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Network Working Group                                           D. Zhang
Internet-Draft                               Huawei Technologies Co.,Ltd
Intended status: Informational                                D. Kuptsov
Expires: April 26, 2011
                                                                 S. Shen
                                                        October 23, 2010

                   Host Identifier Revocation in HIP


   This document mainly analyzes the key revocation issue with host
   identities (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 antique cryptographic
   keys from operational usages 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 key issues that a
   designer of HI revocation mechanisms need to carefully consider.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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 April 26, 2011.

Copyright Notice

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   Copyright (c) 2010 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.  Terminologies  . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Key Management . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Key Revocation . . . . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Classification of permanent Key Revocation Mechanisms  . .  4
     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 . . . . . . . . . . . . . . . . . . . . . . 14
   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 a HIP architecture [RFC5201], a HIP host needs to generate a
   "permanent" public key pair before it communicates with other HIP
   hosts.  The public key is used as its HI while the private key is
   kept securely by the host.  When two HIP hosts attempt to initiate a
   communication (e.g., a TCP session), they can take advantage of their
   HI key pairs to perform mutual authentication and distribute keying
   materials for securing subsequent data and signaling packets.
   Therefore, the security of HIP architectures largely relies on the
   security of those HI key pairs.  If the HI key pair of a HIP host is
   revealed, an attacker can easily impersonate the victim to carry out
   malicious attacks without being detected.

   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 having been employed for a
   certain period, a cryptographic key will be in more dangers of
   compromise.  As time elapses, an attacker can collect more materials
   (e.g., encrypted data, signatures and associated plain texts, etc.)
   and obtain more time to compromise the key.  In addition, unexpected
   key disclosure is a common practical issue, which may be caused by,
   e.g., improper key management policies or hardware stealing.
   Consequently, in the design of a security system which is expected to
   execute for a long period, the issues with revoking the cryptographic
   keys which do not have enough security strengths must be considered.

   In current HIP architectures, the key revocation issues with
   transient (session) keys have been well discussed.  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.  No facility is provided for HI
   revocation either.

2.  Terminologies

   BEX: Base Exchange

   HIP: Host Identity Protocol

   PKI: Public Key Infrastructure

3.  Key Management

   Key management aims at guaranteeing the security of cryptographic
   keys during the period of their application and includes all of the

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   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 antique 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, and etc.

4.1.  Classification of permanent Key Revocation Mechanisms

   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 be various from months
   to years.  Because frequent usages 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 antique 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 proposed.  Using it, two
   communicating HIP hosts can employ the authenticated Diffie-Hellman

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   algorithm to securely distribute keying materials which will be used
   to generate new cryptographic keys in the following communications.
   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, without mentioned otherwise.

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

   o  Whether additional operations are needed.  If a key revocation
      mechanism does not need any additional operation in the revoking
      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
      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 secure 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

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      party provides up-to-date information.  Online Certificate Status
      Protocol (OCSP) for X.509 certificate is such a mechanism.  An
      OSPF client generates an OCSP request that primary 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 uploaded it
      to a key repository server.  The server is regarded as "non-
      secured" only because the server only provides a repository
      service and does not make any assertion.  Certificates themselves
      are individually secured by the signatures thereon, and need not
      be transferred over secured channels.  In fact, authorization
      policies to a repository server in the form of write and delete
      protection is mandatory so as to enable maintenance and update
      without denial of service.

   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.

   o  The way of distributing revocation information.  In a key
      revocation mechanism applying the push model, upon 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),

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   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 simplest 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 that the HI is
   compromised.  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 is enable host managers to define more specific security

   Note that the HI and the HIT of a host are cryptographic associated.
   A revocation of an HI will cause the revocation of the correspondent
   HIT, and vice versa.  Therefore, without losing generality, in this
   document we assume that the life periods of an HI and its HIT are
   identical and they are generated and revoked concurrently in a same

   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 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 extended HOST_ID parameter which is able to
   transport an HI and the associated life period.  This parameter can
   be adopted in the cases where the life period of the HI is specified
   by its holder.  Similar with the live periods of X.509 certificates,
   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 Responser' s HI and the
   associated life period information is transported in the R1 packet.

