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.
ASN.1 Message Syntax for 5G NAS - Authentication Request
This message is not typically analyzed as ASN.1 on the wire. It is usually read as a NAS or protocol field structure instead.
Authentication Request follows the NAS 24.501 IE structure and is not an ASN.1 message.
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
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
ABBA
Anti-bidding down between architectures parameter used in 5G authentication handling.
Presence: Required
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
Authentication parameter RAND
Random challenge value the UE uses to compute the authentication response.
Presence: Required
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
Authentication parameter AUTN
Authentication token that lets the UE verify the network and derive response values.
Presence: Required
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
EAP message
Present when the network is using EAP-based primary authentication rather than pure 5G AKA challenge handling.
Presence: Optional
In practice: If present, it reflects the external DN authentication result. It is worth checking when the session succeeds on NAS but service access still depends on an external authentication flow.
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.
FAQ
What does Authentication Request do in 5G?
It challenges the UE so the network can verify subscriber identity before allowing registration to continue.
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.