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Network Working Group                                           D. Zhang
Internet-Draft                               Huawei Technologies Co.,Ltd
Intended status: Informational                                D. Kuptsov
Expires: September 10, 2012                                         HIIT
                                                                 S. Shen
                                                                   CNNIC
                                                           March 9, 2012


                   Host Identifier Revocation in HIP
                     draft-irtf-hiprg-revocation-05

Abstract

   This document mainly analyzes the key revocation issue with host
   identifiers (HIs) in the Host Identity Protocol (HIP).  Generally,
   key revocation is an important functionality of key management
   systems; it is concerned with the issues of removing cryptographic
   keys from operational use when they are not secure or not secure
   enough any more.  This functionality is particularly important for
   the security systems expected to execute for long periods.  This
   document also attempts to investigate several issues that a designer
   of HI revocation mechanisms need to carefully consider.

Requirements Language

   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 RFC 2119 [RFC2119].

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on September 10, 2012.

Copyright Notice



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   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Key Management . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Key Revocation . . . . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Background . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.2.  Classification of permanent Key Revocation Mechanisms  . .  5
   5.  Implicit HI Revocation in HIP  . . . . . . . . . . . . . . . .  7
   6.  Explicit HI Revocation in HIP  . . . . . . . . . . . . . . . . 11
   7.  Related Discussions  . . . . . . . . . . . . . . . . . . . . . 13
     7.1.  Influence of HI revocation on Already Generated HIP
           Associations . . . . . . . . . . . . . . . . . . . . . . . 13
     7.2.  HI Refreshment . . . . . . . . . . . . . . . . . . . . . . 13
   8.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     12.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17















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1.  Introduction

   In HIP architecture [RFC5201] along with the ephemeral keys, derived
   during the protocol run between two peers, a HIP-enabled host is
   provided with a public/private key pair which are considered to be
   used for a long period of time.  When two HIP hosts attempt to
   establish a connection (e.g., a TCP session) they use the public half
   of the key to represent it's identify (HI).  In literature this phase
   is usually referred to as identification process.  Usually, HIs being
   the public information can be communicated as a plaintext, unless one
   requires also support for privacy.  On the other hand, hosts use the
   private halves of the keys to prove that they are the genuine holders
   of the corresponding HIs.  This process is commonly defined as
   authentication.  As the name implies the private halves should always
   be kept in a secret.  Clearly, the security of many HIP deployments
   largely depends on the security of the private/public key pairs.  If
   the private key pair of a HIP host is revealed (by accident or
   intentionally), an attacker can potentially impersonate the victim to
   carry out attacks without being detected for long period of time.

   It has been widely recognized that a cryptographic key (which can be
   either a symmetric key or a public key) should have a reasonable
   valid period [Recommendations].  After being employed for a certain
   period, a cryptographic may become susceptible to cryptanalysis: As
   time elapses, an attacker can collect enough material (e.g.,
   encrypted data, signatures and associated plain texts, etc.) to
   compromise the key.  On the other extreme, the accidental key
   disclosures is yet another threat.  For instance, such situation can
   occur as a result of improper key management policies or hardware
   compromise.  It is therefore inevitable that the design of a security
   system, which is expected to be operational for a long period of
   time, will include the mechanisms for efficient and secure
   cryptographic keys management.

   It is reasonable to assume that after HIP has been widely adopted
   lots of users may not have enough security knowledge to correctly
   deal with their insecure cryptographic keys, and thus an automatic
   key revocation solution will be desired.  So far, only transient
   (session) key revocation issues have been discussed within the HIP
   framework.  Briefly, HIP allows two communicating hosts to update
   their transient keys securely at run time.  However, the key
   revocation issues with permanent keys (i.e., HIs) have not been well
   explored yet.

