Internet Engineering Task Force G. Montenegro INTERNET DRAFT C. Castelluccia April, 2001 Statistically Unique and Cryptographically Verifiable Identifiers and Addresses draft-montenegro-sucv-00.txt Status of This Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. Comments should be submitted to the HIP and Mobile IP mailing lists at hipsec@mail.freeswan.org and mobile-ip@sunroof.eng.sun.com, respectively. Distribution of this memo is unlimited. This document is an Internet-Draft. 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. Abstract This document addresses the identifier ownership problem. It does so by using characteristics of Statistic Uniqueness and Cryptographic Verifiability (SUCV) of certain entities which this document calls SUCV Identifiers (SUCV ID's). This note also proposes using these SUCV characteristics in related entities called SUCV Addresses in order to severely limit certain classes of denial of service attacks and hijacking attacks. SUCV addresses are particularly applicable to solve the 'address ownership' problem that severely undermines confidence in mechanisms like Binding Updates in Mobile IP for IPv6. Montenegro, Castelluccia Expires October, 2001 [Page 1] INTERNET DRAFT SUCV Addresses April 2001 Table of Contents 1.0 Introduction .................................................. 3 2.0 Related Issues ................................................ 3 2.1 Address Ownership .......................................... 3 2.2 Purpose Built Keys [PBK] ................................... 4 3.0 Overview of the Proposal ...................................... 4 4.0 Statistical Uniqueness and Cryptographic Verifiability (SUCV) ............................................................ 5 4.1 SUCV ID's .................................................. 6 4.2 SUCV ID's versus Addresses ................................. 7 4.3 SUCV Addresses ............................................. 8 4.4 Hash ID Size Considerations ................................ 8 5.0 HIP IPv6 Accomodation Mode .................................... 9 6.0 Use of SUCV Addresses for Mobile IPv6 ......................... 11 6.1 Protocol Overview .......................................... 12 7.0 Security Considerations ....................................... 14 8.0 Conclusions ................................................... 14 References ........................................................ 14 Authors' addresses ................................................ 15 Montenegro, Castelluccia Expires October, 2001 [Page 2] INTERNET DRAFT SUCV Addresses April 2001 1.0 Introduction This document addresses the identifier ownership problem [ADDROWN] by using characteristics of Statistic Uniqueness and Cryptographic Verifiability (SUCV) of certain entities which this document calls SUCV Identifiers (SUCV ID's). This note also proposes using these SUCV characteristics in related entities called SUCV Addresses in order to severely limit certain classes of denial of service attacks and hijacking attacks. SUCV addresses can solve the 'address ownership' problem that severely undermines confidence in mechanisms like Binding Updates in Mobile IP for IPv6. 2.0 Related Issues 2.1 Address Ownership [ADDROWN] argues that there is a fundamental problem in handling operations like Binding Updates (BU's) in Mobile IP for IPv6 [MIPv6], source routing, etc) that allows hosts to modify how other hosts route packets to a certain destination. The problem is that these operations can be misused by rogue nodes to redirect traffic away from its legitimate destination. Authentication does not solve this problem. Even if a node unequivocally identifies itself, this has no bearing on its rights to modify how packets to any given address are routed. This is true even if its packets currently seem to emanate from the address in question. This last point is obvious if one considers DHCP leased addresses. It is imperative not to allow any node to redirect traffic for a DHCP address for which it held a valid lease previously. This would allow it to hijack traffic meant for the current valid user of the address in question. Hence, protection against hijacking of valid addresses requires cryptographic authorization for operations that modify routing (BU's, source routing, etc). One way to show authorization is by showing that the requesting node owns the address for which routing information is being altered. Quoting [ADDROWN]: Currently there exists no specified mechanism for proving address ownership in Internet-wide scale. Montenegro, Castelluccia Expires October, 2001 [Page 3] INTERNET DRAFT SUCV Addresses April 2001 2.2 Purpose Built Keys [PBK] Purpose built keys [PBK] have been proposed as a foundation for solving scaling and cost concerns associated with some uses of BU's [MIPv6]. It has also been suggested that such keys [PBK] can solve the authorization problem that is at the heart of the address ownership issue. The proposal is succintly to: 1. Generate a temporary public/private pair 2. Generate an EID = hash(public component) a. the initiator sends EID to responder along with initial protocol exchanges. b. the initiator sends public component to responder along with subsequent exchanges. 