5 and 6 define the detailed packet 230 formats and rules for packet processing. Finally, Sections 7, 8, and 231 9 discuss policy, security, and IANA considerations, respectively. 233 2. Terms and Definitions 235 2.1. Requirements Terminology 237 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 238 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 239 document are to be interpreted as described in RFC 2119 [RFC2119]. 241 2.2. Notation 243 [x] indicates that x is optional. 245 {x} indicates that x is encrypted. 247 X(y) indicates that y is a parameter of X. 249 i indicates that x exists i times. 251 --> signifies "Initiator to Responder" communication (requests). 253 <-- signifies "Responder to Initiator" communication (replies). 255 | signifies concatenation of information - e.g., X | Y is the 256 concatenation of X and Y. 258 FOLD (X, K) denotes the partitioning of X into n K-bit segments and 259 the iterative folding of these segments via XOR. I.e., X = x_1, 260 x_2, ..., x_n, where x_i is of length K and the last segment x_n 261 is padded to length K by appending 0 bits. FOLD then is computed 262 as FOLD(X, K) = t_n, where t_i = t_i-1 XOR x_i and t_1 = x_1. 264 Ltrunc (M(x), K) denotes the lowest order K bits of the result of 265 the MAC function M on the input x. 267 2.3. Definitions 269 HIP Diet Exchange (DEX): The ECDH-based HIP handshake for 270 establishing a new HIP association. 272 Host Identity (HI): The static ECDH public key that represents the 273 identity of the host. In HIP DEX, a host proves ownership of the 274 private key belonging to its HI by creating a HIP_MAC with the 275 derived ECDH key (c.f. Section 3). 277 Host Identity Tag (HIT): A shorthand for the HI in IPv6 format. It 278 is generated by folding the HI (c.f. Section 3). 280 HIT Suite: A HIT Suite groups all algorithms that are required to 281 generate and use an HI and its HIT. In particular, these 282 algorithms are: 1) ECDH and 2) FOLD. 284 HIP association: The shared state between two peers after completion 285 of the HIP DEX handshake. 287 Initiator: The host that initiates the HIP DEX handshake. This role 288 is typically forgotten once the handshake is completed. 290 Responder: The host that responds to the Initiator in the HIP DEX 291 handshake. This role is typically forgotten once the handshake is 292 completed. 294 Responder's HIT Hash Algorithm (RHASH): In HIP DEX, RHASH is 295 redefined as CMAC. Still, note that CMAC is a message 296 authentication code and not a cryptographic hash function. Thus, 297 a mapping from CMAC(x,y) to RHASH(z) must be defined where RHASH 298 is used. Moreover, RHASH has different security properties in HIP 299 DEX and is not used for HIT generation. 301 Length of the Responder's HIT Hash Algorithm (RHASH_len): The 302 natural output length of RHASH in bits. 304 CKDF: CMAC-based Key Derivation Function. 306 3. Host Identity (HI) and its Structure 308 In this section, the properties of the Host Identity and Host 309 Identity Tag are discussed, and the exact format for them is defined. 310 In HIP, the public key of an asymmetric key pair is used as the Host 311 Identity (HI). Correspondingly, the host itself is defined as the 312 entity that holds the private key of the key pair. See the HIP 313 architecture specification [I-D.ietf-hip-rfc4423-bis] for more 314 details on the difference between an identity and the corresponding 315 identifier. 317 HIP DEX implementations MUST support the Elliptic Curve Diffie- 318 Hellman (ECDH) [RFC6090] key exchange for generating the HI as 319 defined in Section 5.2.3. No additional algorithms are supported at 320 this time. 322 A compressed encoding of the HI, the Host Identity Tag (HIT), is used 323 in the handshake packets to represent the HI. The DEX Host Identity 324 Tag (HIT) is different from the BEX HIT in two ways: 326 o The HIT suite ID MUST only be a DEX HIT ID (see Section 5.2.4). 328 o The DEX HIT is not generated via a cryptographic hash. Rather, it 329 is a compression of the HI. 331 Due to the latter property, an attacker may be able to find a 332 collision with a HIT that is in use. Hence, policy decisions such as 333 access control MUST NOT be based solely on the HIT. Instead, the HI 334 of a host SHOULD be considered. 336 Carrying HIs and HITs in the header of user data packets would 337 increase the overhead of packets. Thus, it is not expected that 338 these parameters are carried in every packet, but other methods are 339 used to map the data packets to the corresponding HIs. In some 340 cases, this allows to use HIP DEX without any additional headers in 341 the user data packets. For example, if ESP is used to protect data 342 traffic, the Security Parameter Index (SPI) carried in the ESP header 343 can be used to map the encrypted data packet to the correct HIP DEX 344 association. 346 3.1. Host Identity Tag (HIT) 348 With HIP DEX, the HIT is a 128-bit value - a compression of the HI 349 prepended with a specific prefix. There are two advantages of using 350 a hashed encoding over the actual variable-sized public key in 351 protocols. First, the fixed length of the HIT keeps packet sizes 352 manageable and eases protocol coding. Second, it presents a 353 consistent format for the protocol, independent of the underlying 354 identity technology in use. 356 The structure of the HIT is based on RFC 7343 [RFC7343], called 357 Overlay Routable Cryptographic Hash Identifiers (ORCHIDs), and 358 consists of three parts: first, an IANA assigned prefix to 359 distinguish it from other IPv6 addresses. Second, a four-bit 360 encoding of the algorithms that were used for generating the HI and 361 the compressed representation of the HI. Third, a 96-bit hashed 362 representation of the HI. In contrast to HIPv2, HIP DEX employs HITs 363 that are NOT generated by means of a cryptographic hash. Instead, 364 the HI is compressed to 96 bits as defined in the following section. 366 3.2. Generating a HIT from an HI 368 The HIT does not follow the exact semantics of an ORCHID as there is 369 no hash function in HIP DEX. Still, its structure is strongly 370 aligned with the ORCHID design. The same IPv6 prefix used in HIPv2 371 is used for HIP DEX. The HIP DEX HIT suite (see Section 9) is used 372 for the four bits of the Orchid Generation Algorithm (OGA) field in 373 the ORCHID. The hash representation in an ORCHID is replaced with 374 FOLD(HI,96). 376 4. Protocol Overview 378 This section gives a simplified overview of the HIP DEX protocol 379 operation and does not contain all the details of the packet formats 380 or the packet processing steps. Section 5 and Section 6 describe 381 these aspects in more detail and are normative in case of any 382 conflicts with this section. Importantly, the information given in 383 this section focuses on the differences between the HIPv2 and HIP DEX 384 protocol specifications. 386 4.1. Creating a HIP Association 388 By definition, the system initiating a HIP Diet EXchange is the 389 Initiator, and the peer is the Responder. This distinction is 390 typically forgotten once the handshake completes, and either party 391 can become the Initiator in future communications. 393 The HIP Diet EXchange serves to manage the establishment of state 394 between an Initiator and a Responder. The first packet, I1, 395 initiates the exchange, and the last three packets, R1, I2, and R2, 396 constitute an authenticated Diffie-Hellman [DH76] key exchange for 397 the Master Key SA generation. This Master Key SA is used by the 398 Initiator and the Responder to wrap secret keying material in the I2 399 and R2 packets. Based on the exchanged keying material, the peers 400 then derive a Pair-wise Key SA if cryptographic keys are needed, 401 e.g., for ESP-based protection of user data. 403 The Initiator first sends a trigger packet, I1, to the Responder. 404 This packet contains the HIT of the Initiator and the HIT of the 405 Responder, if it is known. Moreover, the I1 packet initializes the 406 negotiation of the Diffie-Hellman group that is used for generating 407 the the Master Key SA. Therefore, the I1 packet contains a list of 408 Diffie-Hellman Group IDs supported by the Initiator. Note that in 409 some cases it may be possible to replace this trigger packet by some 410 other form of a trigger, in which case the protocol starts with the 411 Responder sending the R1 packet. In such cases, another mechanism to 412 convey the Initiator's supported DH Groups (e.g., by using a default 413 group) must be specified. 415 The second packet, R1, starts the actual authenticated Diffie-Hellman 416 key exchange. It contains a puzzle - a cryptographic challenge that 417 the Initiator must solve before continuing the exchange. The level 418 of difficulty of the puzzle can be adjusted based on level of trust 419 with the Initiator, current load, or other factors. In addition, the 420 R1 contains the Responder's Diffie-Hellman parameter and lists of 421 cryptographic algorithms supported by the Responder. Based on these 422 lists, the Initiator can continue, abort, or restart the handshake 423 with a different selection of cryptographic algorithms. 425 In the I2 packet, the Initiator MUST display the solution to the 426 received puzzle. Without a correct solution, the I2 packet is 427 discarded. The I2 also contains a key wrap parameter that carries a 428 secret keying material of the Initiator. This keying material is 429 only half the final session key. The packet is authenticated by the 430 sender (Initiator) via a MAC. 432 The R2 packet acknowledges the receipt of the I2 packet and completes 433 the handshake. The R2 contains a key wrap parameter that carries the 434 rest of the keying material of the Responder. The packet is 435 authenticated by the sender (Responder) via a MAC. 437 The HIP DEX handshake is illustrated below. The terms "ENC(DH,x)" 438 and "ENC(DH,y)" refer to the random values x and y that are wrapped 439 based on the Master Key SA (indicated by ENC and DH). Note that x 440 and y each constitute half the final session key material. The 441 packets also contain other parameters that are not shown in this 442 figure. 