Network Working Group R. Moskowitz Internet-Draft ICSAlabs, a Division of TruSecure Expires: November 13, 2003 Corporation P. Nikander Ericsson Research Nomadic Lab May 15, 2003 Host Identity Protocol draft-moskowitz-hip-06 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http:// www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on November 13, 2003. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This memo specifies the details of the Host Identity Protocol (HIP). The overall description of protocol and the underlying architectural thinking is available in the HIP architecture [16] specification. The Host Identity Protocol is used to establish a rapid authentication between two hosts and to provide continuity of communications between those hosts independent of the networking layer. The various forms of the Host Identity, HI, HIT, and LSI, are covered in detail. It is described how they are used to support Moskowitz & Nikander Expires November 13, 2003 [Page 1] Internet-Draft Host Identity Protocol May 2003 authentication and the establishment of keying material, which is then used by IPsec ESP [5] to establish a two-way secured communication channel between the hosts. The basic state machine for HIP provides a HIP compliant host with the resiliency to avoid many DoS attacks. The basic HIP exchange for two public hosts shows the actual packet flow. Other HIP exchanges, including those that work across NATs are covered elsewhere, such as in the HIP implementation document [17]. Table of Contents 1. Status Warning . . . . . . . . . . . . . . . . . . . . . . . 4 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 A new name space and indentifiers . . . . . . . . . . . . . 5 2.2 The HIP protocol . . . . . . . . . . . . . . . . . . . . . . 5 3. Conventions used in this document . . . . . . . . . . . . . 7 4. Host Identifiers . . . . . . . . . . . . . . . . . . . . . . 8 4.1 Host Identity Tag (HIT) . . . . . . . . . . . . . . . . . . 8 4.1.1 Generating a HIT from a HI . . . . . . . . . . . . . . . . . 9 4.1.2 Storing HIT in DNS . . . . . . . . . . . . . . . . . . . . . 9 4.1.3 Host Assigning Authority (HAA) field . . . . . . . . . . . . 10 4.2 Local Scope Identity (LSI) . . . . . . . . . . . . . . . . . 10 4.3 Security Parameter Index (SPI) . . . . . . . . . . . . . . . 11 4.4 Difference between an LSI and the SPI . . . . . . . . . . . 12 5. The Host Identity Protocol . . . . . . . . . . . . . . . . . 13 5.1 Payload format . . . . . . . . . . . . . . . . . . . . . . . 13 5.2 Base HIP exchange . . . . . . . . . . . . . . . . . . . . . 13 5.2.1 HIP Cookie Mechanism . . . . . . . . . . . . . . . . . . . . 14 5.2.2 HIP Controls . . . . . . . . . . . . . . . . . . . . . . . . 17 5.2.3 HIP Birthday . . . . . . . . . . . . . . . . . . . . . . . . 17 5.3 Piggypacking data on I2 and R2 . . . . . . . . . . . . . . . 18 5.4 Distributing certificates . . . . . . . . . . . . . . . . . 18 6. The Host Identity Protocol packet flow and state machine . . 19 6.1 HIP Scenarios . . . . . . . . . . . . . . . . . . . . . . . 19 6.2 Refusing a HIP exchange . . . . . . . . . . . . . . . . . . 20 6.3 Reboot and SA timeout restart of HIP . . . . . . . . . . . . 20 6.4 HIP State Machine . . . . . . . . . . . . . . . . . . . . . 21 6.4.1 HIP States . . . . . . . . . . . . . . . . . . . . . . . . . 21 6.4.2 HIP State Processes . . . . . . . . . . . . . . . . . . . . 21 6.4.3 Simplified HIP State Diagram . . . . . . . . . . . . . . . . 23 7. HIP Packets . . . . . . . . . . . . . . . . . . . . . . . . 24 7.1 I1 - the HIP Initiator packet . . . . . . . . . . . . . . . 24 7.2 R1 - the HIP Responder packet . . . . . . . . . . . . . . . 25 7.3 I2 - the HIP Second Initiator packet . . . . . . . . . . . . 26 7.4 R2 - the HIP Second Responder packet . . . . . . . . . . . . 27 7.5 NES - the HIP New SPI Packet . . . . . . . . . . . . . . . . 28 7.6 BOS - the HIP Bootstrap Packet . . . . . . . . . . . . . . . 29 8. Packet processing . . . . . . . . . . . . . . . . . . . . . 30 Moskowitz & Nikander Expires November 13, 2003 [Page 2] Internet-Draft Host Identity Protocol May 2003 8.1 R1 Management . . . . . . . . . . . . . . . . . . . . . . . 30 8.2 Processing NES packets . . . . . . . . . . . . . . . . . . . 30 9. HIP KEYMAT . . . . . . . . . . . . . . . . . . . . . . . . . 32 10. HIP Fragmentation Support . . . . . . . . . . . . . . . . . 34 11. ESP with HIP . . . . . . . . . . . . . . . . . . . . . . . . 35 11.1 Security Association Management . . . . . . . . . . . . . . 35 11.2 Security Parameters Index (SPI) . . . . . . . . . . . . . . 35 11.3 Supported Transforms . . . . . . . . . . . . . . . . . . . . 35 11.4 Sequence Number . . . . . . . . . . . . . . . . . . . . . . 36 11.5 ESP usage with non-cryptographic HI . . . . . . . . . . . . 36 12. HIP Policies . . . . . . . . . . . . . . . . . . . . . . . . 37 13. Security Considerations . . . . . . . . . . . . . . . . . . 38 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . 41 15. ICANN Considerations . . . . . . . . . . . . . . . . . . . . 42 16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 43 References . . . . . . . . . . . . . . . . . . . . . . . . . 44 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 45 A. Backwards compatibility API issues . . . . . . . . . . . . . 46 B. Probabilities of HIT collisions . . . . . . . . . . . . . . 47 C. Probabilities in the cookie calculation . . . . . . . . . . 48 D. Using responder cookies . . . . . . . . . . . . . . . . . . 49 Intellectual Property and Copyright Statements . . . . . . . 52 Moskowitz & Nikander Expires November 13, 2003 [Page 3] Internet-Draft Host Identity Protocol May 2003 1. Status Warning This document is an interim version of the to-be HIP protocol specification including most but not necessarily all of the issues discussed at the maling list and among the early implementors. However, at the present stage this document contains a largish number of open issues. Many of these issues are marked with XXX, or enclosed in brackets, [like this], but not necessarily all. The purpose of publishing this draft at this stage is to make it available to the community outside of the group of early implementors. Based on the current implementation experiences, it is possible that there may be substantial changes to this specification before it is completed. The description of the REA packet has been removed from this document, with the intention of publishing a separate draft on HIP based mobility and multi-homing. Moskowitz & Nikander Expires November 13, 2003 [Page 4] Internet-Draft Host Identity Protocol May 2003 2. Introduction The Host Identity Protocol (HIP) provides a rapid exchange of Host Identities (HI) between two hosts. The exchange also establishes a pair IPsec Security Associations (SA), to be used with IPsec ESP. The HIP protocol is designed to be resistant to Denial-of-Service (DoS) and Man-in-the-middle (MitM) attacks, and when used to enable ESP, provides DoS and MitM protection to upper layer protocols, such TCP and UDP. 2.1 A new name space and indentifiers The Host Identity Protocol introduces a new namespace, the Host Identity. The affects of this change are explained in the companion document, the HIP architecture [16] specification. There are three representations of the Host Identity, the full Identifier (HI), the Host Identity Tag (HIT), and the Local Scope Identity (LSI). Three representations are used, as each meets a different design goal of HIP, and none of them can be removed and meet these goals. The HI represents directly the Identity, normally being a public key. Since there are different public key algorithms that can be used with different key lengths, the HI is not good for using as the HIP packet identifier, or as a index into the various operational tables needed to support HIP. A hash of the HI, the Host Identity Tag (HIT), thus becomes the operational representation. It is 128 bits long. It is used in the HIP payloads, and it is intended be used to index the corresponding state in the end hosts. In many environments, 128 bits is still considered large. For example, currently used IPv4 based applications are constrained with 32 bit API fields. Thus, the third representation, the 32 bit LSI, is needed. The LSI provides a compression of the HIT with only a local scope so that it can be carried efficiently in any application level packet and used in API calls. 2.2 The HIP protocol The base HIP exchange consists of only four packets. The four-packet design helps to make HIP DoS resilient. The protocol exchanges Diffie-Hellman keys in the 2nd and 3rd packets, and authenticates the parties in the 3rd and 4th packets. Additionally, it starts the cookie exchange in the 2nd packet, completing it with the 3rd packet. The exchange uses the Diffie-Hellman exchange to hide the Host Identity of the Initiator in packet 3. The Responder's Host Identity Moskowitz & Nikander Expires November 13, 2003 [Page 5] Internet-Draft Host Identity Protocol May 2003 is not protected. It should be noted, however, that both the Initiator and the Responder HITs are transported as such in the packets, allowing an eavesdropper with a priori knowledge about the parties to verify their identies. Data packets start after the 4th packet. In some cases, the 3rd and 4th HIP packets can carry a data payload. However, the details of that may need to be revised as more implementation experience is gained. Finally, HIP is designed as an end-to-end authentication and key establishment protocol. It lacks much of the fine-grain policy control found in IKE that allows IKE to support complex gateway policies. Thus, HIP is not a complete replacement for IKE. In many cases, particularly in spanning addressing realms, HIP would be the preferred key establishment protocol. Moskowitz & Nikander Expires November 13, 2003 [Page 6] Internet-Draft Host Identity Protocol May 2003 3. