EAP Methods Explained: PEAP, EAP-TLS and EAP-TTLS for Network+

EAP — Extensible Authentication Protocol — is a framework, not a single protocol. It defines how authentication messages are carried; the individual methods define what actually happens. WPA2 and WPA3 Enterprise use EAP over 802.1X, and the exam expects you to distinguish the common methods.

Where EAP sits

In 802.1X the roles are supplicant (the client), authenticator (the access point or switch) and authentication server (usually RADIUS).

EAP messages travel from supplicant to authenticator encapsulated in EAPOL — EAP over LAN — and from authenticator to server inside RADIUS. The authenticator relays without understanding the method, which is why adding a new EAP method needs changes only at the client and the server.

The methods

Method	Server certificate	Client certificate	Security
EAP-TLS	Required	Required	Strongest
PEAP	Required	No	Strong
EAP-TTLS	Required	Optional	Strong
EAP-FAST	Optional (PAC)	Optional	Moderate
LEAP	No	No	Broken — do not use
EAP-MD5	No	No	Broken — do not use

EAP-TLS

The strongest, and the one to name when a question asks for the most secure option. It uses mutual certificate authentication: the server proves its identity to the client and the client proves its identity to the server, both with X.509 certificates.

There is no password anywhere in the exchange, which eliminates password-based attacks entirely — nothing to phish, spray or crack.

The cost is a PKI. Every client device needs a certificate, which must be issued, deployed, renewed and revoked. That operational burden is the only real argument against it, and it is why organisations with mature device management use EAP-TLS while others do not.

PEAP

Protected EAP. The most widely deployed method, because it authenticates users with a username and password while keeping the credentials protected.

It works in two phases. First, a TLS tunnel is established using the server's certificate only. Second, the actual authentication happens inside that tunnel, normally using MSCHAPv2.

Only the server needs a certificate, so there is no client PKI — which is why it is popular. Credentials commonly come from the existing directory, so users authenticate with their normal account.

The critical caveat, and a likely exam point: PEAP is only secure if the client validates the server certificate. A client configured to accept any certificate can be lured onto a rogue access point presenting its own certificate, and the MSCHAPv2 exchange inside that tunnel yields credentials to the attacker. Server validation and trusted-root configuration must be enforced by policy, not left to the user — because a user faced with a certificate warning will click through it.

EAP-TTLS

Tunneled TLS. Similar to PEAP: a TLS tunnel using the server certificate, then inner authentication. It is more flexible about the inner method, supporting legacy mechanisms such as PAP and CHAP, which makes it useful when the back-end identity store requires them.

Client certificates are optional. The same server-validation requirement applies.

EAP-FAST

Flexible Authentication via Secure Tunneling, developed by Cisco to replace LEAP. It establishes the tunnel using a Protected Access Credential — a shared secret provisioned to the client — instead of requiring a server certificate.

The security hinge is how that PAC arrives. Anonymous in-band provisioning is convenient and vulnerable to a man-in-the-middle during provisioning. Authenticated provisioning or out-of-band distribution is secure but reintroduces the management overhead EAP-FAST was meant to avoid.

It is worth recognising for the exam; it is not the answer to "most secure".

The broken ones

LEAP — Cisco's lightweight method, based on MSCHAPv1 with no tunnel. Credentials are exposed to offline dictionary attack. Deprecated for many years.

EAP-MD5 — a simple challenge-response with no tunnel, no server authentication and no key derivation. Unsuitable for wireless and vulnerable to offline attack.

Both appear as distractors. Neither is ever the right answer.

Choosing

Use EAP-TLS where you can issue and manage certificates — strongest, and no passwords in play. Use PEAP with enforced server validation where you cannot. Use EAP-TTLS where a legacy inner authentication method is required. Never use LEAP or EAP-MD5.

What to remember

EAP is a framework; the methods differ in what they authenticate and how. EAP-TLS requires certificates on both sides and is the strongest. PEAP and EAP-TTLS need only a server certificate and tunnel a password-based inner method — and both depend entirely on the client validating that certificate. EAP-FAST uses a PAC, whose provisioning method determines its security. LEAP and EAP-MD5 are broken.

Related reading: Wireless security: WEP, WPA2 and WPA3 · AAA and RADIUS · SSID, BSSID and ESSID