444 Initiator Responder 446 I1: 447 ---------------------------------> 448 remain stateless 449 R1: puzzle, HI 450 <-------------------------------- 451 solve puzzle 452 perform ECDH 453 encrypt x 454 I2: solution, HI, ENC(DH,x), mac 455 ---------------------------------> 456 check puzzle 457 perform ECDH 458 check mac 459 decrypt x 460 encrypt y 461 R2: ENC(DH,y), mac 462 <--------------------------------- 463 check mac 464 decrypt y 466 4.1.1. HIP Puzzle Mechanism 468 The purpose of the HIP puzzle mechanism is to protect the Responder 469 from a number of denial-of-service threats. It allows the Responder 470 to delay state creation until receiving the I2 packet. Furthermore, 471 the puzzle allows the Responder to use a fairly cheap calculation to 472 check that the Initiator is "sincere" in the sense that it has 473 churned enough CPU cycles in solving the puzzle. 475 The puzzle mechanism enables a Responder to immediately reject an I2 476 packet if it does not contain a valid puzzle solution. To verify the 477 puzzle solution, the Responder only has to compute a single CMAC 478 operation. After a successful puzzle verification, the Responder can 479 securely create session-specific state and perform CPU-intensive 480 operations such as a Diffie-Hellman key generation. By varying the 481 difficulty of the puzzle, the Responder can frustrate CPU or memory 482 targeted DoS attacks. Under normal network conditions, the puzzle 483 difficulty SHOULD be zero, thus effectively reverting the puzzle 484 mechanism to a cookie-based DoS protection mechanism. Without 485 setting the puzzle difficulty to zero under normal network 486 conditions, potentially scarce computation resources at the Initiator 487 would be churned unnecessarily. 489 Conceptually, the puzzle mechanism in HIP DEX is the same as in 490 HIPv2. Hence, this document refers to Sections 4.1.1 and 4.1.2 in 491 [RFC7401] for more detailed information about the employed mechanism. 492 Notably, the only difference between the puzzle mechanism in HIP DEX 493 and HIPv2 is that HIP DEX uses CMAC instead of a hash function for 494 solving and verifying a puzzle. The implications of this change on 495 the puzzle implementation are discussed in Section 6.1. 497 4.1.2. HIP State Machine 499 The HIP DEX state machine has the same states as the HIPv2 state 500 machine (see 4.4. in [RFC7401]). However, HIP DEX features a 501 retransmission strategy with an optional reception acknowledgement 502 for the I2 packet. The goal of this additional acknowledgement is to 503 reduce premature I2 retransmissions in case of devices with low 504 computation resources [HWZ13]. As a result, there are minor changes 505 regarding the transitions in the HIP DEX state machine. The 506 following section documents these differences compared to HIPv2. 508 4.1.2.1. HIP DEX Retransmission Mechanism 510 For the retransmission of I1 and I2 packets, the Initiator adopts the 511 retransmission strategy of HIPv2 (see Section 4.4.3. in [RFC7401]). 512 This strategy is based on a timeout that is set to a value larger 513 than the worst-case anticipated round-trip time (RTT). For each 514 received I1 or I2 packet, the Responder sends an R1 or R2 packet, 515 respectively. This design trait enables the Responder to remain 516 stateless until the reception and successful processing of the I2 517 packet. The Initiator stops retransmitting I1 or I2 packets after 518 the reception of the corresponding R1 or R2. If the Initiator did 519 not receive an R1 packet after I1_RETRIES_MAX tries, it stops I1 520 retransmissions. Likewise, it stops retransmitting the I2 packet 521 after I2_RETRIES_MAX unsuccessful tries. 523 For repeatedly received I2 packets, the Responder SHOULD NOT perform 524 operations related to the Diffie-Hellman key exchange or the keying 525 material wrapped in the ENCRYPTED_KEY parameters. Instead, it SHOULD 526 re-use the previously established state to re-create the 527 corresponding R2 packet in order to prevent unnecessary computation 528 overhead. 530 The potentially high processing time of an I2 packet at a (resource- 531 constrained) Responder may cause premature retransmissions if the 532 time required for I2 transmission and processing exceeds the RTT- 533 based retransmission timeout. Thus, the Initiator should also take 534 the processing time of the I2 packet at the Responder into account 535 for retransmission purposes. To this end, the Responder MAY notify 536 the Initiator about the anticipated delay once the puzzle solution 537 was successfully verified and if the remaining I2 packet processing 538 incurs a high processing delay. The Responder MAY therefore send a 539 NOTIFY packet (see Section 5.3.6. in [RFC7401]) to the Initiator 540 before the Responder commences the ECDH operation. The NOTIFY packet 541 serves as an acknowledgement for the I2 packet and consists of a 542 NOTIFICATION parameter with Notify Message Type I2_ACKNOWLEDGEMENT 543 (see Section 5.2.19. in [RFC7401]). The NOTIFICATION parameter 544 contains the anticipated remaining processing time for the I2 packet 545 in milliseconds as two-octet Notification Data. This processing time 546 can, e.g., be estimated by measuring the computation time of the ECDH 547 key derivation operation at Responder boot-up. After the I2 548 processing has finished, the Responder sends the regular R2 packet. 550 When the Initiator receives the NOTIFY packet, it sets the I2 551 retransmission timeout to the I2 processing time indicated in the 552 NOTIFICATION parameter plus half the RTT-based timeout value. In 553 doing so, the Initiator MUST NOT set the retransmission timeout to a 554 higher value than allowed by a local policy. This is to prevent 555 unauthenticated NOTIFY packets from maliciously delaying the 556 handshake beyond a well-defined upper bound in case of a lost R2 557 packet. At the same time, this extended retransmission timeout 558 enables the Initiator to defer I2 retransmissions until the point in 559 time when the Responder should have completed its I2 packet 560 processing and the network should have delivered the R2 packet 561 according to the employed worst-case estimates. 563 4.1.2.2. HIP State Processes 565 HIP DEX clarifies or introduces the following new transitions. 567 System behavior in state I2-SENT, Table 1. 569 +---------------------+---------------------------------------------+ 570 | Trigger | Action | 571 +---------------------+---------------------------------------------+ 572 | Receive NOTIFY, | Set I2 retransmission timer to value in | 573 | process | I2_ACKNOWLEDGEMENT Notification Data plus | 574 | | 1/2 RTT-based timeout value and stay at | 575 | | I2-SENT | 576 | | | 577 | Timeout | Increment trial counter | 578 | | | 579 | | If counter is less than I2_RETRIES_MAX, | 580 | | send I2, reset timer to RTT-based timeout, | 581 | | and stay at I2-SENT | 582 | | | 583 | | If counter is greater than I2_RETRIES_MAX, | 584 | | go to E-FAILED | 585 +---------------------+---------------------------------------------+ 587 Table 1: I2-SENT - Waiting to finish the HIP Diet EXchange 589 4.1.2.3. Simplified HIP State Diagram 591 The following diagram shows the major state transitions. Transitions 592 based on received packets implicitly assume that the packets are 593 successfully authenticated or processed. 595 +--+ +----------------------------+ 596 recv I1, send R1 | | | | 597 | v v | 598 +--------------+ recv I2, send R2 | 599 +----------------| UNASSOCIATED |----------------+ | 600 datagram | +--+ +--------------+ | | 601 to send, | | | Alg. not supported, | | 602 send I1 | | | send I1 | | 603 . v | v | | 604 . +---------+ recv I2, send R2 | | 605 +---->| I1-SENT |--------------------------------------+ | | 606 | +---------+ +----------------------+ | | | 607 | | recv R1, | recv I2, send R2 | | | | 608 | v send I2 | v v v | 609 | +---------+ | +---------+ | 610 | +--->| I2-SENT |----------+ +--------------| R2-SENT |<---+ | 611 | | +---------+ | +---------+ | | 612 | | | |recv R2 | data or| | | 613 | |recv R1, | | | EC timeout| | | 614 | |send I2 +--|-----------------+ | receive I2,| | 615 | | | | +-------------+ | send R2| | 616 | | | +------>| ESTABLISHED |<----------+ | | 617 | | | +-------------+ | | 618 | | | | | | receive I2, send R2 | | 619 | | +------------+ | +-------------------------------+ | 620 | | | +-----------+ | | 621 | | | no packet sent/received| +---+ | | 622 | | | for UAL min, send CLOSE| | |timeout | | 623 | | | v v |(UAL+MSL) | | 624 | | | +---------+ |retransmit | | 625 +--|----------|------------------------| CLOSING |-+CLOSE | | 626 | | +---------+ | | 627 | | | | | | | | 628 +----------|-------------------------+ | | +----------------+ | 629 | | +-----------+ +------------------|--+ 630 | | |recv CLOSE, recv CLOSE_ACK | | 631 | +-------------+ |send CLOSE_ACK or timeout | | 632 | recv CLOSE, | | (UAL+MSL) | | 633 | send CLOSE_ACK v v | | 634 | +--------+ receive I2, send R2 | | 635 +---------------------| CLOSED |------------------------------+ | 636 +--------+ | 637 ^ | | | 638 recv CLOSE, send CLOSE_ACK| | | timeout (UAL+2MSL) | 639 +-+ +------------------------------------+ 641 4.1.3. HIP DEX Security Associations 643 HIP DEX establishes two Security Associations (SA), one for the 644 Diffie-Hellman derived key, or Master Key, and one for the session 645 key, or Pair-wise Key. 647 4.1.3.1. Master Key SA 649 The Master Key SA is used to authenticate HIP packets and to encrypt 650 selected HIP parameters in the HIP DEX packet exchanges. Since only 651 little data is protected by this SA, it can be long-lived with no 652 need for rekeying. 654 The Master Key SA contains the following elements: 656 o Source HIT 658 o Destination HIT 660 o HIP_Encrypt Key 662 o HIP_MAC Key 664 The HIP_Encrypt and HIP_MAC keys are extracted from the Diffie- 665 Hellman derived key as described in Section 6.3. Their length is 666 determined by the HIP_CIPHER. 668 4.1.3.2. Pair-wise Key SA 670 The Pair-wise Key SA is used to authenticate and to encrypt user 671 data. It is refreshed (or rekeyed) using an UPDATE packet exchange. 672 The Pair-wise Key SA elements are defined by the data transform (e.g. 673 ESP_TRANSFORM [RFC7402]). 