PPPEXT Working Group L. Blunk INTERNET-DRAFT Merit Networks, Inc. Category: Standards Track J. Vollbrecht <draft-ietf-pppext-rfc2284bis-04.txt> Interlink Networks, Inc. 4 April 2002 Bernard Aboba Obsoletes: RFC 2284 Microsoft Extensible Authentication Protocol (EAP) This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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 progress." 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. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract This document defines the Extensible Authentication Protocol (EAP), an authentication protocol which supports multiple authentication mechanisms. EAP typically runs directly over the link layer without requiring IP and therefore includes its own support for in-order delivery and retransmission. Fragmentation is not supported within EAP itself; however, individual EAP methods may support this. While EAP was originally developed for use with PPP, it is also now in use with IEEE 802. This document obsoletes RFC 2284. Blunk, Vollbrecht & Aboba Standards Track [Page 1]
INTERNET-DRAFT RFC2284bis 4 April 2002 Table of Contents 1. Introduction .......................................... 3 1.1 Specification of Requirements ................... 3 1.2 Terminology ..................................... 3 2. Extensible Authentication Protocol (EAP) .............. 4 2.1 EAP State Machine ............................... 6 3. Media-specific issues ................................. 6 3.1 Lower layer assumptions ......................... 6 3.2 EAP usage within PPP ............................ 7 3.3 EAP usage within IEEE 802 ....................... 8 4. EAP Packet Format ..................................... 9 4.1 Request and Response ............................ 10 4.2 Success and Failure ............................. 12 5. Initial EAP Request/Response Types .................... 12 5.1 Identity ........................................ 13 5.2 Notification .................................... 14 5.3 Nak ............................................. 15 5.4 MD5-Challenge ................................... 15 5.5 Vendor-specific ................................. 16 6. IANA Considerations ................................... 18 6.1 Definition of Terms ............................. 18 6.2 Recommended Registration Policies ............... 18 7. Normative references .................................. 19 8. Informative references ................................ 20 9. Security considerations ............................... 21 9.1 Identity protection ............................. 21 9.2 Packet modification attacks ..................... 22 9.3 Denial of service attacks ....................... 22 9.4 Dictionary attacks .............................. 23 9.5 Connection to an untrusted network .............. 23 9.6 Negotiation attacks ............................. 24 9.7 Implementation idiosyncracies ................... 24 9.8 Key derivation .................................. 25 9.9 Weak ciphersuites ............................... 26 ACKNOWLEDGMENTS .............................................. 26 AUTHORS' ADDRESSES ........................................... 26 Full Copyright Statement ..................................... 27 Blunk, Vollbrecht & Aboba Standards Track [Page 2]
INTERNET-DRAFT RFC2284bis 4 April 2002 1. Introduction This document defines the Extensible Authentication Protocol (EAP), an authentication protocol which supports multiple authentication mechanisms. EAP typically runs directly over the link layer without requiring IP and therefore includes its own support for in-order delivery and retransmission. Fragmentation is not supported within EAP itself; however, individual EAP methods may support this. While EAP was originally developed for use with PPP, it is also now in use with IEEE 802. 1.1. Specification of Requirements In this document, several words are used to signify the requirements of the specification. These words are often capitalized. 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]. 1.2. Terminology This document frequently uses the following terms: Authenticator The end of the link requiring the authentication. Peer The other end of the point-to-point link (PPP), point-to-point LAN segment (IEEE 802.1x) or 802.11 wireless link, which being authenticated by the Authenticator. In IEEE 802.1X, this end is known as the Supplicant. Authentication Server An Authentication Server is an entity that provides an Authentication Service to an Authenticator. This service verifies from the credentials provided by the peer, the claim of identity made by the peer. Port Access Entity (PAE) The protocol entity associated with a physical or virtual (802.11) Port. A given PAE may support the protocol functionality associated with the Authenticator, Peer or both. Silently Discard This means the implementation discards the packet without further processing. The implementation SHOULD provide the capability of logging the error, including the contents of the silently discarded packet, and SHOULD record the event in a statistics counter. Blunk, Vollbrecht & Aboba Standards Track [Page 3]
INTERNET-DRAFT RFC2284bis 4 April 2002 Displayable Message This is interpreted to be a human readable string of characters, and MUST NOT affect operation of the protocol. The message encoding MUST follow the UTF-8 transformation format [RFC2044]. 