Category Archives: IMS / VoLTE

Using Wireshark to peer inside IPsec ESP VoLTE data from the P-CSCF

EPC, EUTRAN, IMS / VoLTE, LTE, Mobile Networks, Security, VoIP, , , , , , ,

IPsec ESP can be used in 3 different ways on the Gm interface between the Ue and the P-CSCF:

  • Integrity Protection – To prevent tampering
  • Ciphering – To prevent inception / eavesdropping
  • Integrity Protection & Ciphering

On Wireshark, you’ll see the ESP, but you won’t see the payload contents, just the fact it’s an Encapsulated Security Payload, it’s SPI and Sequence number.

By default, Kamailio’s P-CSCF only acts in Integrity Protection mode, meaning the ESP payloads aren’t actually encrypted, with a few clicks we can get Wireshark to decode this data;

Just open up Wireshark Preferences, expand Protocols and jump to ESP

Now we can set the decoding preferences for our ESP payloads,

In our case we’ll tick the “Attempt to detect/decode NULL encrypted ESP payloads” box and close the box by clicking OK button.

Now Wireshark will scan through all the frames again, anything that’s an ESP payload it will attempt to parse.

Now if we go back to the ESP payload with SQN 1 I showed a screenshot of earlier, we can see the contents are a TCP SYN.

Now we can see what’s going on inside this ESP data between the P-CSCF and the UE!

As a matter of interest if you can see the IK and CK values in the 401 response before they’re stripped you can decode encrypted ESP payloads from Wireshark, from the same Protocol -> ESP section you can load the Ciphering and Integrity keys used in that session to decrypt them.

If you’re leaning about VoLTE & IMS networks, or building your own, I’d suggest checking out my other posts on the topic.

Kamailio Bytes – Configuring Diameter Peers with CDP

EPC, IMS / VoLTE, Kamailio, LTE, Mobile Networks, SDM, , , ,

I’ve talked a little about my adventures with Diameter in the past, the basics of Diameter, the packet structure and the Python HSS I put together.

Kamailio is generally thought of as a SIP router, but it can in fact handle Diameter signaling as well.

Everything to do with Diameter in Kamailio relies on the C Diameter Peer and CDP_AVP modules which abstract the handling of Diameter messages, and allow us to handle them sort of like SIP messages.

CDP on it’s own doesn’t actually allow us to send Diameter messages, but it’s relied upon by other modules, like CDP_AVP and many of the Kamailio IMS modules, to handle Diameter signaling.

Before we can start shooting Diameter messages all over the place we’ve first got to configure our Kamailio instance, to bring up other Diameter peers, and learn about their capabilities.

C Diameter Peer (Aka CDP) manages the Diameter connections, the Device Watchdog Request/Answers etc, all in the background.

We’ll need to define our Diameter peers for CDP to use so Kamailio can talk to them. This is done in an XML file which lays out our Diameter peers and all the connection information.

In our Kamailio config we’ll add the following lines:

loadmodule "cdp.so"
modparam("cdp", "config_file", "/etc/kamailio/diametercfg.xml")
loadmodule "cdp_avp.so"

This will load the CDP modules and instruct Kamailio to pull it’s CDP info from an XML config file at /etc/kamailio/diametercfg.xml

Let’s look at the basic example given when installed:

<?xml version="1.0" encoding="UTF-8"?>
<!-- 

 DiameterPeer Parameters 
  - FQDN - FQDN of this peer, as it should apper in the Origin-Host AVP
  - Realm - Realm of this peer, as it should apper in the Origin-Realm AVP
  - Vendor_Id - Default Vendor-Id to appear in the Capabilities Exchange
  - Product_Name - Product Name to appear in the Capabilities Exchange 
  - AcceptUnknownPeers - Whether to accept (1) or deny (0) connections from peers with FQDN 
    not configured below
  - DropUnknownOnDisconnect - Whether to drop (1) or keep (0) and retry connections (until restart)
    unknown peers in the list of peers after a disconnection.
  - Tc - Value for the RFC3588 Tc timer - default 30 seconds
  - Workers - Number of incoming messages processing workers forked processes.
  - Queue - Length of queue of tasks for the workers:
     - too small and the incoming messages will be blocked too often;
     - too large and the senders of incoming messages will have a longer feedback loop to notice that
     this Diameter peer is overloaded in processing incoming requests;
     - a good choice is to have it about 2 times the number of workers. This will mean that each worker
     will have about 2 tasks in the queue to process before new incoming messages will start to block.
  - ConnectTimeout - time in seconds to wait for an outbound TCP connection to be established.
  - TransactionTimeout - time in seconds after which the transaction timeout callback will be fired,
    when using transactional processing.
  - SessionsHashSize - size of the hash-table to use for the Diameter sessions. When searching for a 
    session, the time required for this operation will be that of sequential searching in a list of 
    NumberOfActiveSessions/SessionsHashSize. So higher the better, yet each hashslot will consume an
    extra 2xsizeof(void*) bytes (typically 8 or 16 bytes extra).
  - DefaultAuthSessionTimeout - default value to use when there is no Authorization Session Timeout 
  AVP present.
  - MaxAuthSessionTimeout - maximum Authorization Session Timeout as a cut-out measure meant to
  enforce session refreshes.
      