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119 [2]. Moskowitz & Nikander Expires November 13, 2003 [Page 7] Internet-Draft Host Identity Protocol May 2003 4. Host Identifiers The structure of the Host Identifier is the public key of a public key pair. Correspondingly, the Host Identity itself can be considered to be the abstract entity that holds the private key from the key pair. DSA is the MUST implement algorithm for all HIP implementations, other algorithms MAY be supported. DSA was chosen as the default algorithm due to its small signature size. A Host Identity Tag (HIT) is used in protocols to represent the Host Identity. Another representation of the Host Identity, the Local Scope Identity (LSI), can also be used in protocols and APIs. LSI's advantage over HIT is its size; its disadvantage is its local scope. 4.1 Host Identity Tag (HIT) The Host Identity Tag is a 128 bit entity. There are two advantages of using a hash over the actual Identity in protocols. First its fix length makes for easier protocol coding and also better manages the packet size cost of this technology. Secondly, it presents a consistent format to the protocol whatever underlying identity technology is used. There are two types of HITs. HITs of the first type consist just of the hash of the public key. HITs of the second type consist of a Host Assigning Authority (HAA) field, and only the last 64 bits come from a SHA-1 hash of the Host Identity. This latter format for HIT is recommended for 'well known' systems. It is possible to support a resolution mechanism for these names in directories like DNS. Another use of HAA is in policy controls, see Section 12. [XXX: Revise to define "pure" HITs and IPv6 compatible HITs.] The formats of the HITs are designed to avoid the most commonly occurring IPv6 addresses in RFC2373 [3]. Bits 0 and 1 are used to differentiate the formats. If Bit 0 is zero and Bit 1 is one, then the rest of HIT is a 126 bits of a SHA-1 hash of the Host Identity. If Bit 0 is one and Bit 1 is zero, then the next 62 bits is the HAA field, and only the last 64 bits come from the hash of the Host Identity. Allocation Prefix Fraction of IPv6 (binary) Address Space ------------------------ -------- ------------- IPv6 Address space 00 1/4 126 bit HIT 01 1/4 HAA assigned 64 bit HIT 10 1/4 IPv6 Address space 11 1/4 Moskowitz & Nikander Expires November 13, 2003 [Page 8] Internet-Draft Host Identity Protocol May 2003 4.1.1 Generating a HIT from a HI The 126 or 64 hash bits in a HIT MUST be generated by taking the least significant 126 or 64 bits of the SHA-1 [15] hash of the Host Identity as it is represented in the Host Identity field in a HIP payload packet. For Identities that are DSA public keys, the HIT is formed as follows. 1. The DSA public key is encoded as defined in RFC2536 [9] Section 2, taking the fields T, Q, P, G, and Y, concatenated. Thus, the length of the data to be hashed is 1 + 20 + 3 * 64 + 3 * 8 * T octets long, where T is the size parameter as defined in RFC2536 [9]. The size parameter T, affecting the field lengths, MUST be selected as the minimum value that is long enough to accomodate P, G, and Y. The fields MUST be encoded in network byte order, as defined in RFC2536 [9]. 2. A SHA-1 hash [15] is calculated over the encoded key. 3. The least signification 126 or 64 bits of the hash result are used to create the HIT, as defined above. The following pseudo-code illustrates the process. The symbol := denotes assignment; the symbol += denotes appending. The pseudo-function encode_in_network_byte_order takes two parameters, an integer (bignum) and length, and returns the integer encoded into a byte string of the given length. buffer := encode_in_network_byte_order ( DSA.T , 1 ) buffer += encode_in_network_byte_order ( DSA.Q , 20 ) buffer += encode_in_network_byte_order ( DSA.P , 64 + 8 * T ) buffer += encode_in_network_byte_order ( DSA.G , 64 + 8 * T ) buffer += encode_in_network_byte_order ( DSA.Y , 64 + 8 * T ) digest := SHA-1 ( buffer ) hit_126 := concatenate ( 01 , low_order_bits ( digest, 126 ) ) hit_haa := concatenate ( 10 , HAA, low_order_bits ( digest, 64 ) ) 4.1.2 Storing HIT in DNS Any conforming implementation SHOULD be able to store Host Identifiers in a DNS IPSECKEY RDATA [14] format. If a particular form of a HI does not already have a specified RDATA format, a new RDATA-like format SHOULD be defined for the HI. Moskowitz & Nikander Expires November 13, 2003 [Page 9] Internet-Draft Host Identity Protocol May 2003 During a transition period, instead of storing the HI, the HIT MAY be stored in an AAAA RR. If a HIT is stored in an AAAA RR, it MUST be returned as the last item in the set of AAAA RRs returned. 4.1.3 Host Assigning Authority (HAA) field The 62 bits of HAA supports two levels of delegation. The first is a registered assigning authority (RAA). The second is a registered identity (RI, commonly a company). The RAA is 22 bits with values assign sequentially by ICANN. The RI is 40 bits, also assigned sequentially but by the RAA. This can be used to create a resolution mechanism in the DNS. For example if FOO is RAA number 100 and BAR is FOO's 50th registered identity, and if 1385D17FC63961F5 is the hash of the Host Identity for www.foo.com, then by using DNS Binary Labels [11] there could be a reverse lookup record like: \[x1385D17FC63961F5/64].\[x32/40].\[x64/22].HIT.int IN PTR www.foo.com. (Note that RFC2673 [11] is Experimental, and that there are some bad experiences with binary DNS labels. [12]) 4.2 Local Scope Identity (LSI) LSIs are 32-bit localized representations of a Host Identity. The purpose of an LSI is to facilitate using Host Identities in existing IPv4 based protocols and APIs. The owner of the Host Identity does not set its own LSI; each host selects its partner's 32 bit representation for a Host Identity. LSI assignment is sequential off of a random starting point. That is, at initialisation time, a random starting point is selected for LSIs, and they are assigned sequentially thereafter. This avoids collisions if LSIs are assigned sequentially starting from zero, and even collisions on a busy host if assigned randomly. The LSIs SHOULD be allocated from the 1.0.0.0/8 subnet. That makes it easier to differentiate between LSIs and IPv4 addresses at the API level. If the LSI assigned by a peer to represent a host is unccapteble, the host MAY terminate the HIP four-way handshake and start anew. [XXX: The details probably need to be worked out.] [XXX: There are still different opinions on how exactly to generate LSIs. The proposed options include the following: east 32 significant bits of HIT a monotonically increasing number from a random seed Moskowitz & Nikander Expires November 13, 2003 [Page 10] Internet-Draft Host Identity Protocol May 2003 1.0.0.0/8 in the IPv4 private address space When computing TCP and UDP checksums on sockets bound to LSIs, the LSIs MUST be used in the place of the IPv4 addresses in the IPv4 pseudoheader. Other examples of how LSIs can be used include the following: as the address in a FTP command and as the address in a socket call. Thus, LSIs act as a bridge for Host Identity into old protocols and APIs. XXX: Recalculate the risk of a collision. The risk of collisions for random assignment would be 1% in a population of 10,000, if all of the IPv4 address space was used. [XXX Question: Does the risk of collisions between LSIs really matter? Since each host selects the representation of its peers, there can't be collisions between the LSIs that are locally used to represent the peers. On the other hand, the host itself is represented by a number of LSIs, each selected separately by its peers. To the IPv4 stack this might look like the host has a large numer of local address aliases. It looks like a collision becomes a problem if a new LSI, selected by a new peer, happens to have a collision with some other LSI, already locally selected to represent some other peer. In that case the host cannot create a new IPv4 alias for the LSI, since it is already used to represent a remote host. In that case the LSI must be rejected.] 4.3 Security Parameter Index (SPI) SPIs are used in ESP to find the right security association for received packets. The ESP SPIs have added significance when used with HIP; they are a compressed representation of the HIT in every packet. Thus they MAY be used by intermediary systems in providing services like address mapping. Note that since the SPI has significance at the receiver, only the < DST, SPI > uniquely identifies the receiver HIT at every given point of time. The same SPI value may be used by several hosts. The same < DST, SPI > may denote different hosts at different points of time, depending on which host is currently reachable at the DST. Each host selects itself the SPI it wants to see in packets received from its peer. This allows it to select different SPIs for different peers. The SPI selection MUST be random. A different SPI MUST be used for each HIP exchange with a particular host; this is to avoid a replay attack. Additionally, when a host rekeys, the SPI MUST change. Furthermore, if a host changes over to use a different IP address, it MAY change the SPI used. One method for SPI creation that meets these criteria, would be to concatenate the HIT with a 32 Moskowitz & Nikander Expires November 13, 2003 [Page 11] Internet-Draft Host Identity Protocol May 2003 bit random or sequential number, hash this (using SHA1), and then use the high order 32 bits as the SPI. [XXX: It is not clear where the requirement for a random SPI comes from. One possible reason is that the sequence numbers always start at one, and therefore using the same SPI values soon again might cause confusion? SPIs should be unique on incoming SAs, for demultiplexing (unlike IPsec, cannot reuse SPI value over different IP addresses).] The selected SPI is communicated to the peer in the third (I2) and fourth (R2) packets of the base HIP exchange. Changes in SPI are signalled with NES or REA packets. [Question: Do we really need separate NES and REA packets? Could their functions be integrated? Partial answer: We do need the version of NES that includes Diffie-Hellman. However, it looks like the REAs need to be able to define new SPIs, too. Thus, the simple case of using NES just to establish a new SPI from existing keymat is probably not needed.] 