INTERNET-DRAFT Bryan D. Payne
Category: Informational Univ of Maryland
<draft-payne-eap-sm-00.txt>
9 May 2002 Nick L. Petroni, Jr.
Univ of Maryland
Extensible Authentication Protocol State Machine
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
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1. Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
2. Abstract
The specification for the Extensible Authentication Protocol (EAP) [2]
omits a state machine description. This omission has led to ambiguity
in the specification and potential security problems in EAP
implementations. This document outlines a state machine to be
integrated into the next revision of the EAP RFC.
3. Introduction
This document offers a proposed state machine for RFC 2284bis (EAP).
There is a state machine for the peer and one for the authenticator.
Accompanying each state machine diagram is a description of the notation
used. Whenever possible, the same notation has been used in both the
peer and authenticator state machines.
Each state machine provides explicit details regarding state transitions
that are associated with each message. Therefore, if a message arrives
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that is not handled by the current state, then the message MUST be
dropped and SHOULD be logged. All malformed messages (i.e., messages
not complying to the format specified in RFC 2284bis) MUST be dropped
and SHOULD be logged.
One type of EAP message has been purposely omitted from the state
machines: the notification messages. This was done because notification
request messages can be sent from the authenticator to the peer at any
point in the state machine. Upon receipt of this request, the peer MUST
send a notification reply. This transaction does not induce any changes
to the peer or authenticator state machines.
For simplicity, the state machine diagrams do not include any
information about timeouts. However, any implementation SHOULD
transition to the initialization state after waiting n seconds in a
given state, where "n" is defined by the implementation. Failure to
implement timeouts can cause the implementation to become stuck in a
state, so this is not recommended.
Additional details about the meaning of each state, and how to handle
messages in each state are included in the text accompanying each state
machine diagram.
3.1. 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 [1].
A protocol submission is not compliant if it fails to satisfy one or
more of the MUST or MUST NOT requirements for the capabilities that it
implements. A protocol submission that satisfies all the MUST, MUST
NOT, SHOULD and SHOULD NOT requirements for its capabilities is said to
be "unconditionally compliant"; one that satisfies all the MUST and MUST
NOT requirements but not all the SHOULD or SHOULD NOT requirements for
its protocols is said to be "conditionally compliant."
4. Policy
As noted in the title, EAP is extensible in nature. Although originally
designed to allow for a series of one-way authentications, current uses
and new EAP methods allow for mutual authentication via a single run of
the protocol. As a direct result of the extensibility of the protocol,
both authenticators and peers are given the opportunity to enforce
policies via the protocol. For example, a peer may choose to only allow
EAP methods that provide mutual authentication or an authenticator may
choose to mandate three successful method authentications out of the
five it supports before allowing access. While an advantage of this is
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more flexibility, one disadvantage is the difficulty involved in
formalizing the process. In order to provide for such flexibility, we
have introduced the concept of a policy for both the authenticator and
the peer. Given the procedures described below, the the authenticator
can insure that its policy is met before sending a success message and
the peer can insure that any success message it receives comes at an
allowable time. The result is a well-defined state machine that
maintains the extensibility of the protocol. Of course, the security of
the protocol directly depends on the effectiveness of the policy being
enforced.
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5. EAP Peer State Machine
The following is a diagram of the EAP Peer state machine. Also included
is an explanation of the primitives and procedures referenced in the
diagram, as well as a clarification of notation.
--------------------
| Initialization |
|====================|
------------------------->|initPolicy() |
| --------------------
| |
| |UCT
| Rec(ID Req.) V !Valid(Rec(Auth Req.))
| --------------- --------------------- -----------------
| |Peer Ident. |<---| Unauthenticated |--->| Invalid Request |
| |===============| |=====================| |=================|
| | Send(ID resp.)|<---|authMethodSuccess = 0|<---| Send(NAK resp.) |
| --------------- --------------------- -----------------
| UCT | ^ | UCT
| | | |
| | | |
| Valid(Rec(Auth Req.)) | | | Rec(Success)
| V | | && policySatisfied()
| ***************** | |
| * * | V
| * EAP Method * | -----------------
| * Peer State * | | Authenticated |
| * Machine * | |=================|
| *===============* | | |
| * * | -----------------
---------------- * updatePolicy()* |
Rec(Failure) * * |
* * |
* * |
* * |
***************** |
| |
-----------
isSet(authMethodSuccess)
5.1. Variables
authMethodSuccess
Set when the current authentication method has been determined to
succeed. SHOULD be set by the EAP Method Peer.
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5.2. Procedures
Send
Send the EAP Response Packet with the ID field corresponding to the
appropriate EAP Request Packet to the lower layer for delivery.
Receive
Determine if a previously unprocessed packet of the specified type
has been delivered from the lower level.
isSet
Evaluate the specified variable to determine its truth value.
Valid
Based on the peer's own policy, determine if the specified request
type is supported and allowed. Return true if so, false if not.
initPolicy
Reset the variables associated with the Peer's policy to their
initial values.
updatePolicy
Generic procedure call relating to the update of any/all necessary
variables related to the Peer's policy.
policySatisfied
Determine if the Peer's policy has been satisfied to the point that
Success is an allowable transition.
5.3. States
INITIALIZATION
This state is entered when the peer protocol begins and anytime the
protocol resets due to a failure.
UNAUTHENTICATED
The state of the protocol after being initialized, but before
entering a specific method's state machine.
PEER IDENTIFICATION
State that handles the response to an ID request.
INVALID REQUEST
State that responds to a request of an invalid type. Invalid is as
defined by the Valid procedure and refers to the peer's policy for
certain EAP methods (either unsupported or disallowed).
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EAP METHOD PEER STATE MACHINE
Each EAP method has its own state machine specific to that method.