675 The keys for the Pair-wise Key SA are derived based on the wrapped 676 keying material exchanged in the ENCRYPTED_KEY parameter (see 677 Section 5.2.5) of the I2 and R2 packets. Specifically, the exchanged 678 keying material of the two peers is concatenated. This concatenation 679 forms the input to a Key Derivation Function (KDF). If the data 680 transform does not specify its own KDF, the key derivation function 681 defined in Section 6.3 is used. Even though this input is randomly 682 distributed, a KDF Extract phase may be needed to get the proper 683 length for the input to the KDF Expand phase. 685 4.1.4. User Data Considerations 687 The User Data Considerations in Section 4.5. of [RFC7401] also apply 688 to HIP DEX. There is only one difference between HIPv2 and HIP DEX. 689 Loss of state due to system reboot may be a critical performance 690 issue for resource-constrained devices. Thus, implementors MAY 691 choose to use non-volatile, secure storage for HIP states in order 692 for them to survive a system reboot. This will limit state loss 693 during reboots to only those situations with an SA timeout. 695 5. Packet Formats 697 5.1. Payload Format 699 HIP DEX employs the same fixed HIP header and payload structure as 700 HIPv2. As such, the specifications in Section 5.1 of [RFC7401] also 701 apply to HIP DEX. 703 5.2. HIP Parameters 705 The HIP parameters carry information that is necessary for 706 establishing and maintaining a HIP association. For example, the 707 peer's public keys as well as the signaling for negotiating ciphers 708 and payload handling are encapsulated in HIP parameters. Additional 709 information, meaningful for end-hosts or middleboxes, may also be 710 included in HIP parameters. The specification of the HIP parameters 711 and their mapping to HIP packets and packet types is flexible to 712 allow HIP extensions to define new parameters and new protocol 713 behavior. 715 In HIP packets, HIP parameters are ordered according to their numeric 716 type number and encoded in TLV format. 718 HIP DEX reuses the HIP parameters of HIPv2 defined in Section 5.2. of 719 [RFC7401] where possible. Still, HIP DEX further restricts and/or 720 extends the following existing parameter types: 722 o DH_GROUP_LIST and HOST_ID are restricted to ECC-based suites. 724 o HIP_CIPHER is restricted to AES-128-CTR and NULL-ENCRYPT. 726 o HIT_SUITE_LIST is limited to the HIT suite ECDH/FOLD. 728 o RHASH and RHASH_len are redefined to CMAC for the PUZZLE, 729 SOLUTION, and HIP_MAC parameters (see Section 6.1 and 730 Section 6.2). 732 In addition, HIP DEX introduces the following new parameter: 734 +------------------+------+----------+------------------------------+ 735 | TLV | Type | Length | Data | 736 +------------------+------+----------+------------------------------+ 737 | ENCRYPTED_KEY | 643 | variable | Encrypted container for the | 738 | | | | session key exchange | 739 +------------------+------+----------+------------------------------+ 741 5.2.1. DH_GROUP_LIST 743 The DH_GROUP_LIST parameter contains the list of supported DH Group 744 IDs of a host. It is defined in Section 5.2.6 of [RFC7401]. With 745 HIP DEX, the DH Group IDs are restricted to: 747 Group KDF Value 749 NIST P-256 [RFC5903] CKDF 7 750 NIST P-384 [RFC5903] CKDF 8 751 NIST P-521 [RFC5903] CKDF 9 752 SECP160R1 [SECG] CKDF 10 754 The ECDH groups 7 - 9 are defined in [RFC5903] and [RFC6090]. ECDH 755 group 10 is covered in [SECG] and Appendix D of [RFC7401]. Any ECDH 756 used with HIP MUST have a co-factor of 1. 758 5.2.2. HIP_CIPHER 760 The HIP_CIPHER parameter contains the list of supported cipher 761 algorithms to be used for encrypting the contents of the ENCRYPTED 762 and ENCRYPTED_KEY parameters. The HIP_CIPHER parameter is defined in 763 Section 5.2.8 of [RFC7401]. With HIP DEX, the Suite IDs are limited 764 to: 766 Suite ID Value 768 RESERVED 0 769 NULL-ENCRYPT 1 ([RFC2410]) 770 AES-128-CTR 5 ([RFC3686]) 772 Mandatory implementation: AES-128-CTR. Implementors SHOULD support 773 NULL-ENCRYPT ([RFC2410]) for testing/debugging purposes but MUST NOT 774 offer or accept this value unless explicitly configured for testing/ 775 debugging of HIP. 777 5.2.3. HOST_ID 779 The HOST_ID parameter conveys the Host Identity (HI) along with 780 optional information about a host. It is defined in Section 5.2.9 of 781 [RFC7401]. 783 HIP DEX uses the public portion of a host's static ECDH key-pair as 784 the HI. Correspondingly, HIP DEX limits the HI algorithms to the 785 following profile: 787 Algorithm profiles Value 789 ECDH 11 [RFC6090] (REQUIRED) 791 HIP DEX HIs are serialized equally to the ECC-based HIs in HIPv2 (see 792 Section 5.2.9. of [RFC7401]). The Group ID of the HIP DEX HI is 793 encoded in the "ECC curve" field of the HOST_ID parameter. The 794 supported DH Group IDs are defined in Section 5.2.1. 796 5.2.4. HIT_SUITE_LIST 798 The HIT_SUITE_LIST parameter contains a list of the supported HIT 799 suite IDs of the Responder. Based on the HIT_SUITE_LIST, the 800 Initiator can determine which source HIT Suite IDs are supported by 801 the Responder. The HIT_SUITE_LIST parameter is defined in 802 Section 5.2.10 of [RFC7401]. 804 The following HIT Suite IDs are defined for HIP DEX, and the 805 relationship between the four-bit ID value used in the OGA ID field 806 and the eight-bit encoding within the HIT_SUITE_LIST ID field is 807 clarified: 809 HIT Suite Four-bit ID Eight-bit encoding 811 ECDH/FOLD 8 0x80 813 Note that the HIP DEX HIT Suite ID allows the peers to distinguish a 814 HIP DEX handshake from a HIPv2 handshake. The Responder MUST respond 815 with a HIP DEX HIT suite ID when the HIT of the Initiator is a HIP 816 DEX HIT. 818 5.2.5. ENCRYPTED_KEY 819 0 1 2 3 820 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 1 821 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 822 | Type | Length | 823 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 824 / Encrypted value / 825 / / 826 / +-------------------------------+ 827 / | Padding | 828 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 830 Type 643 831 Length length in octets, excluding Type, Length, and 832 Padding 833 Encrypted The value is encrypted using an encryption algorithm 834 value as defined in the HIP_CIPHER parameter. 836 The ENCRYPTED_KEY parameter encapsulates a random value that is later 837 used in the session key creation process (see Section 6.3). This 838 random value MUST have a length of at least 64 bit. The puzzle value 839 #I and the puzzle solution #J (see [RFC7401]) are used as the 840 initialization vector (IV) for the encryption process. To this end, 841 the IV is computed as FOLD(I | J, 128). The AES-CTR counter is a 16 842 bit value that is initialized to zero with the first use. 844 Once this encryption process is completed, the "encrypted value" data 845 field is ready for inclusion in the Parameter. If necessary, 846 additional Padding for 8-byte alignment is then added according to 847 the rules of TLV Format in [RFC7401]. 849 5.3. HIP Packets 851 HIP DEX uses the same eight basic HIP packets as HIPv2 (see 852 Section 5.3 of [RFC7401]). Four of them are for the HIP handshake 853 (I1, R1, I2, and R2), one is for updating an association (UPDATE), 854 one is for sending notifications (NOTIFY), and two are for closing 855 the association (CLOSE and CLOSE_ACK). There are some differences 856 regarding the HIP parameters that are included in the handshake 857 packets concerning HIP BEX and HIP DEX. This section covers these 858 differences for the DEX packets. Packets not discussed here, follow 859 the structure defined in [RFC7401]. 861 An important difference between packets in HIP BEX and HIP DEX is 862 that the DIFFIE_HELLMAN and the HIP_SIGNATURE parameters are not 863 included in HIP DEX. Thus, the R1 packet is completely unprotected 864 and can be spoofed. As a result, negotiation parameters contained in 865 the R1 packet have to be re-included in later, protected packets in 866 order to detect and prevent potential downgrading attacks. Moreover, 867 the I2, R2, UPDATE, NOTIFY, CLOSE, and CLOSE_ACK packets are not 868 covered by a signature and purely rely on the HIP_MAC parameter for 869 packet authentication. The processing of these packets is changed 870 accordingly. 872 In the future, an optional upper-layer payload MAY follow the HIP 873 header. The Next Header field in the header indicates if there is 874 additional data following the HIP header. 876 5.3.1. I1 - the HIP Initiator Packet 878 The HIP header values for the I1 packet: 880 Header: 881 Packet Type = 1 882 SRC HIT = Initiator's HIT 883 DST HIT = Responder's HIT, or NULL 885 IP ( HIP ( DH_GROUP_LIST ) ) 887 Valid control bits: none 889 The I1 packet contains the fixed HIP header and the Initiator's 890 DH_GROUP_LIST. The Initiator's HIT Suite ID MUST be of a HIP DEX 891 type as defined in Section 5.2.4. 893 Regarding the Responder's HIT, the Initiator may receive this HIT 894 either from a DNS lookup of the Responder's FQDN, from some other 895 repository, or from a local table. The Responder's HIT also MUST be 896 of a HIP DEX type. If the Initiator does not know the Responder's 897 HIT, it may attempt to use opportunistic mode by using NULL (all 898 zeros) as the Responder's HIT. See Section 4.1.8 of [RFC7401] for 899 detailed information about the "HIP Opportunistic Mode". 901 As a compression of the employed HIs, the Initiator's and the 902 Responder's HITs both determine the DH group ID that must be used in 903 order to successfully conclude the triggered handshake. HITs, 904 however, only include the OGA ID identifying a HIP DEX HIT. They do 905 not include information about the specific DH group ID of the 906 corresponding HI. To inform the Responder about its employed and its 907 otherwise supported DH Group IDs, the Initiator therefore includes 908 the DH_GROUP_LIST parameter in the I1 packet. This parameter MUST 909 include the DH group ID that corresponds to the currently employed 910 Initiator HIT as the first list element. With HIP DEX, the 911 DH_GROUP_LIST parameter MUST only include ECDH groups defined in 912 Section 5.2.1. 914 Since this packet is so easy to spoof even if it were protected, no 915 attempt is made to add to its generation or processing cost. As a 916 result, the DH_GROUP_LIST in the I1 packet is not protected. 918 Implementations MUST be able to handle a storm of received I1 919 packets, discarding those with common content that arrive within a 920 small time delta. 