4.4 Difference between an LSI and the SPI There is a subtle difference between an LSI and a SPI. The LSI is relatively longed lived. A system selects the LSI it locally uses to represent its peer, it SHOULD reuse a previous LSI for a HIT during a HIP exchange. This COULD be important in a timeout recovery situation. The LSI ONLY appears in the 3rd and 4th HIP packets (each system providing the other with its LSI). The LSI is used anywhere in system processes where IP addresses have traditionally have been used, like in TCBs and FTP port commands. The SPI is short-lived. It changes with each HIP exchange and with a HIP rekey and/or movement. A system notifies its peer of the SPI to use in ESP packets sent to it. Since the SPI is in all but the first two HIP packets, it can be used in intermediary systems to assist in address remapping. Moskowitz & Nikander Expires November 13, 2003 [Page 12] Internet-Draft Host Identity Protocol May 2003 5. The Host Identity Protocol The Host Identity Protocol is IP protocol TBD. The HIP payload could be carried in every datagram. However, since HIP datagrams are relatively large (at least 40 bytes), and ESP already has all of the functionality to maintain and protect state, the HIP payload is 'compressed' into an ESP payload after the HIP exchange. Thus in practice, HIP packets only occur in datagrams to establish or change HIP state. 5.1 Payload format 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Payload Len | Type | VER. | RES. | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Controls | CRC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sender's Host Identity Tag (HIT) | | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Receiver's Host Identity Tag (HIT) | | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | / HIP Parameters / / / | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The exact contents of the HIP payload is defined in [13] 5.2 Base HIP exchange The base HIP exchange serves to manage the establishment of state between an Initiator and a Responder. The Initiator first sends a trigger packet, I1, to the responder. The second packet, R1, starts the actual exchange. In contains a puzzle, a cryptographic challenge that the Initiator must solve before continuing the exchange. In its reply, I2, the Initiator must display the solution. Without a solution the I2 message is simply discarded. Moskowitz & Nikander Expires November 13, 2003 [Page 13] Internet-Draft Host Identity Protocol May 2003 The last three packets of the exchange, R1, I2, and R2, constitute a standard authenticated Diffie-Hellman key exchange. The base exchange is illustrated below. Initiator Responder I1: trigger exchange --------------------------> select pre-computed R1 R1: puzzle, D-H, sig <------------------------- check sig remain stateless solve puzzle I2: solution, D-H, sig --------------------------> compute D-H check cookie check puzzle check sig R2: sig <-------------------------- check sig compute D-H 5.2.1 HIP Cookie Mechanism The purpose of the HIP cookie mechanism is to protect the Responder from a number of denial-of-service threats. It allows the Responder to delay state creation until receiving I2. Furthermore, the puzzle included in the cookie allows the Responder to use a fairly cheap calculation to check that the Initiator is "sincere" in the sense that it has churned CPU cycles in solving the puzzle. The Cookie mechanism has been explicitly designed to give space for various implementation options. It allows a responder implementation to completely delay session specific state creation until a valid I2 is received. In such a case a validly formatted I2 can be rejected earliest only once the responder has checked its validity by computing one hash function. On the other hand, the design also allows a responder implementation to keep state about received I1s, and match the received I2s against the state, thereby allowing the implementation to avoid the computational cost of the hash function. The drawback of this latter approach is the requirement of creating state. Finally, it also allows an implementation to use any combination of the space-saving and computation-saving mechanism. One possible way how a Responder can remain stateless but drop most spoofed I2s is to base the selection of the cookie on some function Moskowitz & Nikander Expires November 13, 2003 [Page 14] Internet-Draft Host Identity Protocol May 2003 over the Initiator's identity. The idea is that the Responder has a (perhaps varying) number of pre-calculated R1 packets, and it selects one of these based on the information carried in I1. When the Responder then later receives I2, it checks that the cookie in the I2 matches with the cookie send in the R1, thereby making it impractical for the attacker to first exchange one I1/R1, and then generate a large number of spoofed I2s that seemingly come from different IP addresses or use different HITs. The method does not protect from an attacker that uses fixed IP addresses and HITs, though. Against such an attacker it is probably best to create a piece of local state, and remember that the puzzle check has previously failed. See Appendix D for one possible implementation. The Responder can set the difficulty for Initiator, based on its concern of trust of the Initiator. The Responder SHOULD use heuristics to determine when it is under a denial-of-service attack, and set the difficulty value K appropriately. The Responder starts the cookie exchange when it receives an I1. The Responder supplies a random number I, and requires the Initiator to find a number J. To select a proper J, the Initator must create the concatenation of I, the HITs of the parties, and J, and take a SHA-1 hash over this concatenation. The lowest order K bits of the result MUST be zeros. To accomplish this, the Initiator will have to generate a number of Js until one produces the hash target. The Initiator SHOULD give up after trying 2^(K+2) times, and start over the exchange. (See Appendix C.) The Responder needs to re-create the contactenation of I, the HITs, and the provided J, and compute the hash once to prove that the Initiator did its assigned task. To prevent pre-computation attacks, the Responder MUST select I in such a way that the Inititiator cannot guess it. Furthermore, the construction MUST allow the Responder to verify that the value were indeed selected by it and not by the Initiator. See Appendix D for an example on how to implement this. It is RECOMMENDED that the Responder generates a new cookie and a new R1 once every few minutes. Furthermore, it is RECOMMENDED that the responder remembers an old cookie at least 60 seconds after it has been deprecated. These time values allow a slower Initiator to solve the cookie puzzle while limiting the usability that an old, solved cookie has to an attacker. [XXX: A question is whether the R1 should include a timestamp so that the Initator would not unnecessarily solve old, expired puzzles, perhaps sent by an attacker?] [XXX. Should we use Mike Burrow's memory bound functions instead of Moskowitz & Nikander Expires November 13, 2003 [Page 15] Internet-Draft Host Identity Protocol May 2003 SHA-1?] In R1, the values I and K are sent in network byte order. Similarily, in I2 the values I and J are sent in network byte order. The SHA-1 hash is created by concatenating, in network byte order, the following data, in the following order: 64-bit random value I, in network byte order, as appearing in R1 and I2. 128-bit Initiator HIT, in network byte order, as appearing in the HIP Payload in R1 and I2. 128-bit Responder HIT, in network byte order, as appearing in the HIP Payload in R1 and I2. 64-bit random value J, in network byte order, as appearing in I2. In order to be a valid response cookie, the K low-order bits of the resulting SHA-1 digest must be zero. Notes: The length of the data to be hashed is 48 bytes. All the data in the hash input MUST be in network byte order. The order of the HITs depend on whether processing an R1 or I2. Care must be taken to copy the values in right order to the hash input. Precomputation by the Responder Sets up the challenge difficulty K. Generates a random number I. Creates a signed R1 and caches it. Responder Sends I and K in a HIP Cookie in an R1. Saves I and K for a Delta time. Initiator Generates repeated attempts to solve the challenge until a matching J is found: Ltrunc( SHA-1( I | HIT-I | HIT-R | J ), K ) == 0 Send I and J in HIP Cookie in I2. Moskowitz & Nikander Expires November 13, 2003 [Page 16] Internet-Draft Host Identity Protocol May 2003 Responder Verify that the received I is a saved one. Match the Response with a K based on I. Compute V := Ltrunc( SHA-1( I | HIT-I | HIT-R | J ), K ) Reject if V != 0 Accept if V == 0 5.2.2 HIP Controls HIP controls are informative items that will influence the HIP exchange and the use of ESP. HIP Controls are assigned a bit location in the Controls field numbered left (MSB) to right (LSB). Currently, there are two controls of value to a HIP exchange: BIT Action 0 If value is 1, the HI is anonymous, i.e., generated for this exchange only. Anonymous HIs SHOULD NOT be stored. This control is set in packets R1 and/or I2. The peer receiving an anonymous HI may choose to refuse it by silently dropping the exchange. 1 If value is 1, the ESP transform requires a 64 bit sequence number. See Sequence Number section for processing this control. 