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Versions: 01 02                                                         
     ROAMOPS Working Group                                      Pat Calhoun
     INTERNET-DRAFT                                        Sun Microsystems
     Category: Standards Track                                Bernard Aboba
     <draft-ietf-roamops-roamsec-02.txt>                          Microsoft
     20 July 1998
     
     
                         End-to-End Security in Roaming
     
     
     1.  Status of this Memo
     
     This document is an Internet-Draft.  Internet-Drafts are working docu-
     ments of the Internet Engineering Task Force (IETF),  its  areas,  and
     its  working groups.  Note that other groups may also distribute work-
     ing documents as Internet-Drafts.
     
     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  mate-
     rial or to cite them other than as "work in progress."
     
     To  learn  the  current status of any Internet-Draft, please check the
     "1id-abstracts.txt" listing contained in  the  Internet-Drafts  Shadow
     Directories   on   ftp.ietf.org   (US  East  Coast),  nic.nordu.net
     (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).
     
     The  distribution  of  this memo is unlimited.  It is filed as <draft-
     ietf-roamops-roamsec-02.txt>, and  expires January 15,  1999.   Please
     send comments to the authors.
     
     
     2.  Abstract
     
     As noted in Roaming Requirements, there is a need for end-to-end secu-
     rity in roaming, including end-to-end integrity protection, and confi-
     dentiality.  In roaming implementations based on proxy chaining, pack-
     ets are routed between the NAS and home server  through  a  series  of
     proxies.   Current  roaming  implementations  provide  only hop-by-hop
     security, guarding only against modification  of  packets  in  transit
     between  hops.  This makes it possible for untrusted proxies to modify
     packets sent between a NAS and a home  server  without  detection,  as
     well  as  to decrypt PAP passwords, Tunnel passwords, and other hidden
     attributes which are available to it in cleartext.
     
     This document provides a framework for end-to-end security in roaming,
     making  it  possible  to  provide  end-to-end  message  integrity  and
     attribute hiding through addition of three new attributes.
     
     
     3.  Introduction
     
     As noted in [2], there is a need for end-to-end security  in  roaming,
     including  end-to-end  integrity  protection, and confidentiality.  In
     
     
     
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     roaming implementations based on proxy chaining,  packets  are  routed
     between  the NAS and home server through a series of proxies.  Current
     roaming implementations, as described in [1] provide  only  hop-by-hop
     security,  guarding  only  against  modification of packets in transit
     between hops.  This makes it possible for untrusted proxies to  modify
     packets  sent  between  a  NAS and a home server without detection, as
     well as to decrypt PAP passwords, Tunnel passwords, and  other  hidden
     attributes which are available to proxies in cleartext.
     
     This document provides a framework for end-to-end security in roaming,
     making  it  possible  to  provide  end-to-end  message  integrity  and
     attribute  hiding  through addition of three new attributes, Security-
     Parameter-Index, Hidden and End-to-End-Signature.  In  this  document,
     it  is  assumed that a key has previously been established between the
     two endpoints. It is this key which is used in calculation of the mes-
     sage   integrity  check,  as  well  as  in  end-to-end  encryption  of
     attributes. Future documents will discuss key exchange issues.
     
     The Security-Parameters-Index attribute is used to identify the  secu-
     rity  association  within  which  the  End-to-End-Signature and Hidden
     attributes are to be evaluated. Note that in the case where an  inter-
     mediate proxy implements policy, it is possible for a security associ-
     ation to be established between the intermediate proxy  and  the  home
     server,  NAS,  or  local proxy. For example, an intermediate proxy may
     immediately send an Access-Reject to the NAS in response to an Access-
     Request, without having first forwarded it to the home server. In this
     case, the intermediate proxy and NAS would need to establish  a  secu-
     rity  association  in order to permit verification of the authenticity
     of the Access-Reject.
     
     Using the Hidden attribute, it is possible for the client or server to
     protect an attribute end-to-end. This is accomplished by encapsulating
     and then encrypting another attribute within the Hidden attribute.
     