   During the discussion of this draft, it is assumed that 1) an
   attacker cannot compromise an HI by brutal force during a reasonable
   long period but may need to be removed from usuage for certain
   reasons, 2) an attacker can intercept and modify the packets



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   transported between honest HIP hosts.


2.  Terminology

   BEX (Base Exchange): The handshaking protocol defined in [RFC5201],
   which enable two HIP hosts use their public keys pairs to generate
   key materials for subsequent communication.

   HI (Host Identifier): A public key kept by a HIP host to represent
   the identity of the host.

   HIT (Host Identity Tag): A 128-bit value generated by hashing the
   associated HI.


3.  Key Management

   Key management aims at guaranteeing the security of cryptographic
   keys during the period of their application and includes all of the
   provisions made in a security system design which are related to
   generation, validation, exchange, storage, safeguard, application,
   and replacement of cryptographic keys.  Appropriate key management is
   critical to security mechanisms providing confidentiality, entity
   authentication, data origin authentication, data integrity, and
   digital signatures.  Specifically, a full-fledged key management
   system should be able to support [Menezes et al. 1996]:

   1.  Initialization of system users within a domain;

   2.  Generation, distribution, and installation of keying material;

   3.  Controlling the use of keying material;

   4.  Update, revocation, and destruction of keying material; and

   5.  Storage, backup/recovery, and archival of keying material.


4.  Key Revocation

   Key revocation is an essential functionality of a security system.
   By refreshing cryptographic keys, a security system can reduce the
   dangers of being compromised.  Key revocation is also an important
   step when a security system attempts to confine and recover from the
   damages caused by attacks.  The criteria measuring a key revocation
   mechanism should include security, efficiency, latency, overheads in
   terms of communication, etc.



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4.1.  Background

   Cryptographic keys adopted in a security system can be classified
   into permanent keys and transient keys according to their life
   periods.  As indicated by the name, permanent keys are maintained by
   holders for relatively long periods which can vary from months to
   years.  Because frequent use of permanent keys can damage their
   security strength and reduce their valid periods, in many security
   mechanisms, permanent keys are employed to generate and distribute
   transient keys which are only valid in relatively short periods
   (e.g., within a single TCP session).  Key revocation issues with
   transient keys have been taken account of in most authentication
   mechanisms (e.g., Kerberos, IPSec, SSL, etc.).  For instance, in
   Kerberos, a user can use her password to obtain a session key from a
   KDC; the session key then can be further used to securely discard and
   update old sub-session keys.  The revocation of transient keys is
   also considered in the design of HIP.  A basic handshaking protocol
   (i.e., the HIP Base Exchange) has been specified.  Using it, two
   communicating HIP hosts can employ the authenticated Diffie-Hellman
   algorithm to securely distribute keying material which will be used
   to generate new cryptographic keys in the following communication.
   After a handshake, the hosts are able to refresh their transient keys
   and the corresponding HIP associations, using Update packets.

   The revocation issues with permanent keys are also taken into account
   in lots of key management mechanisms (e.g., PGP, PKI, Peer-to-Peer
   Key Management for Mobile Ad Hoc Networks [Merwe et al. 2007]).
   Particularly, in PKI, key revocation issues are addressed in
   certificate revocation mechanisms.

4.2.  Classification of permanent Key Revocation Mechanisms

   This draft focuses on the issues with permanent key revocation in
   HIP.  In the remainder of this draft, key revocation indicates
   permanent key revocation, unless mentioned otherwise.

