802.1X and EAP Walkthrough: EAP-TLS, PEAP and EAP-TTLS

Follow an enterprise Wi-Fi login message by message between the client, the AP and the RADIUS server: the TLS tunnel, the certificates, the inner login, and the keys that come out at the end. Then break it on purpose and see where each kind of failure stops the exchange.

Three parties, two hops

In WPA2/WPA3-Enterprise the client (supplicant) proves who it is to a RADIUS server (authentication server), and the AP (authenticator) only relays. Over Wi-Fi the EAP messages travel in EAPOL frames; on the wired side the AP puts them in RADIUS packets. The EAP method decides how the proof works: EAP-TLS uses certificates on both sides; PEAP and EAP-TTLS build a TLS tunnel with only a server certificate, then send a username and password inside it. When the server accepts, it gives the AP the MSK; the first 256 bits become the PMK (384 bits for the 192-bit AKMs such as 00-0F-AC:12, IEEE 802.11-2024 12.7.1.3; with FT the second 256 bits become the MPMK instead, IEEE 802.11-2024 12.7.1.6), and the 4-way handshake makes the session keys.

1 · The login

EAP method

What goes wrong

RADIUS round-trip time (AP ↔ server) Server certificate chain size EAP fragment size (bytes)

2 · The messages

Each arrow is one EAP message. Between the client and the AP it travels in an EAPOL frame; between the AP and the server in a RADIUS packet (Access-Request up, Access-Challenge, Access-Accept or Access-Reject down). Highlighted messages travel inside the TLS tunnel, encrypted.

Clientsupplicant

APauthenticator

RADIUS serverauthentication server

3 · The methods compared

Definitions

↗ opens a term's full entry in the Wi-Fi Glossary, which explains over 400 Wi-Fi terms with their IEEE 802.11 references.

Notes

Sources: EAP (IETF RFC 3748), EAP-TLS with TLS 1.2 (RFC 5216) and TLS 1.3 (RFC 9190), EAP-TTLSv0 (RFC 5281), PEAPv0 (Microsoft [MS-PEAP], 2025), MSCHAPv2 (RFC 2759), RADIUS support for EAP (RFC 3579) and the MS-MPPE key attributes (RFC 2548), IEEE 802.1X-2020 for EAPOL, and how the MSK becomes the PMK (IEEE 802.11-2024 12.7.1.3).
Timing counts one RADIUS round trip per EAP request and response pair, plus about 2 ms on Wi-Fi for each. It leaves out the server's processing (TLS, database or directory lookups), which can add tens of milliseconds, and retransmissions.
Fragmentation: a certificate chain bigger than the EAP fragment size is sent in several pieces, and the receiver acknowledges each piece with an empty message, so every extra fragment costs another round trip. Long chains and large RSA keys make logins noticeably slower, especially with a distant or cloud RADIUS server.
Simplified: TLS session resumption, PEAP fast reconnect, SoH (network health) messages and PEAP capability negotiation are left out. A PEAP server that enforces cryptobinding rejects clients that don't send the Cryptobinding TLV.