5G Authentication Procedure Explained

Introduction

In 5G networks, the Authentication Procedure verifies the identity of the User Equipment (UE) before allowing access to the network.

Authentication ensures that only legitimate subscribers can connect to the network and access services.

The 5G system uses the 5G Authentication and Key Agreement (5G-AKA) protocol to perform authentication between the UE and the network.

This procedure is defined by the 3rd Generation Partnership Project in:

• 3GPP TS 23.501 - System Architecture
• 3GPP TS 23.502 - 5G System Procedures
• 3GPP TS 33.501 - Security Architecture

Why Authentication Is Needed

Authentication protects the network from unauthorized access and ensures subscriber security.

It allows the network to:

• verify subscriber identity
• generate security keys
• establish secure communication

Without authentication, the network cannot trust the device requesting access.

Network Functions Involved

UE (User Equipment)

The device requesting network access.

gNB (Next Generation NodeB)

Forwards signaling messages between the UE and the core network.

AMF (Access and Mobility Management Function)

Manages the authentication procedure in the core network.

AUSF (Authentication Server Function)

Performs authentication verification.

UDM (Unified Data Management)

Stores subscriber credentials and generates authentication vectors.

Interfaces Used

5G Authentication Call Flow

Below is the simplified signaling sequence.

UE            gNB            AMF            AUSF/UDM
 |             |              |               |
 |---Registration Request--->|               |
 |             |-------------N2------------->|
 |             |              |----Auth Req->|
 |             |              |<---Auth Vec--|
 |<--Authentication Request--|               |
 |---Authentication Response->|               |
 |             |              |               |
 |<--Security Mode Command---|               |
 |---Security Mode Complete->|               |

After successful authentication, security keys are established between the UE and the network.

Step-by-Step Explanation

Step 1: Registration Request

The UE sends a Registration Request to the network.

This message includes:

• subscriber identity (SUCI)
• registration type
• security capabilities

Important parameters to check

Engineers should verify:

• SUCI format
• registration type
• UE security capabilities

Step 2: Authentication Request

The AMF requests authentication vectors from the AUSF/UDM.

These vectors include:

• RAND (random challenge)
• AUTN (authentication token)
• XRES* (expected response)

The AMF sends the Authentication Request to the UE.

Important parameters to check

Check:

• RAND value
• AUTN validity
• authentication vector generation

Step 3: Authentication Response

The UE calculates a response using its stored subscriber key.

The UE sends Authentication Response containing:

• RES* (response value)

The network compares RES* with XRES*.

Important parameters to check

Verify:

• RES* vs XRES* match
• authentication integrity
• UE computation result

Step 4: Security Mode Command

After successful authentication, the AMF initiates the Security Mode procedure.

This message configures:

• encryption algorithms
• integrity protection algorithms

Important parameters to check

Check:

• ciphering algorithms
• integrity algorithms
• security context parameters

Step 5: Security Mode Complete

The UE confirms the security configuration by sending Security Mode Complete.

At this point:

• authentication is successful
• secure communication is established

Authentication Parameters

Authentication Methods in 5G

Troubleshooting Authentication Issues

Authentication Failure

Possible causes:

• incorrect subscriber credentials
• mismatched authentication vectors
• synchronization errors

Security Mode Failure

Possible reasons:

• ciphering algorithm mismatch
• integrity algorithm mismatch
• UE capability limitations

Registration Failure After Authentication

Possible causes:

• incorrect subscriber profile
• network configuration errors
• session setup issues

Key Messages in Authentication Procedure

Relevant 3GPP Specifications

The 5G Authentication Procedure is defined by the 3rd Generation Partnership Project in:

• 3GPP TS 23.501 - System Architecture
• 3GPP TS 23.502 - 5G System Procedures
• 3GPP TS 33.501 - Security Architecture

Summary

The 5G Authentication Procedure verifies the identity of the UE before allowing network access.

The process includes:

1. UE sending Registration Request
2. network generating authentication vectors
3. UE responding to authentication challenge
4. network verifying the response
5. establishing secure communication

Authentication is essential for subscriber security and network protection in modern 5G systems.