Target Audience

5G Core Architects

Target Audience

Security Engineers

Target Audience

Protocol Engineers

Target Audience

Cybersecurity Teams

1

Trust Model & Core Architecture Shift

The fundamental security paradigm shift from 4G to 5G represents a move from perimeter-based implicit trust to zero-trust architecture with explicit authorization.

4G LTE / EPC
5G SA / SBA
4G LTE

EPC Architecture

Core Protocols
Diameter (S6a, S11) GTP-C/U SCTP/IP
Security Model
Perimeter-based Hop-by-hop trust IPSec tunnels
Key Risk: Once inside IPX, nodes are implicitly trusted. A compromised node gains full lateral movement capability.
Click for 5G fix
5G Solution: OAuth 2.0 + mTLS on every NF-to-NF API call. Even compromised NFs cannot access unauthorized services.

EPC Trust Assumption

EPC assumes network honesty, not network compromise. Physical security boundaries are the primary defense.

5G SA

SBA Architecture

Core Protocols
HTTP/2 REST APIs JSON/OpenAPI
Security Model
Zero Trust TLS 1.2/1.3 OAuth 2.0
Security Impact: Even if an NF is compromised, unauthorized lateral API access is blocked by OAuth 2.0 authorization.

Zero Trust by Design

Each NF (AMF, SMF, UDM, PCF) must authenticate, present access tokens, and be explicitly authorized per service.

5G SBA Security Architecture with OAuth 2.0
NF Registration
NF registers with NRF, providing NF profile and supported services
Token Request
NF Consumer requests OAuth 2.0 access token from NRF for target NF
Token Issuance
NRF validates and issues JWT with scope, expiry, and audience claims
Service Access
NF Consumer presents token; NF Producer validates before processing
OAuth 2.0 Access Token (JWT)
12345678910
{
  "iss": "nrf.5gc.mnc001.mcc001.3gppnetwork.org",
  "sub": "amf-001",
  "aud": "udm.5gc.mnc001.mcc001.3gppnetwork.org",
  "scope": "nudm-uecm nudm-sdm",
  "exp": 1704931200,
  "claims": {
    "nfType": "AMF",
    "nfInstanceId": "3fa85f64-5717-4562-b3fc-2c963f66afa6",
    "allowedPlmns": [{ "mcc": "001", "mnc": "01" }]
  }
}
2

Identity Privacy: IMSI vs SUCI (ECIES)

5G introduces cryptographic identity protection using ECIES to eliminate IMSI exposure vulnerabilities that enable subscriber tracking and surveillance attacks.

IMSI Catcher Attack (4G) vs SUCI Protection (5G)
4G IMSI Exposure
5G SUCI Protection
4G LTE

IMSI Exposure

Permanent Identifier
IMSI (15 digits) MCC + MNC + MSIN

Cleartext Transmission

IMSI sent in cleartext during Attach Request if GUTI is unavailable or rejected by network.

Attack Vectors: IMSI Catchers, Location Tracking, Silent SMS, Subscriber Profiling, Targeted Surveillance
Click for 5G fix
5G Solution: SUCI uses ECIES encryption. Only the home network SIDF in UDM can de-conceal the identity. IMSI catchers are rendered ineffective.
IMSI Structure
1234
// IMSI: 310150123456789
MCC: 310      // Mobile Country Code (USA)
MNC: 150      // Mobile Network Code
MSIN: 123456789 // Subscriber ID (CLEARTEXT)
5G SA

SUCI Protection

Identity Scheme
SUPI (Permanent) SUCI (Concealed) 5G-GUTI (Temporary)

ECIES Encryption

MSIN encrypted using Home Network public key. Only SIDF (in UDM) can decrypt to reveal SUPI.

Critical Improvement: Serving Network (AMF) never sees the real permanent identity. IMSI catchers are rendered ineffective.
SUCI Structure (TS 23.003)
123456
// SUCI: suci-0-001-01-0-0-0-<encrypted_msin>
suci-0-        // SUPI Type (0 = IMSI)
001-01-       // MCC-MNC (Cleartext)
0-0-          // Routing Indicator
0-            // Protection Scheme (Profile A)
<ECIES_Output> // Ephemeral PK + Ciphertext + MAC
SUCI Generation Flow (ECIES - Profile A/B)
1

USIM Storage

USIM stores Home Network Public Key (HNPK) provisioned by operator. Profile A uses Curve25519, Profile B uses secp256r1.