2 If value is 1, the packet is followed by one or more CER packets. The purpose is to inform the recipient to expect the CER packets, allowing it to delay processing if needed. Various controls will be defined over time. These controls will be added to the end of the Controls field so that older implementations can ignore them. 5.2.3 HIP Birthday The Birthday is a reboot count used to manage state reestablishment when one peer rebooted or timed out its SA. The Birthday is increased every time the system boots. The Birthday also has to be increased in accordance with the system's SA timeout parameter. If the system has open SAs, it MUST increase its Birthday. This impacts a system's approach to precomputing R1 packets. Birthday SHOULD be a counter. It cannot be reset by the user and a system is unlikely to need a birthday larger than 2^64. Date-time in GMT can be used if a cross-boot counter is not possible, but it has a potential problem if the system time is set back by the user. Moskowitz & Nikander Expires November 13, 2003 [Page 17] Internet-Draft Host Identity Protocol May 2003 5.3 Piggypacking data on I2 and R2 5.4 Distributing certificates Moskowitz & Nikander Expires November 13, 2003 [Page 18] Internet-Draft Host Identity Protocol May 2003 6. The Host Identity Protocol packet flow and state machine [XXX: Revise if we use IPSECKEY.] A Host Identity Protocol exchange SHOULD be initiated whenever the DNS lookup returns HIP KEY resource records. Since some hosts may choose not to have information in DNS, hosts MUST implement support opportunistic HIP [17]. At this point of time, actually using opportunistic HIP is OPTIONAL. A typical HIP packet flow is shown below. I --> DNS (lookup R) I <-- DNS (R's addresses, HI, and HIT) I1 I --> R (Hi. Here is my I1, let's talk HIP) R1 I <-- R (OK. Here is my R1, handle this HIP cookie) I2 I --> R (Compute, compute, here is my counter I2) R2 I <-- R (OK. Let's finish HIP with my R2) I --> R (ESP protected data) I <-- R (ESP protected data) 6.1 HIP Scenarios The HIP protocol and state machine is designed to recover from one of the parties crashing and losing its state. The following scenarios describe the main use cases covered by the design. No prior state between the two systems. The system with data to send is the Initiator. The process follows standard 4 packet exchange, establishing the SAs. The system with data to send has no state with receiver, but receiver has a residual SA. Intiator acts as in no prior state, sending I1 and getting R1. When Receiver gets I2, the old SA is 'discovered' and deleted; the new SAs are established. System with data to send has an SA, but receiver does not. Receiver 'detects' when it receives an unknown SPI. Receiver sends an R1 with a NULL Initiator HIT. Sender gets the R1 with a later birthdate, discards old SA and continues exchange to establish new SAs for sending data. A peer determines that it needs to reset Sequence number or rekey. It sends NES. Receiver sends NES response, establishes new SAs Moskowitz & Nikander Expires November 13, 2003 [Page 19] Internet-Draft Host Identity Protocol May 2003 for peers. 6.2 Refusing a HIP exchange A HIP aware host may choose not to accept a HIP exchange. If the host's policy is to only be an initiator, it should begin its own HIP exchange. A host MAY choose to have such a policy since only the Initiator HI is protected in the exchange. There is a risk of a race condition if each host's policy is to only be an initiator, at which point the HIP exchange will fail. If the host's policy does not permit it to enter into a HIP exchange with the Initiator, it should send an ICMP Protocol Unreachable, Administratively Prohibited message. A more complex HIP packet is not used here as it actually opens up more potential DoS attacks than a simple ICMP message. 6.3 Reboot and SA timeout restart of HIP Simulating a loss of state is a potential DoS attack. The following process has been crafted to manage state recovery without presenting a DoS opportunity. If a host reboots or times out, it has lost its HIP state. If the system that lost state has a datagram to deliver to its peer, it simply restarts the HIP exchange. The peer sends an R1 HIP packet, but does not reset its state until it receives the I2 HIP packet. The I2 packet MUST have a Birthday greater than the current SA's Birthday. This is to handle DoS attacks that simulate a reboot of a peer. Note that either the original Initiator or the Responder could end up restarting the exchange, becoming the new Initiator. An example of the initial Responder needing to send a datagram but not having state occurs when the SAs timed out and a server on the Responder sends a keep-alive to the Initiator. If a system receives an ESP packet for an unknown SPI, the assumption is that it has lost the state and its peer did not. In this case, the system treats the ESP packet like an I1 packet and sends an R1 packet. The Initiator HIT is typically NULL in the R1, since the system usually does not know the peer's HIT any more. The system receiving the R1 packet first checks to see if it has an established and recently used SA with the party sending the R1. If such an SA exists, the system checks the Birthday, if the Birthday is greater than the current SA's Birthday, it processes the R1 packet and resends the ESP packet along with or after the I2 packet. The peer system processes the I2 in the normal manner, and replies with Moskowitz & Nikander Expires November 13, 2003 [Page 20] Internet-Draft Host Identity Protocol May 2003 an R2. This will reestablish state between the two peers. [Potential DoS attack if hundreds of peers 'loose' their state and all send R1 packets at once to a server. However, that would require the attacker having specific knowledge about the SAs used, and an ability to trigger R1s as the SAs are used.] 6.4 HIP State Machine HIP has very little state. In the base HIP exchange, there is an Initiator and a Responder. Once the SAs are established, this distinction is lost. If the HIP state needs to be re-established, the controlling parameters are which peer still has state and which has a datagram to send to its peer. The following state machine attempts to capture these processes. The state machine is presented in a single system view, reresenting either an Initiator or a Responder. There is not a complete overlap of processing logic here and in the packet definitions. Both are needed to completely implement HIP. 6.4.1 HIP States E0 State machine start E1 Initiating HIP E2 Waiting to finish HIP E3 HIP SA established E-FAILED HIP SA establishment failed 6.4.2 HIP State Processes +---------+ | E0 | Start state +---------+ Datagram to send, send I1 and go to E1 Receive I1, send R1 and stay at E0 Receive I2, process if successful, send R2 and go to E3 if fail, stay at E0 Receive ESP for unknown SA, send R1 and stay at E0 Receive ANYOTHER, drop and stay at E0 +---------+ Moskowitz & Nikander Expires November 13, 2003 [Page 21] Internet-Draft Host Identity Protocol May 2003 | E1 | Initiating HIP +---------+ Receive I1, send R1 and stay at E1 Receive I2, process if successful, send R2 and go to E3 if fail, stay at E1 Receive R1, process if successful, send I2 and go to E2 if fail, go to E-FAILED Receive ANYOTHER, drop and stay at E1 Timeout, increment timeout counter If counter is less than N1, send I1 and stay at E1 If counter is greater than N1, go to E-FAILED +---------+ | E2 | Waiting to finish HIP +---------+ Receive I1, send R1 and stay at E2 Receive I2, process if successful, send R2 and go to E3 if fail, stay at E2 Receive R2, process if successful, go to E3 if fail, go to E-FAILED Receive ANYOTHER, drop and stay at E2 Timeout, increment timeout counter If counter is less than N2, send I2 and stay at E2 If counter is greater than N2, go to E-FAILED +---------+ | E3 | HIP SA established +---------+ Receive I1, send R1 and stay at E3 Receive I2, process with Birthday check if successful, send R2, drop old SA and cycle at E3 if fail, stay at E3 Receive R1, process with SA and Birthday check if successful, send I2 with last datagram, drop old SA and go to E2 if fail, stay at E3 Receive R2, drop and stay at E3 Receive ESP for SA, process and stay at E3 Receive NES, process if successful, send NES and stay at E3 Moskowitz & Nikander Expires November 13, 2003 [Page 22] Internet-Draft Host Identity Protocol May 2003 if failed, stay at E3 Receive REA, process and stay at E3 6.4.3 Simplified HIP State Diagram Receive packets cause a move to new state +---------+ | E0 |>---+ +---------+ | | ^ | | | | | Dgm to | +-+ | send | I1 | | (note: ESP- means ESP with unknown SPI) ESP- | | v | +---------+ | | E1 |>---|----------+ +---------+ | | | | | | R1 | | | |I2 |I2 v | | +---------+ | | | E2 |>---|----------|-----+ | |<---|-----+ | | +---------+ | | | | | | | | | | R2 | |R1 | |I2 | | | | | v | | | | +---------+<---+ | | | | |----------+ | | | E3 |<--------------+ | | |<--------------------+ +---------+ | ^ | | +--+ ESP, NES, REA, I1, I2 Moskowitz & Nikander Expires November 13, 2003 [Page 23] Internet-Draft Host Identity Protocol May 2003 7. HIP Packets There are 9 HIP packets. Four are for the base HIP exchange, four are for mid-state changes (rekeying and address migration), and one is a broadcast for use when there is no IP addressing (e.g., before DHCP exchange). Packet representation uses the following operations: PPP() payload of type PPP FFF\{contents\} function FFF applied on contents [] optional payload An ESP payload MAY follow some HIP payloads. This transmission optimization SHOULD NOT be used if it results in fragmentation, and there would not be any fragmentation if the ESP payload were sent by itself. All implementations MUST be able to receive and process piggybacked ESP payloads. 