INTERNET-DRAFT Bryan D. Payne Category: Informational Univ of Maryland 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. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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 Payne,Petroni Informational [Page 1] INTERNET-DRAFT EAP State Machine 9 May 2002 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 Payne,Petroni Informational [Page 2] INTERNET-DRAFT EAP State Machine 9 May 2002 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. Payne,Petroni Informational [Page 3] INTERNET-DRAFT EAP State Machine 9 May 2002 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. Payne,Petroni Informational [Page 4] INTERNET-DRAFT EAP State Machine 9 May 2002 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). Payne,Petroni Informational [Page 5] INTERNET-DRAFT EAP State Machine 9 May 2002 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. Payne,Petroni Informational [Page 6] INTERNET-DRAFT EAP State Machine 9 May 2002 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 --------------- ---------------- Payne,Petroni Informational [Page 7] INTERNET-DRAFT EAP State Machine 9 May 2002 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. Payne,Petroni Informational [Page 8] INTERNET-DRAFT EAP State Machine 9 May 2002 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. Payne,Petroni Informational [Page 9] INTERNET-DRAFT EAP State Machine 9 May 2002 [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 12. 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Expiration Date This memo is filed as , and expires November 10, 2002. Payne,Petroni Informational [Page 11]