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Network Working Group                                        A. Friedman
Internet-Draft                                              Technion IIT
Intended status: Experimental                                 Y. Sheffer
Expires: December 22, 2007                                   Check Point
                                                               A. Shaqed
                                                          Correlix, Inc.
                                                           June 20, 2007

                        Short-Term Certificates

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   have been or will be disclosed, and any of which he or she becomes
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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document describes an extension to IKEv2 that allows an endpoint
   which has authenticated to a gateway to request a short-term
   credential, possession of which proves the authentication.  This
   allows it to prove to a security gateway that it was already
   authenticated by another trusted security gateway, thereby allowing

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   the authentication of the endpoint without user intervention.  This
   credential is a certificate issued by the authenticating gateway for
   a short period of time, which can be used to authenticate the user
   with IKE signature based authentication.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Preliminaries  . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Short-Term Certificate Usage . . . . . . . . . . . . . . . . .  4
   4.  Short-Term Certificates Issue Exchange . . . . . . . . . . . .  5
   5.  Using Short-Term Certificates  . . . . . . . . . . . . . . . .  8
   6.  Expiration of Short-Term Certificates  . . . . . . . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   9.  Operational Considerations . . . . . . . . . . . . . . . . . .  9
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     11.1.  Normative References  . . . . . . . . . . . . . . . . . . 10
     11.2.  Informative References  . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12

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1.  Introduction

   Many organizations manage one or more security gateways that provide
   IPsec [RFC4301] services, to allow secured connectivity between
   roaming endpoints and the organizational site, as well as between the
   organizational sites themselves.  In most cases, an endpoint needs to
   connect to only one security gateway to gain access to internal
   resources.  However, several situations may require an endpoint to
   connect to additional security gateways of the same organization.
   For example, this may happen when the organization manages multiple
   entry points to the internal network for failover, or when the
   endpoint needs to connect to hosts lying behind multiple security

   Connection to each of the security gateways requires mutual
   authentication between the endpoint and the security gateway.  The
   IKEv2 Protocol [RFC4306] allows the use of legacy authentication
   systems along with authentications that use public key signatures and
   shared secrets.  Legacy strong authentication systems typically
   require active participation of the user operating the endpoint.  For
   example, methods using a token may require the user to enter a
   passcode appearing on the token.  IKE authentication methods may
   require such an intervention as well, e.g., entering a password to
   get access to a certificate file.  Even if the endpoint was
   previously authenticated by one security gateway, any connection to
   an additional security gateway will require additional

   This document describes an extension to IKEv2 that allows an endpoint
   to perform an IKE exchange to request a "short term certificate".
   This short term certificate can be used in subsequent IKE
   authentications to prove to a security gateway that the endpoint was
   already authenticated by another trusted security gateway, thereby
   allowing the authentication of the endpoint without user
   intervention.  The basic idea is to allow a security gateway to vouch
   for the authenticity of an endpoint, thereby saving the need for user
   involvement in the recurring authentication.

   The protocol presented here is similar in concept to the one
   suggested in the now expired draft of the Pre-IKE Credential
   Provisioning Protocol [PIC].  PIC was proposed as a form of using
   legacy authentication methods to enable certificate enrollment for
   use in IKE.  While PIC is performed outside of IKE, the protocol we
   propose is an extension to IKEv2 used by an entity already
   authenticated in a former IKEV2 exchange.

   A work in progress [I-D.ohba-preauth-ps] focuses on pre-
   authentication in the context of EAP [RFC3748], and is mainly driven

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   by the need to provide seamless and fast inter-technology handovers
   for mobile devices.  In contrast, short-term certificates do not
   assume a common EAP server behind the gateways, and do not require
   gateways to communicate with each other.

2.  Preliminaries

   The methods described in this document depend on trust between
   security gateways.  A security gateway should be able to verify that
   a certificate presented by an endpoint was indeed issued by another
   trusted security gateway, and to establish the integrity of the
   presented certificate.  The way this trust is established and
   maintained (e.g., PKI [RFC3280]) lies outside the scope of this

3.  Short-Term Certificate Usage

   The use of Short-Term Certificates takes the following form:

   1.  At any time following a successful mutual authentication and the
       establishment of an IKE SA, an endpoint MAY send a request for a
       Short Term Certificate.

