3GPP TS 33.402 V18 — Non-3GPP Access Security

Q: What does the Scope section cover in 3GPP TS 33.402 V18 — Non-3GPP Access Security?

The Scope section provides detailed technical specifications, security considerations, and implementation guidance as documented in 3GPP TS 33.402 V18 — Non-3GPP Access Security. This content is part of the BLACKLABS 3GPP Security Library.

Q: What does the Architecture section cover in 3GPP TS 33.402 V18 — Non-3GPP Access Security?

The Architecture section provides detailed technical specifications, security considerations, and implementation guidance as documented in 3GPP TS 33.402 V18 — Non-3GPP Access Security. This content is part of the BLACKLABS 3GPP Security Library.

Q: What does the Authentication Mechanisms section cover in 3GPP TS 33.402 V18 — Non-3GPP Access Security?

The Authentication Mechanisms section provides detailed technical specifications, security considerations, and implementation guidance as documented in 3GPP TS 33.402 V18 — Non-3GPP Access Security. This content is part of the BLACKLABS 3GPP Security Library.

Q: What does the Key Management section cover in 3GPP TS 33.402 V18 — Non-3GPP Access Security?

The Key Management section provides detailed technical specifications, security considerations, and implementation guidance as documented in 3GPP TS 33.402 V18 — Non-3GPP Access Security. This content is part of the BLACKLABS 3GPP Security Library.

Q: What does the Non-3GPP Access Threats section cover in 3GPP TS 33.402 V18 — Non-3GPP Access Security?

The Non-3GPP Access Threats section provides detailed technical specifications, security considerations, and implementation guidance as documented in 3GPP TS 33.402 V18 — Non-3GPP Access Security. This content is part of the BLACKLABS 3GPP Security Library.

BLACKLABS

3GPP TS 33.402

3GPP System Architecture Evolution (SAE) — Security Aspects of Non-3GPP Accesses

V18.1.0 (2024-03) — Release 18 Security Architecture

1. Scope

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Specifies security mechanisms for accessing the 3GPP Evolved Packet Core (EPC) and 5G Core via non-3GPP access networks — primarily WiFi but also other IP-based access technologies.

Key concept: Non-3GPP access allows subscribers to use WiFi, satellite, or fixed-line networks while maintaining their 3GPP security context. GTP-U tunnels are used between the ePDG/N3IWF and the core network.

2. Architecture

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2.1 Trusted Non-3GPP Access (TWAN)

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Trusted WiFi AP │ ┌────▼─────┐ │ TWAN │ (Trusted WLAN Access Network) └────┬─────┘ STa│ (Diameter — EAP) │ ┌────▼─────┐ ┌──────────┐ │ AAA / │── S6b ──│ HSS │ │ 3GPP AAA│ └──────────┘ └────┬─────┘ S2a│ (GTP / PMIP) │ ┌────▼─────┐ │ P-GW │ └──────────┘

Trusted access: the WiFi network is operated by or trusted by the mobile operator. Direct connection to PGW via S2a.

2.2 Untrusted Non-3GPP Access (ePDG)

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Any WiFi / Internet │ │ IPsec tunnel (IKEv2 + ESP) │ ┌────▼─────┐ │ ePDG │ (Evolved Packet Data Gateway) │ │ — terminates IPsec from UE └────┬─────┘ S2b│ (GTP-U / GTPv2-C) │ ┌────▼─────┐ │ P-GW │ └──────────┘ 5G equivalent: ┌────▼─────┐ │ N3IWF │ (Non-3GPP Interworking Function) │ │ — terminates IPsec from UE └────┬─────┘ N3 │ (GTP-U) N2 (NGAP to AMF) │ ┌────▼─────┐ │ UPF │ └──────────┘

ePDG/N3IWF is the trust boundary. It terminates the IPsec tunnel from the UE and creates GTP-U tunnels toward the core. The GTP-U side (S2b/N3) runs within the trusted operator domain.

3. Authentication Mechanisms

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Method	Used In	Details
EAP-AKA	Trusted (TWAN)	EAP tunnel carrying AKA over WiFi 802.1X
EAP-AKA'	Trusted + Untrusted	Enhanced EAP-AKA with AT_KDF for key separation
IKEv2 + EAP-AKA'	Untrusted (ePDG)	IPsec IKEv2 with EAP-AKA' for mutual authentication
5G-AKA via N3IWF	5G untrusted	IKEv2 with EAP-5G encapsulating NAS authentication

EAP-AKA' vs EAP-AKA: EAP-AKA' (RFC 5448) adds key binding to the access network name, preventing key reuse across different access types — critical for non-3GPP security.

4. Key Management

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K (USIM) │ ┌────▼────┐ │ CK', IK'│ (EAP-AKA' derived) └────┬────┘ │ KDF ┌────▼─────┐ │ K_ASME │ (bound to access network name) └────┬─────┘ │ ┌────▼─────┐ │ MSK │ (Master Session Key — for IPsec IKEv2) └────┬─────┘ │ IKEv2 SA ┌────▼─────┐ │ IPsec │ (ESP keys for UE↔ePDG tunnel) │ SA keys │ └──────────┘

Key binding: K_ASME includes access network identity to prevent cross-access key reuse
MSK lifetime: Tied to IKEv2 SA — re-authentication required on SA expiry
Fast re-authentication: EAP-AKA' supports fast re-auth to avoid full AKA on reconnect

5. Non-3GPP Access Threats

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TR 33.926 §5.3.6 Protocol implementation vulnerabilities ▶

Threat	Vector	Impact	Mitigation
Rogue WiFi AP	Evil twin AP intercepts UE connection	Credential theft attempt, MITM	EAP-AKA' mutual auth prevents credential leak; UE must verify server cert
IKEv2 Downgrade	MITM weakens IKEv2 SA to vulnerable cipher	IPsec tunnel compromise	Mandatory cipher suite minimums (AES-256, SHA-256)
ePDG Impersonation	DNS spoofing to redirect UE to fake ePDG	Full MITM on non-3GPP access	ePDG certificate validation, DNSSEC
S2b GTP Manipulation	Attack GTP-U/C between ePDG and PGW	Traffic intercept/redirect	NDS/IP (IPsec) on S2b
Cross-access key reuse	Use keys from 3GPP access on non-3GPP	Security context confusion	EAP-AKA' AT_KDF key binding

6. WLAN Interworking Details

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Key WiFi offload security considerations:

ANQP: UE discovers 3GPP-capable WiFi via Access Network Query Protocol (Hotspot 2.0/Passpoint)
SSID hijacking: Attacker clones ANQP profile — mitigated by EAP-AKA' and certificate validation
Seamless handover: 3GPP↔WiFi handover must maintain security context — K_ASME rebinding required
Multi-access PDU session: In 5G (ATSSS), UE may simultaneously use 3GPP and non-3GPP — both paths must be secured independently

GTP impact: Non-3GPP access generates GTP-U tunnels on S2a/S2b (EPC) or N3 via N3IWF (5GC). These tunnels carry WiFi-offloaded traffic and are subject to the same GTP security considerations as RAN-originated tunnels.