Internet DRAFT - draft-ietf-radext-populating-eapidentity
draft-ietf-radext-populating-eapidentity
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
draft-ietf-radext-populating-eapidentity-00
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. 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
encoding.
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",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
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.
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
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
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 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,
<http://www.rfc-editor.org/info/rfc7542>.
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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.
Winter Expires September 22, 2016 [Page 9]