7.1 I1 - the HIP Initiator packet Next Header = IPPROTO_NONE Type = 1 SRC HIT = Initiator's HIT DST HIT = Responder's HIT, or NULL IP(HIP()) The Initiator gets the Responder's HIT either from a DNS lookup of the responder's FQDN or from a local table. If the initiator does not know the responder's HIT, it may attempt anonymous mode by using NULL (all zeros) as the responder's HIT. Since this packet is so easy to spoof even if it were signed, no attempt is made to add to its generation or processing cost. Implementation MUST be able to handle a storm of reveived I1 packets, discarding those with common content that arrive within a small time delta. Moskowitz & Nikander Expires November 13, 2003 [Page 24] Internet-Draft Host Identity Protocol May 2003 7.2 R1 - the HIP Responder packet Next Header = IPPROTO_NONE Type = 2 SRC HIT = Responder's HIT DST HIT = Initiator's HIT Payload Contains: Birthday and Cookie Responder's Diffie-Hellman public value HIP transform ESP transform Responder's HI Signature IP (HIP ( BIRTHDAY_COOKIE, ( DIFFIE_HELLMAN_FULL | DIFFIE_HELLMAN ), HIP_TRANSFORM, ESP_TRANSFORM, HOST_ID, HIP_SIGNATURE ) ) The R1 packet may be followed by one or more CER packets. In this case, the C-bit in the control field MUST be set. If the Responder has multiple HIs, the HIT used MUST match Initiator's request. If the Initiator used anonymous mode, the Responder may select freely among its HIs. The Initiator HIT MUST match the one received in I1. If the R1 is a response to an ESP packet with an unknown SPI, the Initiator HIT SHOULD be zero. The Birthday is a reboot count used to manage state reestablishment when one peer rebooted or timed out its SA. The Cookie contains random I and difficulty K. K is number of bits that the Initiator must match get zero in the puzzle. The Diffie-Hellman value is ephemeral, but can be reused over a number of connections. In fact, as a defense against I1 storms, an implementation MAY use the same Diffie-Hellman value for a period of time, for example, 15 minutes. By using a small number of different Cookies for a given Diffie-Hellman value, the R1 packets can be pre-computed and delivered as quickly as I1 packets arrive. A scavenger process should clean up unused DHs and Cookies. The HIP_TRANSFORM contains the encryption algorithms supported by the Moskowitz & Nikander Expires November 13, 2003 [Page 25] Internet-Draft Host Identity Protocol May 2003 responder to protect the HI exchange, in order of preference. All implementations MUST support the 3DES [7] transform. The ESP_TRANSFORM contains the ESP modes supported by the responder, in order of preference. All implementations MUST support 3DES [7] with HMAC-SHA-1-96 [4]. The SIG is calculated over the whole HIP envelope, after setting the Initiator HIT and header checksum temporarily to zero. This allows the Responder to use precomputed R1s. The Initiator SHOULD validate this SIG. It SHOULD check that the HI received matches with the one expected, if any. 7.3 I2 - the HIP Second Initiator packet Next Header = IPPROTO_NONE or IPPROTO_ESP Type = 3 SRC HIT = Initiator's HIT DST HIT = Responder's HIT Payload Contains: Responder's SPI and LSI Birthday and Cookie Initiator's Diffie-Hellman public value HIP TRANSFORM ESP TRANSFORM The following data are encrypted using the HIP Transform Initiator's HI Signature Optional data in an ESP envelope IP(HIP(SPI_LSI, BIRTHDAY_COOKIE, DIFFIE_HELLMAN, HIP_TRANSFORM, ESP_TRANSFORM, ENCRYPTED{HOST_ID}, HIP_SIGNATURE)[,ESP(data)]) The HITs used MUST match the ones used previously. The Birthday is a reboot count used to manage state reestablishment when one peer rebooted or timed out its SA. The Cookie contains I from R1 and the computed J. The low order K bits of the SHA-1(I | ... | J) MUST match be zero. The Diffie-Hellman value is ephemeral. If precomputed, a scavenger process should clean up unused DHs. Moskowitz & Nikander Expires November 13, 2003 [Page 26] Internet-Draft Host Identity Protocol May 2003 The HIP_TRANSFORM contains the encryption used to protect the HI exchange selected by the initiator. All implementations MUST support the 3DES transform. The Initiator's HI is encrypted using the HIP_TRANSFORM. The keying material is derived from the Diffie-Hellman exchanged as defined in Section 9. The ESP_TRANSFORM contains the ESP mode selected by the initiator. All implementations MUST support 3DES [7] with HMAC-SHA-1-96 [4]. The HIP SIG is calculated over whole HIP envelope. The Responder MUST validate this SIG. It MAY use either the HI in the packet or the HI acquired by some other means. The optional ESP payload contains the first user datagram that the Initiator is sending to the Responder. The SPI is set to the value TBD, as the real SPI value to be used is not known yet by the Initiator. When the Responder processes the HIP payload, it generates the SPI and replaces the value TBD with this SPI before passing the packet to ESP processing. The Sequence Number SHOULD be set to ONE, as this is the first datagram for this SA. [XXX: Should we keep this paragraph? This seems to be rather complicated, and at least I don't quite understand all the implications. --Pekka] If the ESP transform uses the ESP header for the IV, then special considerations for the ESP header might apply. For example, if the transform requires a random value in the header, expecting it to be the SPI, the Sequence Number can be a random number, and be reset to ONE by the Responder. The Responder would pass the Initiator supplied SPI and Sequence Number to the decryption routine. 7.4 R2 - the HIP Second Responder packet Next Header = IPPROTO_NONE or IPPROTO_ESP Type = 4 SRC HIT = Responder's HIT DST HIT = Initiator's HIT Payload Contains: Initiator's LSI and SPI Signature Optional data in an ESP envelope IP(HIP(SPI_LSI, HIP_SIGNATURE),[ESP(data)]) The signature is calculated over whole HIP envelope. The Initiator Moskowitz & Nikander Expires November 13, 2003 [Page 27] Internet-Draft Host Identity Protocol May 2003 MUST validate this signature. The optional ESP payload contains the first user datagram that the Responder is sending to the Initiator. The SPI is the value that was received within I2. The Sequence Number MUST be set to ONE, as this is the first datagram for this SA. 7.5 NES - the HIP New SPI Packet The HIP New SPI Packet serves three functions. First it provides the peer system with its new SPI. Next, it optionally provides a new Diffie-Hellman key to produce new keying material. Additionally, it provides any intermediate system with the mapping of the old SPI to the new. This is important to systems like NATs [17] that use SPIs to maintain address translation state. The new SPI Packet is a HIP packet with SPI and D-H in the HIP payload. The HIP packet contains the current ESP Sequence Number and SPI to provide DoS and replay protection. Next Header = IPPROTO_NONE Type = 5 SRC HIT = Sender's HIT DST HIT = Recipients's HIT Payload Contains: Sender's ESP Sequence Number Sender's old SPI Sender's new SPI Optionally Sender's Diffie-Hellman public value Signature Optional data in an ESP envelope In reply packet only IP(HIP(NEW_SPI [,DIFFIE_HELLMAN], HIP_SIGNATURE), [ESP(data)]) During the life of an SA established by HIP, one of the hosts may need to reset the Sequence Number to one (to prevent wrapping) and rekey. The reason for rekeying might be an approaching sequence number wrap in ESP, or a local policy on use of a key. A new SPI or rekeying ends the current SAs and starts a new ones on both peers. Intermediate systems that use the SPI will have to inspect HIP packets for a HIP New SPI packet. The packet is signed for the benefit of the Intermediate systems. This packet has a potential DoS attack of a packet within the replay window and proper SPI, but a malformed signature. Implementations Moskowitz & Nikander Expires November 13, 2003 [Page 28] Internet-Draft Host Identity Protocol May 2003 MUST recognize when they are under attack and manage the attack. If it is still receiving ESP packets with increasing Sequence Numbers, the NES packets are obviously attacks and can be ignored. Since intermediate systems may need the new SPI values, the contents of this packet cannot be encrypted. Intermediate systems that use the SPI will have to inspect ALL HIP packets for a NES packet. This is a potential DoS attack against the Intermediate system, as the signature processing may be relatively expensive. A further step against attack for the Intermediate systems is to implement ESP's replay protection of windowing the sequence number. This requires the intermediate system to track ALL ESP packets to follow the Sequence Number. 7.6 BOS - the HIP Bootstrap Packet Next Header = IPPROTO_NONE Type = 7 SRC HIT = Announcer's HIT DST HIT = NULL Payload Contains: Announcer's HI Signature IP(HIP(HOST_ID, HIP_SIGNATURE)) The BOS packet may be followed by a CER packet if the HI is signed. In this case, the C-bit in the control field MUST be set. In some situations, an initiator may not be able to learn of a responder's information from DNS or another repository. Some examples of this are DHCP and NetBios servers. Thus, a packet is needed to provide information that would otherwise be gleaned from a repository. This HIP packet is either self-signed in applications like SoHo, or from a trust anchor in large private or public deployments. This packet SHOULD be broadcasted periodically. The Optional CER packets over the Announcer's HI by a higher level authority known to the Initiator is an alternative method for the Initiator to trust the Announcer's HI (over DNSSEC or PKI). [XXX: Andrew suggested possibility of piggybacked data to create an authenticated UDP.] Moskowitz & Nikander Expires November 13, 2003 [Page 29] Internet-Draft Host Identity Protocol May 2003 8. Packet processing [XXX: This section is currently in its very beginning. It needs much more text.] 8.1 R1 Management All compliant implementations MUST produce R1 packets. An R1 packet MAY be precomputed. An R1 packet MAY be reused for time Delta T. R1 information MUST not be discarded until Delta S after T. Time S is the delay needed for the last I2 to arrive back to the responder. A spoofed I1 can result in an R1 attack on a system. An R1 sender MUST have a mechanism to rate limit R1s to an address. 