2. Extensible Authentication Protocol (EAP) The Extensible Authentication Protocol (EAP) is a general protocol for authentication which supports multiple authentication mechanisms. EAP may be used on dedicated links as well as switched circuits, and wired as well as wireless links. To date, EAP has been implemented with hosts and routers that connect via switched circuits or dial-up lines using PPP [RFC1661]. It has also been implemented with switches and access points using IEEE 802 [IEEE802]. EAP encapsulation on IEEE 802 media is described in [IEEE8021X]. One of the advantages of the EAP architecture is its flexibility. EAP is used to select a specific authentication mechanism, typically after the Authenticator requests more information in order to determine the specific authentication mechanism(s) to be used. Rather than requiring the Authenticator to be updated to support each new authentication method, EAP permits the use of a "back-end" server which actually implements the various mechanisms while the Authenticator merely passes through the authentication exchange. The authentication exchange proceeds as follows: 1. After the link has been established, the Authenticator sends one or more Requests to authenticate the peer. The Request has a type field to indicate what is being requested. Examples of Request types include Identity, MD5-challenge, etc. The MD5-challenge type corresponds closely to the CHAP authentication protocol [RFC1994]. Typically, the Authenticator will send an initial Identity Request followed by one or more Requests for authentication information. However, an initial Identity Request is not required, and MAY be bypassed. Situations in which the identity may not be required include where the identity is determined by the port to which the peer has connected (leased lines, dedicated switch or dial-up ports, etc.) or where the identity is obtained in another fashion (via calling station identity or MAC address, in the Name field of the MD5-Challenge Response, etc.). 2. The Peer sends a Response packet in reply to each Request. As with the Request packet, the Response packet contains a type field which Blunk, Vollbrecht & Aboba Standards Track [Page 4]
INTERNET-DRAFT RFC2284bis 4 April 2002 corresponds to the type field of the Request. 3. The Authenticator ends the authentication phase with a Success or Failure packet. An Authenticator MAY authenticate the Peer using a sequence of methods. A common example of this is an Identity request followed by an EAP authentication method such as an MD5-Challenge. To accomplish this, the Authenticator and Peer first complete an EAP exchange involving the initial method, with a matching EAP type field included in both Request and Response packets. If the initial authentication method completes unsuccessfully, then the Authenticator sends a Failure packet to the peer. If it completes successfully, and additional authentication methods are required, the Authenticator MAY send a Request packet for a subsequent authentication method, or it MAY send another Identity request. The Peer will then respond with a Response packet containing a type field matching the Request. The sequence of authentication methods proceeds until either an authentication method fails (in which case the Authenticator sends a Failure packet to the peer) or the final authentication method completes successfully, in which case the Authenticator sends a Success packet to the peer. Advantages The EAP protocol can support multiple authentication mechanisms without having to pre-negotiate a particular one. Certain devices (e.g. a NAS, switch or access point) do not necessarily have to understand each request type and may be able to simply act as a pass-through agent for a "back-end" server on a host. The device only need look for the success/failure code to terminate the authentication phase. Disadvantages For use in PPP, EAP does require the addition of a new authentication type to PPP LCP and thus PPP implementations will need to be modified to use it. It also strays from the previous PPP authentication model of negotiating a specific authentication mechanism during LCP. Similarly, switch or access point implementations need to support [IEEE8021X] in order to use EAP. Blunk, Vollbrecht & Aboba Standards Track [Page 5]
INTERNET-DRAFT RFC2284bis 4 April 2002 2.1. EAP State Machine A formal description of the EAP state machine will be covered here. Some questions this section will answer: [a] What happens if an Authenticator receives a EAP Response after sending a Failure or Success packet? [b] What happens if a Peer receives a Success or Failure packet outside of an authentication conversation (e.g. before receiving an EAP Request?) [c] What happens if a Peer receives a packet for another EAP Type before completing an authentication in progress for a different Type? [d] Can a Peer send a Success or Failure packet to an Authenticator? What does an Authenticator do if it receives one of these packets? 3. Media-specific issues EAP has been run over a variety of lower layers including PPP, wired IEEE 802 LANs, and IEEE 802.11 wireless LANs [IEEE802.11]; UDP (L2TP [RFC2661], PIC [PIC]) and TCP [PIC]. This section discusses EAP dependencies on lower layers. 3.1. Lower layer assumptions EAP makes the following assumptions about lower layers: [1] Unreliable transmission. With the exception of Success and Failure, EAP packets are acknowledged, so that EAP does not assume that lower layers are reliable. However, if lower layers exhibit a high loss rate, then frequent timeouts are likely to result. Since EAP has its own retransmission behavior, when run over a reliable lower layer, it is possible for retransmission to occur both at the lower layer and the EAP layer. [2] Known MTU. EAP itself does not support fragmentation and reassembly. However, well written EAP methods SHOULD NOT make assumptions about the minimum supported MTU size, and SHOULD be capable of handling fragmentation and reassembly. As a result, EAP is typically capable of functioning across a range of MTU sizes, as long as the MTU is known. [3] Possible duplication. While it is desirable that lower layers provide for non-duplication, this is not a requirement. The Blunk, Vollbrecht & Aboba Standards Track [Page 6]