 -->
<DiameterPeer 
        FQDN="pcscf.ims.smilecoms.com"
        Realm="ims.smilecoms.com"
        Vendor_Id="10415"
        Product_Name="CDiameterPeer"
        AcceptUnknownPeers="0"
        DropUnknownOnDisconnect="1"
        Tc="30"
        Workers="4"
        QueueLength="32"
        ConnectTimeout="5"
        TransactionTimeout="5"
        SessionsHashSize="128"
        DefaultAuthSessionTimeout="60"
        MaxAuthSessionTimeout="300"
>

        <!--
                Definition of peers to connect to and accept connections from. For each peer found in here
                a dedicated receiver process will be forked. All other unkwnown peers will share a single
                receiver. NB: You must have a peer definition for each peer listed in the realm routing section
        -->
        <Peer FQDN="pcrf1.ims.smilecoms.com" Realm="ims.smilecoms.com" port="3868"/>
        <Peer FQDN="pcrf2.ims.smilecoms.com" Realm="ims.smilecoms.com" port="3868"/>
        <Peer FQDN="pcrf3.ims.smilecoms.com" Realm="ims.smilecoms.com" port="3868"/>
        <Peer FQDN="pcrf4.ims.smilecoms.com" Realm="ims.smilecoms.com" port="3868"/>
        <Peer FQDN="pcrf5.ims.smilecoms.com" Realm="ims.smilecoms.com" port="3868"/>
        <Peer FQDN="pcrf6.ims.smilecoms.com" Realm="ims.smilecoms.com" port="3868"/>

        <!--
                Definition of incoming connection acceptors. If no bind is specified, the acceptor will bind
                on all available interfaces.
        -->
        <Acceptor port="3868"  />
        <Acceptor port="3869" bind="127.0.0.1" />
        <Acceptor port="3870" bind="192.168.1.1" />

        <!--
                Definition of Auth (authorization) and Acct (accounting) supported applications. This
                information is sent as part of the Capabilities Exchange procedures on connecting to
                peers. If no common application is found, the peers will disconnect. Messages will only
                be sent to a peer if that peer actually has declared support for the application id of 
                the message.
        -->
        <Acct id="16777216" vendor="10415" />
        <Acct id="16777216" vendor="0" />
        <Auth id="16777216" vendor="10415"/>
        <Auth id="16777216" vendor="0" />

        <!-- 
                Supported Vendor IDs - list of values which will be sent in the CER/CEA in the
                Supported-Vendor-ID AVPs
        -->
        <SupportedVendor vendor="10415" />

        <!--
                Realm routing definition.
                Each Realm can have a different table of peers to route towards. In case the Destination
                Realm AVP contains a Realm not defined here, the DefaultRoute entries will be used.

                Note: In case a message already contains a Destination-Host AVP, Realm Routeing will not be
                applied.
                Note: Routing will only happen towards connected and application id supporting peers.
                
                The metric is used to order the list of prefered peers, while looking for a connected and
                application id supporting peer. In the end, of course, just one peer will be selected.
        -->
        <Realm name="ims.smilecoms.com">
                <Route FQDN="pcrf1.ims.smilecoms.com" metric="3"/>
                <Route FQDN="pcrf2.ims.smilecoms.com" metric="5"/>
        </Realm>

        <Realm name="temp.ims.smilecoms.com">
                <Route FQDN="pcrf3.ims.smilecoms.com" metric="7"/>
                <Route FQDN="pcrf4.ims.smilecoms.com" metric="11"/>
        </Realm>
        <DefaultRoute FQDN="pcrf5.ims.smilecoms.com" metric="15"/>
        <DefaultRoute FQDN="pcrf6.ims.smilecoms.com" metric="13"/>


</DiameterPeer>

First we need to start by telling CDP about the Diameter peer it’s going to be – we do this in the <DiameterPeer section where we define the FQDN and Diameter Realm we’re going to use, as well as some general configuration parameters.