   Mechanisms for key revocation can be classified in various ways,
   according to:

   o  Whether additional operations are needed.  If a key revocation
      mechanism does not need any additional operation in the revocation
      process of a cryptographic key, it is called an implicit key
      revocation mechanism.  The basic idea of an implicit HI revocation
      mechanism is to associate a key with a valid period and use
      cryptographic methods to prove the binding between the key and its
      valid period.  Therefore, after the pre-defined period expires,
      the key is obsolete automatically.  For instance, in PKI, a
      Certificate Authority (CA) can issue a certificate for a user in



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      order to assert the association between the user and its public
      key.  The certificate is associated with a life period.  When the
      period expires, the user's public key is revoked automatically.
      If a key revocation mechanism needs to carry out additional
      operations (e.g., notifications) to revoke a cryptographic key, it
      is called an explicit key revocation mechanism.  In different
      explicit key revocation mechanisms, such operations can be
      performed either by a dedicated server or by the owner of the key.
      Compared with implicit key revocation mechanisms, an explicit key
      revocation mechanism has the capability to revoke a cryptographic
      key before its life period expires.  For instance, in X.509
      [RFC2459] based systems, an issuer can generate a list of
      certificates, which were revoked for some reasons before their
      expiring dates, for users to consult.

   o  Whether a trusted third party is needed.  In some revocation
      mechanisms, the status information of a cryptographic key is
      provided by a secure third party.  A proof of validity is
      performed during each request from users, and the secure third
      party provides up-to-date information.  Online Certificate Status
      Protocol (OCSP) for X.509 certificate is such a mechanism.  An
      OCSP client generates an OCSP request that primarily contains the
      information of one or more queried certificates and send it to a
      trusted OCSP server.  After receiving the OCSP request, the server
      creates an OCSP response containing the updated status information
      of the queried certificates.  In some other revocation mechanism,
      validity information is distributed to the requester by a non-
      secured server.  For example, in PGP, a principal can use its
      revoked key to sign a key revocation certificate and upload it to
      a key repository server which only provides a repository service
      and does not make any assertion.

   o  The list is adopted.  According to the information provided, key
      revocation mechanisms can be classified into black list mechanisms
      and white list mechanisms.  A black list mechanism can provide the
      information of the keys which are not valid anymore.  The
      Certificate Revocation List (CRL) is an example of this kind of
      mechanism.  In a CRL, revoked certificates are listed in a signed
      list, so that users can query the information about the revoked
      keys whenever it is convenient.  White list mechanisms, instead,
      only provide information of valid keys.  For example, SSH specify
      a kind of resource record (RR) called SSHFP [RFC4255].  A SSHFP RR
      contains the information of the fingerprint of a valid
      cryptographic key.  If a key needs to be revoked, the associated
      SSHFP RR is removed.  If a user cannot find the associated SSHFP
      RR from DNS, she will believe that the key inquired about is no
      longer valid.




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   o  The way of distributing revocation information.  In a key
      revocation mechanism applying the push model, when a key is
      revoked, a server proactively contacts the related users to inform
      the case.  In contrast, in a key revocation mechanism applying the
      pull model, a client needs to query a server for particular
      revocation information.  OCSP, CRL, and the key revocation
      mechanisms adopted in PGP and SSH all belong to this category.

   There are few discussions about the HI revocation issues with HIP.
   In the current HIP architecture, hosts are allowed to update their
   identifiers arbitrarily without notifying others.  The lack of HI
   revocation mechanism can be taken advantage of by attackers to, for
   instance, escape tracking, bypass ACLs (Access Control Lists),
   impersonate others using the compromised HIs, etc.  In remainder of
   this document, candidate approaches and related issues are discussed.


5.  Implicit HI Revocation in HIP

   Implicit key revocation is the most basic key revocation approach.
   By associating an HI with a life period, the holder of the HI needs
   to update the HI periodically so as to reduce the risk of HI
   compromization.  In addition, life periods of HIs can help users to
   verify how long an HI has been used and how long the HI will still be
   valid.  This enables host managers to define more specific security
   policies.

   Note that the HI and the HIT of a host are cryptographically
   associated.  A revocation of an HI will cause the revocation of the
   corresponding HIT, and vice versa.  The life periods of an HI and its
   HIT are identical; the revocation of a HI implies the revocation of
   the associated HIT, and vice versa.

