Network Working Group Y. Sheffer
Internet-Draft Check Point
Intended status: Informational August 2, 2009
Expires: February 3, 2010
Using EAP-GTC for Simple User Authentication in IKEv2
draft-sheffer-ipsecme-ikev2-gtc-01.txt
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Abstract
Despite many years of effort, simple username-password authentication
is still prevalent. In many cases a password is the only credential
available to the end user. IKEv2 uses EAP as a sub-protocol for user
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authentication. This provides a well-specified and extensible
architecture. To this day EAP does not provide a simple password-
based authentication method. The only existing password
authentication methods either require the peer to know the password
in advance (EAP-MD5), or are needlessly complex when used within
IKEv2 (e.g. PEAP). This document codifies the common practice of
using EAP-GTC for this type of authentication, with the goal of
achieving maximum interoperability. The various security issues are
extensively analyzed.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Alternatives to EAP-GTC in IKEv2 . . . . . . . . . . . . . . . 4
3.1. Non-password credentials . . . . . . . . . . . . . . . . . 4
3.2. Using the IKE preshared secret . . . . . . . . . . . . . . 4
3.3. EAP-MD5 , EAP-MSCHAPv2 and mutual authentication
schemes . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Using EAP-GTC in IKE: Details . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6.1. Key generation and MITM protection . . . . . . . . . . . . 6
6.2. Protection of credentials between the IKE gateway and
the AAA server . . . . . . . . . . . . . . . . . . . . . . 6
6.3. Server authentication . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . . 7
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . . 8
A.1. draft-sheffer-ipsecme-ikev2-gtc-01 . . . . . . . . . . . . 8
A.2. draft-sheffer-ipsecme-ikev2-gtc-00 . . . . . . . . . . . . 8
A.3. draft-sheffer-ikev2-gtc-00 . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
"Oh dear! It's possible that we have added EAP to IKE to support a
case that EAP can't support." -- C. Kaufman.
Despite many years of effort, simple username-password authentication
is still prevalent. In many cases a password is the only credential
available to the end user.
IKEv2 [RFC4306] uses the Extensible Authentication Protocol (EAP) as
a sub-protocol for user authentication. This provides a well-
specified and extensible architecture and enables useful capabilities
like SIM authentication. Unfortunately, for a number of reasons EAP
still does not provide a simple password-based authentication method.
The only existing password authentication methods either require the
peer to know the password in advance (EAP-MD5), or are needlessly
complex when used within IKEv2 (e.g. PEAP).
Technically, the IKE preshared secret authentication mode can be used
for password authentication. In fact even the IKEv2 RFC winks at
this practice. But this use jeopardizes the protocol's security and
should clearly be avoided (more details below).
EAP is used in IKEv2 at a stage when the remote access gateway has
already been authenticated. At this point the user has a high enough
level of trust to send his or her password to the gateway. Such an
exchange is enabled by the EAP Generic Token Card (GTC) method, which
is a simple text transport between the two EAP peers. To quote
[RFC3748]:
The EAP GTC method is intended for use with the Token Cards
supporting challenge/response authentication and MUST NOT be used
to provide support for cleartext passwords in the absence of a
protected tunnel with server authentication.
IKEv2 does indeed provide "a protected tunnel with server
authentication". The current document updates [RFC3748] by making an
exception and allowing the use of GTC to carry secret credentials, in
this specific situation. Section 6 further elaborates on the
security properties of this solution.
Other protocols provide a similar protected tunnel, for example TLS-
EAP, described in [I-D.nir-tls-eap]. These protocols however are out
of scope for this document.
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2. Terminology
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 [RFC2119].
3. Alternatives to EAP-GTC in IKEv2
This section presents a few of the alternatives to EAP-GTC, and
explains why they are either insecure or impractical given today's
common identity management infrastructure.
3.1. Non-password credentials
Certificate-based authentication, especially when combined with
hardware protection (e.g. a hardware token), can be deployed in a
more secure manner than the form of password authentication which we
discuss. However, due to a host of issues to do with cost,
inconvenience and reliability this solution has not gained wide
market acceptance over the last 10 years.
3.2. Using the IKE preshared secret
Sec. 2.15 of RFC 4306 points out that the generation of the IKE
preshared secret from a weak password is insecure. Such use is
vulnerable to off line password guessing by an active attacker. All
the attacker needs to do is respond correctly to the first IKE_INIT
message, and then record the third IKE message. This is then
followed by a dictionary attack to obtain the password.
3.3. EAP-MD5 , EAP-MSCHAPv2 and mutual authentication schemes
Challenge-response schemes, like EAP-MD5 and EAP-MSCHAPv2, have a
clear security advantage over sending the plaintext password to the
gateway. Password-based mutual authentication schemes like SRP have
a further advantage in that the gateway's authentication is much
stronger than when using certificates alone, since the AAA server
proves its knowledge of a per-client credential, and the gateway
proves that it has been authorized by the AAA server for that
particular client.
Unfortunately all of these methods also suffer from a major drawback:
the gateway must have a priori access to the plaintext password.
