3GPP TS 33.401 V18 — EPS Security Architecture

Q: What does the Scope section cover in 3GPP TS 33.401 V18 — EPS Security Architecture?

The Scope section provides detailed technical specifications, security considerations, and implementation guidance as documented in 3GPP TS 33.401 V18 — EPS Security Architecture. This content is part of the BLACKLABS 3GPP Security Library.

Q: What does the EPS Security Architecture section cover in 3GPP TS 33.401 V18 — EPS Security Architecture?

The EPS Security Architecture section provides detailed technical specifications, security considerations, and implementation guidance as documented in 3GPP TS 33.401 V18 — EPS Security Architecture. This content is part of the BLACKLABS 3GPP Security Library.

Q: What does the EPS Authentication (EPS AKA) section cover in 3GPP TS 33.401 V18 — EPS Security Architecture?

The EPS Authentication (EPS AKA) section provides detailed technical specifications, security considerations, and implementation guidance as documented in 3GPP TS 33.401 V18 — EPS Security Architecture. This content is part of the BLACKLABS 3GPP Security Library.

Q: What does the NAS Security section cover in 3GPP TS 33.401 V18 — EPS Security Architecture?

The NAS Security section provides detailed technical specifications, security considerations, and implementation guidance as documented in 3GPP TS 33.401 V18 — EPS Security Architecture. This content is part of the BLACKLABS 3GPP Security Library.

Q: What does the AS Security (Air Interface) section cover in 3GPP TS 33.401 V18 — EPS Security Architecture?

The AS Security (Air Interface) section provides detailed technical specifications, security considerations, and implementation guidance as documented in 3GPP TS 33.401 V18 — EPS Security Architecture. This content is part of the BLACKLABS 3GPP Security Library.

BLACKLABS

3GPP TS 33.401

3GPP System Architecture Evolution (SAE) — Security Architecture

V18.3.0 (2024-06) — Release 18 Security Architecture

1. Scope

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Defines the security architecture for the Evolved Packet System (EPS), covering authentication, key management, NAS/AS security, network domain security, and GTP tunnel protection for LTE/4G networks.

EPS ↔ 5GS: TS 33.401 is the 4G/LTE equivalent of TS 33.501 (5G Security). Many 5G security mechanisms evolved from EPS security. Understanding TS 33.401 is essential for inter-RAT (4G↔5G) security analysis.

2. EPS Security Architecture

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┌──────────┐ ┌──────────┐ │ HSS │◄──────── S6a ──────────────►│ MME │ │ (Auth │ (Diameter) │ (NAS │ │ Center) │ │ Security│ └──────────┘ │ Anchor) │ └──┬───┬───┘ S1-│ │S11 MME │ │(GTPv2-C) │ │ ┌──▼───▼───┐ ┌──────────┐ S1-U │ S-GW │ │ eNB │◄──────────►│ │ │ (AS Sec) │ (GTP-U) └────┬─────┘ └────┬─────┘ S5/S8│(GTPv2-C + GTP-U) Uu │ │ │ ┌──────▼────┐ ┌────▼────┐ │ P-GW │ │ UE │ │ (Policy, │ │ (USIM) │ │ Charging)│ └─────────┘ └───────────┘

2.1 Security Domains (TS 33.401 §5)

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Domain	Scope	Mechanisms
I — Network Access Security	UE ↔ Network	EPS AKA, NAS/AS security
II — Network Domain Security	Network ↔ Network	IPsec/TLS for backhaul, GTP security
III — User Domain Security	User ↔ USIM	PIN protection, USIM-ME interface
IV — Application Domain	App-level	GBA, TLS for apps
V — Visibility/Configurability	User awareness	Security indicators

3. EPS Authentication (EPS AKA)

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3.1 EPS Key Hierarchy

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K (permanent — in USIM & HSS) │ ┌───▼───┐ │ CK,IK │ (from MILENAGE/TUAK) └───┬───┘ │ KDF ┌───▼───┐ │ K_ASME │ (anchor key at MME) └───┬───┘ ┌────┼────┐ │ │ ┌────▼───┐ ┌───▼────┐ │K_NASenc│ │K_NASint│ (NAS keys at MME) └────────┘ └────────┘ │ ┌────▼────┐ │ KeNB │ (eNB key — derived at handover) └────┬────┘ ┌────┼────┐ │ │ ┌────▼───┐ ┌───▼────┐ │K_RRCenc│ │K_RRCint│ (RRC keys) └────────┘ └────────┘ ┌────▼───┐ │K_UPenc │ (User plane encryption key) └────────┘

Key difference from 5G: EPS does not support user plane integrity protection (K_UPint). This was added in 5G (TS 33.501). EPS only provides UP encryption.