8.2 Processing NES packets The ESP Sequence Number and current SPI are included to provide replay protection for the receiving peer. The old SA MUST NOT be deleted until all ESP packets with a lower Sequence Number have been received and processed, or a reasonable time has elapsed (to account for lost packets). If the Sequence Number is the replay window is greater than the number in the NES packet, the NES packet MUST be ignored. If the SPI number does not match with an existing SPI number used, the NES packet must be ignored. The peer that initiates a New SPI exchange MUST include a Diffie- Hellmen key. Its peer MUST respond with a New SPI packet, an MAY include a Diffie-Hellman key if the receiving system's policy is to increase the new KEYMAT by changing its key pair. When a host receives a New SPI Packet with a Diffie-Hellman, its next ESP packet MUST use the KEYMAT generated by the new Kij. The sending host MUST expect at least a replay window worth of ESP packets using the old Kij. Out of order delivery could result in needing the old Kij after packets start arriving using the new SA's Kij. Once past the rekeying start, the sending host can drop the old SA and its Kij. The first packet sent by the receiving system MUST be a HIP New SPI packet. It MAY also include a datagram, using the new SAs. This packet supplies the new SPI for the rekeying system, which cannot send any packets until it receives this packet. If it does not receive a HIP New SPI packet within a reasonable round trip delta, it MUST assume it or the HIP Rekey packet was lost and MAY resend the HIP New SPI packet or renegotiate HIP as if in a reboot condition. The choice is a local policy decision. This packet MAY contain a Diffie-Hellman key, if the receiving system's policy is to increase the new KEYMAT by changing its key Moskowitz & Nikander Expires November 13, 2003 [Page 30] Internet-Draft Host Identity Protocol May 2003 pair. Moskowitz & Nikander Expires November 13, 2003 [Page 31] Internet-Draft Host Identity Protocol May 2003 9. HIP KEYMAT HIP keying material is derived from the Diffie-Hellman Kij produced during the base qHIP exchange. The initiator has Kij during the creation of the I2 packet, and the responder has Kij once it receives the I2 packet. This is why I2 can already contain encrypted information. The KEYMAT is derived by feeding Kij and the HITs into the following operation; the | operation denotes concatenation. KEYMAT = K1 | K2 | K3 | ... where K1 = SHA-1( Kij | sort(HIT-I | HIT-R) | 0x01 ) K2 = SHA-1( Kij | K1 | 0x02 ) K3 = SHA-1( Kij | K2 | 0x03 ) ... K255 = SHA-1( Kij | K254 | 0xff ) K256 = SHA-1( Kij | K255 | 0x00 ) etc. Sort(HIT-I | HIT-R) is defined as the numeric network byte order comparison of the HITs, with lower HIT preceding higher HIT, resulting in the concatenation of the HITs in the said order. The initial keys are drawn sequentially in the following order: HIP Initiator key HIP Responder key (currently unused) Initiator ESP key Initiator AUTH key Responder ESP key Responder AUTH key The number of bits drawn for a given algorithm is the "natural" size of the keys. For the manatory algorithms, the following sizes apply: 3DES 192 bits SHA-1 160 bits Moskowitz & Nikander Expires November 13, 2003 [Page 32] Internet-Draft Host Identity Protocol May 2003 NULL 0 bits Subsequent rekeys without Diffie-Hellman just requre drawing out more sets of ESP keys. In the situation where Kij is the result of a HIP rekey exchange with Diffie-Hellman, there is only the need from one set of ESP keys, without the HIP keys. These are then the only keys taken from the KEYMAT. Moskowitz & Nikander Expires November 13, 2003 [Page 33] Internet-Draft Host Identity Protocol May 2003 10. HIP Fragmentation Support XXX: What shall we do with fragmentation support? Fragementation makes the protocol fragile and somewhat vulnerable to state space exhausting DoS attacks. A HIP implementation MUST support IP fragmentation/reassembly. HIP packets can get large, and may encounter low MTUs along their routed path. Since HIP does not provide a mechanism to use multiple IP datagrams for a single HIP packet, support of path MTU discovery does not bring any value to HIP. HIP aware NAT systems MUST perform any IP reassembly/fragmentation. Moskowitz & Nikander Expires November 13, 2003 [Page 34] Internet-Draft Host Identity Protocol May 2003 11. ESP with HIP HIP sets up a Security Association (SA) to enable ESP in an end-to- end manner that can span addressing realms (i.e. across NATs). This is accomplished through the various informations that are exchanged within HIP. It is anticipated that since HIP is designed for host usage, that is not for gateways, that only ESP transport mode will be supported with HIP. The SA is not bound to an IP address; all internal control of the SA is by the HIT and LSI. Thus a host can easily change its address using Mobile IP, DHCP, PPP, or IPv6 readdressing and still maintain the SA. And since the transports are bound to the SA (LSI), any active transport is also maintained. So real world conditions like loss of a PPP connection and its reestablishment or a mobile cell change will not require a HIP negotiation or disruption of transport services. Since HIP does not negotiate any lifetimes, all lifetimes are local policy. The only lifetimes a HIP implementation MUST support are sequence number rollover (for replay protection), and SA timeout. An SA times out if no packets are received using that SA. The default timeout value is 15 minutes. Implementations MAY support lifetimes for the various ESP transforms. Note that HIP does not offer any service comparable with IKE's Quick Mode. A Diffie- Hellman calculation is needed for each rekeying. 11.1 Security Association Management An SA is indexed by the 2 SPIs and 2 HITs (both HITs since a system can have more than one HIT). An inactivity timer is recommended for all SAs. If the state dictates the deletion of an SA, a timer is set to allow for any late arriving packets. The SA MUST include the I that created it for replay detection. 11.2 Security Parameters Index (SPI) The SPIs in ESP provide a simple compression of the HIP data from all packets after the HIP exchange. This does require a per HIT- pair Security Association (and SPI), and a decrease of policy granularity over other Key Management Protocols like IKE. When a host rekeys, it gets a new SPI from its partner. 11.3 Supported Transforms All HIP implementations MUST support 3DES [7] and HMAC-SHA-1-96 [4]. If the Initiator does not support any of the transforms offered by the Responder in the R1 HIP packet, it MUST use 3DES and HMAC-SHA-1-96 and state so in the I2 HIP packet. Moskowitz & Nikander Expires November 13, 2003 [Page 35] Internet-Draft Host Identity Protocol May 2003 In addition to 3DES, all implementations MUST implement the ESP NULL encryption and authentication algorithms. These algoritms are provided mainly for debugging purposes, and SHOULD NOT be used in production environments. The default configuration in implementations MUST be to reject NULL encryption or authentication. 11.4 Sequence Number The Sequence Number field is MANDATORY in ESP. Anti-replay protection MUST be used in an ESP SA established with HIP. This means that each host MUST rekey before its sequence number reaches 2^32. Note that in HIP rekeying, unlike IKE rekeying, only one Diffie-Hellman key can be changed, that of the rekeying host. However, if one host rekeys, the other host SHOULD rekey as well. In some instances, a 32 bit sequence number is inadequate. In either the I2 or R2 packets, a peer MAY require that a 64 bit sequence number be used. In this case the higher 32 bits are NOT included in the ESP header, but are simply kept local to both peers. 64 bit sequence numbers must only be used for ciphers that will not be open to cryptoanalysis as a result. AES is one such cipher. 11.5 ESP usage with non-cryptographic HI [XXX: This section needs much more work, if we decide to keep this.] Even if the Host Identity is not cryptographically based, ESP MUST still be used after the HIP exchange between the two hosts. The HIP TRANSFORM in this case will be left out of the HIP exchange, and the ESP envelope will not have any authentication of encryption. The purpose of using ESP in this situation is to have the SPI (LSI) for associating the packets with the HITs, and the sequence # for replay protection. Moskowitz & Nikander Expires November 13, 2003 [Page 36] Internet-Draft Host Identity Protocol May 2003 12. HIP Policies There are a number of variables that will influence the HIP exchanges that each host must support. All HIP implementations MUST support at least 2 HIs, one to publish in DNS and one for anonymous usage. Although anonymous HIs will be rarely used as responder HIs, they will be common for initiators. Support for multiple HIs is RECOMMENDED. Many initiators would want to use a different HI for different responders. The implementations SHOULD provide for an ACL of initiator HIT to responder HIT. This ACL SHOULD also include preferred transform and local lifetimes. For HITs with HAAs, wildcarding SHOULD be supported. Thus if a Community of Interest, like Banking, gets an RAA, a single ACL could be used. A global wildcard would represent the general policy to be used. Policy selection would be from most specific to most general. The value of K used in the HIP R1 packet can also vary by policy. K should never be greater than 20, but for trusted partners it could be as low as 0. Responders would need a similar ACL, representing which hosts they accept HIP exchanges, and the preferred transform and local lifetimes. Wildcarding SHOULD be support supported for this ACL also. Moskowitz & Nikander Expires November 13, 2003 [Page 37] Internet-Draft Host Identity Protocol May 2003 13. Security Considerations HIP is designed to provide secure authentication of hosts and to provide a fast key exchange for IPsec ESP. HIP also attempts to limit the exposure of the host to various denial-of-service and man- in-the-middle attacks. In so doing, HIP itself is subject to its own DoS and MitM attacks that potentially could be more damaging to a host's ability to conduct business as usual. [XXX: Revise this based on the outcome of SPI usage.] The Security Association for ESP is indexed by the LSI-SPI, not the SPI and IP address. HIP enabled ESP is IP address independent. This might seem to make it easier for an attacker, but ESP with