3. The initiator sends the BU and its EID signed with its private key to the responder. Notice that the exchange at step 2 must be secure in order to avoid intruder-in-the-middle attacks, but it is an improvement over cookies. Several problems linger: 1. This does NOT really address the address ownership problem of any publicly routable addresses 2. It is not specified how the EID and public component of the PK are sent by the initiator to the responder 3. Preventing or limiting hijacking and intruder-in-the-middle attacks depend on this sequence if not clearly specified. By the time the issues are worked out, [PBK] will look very similar to an existing proposal [HIPARCH]. Because of this, it may be simpler to base a solution on HIP. 3.0 Overview of the Proposal We assume that we have a network in which the nodes inherently distrust each other, and in which a global or centralized PKI (Public Key Infrastructure) or KDC (Key Distribution Center) is Montenegro, Castelluccia Expires October, 2001 [Page 4] INTERNET DRAFT SUCV Addresses April 2001 not available. The goal is to arrive at some fundamental assumptions about trust on top of which one can build some useful peer-to-peer communication using opportunistic security. But in such a network, is there a default rule we can follow safely? We posit this is it: Default Trust Rule: Redirect operations are allowed only with addresses which are bound (via a cryptographically sound binding) to the requesting entity. The above rule (to be refined later) constitutes the only rule that operates by default, allowing any other more dangerous operation only if authorized by strong cryptographic mechanisms. Furthermore, we suggest it be based on HIP for the following reasons: - HIP provides the types of identifiers required by the above rule. - The HIP protocol handshake [HIPPROT] is a foundation for a very solid sequence resistant to denial-of-service attacks. Nevertheless, this document proposes a specific use or 'profile' of HIP as applied to environments without DNS (particularly secure DNS), a centralized PKI, or a Key Distribution Center. Rather, we favor the opportunistic mode in HIP [HIPIMPL], and adapt and apply it to Mobile IPv6 (as detailed below). In order not to hinder deployment, a primary consideration has been to minimize the modifications of existing protocols and network support. Furthermore, at the end of this document we note that there are other areas that can benefit from similar adaptations of HIP's opportunistic mode. Their details, however, are left for further exploration. 4.0 Statistical Uniqueness and Cryptographic Verifiability (SUCV) In the absence of a third party, how does a principal prove ownership of its identity to a peer? Montenegro, Castelluccia Expires October, 2001 [Page 5] INTERNET DRAFT SUCV Addresses April 2001 Notice that usual owner verification relies on a third party to provide this function. In this proposal, the principal self-generates a private/public key pair. It uses the public key as its identity and proves its ownership by signing it with its private key. The recipient verifies the signature, and, consequently, the ownership of the identity. 4.1 SUCV ID's It is much more practical for protocols to use fixed length identifiers (representations of identities). Because of this, we do not use the public key itself as the identifier, but its hash (as in HIP). These identifiers have a strong cryptographic binding with their public components (of their private-public keys). This is exactly the purpose that certificates have. Let's call them Statistically Unique Cryptographically Verifiable ID's, or SUCV ID's. Because of this, once a CN obtains information about one of these identifiers, it has a strong cryptographic assurance about which entity created it. Not only that, it knows that this identifier is owned and used exclusively by only one node in the universe: its peer in the current exchange. Thus, it is safe to allow that peer to effect changes (via BU's, for example) on how this address or identifier is routed to. Notice that with publically routable addresses, this assurance is much harder to arrive