Internet Draft H. Andersson S. Josefsson RSA Security June 2001 EAP Mechanism using TLS and SASL (Version 1) draft-andersson-eap-tls-sasl-00.txt Status of this Memo 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 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 Distribution of this memo is unlimited. Abstract This document specifies an Extensible Authentication Protocol (EAP) mechanism for mutual authentication and session key generation in a roaming environment. The server authentication and the negotiation of the session key is done using the PPP EAP Transport Layer Security Authentication Protocol. The user authenticates using a SASL mechanism, the SASL authentication being protected by TLS. An important application discussed in this document is to provide a strong form of authentication of access points and stations within an IEEE 802.11 Wireless Local Area Network (WLAN), but other applications such as LAN access over Bluetooth might also be considered in the future. 1. Introduction The PPP Extensible Authentication Protocol [2] defines a general authentication framework. This document specifies an EAP mechanism for mutual authentication and session key generation, with support for a roaming environment. The connection is made, using EAP terminology, between a peer and an authenticator. The (public-key) authentication of the authenticator and the negotiation of the Andersson & Josefsson Expires December 2001 [Page 1] Internet Draft EAP TLS SASL (Version 1) June 2001 session key is done using the PPP EAP Transport Layer Security Authentication Protocol [1]. The user performs a strong authentication using a SASL mechanism, see [6], such as the RSA SecurID SASL mechanism [8], the One-Time-Password SASL mechanism [7], or one of the original mechanisms defined in [6]. Section 2 defines the model and some terminology. A brief overview of the EAP conversation is given in Section 3, and Section 4 gives a detailed description of packet formats. In Section 5, the protocol is applied to an IEEE 802.11 Wireless Local Area Network (WLAN). Finally, Section 6 discusses security issues. 2. Model and Terminology The term peer notifies a client, acting on behalf of a user, that requests access to a network. The entity contacted by the peer is denoted authenticator. The authenticator is in turn connected to an entity called authentication server. In our model, the authenticator is acting merely as a passthrough device during the authentication phase, forwarding each packet received from the peer to the authentication server, and vice versa. In the sequel we will identify the authenticator and its corresponding authentication server, whenever this can be done without any risk of misunderstandings. The realisation of the authentication server and the communication between the authenticator and the server is outside the scope of this document. +---+ | A | | u | | t | | h | +---------------+ +--------+ | e | <-----------> | Authenticator | <-----> | Peer | | n | +---------------+ EAP +--------+ | t | . | i | . | c | . | a | +---------------+ . EAP | t | <-----------> | Authenticator | . | i | +---------------+ | o | | n | | | | S | +---------------+ | e | <-----------> | Authenticator | | r | +---------------+ | v | | e | | r | +---+ Andersson & Josefsson Expires December 2001 [Page 2] Internet Draft EAP TLS SASL (Version 1) June 2001 An overview of the assumed environment is found in the figure above. The peer initially contacts the authenticator at the top of the figure. The dotted line between the peer and the next authenticator symbolizes roaming, i.e. the situation where the peer transits from one authenticator to another while still maintaining server and user authentication. It is assumed that the same authentication server sits behind all of the authenticators contacted by the peer. This document frequently uses the following abbreviations: EAP Extensible Authentication Protocol. After a connection link has been established between two entities, an authentication phase may take place. The PPP EAP protocol [2] is a general authentication protocol. The authenticator sends one or more requests to the peer, and the peer sends a response in reply to each request. The authenticator ends the authentication phase with a success or failure message. SASL Simple Authentication and Security Layer. A method for adding authentication support to connection-based protocols and for optionally negotiating a security layer for subsequent protocol interactions. Defined in [6]. The method is mainly focused on authenticating a user to a server. Several SASL mechanisms have been defined, such as the SecurID SASL mechanism [8], the One-Time-Password SASL mechanism [7], and the original mechanisms of [6]. TLS Transport Layer Security. Internet security protocol for point-to-point connections (enhancement of Secure Sockets Layer, SSL). Defined in [3]. Under this protocol, two entities are able to authenticate each other and to establish a secure link. TLS operates at the transport layer. The protocol PPP EAP TLS [1] describes how to provide for TLS mechanisms within EAP. 3. Overview of the conversation A peer wishes to set up a connection with an authenticator, for the purpose of authenticating itself to e.g. a wireless infrastructure. In our model, the authenticators are in connection with an authentication server. The following describes each EAP packet that is sent between the authenticator and peer during the EAP connection. 