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Draft RSVP Cryptographic Authentication July 1997 RSVP Cryptographic Authentication | draft-ietf-rsvp-md5-04.txt | 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 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 a "work in progress". Comments should be made on the list rsvp@isi.edu. Abstract This document describes the format and use of RSVP's INTEGRITY object to provide hop-by-hop integrity and authentication of RSVP messages. Fred Baker Expiration: January 1998 [Page 1] Draft RSVP Cryptographic Authentication July 1997 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 proposes a mechanism to protect RSVP message integrity hop-by-hop. The proposed scheme transmits the result of applying a cryptographic algorithm to a one-way function or "digest" of the message together with a secret Authentication Key. This scheme affords protection against forgery or message modification, but not replays. It is possible to replay a message until the sequence number changes, but the sequence number makes replays less of an issue. 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. The proposed mechanism is independent of a specific cryptographic algorithm, but the document describes the use of Keyed-Hashing for Message Authentication using H-MD5 [8] for | this purpose. As noted in [8], there exist stronger hashes, such as H-SHA-1; where warranted, implementations will do well to make them available. However, in the general case, [8] | suggests that H-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. The author therefore suggests that H-MD5 should be the baseline, universally implemented by RSVP implementations implementing cryptographic authentication, with other proposals optional. The cost of computing an H-MD5 message digest far exceeds the cost of computing an RSVP checksum; therefore the RSVP checksum should be disabled (set to zero) ifH-MD5 authentication is used, as theH-MD5 digest is a much stronger integrity check. Fred Baker Expiration: January 1998 [Page 2] Draft RSVP Cryptographic Authentication July 1997 Two uses are envisioned: authentication of RSVP messages or message fragments (should a fragmentation procedure be defined in the future), and authentication of sessions. 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 more appropriate documents [1] [7]. | 1.1. Why not use the Standard IPSEC Authentication Header? One obvious question is why, since there exists a standard mechanism for authentication, we would choose to not use it. This was discussed at length in the working group, and was rejected due to the operational impact of manually 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 the source address may be a simpler key management strategy. It is also not clear that RSVP messages are well defined for the 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, as in RSVP traffic may otherwise not follow exactly the same IP path as data traffic. These matters are simplified if a secure key management protocol exists which can be used to open and key the security associations; should such a protocol come into existence, it may be worthwhile reviewing this decision. However, the non- RSVP cloud considerations conspire against using the same solution as one which would work for IPSEC. Therefore, this consideration cannot be understood as a promise that this procedure will go away. 2. Data Structures 2.1. INTEGRITY Object Format The 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 Fred Baker Expiration: January 1998 [Page 3] Draft RSVP Cryptographic Authentication July 1997 carried in an INTEGRITY object. The contents of INTEGRITY object are defined as a "Keyed Message Digest" structure, with one of the following formats: IP4 Keyed Message Digest INTEGRITY Object: Class = 4, C-Type = 1 +-------------+-------------+-------------+-------------+ | Key Identifier | +-------------+-------------+-------------+-------------+ | Sequence Number | +-------------+-------------+-------------+-------------+ | Sending System IP4 Address | +-------------+-------------+-------------+-------------+ | | + + | | + Keyed Message Digest | | | + + | | +-------------+-------------+-------------+-------------+ IP6 Keyed Message Digest INTEGRITY Object: Class = 4, C-Type = 2 +-------------+-------------+-------------+-------------+ | Key Identifier | +-------------+-------------+-------------+-------------+ | Sequence Number | +-------------+-------------+-------------+-------------+ | | + + | | + Sending System IP6 Address + | | + + | | +-------------+-------------+-------------+-------------+ | | + + | | + Keyed Message Digest + | | + + | | Fred Baker Expiration: January 1998 [Page 4] Draft RSVP Cryptographic Authentication July 1997 +-------------+-------------+-------------+-------------+ (1) Key Identifier An unsigned 32-bit number that acts as a key selector. With the key, the system stores an algorithm for its application. (2) Sending System Address This is the same address as would be carried in the Next Hop or Previous Hop object, the address of the interface of the RSVP system that sent this message. (3) Sequence Number unsigned 32-bit non-decreasing sequence number. Any non-decreasing sequence of numbers may be used