INTERNET-DRAFT RFC2284bis 4 April 2002 Identifier field provides both the Peer and Authenticator with the ability to detect duplicates. Where the lower layer is reliable, it SHOULD handle duplicate elimination, providing EAP with a non-duplicated stream of packets. In EAP, the Authenticator is responsible for retransmission. Although EAP Peers MUST NOT retransmit Responses, they do respond to duplicate Requests. Since it is possible for the Authenticator to retransmit before receiving a Response from the Peer, Authenticators MUST silently discard duplicate Responses, so that they are not passed on to the backend authentication server. [4] Possible reordering. EAP provides its own mechanisms to detect reordering and so does not assume that the lower layer provides ordering guarantees. EAP is a "lockstep" protocol, so that the Authenticator MUST NOT send a new Request until a Response is received to an outstanding Request. Since the Peer should not expect a new Request until it has sent a Response, if it receives a new Request with a different Identifier before sending a Response to the outstanding Request, the new Request MUST be silently discarded. Similarly, if the Authenticator receives a Response with an Identifier that does not match the Identifier in the outstanding Request, the Response MUST be silently discarded. 3.2. EAP usage within PPP In order to establish communications over a point-to-point link, each end of the PPP link must first send LCP packets to configure the data link during Link Establishment phase. After the link has been established, PPP provides for an optional Authentication phase before proceeding to the Network-Layer Protocol phase. By default, authentication is not mandatory. If authentication of the link is desired, an implementation MUST specify the Authentication- Protocol Configuration Option during Link Establishment phase. The server can use the identification of the connecting host or router in the selection of options for network layer negotiations. When implemented within PPP, EAP does not select a specific authentication mechanism at PPP Link Control Phase, but rather postpones this until the Authentication Phase. This allows the Authenticator to request more information before determining the specific authentication mechanism. This also permits the use of a "back-end" server which actually implements the various mechanisms while the PPP Authenticator merely passes through the authentication exchange. The PPP Link Establishment and Authentication phases, and the Authentication-Protocol Blunk, Vollbrecht & Aboba Standards Track [Page 7]
INTERNET-DRAFT RFC2284bis 4 April 2002 Configuration Option, are defined in The Point-to-Point Protocol (PPP) [RFC1661]. PPP Configuration Option Format A summary of the PPP Authentication-Protocol Configuration Option format to negotiate the EAP Authentication Protocol is shown below. The fields are transmitted from left to right. Exactly one EAP packet is encapsulated in the Information field of a PPP Data Link Layer frame where the protocol field indicates type hex C227 (PPP EAP). 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Authentication-Protocol | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 3 Length 4 Authentication-Protocol C227 (Hex) for PPP Extensible Authentication Protocol (EAP) 3.3. EAP usage within IEEE 802 The encapsulation of EAP over IEEE 802 link layers is defined in [IEEE8021X]. The IEEE 802 encapsulation of EAP does not involve PPP, and IEEE 802.1X does not include support for link or network layer negotiations. As a result, within IEEE 802.1X it is not possible to negotiate non-EAP authentication mechanisms, such as PAP or CHAP [RFC1994]. Whether authentication is mandatory is determined by the switch or access point configuration. If authentication is not required, or if the identity of the Peer is verified solely based on the MAC address, then the Authenticator MAY respond to a request for EAP authentication with a "canned" Success message. Blunk, Vollbrecht & Aboba Standards Track [Page 8]
INTERNET-DRAFT RFC2284bis 4 April 2002 4. EAP Packet format A summary of the EAP packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Data ... +-+-+-+-+ Code The Code field is one octet and identifies the type of EAP packet. EAP Codes are assigned as follows: 1 Request 2 Response 3 Success 4 Failure Since EAP only defines Codes 1-4, EAP packets with other codes MUST be silently discarded. Identifier The Identifier field is one octet and aids in matching Responses with Requests. Length The Length field is two octets and indicates the length of the EAP packet including the Code, Identifier, Length and Data fields. Octets outside the range of the Length field should be treated as Data Link Layer padding and should be ignored on reception. Data The Data field is zero or more octets. The format of the Data field is determined by the Code field. Blunk, Vollbrecht & Aboba Standards Track [Page 9]
INTERNET-DRAFT RFC2284bis 4 April 2002 4.1. Request and Response Description The Request packet is sent by the Authenticator to the peer. Each Request has a type field which serves to indicate what is being requested. The Authenticator MUST transmit an EAP packet with the Code field set to 1 (Request). Additional Request packets MUST be sent until a valid Response packet is received, or an optional retry counter expires. For IEEE 802.1X, the retry counter is effectively set to zero, so that retransmission never occurs, and instead the Peer times out and authentication is restarted. The contents of the data field is dependent on the Request type. The Peer MUST send a Response packet in reply to a Request packet. Responses MUST only be sent in reply to a received Request and never retransmitted on a timer. The Identifier field of the Response MUST match that of the Request. If a Peer receives a duplicate Request for which it has already sent a Response, it MUST resend its Response. If a Peer receives a duplicate Request before it has sent a Response to the initial Request (i.e. it's waiting for user input), it MUST silently discard the duplicate Request. Retransmitted Requests MUST be sent with the same Identifier value in order to distinguish them from new Requests. In order to avoid confusion between new Requests and retransmissions, the Identifier value chosen for each new Request MUST be unique within the EAP conversation. One way to achieve this is to start the Identifier at an initial value and increment it for each new Request. If the Authenticator receives a Response with an Identifier different from the Identifier within the last Request, then the message MUST be silently discarded. Since it is possible that the Authenticator may retransmit prior to receiving a Response from the Peer, the Authenticator MUST be prepared to detect and silently discard duplicate Responses, without passing them on to the backend authentication server. Note that the Identifier field need only be unique on a per-port basis, so that Authenticators are not restricted to only 255 simultaneous authentication conversations. Implementation Note: Because the authentication process will often involve user input, some care must be taken when deciding upon retransmission strategies and authentication timeouts. When run over an unreliable lower layer, it is suggested that by default the EAP retransmission timer (EAP_RTO) be set to 1 second with a maximum of 5 retransmissions. After each retransmission, the retransmission timer is doubled. One may wish to make these timeouts longer in certain cases, such Blunk, Vollbrecht & Aboba Standards Track [Page 10]
