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Network Working Group                                          M. ThomasRequest for Comments: 3129                                 Cisco SystemsCategory: Informational                                        June 2001Requirements for Kerberized Internet Negotiation of KeysStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2001).  All Rights Reserved.Abstract   The goal of this document is to produce a streamlined, fast, easily   managed, and cryptographically sound protocol without requiring   public key.Motivation   The IPsec working group has defined a number of protocols which   provide the ability to create and maintain cryptographically secure   security associations at layer three (i.e., the IP layer).  This   effort has produced two distinct protocols:   1) a mechanism to encrypt and authenticate IP datagram payloads which      assumes a shared secret between the sender and receiver   2) a mechanism for IPsec peers to perform mutual authentication and      exchange keying material   The IPsec working group has defined a peer to peer authentication and   keying mechanism, IKE (RFC 2409).  One of the drawbacks of a peer to   peer protocol is that each peer must know and implement a site's   security policy which in practice can be quite complex.  In addition,   the lack of a trusted third party requires the use of Diffie Hellman   (DH) to establish a shared secret.  DH, unfortunately, is   computationally quite expensive and prone to denial of service   attacks.  IKE also relies on X.509 certificates to realize scalable   authentication of peers.  Digital signatures are also computationally   expensive and certificate based trust models are difficult to deployThomas                       Informational                      [Page 1]

RFC 3129                 Requirements for KINK                 June 2001   in practice.  While IKE does allow for pre-shared symmetric keys, key   distribution is required between all peers -- an O(n^2) problem --   which is problematic for large deployments.   Kerberos (RFC 1510) provides a mechanism for trusted third party   authentication for clients and servers.  Clients authenticate to a   centralized server -- the Key Distribution Center -- which in turn   issues tickets that servers can decrypt thus proving that the client   is who it claims to be.  One of the elements of a Kerberos ticket is   a session key which is generated by the KDC which may be used by the   client and server to share a secret.  Kerberos also allows for both   symmetric key authentication, as well as certificate based public key   authentication (PKinit).  Since the authentication phase of Kerberos   is performed by the KDC, there is no need to perform expensive DH or   X.509 certificate signatures/verification operations on servers.   While clients may authenticate using X.509 certificates, the   authentication phase can be amortized over the lifetime of the   credentials.  This allows a single DH and certificate exchange to be   used to key security associations with many servers in a   computationally economic way.  Kerberos also support clients with   symmetric keys but unlike IKE, the symmetric keys are stored in the   KDC making the number of keys an O(n) problem rather than O(n^2).   Kerberos also allows security policy to be managed in a more   centralized fashion, rather than expecting each potentially   untrustworthy peer to abide by stated security policies of an   organization.   The KINK working group takes these basic features of Kerberos and   uses them to its advantage to create a protocol which can establish   and maintain IPsec security associations (RFC 2401).  It should be   noted that KINK is not a replacement for IKE.  IKE has one property   which KINK cannot reproduce:  the ability for two peers to mutually   authenticate and exchange keys without the need for an actively   participating third party.  However, there are many situations where   a trusted third party which proxies authentication is viable, and in   fact desirable.   While Kerberos specifies a standard protocol between the client and   the KDC to get tickets, the actual ticket exchange between client and   server is application specific.  KINK is intended to be an   alternative to requiring each application having its own method of   transporting and validating service tickets using a protocol which is   efficient and tailored to the specific needs of Kerberos and the   applications for which it provides keying and parameter negotiation.   Given the above, a new general keying protocol which leverages the   scalability of Kerberos is desirable.  The working group's first task   is to define this protocol and define an domain of interpretation forThomas                       Informational                      [Page 2]

RFC 3129                 Requirements for KINK                 June 2001   IPsec to establish and maintain IPsec security associations.  The   protocol must be able to take full advantage of the features ofRFC2401 but in the context of a centralized keying authority.Requirements   KINK must meet the following requirements at a minimum:   -     The protocol must use the session keys found in Kerberos         tickets as the basis of the keying material used for IPsec         security association keys.   -     The protocol must be able to integrate into security         architecture of IPsec (RFC 2401).   -     The protocol must be able to start up SA's regardless of any         client/server disposition in the keying protocol.  In other         words, either IPsec peer can be the initiator or responder,         regardless of whether it's a Kerberos 'client' (TGT-only) or         Kerberos 'server'(has a keytab).   -     The protocol must support Kerberos using either secret key, or         public key (PKINIT) initial authentication.   -     The protocol must support Kerberos User-to-User mode for cases         in which the initiator cannot obtain an AP_REQ for the         responder (i.e.  the responder is a PKINIT client) or the         responder cannot decrypt and AP_REQ from the initiator (i.e.,         the responder doesn't have a Kerberos Keytab, just a TGT).   -     The protocol must be able to allow a peer to authenticate and         participate in many realms.   -     The protocol must handle absolute time skew gracefully.   -     The protocol must be able to create, modify, rekey, and delete         security associations.   -     The protocol must be capable of setting up both transport and         tunnel modes of IPsec.   -     The protocol must be capable of setting up both AH and ESP         security associations.   -     The protocol must be capable of negotiating cipher suites.   -     The protocol must be capable of setting up IPsec flow         selectors.Thomas                       Informational                      [Page 3]

RFC 3129                 Requirements for KINK                 June 2001   -     The protocol must be capable of rekeying without the assistance         of the KDC if the Kerberos session ticket is still valid.   -     The protocol must make an effort to mitigate third party Denial         of Service attacks (aka Zombies attacks).   -     The protocol must be able to be used for more than IPsec         keying.   -     The protocol must support both IPv4 and IPv6.Security Considerations   These requirements lay out input to define a protocol which allows   the keying of IPsec security associations using Kerberos as the key   distribution mechanism.  As such, the security associations that will   be created by the new protocol will inherit the union of IPsec and   Kerberos's existing security weaknesses.  There is no requirement to   address those weaknesses unless in combination they produce a new   weakness which is not inherent in other keying protocols.Acknowledgments   The original KINK Kabal was:      Michael Thomas (Cisco)      David McGrew (Cisco)      Jan Vilhuber (Cisco)      Jonathan Trostle (Cisco)      Matt Hur (Cybersafe)      Mike Froh (Cybersafe)      Sasha Medvinsky (GI)      Derek Atkins (Telcordia)   It must also be acknowledged that the Packetcable Security   specification PKT-SP-SEC-I01-991201 provided the raw fodder for this   effort in its Kerberized IPsec section, and all of the focus team   members who played a part in the spec.  We must also acknowledge   Nancy Davoust of Cablelabs for keeping order in our normally   disorderly proceedings.Thomas                       Informational                      [Page 4]

RFC 3129                 Requirements for KINK                 June 2001References   [1]  Kohl, J. and C. Neuman, "The Kerberos Network        Authentication Service (V5)",RFC 1510, September 1993.   [2]  Kent, S. and R. Atkinson, "Security Architecture for the        Internet Protocol",RFC 2401, November 1998.   [3]  Harkins, D. and D. Carrel, "The Internet Key        Exchange (IKE)",RFC 2409, November 1998.Author's Address   Michael Thomas   Cisco Systems   375 E Tasman Rd   San Jose, Ca, 95134, USA   Phone: +1 408-525-5386   EMail: mat@cisco.comThomas                       Informational                      [Page 5]

RFC 3129                 Requirements for KINK                 June 2001Full Copyright Statement   Copyright (C) The Internet Society (2001).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Thomas                       Informational                      [Page 6]