     Using the End-to-End-Signature attribute, it is possible for a  client
     or server to provide a keyed MAC which will allow end-to-end integrity
     protection. The keyed MAC is calculated over the immutable portions of
     the  packet  header,  as  well as all of the attributes preceeding the
     End-to-End-Signature attribute. This makes it possible for some or all
     of  the attributes in the packet to be protected, at the sender's dis-
     cretion.
     
     While a proxy may add, delete or  modify  unprotected  attributes,  it
     MUST NOT add, delete or modify protected attributes so that the valid-
     ity of the keyed MAC can be maintained. A proxy also MUST  NOT  modify
     an  End-to-End-Signature  or  Hidden  attribute. On receiving a packet
     including an End-to-End-Signature attribute, an end system  implement-
     ing  end-to-end  security  MUST  check  the  validity  of  the message
     integrity check and MUST silently discard the packet  if  the  message
     integrity check cannot be verified.
     
     By  allowing  the message integrity check to be applied to a subset of
     attributes selected by the sender, and by allowing  attributes  to  be
     hidden  individually,  these  extensions  enable  end-to-end  security
     
     
     
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     functionality while at the same time enabling proxies to  continue  to
     implement  policy.  As  described in [8], policy function is a central
     part of the roaming architecture since roaming is inherently an inter-
     domain activity.
     
     
     4.  Requirements language
     
     In   this   document,  the  key  words  "MAY",  "MUST,   "MUST   NOT",
     "optional", "recommended",  "SHOULD",  and  "SHOULD  NOT", are  to  be
     interpreted as described in [7].
     
     
     5.  Transition
     
     Since  NAS  devices will not initially implement the End-to-End-Signa-
     ture and Hidden attributes, it is envisaged that these attributes will
     initially  be deployed on local proxies and home servers. In this sce-
     nario, the local proxy will be configured to employ  end-to-end  secu-
     rity for those NAS devices that do not support this. In this case, the
     local proxy  would  add  End-to-End  security  attributes  to  Access-
     Requests  destined  for  the  home server and would process End-to-End
     security attributes in an Access-Accept, Access-Reject or Access-Chal-
     lenge originated by the home server.
     
     Note  that  if  a  NAS  is end-to-end security enabled, then the local
     proxy will receive an Access-Request from the NAS with an  End-to-End-
     Signature  and  will  not  need  to add its own. As a result, a packet
     should include at most one End-to-End-Signature  attribute.  A  packet
     may contain more than one Hidden attribute.
     
     
     6.  Proposed attributes
     
     
     
     6.1.  Security-Parameter-Index
     
     Description
     
        The  purpose  of the Security-Parameter-Index attribute is to iden-
        tify the security association within which the End-to-End-Signature
        and Hidden attributes should be evaluated.
     
     A  summary  of  the Security-Parameter-Index attribute is shown below.
     The fields are transmitted from left to right.
     
     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 2
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Type     |    Length     |            Value
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Value (cont'd)           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     
     
     
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     Type
     
        ? for Security-Parameter-Index
     
     Length
     
        6
     
     Value
     
        The Value field is four octets. This serves as an index identifying
        the security association established between the two end-points.
     
     
     6.2.  End-to-End-Signature
     
     Description
     
        This  attribute  provides for the use of a keyed-MAC to be verified
        end-to-end.
     
        A summary of the End-to-End Signature  attribute  is  shown  below.
        The fields are transmitted from left to right.
     
        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 2
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |      Type     |    Length     |   Protocol    |      String
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |      String...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     
        Type
     
           ? for End-to-End-Signature
     
        Length
     
           19 (protocol 1)
     
        Protocol
     
           The  protocol field specifies the authentication algorithm to be
           used in computing the message authentication code (MAC) which is
           placed in the string field.
     
           If  the protocol field is 1, the HMAC protocol, described in [6]
           is used, along with the MD5  algorithm  described  in  [5].  All
           implementations  MUST support HMAC-MD5. No other protocol values
           are defined.
     