as Sequence Number values. For example, a timestamp on the message's creation or a simple message counter might be used. This sequence number is reset to zero upon any key change. Note that when procedures such as the Network Time Protocol [10] are in use to synchronize clocks, it is feasible and advisable to use the current time as a sequence number. Doing this obviates the need to store sequence numbers in memory that survives a system or process failure. (4) Keyed Message Digest The digest must be a multiple of 4 octets long. For H-MD5, it will be 16 bytes long. 2.2. H-MD5 Message Header and Trailer The H-MD5 algorithm requires affixing a header and trailer to the message to be sent before the hash is computed. This extra information is not transmitted, since the receiver can reconstruct it knowing the message length and hash algorithm. The content of this trailer is specified by [8]. | Fred Baker Expiration: January 1998 [Page 5] Draft RSVP Cryptographic Authentication July 1997 3. Message Processing Rules 3.1. Message Generation An RSVP message is created as specified in [1], with these exceptions: (1) The RSVP checksum is not calculated, but is set to zero. (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 is placed in the Sequence Number field of the INTEGRITY object. If several messages are being created simultaneously (for example, in a periodic refresh generated by a router), the messages should all use the same sequence number. This is to assure that message reordering between RSVP peers (in non-FIFO queues or in an RSVP tunnel) does not cause authentication to fail for some of them. (4) The appropriate Authentication Key is selected and placed in the Keyed Message Digest field of the INTEGRITY object. (5) The Key Identifier is placed into the INTEGRITY object. (6) The H-MD5 message header and trailer are placed in memory | as described in [8]. (7) A Keyed Message Digest of the augmented message is calculated using the appropriate hash algorithm. When the H-MD5 algorithm is used, the hash calculation is | described in [8]. (8) The digest is written into the Cryptographic Digest field of the INTEGRITY object, overlaying the Authentication Key. 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 should assume that each sender may send with a different key on each Fred Baker Expiration: January 1998 [Page 6] Draft RSVP Cryptographic Authentication July 1997 numbered interface, and that they keys are simplex - the key that a system uses to sign its messages need he same key that its recievers use to sign theirs. Implementations SHOULD maintain a separate key per ip address that they sign with. This restriction to numbered interfaces is intentional; if an RSVP system peers with another through a set of non-RSVP routers, and it might be able to reach systems through that domain from either a numbered interface or an unnumbered interface using the same address as a router id, the choice of key would otherwise be ambiguous. Therefore, on unnumbered interfaces, an RSVP router must use the same key as it uses on the related numbered interface. User interfaces SHOULD provide convenient ways to configure these keys. 3.2. Message Reception When the message is received, the process is reversed: (1) The RSVP checksum is not calculated. (2) The Cryptographic Digest field of the INTEGRITY object is set aside. (3) The Key Identifer field and Sending System Address are used to determine the Authentication Key and the hash algorithm to be used. Implementations SHOULD maintain a key per neighboring RSVP system address or CIDR prefix, as the keys used by neighbors to sign their messages need not be the same key that the recieving system uses. (4) If the received sequence number is less than the last sequence number received from the sending system with that key identifier, the message is discarded unprocessed. (5) The Cryptographic Digest field of the INTEGRITY object is overlaid with the Authentication Key. (6) The message header and trailer are reconstructed. (7) A new digest is calculated using the indicated algorithm. (8) If the calculated digest does not match the received digest, the message is discarded without further processing. Fred Baker Expiration: January 1998 [Page 7] Draft RSVP Cryptographic Authentication July 1997 If a system detects the loss of a neighbor or interface, or the RSVP process is restarted on a system, the system should start with a new key if possible. In this way, the sequence number may be reset without exposure to a replay attack. In the event that no other key is available, the sequence number should be stored in non-volatile memory around failures, so that it may continue without decreasing, or (if the NTP timestamp is being used as a sequence number), RSVP resynchronization should await NTP synchronization. 4. 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. 4.1. Key Management Procedures Each key has a lifetime associated with it. No key is ever used outside its lifetime. If more than one key is currently alive, then the youngest key (the key whose lifetime most recently started) should be sent, and all "live" keys should be tested upon message receipt. 