replay protection is already as well protected as possible, and the removal of the IP address as a check should not increase the exposure of ESP to DoS attacks. Denial-of-service attacks take advantage of the cost of start of state for a protocol on the responder compared to the 'cheapness' on the initiator. HIP makes no attempt to increase the cost of the start of state on the initiator, but makes an effort to reduce the cost to the responder. This is done by having the responder start the 3-way cookie exchange instead of the initiator, making the HIP protocol 4 packets long. In doing this, packet 2 becomes a 'stock' packet that the responder MAY use many times. The duration of use is a paranoia versus throughput concern. Using the same Diffie- Hellman values and random puzzle I has some risk. This risk needs to be balanced against a potential storm of HIP I1 packets. This shifting of the start of state cost to the initiator in creating the I2 HIP packet, presents another DoS attack. The attacker spoofs the I1 HIP packet and the responder sends out the R1 HIP packet. This could conceivably tie up the 'initiator' with evaluating the R1 HIP packet, and creating the I2 HIP packet. The defense against this attack is to simply ignore any R1 packet where a corresponding I1 or ESP data was not sent. A second form of DoS attack arrives in the I2 HIP packet. Once the attacking initiator has solved the cookie challenge, it can send packets with spoofed IP source addresses with either invalid encrypted HIP payload component or a bad HIP SIG. This would take resources in the responder's part to reach the point to discover that the I2 packet cannot be completely processed. The defense against this attack is after N bad I2 packets, the responder would discard any I2s that contain the given I. Sort of a shutdown on the attack. The attacker would have to request another R1 and use that to launch a new attack. The responder could up the value of K while under attack. On the downside, valid I2s might get dropped too. Moskowitz & Nikander Expires November 13, 2003 [Page 38] Internet-Draft Host Identity Protocol May 2003 A third form of DoS attack is emulating the restart of state after a reboot of one of the partners. To protect against such an attack, a system Birthday is included in the R1 and I2 packets to prove loss of state to a peer. The inclusion of the Birthday creates a very deterministic process for state restart. Any other action is a DoS attack. A fourth form of DoS attack is emulating the end of state. HIP has no end of state packet. It relies on a local policy timer to end state. Man-in-the-middle attacks are difficult to defend against, without third-party authentication. A skillful MitM could easily handle all parts of HIP; but HIP indirectly provides the following protection from a MitM attack. If the responder's HI is retrieved from a signed DNS zone by the initiator, the initiator can use this to validate the R1 HIP packet. Likewise, if the initiator's HI is in a secure DNS zone, the responder can retrieve it after it gets the I2 HIP packet and validate that. However, since an initiator may choose to use an anonymous HI, it knowingly risks a MitM attack. The responder may choose not to accept a HIP exchange with an anonymous initiator. New SPIs and rekeying provide another opportunity for an attacker. Replay protection is included to prevent a system from accepting an old new SPI packet. There is still the opening for an attacker to produce a packet with exactly the right Sequence Number and old SPI with a malformed signature, consuming considerable computing resources. All implementations must design to mitigate this attack. If ESP protected datagrams are still being received, there is an obvious attack. If the peer is quiet, it is easier for an attacker to launch this sort of attack, but again, the system should be able to recognize a regular influx of malformed signatures and take some action. There is a similar attack centered on the readdress packet. Similar defense mechanisms are appropriate here. Since not all hosts will ever support HIP, ICMP 'Destination Protocol Unreachable' are to be expected and present a DoS attack. Against an Initiator, the attack would look like the responder does not support HIP, but shortly after receiving the ICMP message, the initiator would receive a valid R1 HIP packet. Thus to protect against this attack, an initiator should not react to an ICMP message until a reasonable delta time to get the real responder's R1 HIP packet. A similar attack against the responder is more involved. First an ICMP message is expected if the I1 was a DoS attack and the real owner of Moskowitz & Nikander Expires November 13, 2003 [Page 39] Internet-Draft Host Identity Protocol May 2003 the spoofed IP address does not support HIP. The responder SHOULD NOT act on this ICMP message to remove the minimal state from the R1 HIP packet (if it has one), but wait for either a valid I2 HIP packet or the natural timeout of the R1 HIP packet. This is to allow for a sophisticated attacker that is trying to break up the HIP exchange. Likewise, the initiator should ignore any ICMP message while waiting for an R2 HIP packet, deleting state only after a natural timeout. [XXX: This does not exist any more, does it?] Another MitM attack is simulating a Responder's rejection of a HIP initiation. This is a simple ICMP Host Unreachable, Administratively Prohibited message. A HIP packet was not used because it would either have to have unique content, and thus difficult to generate, resulting in yet another DoS attack, or just as spoofable as the ICMP message. The defense against this MitM attack is for the responder to wait a reasonable time period to get a valid R1 HIP packet. If one does not come, then the Initiator has to assume that the ICMP message is valid. Since this is the only point in the HIP exchange where this ICMP message is appropriate, it can be ignored at any other point in the exchange. Moskowitz & Nikander Expires November 13, 2003 [Page 40] Internet-Draft Host Identity Protocol May 2003 14. IANA Considerations IANA has assigned IP Protocol number TBD to HIP. [XXX: Revise if we use IPSECKEY.] A new KEY RR protocol of XX is assigned to HIP and an algorithm of XX is assigned to HIT128. IANA will assign a SPI of TBD for use in the ESP header of the optional I2 HIP packet. Moskowitz & Nikander Expires November 13, 2003 [Page 41] Internet-Draft Host Identity Protocol May 2003 15. ICANN Considerations ICANN will need to set up the HIT.int zone and accredit the registered assigning authorities (RAA) for HAA field. With 21 bits, ICANN can allocate just over 2M registries. Moskowitz & Nikander Expires November 13, 2003 [Page 42] Internet-Draft Host Identity Protocol May 2003 16. Acknowledgments The drive to create HIP came to being after attending the MALLOC meeting at IETF 43. Baiju Patel and Hilarie Orman really gave the original author, Bob Moskowitz, the assist to get HIP beyond 5 paragraphs of ideas. It has matured considerably since the early drafts thanks to extensive input from IETFers. Most importantly, its design goals are articulated and are different from other efforts in this direction. Particular mention goes to the members of the NameSpace Research Group of the IRTF. Noel Chiappa provided the framework for LSIs and Kieth Moore the impetuous to provide resolvability. Steve Deering provided encouragement to keep working, as a solid proposal can act as a proof of ideas for a research group. Many others contributed; extensive security tips were provided by Steve Bellovin. Rob Austein kept the DNS parts on track. Paul Kocher taught the original authors, Bob Moskowitz, how to make the cookie exchange expensive for the Initiator to respond, but easy for the Responder to validate. Bill Sommerfeld supplied the Birthday concept to simplify reboot management. Rodney Thayer and Hugh Daniels provide extensive feedback. In the early times of this draft, John Gilmore kept Bob Moskowitz challenged to provide something of value. During the later stages of this document, when the editing baton was transfered to Pekka Nikander, the input from the early implementors were invaluable. Without having actual implementations, this document would not be on the level it is now. Moskowitz & Nikander Expires November 13, 2003 [Page 43] Internet-Draft Host Identity Protocol May 2003 References [1] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [3] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 2373, July 1998. [4] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within ESP and AH", RFC 2404, November 1998. [5] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998. [6] Maughan, D., Schneider, M. and M. Schertler, "Internet Security Association and Key Management Protocol (ISAKMP)", RFC 2408, November 1998. [7] Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher Algorithms", RFC 2451, November 1998. [8] Eastlake, D., "Domain Name System Security Extensions", RFC 2535, March 1999. [9] Eastlake, D., "DSA KEYs and SIGs in the Domain Name System (DNS)", RFC 2536, March 1999. [10] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, August 1999. [11] Crawford, M., "Binary Labels in the Domain Name System", RFC 2673, August 1999. [12] Bush, R., Durand, A., Fink, B., Gudmundsson, O. and T. Hain, "Representing Internet Protocol version 6 (IPv6) Addresses in the Domain Name System (DNS)", RFC 3363, August 2002. [13] Jokela, P., "Optimized Packet Structure for HIP", draft-jokela-hip-packets-01 (work in progress), November 2002. [14] Richardson, M., "A method for storing IPsec keying material in DNS", draft-ietf-ipseckey-rr-01 (work in progress), April 2003. [15] NIST, "FIPS PUB 180-1: Secure Hash Standard", April 1995. Moskowitz & Nikander Expires November 13, 2003 [Page 44] Internet-Draft Host Identity Protocol May 2003 [16] Moskowitz, R. and P. Nikander, "Host Identity Protocol Architecture", draft-moskowitz-hip-arch-03 (work in progress), May 2003. [17] Moskowitz, R., "Host Identity Payload