Because of the extensible nature of EAP, a particular peer's policy
can significantly alter the operation of the protocol and therefore
alter the peer state machine. For this reason, the state machine
provides for policy and method-specific operation within the general
context of the protocol. As shown in Section 3 , the state "EAP
Method Authenticator State Machine" is left as a black-box
representation of EAP methods. This method-specific state machine is
responsible for updating the global variables associated with
possible policies held by the larger authenticator state machine.
This notion is represented in the Figure with a call to the function
updatePolicy.
AUTHENTICATED
The state reached after a receiving some indication of success, but
only after determining such a success occurs at an allowable time.
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6. EAP Authenticator State Machine
Let AMF = authMethodFail
AMS = authMethodSucc
-----------------
| Initialization |
|=================|
-------------------------------------->| idCount = 0 |
| | initPolicy() |
| -----------------
| |
| | UCT
| V
| --------------------- -------------------
| | Peer Identification | | Unauthenticated |<------------
| |=====================| |===================| |
| ->| Send(ID Req.) |<----|authMethodSucc = 0 |<--- |
| | | idCount++ | |authMethodFail = 0 | | |
| | --------------------- ------------------- | |
| | | | | | | |
| | | | |Valid(Rec(ID Reply)) | | |
| | | | | | | |
| -------- | | ----------- | |
| !Valid(Rec(ID reply)) | | | | |
| && idCount < | | V | |
| idCountMax | | ***************** | |
| | | * * | |
| | | * EAP Method * | |
| | ----->* Authenticator *----------- |
| | * State Machine * Rec(NAK) |
| | *===============* |
| | * *--------------------
| !Valid(Rec(ID reply)))| *updatePolicy() * isSet(AMS)
| && idCount >= | * * &&
| idCountMax | * * !policySatisfied()
| ------- *****************
| | | |
| | | |
| | isSet(AMF)| |isSet(AMS)
| | | |&& policySatisfied()
| | V V
| | --------------- ----------------
| ------->| Failure | | Authenticated |
| |===============| |================|
---------------------------| Send(Failure) | | Send(Success) |
UCT --------------- ----------------
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6.1. Variables
idCounter
Counts the number of times that a client is allowed to return an
invalid ID reply. RFC 2284bis recommends allowing the client at
least three invalid ID replies to account for user error while typing
in an identity.
authMethodFailure
Set when the current authentication method has been determined to
fail. SHOULD be set by the EAP Method Authenticator.
authMethodSuccess
Set when the current authentication method has been determined to
succeed. SHOULD be set by the EAP Method Authenticator.
(Note that authMethodSuccess and authMethodFailure can not both be
set to true at the same time. This requirement MUST be maintained by
the EAP Method Authenticator State Machine.)
6.2. Constants
idCounterMax
Specifies the max number of times that a peer can send an invalid ID
reply before moving to a failure state.
6.3. Procedures
Send
Send the EAP Request Packet to the lower layer for delivery.
Receive
Read a previously unprocessed packet of the specified type from the
lower level once it is available.
Valid
specified reply is what is expected. Return true if so, false if not.
initPolicy
Reset the variables associated with the Authenticator's policy to
their initial values.
updatePolicy
Generic procedure call relating to the update of any/all necessary
variables related to the Authenticator's policy.
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policySatisfied
Determine if the Authenticator's policy has been satisfied to the
point that Success is an allowable transition.
6.4. States
INITIALIZATION
This state is entered when the authenticator protocol begins and
anytime the protocol resets due to a failure.
UNAUTHENTICATED
Under this state, the authenticator must decide if it will send an ID
Request or proceed directly to the Auth Request. If multiple Auth
Requests are required by the authenticator's policy, then this state
may be reached multiple times.
PEER IDENTIFICATION
State that sends an ID request.
EAP METHOD AUTHENTICATOR STATE MACHINE
Each EAP method has its own state machine specific to that method.
Because of the extensible nature of EAP, a particular authenticator's
policy can significantly alter the operation of the protocol and
therefore alter the authenticator state machine. For this reason, the
state machine provides for policy and method-specific operation
within the general context of the protocol. As shown in Section 4,
the state "EAP Method Peer State Machine" is left as a black-box
representation of EAP methods. This method-specific state machine is
responsible for updating the global variables associated with
possible policies held by the larger peer state machine. This notion
is represented in the Figure with a call to the function
updatePolicy.
FAILURE
The state reached after sending the failure message. The
authenticator should reinitialize the state machine after sending a
failure.
AUTHENTICATED
The state reached after sending a success message.
7. References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
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[2] Blunk, L., Vollbrecht, J., Aboba, B., "Extensible Authentication
Protocol (EAP)", RFC 2284, March 1998.
8. Security Considerations
This document's intent is to describe the EAP state machine fully. To
this end, any security concerns with this document are likely a
reflection of security concerns with EAP itself.
9. IANA Considerations
This draft does not create any new number spaces for IANA
administration.
10. Acknowledgments
Thanks to the members of the EAP working group who have discussed and
commented on this document. We would also like to thank Chuk Yang Seng,
Univ of Maryland, for his through review of the state machines and
William Arbaugh, Univ of Maryland, for providing resources needed to
complete this document. Finally, we would like to thank Bernard Aboba,
Microsoft, for being both flexible and helpful as we begin working with
the IETF.
11. Authors' Addresses
Bryan D. Payne
Department of Computer Science
University of Maryland
A.V. Williams Building
College Park, MD 20742
Email: bdpayne@cs.umd.edu
Nick L. Petroni, Jr.
Department of Computer Science
University of Maryland
A.V. Williams Building
College Park, MD 20742
Email: npetroni@cs.umd.edu
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This memo is filed as <draft-payne-eap-sm-00.txt>, and expires November
10, 2002.
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