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Network Working Group M. Thomas Request for Comments: 3129 Cisco Systems Category: Informational June 2001

      Requirements for Kerberized Internet Negotiation of Keys

Status 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 deploy

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 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 for

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 IPsec to establish and maintain IPsec security associations.  The
 protocol must be able to take full advantage of the features of RFC
 2401 but in the context of a centralized keying authority.

Requirements

 KINK must meet the following requirements at a minimum:

1. The protocol must use the session keys found in Kerberos

tickets as the basis of the keying material used for IPsec

       security association keys.

1. The protocol must be able to integrate into security

architecture of IPsec (RFC 2401).

1. 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).

1. The protocol must support Kerberos using either secret key, or

public key (PKINIT) initial authentication.

1. 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).

1. The protocol must be able to allow a peer to authenticate and

participate in many realms.

1. The protocol must handle absolute time skew gracefully.

1. The protocol must be able to create, modify, rekey, and delete

security associations.

1. The protocol must be capable of setting up both transport and

tunnel modes of IPsec.

1. The protocol must be capable of setting up both AH and ESP

security associations.

1. The protocol must be capable of negotiating cipher suites.

1. The protocol must be capable of setting up IPsec flow

selectors.

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1. The protocol must be capable of rekeying without the assistance

of the KDC if the Kerberos session ticket is still valid.

1. The protocol must make an effort to mitigate third party Denial

of Service attacks (aka Zombies attacks).

1. The protocol must be able to be used for more than IPsec

keying.

1. 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.

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References

 [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.com

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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

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