INTERNET-DRAFT RFC2284bis 4 April 2002 as when EAP is run over a reliable lower layer (e.g. EAP over ISAKMP/TCP, as within [PIC]), or where Token Cards are involved. Additionally, the Peer MUST be prepared to silently discard received retransmissions while waiting for user input. A summary of the Request and Response packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Type-Data ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- Code 1 for Request 2 for Response Identifier The Identifier field is one octet and aids in matching Responses with Requests. Length The Length field is two octets and indicates the length of the EAP packet including the Code, Identifier, Length, Type, and Type-Data fields. Octets outside the range of the Length field should be treated as Data Link Layer padding and should be ignored on reception. Type The Type field is one octet. This field indicates the Type of Request or Response. A single Type MUST be specified for each EAP Request or Response. Normally, the Type field of the Response will be the same as the Type of the Request. However, there is also a Nak Response Type for indicating that a Request type is unacceptable to the peer. An initial specification of Types follows in a later section of this document. Type-Data The Type-Data field varies with the Type of Request and the associated Response. Blunk, Vollbrecht & Aboba Standards Track [Page 11]
INTERNET-DRAFT RFC2284bis 4 April 2002 4.2. Success and Failure The Success packet is sent by the Authenticator to the Peer to acknowledge successful completion of an authentication method. The Authenticator MUST transmit an EAP packet with the Code field set to 3 (Success). If the Authenticator cannot authenticate the Peer(unacceptable Responses to one or more Requests) then the implementation MUST transmit an EAP packet with the Code field set to 4 (Failure). An Authenticator MAY wish to issue multiple Requests before sending a Failure response in order to allow for human typing mistakes. Success and Failure packets MUST NOT contain additional data. Implementation Note: Because the Success and Failure packets are not acknowledged, the Authenticator cannot know whether they have been received. As a result, these packets are not retransmitted by the Authenticator, and if they are lost, the Peer will timeout and retry the authentication. A summary of the Success and Failure packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 3 for Success 4 for Failure Identifier The Identifier field is one octet, and aids in matching Responses with Requests. In order to avoid confusion between Success or Failure packets and retransmissions, the Identifier value chosen for a Success or Failure packet MUST be unique within the EAP conversation. Length 4 5. Initial EAP Request/Response Types This section defines the initial set of EAP Types used in Request/Response exchanges. More Types may be defined in follow-on Blunk, Vollbrecht & Aboba Standards Track [Page 12]
INTERNET-DRAFT RFC2284bis 4 April 2002 documents. The Type field is one octet and identifies the structure of an EAP Request or Response packet. The first 3 Types are considered special case Types. The remaining Types define authentication exchanges. The Nak Type is valid only for Response packets, it MUST NOT be sent in a Request. The Nak Type MUST only be sent in response to a Request which uses an authentication Type code (i.e., Type > 3). All EAP implementations MUST support Types 1-4, which are defined in this document, and SHOULD support Type 255. Follow-on RFCs MAY define additional EAP Types. 1 Identity 2 Notification 3 Nak (Response only) 4 MD5-Challenge 255 Vendor-specific Within EAP, the Type functions much like a port number in UDP or TCP. It is assumed that EAP implementations multiplex incoming EAP packets according to their Type, and deliver them to the appropriate EAP method. Peers respond to an EAP Request for an unsupported Type with a Nak Response. Authenticators receiving an EAP Request with an unsupported Type silently discard the Response. EAP packets with codes of Success or Failure do not include a Type, and therefore are not assumed to be delivered to an EAP method. Similarly, EAP messages of Types Identity, Notification and Nak are assumed to be handled by code that is specific to those Types. As a result, these messages MUST NOT be used to carry data destined for delivery to other EAP Types. 5.1. Identity Description The Identity Type is used to query the identity of the peer. Generally, the Authenticator will issue this as the initial Request. An optional displayable message MAY be included to prompt the Peer in the case where there expectation of interaction with a user. A Response MUST be sent to this Request with a Type of 1 (Identity). Implementation Note: The Peer MAY obtain the Identity via user input. It is suggested that the Authenticator retry the Identity Request in the case of an invalid Identity or authentication failure to allow for potential typos on the part of the user. It is suggested that the Identity Request be retried a minimum of 3 times before terminating the authentication phase with a Failure reply. The Notification Request MAY be used to indicate an Blunk, Vollbrecht & Aboba Standards Track [Page 13]