2

Ephemeral Key Generation

UE generates fresh ephemeral key pair (ePK, eSK) for each SUCI computation. Ensures unlinkability between sessions.

3

ECDH Key Agreement

Shared secret derived: SS = ECDH(eSK, HNPK). Used to derive encryption key (EK) and MAC key (MK) via KDF.

4

MSIN Encryption

MSIN encrypted using AES-128-CTR with EK. MAC computed over ciphertext using HMAC-SHA-256 with MK.

5

SUCI Output

SUCI = MCC || MNC || Routing Ind || Protection Scheme || ePK || Ciphertext || MAC. Only SIDF can decrypt.

3GPP TS 33.501 Section 6.12

SUCI concealment is mandatory for 5G SA. The SIDF (Subscription Identifier De-concealing Function) resides in UDM and is the only entity capable of de-concealing SUPI from SUCI.

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3

Authentication & Key Hierarchy

5G introduces a split anchor key model that prevents serving network compromise from exposing home network key material, fundamentally changing the trust boundary.

5G-AKA Authentication Flow (Animated)
Key Hierarchy: 4G EPS-AKA (Red) vs 5G-AKA (Green)
4G EPS-AKA
5G-AKA Split Key
4G LTE

EPS-AKA Key Hierarchy

K (USIM/AuC)
CK, IK
K_ASME (stored in MME)
K_NASenc
K_NASint
K_eNB
Vulnerability: K_ASME stored in MME. Compromised MME = attacker can derive all access stratum and NAS keys.
Click for 5G fix
5G Fix: Home network (AUSF/UDM) never exposes K_AUSF. Serving network only receives K_SEAF -- context-limited, non-reversible to K.
5G SA

5G-AKA Split Key Model

K (USIM/UDM)
K_AUSF (Home Network Only)
K_SEAF (Sent to AMF)
K_AMF
K_NASenc
K_NASint
K_gNB
Key Improvement: Home network (AUSF/UDM) never exposes K_AUSF. Serving network only receives K_SEAF -- context-limited, non-reversible to K.
5G Authentication Methods (TS 33.501)

5G-AKA

Primary method for SIM-based devices. Enhanced EPS-AKA with home network confirmation and concealed identity support.

Key Features
HRES* verification SUCI support Anchor key split

EAP-AKA'

EAP framework method enabling WLAN interworking and non-3GPP access. Binds to access network name for serving network authentication.

Key Features
AT_KDF binding Non-3GPP access WLAN integration

EAP-TLS

Certificate-based authentication for non-SIM IoT devices and enterprise deployments. Mutual authentication via X.509 certificates.

Key Features
No SIM required X.509 certs Enterprise IoT

5G-AKA Extensions

Extensibility for future authentication methods including AKMA (Authentication and Key Management for Applications) per TS 33.535.

Key Features
AKMA GBA successor App-level keys
5G Key Derivation Functions (TS 33.501 Annex A)
1234567891011121314
// K_AUSF derivation (in UDM/ARPF)
K_AUSF = KDF(CK || IK, "6A" || SN_name || SQN ^ AK)

// K_SEAF derivation (in AUSF)
K_SEAF = KDF(K_AUSF, "6C" || SN_name)

// K_AMF derivation (in SEAF/AMF)
K_AMF = KDF(K_SEAF, "6D" || SUPI || ABBA)

// NAS keys derivation
K_NASint = KDF(K_AMF, "69" || alg_type || alg_id)
K_NASenc = KDF(K_AMF, "69" || alg_type || alg_id)

// K_gNB derivation
K_gNB = KDF(K_AMF, "6E" || NAS_UL_COUNT || Access_Type)
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4

Radio Interface Security (PDCP Layer)

5G NR introduces mandatory user plane integrity protection and 256-bit algorithm support to address bit-flip attacks and provide quantum-resistant cryptographic options.

Encryption & Integrity Algorithms (TS 33.501 Section 5.6)

4G LTE Algorithms

NEA1 / NIA1
SNOW 3G (128-bit)
NEA2 / NIA2
AES-128
NEA3 / NIA3
ZUC-128
NEA0 / NIA0
NULL (No protection)

5G NR Algorithms

NEA1 / NIA1
SNOW 3G (128-bit)
NEA2 / NIA2
AES-128
NEA3 / NIA3
ZUC-128
NEA4 / NIA4
AES-256
NEW
NEA5 / NIA5
SNOW-256
NEW
NEA6 / NIA6
ZUC-256
NEW

Quantum Resistance Considerations

256-bit symmetric algorithms provide defense against Grover's algorithm, which effectively halves the security of symmetric keys. AES-256 provides ~128-bit post-quantum security.