at, given that the address may be 'loaned' to (not owned by) the peer in question, perhaps thanks to the good service of a DHCP server. Despite the fact that currently there is no way to prove address ownership in the Internet, these considerations lead to the following fundamental assumption: Default Trust Rule Redirect operations are only allowed to affect routing for entities which have the SUCV property. The above constitutes perhaps the only rule that operates by default, allowing any other more dangerous operation only if authorized by strong cryptographic mechanisms Montenegro, Castelluccia Expires October, 2001 [Page 6] INTERNET DRAFT SUCV Addresses April 2001 4.2 SUCV ID's versus Addresses What should one use: pure identifiers with no routing significance or addresses? For example, in the Mobile IPv6 case, a node starts using its home address, and issues binding updates as it moves. With a home address that is not an SUCV ID it is never evident to the CN that whoever was sitting on this address actually owns it. At the very most, the mobile node can prove that at some point it was sitting on a certain address, and later it can prove it is still the same node, but now sitting on another address. But it cannot prove ownership. Ignoring this subtle distinction leads to DOS and hijacking attacks. Problems may also arise because of honest mistakes in configuration. For example, say node A was originally sitting on CoA, and then moved on to CoA'. Suppose it then asks its CN's to redirect traffic to CoA'. In the meanwhile, the original CoA may have been assigned to another node B, perhaps by the DHCP server that rightfully 'owns' that address. The result is that now traffic meant for B has been redirected to A at its new location. Relying on ingress filtering may limit the risk, but essentially, the only way for a node to prove ownership of an identifier (in the absence of any other centralized or global mechanism), is for it to prove that it created this statistically unique series of bits. The intent is to use an identifier instead of an address. Using identifiers that satisfy the SUCV conditions outlined above, it is possible to gain the tremendous advantage that other nodes can safely believe the node when it claims ownership of that identifier. Hence they can safely heed its redirects when it says that it is now available at some different CoA (and later at another). Furthermore, you do not rely on ingress filtering to limit exposure. A major advantage to using an address is that the data traffic need not carry extra information in the packet to guarantee proper delivery by routing. Because of this it is advantageous to create addresses that are both routable and satisfy the SUCV property: SUCV addresses. Montenegro, Castelluccia Expires October, 2001 [Page 7] INTERNET DRAFT SUCV Addresses April 2001 Another advantage to using an SUCV address is that this address can be registered in the DNS and the host does not have to worry about communicating securely this identifier to its correspondent node. The correspondent node will just get it from the DNS. 4.3 SUCV Addresses In IPv6, addresses that satisfy the SUCV property may be obtained as follows: - use the top 64 bits from your routing prefix (as in rfc3041) - define the bottom 64 bits as an SUCV ID (called the HID or 'hash' ID). Use these 64 bits instead of the 'interface identifier' in IPv6 [IPV6ADDR]. The resultant 128 bit field is an identifier that is also routable, avoiding the need for taking extra space in the packet by sending routing options. Notice that even after moving, it is possible to reuse the 'HID' portion of the address with the new network prefix at the new location. Thus it is possible to reuse the HID with different CoA's. Nevertheless, by snooping on binding updates, it is possible for an attacker to learn the original network prefix used by the home address. This tells an eavesdropper where this home address began to be used, and to which network it belongs, potentially important information. On the other hand, if you use a 'pure' SUCV ID (without any routing significance), then your packets will always need extra information somewhere to assure they are routed properly. Eavesdroppers may still know where that identity is at any particular point in time. But at least they will not know where (under what prefix) that identity began to be used. For further details on SUCV address please refer to section 5.0. 