        String
     
           The End-to-End-Signature is an calculated  using  the  algorithm
     
     
     
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           specified  in  the  protocol field. The MAC is computed over the
           portion of the RADIUS packet from the beginning until the end of
           the  End-to-End-Signature  attribute, including Type, ID, Length
           and authenticator.  In calculating the End-to-End-Signature, the
           authenticator should be considered to be filled with zeroes, and
           the End-to-End-Signature string should be considered to be  six-
           teen octets of zero.
     
           The  portion of the RADIUS packet after the End-to-End-Signature
           attribute is not included in the calculation of the  MAC.   This
           makes  it  possible  for  a  proxy  to  add,  modify,  or delete
           attributes placed after the End-to-End-Signature attribute. As a
           result,  attributes  placed  prior  to  the End-to-End-Signature
           attribute can be considered "protected" and those  placed  after
           the  attribute  can be considered to be "unprotected." Note that
           in order for the End-to-End-Signature to be verified, the  proxy
           MUST maintain attribute order.
     
     
     6.3.  Hidden
     
     Description
     
        The  purpose of the Hidden attribute is to make possible end-to-end
        encryption of individual attributes. This would  make  it  possible
        for  attributes such as User-Password or Tunnel-Password to be sent
        securely between a RADIUS client and a server, without risk of com-
        promise by an untrusted proxy.
     
        A  summary  of the Hidden attribute is shown below.  The fields are
        transmitted from left to right.
     
        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 2
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |      Type     |    Length     |        String
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |      String...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     
        Type
     
           ? for Hidden
     
        Length
     
           >=4
     
        String
     
           The String field includes the  encapsulated  ciphertext  of  the
           attribute which is to be hidden end-to-end. The hidden attribute
           is encrypted using a key and encryption algorithm which had pre-
           viously  been established between the two endstations. Note that
     
     
     
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     INTERNET-DRAFT                                            20 July 1998
     
     
           due  to  the  253  octet  limitation  on  the  size  of   RADIUS
           attributes,  the  encapsulated attribute may be a maximum of 251
           octets in length.
     
     
     7.  Table of Attributes
     
     The following table provides a guide to which attributes may be  found
     in which kind of packets.
     
        Request   Accept   Reject   Challenge   #    Attribute
        0-1       0-1      0-1      0-1         ?    End-to-End-Signature
        0+        0+       0        0+          ?    Hidden
        0-1       0-1      0-1      0-1         ?    SPI
     
     
     8.  Security issues
     
     The  following  security  threats have been identified in roaming sys-
     tems:
     
        Rogue proxies
        Theft of passwords
        Theft of accounting data
        Replay attacks
        Connection hijacking
        Fraudulent accounting
     
     
     8.1.  Rogue proxies
     
     In conventional ISP application, the NAS, proxy, and home server exist
     within  a  single administrative entity. As a result, the proxy may be
     considered a trusted component.
     
     However, in a roaming system implemented with proxy chaining, the NAS,
     proxies,  and  home  server may be managed by different administrative
     entities. Through the use of shared secrets it is possible for proxies
     operating in different domains to establish a trust relationship. How-
     ever, if packets are only authenticated on a  hop-by-hop  basis,  then
     untrusted  proxies are capable of perpetrating a number of man-in-the-
     middle attacks.
     
     These attacks typically involve the editing of attributes, or the mod-
     ification  or  insertion  of  messages, such as the substitution of an
     Access-Accept for an Access-Reject.  For example, a proxy  may  modify
     an  Access-Accept sent by the home server so as to lessen the security
     obtained by the client. For example, EAP attributes might  be  removed
     or  modified  so  as to cause a client to authenticate with EAP MD5 or
     PAP, instead of a stronger authentication method. Alternatively,  tun-
     nel  attributes  might  be removed or modified so as to remove encryp-
     tion, redirect the tunnel to  a  rogue  tunnel  server,  or  otherwise
     lessen the security provided to the client.
     
     
     
     
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     Through  implementation  of  the End-to-End-Signature attribute, it is
     possible to detect unauthorized addition, deletion, or modification of
     protected attributes. Note that it is still possible for a rogue proxy
     to add, delete, or modify unprotected attributes.
     