Implementation", draft-moskowitz-hip-impl-02 (work in progress), January 2001. Authors' Addresses Robert Moskowitz ICSAlabs, a Division of TruSecure Corporation 1000 Bent Creek Blvd, Suite 200 Mechanicsburg, PA USA EMail: rgm@icsalabs.com Pekka Nikander Ericsson Research Nomadic Lab JORVAS FIN-02420 FINLAND Phone: +358 9 299 1 EMail: pekka.nikander@nomadiclab.com Moskowitz & Nikander Expires November 13, 2003 [Page 45] Internet-Draft Host Identity Protocol May 2003 Appendix A. Backwards compatibility API issues Tom floated again the thought that that the LSI could be completely local and does not need to be exchanged, as long as each host can determine from local information what value for LSI that the peer will use in its checksum computations. Applications continue to use IP addresses in socket calls, and kernel does whatever NATting (including application NATting) is required. It was pointed out that this approach was going to be prone to some kinds of data flows escaping the HIP protection, unless the local housekeeping in an implementation was especially good. Example: FTP opens control connection to IP address. One or both parties move. FTP later opens data connection to the old IP address. Kernel must identify that the application really means to connect to the host that was previously at that IP address-- but obviously if the old address is reused by another host, this becomes difficult. Related to this, the discussion also opened up the question of DNS resolution. Should the HIT/LSI be returned to applications as a (spoofed) address in the resolution process, allowing apps to use the socket API with HIT or LSI values instead of an IP address? While this seems to be the original intention of LSIs, there are a couple of difficulties especially in the IPv4 case: how does kernel know whether value being passed in a socket call is an IP address or an LSI? The fact that a name resolver library gave an application an LSI is no guarantee that the application will use that information in its socket call. It may also have cached some IP address from before or received an IP address as side information. This difficulty may be relieved if LSIs are constrained to some well- known private subnet space. this may confuse legacy applications that assume that what is being passed to them is an IP address. Good examples of this are diagnostic tools such as dig and ping. what does kernel do with an LSI that it cannot map to an address based on information that it has locally cached? It seems that some modification to the resolver library (to explicitly convey HIP information rather than spoofing IP addresses), as well as modifications to socket API to explicitly let the kernel know that the application is HIP aware, are the cleanest long-term solution, but what to do about legacy applications??-- still an open issue. The HUT team has been considering these problems. Moskowitz & Nikander Expires November 13, 2003 [Page 46] Internet-Draft Host Identity Protocol May 2003 Appendix B. Probabilities of HIT collisions The birthday paradox sets a bound for the expectation of collisions. It is based on the square root of the number of values. A 64-bit hash, then, would put the chances of a collision at 50-50 with 2^32 hosts (4 billion). A 1% chance of collision would occur in a population of 640M and a .001% collision chance in a 20M population. A 128 bit hash will have the same .001% collision chance in a 9x10^16 population. Moskowitz & Nikander Expires November 13, 2003 [Page 47] Internet-Draft Host Identity Protocol May 2003 Appendix C. Probabilities in the cookie calculation A question: Is it quaranteed that the Initiator is able to solve the puzzle in this way when the K value is large? No, it is not guaranteed. But it is not guaranteed even in the old mechanism, since the Initiator may start far away from J and arrive to J after far too many steps. If we wanted to make sure that the Initiator finds a value, we would need to give some hint of a suitable J, and I don't think we want to do that. In general, if we model the hash function with a random function, the probability that one iteration gives are result with K zero bits is 2^-K. Thus, the probablity that one iteration does *not* give K zero bits is (1 - 2^-K). Consequently, the probablity that 2^K iterations does not give K zero bits is (1 - 2^-K)^(2^K). Since my calculus starts to be rusty, I made a small experiment and found out that lim (1 - 2^-k)^(2^k) = 0.36788 k->inf lim (1 - 2^-k)^(2^(k+1)) = 0.13534 k->inf lim (1 - 2^-k)^(2^(k+2)) = 0.01832 k->inf lim (1 - 2^-k)^(2^(k+3)) = 0.000335 k->inf Thus, if hash functions were random functions, we would need about 2^(K+3) iterations to make sure that the probability of a failure is less than 1% (actually less than 0.04%). Now, since my perhaps flawed understanding of hash functions is that they are "flatter" than random functions, 2^(K+3) is probably overkill. OTOH, the currently suggested 2^K is clearly too little. I'll change the draft to read 2^(K+2). Moskowitz & Nikander Expires November 13, 2003 [Page 48] Internet-Draft Host Identity Protocol May 2003 Appendix D. Using responder cookies As mentioned in Section 5.2.1, the responder may delay state creation and still reject most spoofed I2s by using a number of pre-calculated R1s and a local selection function. This appendix defines one possible implementation in detail. The purpose of this appendix is to give the implementators an idea on how to implement the mechanism. The method described in this appendix SHOULD NOT be used in any real implementation. If the implementation is based on this appendix, it SHOULD contain some local modification that makes an attacker's task harder. The basic idea is to create a cheap, varying local mapping function f: f( IP-I, IP-R, HIT-I, HIT-R ) -> cookie-index That is, given the Initiators and Responders IP addresses and HITs, the function returns an index to a cookie. When processing an I1, the cookie is embedded in an pre-computed R1, and the Responder simply sends that particular R1 to the Initiator. When processing an I2, the cookie may still be embedded in the R1, or the R1 may be depracated (and replaced with a new one), but the cookie is still there. If the received cookie does not match with the R1 or saved cookie, the I2 is simply dropped. That prevents the Initiator from generating spoofed I2s with a probability that depends on the number of pre-computed R1s. As a concrete example, let us assume that the Responder has an array of R1s. Each slot in the array contains a timestamp, an R1, and an old cookie that was sent in the previous R1 that occupied that particular slot. The Responder replaces one R1 in the array every few minutes, thereby replacing all the R1s gradually. To create a varying mapping function, the Responder generates a random number every few minutes. The octets in the IP addresses and HITs are XORed together, and finally the result is XORed with the random number. Using pseudo-code, the function looks like the following. Pre-computation: r1 := random number Index computation: index := r1 XOR hit_r[0] XOR hit_r[1] XOR ... XOR hit_r[15] index := index XOR hit_i[0] XOR hit_i[1] XOR ... XOR hit_i[15] index := index XOR ip_r[0] XOR ip_r[1] XOR ... XOR ip_r[15] index := index XOR ip_i[0] XOR ip_i[1] XOR ... XOR ip_i[15] Moskowitz & Nikander Expires November 13, 2003 [Page 49] Internet-Draft Host Identity Protocol May 2003 The index gives the slot used in the array. It is possible that an Initator receives an I1, and while it is computing I2, the Responder deprecates an R1 and/or chooses a new random number for the mapping function. Therefore the Responder must remember the cookies used in deprecated R1s and the previous random number. To check an received I2, the Responder can use a simple algorithm, expressed in pseudo-code as follows. If I2.hit_r does not match my_hits, drop the packet. index := compute_index(current_random_number, I2) If current_cookie[index] == I2.cookie, go to cookie check. If previous_cookei[index] == I2.cookie, go to cookie check. index := compute_index(previous_random_number, I2) If current_cookie[index] == I2.cookie, go to cookie check. If previous_cookei[index] == I2.cookie, go to cookie check. Drop packet. cookie_check: V := Ltrunc( SHA-1( I2.I, I2.hit_i, I2.hit_r, I2.J ), K ) if V != 0, drop the packet. Whenever the Responder receives an I2 that fails on the index check, it can simply drop the packet on the floor and forget about it. New I2s with the same or other spoofed parameters will get dropped with a reasonable probability and minimal effort. If a Responder receives an I2 that passes the index check but fails on the puzzle check, it should create a state indicating this. After two or three failures the Responder should cease checking the puzzle but drop the packets directly. This saves the Responder from the SHA-1 calculations. Such block should not last long, however, or there would be a danger that a legitimite Initiator could be blocked from getting connections. A key for the success of the defined scheme is that the mapping function must be considerably cheaper than computing SHA-1. It also must detect any changes in the IP addresses, and preferably most changes in the HITs. Checking the HITs is not that essential, though, since HITs are included in the cookie computation, too. The effectivity of the method can be varied by varying the size of the array containing pre-computed R1s. If the array is large, the Moskowitz & Nikander Expires November 13, 2003 [Page 50] Internet-Draft Host Identity Protocol May 2003 probability that an I2 with a spoofed IP address or HIT happens to map to the same slot is fairly slow. However, a large array means that each R1 has a fairly long life time, thereby allowing an attacker to utilize one solved puzzle for a longer time. Moskowitz & Nikander Expires November 13, 2003 [Page 51] Internet-Draft Host Identity Protocol May 2003 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. 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