<Peers are of course, Diameter peers. Defining them here will mean a connection is established to each one, Capabilities exchanged and Watchdog request/responses managed. We define the usage of each Peer further on in the config.

The Acceptor section – fairly obviously – sets the bindings for the addresses and ports we’ll listen on.

Next up we need to define the Diameter applications we support in the <Acct id=” /> and <SupportedVendor> parameters, this can be a little unintuitive as we could list support for every Diameter application here, but unless you’ve got a module that can handle those applications, it’s of no use.

Instead of using Dispatcher to manage sending Diameter requests, CDP handles this for us. CDP keeps track of the Peers status and it’s capabilities, but we can group like Peers together, for example we may have a pool of PCRF NEs, so we can group them together into a <Realm >. Instead of calling a peer directly we can call the realm and CDP will dispatch the request to an up peer inside the realm, similar to Dispatcher Groups.

Finally we can configure a <DefaultRoute> which will be used if we don’t specify the peer or realm the request needs to be sent to. Multiple default routes can exist, differentiated based on preference.

We can check the status of peers using Kamcmd’s cdp.list_peers command which lists the peers, their states and capabilities.

OTP Authentication required to unlock IMS Debugging and TCPDUMP on Samsung Sysdump tool

Reverse Engineering Samsung Sysdump Utils to Unlock IMS Debug & TCPdump on Samsung Phones

IMS / VoLTE, LTE, Mobile Networks, Security, Software, , , ,

Note: This post is just about the how I reverse engineered the tool, for info on how to use it, you want this post.

While poking around the development and debugging features on Samsung handsets I found the ability to run IMS Debugging directly from the handset.

Alas, the option is only available in the commercial version, it’s just there for carriers, and requires a One Time Password to unlock.

OTP Authentication required to unlock IMS Debugging and TCPDUMP on Samsung Sysdump tool "This menu is not allowed for commercial version. You can activate this menu after OTP Authentication enabled"

When tapping on the option a challenge is generated with a key.

Interestingly I noticed that the key changes each time and can reject you even in aeroplane mode, suggesting the authentication happens client side.

This left me thinking – If the authentication happens client side, then the App has to know what the valid password for the key shown is…

Some research revealed you can pull APKs off an Android phone, so I downloaded a utility called “APK Extractor” from the Play store, and used it to extract the Samsung Sysdump utility.

So now I was armed with the APK on my local machine, the next step was to see if I could decompile the APK back into source code.

Some Googling found me an online APK decompiler, which I fed the compiled APK file and got back the source code.

I did some poking around inside the source code, and then I found an interesting directory:

Here’s a screenshot of the vanilla code that came out of the app.

Samsung OTPSecurty Source Code

I’m not a Java expert, but even I could see the “CheckOTP” function and understand that that’s what validates the One Time Passwords.

The while loop threw me a little – until I read through the rest of the code; the “key” in the popup box is actually a text string representing the current UNIX timestamp down to the minute level. The correct password is an operation done on the “key”, however the CheckOTP function doesn’t know the challenge key, but has the current time, so generates a challenge key for each timestamp back a few minutes and a few minutes into the future.

I modified the code slightly to allow me to enter the presented “key” and get the correct password back. It’s worth noting you need to act quickly, enter the “key” and enter the response within a minute or so.

In the end I’ve posted the code on an online Java compiler,

Generate OTP Response from Key (Challenge)

Replace yy182 with your challenge. I suggest you try the 0 offset and type it in quickly.

I did a write up on how to use the features this unlocks in this post.

If you’re leaning about VoLTE & IMS networks, or building your own, I’d suggest checking out my other posts on the topic.

Samsung-Sysdump-IMS-Debug-DM-View_Cropped

VoLTE/IMS Debugging on Samsung Handsets using Sysdump & Samsung IMS Logger

EPC, EUTRAN, IMS / VoLTE, LTE, Mobile Networks, Security, Software, VoIP, , , , , , , ,

Samsung handsets have a feature built in to allow debugging from the handset, called Sysdump.

Entering *#9900# from the Dialing Screen will bring up the Sysdump App, from here you can dump logs from the device, and run a variety of debugging procedures.