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 Fred Baker Expiration: January 1998 [Page 8] Draft RSVP Cryptographic Authentication July 1997 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 uniquely identifies the cryptographic algorithm and Authentication Key in use by RSVP. As noted above, the party creating the RSVP message will select a valid key from the set of valid keys for that interface. The receiver will use the Key Identifier and interface 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 the new key will begin being used and the current key will go out of use before the current key's lifetime expires. 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: o KeyStartReceive: the time the system starts accepting received packets signed with the key. o KeyStartSign: the time the system starts signing packets with the key. o KeyStopSign: the time the system stops signing packets with the key, which implies that it starts signing with the next key, if any. o 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. Each system Fred Baker Expiration: January 1998 [Page 9] Draft RSVP Cryptographic Authentication July 1997 sends using the key with the most recent "start" time and makes its first attempt at validation of incoming traffic with this same key. If this validation fails and another (older) key is also active, the system should attempt to validate with any other active keys it may possess. 4.2. 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, and an acceptable sequence number SHOULD be derivable either from non- volatile memory or a procedure such as NTP. Fred Baker Expiration: January 1998 [Page 10] Draft RSVP Cryptographic Authentication July 1997 4.3. Pathological Cases An implementation of this document must handle two pathological cases. Both of these should be exceedingly rare. (1) During key switch-over, devices may exist which have not yet been successfully configured with the new key. Therefore, systems MAY implement (and would be well advised to implement) an algorithm that detects the set of keys being used by its neighbors, and transmits its messages using both the new and old keys until all the neighbors are using the new key or the lifetime of the old key expires. Under normal circumstances, this elevated transmission rate will exist for a single refresh interval. (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. 5. Conformance Requirements To conform to this specification, an implementation MUST support all of its aspects. The H-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 rollover described under "Key Change Procedures." The user documentation provided with the implementation MUST contain clear instructions on how to ensure that smooth key rollover occurs. Implementations SHOULD support a standard key management protocol for secure distribution of RSVP Authentication Keys Fred Baker Expiration: January 1998 [Page 11] Draft RSVP Cryptographic Authentication July 1997 once such a key management protocol is standardized by the IETF. 6. Acknowledgments This document is derived directly from similar work done for OSPF and RIP Version II, jointly by Ran Atkinson and Fred Baker. Significant editing was sone by Bob Braden, resulting in increased clarity. (if you think this document was hard to read, think about what Bob read). Signifiant comments were submitted by Steve Bellovin, who actually understands this stuff. 7. References [1] Braden, R., Ed., Zhang, L., Estrin, D., Herzog, S., and S. Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specificationq. 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. Fred Baker Expiration: January 1998 [Page 12] Draft RSVP Cryptographic Authentication July 1997 8. 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. Confidentiality is not provided by this mechanism; if this is required, IPSEC ESP may be the best approach, although it is subject to the same criticisms as IPSEC Authetication, 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. 9. Author's Address Fred Baker Cisco Systems 519 Lado Drive Santa Barbara, California 93111 Phone: (408) 526-4257 Email: fred@cisco.com Fred Baker Expiration: January 1998 [Page 13] Draft RSVP Cryptographic Authentication July 1997 Table of Contents 1 Introduction .......................................... 2 1.1 Why not use the Standard IPSEC Authentication Header? ........................................... 3 2 Data Structures ....................................... 3 2.1 INTEGRITY Object Format ............................. 3 2.2 H-MD5 Message Header and Trailer .................... 5 3 Message Processing Rules .............................. 6 3.1 Message Generation .................................. 6 3.2 Message Reception ................................... 7 4 Key Management ........................................ 8 4.1 Key Management Procedures ........................... 8 4.2 Key Management Requirements ......................... 10 4.3 Pathological Cases .................................. 11 5 Conformance Requirements .............................. 11 6 Acknowledgments ....................................... 12 7 References ............................................ 12 8 Security Considerations ............................... 13 9 Author's Address ...................................... 13 Fred Baker Expiration: January 1998 [Page 14]