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Versions: 00 01                                                         
RADIUS Extensions Working Group                                S. Winter
Internet-Draft                                                   RESTENA
Intended status: Best Current Practice                    March 21, 2016
Expires: September 22, 2016

   Considerations regarding the correct use of EAP-Response/Identity


   There are some subtle considerations for an EAP peer regarding the
   content of the EAP-Response/Identity packet when authenticating with
   EAP to an EAP server.  This document describes two such
   considerations and suggests workarounds to the associated problems.

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
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   This Internet-Draft will expire on September 22, 2016.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Problem Statement . . . . . . . . . . . . . . . . . . . .   2
     1.2.  Taxonomy of identities in EAP . . . . . . . . . . . . . .   2
     1.3.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  EAP-Response/Identity: Effects on EAP type negotiation  . . .   5
   3.  Character (re-)encoding may be required . . . . . . . . . . .   6
   4.  Recommendations for EAP peer implementations  . . . . . . . .   6
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8

1.  Introduction

1.1.  Problem Statement

   An Extensible Authentication Protocol (EAP, [RFC3748]) conversation
   between an EAP peer and an EAP server starts with an (optional)
   request for identity information by the EAP server (EAP-Request/
   Identity) followed by the peer's response with identity information
   (EAP-Response/Identity).  Only after this identity exchange are EAP
   types negotiated.

   EAP-Response/Identity is sent before EAP type negotiation takes
   place, but it is not independent of the later-negotiated EAP type.
   Two entanglements between EAP-Response/Identity and EAP methods'
   notions of a user identifier are described in this document.

   1.  The choice of identity to send in EAP-Response/Identity may have
       detrimental effects on the subsequent EAP type negotiation.

   2.  Using identity information from the preferred EAP type without
       thoughtful conversion of character encoding may have detrimental
       effects on the outcome of the authentication.

   The following two chapters describe each of these issues in detail.
   The last chapter contains recommendations for implementers of EAP
   peers to avoid these issues.

1.2.  Taxonomy of identities in EAP

   The notion of identity occurs numerous times in the EAP protocol
   stack (EAP-Response/Identity, Outer identity, method-specific

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   identity, tunneled identity).  This document uses the following
   terminology when discussing EAP identities.

   o  Method-specific Identity: Each EAP method has a means to identify
      the user or machine that tries to authenticate.  There are no
      restrictions on the format or encoding of this method-specific
      identity.  If an EAP methods distinguishes between this actual
      identity and a outer identity (see next bullet), then the Method-
      specific Identity is also often called the Inner Identity.

   o  Method-specific Outer Identity: Some EAP methods allow privacy-
      preserving enhancements where a string is sent as "identity" which
      is actually not necessarily related to the user or machine that
      tries to authenticate.  There is often a relationship between the
      Method-specific Outer Identity and the Inner Identity (e.g. they
      often share the same NAI realm suffix); but this is not a
      requirement.  There are no restrictions on the format or encoding
      of this method-specific identity.  Method-specific outer
      identities are either

      *  explicitly configured (e.g. string input UI: "Outer Identity")

      *  implicitly configured by copying the actual Method-specific
         (Inner) Identity

      *  implicitly configured by copying the NAI realm of the Method-
         specific (Inner) Identity and prefixing it non-configurably
         with a fixed privacy-preserving local username part like
         "anonymous" or the empty string (see [RFC7542])

      *  configured in a mixed way, e.g. using a explicit string input
         UI for the local part of the outer identity and combining it
         implicitly with a copy of the NAI realm part of the Method-
         specific (Inner) Identity

   o  EAP-Response/Identity: a string representing the user or machine
      that tries to authenticate, used outside the EAP method-specific
      context for the entire EAP session.  There can be only one EAP-
      Response/Identity per EAP session, even if that session is
      configured with more than one EAP method to authenticate with.  As
      per [RFC3748] there is no encoding requirement on EAP-Response/
      Identity.  In AAA protocol routing contexts, the content of EAP-
      Response/Identity is often used for request routing purposes.
      EAP-Response/Identity is chosen from the set:

      *  all method-specific outer identities from all configured EAP
         types supporting the notion of an outer identity union

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      *  all method-specific identities from all configured EAP types
         without the notion of an outer identity

      One of the two problems addressed in this document stems from this
      fact: the set of identities may contain more than one element.
      The resulting EAP-Response/Identity always routes all configured
      EAP types to only one destination, even if different EAP types
      would need routing to different destinations.