922 5.3.2. R1 - the HIP Responder Packet 924 The HIP header values for the R1 packet: 926 Header: 927 Packet Type = 2 928 SRC HIT = Responder's HIT 929 DST HIT = Initiator's HIT 931 IP ( HIP ( [ R1_COUNTER, ] 932 PUZZLE, 933 DH_GROUP_LIST, 934 HIP_CIPHER, 935 HOST_ID, 936 HIT_SUITE_LIST, 937 TRANSPORT_FORMAT_LIST, 938 [ <, ECHO_REQUEST_UNSIGNED >i ]) 940 Valid control bits: A 942 If the Responder's HI is an anonymous one, the A control MUST be set. 944 The Initiator's HIT MUST match the one received in the I1 packet if 945 the R1 is a response to an I1. If the Responder has multiple HIs, 946 the Responder's HIT MUST match the Initiator's request. If the 947 Initiator used opportunistic mode, the Responder may select among its 948 HIs as described below. See Section 4.1.8 of [RFC7401] for detailed 949 information about the "HIP Opportunistic Mode". 951 The R1 packet generation counter is used to determine the currently 952 valid generation of puzzles. The value is increased periodically, 953 and it is RECOMMENDED that it is increased at least as often as 954 solutions to old puzzles are no longer accepted. 956 The Puzzle contains a Random value #I and the puzzle difficulty K. 957 The difficulty K indicates the number of lower-order bits, in the 958 puzzle CMAC result, that MUST be zeros (see [RFC7401]). Responders 959 SHOULD set K to zero by default and only increase the puzzle 960 difficulty to protect against a DoS attack targeting the HIP DEX 961 handshake. A puzzle difficulty of zero effectively turns the puzzle 962 mechanism into a return-routablility test and is strongly encouraged 963 during normal operation in order to conserve energy resources as well 964 as to prevent unnecessary handshake delay in case of a resource- 965 constrained Initiator. 967 The DH_GROUP_LIST parameter contains the Responder's order of 968 preference based on which it chose the ECDH key contained in the 969 HOST_ID parameter (see below). This allows the Initiator to 970 determine whether its own DH_GROUP_LIST in the I1 packet was 971 manipulated by an attacker. There is a further risk that the 972 Responder's DH_GROUP_LIST was manipulated by an attacker, as the R1 973 packet cannot be authenticated in HI DEX. Thus, this parameter is 974 repeated in the R2 packet to allow for a final, cryptographically 975 secured validation. 977 The HIP_CIPHER contains the encryption algorithms supported by the 978 Responder to protect the key exchange, in the order of preference. 979 All implementations MUST support the AES-CTR [RFC3686]. 981 The HIT_SUITE_LIST parameter is an ordered list of the Responder's 982 supported and preferred HIT Suites. It enables a Responder to notify 983 the Initiator about other available HIT suites than the one used in 984 the current handshake. Based on the received HIT_SUITE_LIST, the 985 Initiator MAY decide to abort the current handshake and initiate a 986 new handshake with a different mutually supported HIT suite. This 987 mechanism can, e.g., be used to move from an initial HIP DEX 988 handshake to a HIP BEX handshake for peers supporting both protocol 989 variants. 991 The HOST_ID parameter depends on the received DH_GROUP_LIST parameter 992 and the Responder HIT in the I1 packet. Specifically, if the I1 993 contains a Responder HIT, the Responder verifies that this HIT 994 matches the required DH group based on the received DH_GROUP_LIST 995 parameter. In case of a positive result, the Responder selects the 996 corresponding HOST_ID for inclusion in the R1 packet. Likewise, if 997 the Responder HIT in the I1 packet is NULL (i.e., during an 998 opportunistic handshake), the Responder chooses its HOST_ID according 999 to the Initiator's employed DH group as indicated in the received 1000 DH_GROUP_LIST parameter and sets the source HIT in the R1 packet 1001 accordingly. If the Responder however does not support the DH group 1002 required by the Initiator or if the Responder HIT in the I1 packet 1003 does not match the required DH group, the Responder selects the 1004 mutually preferred and supported DH group based on the DH_GROUP_LIST 1005 parameter in the I1 packet. The Responder then includes the 1006 corresponding ECDH key in the HOST_ID parameter. This parameter also 1007 indicates the selected DH group. Moreover, the Responder sets the 1008 source HIT in the R2 packet based on the destination HIT from the I1 1009 packet. Based on the deviating DH group ID in the HOST_ID parameter, 1010 the Initiator then SHOULD abort the current handshake and initiate a 1011 new handshake with the mutually supported DH group as far as local 1012 policies (see Section 7) permit. 1014 The TRANSPORT_FORMAT_LIST parameter is an ordered list of the 1015 Responder's supported and preferred transport format types. The list 1016 allows the Initiator and the Responder to agree on a common type for 1017 payload protection. Currently, the only transport format defined is 1018 IPsec ESP [RFC7402]. 1020 The ECHO_REQUEST_UNSIGNED parameters contain data that the sender 1021 wants to receive unmodified in the corresponding response packet in 1022 the ECHO_RESPONSE_UNSIGNED parameter. The R1 packet may contain zero 1023 or more ECHO_REQUEST_UNSIGNED parameters. 1025 5.3.3. I2 - the Second HIP Initiator Packet 1027 The HIP header values for the I2 packet: 1029 Header: 1030 Type = 3 1031 SRC HIT = Initiator's HIT 1032 DST HIT = Responder's HIT 1034 IP ( HIP ( [R1_COUNTER,] 1035 SOLUTION, 1036 HIP_CIPHER, 1037 ENCRYPTED_KEY, 1038 HOST_ID, 1039 TRANSPORT_FORMAT_LIST, 1040 HIP_MAC, 1041 [<, ECHO_RESPONSE_UNSIGNED>i )] ) 1043 Valid control bits: A 1045 The HITs MUST match the ones used in the R1 packet. 1047 If the Initiator's HI is an anonymous one, the A control bit MUST be 1048 set. 1050 If present in the R1 packet, the Initiator MUST include an unmodified 1051 copy of the R1_COUNTER parameter into the I2 packet. 1053 The Solution contains the Random #I from the R1 packet and the 1054 computed #J value. The low-order #K bits of the RHASH(I | ... | J) 1055 MUST be zero. 1057 The HIP_CIPHER contains the single encryption transform selected by 1058 the Initiator that it uses to encrypt the ENCRYPTED and ENCRYPTED_KEY 1059 parameters. The chosen cipher MUST correspond to one of the ciphers 1060 offered by the Responder in the R1. All implementations MUST support 1061 the AES-CTR transform [RFC3686]. 1063 The HOST_ID parameter contains the Initiator HI corresponding to the 1064 Initiator HIT. 1066 The ENCRYPTED_KEY parameter contains an Initiator generated random 1067 value that MUST be uniformly distributed. This random value is 1068 encrypted with the Master Key SA using the HIP_CIPHER encryption 1069 algorithm. 1071 The ECHO_RESPONSE_UNSIGNED parameter(s) contain the unmodified Opaque 1072 data copied from the corresponding echo request parameter(s). This 1073 parameter can also be used for two-factor password authentication as 1074 shown in Appendix A. 1076 The TRANSPORT_FORMAT_LIST parameter contains the single transport 1077 format type selected by the Initiator. The chosen type MUST 1078 correspond to one of the types offered by the Responder in the R1 1079 packet. Currently, the only transport format defined is the ESP 1080 transport format [RFC7402]. 1082 The MAC is calculated over the whole HIP envelope, excluding any 1083 parameters after the HIP_MAC parameter as described in Section 6.2. 1084 The Responder MUST validate the HIP_MAC parameter. 1086 5.3.4. R2 - the Second HIP Responder Packet 1088 The HIP header values for the R2 packet: 1090 Header: 1091 Packet Type = 4 1092 SRC HIT = Responder's HIT 1093 DST HIT = Initiator's HIT 1095 IP ( HIP ( DH_GROUP_LIST, 1096 HIP_CIPHER, 1097 ENCRYPTED_KEY, 1098 HIT_SUITE_LIST, 1099 TRANSPORT_FORMAT_LIST, 1100 HIP_MAC) 1102 Valid control bits: none 1104 The HITs used MUST match the ones used in the I2 packet. 1106 The Responder repeats the DH_GROUP_LIST, HIP_CIPHER, HIT_SUITE_LIST, 1107 and TRANSPORT_FORMAT_LIST parameters in the R2 packet. These 1108 parameters MUST be the same as included in the R1 packet. The 1109 parameter are re-included here because the R2 packet is MACed and 1110 thus cannot be altered by an attacker. For verification purposes, 1111 the Initiator re-evaluates the selected suites and compares the 1112 results against the chosen ones. If the re-evaluated suites do not 1113 match the chosen ones, the Initiator acts based on its local policy. 1115 The ENCRYPTED_KEY parameter contains an Responder generated random 1116 value that MUST be uniformly distributed. This random value is 1117 encrypted with the Master Key SA using the HIP_CIPHER encryption 1118 algorithm. 1120 The MAC is calculated over the whole HIP envelope, excluding any 1121 parameters after the HIP_MAC, as described in Section 6.2. The 1122 Initiator MUST validate the HIP_MAC parameter. 1124 5.4. ICMP Messages 1126 When a HIP implementation detects a problem with an incoming packet, 1127 and it either cannot determine the identity of the sender of the 1128 packet or does not have any existing HIP association with the sender 1129 of the packet, it MAY respond with an ICMP packet. Any such reply 1130 MUST be rate-limited as described in [RFC4443]. In most cases, the 1131 ICMP packet has the Parameter Problem type (12 for ICMPv4, 4 for 1132 ICMPv6), with the Pointer field pointing to the field that caused the 1133 ICMP message to be generated. The problem cases specified in 1134 Section 5.4. of [RFC7401] also apply to HIP DEX. 1136 6. Packet Processing 1138 Due to the adopted protocol semantics and the inherited general 1139 packet structure, the packet processing in HIP DEX only differs from 1140 HIPv2 in very few places. Here, we focus on these differences and 1141 refer to Section 6 in [RFC7401] otherwise. 1143 The processing of outgoing and incoming application data remains the 1144 same as in HIP BEX (see Sections 6.1 and 6.2 in [RFC7401]). 