Samsung share information about this app publicly on their website,

Sysdump App in Samsung handsets used for debugging the device

But for private LTE operators, the two most interesting options are by far the TCPDUMP START option and IMS Logger, but both are grayed out.

Tapping on them asks for a one-time password and has a challenge key.

OTP Authentication required to unlock IMS Debugging and TCPDUMP on Samsung Sysdump tool

These options are not available in the commercial version of the OS and need to be unlocked with a one time key generated by a tool Samsung for unlocking engineering firmware on handsets.

Luckily this authentication happens client side, which means we can work out the password it’s expecting.

For those interested I’ve done a write up of how I reversed the password validation algorithm to take the key given in the OTP challenge and generate a valid response.

For those who just want to unlock these features you can click here to run the tool that generates the response.

Once you’ve entered the code and successfully unlocked the IMS Debugging tool there’s a few really cool features in the hamburger menu in the top right.

DM View

This shows the SIP / IMS Messaging and the current signal strength parameters (used to determine which RAN type to use (Ie falling back from VoLTE to UMTS / Circuit Switched when the LTE signal strength drops).

Screenshot of Samsung Sysdump tool in the IMS Debug - DM View section

Tapping on the SIP messages expands them and allows you to see the contents of the SIP messages.

Viewing SIP Messaging directly from the handset

Interesting the actual nitty-gritty parameters in the SIP headers are missing, replaced with X for anything “private” or identifiable.

Luckily all this info can be found in the Pcap.

The DM View is great for getting a quick look at what’s going on, on the mobile device itself, without needing a PC.

Logging

The real power comes in the logging functions,

There’s a lot of logging options, including screen recording, TCPdump (as in Packet Captures) and Syslog logging.

From the hamburger menu we can select the logging parameters we want to change.

Settings for Samsung IMS Logger

From the Filter Options menu we can set what info we’re going to log,

Filter options used in Dump output of Samsung IMS Logger application

If you’re leaning about VoLTE & IMS networks, or building your own, I’d suggest checking out my other posts on the topic.

Roll your own USIMs for Private LTE Networks

EPC, EUTRAN, IMS / VoLTE, LTE, Mobile Networks, SDM, Security, SIM Cards, , , , , , , ,

I wrote a while ago about USIM basics and talked about what each of the fields stored on a USIM manage, but I thought I’d talk a little about my adventures in getting custom USIMs.

I started working on a private LTE project a while ago; RAN hardware (eNodeBs) were on the way, down to a shortlist of a few EPC platforms, but I still needed USIMs before anyone was connecting to the network.

So why are custom USIMs a requirement? Can’t you just use any old USIM/SIMs?

In UMTS / LTE / NR networks there’s mutual network authentication, again I’ve written about this topic before, but unlike GSM where the network authenticates the UE, in later RAN standards, the UE also authenticates the network. (This mitigates any bad actor from setting up their own base stations and having UEs attach to it and have their traffic intercepted).

For roaming to work between carriers they’ve got to have their HSS / DRA connecting to the DRA or HSS of other carriers, to allow roaming subscribers to access the network, otherwise they too would fall foul of the mutual network authentication and the USIM wouldn’t connect to the network.

The first USIMs I purchased online through a popular online marketplace with a focus on connecting you to Chinese manufacturers.
They listed a package of USIMS, a USB reader/writer that supported all the standard USIM form factors and the software to program it, which I purchased.

The USIMs worked fairly well – They are programmable via a card reader and software that, although poorly translated/documented, worked fairly well.

USIM Programming Interface

K and OP/OPc values could be written to the card but not read, while the other values could be read and written from the software, the software also has the ability to sequentially program the USIMs to make bulk operations easier. The pricing worked out about $8 USD per USIM, which although expensive for the quantity and programmable element is pretty reasonable.

Every now and then the Crypto values for some reason or another wouldn’t get updated, which is exactly as irritating as it sounds.

Pretty quickly into the build I learned the USIMs didn’t include an ISIM service on the card, ISIM being the service that runs on the UCCID responsible for IMS / VoLTE authentication.

Again I went looking and reached out to a few manufacturers of USIMs.

The big vendors, Gemalto, Kona, etc, weren’t interested in providing USIMs in quantities less than 100,000 and their USIMs came from the factory pre-programmed, meaning the values could only be changed through remote SIM provisioning, a form of black magic.

In the end I reached out to an OEM manufacturer from China who provided programmable USIM / ISIMs for less than I was paying on the online marketplace and at any quantity I wanted with custom printing options, allocated ICCIDs, etc.

