Internet Draft Fred Baker Expiration: February 1999 Cisco File: draft-ietf-rsvp-md5-06.txt Bob Lindell USC/ISI Mohit Talwar USC/ISI RSVP Cryptographic Authentication Status of this Memo 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". Comments should be made on the list rsvp@isi.edu. To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). Abstract This document describes the format and use of RSVP's INTEGRITY object to provide hop-by-hop integrity and authentication of RSVP messages. Baker, et. al. Expiration: February 1999 [Page 1]
Internet Draft RSVP Cryptographic Authentication August 1998 1. Introduction The Resource ReSerVation Protocol RSVP [1] is a protocol for setting up distributed state in routers and hosts, and in particular for reserving resources to implement integrated service. RSVP allows particular users to obtain preferential access to network resources, under the control of an admission control mechanism. Permission to make a reservation will depend both upon the availability of the requested resources along the path of the data, and upon satisfaction of policy rules. To protect the integrity of this admission control mechanism, RSVP requires the ability to protect its messages against corruption and spoofing. This document defines a mechanism to protect RSVP message integrity hop-by-hop. The proposed scheme transmits an authenticating digest of the message, computed using a secret Authentication Key and a keyed-hash algorithm. This scheme provides protection against forgery or message modification. The INTEGRITY object of each RSVP message is tagged with a one time use sequence number. This allows the message receiver to identify playbacks and hence to thwart replay attacks. The proposed mechanism does not afford confidentiality, since messages stay in the clear; however, the mechanism is also exportable from most countries, which would be impossible were a privacy algorithm to be used. Note: this document uses the terms "sender" and "receiver" differently from [1]. They are used to refer to systems which face each other across an RSVP hop, the "sender" being the system generating RSVP messages. The message replay prevention algorithm is quite simple. The sender generates packets with increasing sequence numbers. In turn, the receiver only accepts packets which have a larger sequence number than the previous packet. To get this process started, a receiver handshakes with the sender to get a starting sequence number. This memo discusses ways to relax the strictness of the in order delivery of messages as well as techniques to generate monotonically increasing sequence numbers that are robust across sender failures and restarts. The proposed mechanism is independent of a specific cryptographic algorithm, but the document describes the use of Keyed-Hashing for Message Authentication using HMAC-MD5 [8]. As noted in [8], there exist stronger hashes, such as HMAC-SHA1; where warranted, implementations will do well to make them available. However, in the general case, [8] suggests that HMAC-MD5 is adequate to the purpose at hand and has preferable performance characteristics. [8] also offers source code and test vectors for this algorithm, a boon to those who would test for interoperability. HMAC-MD5 is required as a baseline to be universally included in RSVP implementations providing Baker, et. al. Expiration: February 1999 [Page 2]
Internet Draft RSVP Cryptographic Authentication August 1998 cryptographic authentication, with other proposals optional (see the section on Conformance Requirements below). The RSVP checksum may be disabled (set to zero) if HMAC-MD5 authentication is used, as the HMAC-MD5 digest is a much stronger integrity check. Two uses are envisioned for INTEGRITY objects: authentication of RSVP messages (or message fragments, should a fragmentation procedure be defined in the future), and authentication of policy data objects [10]. The INTEGRITY object used in both is the same, and is defined in this document. The use of the INTEGRITY object for those purposes is defined in other documents [1] [7]. 1.1. 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 [9]. 1.2. Why not use the Standard IPSEC Authentication Header? One obvious question is why, since there exists a standard mechanism, IPSEC, for authentication [5], we would choose not to use it. This was discussed at length in the working group, and the use of IPSEC was rejected for the following reasons. The security associations in IPSEC are based on destination address. It is not clear that RSVP messages are well defined for either source or destination based security associations, as a router must forward PATH and PATH TEAR messages using the same source address as the sender listed in the SENDER TEMPLATE. RSVP traffic may otherwise not follow exactly the same path as data traffic. Using either source or destination based associations would require opening a new security association among the routers that a flow traverses for each flow making reservations. In addition, it was noted that neighbor relationships between RSVP systems are not limited to those which face one another across a communication channel. RSVP relationships across non-RSVP clouds, such as those described in section 2.8 of [1], are not necessarily visible to the sending system, suggesting that key management based on RSVP router to RSVP router associations may allow a simpler key management strategy. 2. Data Structures Baker, et. al. Expiration: February 1999 [Page 3]