1146 6.1. Solving the Puzzle 1148 The procedures for solving and verifying a puzzle in HIP DEX are 1149 strongly based on the corresponding procedures in HIPv2. The only 1150 exceptions are that HIP DEX does not use pre-computation of R1 1151 packets and that RHASH is set to CMAC. As a result, the pre- 1152 computation step in in Section 6.3 of [RFC7401] is skipped in HIP 1153 DEX. 1155 Moreover, the Initiator solves a puzzle by computing: 1156 Ltrunc( CMAC( I, HIT-I | HIT-R | J ), K ) == 0 1158 Similarly, the Responder verifies a puzzle by computing: 1159 V := Ltrunc( CMAC( I, HIT-I | HIT-R | J ), K ) 1161 Apart from these modifications, the procedures defined in Section 6.3 1162 of [RFC7401] also apply for HIP DEX. 1164 6.2. HIP_MAC Calculation and Verification 1166 The following subsections define the actions for processing the 1167 HIP_MAC parameter. 1169 6.2.1. CMAC Calculation 1171 The HIP_MAC calculation uses RHASH, i.e., CMAC, as the underlying 1172 cryptographic function. The scope of the calculation for HIP_MAC is: 1174 CMAC: { HIP header | [ Parameters ] } 1176 where Parameters include all HIP parameters of the packet that is 1177 being calculated with Type values ranging from 1 to (HIP_MAC's Type 1178 value - 1) and exclude parameters with Type values greater or equal 1179 to HIP_MAC's Type value. 1181 During HIP_MAC calculation, the following applies: 1183 o In the HIP header, the Checksum field is set to zero. 1185 o In the HIP header, the Header Length field value is calculated to 1186 the beginning of the HIP_MAC parameter. 1188 The parameter order is described in Section 5.2.1 of [RFC7401]. 1190 The CMAC calculation and verification process is as follows: 1192 Packet sender: 1194 1. Create the HIP packet, without the HIP_MAC or any other parameter 1195 with greater Type value than the HIP_MAC parameter has. 1197 2. Calculate the Header Length field in the HIP header. 1199 3. Compute the CMAC using either HIP-gl or HIP-lg integrity key 1200 retrieved from KEYMAT as defined in Section 6.3. 1202 4. Add the HIP_MAC parameter to the packet and any parameter with 1203 greater Type value than the HIP_MAC's that may follow. 1205 5. Recalculate the Length field in the HIP header. 1207 Packet receiver: 1209 1. Verify the HIP header Length field. 1211 2. Remove the HIP_MAC parameter, as well as all other parameters 1212 that follow it with greater Type value, saving the contents if 1213 they will be needed later. 1215 3. Recalculate the HIP packet length in the HIP header and clear the 1216 Checksum field (set it to all zeros). 1218 4. Compute the CMAC using either HIP-gl or HIP-lg integrity key as 1219 defined in Section 6.3 and verify it against the received CMAC. 1221 5. Set Checksum and Header Length fields in the HIP header to 1222 original values. Note that the Checksum and Length fields 1223 contain incorrect values after this step. 1225 6.3. HIP DEX KEYMAT Generation 1227 The HIP DEX KEYMAT process is used to derive the keys for the Master 1228 Key SA as well as for the Pair-wise Key SA. The keys for the Master 1229 Key SA are based from the Diffie-Hellman derived key, Kij, produced 1230 during the HIP DEX handshake. The Initiator generates Kij during the 1231 creation of the I2 packet and the Responder generates Kij once it 1232 receives the I2 packet. Hence, I2, R2, UPDATE, CLOSE, and CLOSE_ACK 1233 packets can contain authenticated and/or encrypted information. 1235 The keys of the Pair-wise Key SA are not directly used in the HIP DEX 1236 handshake. Instead, these keys are made available as payload 1237 protection keys. Some payload protection mechanisms have their own 1238 Key Derivation Function, and if so this mechanism SHOULD be used. 1239 Otherwise, the HIP DEX KEYMAT process MUST be used to derive the keys 1240 of the Pair-wise Key SA based on the concatenation of the random 1241 values that are contained in the exchanged ENCRYPTED_KEY parameters. 1243 The HIP DEX KEYMAT process consists of two components, CKDF-Extract 1244 and CKDF-Expand. The Extract function compresses a non-uniformly 1245 distributed key, as is the output of a Diffie-Hellman key derivation, 1246 to extract the key entropy into a fixed length output. The Expand 1247 function takes either the output of the Extract function or directly 1248 uses a uniformly distributed key and expands the length of the key, 1249 repeatedly distributing the key entropy, to produce the keys needed. 1251 The key derivation for the Master Key SA employs both the Extract and 1252 Expand phases, whereas the Pair-wise Key SA MAY need both the Extract 1253 and Expand phases if the key is longer than 128 bits. Otherwise, it 1254 only requires the Expand phase. 1256 The CKDF-Extract function is the following operation: 1258 CKDF-Extract(I, IKM, info) -> PRK 1260 where 1262 I Random #I from the PUZZLE parameter 1263 IKM Input keying material, i.e., either the Diffie-Hellman 1264 derived key or the concatenation of the random values 1265 of the ENCRYPTED_KEY parameters in the same order as 1266 the HITs with sort(HIT-I | HIT-R) 1267 info sort(HIT-I | HIT-R) | "CKDF-Extract" 1268 PRK a pseudorandom key (of RHASH_len/8 octets) 1269 | denotes the concatenation 1271 The pseudorandom key PRK is calculated as follows: 1273 PRK = CMAC(I, IKM | info) 1275 The CKDF-Expand function is the following operation: 1277 CKDF-Expand(PRK, info, L) -> OKM 1279 where 1281 PRK a pseudorandom key of at least RHASH_len/8 octets 1282 (either the output from the extract step or the 1283 concatenation of the random values of the 1284 ENCRYPTED_KEY parameters in the same order as the 1285 HITs with sort(HIT-I | HIT-R)) 1286 info sort(HIT-I | HIT-R) | "CKDF-Expand" 1287 L length of output keying material in octets 1288 (<= 255*RHASH_len/8) 1289 | denotes the concatenation 1291 The output keying material OKM is calculated as follows: 1293 N = ceil(L/RHASH_len/8) 1294 T = T(1) | T(2) | T(3) | ... | T(N) 1295 OKM = first L octets of T 1297 where 1299 T(0) = empty string (zero length) 1300 T(1) = CMAC(PRK, T(0) | info | 0x01) 1301 T(2) = CMAC(PRK, T(1) | info | 0x02) 1302 T(3) = CMAC(PRK, T(2) | info | 0x03) 1303 ... 1305 (where the constant concatenated to the end of each T(n) is a 1306 single octet.) 1308 sort(HIT-I | HIT-R) is defined as the network byte order 1309 concatenation of the two HITs, with the smaller HIT preceding the 1310 larger HIT, resulting from the numeric comparison of the two HITs 1311 interpreted as positive (unsigned) 128-bit integers in network byte 1312 order. 1314 The initial keys are drawn sequentially in the order that is 1315 determined by the numeric comparison of the two HITs, with the 1316 comparison method described in the previous paragraph. HOST_g 1317 denotes the host with the greater HIT value, and HOST_l the host with 1318 the lower HIT value. 1320 The drawing order for initial keys: 1322 1. HIP-gl encryption key for HOST_g's outgoing HIP packets 1324 2. HIP-gl integrity (CMAC) key for HOST_g's outgoing HIP packets 1325 3. HIP-lg encryption key for HOST_l's outgoing HIP packets 1327 4. HIP-lg integrity (CMAC) key for HOST_l's outgoing HIP packets 1329 The number of bits drawn for a given algorithm is the "natural" size 1330 of the keys. For the mandatory algorithms, the following sizes 1331 apply: 1333 AES 128 or 256 bits 1335 If other key sizes are used, they must be treated as different 1336 encryption algorithms and defined separately. 1338 6.4. Initiation of a HIP Diet EXchange 1340 The initiation of a HIP DEX handshake proceeds as described in 1341 Section 6.6 of [RFC7401]. The I1 packet contents are specified in 1342 Section 5.3.1. 1344 6.5. Processing Incoming I1 Packets 1346 I1 packets in HIP DEX are handled almost identical to HIPv2 (see 1347 Section 6.7 of [RFC7401]). The main differences are that the 1348 Responder SHOULD select a HIP DEX HIT Suite in the R1 response. 1349 Moreover, as R1 packets are neither covered by a signature nor incur 1350 the overhead of generating an ephemeral Diffie-Hellman key-pair, pre- 1351 computation of an R1 is only marginally beneficial, but would incur 1352 additional memory resources at the Responder. Hence, the R1 pre- 1353 computation SHOULD be omitted in HIP DEX. 1355 Correspondingly, the modified conceptual processing rules for 1356 responding to an I1 packet are as follows: 1358 1. The Responder MUST check that the Responder's HIT in the received 1359 I1 packet is either one of its own HITs or NULL. Otherwise, it 1360 must drop the packet. 1362 2. If the Responder is in ESTABLISHED state, the Responder MAY 1363 respond to this with an R1 packet, prepare to drop an existing 1364 HIP security association with the peer, and stay at ESTABLISHED 1365 state. 1367 3. If the Responder is in I1-SENT state, it MUST make a comparison 1368 between the sender's HIT and its own (i.e., the receiver's) HIT. 1369 If the sender's HIT is greater than its own HIT, it should drop 1370 the I1 packet and stay at I1-SENT. If the sender's HIT is 1371 smaller than its own HIT, it SHOULD send the R1 packet and stay 1372 at I1-SENT. The HIT comparison is performed as defined in 1373 Section 6.3. 1375 4. If the implementation chooses to respond to the I1 packet with an 1376 R1 packet, it creates a new R1 according to the format described 1377 in Section 5.3.2. It chooses the HI based on the destination HIT 1378 and the DH_GROUP_LIST in the I1 packet. If the implementation 1379 does not support the DH group required by the Initiator or if the 1380 destination HIT in the I1 packet does not match the required DH 1381 group, it selects the mutually preferred and supported DH group 1382 based on the DH_GROUP_LIST parameter in the I1 packet. The 1383 implementation includes the corresponding ECDH public key in the 1384 HOST_ID parameter. If no suitable DH Group ID was contained in 1385 the DH_GROUP_LIST in the I1 packet, it sends an R1 packet with 1386 any suitable ECDH public key. 1388 5. If the received Responder's HIT in the I1 packet is not NULL, the 1389 Responder's in the R1 packet HIT MUST match the destination HIT 1390 in the I1 packet. Otherwise, the Responder MUST select a HIT 1391 with the same HIT Suite as the Initiator's HIT. If this HIT 1392 Suite is not supported by the Responder, it SHOULD select a 1393 REQUIRED HIT Suite from Section 5.2.10 of [RFC7401], which is 1394 currently RSA/DSA/SHA-256. Other than that, selecting the HIT is 1395 a local policy matter. 1397 6. The Responder expresses its supported HIP transport formats in 1398 the TRANSPORT_FORMAT_LIST as described in Section 5.2.11 of 1399 [RFC7401]. The Responder MUST provide at least one payload 1400 transport format type. 1402 7. The Responder sends the R1 packet to the source IP address of the 1403 I1 packet. 