   The life period of an HI can be specified either by the holder of the
   HI or by a trusted authority.  During HIP BEXs, such life period
   information can be encapsulated in (to be specified) parameters and
   transported within HIP packets.  If the life period of the HI is
   specified by its holder, the holder needs to use the associated
   private key to sign the parameter.  If the life period of the HI is
   specified by a trusted authority, the authority needs to use its
   private key to sign a life period certificate for the HI.  The
   certificate can be encapsulated within a CERT parameter and
   transported in HIP packets.

   Figure 1 illustrates an example HOST_ID parameter which is extended
   to transport the associated life period of an HI.  This parameter can
   be applied in the cases where the life period of the HI is specified
   by its holder.  Similar to the life periods of X.509 certificates,



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   the life period of an HI is specified by a Not Before Time and a Not
   After Time.  In this parameter, the NB Length and NA Length fields
   indicate the lengths of Not Before Time and Not After Time fields
   respectively.  The Not-Before-Time and the Not-After-Time can be in a
   format of either UTCTime or GeneralizedTime defined in [RFC2459].

   During a HIP base exchange, the parameter containing Initiator' s HI
   and the associated life period information is transported in the I2
   packet, while the parameter containing Responder' s HI and the
   associated life period information is transported in the R1 packet.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             HI Length         |DI-type|     DI Length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             NB Length         |           NA Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               Host Identity                   /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /               |              Domain Identifier                /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                               |          Not Before Time      /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                               |          Not After Time       /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                             |          Padding                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Figure 1. An extension of HOST_ID parameter

   This approach enables a holder to specify the life period of its HI.
   It does not rely on any dedicated trusted authority and introduces
   little performance penalty in verifying the life period.  However,
   this solution is less effective in the environments where
   communicating HIP hosts lack sufficient trust; it is difficult for a
   HIP host to identify either the remote host has appropriately defined
   and managed its HI life period or the HI used by the remote host has
   not been compromised.  In order to reduce memory consumption and foil
   deny-of-service attacks, HIP hosts normally do not maintain the
   information of the HIP hosts that they used to communicated with for
   a long period.  In addition, in the current HIP resolution solutions
   (e.g., HIP RR), no information about the life periods of HIs is
   provided.  If a user of a HIP host assigns a new life period with a
   reasonable length for the HI before the expiration of the old life
   period, the update of the life period is unlikely to be detected.
   Moreover, because HITs are treated by applications as ordinary IP
   addresses which have no expiration date, in referral scenarios the



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   receiver of a HIT may not be able to obtain the knowledge of the life
   period of a HIT from the referrer.  In the current HIP resolution
   solutions (e.g., HIP RR), there is no concern about the life periods
   of HIs.  Therefore, in current HIP architectures, the approach cannot
   work properly unless there has already been a certain level of trust
   between two HIP hosts beforehand, that is, a HIP host can believe the
   HI of its communicating partner is within the declared life period
   and has sufficient security strength.

   The issues mentioned above can be largely addressed by assigning a
   trusted authority to manage the life periods of HIs and the binding
   between HIs and HITs.  Dedicated trusted auhtorities may introduce
   complexity into the current HIP architecture, impose additional
   communications (e.g., registration process, generation of certificate
   chain, etc.), and cause issues in terms of scalability and trust.
   However, in many cases they seem to be the only choice.  The benefit
   and the issues brought by dedicated authorities are discussed in
   section 6 in detail.

   The remainder of this sub-section introduces two complementary
   solutions which are able to mitigate the issues of arbitrarily
   modifying HI life periods but impose little performance penalty.  The
   first approach is to facilitate the implicit HI revocation
   functionality with resolution systems systems (i.e., to extend
   resolution systems to provide trustable life-period information of
   HIs).  For example, the HI life-period information could be
   maintained by DNS servers and provided to users just like other
   mapping information.  In order to achieve this, space for the life
   period information needs to be allocated in the resource records sent
   back to users.  In Figure 2, an example extension of the HIP RR with
   life period information is illustrated.  Same as the extended HOST_ID
   parameter in Figure 1, the NB Length and NA Length fields indicate
   the lengths of Not Before Time and Not After Time fields
   respectively.  The Not-Before-Time and the Not-After-Time can be in a
   format of either UTCTime or GeneralizedTime defined in [RFC2459].
