   2.  Subject to security gateway configuration and policy, the gateway
       issues a Short Term Certificate and sends it back to the
       endpoint.  The lifetime of the Short Term Certificate will
       typically be the timeout until re-authentication is required.
       According to the IKEv2 specification, a gateway that does not
       support this type of request will send an empty CFG_REPLY or a
       response with no CFG_REPLY at all.

   3.  During the validity period of the certificate, the endpoint MAY
       use the certificate for signature-based authentication with any
       security gateway that trusts the issuer of the certificate,
       instead of using any default authentication method.  Note that
       any security gateway that conforms to IKEv2 specification can
       authenticate this certificate, whether or not it conforms to the
       Short Term Credentials specification.

   The following sections describe the message exchange required for
   issuing a Short-Term Certificate, how the certificate is used and how
   expiration of a certificate should be handled.

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4.  Short-Term Certificates Issue Exchange

   At any point after the security gateway authenticated the endpoint
   and the IKE SA was established, the endpoint MAY initiate a Short
   Term Certificate request and send it to the security gateway.  A
   Short Term Certificate may also be requested from a security gateway
   which did not authenticate the endpoint directly, but authenticated
   it based on another Short Term Certificate (i.e., authentication with
   Short Term Certificates is transitive).

   A Short Term Certificate Request is sent as a Configuration Payload
   of type CFG_REQUEST in an INFORMATIONAL exchange.  The reply is sent
   as a Configuration Payload of type CFG_REPLY in an INFORMATIONAL
   exchange.  The following attribute types have been defined for the
   Short Term Certificate issue exchange:

    | Attribute Type       | Value | Request Length | Response Length |
    | STC_CERTIFICATE_TYPE | TBD+0 | 1 octet        | 1 octet         |
    | STC_ROOT_CA          | TBD+1 | 0 or more      | --              |
    | STC_CERTREQ          | TBD+2 | 0 or more      | --              |
    | STC_CHAIN            | TBD+3 | 1 octet        | --              |
    | STC_CERTIFICATE      | TBD+4 | --             | 0 or more       |
    | STC_LIFETIME         | TBD+5 | --             | 4 octets        |

   o  STC_CERTIFICATE_TYPE - An encoding of a certificate type which
      indicates the type of certificate provided in the STC_CERTIFICATE
      attribute, according to the Certificate Encoding values provided
      for the Certificate Payload in Sec. 3.6 of IKEv2 [RFC4306] .  In a
      request message, this field defines the encodings of all requested
      attributes.  In a reply message, this value MUST be identical to
      the one appearing in the request, and MUST determine the encodings
      of all included attributes.  For interoperability, all
      implementations MUST support type 1, "PKCS #7 wrapped X.509
      certificate" [RFC2315].

   o  STC_ROOT_CA - MUST NOT be sent in a reply.  An encoding
      identifying the Certificate Authority on whose trust chain a
      signature is requested.  If STC_CERTIFICATE_TYPE is type 1, this
      field MUST contain the Binary Distinguished Encoding Rules (DER)
      encoding of an ASN.1 X.500 Distinguished Name [ITU.X501.1993] that
      identifies a Certificate Authority.  In a request message, one
      trusted Certificate Authority MAY be provided.

   o  STC_CERTREQ - MUST NOT be sent in a reply.  An encoding of a
      certification request, including the requesting endpoint identity,

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      a public key to be associated with this identity, and a proof of
      possession of the matching private key.  If STC_CERTIFICATE_TYPE
      is type 1, this field MUST contain a PKCS #10 [RFC2986] encoded
      certification request.

   o  STC_CHAIN - MUST NOT be sent in a reply.  A flag used to denote
      whether a single certificate is required (NO_CHAIN=0) or a full
      certificate chain (FULL_CHAIN=1).  If the client already possesses
      the certificates required to construct a certificate chain, it may
      set this variable to NO_CHAIN (0) in the request message to save
      bandwidth.  This flag is a hint: the responder MAY reply with a
      full chain even if no chain was requested, and vice versa.

   o  STC_CERTIFICATE - MUST NOT be sent in a request.  Whenever the
      request is accepted and a short term certificate is issued, the
      responder MUST set this attribute with an encoding of the issued
      certificate according to the type that appeared in the request
      message, and set the STC_LIFETIME attribute in accordance with the
      certificate contents.  If STC_CERTIFICATE_TYPE is type 1, this
      field MUST contain a PKCS #7 encoding of the issued certificate.