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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
   |              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

   The approach to enabling a holder to specify the life period of its
   HI does not rely on any trusted third party and introduces little
   performance penalty in verifying the life period.  However, a concern
   about this approach is how to ensure that HIP hosts will
   appropriately define and manage the life periods of their HIs.  In
   practice, the revocation and refreshment of an HI can be quite
   complex.  Apart from updating the key material locally, additional
   operations also need to be performed (e.g., updating the associated
   HIP resource record in DNS, proactively informing the partners which
   may be affected by the revocation, etc.).  Therefore, a lazy manager
   of a HIP host may attempt to avoid refreshing the HI and HIT of her
   host.  If the manager assigns an extremely long life period for its
   HI, other HIP hosts can easily detect the problem and refuse to
   communicate with the host.  However, if the manager selects to assign
   a new life period with a reasonable length for her HI whenever the
   old life period has expired, the renewal of the life period can be
   difficult to be detected in current HIP architectures.  For instance,
   in practice a HIP host normally does not maintain the HIs and other
   related information of its communicating partners for a long period,
   in order to reduce memory consumption and foil deny-of-service
   attacks.  Moreover, because HITs are treated by applications as
   ordinary IP addresses which have no expire date, In referral
   scenarios the 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.  On such occasions, a host can only
   obtain the life period information from its communicating partner

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   (i.e., the holder of the HI).  Therefore, in current HIP
   architectures, the approach which allows a holder to specify the life
   period of its HI can only be feasible in the environments where there
   has already been a certain level of trust between two HIP hosts
   beforehand, that is, a HIP host can believe its communicating partner
   has specified an appropriate life period for its HI and will only
   attempt to use it within the valid period.

   The issues mentioned above can be largely addressed by assigning a
   trusted authority to manage the life periods of HIs.  However, a
   dedicated trust third party 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.  The details of the
   issues imposed by such dedicated authorities are discussed in section

   In the remainder of this sub-section, we introduce two complementary
   approaches to mitigating the issues of arbitrarily modifying HI life
   periods while imposing little performance penalty to HIP hosts.  The
   first approach is to extend resolution systems (e.g., DNS servers) to
   provide trustable life-period information of HIs.  In this approach,
   the life-period information can be encapsulated in the same packet
   with other mapping information and sent back to users so as to
   eliminate addition communicating overheads between users and
   resolutions systems.

   In order to achieve this, spaces for the life period information
   needs to be allocated in the resource records sent back to users.  In
   Figure 2, an 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

   The basic functionality of a resolution server is to provide mapping
   information for users.  In practice, it is normally the
   responsibility of authorized users to maintain and update the
   contents of RRs while resolution severs can verify the contents of
   RRs against certain security policies.  Therefore, 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.  After the registration, the life period
   information is only allowed to be updated by the ones who have higher
   privileges (e.g., server managers).

   Let us use DNS servers as an example.  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, the associated RRs needs to be removed.

   Until now, the ID to Locator mapping solutions in HIP has not been
   standardized yet.  We argue that it is desired to integrate the
   implicit key revocation functionality into such systems.

   The second approach is to introduce the life periods of HIs into the
   generating process of HITs.  For instance, the life period of an HI
   can be used as a part of the input for generating the associated HIT.
   This approach makes it computational difficult for the holder of an
   HI to modify the life period without modifying the associated HIT.
   Therefore, after a host advertises its contacting information in

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   resolution servers, any attempts to modify the life period of the HI
   can be easily detected.  For instance, in the case 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.

6.  Explicit HI Revocation in HIP

   As mentioned previously, in many typical scenarios a cryptographic
   key should not be used any more even when it is still in its valid
   life period.  For instance, when a key is detected to be compromised,
   it must to be revoked immediately even if it has not reached its
   expiration date.  In such a case, 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 of all, this
   solution requires the holder an HI to maintain a long list of
   information about the partner which may be affected by the
   revocation; 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,

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   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

   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
   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 chains, communications
   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 antique 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.  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

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   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
   server 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, just similar with what specified
   in 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 this sub-section, we investigate the possibility of using already
   generated HIP associations to transport the update information after
   the correspondent HI key pair is no longer valid.

   In a BEX, HI key pairs of the both communicating partners are used to
   carry out mutual authentication while the key materials 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 materials.  Assume there
   is an attacker which has compromised the HI key pair.  It is still
   computational 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 communications.  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 will not affect the security
   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 materials 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.

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   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

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, we briefly discuss the issues with HI refreshment in

   Ideally, the refreshment of an operational HI should be performed
   before its crypt-period is expired.  That is, when an HI refreshment
   process is performed, the HI expected to be updated is still valid.
   The holder then 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, as a
   host cannot use the invalid key to securely transport the refreshment
   information any more.  If a host has multiple HIs, it can also select
   a valid HI to generate secure channels to transport the refreshment
   information of another HI.  The refreshment information can be
   transported in an Update packet in which both the new HI and the old
   HI are contained.  This solution require that the partner
   communicating with the host can ensure that the HI used to generate
   secure channel and the HI going to be refreshed are possessed by the
   same HIP host.  Such knowledge can be obtained from resolution
   systems or provided by the host.  In the cases where all the HIs of a

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   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 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 provide the
   life period of the inquired HIT in a query.

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 con/
   pro of different key revocations and analyzes their performances 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
   predicable 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 whole document is about security.

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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.

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

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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

   Email: zhangdacheng@huawei.com

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

   Email: dmitriy.kuptsov@hiit.fi

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

   Email: shenshuo@cnnic.cn

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