3.1. Initial registration The first two steps are described in detail in Section 3.1 of [2], we repeat them here for completeness. Andersson & Josefsson Expires December 2001 [Page 3] Internet Draft EAP TLS SASL (Version 1) June 2001 1. The first EAP Request packet sent by the authenticator to the peer is of type Identity. The data field may optionally contain a displayable message. 2. The peer responds with an EAP-Response packet of type Identity. The data field containing Identity is not used here (the peer identity is transferred under TLS protection by the SASL mechanism below) and should be of zero length. The entities now initiate an EAP-TLS conversation. The following is an example of a TLS handshake within EAP -- the packets are described in detail in Section 4 of [1]. Note that the EAP method defined in this document does not terminate the TLS connection once the TLS handshake is concluded (and thus differs subtly from how TLS is used in [1]). 3. The authenticator sends an EAP-TLS packet of type Start with empty data field. The data field of following packets will encapsulate TLS Handshake Protocol messages. 4. Client hello: The peer sends a preferred TLS protocol version number, an empty Session ID field, a list of preferred cryptographic algorithms, and a random number to initialize the TLS handshake. 5. Server hello: The authenticator responds with a selected TLS protocol version number, a new Session ID, a list of selected cryptographic algorithms, and another random number. Server certificate: The authenticator then sends a chain of X.509v3 certificates, starting with its own certificate. Server hello done: Finally, the authenticator indicates the end of this message stream. (Note that the authenticator must NOT send any certificate request.) 6. Client key exchange: The peer generates a premaster secret, encrypts it using the public key obtained from the server certificate, and sends the result. Client finished: The peer also calculates a master secret from the premaster secret, and sends a hash of a message consisting of the master secret; all of the data from all previous handshake messages; the string "client finished". 7. Server finished: Finally, the authenticator itself generates the master secret from the premaster secret and responds with a hash of a message consisting of the master secret; all of the data from all previous handshake messages; the string "server finished". This concludes the TLS handshake and the authentication of the authenticator. It remains to perform user authentication. Note that it is not until now that we will deviate from the specification [1]. The following packets will be sent within the TLS Record Protocol (and will therefore be protected by the encryption negotiated by TLS). Andersson & Josefsson Expires December 2001 [Page 4] Internet Draft EAP TLS SASL (Version 1) June 2001 8. The authenticator sends a list of supported SASL mechanisms. 9. The SASL mechanism at the peer gathers information and constructs a SASL token that is sent to the authenticator protected by the TLS Record Protocol layer. Typically the token consists of user name, one time password, PIN code, etc. 10. The SASL mechanism at the authenticator reads the SASL token, and can proceed in one of three ways. The first path, taken upon successful verification, returns EAP-Success and finishes the authentication phase. The second path is when verification of the user credentials has terminally failed, and an error message is returned to the peer. The peer may choose to re-start the SASL dialogue by resuming at step 9 above (or give up by disconnecting). The third path is when the SASL mechanism requires additional information from the peer. A new SASL token will then be sent to the peer. The entities resume at step 9, awaiting the eventual outcome of either success or failure. This concludes the mutual authentication, and upon success both authenticator and peer may generate any amount of new key material to be forwarded to the underlying transport. This is accomplished within the TLS Record Protocol by using the so-called PRF (Pseudo-Random Function), see [3]. 3.2. Roaming We now describe the case where the peer is transiting between two authenticators during a session. In order to obtain a seamless transition to a connection between the peer and the new authenticator, we use the connection re-establishment mechanism provided by the TLS Handshake Protocol. Note that the new authenticator is assumed to use the same authentication server as the old one, hence the old security parameters are still available. The steps 1-3 above are repeated without change. Then the following condensed TLS handshake follows. 4. Client hello: The peer sends the TLS protocol version number, the Session ID of the old connection, the previously negotiated cryptographic algorithms, and a random number. 5. Server hello: The authenticator responds with the TLS protocol version number, the Session ID, the negotiated cryptographic algorithms, and another random number. If the old Session ID has expired, then a new Session ID is presented to the peer and full authentication takes place, as described in Subsection 3.1. 