Message Fact Sheet

Protocol	nas	Network	5g
Spec	3GPP TS 24.501	Spec Section	8.2.1
Direction	AMF to UE	Message Type	5GMM signaling

Full message name	5G NAS - Authentication Request
Protocol	NAS
Technology	5G
Direction	AMF to UE
Interface	N1
Signaling bearer / channel	NAS signaling / Dedicated NAS message, commonly transported via DL Information Transfer
Typical trigger	Sent after Registration Request or another 5GMM procedure when the network needs to authenticate the UE.
Main purpose	Carries the network authentication challenge and related security context so the UE can prove subscriber legitimacy.
Main specification	3GPP TS 24.501, 8.2.1
Release added	Release 15
Procedures where used	5G Initial Registration, Mobility Registration Update, 5G Authentication Procedure
Related timers	T3560
Related cause values	Authentication parameter / challenge context

What is Authentication Request in simple terms?

Authentication Request is the NAS message the AMF uses to challenge the UE and start the 5G primary authentication procedure.

Carries the network authentication challenge and related security context so the UE can prove subscriber legitimacy.

Why this message matters

Authentication Request is the network challenging the UE to prove that it is a valid subscriber.

Where this message appears in the call flow

5G Authentication Procedure

Call flow position: Network challenge message that starts the UE side of 5G primary authentication.

Typical state: UE is attempting to register or refresh 5GMM context and has not yet completed authentication.

Preconditions:

The AMF has enough identity context to start authentication.

Next likely message: Authentication Response or Authentication Failure

Call flow position

Previous message(s): Registration Request, Identity Response

Next message(s): Authentication Response, Authentication Failure

Message direction and transport

Sender and receiver: AMF to UE

Interface: N1

Domain: Core-side mobility management signaling with radio-side NAS transport

Signaling bearer: NAS signaling

Logical channel: Dedicated NAS message, commonly transported via DL Information Transfer

Transport / encapsulation: NAS 5GS message carried end-to-end between AMF and UE

Security context: Often sent before full NAS security is active, so the exact protection state depends on the authentication stage and available context.

Message Structure Overview

Authentication Request is a NAS 5GMM challenge message rather than an ASN.1-encoded RRC or NGAP structure.
In practice, engineers first inspect RAND, AUTN, ngKSI, and whether the procedure is normal 5G AKA or EAP based.

Structure Syntax

This message is not typically analyzed as ASN.1 on the wire. Engineers usually inspect it as a NAS or protocol field decode rather than an ASN.1 tree.

Authentication Request follows the NAS 24.501 IE structure and is not an ASN.1 message.

Decoded Message Dump

Authentication Request
  Extended Protocol Discriminator: 5G Mobility Management
  Security Header Type: Plain NAS
  Message Type: Authentication Request
  ngKSI: 3
  ABBA: 0x0000
  RAND: 9f76b5c4a102f6d1557a4f2cb9d0e841
  AUTN: 8d34fe2210ca7f118e4c22aa119b55f0

How to read this dump

Read RAND and AUTN together because they define the authentication challenge.
Check whether the message is part of 5G AKA or EAP-based authentication before interpreting the following result.

Important Information Elements

IE	Required	Description
`ngKSI`	Yes	Identifies the NAS key set context associated with the authentication procedure.
`ABBA`	Yes	Anti-bidding down between architectures parameter used in 5G authentication handling.
`Authentication parameter RAND`	Yes	Random challenge value the UE uses to compute the authentication response.
`Authentication parameter AUTN`	Yes	Authentication token that lets the UE verify the network and derive response values.
`EAP message`	Optional	Present when the network is using EAP-based primary authentication rather than pure 5G AKA challenge handling.

Detailed field explanation

`ngKSI`

Identifies the NAS key set context associated with the authentication procedure.

Presence: Required

`ABBA`

Anti-bidding down between architectures parameter used in 5G authentication handling.

Presence: Required

`Authentication parameter RAND`

Random challenge value the UE uses to compute the authentication response.

Presence: Required

`Authentication parameter AUTN`

Authentication token that lets the UE verify the network and derive response values.

Presence: Required

`EAP message`

Present when the network is using EAP-based primary authentication rather than pure 5G AKA challenge handling.