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   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | HIT Length    |  PK algorithm |           PK Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            NB Length          |           NA Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              HIT                              /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                               |        Public Key             /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                               |     Rendezvous Server         /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                               |       Not Before Time         /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                               |       Not After Time          /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /       |
   +-+-+-+-+
   Figure 2. An Extension of HIP RR

   In this approach, information of the life period of an HI, just like
   the other information in the RR, can be provided by an authorized
   user at the registration time.  But after the registration, the life
   period information is only allowed to be updated by the ones who have
   higher privileges (e.g., server managers).  After a user uploads the
   information of a HIP host in an authoritative DNS server, the user is
   not allowed to modify the Not Before Time and Not After Time fields
   of the HI any more.  Moreover, after the life period of the HI has
   expired and is not allowed to be extended, the associated RRs should
   be removed.

   The other approach is to introduce the life period of a HI into the
   generating process of the associated HIT.  For instance, the life
   period of an HI can be used as a part of the input for generating the
   associated HIT.  Therefore it is computationally difficult even for
   the holder of the HI to modify the life period without modifying the
   HIT.  For example, after a host advertises its HIP RR, any attempts
   to modify the life period of the HI can be easily detected, even no
   life period information is provided by the DNS server.  For instance,
   in the case that a host obtains a HIT from its referrer, it needs to
   first obtain the knowledge to access the host holding the HIT from
   resolution servers.  Then it can get the associated HI and the life
   period from the HIT holder, and re-calculate the HIT to verify
   whether the life period of the HIT is valid.  This approach needs
   little modification on the resolution servers and can be applied
   independently.  A disadvantage of this approach is its inflexibility
   in the cases where the life periods of HIs need to be extended.



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6.  Explicit HI Revocation in HIP

   As mentioned previously, in many typical scenarios (e.g., the
   compromise of a key is detected), a cryptographic key need to be
   revoked before its life period expires.  In such cases, explicit key
   revocation is needed.

   When an HI needs to be removed from operational use prior to its
   originally scheduled expiry, the revocation of the HI needs to be
   informed to all the hosts which might be affected.  If there is no
   dedicated third party to rely on, the holder of the HI needs to
   deliver the revocation certificate signed by the associated private
   key to all the affected partners.  The poor scalability of this type
   of solution is always a subject of debates.  First, using this
   solution, the holder an HI may need to maintain a long list of
   information about the partners which will be affected by the
   revocation.  Especially when the number of the partners is big, this
   job can be onerous and error prone.  In addition, because HIP does
   not support multicast, the holder has to generate a notification
   packet for each of its partners, and send them out during the
   revocation.  When the number of related partners increases, the
   holder may have to spend a large amount of bandwidth, memory and
   computing resources in generating and delivering the notification
   packets.  In order to improve the performance of this solution, the
   holder can send the certificate to a limited set of partners.  These
   partners then relay the certificate to others.  However, this
   solution may introduce additional latency and make the delivery of
   the certificate un-reliable.  Besides the above issues, this solution
   requires all the involved partners to be online during an HI
   revocation process, which can be hardly fulfilled on many occasions.
   Basically, this solution is only suitable in the circumstances where
   the number of involved hosts is relatively small and stable.

   The experiences in PKI demonstrate that pull models can be more
   scalable in dealing with a large amount of users, and as a result,
   most of the certification revocation mechanisms (e.g., Certification
   Revocation Lists (CRLs), delta CRLs [RFC2459], and the On-Line
   Certificate Status Protocol (OCSP)) proposed in PKI are based on pull
   models.  In these mechanisms, the revocation information is
   maintained in a third party for users to query whenever it is
   convenient.