The non-programmable USIMs worked out less than $0.40 USD each in larger quantities, and programmable USIM/ISIMs for about $5 USD.

The software was almost identical except for the additional tab for ISIM operations.

USIM / ISIM programming
ISIM parameters

Smart Card Readers

In theory this software and these USIMs could be programmed by any smart card reader.

In practice, the fact that the ISO standard smart card is the same size as a credit card, means most smart card readers won’t fit the bill.

I tried a few smart card readers, from the one built into my Thinkpad, to a Bluedrive II from one of the USIM vendors, in the end the MCR3516 Smart Card Reader which reads 4FF USIMs (Standard ISO size smart card, full size SIM, Micro SIM and Nano SIM form factors, which saved on so much mucking about with form factor adapters etc.

4FF Smart Card Reader for programming SIM/USIM/ISIM

Future Projects

I’ve got some very calls “Multi Operator Neutral Host” (MoNEH) USIMs from the guys at Telet Research I’m looking forward to playing with,

eSIMs are on my to-do list too, and the supporting infrastructure, as well as Over the Air updating of USIMs.

The case for Header Compression in VoIP/VoLTE

EPC, EUTRAN, IMS / VoLTE, LTE, Mobile Networks, RFCs & Standards, VoIP, , , , , ,

On a PCM (G.711) RTP packet the payload is typically 160 bytes per packet.

But the total size of the frame on the wire is typically ~214 bytes, to carry a 160 byte payload that means 25% of the data being carried is headers.

This is fine for VoIP services operating over fixed lines, but when we’re talking about VoLTE / IMS and the traffic is being transferred over Radio Access Networks with limited bandwidth / resources, it’s important to minimize this as much as possible.

IMS uses the AMR codec, where the RTP payload for each packet is around 90 bytes, meaning up to two thirds of the packet on the wire (Or in this case the air / Uu interface) is headers.

Enter Robust Header Compression which compresses the headers.

Using ROHC the size of the headers are cut down to only 4-5 bytes, this is because the IPv4 headers, UDP headers and RTP headers are typically the same in each packet – with only the RTP Sequence number, RTP timestamp IPv4 & UDP checksum and changing between frames.

Building Android APN / Carrier Config

EPC, EUTRAN, GSM, IMS / VoLTE, LTE, Mobile Networks, SIM Cards, Software, ,

As anyone who’s setup a private LTE network can generally attest, APNs can be a real headache.

SIM/USIM cards, don’t store any APN details. In this past you may remember having to plug all these settings into your new phone when you upgraded so you could get online again.

Today when you insert a USIM belonging to a commercial operator, you generally don’t need to put APN settings in, this is because Android OS has its own index of APNs. When the USIM is inserted into the baseband module, the handset’s OS looks at the MCC & MNC in the IMSI and gets the APN settings automatically from Android’s database of APN details.

There is an option for the network to send the connectivity details to the UE in a special type of SMS, but we won’t go into that.

All this info is stored on the Android OS in apns-full-conf.xml which for non-rooted (stock) devices is not editable.

Instead the devices get updates through the OS updates which pull the latest copy of this file from Google’s Android Open Source Git repo, you can view the current master file here.

This file can override the user’s APN configuration, which can lead to some really confusing times as your EPC rejects the connection due to an unrecognized APN which is not what you have configured on the UE’s operating system, but it instead uses APN details from it’s database.

The only way around this is to change the apns-full-conf.xml file, either by modifying it per handset or submitting a push request to Android Open Source with your updated settings.

(I’ve only tried the former with rooted devices)

The XML file itself is fairly self explanatory, taking the MCC and MNC and the APN details for your network:

<apn carrier="CarrierXYZ"
      mcc="123"
      mnc="123"
      apn="carrierxyz"
      type="default,supl,mms,ims,cbs"
      mmsc="http://mms.carrierxyz.com"
      mmsproxy="0.0.0.0"
      mmsport="80"
      bearer_bitmask="4|5|6|7|8|12"
/>

Once you’ve added yours to the file, inserting the USIM, rebooting the handset or restarting the carrier app is all that’s required for it to be re-read and auto provision APN settings from the XML file.

Further reading

APN and CarrierConfig | Android Open Source Project

Carrier Configuration | Android Open Source Project

UICC Carrier Privileges | Android Open Source Project

/etc/apns-full-conf.xml – Master Branch

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
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