Help Required: Encountering AEADBadTagException ("Tag mismatch!") with FreeRADIUS Integration

[Alan DeKok]

On Jul 21, 2025, at 2:19 PM, Kalyani Borkar <kalyaniborkar2205 at gmail.com> wrote:
> I'm currently working on integrating FreeRADIUS with a Java-based
> authentication system, and I'm running into an issue related to TLS
> decryption during the EAP-TTLS handshake.
> 
> Specifically, I'm seeing the following exception on the server side:
> 
> javax.crypto.AEADBadTagException: Tag mismatch!

  To a large extent, that's an issue with the local code.  i.e. the Java code which isn't FreeRADIUS.

> From my understanding, this typically indicates a problem with key mismatch
> or incorrect handling of encrypted TLS data, possibly during the decryption
> of the ClientKeyExchange or in the derivation of the pre-master/master
> secrets.

  Quite possibly.

> Would you be able to guide me on what might be causing this or how I can
> debug it further within the FreeRADIUS or TLS stack? I’d greatly appreciate
> any pointers or direction, even if it's just confirming what part of the
> handshake to focus on.

  I would suggest running the same test with eapol_test.  That's a supplicant which implements EAP-TTLS.  You can follow the code and packet flow there.  Then, compare it to your tool.

 But ultimately this is a TLS issue on your system, and Java, and on custom code that we didn't write.  So we can only give very vague advice here.

  Alan DeKok.