1405 Note that only steps 4 and 5 have been changed with regard to the 1406 processing rules of HIPv2. The considerations about R1 management 1407 (except pre-computation) and malformed I1 packets in Sections 6.7.1 1408 and 6.7.2 of [RFC7401] likewise apply to HIP DEX. 1410 6.6. Processing Incoming R1 Packets 1412 R1 packets in HIP DEX are handled identically to HIPv2 (see 1413 Section 6.8 in [RFC7401]) with the following exceptions: HIP DEX uses 1414 ECDH public keys as HIs and does not employ signatures. 1416 The modified conceptual processing rules for responding to an R1 1417 packet are as follows: 1419 1. A system receiving an R1 MUST first check to see if it has sent 1420 an I1 packet to the originator of the R1 packet (i.e., it has a 1421 HIP association that is in state I1-SENT and that is associated 1422 with the HITs in the R1). Unless the I1 packet was sent in 1423 opportunistic mode (see Section 4.1.8 of [RFC7401]), the IP 1424 addresses in the received R1 packet SHOULD be ignored by the R1 1425 processing and, when looking up the right HIP association, the 1426 received R1 packet SHOULD be matched against the associations 1427 using only the HITs. If a match exists, the system should 1428 process the R1 packet as described below. 1430 2. Otherwise, if the system is in any state other than I1-SENT or 1431 I2-SENT with respect to the HITs included in the R1 packet, it 1432 SHOULD silently drop the R1 packet and remain in the current 1433 state. 1435 3. If the HIP association state is I1-SENT or I2-SENT, the received 1436 Initiator's HIT MUST correspond to the HIT used in the original 1437 I1 packet. Also, the Responder's HIT MUST correspond to the one 1438 used in the I1 packet, unless this packet contained a NULL HIT. 1440 4. If the HIP association state is I1-SENT, and multiple valid R1 1441 packets are present, the system MUST select from among the R1 1442 packets with the largest R1 generation counter. 1444 5. The system MUST check that the Initiator's HIT Suite is 1445 contained in the HIT_SUITE_LIST parameter in the R1 packet 1446 (i.e., the Initiator's HIT Suite is supported by the Responder). 1447 If the HIT Suite is supported by the Responder, the system 1448 proceeds normally. Otherwise, the system MAY stay in state 1449 I1-SENT and restart the HIP DEX handshake by sending a new I1 1450 packet with an Initiator HIT that is supported by the Responder 1451 and hence is contained in the HIT_SUITE_LIST in the R1 packet. 1452 The system MAY abort the handshake if no suitable source HIT is 1453 available. The system SHOULD wait for an acceptable time span 1454 to allow further R1 packets with higher R1 generation counters 1455 or different HIT and HIT Suites to arrive before restarting or 1456 aborting the HIP DEX handshake. 1458 6. The system MUST check that the DH Group ID in the HOST_ID 1459 parameter in the R1 matches the first DH Group ID in the 1460 Responder's DH_GROUP_LIST in the R1 packet, and also that this 1461 Group ID corresponds to a value that was included in the 1462 Initiator's DH_GROUP_LIST in the I1 packet. If the DH Group ID 1463 of the HOST_ID parameter does not express the Responder's best 1464 choice, the Initiator can conclude that the DH_GROUP_LIST in the 1465 I1 or R1 packet was adversely modified. In such a case, the 1466 Initiator MAY send a new I1 packet; however, it SHOULD NOT 1467 change its preference in the DH_GROUP_LIST in the new I1 packet. 1468 Alternatively, the Initiator MAY abort the HIP DEX handshake. 1469 Moreover, if the DH Group ID indicated in the HOST_ID parameter 1470 does not match the DH Group ID of the HI employed by the 1471 Initiator, the system SHOULD wait for an acceptable time span to 1472 allow further R1 packets with different DH Group IDs to arrive 1473 before restarting or aborting the HIP DEX handshake. When 1474 restarting the handshake, the Initiator MUST consult local 1475 policies (see Section 7) regarding the use of another, mutually 1476 supported DH group for the subsequent handshake with the 1477 Responder. 1479 7. If the HIP association state is I2-SENT, the system MAY re-enter 1480 state I1-SENT and process the received R1 packet if it has a 1481 larger R1 generation counter than the R1 packet responded to 1482 previously. 1484 8. The R1 packet may have the A-bit set - in this case, the system 1485 MAY choose to refuse it by dropping the R1 packet and returning 1486 to state UNASSOCIATED. The system SHOULD consider dropping the 1487 R1 packet only if it used a NULL HIT in the I1 packet. If the 1488 A-bit is set, the Responder's HIT is anonymous and SHOULD NOT be 1489 stored permanently. 1491 9. The system SHOULD attempt to validate the HIT against the 1492 received Host Identity by using the received Host Identity to 1493 construct a HIT and verify that it matches the Sender's HIT. 1495 10. The system MUST store the received R1 generation counter for 1496 future reference. 1498 11. The system attempts to solve the puzzle in the R1 packet. The 1499 system MUST terminate the search after exceeding the remaining 1500 lifetime of the puzzle. If the puzzle is not successfully 1501 solved, the implementation MAY either resend the I1 packet 1502 within the retry bounds or abandon the HIP base exchange. 1504 12. The system computes standard Diffie-Hellman keying material 1505 according to the public value and Group ID provided in the 1506 HOST_ID parameter. The Diffie-Hellman keying material Kij is 1507 used for key extraction as specified in Section 6.3. 1509 13. The system selects the HIP_CIPHER ID from the choices presented 1510 in the R1 packet and uses the selected values subsequently when 1511 generating and using encryption keys, and when sending the I2 1512 packet. If the proposed alternatives are not acceptable to the 1513 system, it may either resend an I1 packet within the retry 1514 bounds or abandon the HIP base exchange. 1516 14. The system chooses one suitable transport format from the 1517 TRANSPORT_FORMAT_LIST and includes the respective transport 1518 format parameter in the subsequent I2 packet. 1520 15. The system initializes the remaining variables in the associated 1521 state, including Update ID counters. 1523 16. The system prepares and sends an I2 packet as described in 1524 Section 5.3.3. 1526 17. The system SHOULD start a timer whose timeout value SHOULD be 1527 larger than the worst-case anticipated RTT, and MUST increment a 1528 trial counter associated with the I2 packet. The sender SHOULD 1529 retransmit the I2 packet upon a timeout and restart the timer, 1530 up to a maximum of I2_RETRIES_MAX tries. 1532 18. If the system is in state I1-SENT, it SHALL transition to state 1533 I2-SENT. If the system is in any other state, it remains in the 1534 current state. 1536 Note that step 4 from the original processing rules of HIPv2 1537 (signature verification) has been removed in the above processing 1538 rules for HIP DEX. Moreover, step 7 of the HIPv2 processing rules 1539 has been adapted to account for the fact that HIP DEX uses ECDH 1540 public keys as HIs. The considerations about malformed R1 packets in 1541 Sections 6.8.1 of [RFC7401] also apply to HIP DEX. 1543 6.7. Processing Incoming I2 Packets 1545 The processing of I2 packets follows similar rules as HIPv2 (see 1546 Section 6.9 of [RFC7401]). The main differences to HIPv2 are that 1547 HIP DEX introduces a new session key exchange via the ENCRYPTED_KEY 1548 parameter as well as an I2 reception acknowledgement for 1549 retransmission purposes. Moreover, with HIP DEX the Initiator is 1550 responsible for triggering retransmissions, whereas the Responder 1551 merely replies to received I2 packets. 1553 The modified HIP DEX conceptual processing rules for responding to an 1554 I2 packet are: 1556 1. The system MAY perform checks to verify that the I2 packet 1557 corresponds to a recently sent R1 packet. Such checks are 1558 implementation dependent. See Appendix A in [RFC7401] for a 1559 description of an example implementation. 1561 2. The system MUST check that the Responder's HIT corresponds to 1562 one of its own HITs and MUST drop the packet otherwise. 1564 3. The system MUST further check that the Initiator's HIT Suite is 1565 supported. The Responder SHOULD silently drop I2 packets with 1566 unsupported Initiator HITs. 1568 4. If the system's state machine is in the R2-SENT state, the 1569 system MUST check to see if the newly received I2 packet is 1570 similar to the one that triggered moving to R2-SENT. If so, it 1571 MUST retransmit a previously sent R2 packet and reset the 1572 R2-SENT timer. The system SHOULD re-use the previously 1573 established state to re-create the corresponding R2 packet in 1574 order to prevent unnecessary computation overhead. 1576 5. If the system's state machine is in the I2-SENT state, the 1577 system MUST make a comparison between its local and sender's 1578 HITs (similarly as in Section 6.3). If the local HIT is smaller 1579 than the sender's HIT, it should drop the I2 packet, use the 1580 peer Diffie-Hellman key, ENCRYPTED_KEY keying material and nonce 1581 #I from the R1 packet received earlier, and get the local 1582 Diffie-Hellman key, ENCRYPTED_KEY keying material, and nonce #J 1583 from the I2 packet sent to the peer earlier. Otherwise, the 1584 system should process the received I2 packet and drop any 1585 previously derived Diffie-Hellman keying material Kij and 1586 ENCRYPTED_KEY keying material it might have generated upon 1587 sending the I2 packet previously. The peer Diffie-Hellman key, 1588 ENCRYPTED_KEY, and the nonce #J are taken from the just arrived 1589 I2 packet. The local Diffie-Hellman key, ENCRYPTED_KEY keying 1590 material, and the nonce #I are the ones that were sent earlier 1591 in the R1 packet. 1593 6. If the system's state machine is in the I1-SENT state, and the 1594 HITs in the I2 packet match those used in the previously sent I1 1595 packet, the system uses this received I2 packet as the basis for 1596 the HIP association it was trying to form, and stops 1597 retransmitting I1 packets (provided that the I2 packet passes 1598 the additional checks below). 