4.4 Hash ID Size Considerations In SUCV addresses, one of the lower 64 bits is reserved as the local/universal bit (the 'u' bit), so only 63 bits are actually usable as a hash. Suppose the hash function produces an n-bit long output. If we Montenegro, Castelluccia Expires October, 2001 [Page 8] INTERNET DRAFT SUCV Addresses April 2001 are trying to find some input which will produce some target output value y, then since each output is equally likely we expect to have to try 2^(n-1) possible input values on average. If we are trying to find a collision, then by the birthday paradox we would expect that after trying 1.2*2^n/2 possible input values we would a 50% probability of collision [BIRTH]. So if n=63, you need 1.2*2^31.5 i.e. 3.64*10^9 tries on average before having a collision. This is acceptable especially if you consider that this collision is actually harmfull only if the 2 hosts (that collide) are in the same site (i.e. they have the same network prefix), and have the same correspondent nodes. This is very unlikely. Additionally, if the collision is not deliberate the duplicate address detection (DAD) will detect it. If an attacker wishes to impersonate a given SUCV address, it must attempt 2^62 (i.e. approximately 4.8*10^18) tries to find a public key that hashes to this SUCV address. If the attacker can do 1 million hashes per second it will need 142,235 years. If the attacker can hash 1 billion hashes per second it will still need 142 years If we consider that the SUCV Addresses are renewed every 24 hours (as suggested in RFC3041), an attacker would then be able to hash 5.3*.10^13 hashes/second in order to be able to find a public key that hashes to the SUCV HID of a given host... Note that the previous analysis only considers the cost of computing the hash of the public key. Prior to this step, an attacker must also generate a valid (public, private) key pair. This is also a very computionally expensive operation. As a conclusion SUCV addresses as used in this document provide more than enough security. 5.0 HIP IPv6 Accomodation Mode Using these ID's or addresses depends on also communicating safely the SUCV portion, and this, in turn is dependent on the packet sequencing, etc. These concerns are not addresses at all in the PBK draft. On the other hand, HIP includes mechanisms and detailed considerations in this regard (protection against replay, DOS and MITM attacks). This is why this note proposes that a solution be drafted based heavily on HIP [HIPARCH, HIPPROT, HIPIMPL]. Montenegro, Castelluccia Expires October, 2001 [Page 9] INTERNET DRAFT SUCV Addresses April 2001 To obtain an IPv6 SUCV address, we define a HIT-64 format and use its lower 64 bits to form the relevant IPv6 addresses (this constitutes HIP's IPv6 accomodation mode). So, for example, the HIT-64 is used to form global aggregatable addresses which start thus [IPV6ADDR]: Aggregatable Global Unicast Addresses 001 As well as to derive link-local and site-local addresses which start thus: Link-Local Unicast Addresses 1111 1110 10 Site-Local Unicast Addresses 1111 1110 11 The HIT-64 format (section 5.2 of [HIPARCH]) is defined as follows: - first 64 bits: - Bit 0 is one - HAA field (next 63 bits) formed as follows: - RAA: 23 bits of registered assigning authority assigned by ICANN (suggested value: 0) - RI: 40 bits of registered identity assigned sequentially by the RAA. (suggested value: 0) - last 64 bits: - derived from a hash of the host identity - used as the interface identifier in IPv6 addresses The IPv6 accomodation mode consists in using a HIT-64 to form an IPv6 address. A HIT-64 derived IPv6 Aggregatable Global Unicast Address (RFC2374) is formed as follows: - top 64 bits: as per the valid prefix at the link the device is on - bottom 64 bits: interface identifier obtained by taking the last 64 bits of the above HIT-64, and setting bit 6 (the left-most bit is numbered 0) to one. This creates an interface identifier with universal significance. From this IPv6 address, other non-global scope addresses are derived. For example, a node uses the bottom 64 bits to form the site-local and link-local addresses on the same prefix (link) as the aggregatable global unicast address (alternatively, if a non-global address is formed first, it is used to form the others). Furthermor, if this address is used in conjuction with Mobile IP for IPv6 [MIPV6], the Home Agent uses Montenegro, Castelluccia Expires October, 2001 [Page 10] INTERNET DRAFT SUCV Addresses April 2001 the Prefix Length information within a "home registration" binding update to form the corresponding link-local and site-local addresses for the Mobile Node, and defend them for purposes of Duplicate Address Detection. Any of the addresses formed as described above constitute SUCV addresses. 