   o  STC_LIFETIME - MUST NOT be sent in a request.  In the reply
      message, this attribute contains the remaining lifetime of the
      Short Term Certificate, in seconds.  This value can be used by the
      endpoint to keep track of the Short Term Certificate expiration
      time, so a new certificate can be requested and the old
      certificate deleted by expiration.  This attribute is required
      since the client cannot rely on the notBefore and notAfter fields
      supplied in the certificate: there is no guarantee that the
      endpoint's clock is synchronized with the security gateway's
      clock.  In addition, the notBefore field may be set several
      minutes prior to certificate creation time (to compensate for
      minor clock deviation between security gateways).

   The endpoint MAY use a fixed private/public key pair for all Short
   Term Credential exchanges, or create a different key pair for each
   Short Term Certificate in use.  Security considerations (see the
   Security Considerations section (Section 8)) may apply in case a
   fixed private/public key pair is used for more than one Short Term
   Credential exchange.

   In case a security gateway receives a malformed Short Term
   Certificate request, it MUST send a notification payload of type
   INVALID_SYNTAX in the response message.  If the Short Term
   Certificate request is rejected for any other reason (e.g., gateway
   configuration or security policy), the gateway MUST send a
   notification payload of type STC_UNSUPPORTED in the response message
   to inform that the request was rejected.  In both cases, the

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   CFG_REPLY payload MUST either be sent empty or dropped altogether
   from the response.

   When a security gateway issues a Short Term Certificate, the
   following restrictions apply:

   o  The Short Term Certificate MUST be a legal IKEv2 certificate, per
      IKEv2 [RFC4306] and PKI4IPsec

   o  It is RECOMMENDED that the private/public keys used by the
      security gateway to issue Short Term Certificates will be
      different from those used for authenticating the security gateway.

   o  The gateway MUST check that the identity appearing in the received
      certification request matches the identity of the endpoint
      associated with the IKE SA used to perform the exchange (similarly
      to the specification in PKI4IPsec
      [I-D.ietf-pki4ipsec-ikecert-profile]).  In case of a mismatch, the
      gateway MUST reject the request.  In other words, a gateway MUST
      NOT issue a certificate which identifies a different entity than
      the one associated with the IKE SA.

   o  The gateway MUST verify the signature in the certification
      request, to assert that the client possesses the private key
      corresponding to the public key being certified.  In case of an
      invalid signature, the gateway MUST reject the request.

   o  The Short Term Certificate expiration time MUST NOT exceed the
      remaining time for repeated authentication [RFC4478], if such time
      is defined.  If there is no defined time for repeated
      authentication, the gateway MUST limit the expiration time to no
      more than 24 hours.  Note that IKE SA lifetime is irrelevant for
      determining Short Term Certificates expiration time, since
      rekeying IKE SAs does not require re-authentication.

   o  If the security gateway has a signing key that was certified by
      the root CA described in the Short Term Certificate request, then
      it MUST use this key to sign the Short Term Certificate.  If no
      signing key matches a requested root CA, then the gateway MUST
      reject the request.  If no STC_ROOT_CA was specified in the
      request, the security gateway MAY choose the root CA to use.
      Alternatively, it MAY reject the request.

   o  The issued certificate MUST be of the type requested in a
      STC_CERTIFICATE_TYPE attribute of the request message.  In other
      words, the STC_CERTIFICATE_TYPE attribute has the same value in
      the request and the response.

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   o  The key associated with the subject of the certificate MUST be the
      public key sent in the Short Term Certificate request.

   o  When a full certificate chain should be sent to the endpoint, it
      is RECOMMENDED to make use of "Hash and URL" formats for the
      certificate chain to keep message size below the maximum UDP
      message size supported.

5.  Using Short-Term Certificates

   Once an endpoint acquired a Short Term Certificate from a security
   gateway, this certificate can be used for signature based
   authentication with any other security gateway that trusts the
   issuing gateway.

   Note that certificate specifications are flexible enough to allow for
   transferring proprietary authentication-related information from the
   issuing security gateway to any other security gateway which will
   validate the Short Term Certificate.  Such proprietary extensions and
   their implications on security are out of the scope of this document.