   PKI has provided a set of certificate management mechanisms.  On many
   occasions, it is feasible for HIP to take advantage of PKI style
   solutions to address the issues with HI management.

   However, it should be realized that PKI oriented solutions are not
   silver bullets and cannot be utilized to address all the issues that



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   HIP has to encounter.  After HIP has been globally deployed, it is
   expected that there will be billions of HIP users which may belong to
   different organizations and attach to the Internet through different
   ISPs.  Due to the poor scalability of PKI and lack of trust, it is
   extremely difficult (if possible) to put such a big amount of
   geographically distributed users under the control of a unique PKI
   security domain.  Therefore, it is reasonable to assume that there
   will be many different security domains all over the world.  When two
   HIP hosts belong to two different security domains, it may be
   difficult for a host to verify the assertion made by the security
   server in the domain of the other one.  Although there have been
   solutions of generating trust relationship across various security
   domains, all of them impose additional overheads with respect to the
   construction and verification of credential chain and communication
   with remote security servers, which negatively influences the
   performance of HIP.  Therefore, the HIP community argues that two
   HIP-aware hosts should be able to communicate without any additional
   security facilities.  Actually, the only third party server
   introduced in the base-line HIP architecture is the Rendezvous Server
   (RVS) [RFC5204].  A RVS only relays messages for the hosts which
   attempts to communicate with mobile hosts and provides little
   security functionality.  The HIP hosts intending to communicate with
   each other still need to use the HIP Base Exchange protocol to carry
   out authentication and exchange keying material for future
   communications.  However, RVSes can be extended to support HI
   revocation if necessary.  When a mobile host changes its HI, it can
   inform its RVS.  Therefore, when the RVS find that a host attempts to
   access the mobile host with the old HI, the RVS can send the mapping
   information of the antique HI and the new HI to the host.  The RVS
   needs to use its private key to sign the mapping information in order
   to ensure the information will not be tampered with.  Upon receiving
   the mapping information, the remote host can use the new HI in the
   subsequent communications.  Additionally, since it is suggested in
   [RFC5204] that a user get the information of RVSes from DNS, the
   security of the communication between the remote host and DNS servers
   needs to be protected.  Otherwise, an attacker can easily convince a
   witness that she is a legal RVS by forwarding a bogus DNS RR
   consisting of its information to the witness.  DNSSEC can be applied
   to address this issue.

   Also, resolution servers can be potentially adopted to construct a
   global explicit HI revocation mechanism applying a pull model.  For
   instance, when a host intends to revoke its HI, it can send a
   revocation certificate signed by its private key to an authoritative
   DNS server.  After receiving the certificate, the correspondent RR
   will be removed, and thus users will not obtain the information about
   the revoked HI any more.  Therefore, DNS servers can perform as a
   white list HI revocation mechanism, similar to what is specified in



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   SSH.  To avoid the long delay in the spread of revocation information
   caused by caching RRs on DNS resolvers, the TTL (Time To Life) of RRs
   can be set to zero.  In order to secure the revocation information,
   DNSSEC should be adopted.