1600 7. If the system's state machine is in any state other than 1601 R2-SENT, the system SHOULD check that the echoed R1 generation 1602 counter in the I2 packet is within the acceptable range if the 1603 counter is included. Implementations MUST accept puzzles from 1604 the current generation and MAY accept puzzles from earlier 1605 generations. If the generation counter in the newly received I2 1606 packet is outside the accepted range, the I2 packet is stale 1607 (and perhaps replayed) and SHOULD be dropped. 1609 8. The system MUST validate the solution to the puzzle as described 1610 in Section 6.1. 1612 9. The I2 packet MUST have a single value in the HIP_CIPHER 1613 parameter, which MUST match one of the values offered to the 1614 Initiator in the R1 packet. 1616 10. The system MUST derive Diffie-Hellman keying material Kij based 1617 on the public value and Group ID in the HOST_ID parameter. This 1618 keying material is used to derive the keys of the Master Key SA 1619 as described in Section 6.3. If the Diffie-Hellman Group ID is 1620 unsupported, the I2 packet is silently dropped. If the 1621 processing time for the derivation of the Diffie-Hellman keying 1622 material Kij is likely to cause premature I2 retransmissions by 1623 the Initiator, the system MAY send a NOTIFY packet before 1624 starting the key derivation process. The NOTIFY packet contains 1625 a NOTIFICATION parameter with Notify Message Type 1626 I2_ACKNOWLEDGEMENT. The NOTIFICATION parameter indicates the 1627 anticipated remaining processing time for the I2 packet in 1628 milliseconds as two-octet Notification Data. 1630 11. The implementation SHOULD also verify that the Initiator's HIT 1631 in the I2 packet corresponds to the Host Identity sent in the I2 1632 packet. (Note: some middleboxes may not be able to make this 1633 verification.) 1635 12. The system MUST process the TRANSPORT_FORMAT_LIST parameter. 1636 Other documents specifying transport formats (e.g., [RFC7402]) 1637 contain specifications for handling any specific transport 1638 selected. 1640 13. The system MUST verify the HIP_MAC according to the procedures 1641 in Section 6.2. 1643 14. If the checks above are valid, then the system proceeds with 1644 further I2 processing; otherwise, it discards the I2 and its 1645 state machine remains in the same state. 1647 15. The I2 packet may have the A-bit set - in this case, the system 1648 MAY choose to refuse it by dropping the I2 and the state machine 1649 returns to state UNASSOCIATED. If the A-bit is set, the 1650 Initiator's HIT is anonymous and should not be stored 1651 permanently. 1653 16. The system MUST decrypt the keying material from the 1654 ENCRYPTED_KEY parameter. This keying material is a partial 1655 input to the key derivation process for the Pair-wise Key SA 1656 (see Section 6.3). 1658 17. The system initializes the remaining variables in the associated 1659 state, including Update ID counters. 1661 18. Upon successful processing of an I2 packet when the system's 1662 state machine is in state UNASSOCIATED, I1-SENT, I2-SENT, or 1663 R2-SENT, an R2 packet is sent as described in Section 5.3.4 and 1664 the system's state machine transitions to state R2-SENT. 1666 19. Upon successful processing of an I2 packet when the system's 1667 state machine is in state ESTABLISHED, the old HIP association 1668 is dropped and a new one is installed, an R2 packet is sent as 1669 described in Section 5.3.4, and the system's state machine 1670 transitions to R2-SENT. 1672 20. Upon the system's state machine transitioning to R2-SENT, the 1673 system starts a timer. The state machine transitions to 1674 ESTABLISHED if some data has been received on the incoming HIP 1675 association, or an UPDATE packet has been received (or some 1676 other packet that indicates that the peer system's state machine 1677 has moved to ESTABLISHED). If the timer expires (allowing for a 1678 maximal amount of retransmissions of I2 packets), the state 1679 machine transitions to ESTABLISHED. 1681 Note that steps 11 (encrypted HOST_ID) and 15 (signature 1682 verification) from the original processing rules of HIPv2 have been 1683 removed in the above processing rules for HIP DEX. Moreover, step 10 1684 of the HIPv2 processing rules has been adapted to account for 1685 optional extension of the retransmission mechanism. Step 16 has been 1686 added to the processing rules. The considerations about malformed I2 1687 packets in Sections 6.9.1 of [RFC7401] also apply to HIP DEX. 1689 6.8. Processing Incoming R2 Packets 1691 R2 packets in HIP DEX are handled identically to HIPv2 (see 1692 Section 6.10 of [RFC7401]) with the following exceptions: HIP DEX 1693 introduces a new session key exchange via the ENCRYPTED_KEY parameter 1694 and does not employ signatures. 1696 The modified conceptual processing rules for responding to an R2 1697 packet are as follows: 1699 1. If the system is in any other state than I2-SENT, the R2 packet 1700 is silently dropped. 1702 2. The system MUST verify that the HITs in use correspond to the 1703 HITs that were received in the R1 packet that caused the 1704 transition to the I2-SENT state. 1706 3. The system MUST verify the HIP_MAC according to the procedures in 1707 Section 6.2. 1709 4. The system MUST re-evaluate the DH_GROUP_LIST, HIP_CIPHER, 1710 HIT_SUITE_LIST, and TRANSPORT_FORMAT_LIST parameters in the R2 1711 packet and compare the results against the chosen suites. 1713 5. If any of the checks above fail, there is a high probability of 1714 an ongoing man-in-the-middle or other security attack. The 1715 system SHOULD act accordingly, based on its local policy. 1717 6. The system MUST decrypt the keying material from the 1718 ENCRYPTED_KEY parameter. This keying material is a partial input 1719 to the key derivation process for the Pair-wise Key SA (see 1720 Section 6.3). 1722 7. Upon successful processing of the R2 packet, the state machine 1723 transitions to state ESTABLISHED. 1725 Note that step 4 (signature verification) from the original 1726 processing rules of HIPv2 has been replaced with a negotiation re- 1727 evaluation in the above processing rules for HIP DEX. Moreover, step 1728 6 has been added to the processing rules. 1730 6.9. Processing Incoming NOTIFY Packets 1732 Processing of NOTIFY packets is OPTIONAL. If processed, any errors 1733 in a received NOTIFICATION parameter SHOULD be logged. Received 1734 errors MUST be considered only as informational, and the receiver 1735 SHOULD NOT change its HIP state purely based on the received NOTIFY 1736 packet. 1738 If a NOTIFY packet is received in state I2-SENT, this packet may be 1739 an I2 reception acknowledgement of the optional retransmission 1740 mechanism extension and SHOULD be processed. The following steps 1741 define the conceptual processing rules for such incoming NOTIFY 1742 packets in state I2-SENT: 1744 1. The system MUST verify that the HITs in use correspond to the 1745 HITs that were received in the R1 packet that caused the 1746 transition to the I2-SENT state. If this check fails, the NOTIFY 1747 packet SHOULD be dropped silently. 1749 2. If the NOTIFY packet contains a NOTIFICATION parameter with 1750 Notify Message Type I2_ACKNOWLEDGEMENT, the system SHOULD set the 1751 I2 retransmission timer to the I2 processing time indicated in 1752 the NOTIFICATION parameter plus half the RTT-based timeout value. 1753 The system MUST NOT set the retransmission timeout to a higher 1754 value than allowed by a local policy. Moreover, the system 1755 SHOULD reset the I2 retransmission timer to the RTT-based timeout 1756 value after the next I2 retransmission. 1758 6.10. Processing UPDATE, CLOSE, and CLOSE_ACK Packets 1760 UPDATE, CLOSE, and CLOSE_ACK packets are handled similarly in HIP DEX 1761 as in HIP BEX (see Sections 6.11, 6.12, 6.14, and 6.15 of [RFC7401]). 1762 The only difference is the that the HIP_SIGNATURE is never present 1763 and, therefore, is not required to be processed by the receiving 1764 party. 1766 6.11. Handling State Loss 1768 Implementors MAY choose to use non-volatile, secure storage for HIP 1769 states in order for them to survive a system reboot. If no secure 1770 storage capabilities are available, the system SHOULD delete the 1771 corresponding HIP state, including the keying material. If the 1772 implementation does drop the state (as RECOMMENDED), it MUST also 1773 drop the peer's R1 generation counter value, unless a local policy 1774 explicitly defines that the value of that particular host is stored. 1775 An implementation MUST NOT store a peer's R1 generation counters by 1776 default, but storing R1 generation counter values, if done, MUST be 1777 configured by explicit HITs. 1779 7. HIP Policies 1781 There are a number of variables that will influence the HIP exchanges 1782 that each host must support. All HIP DEX implementations SHOULD 1783 provide for an ACL of Initiator's HI to Responder's HI. This ACL 1784 SHOULD also include preferred transform and local lifetimes. 1785 Wildcards SHOULD also be supported for this ACL. 1787 The value of #K used in the HIP R1 must be chosen with care. Values 1788 of #K that are too high will exclude clients with weak CPUs because 1789 these devices cannot solve the puzzle within a reasonable amount of 1790 time. #K should only be raised if a Responder is under high load, 1791 i.e., it cannot process all incoming HIP handshakes any more. If a 1792 Responder is not under high load, #K SHOULD be 0. 1794 8. Security Considerations 1796 HIP DEX closely resembles HIPv2. As such, the security 1797 considerations discussed in Section 8 of [RFC7401] similarly apply to 1798 HIP DEX. HIP DEX, however, replaces the SIGMA-based authenticated 1799 Diffie-Hellman key exchange of HIPv2 with an exchange of random 1800 keying material that is encrypted by a Diffie-Hellman derived key. 1801 Both the Initiator and Responder contribute to this keying material. 1802 As a result, the following additional security considerations apply 1803 to HIP DEX: 1805 o The strength of the keys for the Pair-wise Key SA is based on the 1806 quality of the random keying material generated by the Initiator 1807 and the Responder. Since the Initiator is expected to be a sensor 1808 or an actuator device, there is a natural concern about the 1809 quality of its random number generator. 