     While a proxy MUST NOT send an Access-Accept to the NAS after  receiv-
     ing an Access-Reject from the home server, a proxy MAY send an Access-
     Reject to the NAS after  receiving  an  Access-Accept  from  the  home
     server.  Note  that  in  the  latter  case, a Security-Parameter-Index
     attribute should be used denoting the security association between the
     proxy  and  the  NAS, rather than that between the home server and the
     NAS, since the proxy has originated the packet. This  will  allow  the
     NAS  to  verify  the End-to-End-Signature attribute within the packet,
     and decide whether to silently discard the packet.  As noted  earlier,
     an  Access-Accept  originated by a proxy MUST be silently discarded by
     the NAS, even if the End-to-End-Signature attribute can be verified.
     
     The determination of whether end-to-end security is to be  used  in  a
     conversation  is  made using out-of-band mechanisms. Typically this is
     based either on static configuration  or  on  the  outcome  of  a  key
     exchange  conversation between the two endpoints.  However, once it is
     determined that the end systems wish to use end-to-end  security,  all
     packets  sent MUST include an End-to-End-Signature attribute and pack-
     ets  received  without  an  End-to-End-Signature  attribute  MUST   be
     silently  discarded.  Note  that policy determination using an out-of-
     band mechanism rather than a proxied conversation limits  the  ability
     of  a  rogue  proxy to interfere with the security negotiation between
     the two end systems.
     
     
     8.2.  Theft of passwords or keys
     
     Unless the Hidden attribute is used, where clients authenticate  using
     PAP,  or  where  the  Tunnel-Password  attribute  is included with the
     Access-Accept, each proxy along the path between the local NAS and the
     home server will have access to the cleartext password or key. In many
     circumstances, this represents an unacceptable security  risk.   As  a
     result, the Hidden attribute SHOULD be used to provide end-to-end con-
     fidentiality for User-Password or Tunnel-Password attributes.
     
     
     8.3.  Integrity and confidentiality of accounting data
     
     Typically in roaming systems, accounting packets are provided  to  all
     the  participants  along  the  roaming  relationship path, in order to
     allow them to audit subsequent invoices. In order to prevent modifica-
     tion of accounting packets by untrusted proxies, the End-to-End-Signa-
     ture attribute MAY be used. If it is desired that accounting  data  be
     kept  confidential  from a proxy, the Hidden attribute MAY be used. If
     the objective is to prevent snooping of accounting data on  the  wire,
     then IPSEC ESP MAY be used.
     
     
     
     
     
     
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     8.4.  Replay attacks
     
     In this attack, a man in the middle or rogue proxy collects CHAP-Chal-
     lenge and CHAP-Response attributes, and later replays  them.  If  this
     attack  is performed in collaboration with an unscrupulous ISP, it can
     be used to subsequently submit fraudulent accounting  records  to  the
     accounting  agent  for payment.  The system performing the replay need
     not necessarily be the one that  initially  captured  the  CHAP  Chal-
     lenge/Response pair.
     
     While  such  an  attack  has always been possible, without roaming the
     threat is restricted to proxies operating in the home server's domain.
     With  roaming,  such  an attack can be mounted by any proxy capable of
     reaching the home server.
     
     In order to protect against replay attacks, CHAP-Challenge  and  CHAP-
     Response attributes MAY be protected using the Hidden attribute.  CHAP
     replay attacks can also be defeated by means of  an  end-to-end  chal-
     lenge-response  exchange.   For example, if the home server returns an
     Access-Challenge packet  containing  a  CHAP-Challenge  attribute  and
     maintains state with respect to outstanding challenges, replay attacks
     cannot succeed.
     
     However, it should also be noted that end-to-end challenges (as  prac-
     ticed  within  the EAP MD5 authentication method, or in the CHAP-Chal-
     lenge method described above) are also subject  to  attacks  by  rogue
     proxies.  In such an attack a proxy substitutes a static challenge for
     the challenge sent by the home server, and on receiving the  response,
     checks  it against a databases of hashes applied against a dictionary.
     This attack may be prevented through use of  the  End-to-End-Signature
     attribute.
     