Internet Draft RSVP Cryptographic Authentication August 1998 2.1. INTEGRITY Object Format An RSVP Message consists of a sequence of "objects," which are type- length-value encoded fields having specific purposes. The information required for hop-by-hop integrity checking is carried in an INTEGRITY object. The same INTEGRITY object type is used for both IPv4 and IPv6. The contents of an INTEGRITY object are defined as a "Keyed Message Digest" structure, with the following format: Keyed Message Digest INTEGRITY Object: Class = 4, C-Type = 1 +-------------+-------------+-------------+-------------+ | Key Identifier | | | +-------------+-------------+-------------+-------------+ | Sequence Number | | | +-------------+-------------+-------------+-------------+ | | + + | | + Keyed Message Digest | | | + + | | +-------------+-------------+-------------+-------------+ (1) Key Identifier An unsigned 64-bit number that acts as a key selector. With the key, the system stores an algorithm for its application. Senders SHOULD pick globally unique key identifiers. This can be accomplished by using an IP address, a hash of an IP address, or a globally unique MAC address concatenated with a key number. (2) Sequence Number An unsigned 64-bit monotonically increasing, unique sequence number. Any monotonically increasing sequence of numbers, that provides the INTEGRITY object of each RSVP message with a unique tag, may be used as Sequence Number values. Details Baker, et. al. Expiration: February 1999 [Page 4]
Internet Draft RSVP Cryptographic Authentication August 1998 on sequence number generation are presented in the next section. (3) Keyed Message Digest The digest must be a multiple of 4 octets long. For HMAC- MD5, it will be 16 bytes long. 3. Generating Sequence Numbers In this section we describe methods which could be chosen to generate the sequence numbers used in the INTEGRITY object of an RSVP message. As previous stated, there are two important properties that MUST be satisfied by the generation procedure. The first property is that the sequence numbers are unique, or one time, for the lifetime of the integrity key that is in current use. A receiver can use this property to unambiguously distinguish between a new or a replayed message. The second property is that the sequence numbers are generated in monotonically increasing order. This is required to greatly simplify the message processing rules, since a receiver only needs to save the value of the highest sequence number seen to avoid a replay attack. The size of the sequence number field is chosen to be a 64-bit unsigned quantity. This is large enough to avoid exhaustion over the key lifetime. For example, if a key lifetime was conservatively defined as one year, there would be enough sequence number values to send RSVP messages at an average rate of about 585 gigaMessages per second. A 32-bit sequence number would limit this average rate to about 136 messages per second. The ability to generate unique monotonically increasing sequence numbers across a failure and restart implies some form of stable storage, either local to the device or remotely over the network. Three state-based sequence number generation procedures are described below. 3.1. Simple Sequence Numbers The most straight forward approach would be to generate a unique sequence number using a message counter. Each time a message is transmitted for a given key, the sequence number counter is incremented. The current value of this counter is continually or periodically saved to stable storage. Recovery of this counter after a restart is accomplished by using this stable storage. If the counter was saved periodically to stable storage, the count should be recovered by increasing the saved value to be larger than any possible value of the counter at the time of the failure. This can Baker, et. al. Expiration: February 1999 [Page 5]
Internet Draft RSVP Cryptographic Authentication August 1998 be computed knowing the interval at which the counter was saved to stable storage and incrementing the stored value by that amount. 3.2. Sequence Numbers Based on a Real Time Clock Most devices will probably not have the capability to save sequence number counters to stable storage for each key. A more universal solution is to base sequence numbers on the stable storage of a real time clock. Many computing devices have a real time clock module that includes stable storage of the clock. These modules generally include some form of nonvolatile memory to retain clock information in the event of a power failure. In this approach, we use an NTP based timestamp value as the sequence number. The rollover period of an NTP timestamp is about 136 years, much longer than any reasonable lifetime of a key. In addition, the granularity of the NTP timestamp is fine enough to allow the generation of an RSVP message every 200 picoseconds for a given key. Many real time clock modules do not have the resolution of an NTP timestamp. In these cases, the least significant bits of the timestamp can be generated using a message counter, which is reset every clock tick. For example, when the real time clock provides a resolution of 1 second, the 32 least significant bits of the sequence number can be generated using a message counter. The remaining 32 bits are filled with the 32 least significant bits of the timestamp. Assuming that the recovery time after failure takes longer than one tick of the real time clock, the message counter for the low order bits can be reset to zero. 3.3. Sequence Numbers Based on a Network Recovered Clock If the device does not contain any stable storage, it could recover the real time clock from the network using NTP. Once the clock has been recovered, following a restart, the sequence number generation procedure would be identical to the procedure described in section 6.2. 