INTERNET-DRAFT RFC2284bis 4 April 2002 invalid authentication attempt prior to transmitting a new Identity Request (optionally, the failure MAY be indicated within the message of the new Identity Request itself). Type 1 Type-Data This field MAY contain a displayable message in the Request, containing UTF-8 encoded 10646 characters [RFC2279]. The Response uses this field to return the Identity. If the Identity is unknown, this field should be zero bytes in length. The field MUST NOT be null terminated. The length of this field is derived from the Length field of the Request/Response packet and hence a null is not required. 5.2. Notification Description The Notification Type is optionally used to convey a displayable message from the Authenticator to the peer. The Peer SHOULD display this message to the user or log it if it cannot be displayed. It is intended to provide an acknowledged notification of some imperative nature. Examples include a password with an expiration time that is about to expire, an OTP sequence integer which is nearing 0, an authentication failure warning, etc. In most circumstances, notification should not be required. Type 2 Type-Data The Type-Data field in the Request contains a displayable message greater than zero octets in length, containing UTF-8 encoded 10646 characters [RFC2279]. The length of the message is determined by Length field of the Request packet. The message MUST NOT be null terminated. A Response MUST be sent in reply to the Request with a Type field of 2 (Notification). The Type-Data field of the Response is zero octets in length. The Response should be sent immediately (independent of how the message is displayed or logged). Blunk, Vollbrecht & Aboba Standards Track [Page 14]
INTERNET-DRAFT RFC2284bis 4 April 2002 5.3. Nak Description The Nak Type is valid only in Response messages. It is sent in reply to a Request where the desired authentication Type is unacceptable. Authentication Types are numbered 4 and above. The Response contains zero or more authentication Types desired by the peer. A Nak with no authentication Type(s) indicates that the Peer does not wish to authenticate using the proposed method but is not proposing an alternative. Since the Nak Type is only valid in Responses and has very limited functionality, it MUST NOT be used as a general purpose error indication, such as for communication of error messages, or negotiation of parameters specific to a particular EAP method. Code 2 for Response. Identifier The Identifier field is one octet and aids in matching Responses with Requests. The Identifier field of a Nak Response MUST match the Identifier field of the Request packet that it is sent in response to. Length >=5 Type 3 Type-Data This field MUST contain zero or more octets indicating the desired authentication Types, in order of preference, with the most preferred method first. In order to avoid an interminable negotiation, the Peer MUST only include Types that it is willing to accept. 5.4. MD5-Challenge Description The MD5-Challenge Type is analogous to the PPP CHAP protocol Blunk, Vollbrecht & Aboba Standards Track [Page 15]
INTERNET-DRAFT RFC2284bis 4 April 2002 [RFC1994] (with MD5 as the specified algorithm). The Request contains a "challenge" message to the peer. A Response MUST be sent in reply to the Request. The Response MAY be either of Type 4 (MD5-Challenge) or Type 3 (Nak). The Nak reply indicates zero or more of the peer's desired authentication Types. All EAP implementations MUST support the MD5-Challenge mechanism. Note that the use of the Identifier field in the MD5-Challenge Type is different from that described in [RFC1994]. EAP allows for retransmission of MD5-Challenge Request packets while RFC 1994 states that both the Identifier and Challenge fields MUST change each time a Challenge (the CHAP equivalent of the MD5-Challenge Request packet) is sent. Type 4 Type-Data The contents of the Type-Data field is summarized below. For reference on the use of this fields see the PPP Challenge Handshake Authentication Protocol [RFC1994]. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Value-Size | Value ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Name ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 5.5. Vendor-specific Description Due to EAP's popularity, the original Method Type space, which only provides for 255 values, is being allocated at a pace, which if continued, would result in exhaustion within a few years. Since many of the existing uses of EAP are vendor-specific, the Vendor-Specific Method Type is available to allow vendors to support their own extended Types not suitable for general usage. The Vendor-specific Type may also be used to expand the global Method Type space beyond the original 255 values. Peers not equipped to interpret the Vendor-specific Type MUST send a Nak, and negotiate a more suitable authentication method. Blunk, Vollbrecht & Aboba Standards Track [Page 16]
INTERNET-DRAFT RFC2284bis 4 April 2002 A summary of the Vendor-specific Type format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Vendor-Id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | String... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 255 for Vendor-specific Vendor-Id The Vendor-Id is 3 octets and represents the SMI Network Management Private Enterprise Code of the Vendor in network byte order, as allocated by IANA. A Vendor-Id of zero is reserved for use by the IETF in providing an expanded global EAP Type space. String The String field is one or more octets. The actual format of the information is site or application specific, and a robust implementation SHOULD support the field as undistinguished octets. The codification of the range of allowed usage of this field is outside the scope of this specification. It SHOULD be encoded as follows. The Vendor-Specific field is dependent on the vendor's definition of that attribute. An example encoding of the Vendor-Specific attribute using this method follows. Example Implementation 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Vendor-Id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Vendor-Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Vendor-Specific... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Blunk, Vollbrecht & Aboba Standards Track [Page 17]