User Plane Integrity Protection (UPIP)
4G LTE - No UPIP
5G NR - Mandatory UPIP

4G LTE - No UPIP

Protection Scope
RRC Signaling OK User Plane Data NONE
aLTEr Attack (CVE-2019-15619): DNS redirection via bit-flip on encrypted user plane without detection.
Click for 5G fix
5G Fix: Mandatory UPIP. UE and gNB must support user plane integrity. Critical for URLLC (autonomous vehicles, industrial control).

5G NR - Mandatory UPIP

Protection Scope
RRC Signaling OK NAS Signaling OK User Plane Data OK
TS 33.501 Mandate: UE and gNB must support UPIP. Critical for URLLC (autonomous vehicles, industrial control) where data integrity is paramount.
PDCP Integrity Protection (TS 33.501 Section 5.6.3)
12345678910
// User Plane Integrity Protection MAC calculation
MAC-I = NIA(K_UP-int, COUNT, BEARER, DIRECTION, MESSAGE)

// Where:
K_UP-int   = User Plane Integrity Key (derived from K_gNB)
COUNT      = 32-bit PDCP COUNT (HFN || PDCP SN)
BEARER     = 5-bit Radio Bearer ID
DIRECTION  = 1-bit (0=UL, 1=DL)
MESSAGE    = PDCP PDU (Header + Data)

// MAC-I appended to PDCP PDU for receiver verification
5

Roaming & Interconnect Security

5G replaces vulnerable SS7/Diameter interconnects with SEPP-based N32 interface, providing end-to-end protection against IPX-based attacks.

SEPP-Based N32 Roaming Architecture (Animated)
4G SS7/Diameter
5G SEPP + N32
4G LTE

SS7/Diameter Roaming

Interconnect Protocols
SS7 (MAP) Diameter (S6a/d) GTP (S8)
Security Model
IPX filtering DRA/DEA proxies Implicit trust
Known Attacks: Location leakage, Silent interception, DRA spoofing, Subscriber DoS, Fraud (SS7 MAP attacks)
Click for 5G fix
5G Fix: SEPP + PRINS (N32) provides end-to-end protection. IPX nodes cannot read or modify signaling. JWS for integrity, JWE for confidentiality.
5G SA

SEPP + N32 Interface

Interconnect Protocol
N32-c (Control) N32-f (Forwarding) HTTP/2 + JSON
Security Stack
Mutual TLS PRINS (App Layer) JWS/JWE
Protection: Intermediate IPX nodes cannot read or alter signaling content. End-to-end integrity and optional confidentiality.
SEPP-Based N32 Roaming Architecture (TS 33.501 Section 13)

Home PLMN (HPLMN)

UDM AUSF PCF
hSEPP
N32 Interface
mTLS JWS JWE
IPX cannot inspect/modify

Visited PLMN (VPLMN)

AMF SMF vPCF
vSEPP
N32-f PRINS Message Protection (TS 29.573)
1234567891011121314
// N32-f message with PRINS protection
{
  "n32fContextId": "ctx-12345",
  "dataToIntegrityProtectBlock": {
    "metaData": { // Visible to IPX (routing)
      "n32fPeerId": "hsepp.mnc001.mcc001.3gppnetwork.org"
    },
    "requestLine": {
      "method": "POST",
      "uri": "/nudm-uecm/v1/imsi-001010123456789/registrations/amf-3gpp-access"
    },
    "payload": "<JWS-protected JSON>" // Integrity protected
  },
  "dataToIntegrityProtectAndCipherBlock": {
    "payload": "<JWE-encrypted JSON>" // Confidential + Integrity
  }
}

PRINS Modes (TS 33.501 Section 13.2)

TLS-only: Transport layer protection only. PRINS: Application layer protection using JWS (integrity) and JWE (confidentiality). PRINS enables sensitive IEs to remain protected even when IPX proxies terminate TLS.

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6

Anti-Bidding Down Protection

5G implements cryptographic detection of capability downgrade attacks that allow MITM attackers to force null encryption or legacy RAT fallback.

