EAP

Figure 7-5 shows the success/failure frame.

The EAP message exchange is basic, as shown in Figure 7-6. EAP starts after the supplicant has data and link layer connectivity (Step 0 in Figure 7-6). The communication between the authenticator and the supplicant is done as a request-response paradigm, meaning a message is sent and the sender waits for a response before sending another message.

The first exchange (Step 1 in Figure 7-6) could be an identity exchange. Even though there is an identity message type, the RFC does not guarantee identity semantics and encourages that the authentication mechanisms not depend on this exchange for identity and have their own identity-recognition mechanisms. Moreover, the initial exchange would most likely be in cleartext; therefore, it is a security vulnerability.

In Step 2, all the exchanges between the supplicant, authenticator, and back-end authentication systems are defined by a wide variety of specific RFCs or drafts and authentication mechanisms.

Finally, at some point, the authenticator determines whether the authentication is a success or failure and sends an appropriate message to the supplicant (Step 3 in Figure 7-6).

EAP Authentication Mechanisms

This section examines in detail some of the most relevant EAP authentication frameworks. The typical mechanisms using EAP over LANS are EAP-MD5, EAP-One-Time Password (EAP-OTP), EAP-TLS, EAP-TTLS, EAP-Generic Token Card (EAP-GTC), Microsoft CHAP (EAP-MSCHAPv2), and EAP-FAST.

EAP-MD5

The EAP-MD5 is a Challenge Handshake Authentication Protocol (CHAP), as defined in RFC 1994. Figure 7-7 shows the choreography of the EAP-MD5 mechanism.

EAP-MD5 is a pure authentication protocol; after the authentication, the messages are transmitted in cleartext. It is also a client authentication protocol?the server side (authenticator) is not authenticated; therefore, it cannot detect a rogue AP.

EAP-MD5 also contains a set of good features: It requires only lightweight processing (which translates to less hardware) and does not require a key/certificate infrastructure. Although pure EAP-MD5 has some value in the PPP world, it is of limited use in the wireless world. For example, Microsoft has dropped the support for EAP-MD5 for the wireless interface in Windows XP. Support was dropped because of security problems; EAP-MD5 is vulnerable to dictionary and brute-force attacks when used with Ethernet and wireless.

EAP-OTP

EAP-OTP is similar to MD5, except it uses the OTP as the response. The request contains a displayable message. The OTP method is defined in RFC 2289. The OTP mechanism is employed extensively in VPN and PPP scenarios but not in the wireless world.

EAP-GTC

The EAP-GTC (Generic Token Card) is similar to the EAP-OTP except with hardware token cards. The request contains a displayable message, and the response contains the string read from the hardware token card.

EAP-TLS

As you have seen, methods such as EAP-MD5 and EAP-GTC are specific to authentication and are confined to authenticating only the client. EAP-TLS adds more capabilities such as mutual authentication, which provides an encrypted transport layer and the capability to dynamically change the keys. On the other hand, EAP-TLS is based on digital certificates and thus requires an infrastructure to manage?issue, revoke, and verify?certificates and keys.

EAP-TLS is based on the TLS protocol that is defined in RFC 2246. The following section talks a little bit about TLS, and then you will look at which of its features carry over into EAP-TLS.

A Brief Introduction to TLS

TLS has the concept of sessions and connection. A connection is a channel, whereas a session is governed by security context?session identifier, peer certificate, compression method, cipher spec for the session key, and MAC algorithm parameters and the shared master secret. TLS can and will securely negotiate different session parameters while maintaining the same connection?usually a TCP connection. The handshake phase establishes a session, and the session keys (symmetric) encrypt the transport during the data transfer phase. In addition to providing confidentiality, TLS provides integrity check. TLS, of course, is a point-to-point method.

TLS defines two layers: a record layer (which exchanges messages dealing with things such as fragmentation, MAC, and encryption) and a message layer (which defines different types of messages). The four message types are as follows:

Figure 7-8 shows the TLS choreography, through the lifetime of a connection, in some detail.

The handshake protocol (Steps 3, 4, 5, and 6 in Figure 7-8) accomplishes server authentication, algorithm negotiation, establishing session context, and (optional) client authentication. Of course, to successfully complete the handshake and arrive at the keys and secrets, the client and server should have digital certificates (Step 1 in Figure 7-8) and connectivity (Step 2 in Figure 7-8).

After the handshake is successfully completed, the client and server can exchange application data (Step 7 in Figure 7-8) using the established secure transport. Occasionally, renegotiation of session context might happen, usually for new session keys. Finally, the client or server with the close message closes the connection (Step 8 in Figure 7-8).

EAP-TLS Choreography

EAP-TLS employs selected parts of the TLS. For example, it uses the TLS handshake for mutual authentication, cipher suit negotiation, and to derive session keys; however, it does not use all parts of the TLS record protocol.

Figure 7-9 shows the frame format for EAP-TLS. The EAP type is 13 (see Table 7-2). The EAP data frame consists of TLS-specific fields. A similar approach is taken for the choreography, as shown in Figure 7-10. As expected, Figure 7-10 is a combination of Figures 7-6 and 7-8.