INTERNET-DRAFT RFC2284bis 4 April 2002 Vendor-Type The Vendor-Type field is four octets and represents the vendor- specific Method Type. Where a Vendor-Id of zero is present, the Vendor-Type field provides an expanded global EAP Type space, beginning with EAP Type values of 256. Vendor-Specific The Vendor-Specific field is dependent on the vendor's definition of that attribute. Where a Vendor-Id of zero is present, the Vendor- Specific field will be used for transporting the contents of EAP Methods of Types 256 or greater. 6. IANA Considerations This section provides guidance to the Internet Assigned Numbers Authority (IANA) regarding registration of values related to the EAP protocol, in accordance with BCP 26, [RFC2434]. There are two name spaces in EAP that require registration: Packet Codes and Method Types. EAP is not intended as a general-purpose protocol, and allocations SHOULD NOT be made for purposes unrelated to authentication. 6.1. Definition of Terms The following terms are used here with the meanings defined in BCP 26: "name space", "assigned value", "registration". The following policies are used here with the meanings defined in BCP 26: "Private Use", "First Come First Served", "Expert Review", "Specification Required", "IETF Consensus", "Standards Action". 6.2. Recommended Registration Policies For registration requests where a Designated Expert should be consulted, the responsible IESG Area Director should appoint the Designated Expert. For registration requests requiring Expert Review, the eap mailing list should be consulted. Packet Codes have a range from 1 to 255, of which 1-4 have been allocated. Because a new Packet Code has considerable impact on interoperability, a new Packet Code requires Standards Action, and should be allocated starting at 5. Blunk, Vollbrecht & Aboba Standards Track [Page 18]
INTERNET-DRAFT RFC2284bis 4 April 2002 The original EAP Method Type space has a range from 1 to 255, and is the scarcest resource in EAP, and thus must be allocated with care. Method Types 1-32 have been allocated, with 20 available for re-use. Method Types 33-191 may be allocated following Expert Review, with Specification Required. Release of blocks of Method Types (more than 2 at a time for a given purpose) should require IETF Consensus. EAP Type Values 192-254 are reserved and allocation requires Standards Action. Method Type 255 is allocated for Vendor-Specific extensions and the use of that should be encouraged instead of allocation from the original global Method Type space, for functions specific only to one vendor's implementation of EAP, where no interoperability is deemed useful. When used with a Vendor-Id of zero, Method Type 255 can also be used to provide for an expanded Method Type space. Expanded Method Type values 256-4294967295 may be allocated after Type values 1-191 have been allocated. 7. Normative references [RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [RFC1994] Simpson, W., "PPP Challenge Handshake Authentication Protocol (CHAP)", RFC 1994, August 1996. [RFC2044] Yergeau, F., "UTF-8, a transformation format of Unicode and ISO 10646", RFC 2044, October 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997.] [RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 2279, January 1998. [RFC2434] Alvestrand, H. and Narten, T., "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [IEEE802] IEEE Standards for Local and Metropolitan Area Networks: Overview and Architecture, ANSI/IEEE Std 802, 1990. [IEEE8021X] IEEE Standards for Local and Metropolitan Area Networks: Port based Network Access Control, IEEE Std 802.1X-2001, June 2001. Blunk, Vollbrecht & Aboba Standards Track [Page 19]
INTERNET-DRAFT RFC2284bis 4 April 2002 8. Informative references [RFC1510] Kohl, J., Neuman, C., "The Kerberos Network Authentication Service (V5)", RFC 1510, September 1993. [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, November 1998. [RFC2486] Beadles, M., Aboba, B., "The Network Access Identifier", RFC 2486, January 1999. [RFC2401] Atkinson, R., Kent, S., "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [RFC2408] Maughan, D., Schertler, M., Schneider, M., Turner, J., "Internet Security Association and Key Management Protocol (ISAKMP)", RFC 2408, November 1998. [RFC2433] Zorn, G., Cobb, S., "Microsoft PPP CHAP Extensions", RFC 2433, October 1998. [RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G., and Palter, B., "Layer Two Tunneling Protocol L2TP", RFC 2661, August 1999. [RFC2716] Aboba, B., Simon, D.,"PPP EAP TLS Authentication Protocol", RFC 2716, October 1999. [KRBATTACK] Wu, T., "A Real-World Analysis of Kerberos Password Security", Stanford University Computer Science Department, http://theory.stanford.edu/~tjw/krbpass.html [KRBLIM] Bellovin, S.M., Merritt, M., "Limitations of the kerberos authentication system", Proceedings of the 1991 Winter USENIX Conference, pp. 253-267, 1991. [KERB4WEAK] Dole, B., Lodin, S., and Spafford, E., "Misplaced trust: Kerberos 4 session keys", Proceedings of the Internet Society Network and Distributed System Security Symposium, pp. 60-70, March 1997. [PIC] Sheffer, Y., Krawczyk, H., Aboba, B., "PIC, A Pre-IKE Credential Provisioning Protocol", Internet draft (work in progress), draft-ietf-ipsra-pic-05.txt, February 2002. Blunk, Vollbrecht & Aboba Standards Track [Page 20]
INTERNET-DRAFT RFC2284bis 4 April 2002 [PPTPv1] Schneier, B, Mudge, "Cryptanalysis of Microsoft's Point-to- Point Tunneling Protocol", Proceedings of the 5th ACM Conference on Communications and Computer Security, ACM Press, November 1998. [IEEE80211] Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific Requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, IEEE Std. 802.11-1997, 1997. 9. Security Considerations Examples of attacks against EAP include: [1] An adversary may try to discover user identities by snooping data packets. [2] An adversary may try to modify or spoof EAP packets. [3] An adversary may launch denial of service attacks by terminating EAP conversations. [4] An adversary might attempt to recover the passphrase by mounting an offline dictionary attack. [5] An adversary may attempt to convince the Peer to connect to an untrusted network. [6] An adversary may attempt to disrupt the EAP negotiation in order to weaken the authentication, gain access to user passwords or remove confidentiality protection. [7] An attacker may attempt to take advantage of implementation idiosyncracies. [8] An attacker may attempt to take advantage of weak key derivation techniques used within EAP methods. [9] An attacker may attempt to take advantage of weak ciphersuites. In the sections that follow, each of these threats is discussed in turn. 9.1. Identity protection This specification does not require an Identity exchange as part of the EAP conversation. Therefore, it is possible to omit the Identity Blunk, Vollbrecht & Aboba Standards Track [Page 21]