3.4. Sequence Number Wraparound Sequence numbers might roll over within some key's lifetime. To accomodate this behavior, each receiver MUST accept monotonically increasing sequence numbers, modulo 2^64. In addition, we assume that a key's lifetime is short enough for the key to expire within half the sequence number space. With this assumption, receivers MUST reject sequence numbers which lie in the wrong half of the sequence number space. This solution draws directly from the TCP sequence number algorithm [11]. Baker, et. al. Expiration: February 1999 [Page 6]
Internet Draft RSVP Cryptographic Authentication August 1998 4. Message Processing Rules 4.1. Message Generation An RSVP message is created as specified in [1], with these exceptions: (1) The RSVP checksum field is set to zero. If required, an RSVP checksum can be calculated after step (7), when the processing of the INTEGRITY object is complete. (2) The INTEGRITY object is inserted in the appropriate place, and its location in the message is remembered for later use. (3) The current sequence number must be updated, if required, to ensure a unique, monotonically increasing number. It is then placed in the Sequence Number field of the INTEGRITY object. (4) The Keyed Message Digest field is set to zero. (5) The Key Identifier is placed into the INTEGRITY object. (6) An authenticating digest of the is computed using the appropriate Authentication Key in conjunction with the keyed- hash algorithm. When the HMAC-MD5 algorithm is used, the hash calculation is described in [8]. (7) The digest is written into the Cryptographic Digest field of the INTEGRITY object. In the sender, Authentication Key selection is based on the interface through which the message is sent, there being a key configured per interface. While administrations may configure all the routers and hosts on a subnet (or for that matter, in their network) with the same key, implementations MUST assume that each sender may send with a different key on each interface (be it numbered or unnumbered), and that the keys are simplex - the key that a system uses to sign its messages need not be same key that its receivers use to sign theirs. Implementations SHOULD maintain a separate key per interface they send on. User interfaces SHOULD provide convenient ways to configure these keys. An RSVP router may have unnumbered interfaces which carry a common IP address but distinct Logical Interface Handles (LIHs) for RSVP. In these cases, an implementation SHOULD maintain a separate key per LIH. In addition, Key Identifier values should be formed using some combination of the common IP address, the LIH, and the key number in an attempt to produce a global unique value. Baker, et. al. Expiration: February 1999 [Page 7]
Internet Draft RSVP Cryptographic Authentication August 1998 4.2. Message Reception When the message is received, the process is reversed: (1) The RSVP checksum field is set to zero. (2) The Cryptographic Digest field of the INTEGRITY object is set aside. (3) The Key Identifier field is used to determine the Authentication Key and the hash algorithm to be used. In the rare event that the key identifier is not unique, each matching Authentication Key and associated hash algorithm is applied in an attempt to find a match. (4) The sequence number is validated to prevent replay attacks, and messages with invalid sequence numbers are ignored by the receiver. Every time a message is accepted, the sequence number of that message updates the stored value corresponding to the largest sequence number received to date. Each subsequent message must have a larger sequence number to be accepted (see section 3.4). This simple processing rule prevents message replay attacks, it must be modified to be tolerant to limited out of order message delivery. If several messages were sent simultaneously (for example, in a periodic refresh generated by a router, or as a result of a tear down function), a reordering problem might arise either due to the use of CBQ/WFQ queuing algorithms in the sender, or due to reordering in an intervening non-RSVP cloud. Therefore, the sequence number received may not be higher than the number last seen. An implementation SHOULD allow administrative configuration which sets the tolerance to out of order message delivery. A simple approach would allow administrators to specify a message window corresponding to the worst case reordering behavior. For example, one might specify that packets reordered within a 32 message window would be accepted. If no reordering can occur, the window is set to one. Since sequence numbers might not be strictly sequential, it is necessary to store a list of all sequence numbers seen during the reordering window. Acceptance of a sequence number implies adding it to the list and removing a number from the lower end of the list. A received sequence number is valid if (a) it is greater than Baker, et. al. Expiration: February 1999 [Page 8]