6.  Expiration of Short-Term Certificates

   An endpoint SHOULD NOT keep a Short Term Certificate after its
   expiration time was reached, since trying to use this certificate
   would result in a failed authentication.  It is RECOMMENDED to
   refrain from using a Short Term Certificate for authentication if its
   expiration time is very close (e.g., less than ten minutes), since a
   repeated authentication [RFC4478] may take place once the newly
   created IKE SA is expired.

   When the user terminates communications with a site ("logs off"), the
   Short Term Credentials associated with the site MUST be destroyed.
   Typically this will occur in conjunction with deletion of IKE_SAs
   with the site.  However IKE_SAs may also be deleted without explicit
   user action.

7.  IANA Considerations

   The proposed extension requires IANA allocations for the "TBD"
   attribute types and the STC_UNSUPPORTED notify message type described
   in Section 4.

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8.  Security Considerations

   Since certificates are widely used as long term credentials, special
   care should be taken to prevent abuse of Short Term Certificates
   which would lead to security risks.

   The expiration time of the Short Term Certificate can never be later
   than a time limit defined for repeated authentication.  This
   restriction prevents the use of Short Term Credentials for artificial
   extension of the IKE SA validity time, bypassing actual
   authentication of the user.

   The Short Term Certificate issue exchange is protected with the
   established IKE SA.  This maintains the confidentiality and the
   integrity of the exchange.  Impersonating a security gateway would
   not allow a malicious user to abuse a Short Term Certificate and
   impersonate a valid user.  Even if a malicious user was able to
   acquire a Short Term Certificate of another user, knowledge of the
   private key is still required to be able to use the Short Term
   Certificate successfully.

   Lifetime of private/public key pairs needs to be considered if the
   same key pair is used for more than a single Short Term Certificate
   exchange.  The client SHOULD generate new private/public key pairs at
   regular intervals, in accordance with security policy.  This is the
   same situation as applies in all certificate request protocols: The
   same private/public key pair can be used in multiple requests, but
   its lifetime should nonetheless be considered limited.

9.  Operational Considerations

   Because of the granularity of Short Term Certificates expiration
   time, the administrator MUST prevent clock rollback on gateways and
   synchronize the clocks of mutually trusting security gateways.  For
   example, the NTPv3 [RFC1305] or SNTPv4 [RFC4330] protocols can be
   implemented to provide this functionality.  When such protocols are
   implemented to provide gateway synchronization, they SHOULD be
   properly secured to prevent attacks based on desynchronizing security
   gateway clocks.

10.  Acknowledgements

11.  References

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11.1.  Normative References

              Korver, B., "The Internet IP Security PKI Profile of
              IKEv1/ISAKMP, IKEv2, and PKIX",
              draft-ietf-pki4ipsec-ikecert-profile-12 (work in
              progress), February 2007.

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

   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 2005.

11.2.  Informative References

              Ohba, Y., "EAP Pre-authentication Problem Statement",
              draft-ohba-preauth-ps-01 (work in progress), March 2007.

              International Telecommunications Union, "Information
              Technology - Open Systems Interconnection - The Directory:
              Models", ITU-T Recommendation X.501, ISO Standard 9594-2,

   [PIC]      Sheffer, Y., Krawczyk , H., and B. Aboba , "PIC, A Pre-IKE
              Credential Provisioning Protocol, Internet-draft
              (expired), draft-ietf-ipsra-pic-06.txt", October 2002.

   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
              Specification, Implementation", RFC 1305, March 1992.

   [RFC2315]  Kaliski, B., "PKCS #7: Cryptographic Message Syntax
              Version 1.5", RFC 2315, March 1998.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              November 2000.

   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)",
              RFC 3748, June 2004.

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   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4330]  Mills, D., "Simple Network Time Protocol (SNTP) Version 4
              for IPv4, IPv6 and OSI", RFC 4330, January 2006.

   [RFC4478]  Nir, Y., "Repeated Authentication in Internet Key Exchange
              (IKEv2) Protocol", RFC 4478, April 2006.

Authors' Addresses

   Arik Friedman
   Technion IIT
   Haifa 32000

   Email: arikf@cs.technion.ac.il

   Yaron Sheffer
   Check Point Software Technologies Ltd.
   5 Hasolelim st.
   Tel Aviv  67897

   Email: yaronf at checkpoint dot com

   Ariel Shaqed (Scolnicov)
   Correlix, Inc.
   Herzelia Pituah

   Email: ariel.shaqed+ietf@gmail.com

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