     
     8.5.  Connection hijacking
     
     In  this  form of attack, the attacker attempts to inject packets into
     the conversation between the  NAS  and  the  home  server.  RADIUS  as
     described in [3] is vulnerable to such attacks since only Access-Reply
     and Access-Challenge packets are authenticated.  This  attack  may  be
     defeated via use of an End-to-End-Signature attribute.
     
     
     8.6.  Fraudulent accounting
     
     In  this form of attack, a local proxy transmits fraudulent accounting
     packets or session records in an effort to collect fees to which  they
     are  not  entitled.  This  includes  submission  of packets or session
     records for non-existent sessions, or submission of  exaggerated  ses-
     sion times for actual sessions.
     
     Such behavior will only be easily detectable in the event that roaming
     users are making use of voluntary or compulsory  tunneling,  in  which
     case  the tunnel server will generate its own accounting record, which
     may be compared to that generated by the local ISP. However, tunneling
     
     
     
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     is expected to represent only a small percentage of roaming use.
     
     In  order to detect submisson of fraudulent accounting packets or ses-
     sion records, the the accounting gateway SHOULD send copies of session
     records to all parties with a financial interest in the session.  Par-
     ties receiving copies of these session records SHOULD  reconcile  them
     with logs of proxied authentications.
     
     In  order to make it easier to match authentication logs with account-
     ing records, home servers involved in roaming SHOULD include  a  Class
     attribute  in  the Access-Accept.  The Class attribute should uniquely
     identify a session, so as to allow an authentication log entry  to  be
     matched with a corresponding accounting log.
     
     In  order  to  be  able to  match accounting logs with logs of proxied
     authentications, accounting agents SHOULD require that they act as  an
     authentication  proxy  for all sessions for which an accounting record
     will subsequently be submitted.
     
     
     9.  Acknowledgments
     
     Thanks to Glen Zorn of Microsoft for useful discussions of this  prob-
     lem space.
     
     
     10.  References
     
     [1]   B. Aboba, J. Lu, J. Alsop, J. Ding, W. Wang.  "Review of Roaming
     Implementations." RFC 2194, Microsoft, Aimnet, i-Pass  Alliance,  Asi-
     ainfo, Merit, September 1997.
     
     [2]   B. Aboba, G. Zorn.  "Roaming Requirements." Internet-Draft (work
     in progress) draft-ietf-roamops-romreq-09.txt, Microsoft, April  1998.
     
     [3]   C. Rigney, A. Rubens, W. Simpson, S. Willens.  "Remote Authenti-
     cation Dial In User Service (RADIUS)." RFC  2138,  Livingston,  Merit,
     Daydreamer, April 1997.
     
     [4]   C.  Rigney.   "RADIUS  Accounting."  RFC 2139, Livingston, April
     1997.
     
     [5] R. Rivest, S. Dusse.   "The  MD5  Message-Digest  Algorithm",  RFC
     1321,  MIT  Laboratory  for  Computer Science, RSA Data Security Inc.,
     April 1992.
     
     [6] Krawczyk H., M. Bellare and R. Canetti, "HMAC:  Keyed-Hashing  for
     Message  Authentication,"  Internet  Draft  (work in progress), draft-
     ietf-ipsec-hmac-md5-01.txt, August 1996.
     
     [7] S. Bradner.  "Key words for use in RFCs  to  Indicate  Requirement
     Levels."  RFC 2119, Harvard University, March, 1997.
     
     [8]   B.   Aboba,   J.R.   Vollbrecht,   "Proxy  Chaining  and  Policy
     
     
     
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     Implementation in Roaming," Internet Draft (work in progress),  draft-
     ietf-roamops-auth-05.txt, Microsoft, MERIT, July 1998.
     
     
     11.  Authors' Addresses
     
     Pat R. Calhoun
     Technology Development
     Sun Microsystems, Inc.
     15 Network Circle
     Menlo Park, CA 94025
     
     Phone: 650-786-7733
     Fax:   650-786-6445
     EMail: pcalhoun@eng.sun.com
     
     Bernard Aboba
     Microsoft Corporation
     One Microsoft Way
     Redmond, WA 98052
     
     Phone: 206-936-6605
     EMail: bernarda@microsoft.com
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     Calhoun & Aboba                                              [Page 10]