Internet Draft RSVP Cryptographic Authentication August 1998 the maximum sequence number received as yet or (b) a past sequence number lying within the reordering window and not recorded in the list. Messages received with sequence numbers lying below the lower end of the window or marked seen in the list are silently discarded. (5) The Cryptographic Digest field of the INTEGRITY object is set to zero. (6) A new keyed-digest is calculated using the indicated algorithm and the Authentication Key. (7) If the calculated digest does not match the received digest, the message is discarded without further processing. 4.3. Handshake at Restart or Initialization of the Receiver During initialization or restart, a receiver must obtain a starting sequence number for a sender whose messages require an integrity check. The receiver SHOULD initiate a handshake with the sender to attain this information. An RSVP capable device challenges another device with an RSVP handshake message containing a unique sequence number generated by the standard methods outlined earlier. The remote device receives the challenge and returns an INTEGRITY-checked RSVP handshake message which contains the original sequence number. The response is accepted only if the original sequence number matches the returned sequence number in the message. This prevents replay of old handshake responses. If the sequence number matches, the device saves the remote sequence number from the INTEGRITY object, along with the key for the remote device. If a response is not received within a given period of time, the challenge is repeated. When the handshake is successfully completed, a device will begin accepting normal RSVP signaling messages from that sender and ignore any other handshake responses. An RSVP handshake message will carry a message type of 11. The message format is as follows: <INTEGRITY Handshake message> ::= <Common Header> [ <INTEGRITY> ] <CHALLENGE> The contents of a CHALLENGE object is defined with the following format: Baker, et. al. Expiration: February 1999 [Page 9]
Internet Draft RSVP Cryptographic Authentication August 1998 CHALLENGE Object: Class = 16, C-Type = 1 +-------------+-------------+-------------+-------------+ | Sequence Number | | | +-------------+-------------+-------------+-------------+ The use of a handshake may not be necessary in all environments. A common use of RSVP integrity will be between peering domain routers, which are likely to be processing a steady stream of RSVP messages due to aggregation effects. If a router crashes and restarts, there will probably be valid RSVP messages from peering senders arriving within a short duration of restart. Assuming that replay messages are injected into the stream of valid RSVP messages, there may only be a small window of opportunity for a replay attack before a valid message is processed. This valid message will set the largest sequence number seen to a value greater than any number that had been stored prior to the crash, preventing any further replays. On the other hand, not using a handshake could allow exposure to replay attacks if there is a long period of silence from a given sender following a restart of a receiver. It SHOULD be an administrative decision whether or not a handshake is performed. That decision will be based on assumptions related to a particular network environment. 5. Key Management It is likely that the IETF will define a standard key management protocol. It is strongly desirable to use that key management protocol to distribute RSVP Authentication Keys among communicating RSVP implementations. Such a protocol would provide scalability and significantly reduce the human administrative burden. The Key ID can be used as a hook between RSVP and such a future protocol. Key management protocols have a long history of subtle flaws that are often discovered long after the protocol was first described in public. To avoid having to change all RSVP implementations should such a flaw be discovered, integrated key management protocol techniques were deliberately omitted from this specification. 5.1. Key Management Procedures Each key has a lifetime associated with it. In general, no key is ever used outside its lifetime (but see section 5.3). If more than one key is currently alive, then the youngest key (the key whose lifetime most recently started) should be sent. Baker, et. al. Expiration: February 1999 [Page 10]