1811 o HIP DEX lacks the Perfect Forward Secrecy (PFS) property of HIPv2. 1812 Consequently, if an HI is compromised, all HIP connections 1813 protected with that HI are compromised. 1815 o The puzzle mechanism using CMAC may need further study regarding 1816 the level of difficulty. 1818 o The HIP DEX HIT generation may present new attack opportunities. 1820 o The R1 packet is unauthenticated and offers an adversary a new 1821 attack vector against the Initiator. This is mitigated by only 1822 processing a received R1 packet when the Initiator has previously 1823 sent a corresponding I1 packet. Moreover, the Responder repeats 1824 the DH_GROUP_LIST, HIP_CIPHER, HIT_SUITE_LIST, and 1825 TRANSPORT_FORMAT_LIST parameters in the R2 packet in order to 1826 enable the Initiator to verify that these parameters have not been 1827 modified by an attacker in the unprotected R1 packet. 1829 The optional retransmission extension of HIP DEX is based on a NOTIFY 1830 packet that the Responder can use to inform the Initiator about the 1831 reception of an I2 packet. The Responder, however, cannot protect 1832 the authenticity of this packet as it did not yet set up the Master 1833 Key SA. Hence, an eavesdropping adversary may send spoofed reception 1834 acknowledgements for an overheard I2 packet and signal an arbitrary 1835 I2 processing time to the Initiator. The adversary can, e.g., 1836 indicate a lower I2 processing time than actually required by the 1837 Responder in order to cause premature retransmissions. To protect 1838 against this attack, the Initiator SHOULD set the NOTIFY-based 1839 timeout value to the maximum indicated packet processing time in case 1840 of conflicting NOTIFY packets. This allows the legitimate Responder 1841 to extend the retransmission timeout to the intended length. The 1842 adversary, however, can still arbitrarily delay the protocol 1843 handshake beyond the Responder's actual I2 processing time. To limit 1844 the extend of such a maliciously induced handshake delay, this 1845 specification additionally requires the Initiator not to set the 1846 NOTIFY-based timeout value higher than allowed by a local policy. 1848 9. IANA Considerations 1850 The following changes to the "Host Identity Protocol (HIP) 1851 Parameters" registries have been made: 1853 HIT Suite ID This document defines the new HIT Suite "ECDH/FOLD" 1854 (see Section 5.2.4). 1856 Parameter Type This document defines the new HIP parameter 1857 "ENCRYPTED_KEY" with type number 643 (see Section 5.2.5). 1859 HIP Cipher ID This document defines the new HIP Cipher ID "AES- 1860 128-CTR" (see Section 5.2.2). 1862 HI Algorithm This document defines the new HI Algorithm "ECDH" (see 1863 Section 5.2.3). 1865 ECC Curve Label This document specifies a new algorithm-specific 1866 subregistry named "ECDH Curve Label". The values for this 1867 subregistry are defined in Section 5.2.1. 1869 10. Acknowledgments 1871 The drive to put HIP on a cryptographic 'Diet' came out of a number 1872 of discussions with sensor vendors at IEEE 802.15 meetings. David 1873 McGrew was very helpful in crafting this document. 1875 11. Changelog 1877 This section summarizes the changes made from draft-moskowitz-hip-rg- 1878 dex-05, which was the first stable version of the draft. Note that 1879 the draft was renamed after draft-moskowitz-hip-rg-dex-06. 1881 11.1. Changes in draft-moskowitz-hip-rg-dex-06 1883 o A major change in the ENCRYPT parameter to use AES-CTR rather than 1884 AES-CBC. 1886 11.2. Changes in draft-moskowitz-hip-dex-00 1888 o Draft name change. HIPRG ended in IRTF, HIP DEX is now individual 1889 submission. 1891 o Added the change section. 1893 o Added a Definitions section. 1895 o Changed I2 and R2 packets to reflect use of AES-CTR for 1896 ENCRYPTED_KEY parameter. 1898 o Cleaned up KEYMAT Generation text. 1900 o Added Appendix with C code for the ECDH shared secret generation 1901 on an 8 bit processor. 1903 11.3. Changes in draft-moskowitz-hip-dex-01 1905 o Numerous editorial changes. 1907 o New retransmission strategy. 1909 o New HIT generation mechanism. 1911 o Modified layout of ENCRYPTED_KEY parameter. 1913 o Clarify to use puzzle difficulty of zero under normal network 1914 conditions. 1916 o Align inclusion directive of R1_COUNTER with HIPv2 (from SHOULD to 1917 MUST). 1919 o Align inclusion of TRANSPORT_FORMAT_LIST with HIPv2 (added to R1 1920 and I2). 1922 o HIP_CIPHER, HIT_SUITE_LIST, and TRANSPORT_FORMAT_LIST must now be 1923 echoed in R2 packet. 1925 o Added new author. 1927 11.4. Changes in draft-moskowitz-hip-dex-02 1929 o Introduced formal definition of FOLD function. 1931 o Clarified use of CMAC for puzzle computation in section "Solving 1932 the Puzzle". 1934 o Several editorial changes. 1936 11.5. Changes in draft-moskowitz-hip-dex-03 1938 o Addressed HI crypto agility. 1940 o Clarified purpose of secret exchanged via ENCRYPTED_KEY parameter. 1942 o Extended the IV in the ENCRYPTED_KEY parameter. 1944 o Introduced forward-references to HIP DEX KEYMAT process and 1945 improved KEYMAT section. 1947 o Replaced Appendix A on "C code for ECC point multiplication" with 1948 short discussion in introduction. 1950 o Updated references. 1952 o Further editorial changes. 1954 11.6. Changes in draft-moskowitz-hip-dex-04 1956 o Improved retransmission extension. 1958 o Updated and strongly revised packet processing rules. 1960 o Updated security considerations. 1962 o Updated IANA considerations. 1964 o Move the HI Algorithm for ECDH to a value of 11. 1966 o Many editorial changes. 1968 12. References 1970 12.1. Normative References 1972 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1973 Requirement Levels", BCP 14, RFC 2119, March 1997. 1975 [RFC2410] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and 1976 Its Use With IPsec", RFC 2410, November 1998. 1978 [RFC3686] Housley, R., "Using Advanced Encryption Standard (AES) 1979 Counter Mode With IPsec Encapsulating Security Payload 1980 (ESP)", RFC 3686, January 2004. 1982 [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control 1983 Message Protocol (ICMPv6) for the Internet Protocol 1984 Version 6 (IPv6) Specification", RFC 4443, March 2006. 1986 [RFC7343] Laganier, J. and F. Dupont, "An IPv6 Prefix for Overlay 1987 Routable Cryptographic Hash Identifiers Version 2 1988 (ORCHIDv2)", RFC 7343, September 2014. 1990 [RFC7401] Moskowitz, R., Heer, T., Jokela, P., and T. Henderson, 1991 "Host Identity Protocol Version 2 (HIPv2)", RFC 7401, 1992 April 2015. 1994 [RFC7402] Jokela, P., Moskowitz, R., and J. Melen, "Using the 1995 Encapsulating Security Payload (ESP) Transport Format with 1996 the Host Identity Protocol (HIP)", RFC 7402, April 2015. 1998 12.2. Informative References 2000 [DH76] Diffie, W. and M. Hellman, "New Directions in 2001 Cryptography", IEEE Transactions on Information Theory 2002 vol. IT-22, number 6, pages 644-654, Nov 1976. 2004 [HWZ13] Hummen, R., Wirtz, H., Ziegeldorf, J., Hiller, J., and K. 2005 Wehrle, "Tailoring End-to-End IP Security Protocols to the 2006 Internet of Things", in Proceedings of IEEE International 2007 Conference on Network Protocols (ICNP 2013), October 2013. 2009 [I-D.ietf-hip-rfc4423-bis] 2010 Moskowitz, R. and M. Komu, "Host Identity Protocol 2011 Architecture", draft-ietf-hip-rfc4423-bis-12 (work in 2012 progress), June 2015. 2014 [IEEE.802-11.2007] 2015 "Information technology - Telecommunications and 2016 information exchange between systems - Local and 2017 metropolitan area networks - Specific requirements - Part 2018 11: Wireless LAN Medium Access Control (MAC) and Physical 2019 Layer (PHY) Specifications", IEEE Standard 802.11, June 2020 2007, . 2023 [IEEE.802-15-4.2011] 2024 "Information technology - Telecommunications and 2025 information exchange between systems - Local and 2026 metropolitan area networks - Specific requirements - Part 2027 15.4: Wireless Medium Access Control (MAC) and Physical 2028 Layer (PHY) Specifications for Low-Rate Wireless Personal 2029 Area Networks (WPANs)", IEEE Standard 802.15.4, September 2030 2011, . 2033 [LN08] Liu, A. and H. Ning, "TinyECC: A Configurable Library for 2034 Elliptic Curve Cryptography in Wireless Sensor Networks", 2035 in Proceedings of International Conference on Information 2036 Processing in Sensor Networks (IPSN 2008), April 2008. 2038 [RFC5903] Fu, D. and J. Solinas, "Elliptic Curve Groups modulo a 2039 Prime (ECP Groups) for IKE and IKEv2", RFC 5903, June 2040 2010. 2042 [RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen, 2043 "Internet Key Exchange Protocol Version 2 (IKEv2)", RFC 2044 5996, September 2010. 2046 [RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic 2047 Curve Cryptography Algorithms", RFC 6090, February 2011. 2049 [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for 2050 Constrained-Node Networks", RFC 7228, May 2014. 2052 [SECG] SECG, "Recommended Elliptic Curve Domain Parameters", SEC 2053 2 , 2000, . 2055 Appendix A. Password-based two-factor authentication during the HIP DEX 2056 handshake 2058 HIP DEX allows to identify authorized connections based on a two- 2059 factor authentication mechanism. With two-factor authentication, 2060 devices that are authorized to communicate with each other are 2061 required to be pre-provisioned with a shared (group) key. The 2062 Initiator uses this pre-provisioned key to encrypt the 2063 ECHO_RESPONSE_UNSIGNED in the I2 packet. Upon reception of the I2, 2064 the Responder verifies that its challenge in the 2065 ECHO_REQUEST_UNSIGNED parameter in the R1 packet has been encrypted 2066 with the correct key. If verified successfully, the Responder 2067 proceeds with the handshake. Otherwise, it silently drops the I2 2068 packet. 2070 Note that there is no explicit signaling in the HIP DEX handshake for 2071 this behavior. Thus, knowledge of two-factor authentication must be 2072 configured externally prior to the handshake. 2074 Authors' Addresses 2076 Robert Moskowitz (editor) 2077 HTT Consulting 2078 Oak Park, MI 2079 USA 2081 EMail: rgm@htt-consult.com 2083 Rene Hummen 2084 Chair of Communication and Distributed Systems, RWTH Aachen 2085 Ahornstrasse 55 2086 Aachen 52074 2087 Germany 2089 EMail: hummen@comsys.rwth-aachen.de 2090 URI: http://www.comsys.rwth-aachen.de/team/rene-hummen/