Internet DRAFT - draft-winter-radext-populating-eapidentity
draft-winter-radext-populating-eapidentity
RADIUS Extensions Working Group S. Winter
Internet-Draft RESTENA
Intended status: Best Current Practice October 27, 2014
Expires: April 30, 2015
Considerations regarding the correct use of EAP-Response/Identity
draft-winter-radext-populating-eapidentity-01
Abstract
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Problem Statement . . . . . . . . . . . . . . . . . . . . 2
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. EAP-Response/Identity: Effects on EAP type negotiation . . . 3
3. Character (re-)encoding may be required . . . . . . . . . . . 4
4. Recommendations for EAP peer implementations . . . . . . . . 5
5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 6
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. Requirements Language
In this document, several words are used to signify the requirements
of the specification. The key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
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and "OPTIONAL" in this document are to be interpreted as described in
RFC 2119. [RFC2119]
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 user identifier.
Issue: if the user identifiers in the set of configured EAP types
differ (e.g. have a different [RFC4282] "realm" portion), and the
authenticator does not send identity selection hints as per
[RFC4284], 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.
Example:
Assume an EAP peer is configured to support two EAP types:
o EAP-AKA' [RFC5448] with user identifier imsi@mnc123.mcc123.3gpp-
network.org
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
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come to a conclusive outcome of the authentication attempt. Multiple
new EAP authentications, each using a different user identifier from
the set of configured user identities, may be required to fully
iterate through the list of usable identities.
3. Character (re-)encoding may be required
The user identifier as configured in the EAP method configuration is
not always suited as user identifier to choose as EAP-Response/
Identity. This is trivially true when using tunneled EAP types and
configuring anonymous outer identity for the tunneling EAP type.
There is at least one additional, non-trivial, case to consider
however:
EAP methods define the encoding of their user identifiers; in
particular, the encoding of the user identifiers as defined the EAP
method may or may not be UTF-8; some EAP methods are even known not
to put any encoding restrictions on their user identifiers at all.
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 a user identifier. 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 user identifier is not encoded in
UTF-8, and the EAP peer verbatimly uses that method's notion of a
user identifier 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 configuration sets an 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.
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4. Recommendations for EAP peer implementations
Where user identifiers between 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 user identifiers from the entire set of configured EAP types
before declaring final authentication failure.
EAP peers need to maintain state on the encoding of the user
identifiers which are used in their locally configured EAP types.
When constructing an EAP-Response/Identity from that user identifier,
they MUST (re-)encode that user identifier as UTF-8 and use the
resulting value for the EAP-Response/Identity. If the EAP type is
configured for the use of anonymous outer identities, the desired
outer identity MUST also be (re-)encoded in UTF-8 encoding before
being put into the EAP-Response/Identity.
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 configured outer identity should be
tried before other EAP types which would reveal actual user
identities.
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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
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.
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
[RFC4284] Adrangi, F., Lortz, V., Bari, F., and P. Eronen, "Identity
Selection Hints for the Extensible Authentication Protocol
(EAP)", RFC 4284, January 2006.
[RFC5281] Funk, P. and S. Blake-Wilson, "Extensible Authentication
Protocol Tunneled Transport Layer Security Authenticated
Protocol Version 0 (EAP-TTLSv0)", RFC 5281, August 2008.
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[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.
Author's Address
Stefan Winter
Fondation RESTENA
6, rue Richard Coudenhove-Kalergi
Luxembourg 1359
LUXEMBOURG
Phone: +352 424409 1
Fax: +352 422473
EMail: stefan.winter@restena.lu
URI: http://www.restena.lu.
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