Internet Draft RSVP Cryptographic Authentication August 1998 Possible mechanisms for managing key lifetime include: the use of the Network Time Protocol, hardware time-of-day clocks, or waiting some time before emitting the first message to determine what key other systems are signing with. The matter is left for the implementor. Note that the concept of a "key lifetime" does not require a hardware time-of-day clock or the use of NTP, although one or the other is advised; it merely requires that the earliest and latest times that the key is valid must be programmable in a way the system understands. To maintain security, it is advisable to change the RSVP Authentication Key on a regular basis. It should be possible to switch the RSVP Authentication Key without loss of RSVP state or denial of reservation service, and without requiring people to change all the keys at once. This requires the RSVP implementation to support the storage and use of more than one RSVP Authentication Key on a given interface at the same time. For each key there will be a locally-stored Key Identifier. The combination of the Key Identifier and the interface associated with the message identifies the cryptographic algorithm and Authentication Key in use by RSVP. As noted above, the sender will select a valid key from the set of valid keys for that interface. The receiver will use the Key Identifier to determine which key to use for authentication of the received message. More than one key may be associated with an interface at the same time. To ensure a smooth switch-over, each communicating RSVP system must be updated with the new key before the current key will expire and before the new key lifetime begins. The new key should have a lifetime that starts several minutes before the old key expires. This gives time for each system to learn of the new RSVP Authentication Key before that key will be used. It also ensures that at the time that the current key's lifetime has expired, all systems have prepared to send and receive data using the new key. For the duration of the overlap in key lifetimes, a system may receive messages using either key and authenticate the message. There are four important times for each key: + KeyStartReceive: the time the system starts accepting received packets signed with the key. + KeyStartSign: the time the system starts signing packets with the key. + KeyStopSign: the time the system stops signing packets with the key, which implies that it starts signing with the next key, if Baker, et. al. Expiration: February 1999 [Page 11]
Internet Draft RSVP Cryptographic Authentication August 1998 any. + KeyStopReceive: the time the system stops accepting received packets signed with the key. The times in the order listed SHOULD form a non-decreasing sequence. There needs to be some distance between start times and stop times, to achieve a seamless transition. 5.2. Key Management Requirements Requirements on an implementation are as follows. (1) It is strongly desirable that a hypothetical security breach in one Internet protocol not automatically compromise other Internet protocols. The Authentication Key of this specification SHOULD NOT be stored using protocols or algorithms that have known flaws. (2) An implementation MUST support the storage of more than one key at the same time, although normally only one key will be active on an interface. (3) An implementation MUST associate a specific lifetime (i.e., KeyStartSign and KeyStopSign) with each key and corresponding Key Identifier. (4) An implementation MUST support manual key distribution (e.g., the privileged user manually typing in the key, key lifetime, and key identifier on the console). The lifetime may be infinite. (5) If more than one algorithm is supported, then the implementation MUST require that the algorithm be specified for each key at the time the other key information is entered. (6) Keys that are out of date MAY be deleted at will by the implementation without requiring human intervention. (7) Manual deletion of active keys SHOULD also be supported. (8) Key storage SHOULD persist across a system restart, warm or cold, to avoid operational issues. 5.3. Pathological Cases An implementation of this document must handle two pathological cases. Both of these should be exceedingly rare. Baker, et. al. Expiration: February 1999 [Page 12]
Internet Draft RSVP Cryptographic Authentication August 1998 (1) During key switch-over, devices may exist which have not yet been successfully configured with the new key. If a key is shared with multiple receivers, there is a region of uncertainty around the time of key switch-over during which some receivers may still be using the old key and others have switched to the new key. The size of this uncertainity region is related to clock synchrony of the receivers. Administrators should configure the overlap between the expiration time of the old key (KeyStopReceive) and the validity of the new key (KeyStartReceive) to be at least twice the size of this uncertainity interval. This will allow a sender to make the key switch-over at the midpoint of this interval and be confident that all receivers are now accepting the new key. (2) It is possible that the last key associated with an interface may expire. When this happens, it is unacceptable to revert to an unauthenticated condition, and not advisable to disrupt current reservations. Therefore, the system should send a "last authentication key expiration" notification to the network manager and treat the key as having an infinite lifetime until the lifetime is extended, the key is deleted by network management, or a new key is configured. 6. Conformance Requirements To conform to this specification, an implementation MUST support all of its aspects. The HMAC-MD5 authentication algorithm defined in [8] MUST be implemented by all conforming implementations. A conforming implementation MAY also support other authentication algorithms such as NIST's Secure Hash Algorithm (SHA). Manual key distribution as described above MUST be supported by all conforming implementations. All implementations MUST support the smooth key roll over described under "Key Change Procedures." The user documentation provided with the implementation MUST contain clear instructions on how to ensure that smooth key roll over occurs. Implementations SHOULD support a standard key management protocol for secure distribution of RSVP Authentication Keys once such a key management protocol is standardized by the IETF. 7. Acknowledgments This document is derived directly from similar work done for OSPF and RIP Version II, jointly by Ran Atkinson and Fred Baker. Significant Baker, et. al. Expiration: February 1999 [Page 13]