   o  User-Name: when using EAP in AAA protocol contexts (e.g. RADIUS
      [RFC2865], Diameter [RFC6733]), this additional identity is
      created outside the EAP peer (typically in a pass-through
      authenticator) by copying EAP-Response/Identity content to the AAA
      protocol's User-Name attribute.  There is no format requirement on
      User-Name, but there is an encoding requirement: the string MUST
      be UTF-8 encoded.  One of the two problems addressed in this
      document stems from this fact: EAP-Response/Identity does not have
      an encoding requirement, nor does it carry meta-information about
      the encoding used - and yet, it needs to be coerced into a UTF-8

   o  Further identities: Some EAP methods establish an EAP session
      inside EAP (e.g. PEAP first establishes a TLS tunnel using a
      method-specific outer identity, and then starts an EAP exchange
      inside the tunnel).  This being a new, independent EAP session, it
      contains its own EAP-Response/Identity, can invoke EAP method
      negotiation with different (inner) EAP types (this happens e.g.
      with EAP-FAST and its configurable choice of EAP-GTC or EAP-
      MSCHAPv2 inside the inner EAP session), and those inner EAP
      methods then have their own (inner) method-specific identities.
      Where the inner EAP method itself supports the notion of method-
      specific outer identities, another identity could be configured.
      For the purposes of this document, none of those details are
      considered and the process by which the (outer) EAP method selects
      its method-specific identity is left entirely to that EAP type.
      This document does not consider the (inner) EAP-Response/Identity
      in scope; the recommendations in this document to not apply to
      such (inner) occurences of EAP-Response/Identity.

1.3.  Requirements Language

   In this document, several words are used to signify the requirements
   of the specification.  The key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" in this document are to be interpreted as described in
   RFC 2119.  [RFC2119]

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2.  EAP-Response/Identity: Effects on EAP type negotiation

   Assuming the EAP peer's EAP type selection is not the trivial case
   (i.e. it has more than one configured EAP type for a given network or
   application, and needs to make a decision which one to use), an issue
   arises when the configured EAP types are not all configured with the
   same method-specific outer identity (or method-specific identity for
   EAP types not supporting the notion of an outer identity).

   Issue: if the identities in the set of configured EAP types differ
   (e.g. have a different [RFC7542] "realm" portion), and the
   authenticator does not send identity selection hints as per
   [RFC7542], then EAP type negotiation may be limited to those EAP
   types which are terminated in the same EAP server.  The reason for
   that is because the information in the EAP-Response/Identity is used
   for request routing decisions and thus determines the EAP server - a
   given user identifier may be routed to a server which exclusively
   serves the matching EAP type.  Negotiating another EAP type from the
   set of configured EAP types during the running EAP conversation is
   then not possible.


   Assume an EAP peer is configured to support two EAP types:

   o  EAP-AKA' [RFC5448] with user identifier imsi@mnc123.mcc123.3gpp-

   o  EAP-TTLS [RFC5281] with user identifier john@realm.example

   The user connects to hotspot of a roaming consortium which could
   authenticate him with EAP-TTLS and his john@realm.example identity.
   The hotspot operator has no business relationship at all with the
   3GPP consortium; incoming authentication requests for realms ending
   in 3gppnetwork.org will be immediately rejected.  Identity selection
   hints are not sent.

   Consequence: If the EAP peer consistently chooses the
   imsi@mnc123.mcc123.3gpp-network.org user identifier as choice for its
   initial EAP-Response/Identity, the user will be consistently and
   perpetually rejected, even though in possession of a valid credential
   for the hotspot.

   An EAP peer should always try all options to authenticate.  As the
   example above shows, it may not be sufficient to rely on EAP method
   negotiation alone to iterate through all configured EAP types and
   come to a conclusive outcome of the authentication attempt.  Multiple
   new EAP authentications, each using an EAP-Response/Identity from a

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   different element of the set of method-specific outer identities, may
   be required to fully iterate through the list of usable identities.

3.  Character (re-)encoding may be required

   The method-specific identities as configured in the EAP method
   configuration are not always suited as identities to choose as EAP-
   Response/Identity: EAP methods define the encoding of their method-
   specific outer identities at their leisure; in particular, the chosen
   encoding may or may not be UTF-8.

   It is not the intention of EAP, as a mere method-agnostic container
   which simply carries EAP types, to restrict an EAP method's choice of
   encoding of method-specific identities.  However, there are
   restrictions in what should be contained in the EAP-Response/
   Identity: EAP is very often carried over a AAA protocol (e.g over
   RADIUS as per [RFC3579]).  The typical use for the contents of EAP-
   Response/Identity inside AAA protocols like RADIUS [RFC2865] and
   Diameter [RFC6733] is to copy the content of EAP-Response/Identity
   into a "User-Name" attribute; the encoding of the User-Name attribute
   is required to be UTF-8.  EAP-Response/Identity does not carry
   encoding information itself, so a conversion between a non-UTF-8
   encoding and UTF-8 is not possible for the AAA entity doing the EAP-
   Response/Identity to User-Name copying.