INTERNET-DRAFT RFC2284bis 4 April 2002 exchange entirely, or to postpone it until later in the conversation one a protected channel has been negotiated. However, within networks where proxies or relays are present, it may be necessary to locate the appropriate backend authentication server before the authentication conversation can succeed. The realm portion of the Network Access Identifier (NAI) [RFC2486] is typically included within the Response-Identity in order to enable the authentication exchange to be routed to the appropriate backend authentication server. As a result, while the user-name portion of the NAI may be ommitted in the Identity- Response, where proxies or relays are present, the realm portion may be required. 9.2. Packet modification attacks While individual EAP methods such as EAP TLS [RFC2716] may provide for authentication, integrity and replay protection of data placed within the Type-Data portion of EAP Request and Response packets, EAP itself does not provide built-in support for per-packet data origin authentication, replay or integrity protection. This means that an attacker may inject or modify EAP packets, including Request and Response packets of Types Identity, Notification, Nak, MD5-Challenge as well as Success and Failure packets. An attacker may also modify the EAP headers within EAP packets where the Type-Data field is protected. In the case of PPP and IEEE 802 wired links, it is assumed that such attacks are restricted to attackers who can gain access to the physical link. However, where EAP is run over wireless media such as IEEE 802.11, or over IP, such as within protocols supporting PPP or Ethernet tunneling [RFC2661], this assumption is no longer valid and the vulnerability to attack is much greater. As a result, when EAP is used with wireless media or over IP, the EAP conversation SHOULD be authenticated, integrity and replay protected, on a per-packet basis. This can be accomplished using mechanisms such as ISAKMP [RFC2408], as is done in PIC [PIC], or using TLS [RFC2246]. 9.3. Denial of service attacks EAP headers are unprotected, as are packets of Types Identity, Nak, Notification as well as Success and Failure. Since the Identifier is only a single octet, and implementations typically start it at zero (0), incrementing with each new Request, it is very easy to guess. This makes it easy for an attacker to deny service by spoofing Failure packets, as well as by inserting Nak packets so as to prolong the method negotiation. It is also possible for the attacker to attempt to replay packets from previous conversations. These attacks are easier to carry Blunk, Vollbrecht & Aboba Standards Track [Page 22]
INTERNET-DRAFT RFC2284bis 4 April 2002 out where EAP is run over a wireless medium or over the Internet. Several mechanisms are recommended for addressing this vulnerability: [1] Silent discard. Since only a single EAP Request can be in progress between an Authenticator and a Peer at a given time, if a Peer receives a new Request before sending a Response, the new Request can be silently discarded. This increases resilience against spoofed Requests. Similarly, an Authenticator can silently discard Responses with Identifiers that do not correspond to the Identifier included in the last Request, or that represent duplicate Responses. [2] Where the EAP conversation is protected via per-packet data origin authentication, integrity and replay protection, spoofing or data modification attacks can be detected. 9.4. Dictionary attacks Password authentication algorithms such as EAP-MD5, MS-CHAPv1 [RFC2433] and Kerberos V [RFC1510] are known to be vulnerable to offline dictionary attacks. MS-CHAPv1 vulnerabilities are documented in [PPTPv1]; Kerberos vulnerabilities are described in [KRBATTACK], [KRBLIM], and [KERB4WEAK]. As a result, EAP authentication algorithms vulnerable to offline dictionary attack SHOULD NOT be used without encrypting and integrity protecting the EAP exchange, and methods vulnerable to such attacks should contain a warning to that effect within the specification. 9.5. Connection to an untrusted network Lf a one-way authentication method is negotiated, such as EAP-MD5, then the Authenticator's identity will not be verified. While this might be acceptable on a wired network, this level of security is not acceptable where EAP runs over wireless media or IP, since this leaves the Peer vulnerable to connection to an untrusted network. For use on wireless media such as 802.11 [IEEE80211], or over the Internet, where physical security can no longer be assumed, EAP methods providing mutual authentication SHOULD be used. Where a mutually authenticating method is selected, the Peer SHOULD NOT accept a Success packet from an Authenticator that has not yet authenticated to the Peer. Since the Success message is not protected, it may be spoofed, or sent by a rogue Authenticator seeking to avoid having to authenticate to the Peer. It should be understood that nothing within the EAP specification precludes the Peer from silently discarding such a premature Success packet. Blunk, Vollbrecht & Aboba Standards Track [Page 23]
INTERNET-DRAFT RFC2284bis 4 April 2002 In EAP there is no requirement that authentication be full duplex or that the same protocol be used in both directions. It is perfectly acceptable for different protocols to be used in each direction. This will, of course, depend on the specific protocols negotiated. 9.6. Negotiation attacks Vulnerability to negotiation attacks is particularly acute where EAP runs over wireless media or IP since EAP method negotiation is unprotected. For use on wireless media such as 802.11 [IEEE80211], or over the Internet, where physical security can no longer be assumed, the EAP conversation SHOULD be authenticated, replay and integrity protected on a per-packet basis. In addition, in order to avoid downgrading from a mutually authenticating method to one that only authenticates the peer, the peer SHOULD only accept negotiation of a mutually authenticating method. In practice, within or associated with each EAP server, it is anticipated that a particular named user will be authenticated by a predefined method or sequence of methods, without leaving the user any choice. Enabling negotiation would make the user vulnerable to attacks which negotiate the least secure method from among a set (such as EAP- MD5 instead of a mutually authenticating method). Instead, for each named user there should be an indication of exactly one method or sequence of methods used to authenticate that user name. If a user needs to make use of different authentication methods under different circumstances, then distinct identities SHOULD be employed, each of which identifies exactly one authentication method or sequence of methods. 