Internet Draft RSVP Cryptographic Authentication August 1998 editing was done by Bob Braden, resulting in increased clarity. (if you think this document was hard to read, think about what Bob read). Significant comments were submitted by Steve Bellovin, who actually understands this stuff. Matt Crawford and Dan Harkins helped revise the document. 8. References [1] Braden, R., Ed., Zhang, L., Estrin, D., Herzog, S., and S. Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification". Internet Draft draft-ietf-rsvp- spec-14.ps, January 1997. [2] S. Bellovin, "Security Problems in the TCP/IP Protocol Suite", ACM Computer Communications Review, Volume 19, Number 2, pp.32- 48, April 1989. [3] N. Haller, R. Atkinson, "Internet Authentication Guidelines", Request for Comments 1704, October 1994. [4] R. Braden, D. Clark, S. Crocker, & C. Huitema, "Report of IAB Workshop on Security in the Internet Architecture", Request for Comments 1636, June 1994. [5] R. Atkinson, "IP Authentication Header", Request for Comments 1826, August 1995. [6] R. Atkinson, "IP Encapsulating Security Payload", Request for Comments 1827, August 1995. [7] S. Herzog, "RSVP Extensions for Policy Control", draft-ietf- rsvp-policy-ext-02.txt, March 1997. [8] Krawczyk, Bellare, and Canetti, "HMAC: Keyed-Hashing for Message Authentication", Request for Comments 2104, March 1996. [9] [RFC-2119], Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, Harvard University, March 1997. [10] R. Yavatkar, D. Pendarakis, R. Guerin, "A Framework for Policy-based Admission Control", draft-ietf-rap-framework-00.txt November 1997. [11] Postel, Jon, "Transmission Control Protocol", Request for Comments 793, September 1981. Baker, et. al. Expiration: February 1999 [Page 14]
Internet Draft RSVP Cryptographic Authentication August 1998 9. Security Considerations This entire memo describes and specifies an authentication mechanism for RSVP that is believed to be secure against active and passive attacks. Passive attacks are widespread in the Internet at present. Protection against active attacks is also needed even though such attacks are not as widespread. Users need to understand that the quality of the security provided by this mechanism depends completely on the strength of the implemented authentication algorithms, the strength of the key being used, and the correct implementation of the security mechanism in all communicating RSVP implementations. This mechanism also depends on the RSVP Authentication Keys being kept confidential by all parties. If any of these assumptions are incorrect or procedures are insufficiently secure, then no real security will be provided to the users of this mechanism. While the handshake response is required to be INTEGRITY-checked, the handshake challenge is not. This was done intentionally to eliminate a dependency on an integrity key in the opposite direction. Hence an intruder could generate fake handshaking challenges with a certain sequence number. It could then save the response and attempt to play it against a receiver that is in recovery. If it was lucky enough to have guessed the sequence number used by the receiver at recovery time it could use the saved response. This response would be accepted, since it is properly signed, and would have a smaller sequence number for the sender because it was an old message. This opens the receiver up to replays. This seems very difficult to exploit. Not only does it require guessing the challenge sequence number in advance, but also being able to masquerade as the receiver to generated a handshake request with the proper IP address and not being caught. Moreover, since messages flow back and forth there might be keys in both directions. In such cases the receiver can also sign its challenge for the sender to authenticate and cover this attack. Confidentiality is not provided by this mechanism; if this is required, IPSEC ESP [6] may be the best approach, although it is subject to the same criticisms as IPSEC Authentication, and therefore would be applicable only in specific environments. Protection against traffic analysis is also not provided. Mechanisms such as bulk link encryption might be used when protection against traffic analysis is required. Baker, et. al. Expiration: February 1999 [Page 15]
Internet Draft RSVP Cryptographic Authentication August 1998 10. Authors' Addresses Fred Baker Cisco Systems 519 Lado Drive Santa Barbara, California 93111 Phone: (408) 526-4257 Email: fred@cisco.com Bob Lindell USC Information Sciences Institute 4676 Admiralty Way Marina del Rey, CA 90292 Phone: (310) 822-1511 EMail: lindell@ISI.EDU Mohit Talwar USC Information Sciences Institute 4676 Admiralty Way Marina del Rey, CA 90292 Phone: (310) 822-1511 EMail: mtalwar@ISI.EDU Baker, et. al. Expiration: February 1999 [Page 16]