3.2 EPS AKA Procedure

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UE (USIM) MME HSS │ │ │ │── Attach Req ──►│ │ │ (IMSI/GUTI) │ │ │ │── Auth Info ──►│ │ │ Req (IMSI) │ │ │ │ Generate AV: │ │ │ RAND, AUTN, XRES, K_ASME │ │◄── Auth Info ──│ │ │ Resp (AVs) │ │ │ │ │◄── Auth Req ────│ │ │ (RAND, AUTN) │ │ │ │ │ │ Verify AUTN │ │ │ Compute RES │ │ │ Derive K_ASME │ │ │ │ │ │── Auth Resp ───►│ │ │ (RES) │ │ │ │ Verify RES==XRES │ │ Derive NAS keys │ │ │

4. NAS Security

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NAS security protects signaling between UE and MME:

Feature	Algorithm	Key
NAS Encryption	EEA0/EEA1(SNOW)/EEA2(AES)/EEA3(ZUC)	K_NASenc
NAS Integrity	EIA1(SNOW)/EIA2(AES)/EIA3(ZUC)	K_NASint

EIA0 (null integrity) is forbidden for NAS. NAS integrity protection is mandatory — unlike encryption which can be EEA0 (null) in some cases.

NAS Security Mode Command activates ciphering + integrity
NAS COUNT (32-bit) prevents replay — separate UL/DL counters
GUTI reallocation provides temporary identity (privacy)

5. AS Security (Air Interface)

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Access Stratum security protects RRC signaling and user plane between UE and eNB:

Feature	Mandatory	Notes
RRC Encryption	Optional (can be EEA0)	Uses K_RRCenc
RRC Integrity	Mandatory	Uses K_RRCint
UP Encryption	Optional (can be EEA0)	Uses K_UPenc
UP Integrity	NOT SUPPORTED in EPS	Added in 5G NR only

No UP integrity in LTE means user plane data between UE and eNB can be modified if the encryption is weak or NULL (EEA0). This is a known limitation addressed in 5G.

6. Inter-RAT Security (4G↔5G)

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Security during handover between EPS and 5GS:

Scenario	Key Derivation	Security Risk
5G → 4G	K_ASME derived from K_AMF via mapped security context	Capability downgrade (no UP integrity in LTE)
4G → 5G	K_AMF derived from K_ASME (or full re-authentication)	May inherit weaker EPS security context
EN-DC	S_KgNB derived from KeNB for SCG bearer	LTE anchor controls gNB key derivation

Bidding-down attack: An attacker may force UE from 5G to 4G to exploit the lack of UP integrity protection. TS 33.501 §6.6.1 includes countermeasures.

7. Network Domain Security

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TS 33.401 §11 specifies protection for backhaul and inter-node interfaces:

S1 (eNB↔MME/SGW): IPsec ESP recommended; mandatory when eNB is in untrusted location
X2 (eNB↔eNB): IPsec for inter-eNB handover signaling
S6a (MME↔HSS): Diameter/TLS or IPsec
S5/S8 (SGW↔PGW): GTP-C/U — IPsec for roaming (S8)

NDS/IP (TS 33.210) defines the SEcurity Gateway (SEG) architecture for inter-PLMN protection.

8. GTP Security in EPS

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TR 33.926 §5.3.6.5 GTP-U/C attacks on S1/S5/S8 ▶

Interface	Protocol	Security
S1-U	GTP-U	IPsec ESP (conditional — based on eNB trust)
S5/S8	GTPv2-C + GTP-U	IPsec for S8 (roaming); intra-PLMN = operator policy
S11	GTPv2-C	Within trusted domain — IPsec optional
S3/S4/S10/S16	GTPv2-C	IPsec for inter-PLMN (roaming)

Core EPS GTP vulnerability: GTP has no built-in authentication or encryption. Without IPsec, any node with IP connectivity to the S1-U/S5/S8 interface can inject, modify, or eavesdrop on GTP tunnels. See TS 29.274 and TS 29.281 for protocol details.