Presence: Optional

What to check in logs and traces

Confirm the message appears after Registration Request or Identity Response.
Check ngKSI and whether it matches the expected security-context scenario.
Inspect RAND and AUTN formatting if authentication repeatedly fails.
Correlate the message with the UE's next Authentication Response or Authentication Failure.

Common Issues and Troubleshooting

Authentication never progresses after Authentication Request.

Likely cause: The UE may reject the challenge, fail lower-layer delivery, or be unable to verify AUTN.

What to inspect: Check RAND/AUTN values, UE logs, and whether any Authentication Failure follows.

Next step: Compare against a known-good authentication trace and verify subscriber credentials and time synchronization context.

Detailed explanation

5G NAS - Authentication Request Explained

Authentication Request is the NAS message the network sends to challenge the UE during the 5G authentication procedure. It is one of the most important messages in any registration trace because it marks the point where the AMF starts verifying that the UE is a legitimate subscriber.

For beginners, the simple meaning is: the network is asking the UE to prove who it is.
For engineers, this message is the start of the challenge-response branch that decides whether the procedure continues toward security mode and registration success.

What is Authentication Request in simple terms?

The UE asked to register or update context. Before allowing that procedure to continue, the AMF sends a challenge. The UE must process that challenge and respond correctly.

Why Authentication Request matters

This message matters because it sits at the boundary between:

identity handling
subscriber verification
later NAS security activation

If this step fails, the UE never reaches a clean authenticated registration path.

Where Authentication Request appears in the call flow

UE                              gNB / AMF
|--- Registration Request ------>|
|<-- Identity Request (opt) -----|
|--- Identity Response (opt) ---->|
|<-- Authentication Request -----|
|--- Authentication Response ---->|

It usually appears during initial registration, but it can also appear in update procedures depending on context.

Transport characteristics

Direction: AMF to UE
Interface: N1
Transport on access side: commonly via DL Information Transfer
Security expectation: often still plain NAS in early registration, though exact protection depends on the procedure branch

What Authentication Request means operationally

Operationally, this message tells engineers that the AMF has enough identity context to start subscriber authentication. From this point on, the trace should be interpreted as a challenge-response exchange rather than just access or identity handling.

The most useful first checks are:

whether the challenge values are present and well formed
whether the UE answers with Authentication Response or Authentication Failure
whether the procedure is 5G AKA or EAP based

Important Information Elements

IE	Why it matters
`ngKSI`	Tells you which NAS security key set context is associated with the procedure.
`ABBA`	Helps protect against bidding-down between architectures and should be decoded correctly.
`RAND`	Main random challenge value used by the UE to compute its response.
`AUTN`	Lets the UE verify the network and derive authentication context.
`EAP message`	Indicates EAP-based authentication when present.

Example message dump

Authentication Request
  Extended Protocol Discriminator: 5G Mobility Management
  Security Header Type: Plain NAS
  Message Type: Authentication Request
  ngKSI: 3
  ABBA: 0x0000
  RAND: 9f76b5c4a102f6d1557a4f2cb9d0e841
  AUTN: 8d34fe2210ca7f118e4c22aa119b55f0

How to read this dump

Start with RAND and AUTN because they define the challenge.
Check ngKSI to understand expected security context.
Then decide whether the next useful branch is Authentication Response, Authentication Failure, or an EAP-specific result path.

What to check in logs

verify that the message appears after the correct registration or identity stage
inspect RAND, AUTN, and ABBA if authentication repeatedly fails
check whether the UE answers or silently stalls
correlate the exchange with the later Authentication Result or Security Mode Command

FAQ

What does Authentication Request do in 5G?

It challenges the UE so the network can verify subscriber identity before allowing registration to continue.

Summary

Authentication Request is the NAS message the AMF uses to challenge the UE and start the 5G primary authentication procedure.

Advanced Engineer Notes

A malformed or inconsistent challenge can produce failures that look like generic registration problems unless the authentication exchange is decoded carefully.

Related message pages

Related Procedures

Related Tools

Use This Reference in Practice

Decode this message with the 3GPP Decoder, inspect the related message database, or open the matching call flow to see where this signaling step fits in the full procedure.