Security (Advanced) — A Bird's-Eye View of 5GC Defense in Depth¶

Learning objectives¶

After reading this chapter, you will be able to do the following.

• Explain, as a map, that 5GC security is composed of multiple layers — the access layer, NAS layer, SBA layer, and roaming boundary.
• Explain why SUCI/SUPI concealment is important (countermeasure against IMSI catchers, privacy protection).
• Take a bird's-eye view of the whole key hierarchy (K → CK/IK → KAUSF → KSEAF → KAMF → NAS keys/AS keys).
• Explain the purpose of security across the SBA (inter-NF TLS, OAuth 2.0 token authorization by the NRF, mutual TLS with a Service Mesh).
• Explain that NIA0/NEA0 (Null integrity/Null ciphering) are limited to special cases such as emergency calls.

Prerequisites¶

• Authentication (5G-AKA / EAP-AKA') — this chapter treats this primary authentication as "already done."
• NAS/AS Security · SUCI generation — the details of key derivation and SUCI generation are delegated here.
• Supplementary: AUSF (authentication NF) / NRF (NF registration · token issuance) / SEPP (roaming-boundary NF) / term lookup: Security Dictionary

Why — why defend in multiple layers¶

5GC security cannot be protected by a single mechanism. That is because what must be protected, and the threats, differ from place to place.

• In the radio segment, the threats are eavesdropping, tampering, and impersonation (a fake base station = an IMSI catcher). → Defend with ciphering, integrity protection, and concealment of the permanent identifier.
• In the control signaling (NAS) between the UE and the core (AMF), the location-registration and session-control messages need to be protected. → Defend with NAS Security.
• In the inter-NF communication inside the core (SBA), the threats are impersonation and eavesdropping by illegitimate NFs. → Defend with TLS and OAuth authorization.
• At the operator-network boundary (roaming), the signaling that passes through another operator's network needs to be protected. → Defend with SEPP/N32.

Analogy: 5GC security is "the multilayered defense of a castle." The outer moat (radio segment), the pass to enter the castle gate (NAS), the entry permit to each room inside the castle (SBA authorization), and the border checkpoint with the neighboring country (roaming boundary) — each layer defends in a different way. The aim is that even if one of them is breached, it is stopped at the next layer.

What — the four layers and the special cases¶

5GC security can broadly be grasped in four layers (TS 33.501).

graph TD
    subgraph Access["Access layer — radio segment"]
        A1["AS Security
UE ⇔ gNB
RRC/User Plane ciphering · integrity"]
    end
    subgraph NAS["NAS layer — UE⇔core control signaling"]
        N1["NAS Security
UE ⇔ AMF
NAS ciphering · integrity + SUCI concealment"]
    end
    subgraph SBA["SBA layer — inter-NF inside the core"]
        S1["Inter-NF TLS + OAuth 2.0
NRF issues tokens
mutual TLS (Service Mesh)"]
    end
    subgraph Roam["Roaming boundary — between operator networks"]
        R1["SEPP / N32
PRINS or TLS"]
    end
    Access --> NAS --> SBA
    NAS -. "when roaming" .-> Roam
    SBA -. "when roaming" .-> Roam

Contrasting the role of each layer.

How — surveying each topic¶

SUCI/SUPI concealment — privacy protection¶

SUPI (Subscription Permanent Identifier) is the subscriber's permanent identifier (equivalent to IMSI). If it is sent in cleartext over the radio segment, a fake base station (IMSI catcher) can capture it, making it possible to track an individual.

In 5G, the SUPI is sent as SUCI (Subscription Concealed Identifier), encrypted with the home network's (hPLMN's) public key. Because only the home network (UDM/SIDF) can decrypt it, the SUPI is not visible to anyone in transit. The encryption scheme used is ECIES (Elliptic Curve Integrated Encryption Scheme) (TS 33.501).

Overview of the key hierarchy¶

When authentication (5G-AKA/EAP-AKA') succeeds, keys for specific purposes are derived hierarchically from a single root key. The lower you go, the narrower the scope of use, so that even if a higher key leaks, not all keys are immediately broken (TS 33.501).

Security across the SBA — preventing NFs from impersonating each other¶

Inside the core too, the SBI communication among NFs (AMF, SMF, UDM…) needs to be protected. In the SBA, three mechanisms overlap (TS 33.501).

• TLS — Ciphers and integrity-protects the inter-NF SBI (HTTP/2) communication. Prevents eavesdropping and tampering.
• OAuth 2.0 token authorization — When an NF calls a service, it presents an access token issued by the NRF. The called side verifies the token and accepts only legitimately authorized NFs. This prevents "impersonation by an illegitimate NF."
• Mutual TLS (mTLS) / Service Mesh — Depending on the implementation, a Service Mesh may set up mutual TLS to authenticate NFs in both directions.

Security at the roaming boundary¶

When roaming, the signaling between the serving network (vPLMN) and the home network (hPLMN) goes not directly but through the SEPP (Security Edge Protection Proxy), and is protected over the N32 interface. The protection schemes are PRINS (application-layer message protection, supporting IPX relay) and TLS (direct connection).

NIA0 / NEA0 — the special cases where "no protection" is allowed¶

• NEA0 (Null Encryption Algorithm) — An algorithm that performs no ciphering.
• NIA0 (Null Integrity Algorithm) — An algorithm that performs no integrity protection.

These are normally not used. They are permitted only in special cases where authentication is not possible but communication should still be allowed, such as emergency calls (e.g., an emergency call from an unauthenticated UE) (TS 33.501).

3GPP Specification¶

FAQ¶

Summary¶

• 5GC security is defense in depth across the access layer, NAS layer, SBA layer, and roaming boundary (TS 33.501).
• SUCI/SUPI concealment is the linchpin of privacy protection. ECIES encryption with the home network's public key neutralizes IMSI catchers.
• In the key hierarchy (K → CK/IK → KAUSF → KSEAF → KAMF → NAS/AS keys), the lower you go, the narrower the scope.
• The SBA layer prevents NF impersonation with TLS (ciphering) + OAuth 2.0 tokens (NRF-issued authorization) + mutual TLS.
• The roaming boundary is protected by SEPP/N32 (PRINS or TLS) (details in roaming.md).
• NIA0/NEA0 are "no protection," limited to special cases such as emergency calls. Normally not used.

Comprehension check¶

Next Step¶

• Return to the details of the prerequisites: confirm key derivation and SUCI generation in NAS/AS Security · SUCI.
• Dig deeper into the roaming-boundary defense: go to SEPP/N32 · PRINS/TLS in the same-level Roaming.
• For the handling of NIA0/NEA0 in emergency calls, go to Emergency calls.
• For term lookup, go to the Security Dictionary.