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Network Working Group                                      Scott Bradner
                                                      Harvard University
                                                          Allison Mankin
                                                     Jeffrey I. Schiller
                                   Massachusetts Institute of Technology
                                                            January 2003

                A Framework for Purpose Built Keys (PBK)


Status of this Memo

   This document is an Internet-Draft and is subject to the provisions
   of Section 10 of RFC2026.

   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-

   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

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Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   This memo considers the need to authenticate the source of a network
   communication where the actual identity of the source is not
   important but it is important to be sure that the source can not be
   spoofed and that successive messages in the communication come from
   the same source.  This memo defines the use of specially generated
   public/private key pairs, known as Purpose Built Keys (PBKs), to
   provide this assurance.  This memo is not a full specification of a

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Purpose Built Keys Framework          January 2003

   PBK protocol, but rather a model or framework for development of PBK
   in applications.

1.0 Introduction

   There are many cases in Internet protocols where cryptographic
   mechanisms can add significant security improvement. However most
   such mechanisms rely on associating keys to entities, ultimately
   requiring an enterprise-wide, multi-enterprise, or even more widely
   deployed Public Key Infrastructure (PKI).

   In the absence of security mechanisms, many protocols are
   continuously vulnerable to attack.

   However there are many circumstances where we can improve overall
   security by narrowing the window of vulnerability, so that if we
   assume that some operation is performed securely, we can secure all
   future transactions.

   There are also cases where the actual identity of the initiator of a
   network communication is not an important piece of information, yet
   it is important to know that successive packets are from that same
   source.  One example of this is in mobile IPv6.  Mobile IPv6 contains
   a rebinding option that enables a mobile node to tell the other end
   of a communication that the IP address for the mobile node has
   changed.  It is clearly important to know that any such rebinding
   request actually came from the correct mobile node even if the
   identity of the user of that mobile node does not need to be known.

   Note that it is not that the identity of the user here is unimportant
   to the network (the node user may well authenticate to an AAA service
   or other access manager at the start of network activity), but rather
   that it is unimportant to accomplish that level of authentication for
   the purpose of rebinding.

   This memo describes the use of a temporary public/private key pair
   that is generated by a host for each case where the consistency of
   authentication needs to be assured.  For example, if mobile IP
   binding were to use something like this technique, then a new key
   pair would be generated before each mobile ip session in which the
   mobile was roaming, and discarded after the session was completed.

   This use of host-generated temporary keys is confined to the parties
   in a communication and does not require that the keys be registered
   with or known by any third party.  Thus this mechanism does not
   require that any support infrastructure exist outside of the protocol

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Purpose Built Keys Framework          January 2003

   support in the corresponding hosts and it can be deployed
   incrementally as host support becomes available.  It also scales well
   since the operations are confined to the end systems involved in the

   By not using registered keys, the PBK mechanism preserves user
   pseudonymity as long as the identities of the users are not obtained
   by some other process during the communication. There is extensive
   literature about the lack of anonymity of persons stemming from their
   IP addresses ([Syverson] is a good starting point) as well as work
   that has kinship to the pseudonyms in this in this work [Brands],
   [Chaum88], [HIP], [SUCV].

   The PBK mechanism is susceptible to man-in-the-middle attacks which
   affect its initialization. Such attacks may make it possible for a
   pseudonymous identity to be used by a party other than the issuer.
   There is an "initial leap of faith" about the pseudonymous identity
   since it has no parties, other than the issuer, vouching for it, and
   though only the issuer holds the private key, a man-in-the-middle
   attacker may appear to hold and use the identity without good care
   being taken in a protocol design that makes use of PBK. Therefore,
   the designer of such a protocol should be aware of this risk and
   include a challenge-response confirmation step.  The challenge-
   response step should have the property of needing the private key for
   decryption and include a nonce.

   The PBK mechanism does not require the use of a reliable protocol.
   It is intended to used with transport or application protocols.  It
   differs from IPSec in that it is applied on demand by an application
   or by a transport protocol.

   When this mechanism is used with applications the PBK's public key
   can be used in an identity for a web-cookie like function, but the
   use is under the control of the node that initiates the connection
   rather than under the control of the server.

2.0 Conceptual Overview

   Following is a conceptual step-by-step description of the PBK process
   when operating below the transport layer.

   First some definitions:
      initiating node:  the node initiating the conversation
      receiving node: the node at the other end of the conversation

   Before an initiating node initiates a connection during which it will
   need to prove  that it is the same node that started the connection,
   it creates a public/private key pair for use during the connection.

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Purpose Built Keys Framework          January 2003

   This is known as a purpose-built key (PBK) pair.

   The initiating node then creates a Purpose-Built ID (PBID) by
   performing a cryptographic hash of the public part of the PBK pair.
   This PBID will be used as an identity token for the node.

   The initiating node then initiates the connection.  The PBID is sent
   along with the initial packets in the connection.  In IPv6 this could
   be done in an end-to-end option header, in IPv4 as a header option.
   (These option ideas are for transport level use of the PBK - if the
   PBK was used from within HTTP or another application, the PBID's
   location would be in the application protocol.). The PBID does not
   need to appear in all of the packets; it just has to be reliably
   conveyed to the receiving node.  Reliability may be obtained by
   carrying it on enough packets so that a return packet indicates it
   was received eventually.  This is the simplest approach; depending on
   requirements and the application, the PBID may well be sent using a
   reliable transport protocol. Retransmission algorithms, where they
   are needed, must be conformant with RFC 2914 [RFC2914].