7.  Related Discussions

7.1.  Influence of HI revocation on Already Generated HIP Associations

   In a BEX, HI key pairs of the both communicating partners are used to
   carry out mutual authentication while the key material for securing
   subsequent communication are generated by the Diffie-Hellman
   algorithm.  Therefore, if an HI key pair is secure at the time when a
   HIP association is generated, the later revocation of the HI key pair
   will not affect the security of the keying material.  Assume there is
   an attacker which has compromised the HI key pair.  It is still
   computationally difficult for the attacker to decrypt the packets
   transported between the communicating partners.  Because the Update
   packets are under the protection of HMAC, the attacker cannot forge
   them to interfere with the communication.  Note that the attacker can
   try to forge Notify packets.  However, according to [RFC 5201] Notify
   packets are only informative, which will not affect the state of the
   communicating partners.  Therefore, if no explicit key revocation
   occurs, the expiry of an HI does not have to affect the security
   strength of HIP associations generated using the HI when it is still
   valid.  They still can be used until they reach their expiring time.
   However, if an HI is found to be compromised, the security of the
   keying material of the already generated HIP associations cannot be
   guaranteed.  In practice, the compromise of a cryptographic key can
   be perceived only after the attacks employing the key are detected.
   It is difficult for one to identify the exact time from which the key
   is no longer secure.  Hence, under this circumstance, the pre-
   generated HIP associations can only be used to deliver revocation
   certificates, as it is difficult for the communicating partners to
   know whether the HI is still secure when the HIP associations were
   generated.

7.2.  HI Refreshment

   In key management mechanisms, key refreshment is concerned with the
   issues of using new cryptographic keys to take place of "old" ones.
   Therefore, it closely related with key revocation.  A refreshment
   procedure of a key can occur either before or after the revocation of
   the key (Note that in the first case the key is still valid).  In
   this section, the issues with HI refreshment in HIP are discussed.

   Ideally, an operational HI should be refreshed before its crypt-



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   period is expired.  In thsi case, the holder can use the old HI to
   establish secure channels, and use Update packets to transport the
   refreshment information to related partners (in a push model) or to
   trusted third parties (in a pull model).  In the Update packets, the
   new HI and other related information are encapsulated.  Therefore,
   before the old HI expires, both HIs are valid, and the HIP
   associations generated with the old HI can still be applied.

   In practice, the third parties deployed for HI revocation can also be
   used to support HI refreshment.  For instance, when using a pull
   model, a host can transport the HI revoking and the refreshing
   information to a third party.  Therefore, when a user inquires of the
   third party about the status information of an HI, the user can get
   the status of the HI inquired about as well as the associated
   refreshment information.

   If an HI needs to be revoked due to accident disclosure or
   compromise, the update of the HI can be a little more complex.
   Although the invalid key can be used to send a "suicide" information
   to others (e.g., resolution systems, RVSes, or any entities which may
   be affected by the revocation), it cannot be used to securely
   transport the refreshment information any more.

   If a host has multiple HIs, it can select a HI still valid to
   securely transport the refreshment information.  The refreshment
   information should consist of both the new HI and the compromised HI.
   This solution requires that the partner communicating with the host
   can ensure that the HI used to generate secure channel and the
   compromised HI are possessed by the same HIP host.  Such knowledge
   can be obtained from resolution systems or provided by the host.  It
   is recommended that there is a HI used only for HI refreshment.

   In the cases where all the HIs of a host become invalid (e.g., the
   host is found to compromised), the host only can distribute the
   refreshment information using an out-of-band way.

   A host can also implement a pull model by directly transporting the
   update information to resolution servers.  If the information is
   forwarded to a DNS server, users can query the latest HI using FQDN
   of the host.  In a resolution system providing ID to locator mapping
   services (e.g., DHT), users can only try to query the resolution
   systems using old HITs.  In this case, besides the IP addresses
   inquired, the resolution system should also provide the latest HIs
   and other useful information.  Note that it is assumed that no two
   HITs of different hosts are identical, even if they are adopted in
   different time period.  In practice, because the length of HITs is
   long, the possibility that two hosts select a same HI can be very
   low.  In order to further reduce the possibility, a user can also



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   provide the life period of the inquired HIT to the resolution server.