4G Vulnerable
5G Cryptographic Detection
4G LTE

Vulnerable to Downgrade

Attack Vector

UE capabilities sent unencrypted in Attach Request. MITM can modify to remove strong algorithms, forcing NEA0 (null encryption) or legacy RAT downgrade to 2G/3G.

Attack Consequences
Null encryption 2G/3G fallback Eavesdropping
5G SA

Cryptographic Detection

Detection Mechanism

UE resends integrity-protected hash of original capabilities after security establishment. Network compares with initial capabilities -- mismatch = attack detected.

Protection Features
ABBA parameter UE capabilities hash Session termination
5G Anti-Bidding Down Detection Flow (TS 33.501 Section 6.2)
UE

1. Registration Request (Unprotected)

UE sends 5G NR Security Capabilities, S1 UE Network Capability in cleartext. Potentially modifiable by MITM.

AMF

2. Security Mode Command

AMF selects algorithms, sends SMC with integrity protection. Includes Replayed UE Security Capabilities for verification.

UE

3. Security Mode Complete (Integrity Protected)

UE compares replayed capabilities with original. If mismatch, UE rejects. UE includes HASH(UE capabilities) in protected message.

AMF

4. Verification

AMF computes HASH of received capabilities, compares with UE-provided hash. Mismatch indicates MITM attack.

Hash Match = Secure Session Established

Capabilities verified, no downgrade detected. Proceed with selected algorithms.

Hash Mismatch = Attack Detected

Session terminated. Security event logged. UE may attempt re-registration.

ABBA Parameter (Anti-Bidding down Between Architectures)
1234567891011
// ABBA parameter in K_AMF derivation (TS 33.501 Annex A.7)
K_AMF = KDF(K_SEAF, "6D" || SUPI || ABBA)

// ABBA value indicates security features supported
ABBA = {
  // Bit 0: 5G-AKA vs EAP-AKA'
  // Bit 1: Serving network authentication supported
  // Bits 2-15: Future extensions
}

// If ABBA is modified by MITM, K_AMF will differ between UE and AMF
// -> Security Mode Complete verification will fail

Technical Comparison Summary

Security Domain 4G (LTE/EPC) 5G (SA/SBA) 3GPP Reference
Identity ProtectionIMSI (Cleartext)SUCI (ECIES)TS 33.501 Section 6.12
Trust ModelPerimeter-basedZero TrustTS 33.501 Section 13
Core ProtocolDiameter / GTPHTTP/2 + REST + OAuthTS 29.500
NF AuthorizationImplicit trustOAuth 2.0 (NRF)TS 33.501 Section 13.3
Key ExposureK_ASME in MMEK_SEAF onlyTS 33.501 Section 6.2
UP IntegrityNot supportedMandatory UPIPTS 33.501 Section 5.6.3
256-bit CryptoNot availableAES-256, SNOW-256, ZUC-256TS 33.501 Section 5.6.4
Roaming SecurityIPX trustSEPP + PRINSTS 33.501 Section 13.2
Anti-DowngradeVulnerableABBA + Hash verificationTS 33.501 Section 6.4
AuthenticationEPS-AKA only5G-AKA, EAP-AKA', EAP-TLSTS 33.501 Section 6.1

Key Takeaway for Architects

5G SA security is designed for cloud-native, multi-vendor, sliced, and roaming-heavy environments with zero-trust assumptions. The protocol changes are not incremental improvements -- they represent a fundamental architectural shift to address threats that EPC security was not designed to handle.

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Test Your Knowledge

1. What is the primary function of SIDF in 5G identity privacy?

SIDF (Subscription Identifier De-concealing Function) resides in UDM and is the only entity capable of de-concealing the permanent identity (SUPI) from the concealed identifier (SUCI) using the home network private key.

2. How does the ABBA parameter prevent downgrade attacks?

ABBA (Anti-Bidding down Between Architectures) is included in K_AMF derivation. If a MITM modifies ABBA, the derived K_AMF will differ between UE and AMF, causing Security Mode Complete verification to fail and terminating the session.

3. What vulnerability does 5G UPIP address that 4G LTE cannot?

5G's mandatory User Plane Integrity Protection (UPIP) addresses the aLTEr attack (CVE-2019-15619), where an attacker could perform bit-flip modifications on encrypted user plane data (e.g., DNS redirection) because 4G LTE only provides encryption without integrity checking on the user plane.