6.0 Use of SUCV Addresses for Mobile IPv6 In Mobile IPv6, a mobile host obtains a new address, a CoA, each time it moves. It then registers the binding between its home address and its new CoA with its home agent and correspond nodes. Correspondent nodes in posession of such a binding can send packets to the mobile node directly at its current CoA. Instead, sending packets to the mobile node's home address implies sub-optimal routing as they first proceed to the mobile node's home link, where they are intercepted by the home agent and forwarded to the mobile node's CoA. However, the correspond nodes MUST get the assurance the home address actually belongs to the mobile node. Otherwise, an attacker could send a binding update with a victim's home address, thus redirecting all the victim's packets. Additionally, the correspond nodes SHOULD get the assurance that the CoA actually belongs to the mobile node. Otherwise, any host could use the address of another victim as its CoA. The packets that were initially addressed to the first victim will then be sent to the victim. Depending on how much traffic this implies, this could be used as a denial-of-service attack. These attacks can be avoided if, in order to accept and process a binding update, a correspond host requires a mobile node to prove ownership of its home address and its CoA. If ownership is proven, the correspond node has the assurance that the mobile node is not hijacking some other node's address, and that it is not directing packets at some other node's one's address. The solution that we propose is that a mobile node uses the method describes previously to configure its Home Address and its CoA (the same HID and public/private key pair is used for the home address and the CoA). By verifying the signature and the HID, the correspond host has the assurance that the mobile host is not using some other node's home address and CoA. Montenegro, Castelluccia Expires October, 2001 [Page 11] INTERNET DRAFT SUCV Addresses April 2001 As described in [MIPPRIV], the use of SUCV Identifier for Mobile IPv6 is useful when a mobile node wishes to hide its home address. Indeed the home address can reveal a lot of information about a mobile node. [MIPPRIV] proposes to use a random Temporary Mobile Identifier (TMI) in place of the home address. By using a SUCV ID as a TMI, a mobile node will be able to prove ownership of the TMI and avoid hijacking attacks. 6.1 Protocol Overview The following protocol sequence is based on [HIPPROT]. However, it has been modified for Mobile IPv6 based on the following considerations: - the goal is to secure binding updates sent by a mobile node to an arbitrary correspondent node - the protocol should not rely on a third party (i.e. a home agent, mobility anchor point, global PKI, central key distribution center, etc) - HIP is used for 'opportunistic security' so there is no reliance on DNS - not all nodes need to use SUCV addresses, only those that wish their binding updates to be heeded (mobile nodes) - not all nodes need to verify the validity of SUCV addresses, only those that wish to safely heed binding updates in order to populate its binding cache The proposed protocol that a mobile host uses to send a BU to its CN is the following: msg1- The MN sends a BU HELLO message (just to initiate the exchange) to its correspond node. This message contains a Nonce, N1. msg2- The CN replies with a message that contains the following: N1, HIP Cookie request, SPI, Diffie-Hellman value, ESP transform (list of supported ESP modes). In order to defend against msg1 storms, a host might use the same DH value for a period of time. A HIP cookie request contains a random number I, the hash of I concatenated with a random value J, and K, the number of bits that must match as per [HIPPROT]. Montenegro, Castelluccia Expires October, 2001 [Page 12] INTERNET DRAFT SUCV Addresses April 2001 When the MN receives msg2, it verifies that the nonce N1 is the same than the one that was sent in the msg1. It then computes the HIP Cookie reply by finding J and replies with msg3. msg3- The MN replies with a message that contains the following: HIP Cookie reply, Public key, Diffie-Hellman value, ESP transform (the selected ESP modes) and BU. This message MUST be signed by the MN with its private key. Note that the home address contained in the BU is either a SUCV Address or a SUCV Identifier. The CoA is either a SUCV Address or a regular address. By using a CoA SUCV address, a CN has the assurance the the CoA belongs to the MN and has not been stolen. When the CN receives the msg3, it can verify the ownership of the Home and CoA addresses and authenticate the BU because it is signed. The MN and CN can then derive a session key (using the ephemeral D-H value), and use it in conjunction with IPSec to authenticate other subsequent