While many RADIUS servers may indeed have such access, other very
common deployments do not provide it. One typical example is when
the gateway directly accesses an LDAP directory (or a Microsoft
Active Directory) to authenticate the user. The usual way to do that
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is by issuing an LDAP Bind operation into the directory, using the
just-received plaintext password. Often in this case it is the IKE
gateway that terminates the EAP protocol, and it needs a way to
obtain the raw password.
An additional issue with mutual authentication schemes is their heavy
IP encumbrance, which has resulted in a scarcity of standards using
them and a low rate of market adoption.
4. Using EAP-GTC in IKE: Details
EAP-GTC is specified in [RFC3748], Sec. 5.6. This section is non-
normative, and is merely an interpretation of this specification in
the context of IKEv2.
Simple authentication requires a non secret identity ("user name")
and a secret credential ("password"). Both of these are arbitrary
Unicode strings, although implementations may impose length
constraints.
In the case of EAP-GTC, the user name is conveyed in the IKE IDi
payload. According to [RFC4718], Sec. 3.4, the user name can be
encoded in one of two ways: as a simple user name, in which case the
ID_KEY_ID identification type is used; or as a combination user name
plus realm, in which case the format is a NAI [RFC4282] and the
identification type is ID_RFC822_ADDR. In either case, the user name
is a Unicode string encoded as UTF-8. Using the EAP Identity payload
is redundant, and if it is used, it should be identical to the IDi
payload.
EAP-GTC consists of a simple 2-message exchange. The contents of the
Type-Data field in the Request should not be interpreted in any way,
and should be displayed to the user. This field contains a Unicode
string, encoded as UTF-8.
The password is sent in the EAP Response. The Type-Data field of the
Response is also a Unicode string encoded as UTF-8. Note that none
of the IDi payload, the EAP Request or the EAP Response is null-
terminated.
If either or both the user name and the password are non-ASCII, they
should be normalized by the IKE client before the IKE/EAP message is
constructed. The normalization method is SASLprep, [RFC4013].
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5. IANA Considerations
This document does not require any action by IANA.
6. Security Considerations
6.1. Key generation and MITM protection
Modern EAP methods generate a key shared between the two protocol
peers. GTC does not (and cannot) generate such a key. RFC 4306
mandates that:
EAP methods that do not establish a shared key SHOULD NOT be used,
as they are subject to a number of man-in-the-middle attacks
[EAPMITM] if these EAP methods are used in other protocols that do
not use a server-authenticated tunnel.
However GTC must never be used in such a situation, since the client
would be sending its credentials openly to an unauthenticated server.
When using GTC with IKEv2, the implementation (or local
administrators) MUST ensure that the same credentials are never used
in such a manner.
6.2. Protection of credentials between the IKE gateway and the AAA
server
In the proposed solution, the raw credentials are sent from the IKE
gateway to a AAA server, typically a RADIUS server. These
credentials and the associated messaging MUST be strongly protected.
Some of the existing options include:
o An IPsec tunnel between the gateway and the AAA server.
o RADIUS over TCP with TLS, [I-D.winter-radsec].
o RADIUS over UDP with DTLS, [I-D.dekok-radext-dtls] (expired).
The legacy RADIUS security mechanism (Sec. 5.2 of [RFC2865]) is
considered weak and SHOULD NOT be used when better alternatives are
available.
6.3. Server authentication
The client may only send its cleartext credentials after it has
positively authenticated the server. This authentication is
specified, albeit rather vaguely, in [RFC4306] and is out of scope of
the current document. Unauthenticated (BTNS) derivatives of IKE MUST
NOT be used with EAP-GTC.
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7. Acknowledgments
I would like to thank Yoav Nir and Charlie Kaufman for their helpful
comments.
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.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
and Passwords", RFC 4013, February 2005.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
8.2. Informative References
[EAPMITM] Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle
in Tunneled Authentication Protocols", November 2002,
<http://eprint.iacr.org/2002/163>.
[I-D.dekok-radext-dtls]
DeKok, A., "DTLS as a Transport Layer for RADIUS",
draft-dekok-radext-dtls-01 (work in progress), June 2009.
[I-D.nir-tls-eap]
Nir, Y., Sheffer, Y., Tschofenig, H., and P. Gutmann, "TLS
using EAP Authentication", draft-nir-tls-eap-06 (work in
progress), April 2009.
[I-D.winter-radsec]
Winter, S., McCauley, M., and S. Venaas, "RadSec Version 2
- A Secure and Reliable Transport for the RADIUS
Protocol", draft-winter-radsec-01 (work in progress),
February 2008.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
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[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
[RFC4718] Eronen, P. and P. Hoffman, "IKEv2 Clarifications and
Implementation Guidelines", RFC 4718, October 2006.
Appendix A. Change Log
A.1. draft-sheffer-ipsecme-ikev2-gtc-01
Republished.
A.2. draft-sheffer-ipsecme-ikev2-gtc-00
Document renamed.
A.3. draft-sheffer-ikev2-gtc-00
Initial version.
Author's Address
Yaron Sheffer
Check Point Software Technologies Ltd.
5 Hasolelim St.
Tel Aviv 67897
Israel
Email: yaronf@checkpoint.com
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