8.  Conclusions

   Key revocation is critical for HIP to be secure, practical and
   manageable.  Particularly, HIP hosts are expected to keep working
   securely for a relatively long period, proper key revocation
   mechanisms for HIs must be provided.  This document focuses on pros
   and cons of different key revocations and analyzes their security and
   practicality in different practical scenarios.  Although key
   management has been an active research area for a long period and
   lots of successful key-management systems (e.g., PKI) are widely
   adopted in practice, many issues (e.g., scalability, lack of trust)
   still exist.  There is no solution being found to meet the timeliness
   and performance requirements of all applications and environments
   that HIP is expected to support [McDaniel et al. 2001].  Therefore,
   it is predicted that various HI revocation approaches will be adopted
   after HIP has been globally adopted.


9.  IANA Considerations

   This document makes no request of IANA.


10.  Security Considerations

   The important of HI revocation can be various according to the usage
   of HITs.  When HITs are used for authentication/access control, the
   HI revocation is critical to prevent attackers from using compromised
   HIs to access certain resources illegally.  In the scenarios where
   HIP is purely used as an ID/Locator separation solution to support
   mobility or multi-homing and the authentication issues are addressed
   by other security mechanisms, the HI revocation is less important.

   In the existing HIP architectures, the HI of a HIP host acts as both
   the identifier and the public key of the HIP host at the same time.
   The revocation of the host's public key will result in the change of
   the identifier of the host.  Without the assistance of other
   measures, the host will be regarded as a different one by others.
   The instability issue introduced by the HI revocation must be
   considered in designing identity management and resolution systems
   for HIP hosts.  For instance, during the revocation of a HI, all the
   TCP sessions identified with the associated HIT have to be broken.
   There are two solutions can be considered in addressing this
   problem.The first one is to check the life period of a HI before
   using it to construct a TCP session and guarantee that the HI can be



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   stable during the communication.  The second one is to introduce a
   stable identifier to represent a HIP host for up layer protocols.
   The new identifier should not have to be changed during the update of
   a HI.


11.  Acknowledgements

   Many Thanks to Thomas.R.Henderson for his kindly revision and
   precious comments.


12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2459]  Housley, R., Ford, W., Polk, T., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and CRL
              Profile", RFC 2459, January 1999.

   [RFC5201]  Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
              "Host Identity Protocol", RFC 5201, April 2008.

   [RFC5204]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Rendezvous Extension", RFC 5204, April 2008.

   [RFC5205]  Nikander, P. and J. Laganier, "Host Identity Protocol
              (HIP) Domain Name System (DNS) Extensions", RFC 5205,
              April 2008.

12.2.  Informative References

   [McDaniel et al. 2001]
              McDaniel, P. and A. Rubin, "A Response to "can we
              eliminate certificate revocation list?"", 2001.

   [Menezes et al. 1996]
              MENEZES, A., VAN OORSCHOT, P., and S. AND VANSTONE,
              "Handbook in Applied Cryptography", 1996.

   [Merwe et al. 2007]
              Merwe, J., Dawoud, D., and S. McDONALD, "A Survey on Peer-
              to-Peer Key Management for Mobile Ad Hoc Networks", 2007.

   [Recommendations]



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              Barker, E., Barker, W., Burr, W., Polk, W., and M. Smid,
              "Recommendation for Key Management-Part1-
              General(Revised)", March 2007.


Authors' Addresses

   Dacheng Zhang
   Huawei Technologies Co.,Ltd
   HuaWei Building, No.3 Xinxi Rd., Shang-Di Information Industry Base, Hai-Dian District
   Beijing,   100085
   P. R. China

   Phone:
   Fax:
   Email: zhangdacheng@huawei.com
   URI:


   Dmitriy Kuptsov
   Helsinki Institute for Information Technology
   PO. Box 9800,   TKK FI-02015
   Finland

   Phone:
   Fax:
   Email: dmitriy.kuptsov@hiit.fi
   URI:


   Sean Shen
   CNNIC
   4, South 4th Street, Zhongguancun
   Beijing,   100190
   P.R. China

   Phone:
   Fax:
   Email: shenshuo@cnnic.cn
   URI:











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