BU (if any) as it is done in current MIPv6. As long as the MN uses the same HID interface identifier for its CoA, it does not have to prove the CoA ownership and IPSec authentication mechanism is fine. If for any reason the MN configures its CoA with a new interface identifier, it MUST restart the whole protocol (i.e. msg1, msg2, msg3). This proposal does not require any prerequisite between the MN and the CN. By using a Home Address SUCV, that is generated by hashing a public key, and signing message 2 with the corresponding private key a MN can prove the ownership of its Home Address. Because our proposal is heavily based on HIP, it is resistant to denial-of service attacks. Because our proposal is based on SUCV Home Address, it is resistant to Man-in the Middle attacks. An attacker won't be able to redirect the traffic destined to a particular SUCV Home Address unless it can find a (public, private) key pair such that the hash of the public component is equal to the least significant 64 bits in the SUCV Home Address. This is computationally infeasable (see section 4.4). Montenegro, Castelluccia Expires October, 2001 [Page 13] INTERNET DRAFT SUCV Addresses April 2001 7.0 Security Considerations The technique introduced in this document is meant to increase the level of security in the Internet. This document explains the concept of statistical uniqueness and cryptographic verifiability (SUCV), specially as it applies to IPv6 addresses in the form of SUCV addresses. The SUCV characteristics are used to prove address ownership, thus preventing a class of attacks which exploit this fault in many types of commands. In particular, commands which alter how an address is treated by peers or by the routing infrastructure can be used to launch denial of service attacks or hijacking attacks. Proving address ownership eliminates these attacks. However, given that this technique is meant to be used primarily in the absence of global infrastructures, the possibility of man in the middle attacks does remain. Nevertheless, since the protocol used here is based on HIP, these attacks are limited by the use of cookies and client puzzles. 8.0 Conclusions The present document focuses on the use of the SUCV property to enhance the security of exchanges between an arbitrary pair of peers in the absence of any third party. In particular, we propose that SUCV addresses be used to solve the issue of securing binding updates in Mobile IPv6. Recent micro-mobility management protocols (such as HAWAII or Cellular IP) propose to use specialized path setup schemes which install host-based forwarding entries in specific routers to support intra-domain micro-mobility. In order to avoid trafic redirection, routers need to verify the ownership of an address used by a mobile host before adding an entry for that particular mobile host in its routing table. SUCV addresses or identifiers also can be very useful for that purpose. References [ADDROWN] Pekka Nikander, "An Address Ownership Problem in IPv6", draft-nikander-ipng-address-ownership-00.txt, February 2001. [BIRTH] http://www.rsasecurity.com/rsalabs/faq/2-4-6.html [HIPARCH] Bob Moskowitz, "HIP Architecture," draft-ietf-moskowitz-hip-arch-02.txt Montenegro, Castelluccia Expires October, 2001 [Page 14] INTERNET DRAFT SUCV Addresses April 2001 [HIPIMPL] Bob Moskowitz, "HIP Implementation," draft-moskowitz-hip-impl-01.txt [HIPPROT] Bob Moskowitz, "Host Identity Payload and Protocol," draft-moskowitz-hip-03.txt [IPV6ADDR] Hinden, Deering, "IPv6 Addressing Architecture," draft-ietf-ipngwg-addr-arch-v3-05.txt [MIPPRIV] Castelluccia, Dupont, "A Simple Privacy Extension for Mobile IPv6," draft-castelluccia-mobileip-privacy-00.txt, February 2001. [MIPV6] C. Perkins, "Mobile IP for IPv6,", draft-ietf-mobileip-ipv6-13.txt [PBK] Bradner, Mankin, Schiller, "Purpose Built Keys," draft-bradner-pbk-frame-00.txt [RFC3041] T. Narten, R. Draves "Privacy Extensions for Stateless Address Autoconfiguration in IPv6," RFC 3041. [WeakMD5] H. Dobbertin, "Cryptanalysis of MD5 Compress," http://www.cs.ucsd.edu/users/bsy/dobbertin.ps Authors' addresses Questions about this document may be directed to: Gabriel Montenegro Sun Microsystems Laboratories, Europe 29, chemin du Vieux Chene 38240 Meylan, FRANCE Voice: +33 476 18 80 45 E-Mail: gab@sun.com Claude Castelluccia INRIA Rhone-Alpes 655 avenue de l'Europe 38330 Montbonnot Saint-Martin FRANCE email: claude.castelluccia@inria.fr phone: +33 4 76 61 52 15 fax: +33 4 76 61 52 52 Montenegro, Castelluccia Expires October, 2001 [Page 15] INTERNET DRAFT SUCV Addresses April 2001 Copyright (c) The Internet Society (2000). All Rights Reserved. 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