Category Archives: IMS / VoLTE

PyHSS – Python 3GPP LTE Home Subscriber Server

EPC, EUTRAN, IMS / VoLTE, LTE, Mobile Networks, Python, RF, RFCs & Standards, SDM, , ,

I recently started working on an issue that I’d seen was to do with the HSS response to the MME on an Update Location Answer.

I took some Wireshark traces of a connection from the MME to the HSS, and compared that to a trace from a different HSS. (Amarisoft EPC/HSS)

The Update Location Answer sent by the Amarisoft HSS to the MME over the S6a (Diameter) interface includes an AVP for “Multiple APN Configuration” which has the the dedicated bearer for IMS, while the HSS in the software I was working on didn’t.

After a bit of bashing trying to modify the S6a responses, I decided I’d just implement my own Home Subscriber Server.

The Diameter interface is pretty straight forward to understand, using a similar structure to RADIUS, and with the exception of the Crypto for the EUTRAN Authentication Vectors, it was all pretty straight forward.

If you’d like to know more you can download PyHSS from my GitLab page, and view my Diameter Primer post and my post on Diameter packet structure.

Wireshark trace showing a "401 Unauthorized" Response to an IMS REGISTER request, using the AKAv1-MD5 Algorithm

All About IMS Authentication (AKAv1-MD5) in VoLTE Networks

EPC, IMS / VoLTE, LTE, Mobile Networks, Notes, RFCs & Standards, SDM, Security, SIM Cards, Software, VoIP, , , , ,

I recently began integrating IMS Authentication functions into PyHSS, and thought I’d share my notes / research into the authentication used by IMS networks & served by a IMS capable HSS.

There’s very little useful info online on AKAv1-MD5 algorithm, but it’s actually fairly simple to understand.

RFC 2617 introduces two authentication methods for HTTP, one is Plain Text and is as it sounds – the password sent over the wire, the other is using Digest scheme authentication. This is the authentication used in standard SIP MD5 auth which I covered ages back in this post.

Authentication and Key Agreement (AKA) is a method for authentication and key distribution in a EUTRAN network. AKA is challenge-response based using symmetric cryptography. AKA runs on the ISIM function of a USIM card.

I’ve covered the AKA process in my post on USIM/HSS authentication.

The Nonce field is the Base64 encoded version of the RAND value and concatenated with the AUTN token from our AKA response. (Often called the Authentication Vectors).

That’s it!

It’s put in the SIP 401 response by the S-CSCF and sent to the UE. (Note, the Cyperhing Key & Integrity Keys are removed by the P-CSCF and used for IPsec SA establishment.

Wireshark trace showing a "401 Unauthorized" Response to an IMS REGISTER request, using the AKAv1-MD5 Algorithm
Click for Full Size version of this image