   Consequence: If an EAP method's method-specific identity is not
   encoded in UTF-8, and the EAP peer verbatimly uses that method-
   specific identity for its EAP-Response/Identity field, then the AAA
   entity is forced to violate its own specification because it has to,
   but can not use UTF-8 for its own User-Name attribute.  If the EAP
   method supports a method-specific outer identity in a non UTF-8
   character set, and the EAP peer verbatimly uses that outer identity
   for its EAP-Response/Identity field, then the same violation occurs.

   This jeopardizes the subsequent EAP authentication as a whole;
   request routing may fail, lead to a wrong destination or introduce
   routing loops due to differing interpretations of the User-Name in
   EAP pass-through authenticators and AAA proxies.

4.  Recommendations for EAP peer implementations

   Where method-specific identities or method-specific outer identities
   in configured EAP types in an EAP peer differ, the EAP peer can not
   rely on the EAP type negotiation mechanism alone to provide useful
   results.  If an EAP authentication gets rejected, the EAP peer SHOULD
   re-try the authentication using a different EAP-Response/Identity
   than before.  The EAP peer SHOULD try all possible EAP-Response/

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   Identity contents from the entire set of configured EAP types before
   declaring final authentication failure.

   EAP peers need to maintain state on the encoding of the method-
   specific identities and outer identities which are used in their
   locally configured EAP types.  When constructing an EAP-Response/
   Identity from the set of identities, they MUST (re-)encode the
   corresponding identity as UTF-8 and use the resulting value for the

5.  Privacy Considerations

   Because the EAP-Response/Identity content is not encrypted, the
   backtracking to a new EAP-Response/Identity will systematically
   reveal all configured identities to intermediate passive listeners on
   the path between the EAP peer and the EAP server (until one
   authentication round succeeds).

   This additional leakage of identity information is not very
   significant though because where privacy is considered important, the
   additional option for identity privacy which is present in most
   modern EAP methods can be used.

   If the EAP peer implementation is certain that all EAP types will be
   terminated at the same EAP server (e.g. with a corresponding
   configuration option) then the iteration over all identities can be
   avoided, because the EAP type negotiation is then sufficient.

   If a choice of which identity information to disclose needs to be
   made by the EAP peer, when iterating through the list of identities
   the EAP peer SHOULD

      in first priority honour a manually configured order of preference
      of EAP types, if any

      in second priority try EAP types in order of less leakage first;
      that is, EAP types with a method-specific outer identity that
      differs from the method-specific identity should be tried before
      other EAP types which would reveal actual user identities.

6.  Security Considerations

   The security of an EAP conversation is determined by the EAP method
   which is used to authenticate.  This document does not change the
   actual authentication with an EAP method, and all the security
   properties of the chosen EAP method remain.  The format requirements
   (character encoding) and operational considerations (re-try EAP with

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   a different EAP-Response/Identity) do not lead to new or different
   security properties.

7.  IANA Considerations

   There are no IANA actions in this document.

8.  References

8.1.  Normative References

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

8.2.  Informative References

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)", RFC
              2865, June 2000.

   [RFC3579]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
              Dial In User Service) Support For Extensible
              Authentication Protocol (EAP)", RFC 3579, September 2003.

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

   [RFC5281]  Funk, P. and S. Blake-Wilson, "Extensible Authentication
              Protocol Tunneled Transport Layer Security Authenticated
              Protocol Version 0 (EAP-TTLSv0)", RFC 5281, August 2008.

   [RFC5448]  Arkko, J., Lehtovirta, V., and P. Eronen, "Improved
              Extensible Authentication Protocol Method for 3rd
              Generation Authentication and Key Agreement (EAP-AKA')",
              RFC 5448, May 2009.

   [RFC6733]  Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
              "Diameter Base Protocol", RFC 6733, October 2012.

   [RFC7542]  DeKok, A., "The Network Access Identifier", RFC 7542, DOI
              10.17487/RFC7542, May 2015,

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Author's Address

   Stefan Winter
   Fondation RESTENA
   6, rue Richard Coudenhove-Kalergi
   Luxembourg  1359

   Phone: +352 424409 1
   Fax:   +352 422473
   EMail: stefan.winter@restena.lu
   URI:   http://www.restena.lu.

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