9.7. Implementation idiosyncracies The interaction of authentication protocols with link layer technologies such as PPP and IEEE 802 are highly implementation dependent. For example, upon failure of authentication, some PPP implementations do not terminate the link, instead limiting the kind of traffic in the Network-Layer Protocols to a filtered subset, which in turn allows the user opportunity to update secrets or send mail to the network administrator indicating a problem. Similarly, while in IEEE 802.1X an authentication failure will result denied access to the controlled port, limited traffic may be permitted on the uncontrolled port. In EAP there is no provision for retries of failed authentication. However, in PPP the LCP state machine can renegotiate the authentication Blunk, Vollbrecht & Aboba Standards Track [Page 24]
INTERNET-DRAFT RFC2284bis 4 April 2002 protocol at any time, thus allowing a new attempt. Similarly, in IEEE 802.1X the supplicant or Authenticator can re-authenticate at any time. It is recommended that any counters used for authentication failure not be reset until after successful authentication, or subsequent termination of the failed link. 9.8. Key derivation It is possible for the EAP endpoints to mutually authenticate, negotiate a ciphersuite, and derive session key(s) for subsequent use with link layer authentication, integrity protection and encryption. This does not present an issue on the Peer, since the Peer and EAP client reside on the same machine; all that is required is for the EAP client module to pass the session key to the link layer security module. The situation is more complex when the Authenticator does not reside on the same machine as the EAP server. For example, the EAP server may be a backend security server. In the case where the EAP server and Authenticator reside on different machines, there are several implications for security. Firstly, mutual authentication will occur between the Peer and the EAP server, not between the Peer and the Authenticator. This means that it is not possible for the Peer to validate the identity of the Authenticator. The second issue that arises in the case of an EAP server and Authenticator residing on different machines is that the session key negotiated between the Peer and EAP server will need to be transmitted to the Authenticator. Therefore a mechanism needs to be provided to transmit the session key from the EAP server to the Authenticator that needs to use the key. The specification of this transit mechanism is outside the scope of this document. This specification does not provide guidance on how EAP methods are to derive keys. Key derivation is an art that is best practiced by professionals; rather than inventing new key derivation algorithms, reuse of existing algorithms such as those specified in IKE [RFC2409], or TLS [RFC2246] is recommended. However, EAP methods deriving keys SHOULD provide keying material that can be used with any ciphersuite, since EAP methods cannot be assumed to provide protected ciphersuite negotiation and the negotiated ciphersuite may not be known beforehand. In addition, EAP methods deriving keys MUST describe how keys for authentication/integrity, encryption and IVs are to be derived from the provided keying material, and how cryptographic separation is maintained Blunk, Vollbrecht & Aboba Standards Track [Page 25]
INTERNET-DRAFT RFC2284bis 4 April 2002 between keying material used for different purposes. 9.9. Weak ciphersuites EAP authentication may be used in situations where after the initial authentication, data packets are sent without per-packet data origin authentication, integrity and replay protection or confidentiality. These scenarios are inherently insecure. Without per-packet authentication, integrity and replay protection, an attacker with access to the media can inject packets, "flip bits" within existing packets, or even hijack the session completely. Without per-packet confidentiality, it is possible to snoop data packets. Where attackers may easily gain access to the medium (such as in wireless networks, or where EAP is run over the Internet), EAP SHOULD be used in concert with credible ciphersuites in order to provide defensible security. Due to the many of known attacks to which it is vulnerable, Wired Equivalent Privacy (WEP) [IEEE80211] does not qualify as a credible ciphersuite. Acknowledgments This protocol derives much of its inspiration from Dave Carrel's AHA draft as well as the PPP CHAP protocol [RFC1994]. Bill Simpson provided much of the boilerplate used throughout this document. Al Rubens (Merit) also provided valuable feedback, as did Glen Zorn (Cisco) and Ashwin Palekar (Microsoft). Authors' Addresses Larry J. Blunk Merit Network, Inc. 4251 Plymouth Rd., Suite C Ann Arbor, MI 48105 EMail: ljb@merit.edu Phone: 734-763-6056 FAX: 734-647-3185 John R. Vollbrecht Interlink Networks, Inc. 775 Technology Drive, Suite 200 Ann Arbor, MI 48108 USA Phone: +1 734 821 1205 Fax: +1 734 821 1235 EMail: jrv@interlinknetworks.com Blunk, Vollbrecht & Aboba Standards Track [Page 26]
INTERNET-DRAFT RFC2284bis 4 April 2002 Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 EMail: bernarda@microsoft.com Phone: +1 425 706 6605 Fax: +1 425 706 7329 Full Copyright Statement Copyright (C) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." Open issues Open issues relating to this specification are tracked on the following web site: http://www.drizzle.com/~aboba/EAP/eapissues.html Expiration Date This memo is filed as <draft-ietf-pppext-rfc2284bis-04.txt>, and expires November 24, 2002. Blunk, Vollbrecht & Aboba Standards Track [Page 27]
