EAP-TTLS is an EAP (Extensible Authentication Protocol) method that
encapsulates a TLS (Transport Layer Security) session, consisting of
a handshake phase and a data phase. During the handshake phase, the
server is authenticated to the client (or client and server are
mutually authenticated) using standard TLS procedures, and keying
material is generated in order to create a cryptographically secure
tunnel for information exchange in the subsequent data phase. During
the data phase, the client is authenticated to the server (or client
and server are mutually authenticated) using an arbitrary
authentication mechanism encapsulated within the secure tunnel. The
encapsulated authentication mechanism may itself be EAP, or it may be
another authentication protocol such as PAP, CHAP, MS-CHAP, or MS-
CHAP-V2. Thus, EAP-TTLS allows legacy password-based authentication
protocols to be used against existing authentication databases, while
protecting the security of these legacy protocols against
eavesdropping, man-in-the-middle, and other attacks. The data phase
may also be used for additional, arbitrary data exchange.