6. Client finished: The peer sends a hash of a message consisting of the previously calculated master secret; data from all previous handshake messages; the string "client finished". Andersson & Josefsson Expires December 2001 [Page 5] Internet Draft EAP TLS SASL (Version 1) June 2001 7. Server finished: The authenticator responds with a hash of a message consisting of the master secret; data from all previous handshake messages; the string "server finished". Note that mutual authentication is achieved, since both peer and authenticator have to know the old master secret in order to successfully complete the protocol. An alternative to TLS resumption has been discussed, whereby a "Roaming ID" is used to identify the user moving between authenticators. At a new connection, server authentication and generation of new security parameters is mandatory. The advantage of this approach is that the authentication server does not have to store so much key material, since all data except the Roaming ID may be deleted when entities are disconnected. This can be an important issue if there are many peers to be served. On the other hand, having to generate much new key material could be very time consuming for the authentication server, and this potential danger has led us to choose TLS resumption as described above. Finally, the length of time that a Session ID is valid should be limited. The time of validity is application dependent. In some environments it may be desirable that the authenticator notify the peer that the Session ID is about to expire. No mechanism is defined in this document to handle such a scenario, but note that the Session ID validity is checked during connection re-establishment (see 5 above). 4. Packet formats It is assumed that underlying transport protocols has set up the connection so that it is ready to transfer EAP packets. 4.1. TLS in EAP The syntax of EAP packets containing TLS messages is per [1], and the TLS protocol description is per [3]. Note that [1] does not make use of messages in the Record Protocol layer of TLS while this specification does, however this does not affect the EAP protocol syntax. We include the EAP syntax in the following figure, referring to Sections 4.2 and 4.3 of [1] for the definition of the Request and Response packets. 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 | Data ... / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Andersson & Josefsson Expires December 2001 [Page 6] Internet Draft EAP TLS SASL (Version 1) June 2001 Code 1 - Request 2 - 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 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 TBA - EAP TLS SASL Data The format of the Data field is determined by the Code field. 4.2. SASL negotiation inside TLS Record Protocol We now assume that the TLS handshake has been successfully completed and that a secure TLS connection is available within the TLS Record Protocol. The following packets (protected by TLS Record Protocol and sent inside EAP packets) are used to negotiate a SASL authentication. The following figure describes the template packet structure that is used during this communication. 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 | Data ... / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 0 Reserved 1 SASL greeting 2 SASL token 3 Soft error 4-254 Not used by this specification 255 Reserved Andersson & Josefsson Expires December 2001 [Page 7] Internet Draft EAP TLS SASL (Version 1) June 2001 We now describe each of the individual packet types. SASL greeting The data field of this packet contains a list of SASL mechanisms supported by the authenticator. Each entry in the list of SASL mechanisms is terminated by a NUL octet, and contains a SASL mechanism name (from 1 to 20 characters in length). SASL token The data field of this packet contains the name of the selected SASL mechanism, terminated by a NUL octet, together with the SASL token. Soft error This packet (with empty data field) notifies the peer that a soft user authentication error has occurred, such as when a user has typed her username incorrectly. When this message is received during an initial registration, the peer may reset the SASL mechanism and re-try (or disconnect). Note that the EAP type Notify packet may be used if the authenticator wishes to relay a displayable text message to the peer. 5. Example: IEEE 802.11 WLAN IEEE 802.11 Wireless Local Area Network (WLAN) is a standard for wireless computer networks, see [5]. Any device that contains an IEEE 802.11 conformant medium access control and physical layer interface to the wireless medium is called a Station (STA). An entity that has station functionality and also provides access to the distribution services (e.g. a wired LAN) via the wireless medium for associated stations is called an Access Point (AP). The authentication services defined within IEEE 802.11 are discussed below, and the need for higher level authentication is addressed. IEEE 802.11 defines two types of authentication methods -- Open system authentication and Shared key authentication. Open system authentication is essentially a null authentication. The conversation is done in clear, no challenge procedure is performed. The purpose of Shared key authentication is to check that both parties share a pre-negotiated encryption key. The AP sends a challenge and the STA responds by encrypting this challenge. If the AP successfully decrypts that message, the authentication is finished. In other words, the AP is never required to authenticate itself. This opens up for a number of attacks, such as denial of service attacks via rogue APs. It is thus crucial to achieve mutual authentication. The IEEE 802.1X