   The receiving node stores the PBID and the source IP address that
   were in the received packet in a table.

   At some time in the connection before the proof of identity is
   needed, the initiating node sends its public key to the receiving
   node.  This again could be done in IP-level options or in an
   application-level exchange. The receiving node verifies that the
   received public key hashes to the previously provided PBID.

   When the initiating node wants to perform some operation for which it
   wants to prove its identity, it sends the PBID along with the
   operation request.  The message is signed using the private part of
   the PBK. If replay protection is necessary, a nonce value (a
   monotonically increasing value) or timestamp may be included with the
   operation request.

   When the receiving node gets such an operation request it verifies
   the digital signature and returns a challenge packet.  The challenge
   packet is sent to the IP address that was in the source IP address
   field of the packet that contained the request.  The challenge packet
   contains a random number test value generated by the receiving node.

   When the initiating node receives the challenge packet it encrypts
   the test value in its private key and sends the result back to
   receiving node.

   When the receiving node gets the challenge response it decrypts the
   test value using the stored public key associated with the PBID.  If

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Purpose Built Keys Framework          January 2003

   the results match then the receiving node can be sure that the node
   that sent the operation request was the correct initiating node.

   The PBKs would normally be discarded at the end of the communication
   but in those cases where a continuity of identity is needed over
   multiple sessions the PBKs could be retained until the requirement
   was over.

3.0 Notes on the design

   The hash of the public key is used as the PBID so that the
   relationship between an offered PBID and public key can be
   established.  If a receiving node is in possession of the private key
   and the hash of the corresponding public key matches an offered PBID,
   it can be sure that it has the correct PBID for that public key.

   The challenge / response exchange has to be synchronized within the
   data stream if the processing of packets after the operation request
   would be different that before the operation request, as it would be
   for mobile IPv6.  This would mean suspending normal transmission
   until the challenge / response exchange was completed.

   The challenge is sent to the source address in the packet and this
   address is not included in the digital signature on the operation
   request packet so that this mechanism can work through any address
   modifying devices that may be in the path.

   In the cases where commands could be issued by both ends of a
   communication, as would be the case in mobile IPv6 if both ends were
   mobile, separate PBKs would be created by each end and the mechanism
   would be run independently by each end.

4.0 Security Considerations

   This whole document is about security.  Specifically the memo
   discusses how to perform authenticated operations in an environment
   where there is no existing security infrastructure or an environment
   where network addresses might change during the course of the

   In the absence of a security infrastructure such as a PKI, it is not
   always possible to authenticate one party to another.  In the absence
   of any cryptographic security mechanism, internet transactions are
   continuously at risk of compromise.  With PBKs it is possible to
   leverage an initial "leap of faith" so that presuming an initial
   transaction has not been tampered with (say the exchange of PBID's at
   the beginning of an association between two parties), future
   transactions can be secured.

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Purpose Built Keys Framework          January 2003

5.0 Acknowledgements

   Some of these same concepts are explored in [HIP].  We owe concepts
   in this draft to Bob Moskowitz and also (as anyone working in the
   area of privacy does) to David Chaum.

6.0 Author's Addresses

   Scott Bradner
   Harvard University
   Cambridge MA 02138

   Phone +1 617 495 3864
   email sob@harvard.edu

   Allison Mankin
   Bell Labs, Lucent
   Phone: +1 301 728 7199
   email: mankin@psg.com

   Jeffrey I. Schiller
   Massachusetts Institute of Technology
   MIT Room W92-190
   77 Massachusetts Avenue
   Cambridge, MA 02139-4307
   Phone: +1 617 253 0161
   email: jis@mit.edu

Informative References

   [RFC2914] Floyd, S., "Congestion Control Principles", RFC 2914,
   September 2000.

   ([Syverson] is just a good starting point) as well as work that has
   kinship to the pseudonyms in this in this work [Brands], [Chaum88],
   [HIP], [SUCV].

   [Syverson] Syverson, P., Goldschlag, D. and Reed, M., "Anonymous
   Connections and Onion Routing," in 18th Annual Symposium on Security
   and Privacy, Oakland CA, 1997.  http://www.onion-

   [Brands] Brands, S.A., "Rethinking Public Key Infrastructures and
   Digital Certificates - Building In Privacy," MIT Press, 2000.

   [Chaum88] Chaum, D., Fiat, A., and Naor, M.  "Untraceable Electronic

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Purpose Built Keys Framework          January 2003

   Cash", in S. Goldwasser, Editor, Advances in Cryptology - CRYPTO '88.
   Lecture Notes in Computer Science Volume 403, Springer-Verlag, 1988.

   [HIP] Moskowitz, R., "Host Identity Payload Architecture", "Host
   Identity Payload Protocol", http://homebase.htt-consult.com/~hip,

   [SUCV] Montenegro, G., Castellucia, C., "SUCV Identifiers and
   Addresses", IETF Work in Progress, July 2002.

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