draft [4] specifies a general method for the provision of port based network access control. A port in this context is an attachment point to the LAN infrastructure, e.g. an Andersson & Josefsson Expires December 2001 [Page 8] Internet Draft EAP TLS SASL (Version 1) June 2001 association between a STA and an AP. The specification describes the architectural framework within which the authentication takes place, and establishes the requirements for a higher level authentication protocol between the station and the access point. The IEEE 802.1X draft provides a framework, Extensible Authentication Protocol Over Local area networks (EAPOL), that makes it possible to send EAP packets between IEEE 802.11 entities. In a WLAN environment, the "Authenticator" is the AP, and the "Peer" is a STA. An Authentication Server is an entity connected with the AP. The server is communicating with the STA during the authentication -- the AP is sitting in between, acting merely as a passthrough device. In a roaming environment, the STA may connect to several APs during a session. All the APs are assumed to be connected to the same authentication server. The protocol described in this paper may therefore be applied to a WLAN environment, providing authentication of the AP, strong authentication of the user of the STA, and session key negotiation. Note that the present protocol is partly based on [1], which in turn assumes PPP EAP and not EAPOL as the underlying protocol. However, this minor difference will cause no problems whatsoever, since the TLS conversation carries over word by word to the new environment. Let us finally comment on the Wired Equivalent Privacy (WEP) encryption scheme defined in the IEEE 802.11 standard. WEP uses the stream cipher RC4 with key obtained as the concatenation of a 24 bit IV and a 40 bit WEP key. Four WEP keys can be prestored, but it is also possible to use a session key negotiated during the authentication phase, i.e. follow the approach outlined in this work. WEP suffers from some serious security weaknesses, e.g. the WEP key is too short to withstand a brute force attack. Also, the IV is too short -- even if a new random IV is used for each packet, collisions will start appearing within a few seconds (according to the birthday paradox). XORing messages with the same IV results in plaintext difference that can be further analyzed. Finally, there is no real data integrity since the integrity check value used is just a linear checksum. An active attacker wishing to alter the plaintext can easily modify the checksum to be valid for the new plaintext. The IEEE 802.11 working group recognizes the need to improve security, and is currently working on a revision of the standard. 6. Security considerations The Transport Layer Security protocol is presumed to be a strong security protocol and it is widely accepted. Here we discuss some security issues. The Session ID is sent in clear, so an attacker may contact an authenticator, pretending to be the legitimate user. However, by sending correct Finished messages, the parties prove to each other that they know the correct premaster secret. The attacker will not be able to finish the handshake properly (unless the protocol has been completely broken). Andersson & Josefsson Expires December 2001 [Page 9] Internet Draft EAP TLS SASL (Version 1) June 2001 An attacker, acting as an active man-in-the-middle, might try to influence the choice of encryption algorithm by altering the corresponding handshake message. However, this will also be detected in the verification of the Finished messages, since each of these consists of a hash of all previous messages. The hash functions MD5 and SHA-1 are used in tandem wherever possible. The TLS designers claim that this approach ensures that a serious flaw in one of the functions will not lead to failure of the entire TLS protocol. Finally, the SASL mechanism typically provides strong user authentication. With the approach described here, the SASL tokens sent by the peer are not transmitted in clear. This is particularly important in a wireless environment where passive eavesdropping is a serious threat. 7. Acknowledgements We wish to thank Jan-Ove Larsson and Magnus Nystrom for helpful discussions and comments on earlier drafts. References [1] Aboba, B., Simon, D., "PPP EAP TLS Authentication Protocol", RFC 2716, October 1999. [2] Blunk, L., Vollbrecht, J., "PPP Extensible Authentication Protocol (EAP)", RFC 2284, March 1998. [3] Dierks, T., Allen, C., "The TLS Protocol", RFC 2246, January 1999. [4] IEEE Standards for Local and Metropolitan Area Networks: Port based Network Access Control, IEEE Draft 802.1X/D10, January 2001. [5] 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, 1999. [6] Myers, J., "Simple Authentication and Security Layer (SASL)", RFC 2222, October 1997. [7] Newman, C., "The One-Time-Password SASL Mechanism", RFC 2444, October 1998. [8] Nystrom, M., "The SecurID SASL Mechanism", RFC 2808, April 2000. Andersson & Josefsson Expires December 2001 [Page 10] Internet Draft EAP TLS SASL (Version 1) June 2001 Address of the authors Hakan Andersson RSA Security Box 107 04 SE-121 29 Stockholm Sweden E-mail: handersson@rsasecurity.com Phone: +46 8 725 9758 Fax: +46 8 649 4970 Simon Josefsson RSA Security Box 107 04 SE-121 29 Stockholm Sweden E-mail: sjosefsson@rsasecurity.com Phone: +46 8 725 0914 Fax: +46 8 649 4970 Andersson & Josefsson Expires December 2001 [Page 11]