Tag Archives: K

Information stored on USIM / SIM Card for LTE / EUTRAN / EPC - K key, OP/OPc key and SQN Sequence Number

HSS & USIM Authentication in LTE/NR (4G & 5G)

I talked a bit in my last post about using osmo-sim-auth to authenticate against a USIM / SIM card when it’s not in a phone,

I thought I’d expand a little on how the Crypto side of things works in LTE & NR (also known as 4G & 5G).

Authentication primarily happens in two places, one at each end of the network, the Home Subscriber Server and in the USIM card. Let’s take a look at each of them.

On the USIM

On the USIM we’ve got two values that are entered in when the USIM is provisioned, the K key – Our secret key, and an OPc key (operator key).

These two keys are the basis of all the cryptography that goes on, so should never be divulged.

The only other place to have these two keys in the HSS, which associates each K key and OPc key combination with an IMSI.

The USIM also stores the SQN a sequence number, this is used to prevent replay attacks and is incremented after each authentication challenge, starting at 1 for the first authentication challenge and counting up from there.

On the HSS

On the HSS we have the K key (Secret key), OPc key (Operator key) and SQN (Sequence Number) for each IMSI on our network.

Each time a IMSI authenticates itself we increment the SQN, so the value of the SQN on the HSS and on the USIM should (almost) always match.

Authentication Options

Let’s imagine we’re designing the authentication between the USIM and the Network; let’s look at some options for how we can authenticate everyone and why we use the process we use.

Failed Option 1 – Passwords in the Clear

The HSS could ask the USIM to send it’s K and OPc values, compare them to what the HSS has in place and then either accept or reject the USIM depending on if they match.

The obvious problem with this that to send this information we broadcast our supposedly secret K and OPc keys over the air, so anyone listening would get our secret values, and they’re not so secret anymore.

This is why we don’t use this method.

Failed Option 2 – Basic Crypto

So we’ve seen that sending our keys publicly, is out of the question.

The HSS could ask the USIM to mix it’s K key and OPc key in such a way that only someone with both keys could unmix them.

This is done with some cryptographic black magic, all you need to know is it’s a one way function you enter in values and you get the same result every time with the same input, but you can’t work out the input from the result.

The HSS could then get the USIM to send back the result of mixing up both keys, mix the two keys it knows and compare them.

The HSS mixes the two keys itself, and get’s it’s own result called XRES (Expected Result). If the RES (result) of mixing up the keys by the USIM is matches the result when the HSS mixes the keys in the same way (XRES (Expected Result)), the user is authenticated.

The result of mixing the keys by the USIM is called RES (Result), while the result of the HSS mixing the keys is called XRES (Expected Result).

This is a better solution but has some limitations, because our special mixing of keys gets the same RES each time we put in our OPc and K keys each time a subscriber authenticates to the network the RES (result) of mixing the keys is going to be the same.

This is vulnerable to replay attacks. An attacker don’t need to know the two secret keys (K & OPc) that went into creating the RES (resulting output) , the attacker would just need to know the result of RES, which is sent over the air for anyone to hear.
If the attacker sends the same RES they could still authenticate.

This is why we don’t use this method.

Failed Option 3 – Mix keys & add Random

To prevent these replay attacks we add an element of randomness, so the HSS generates a random string of garbage called RAND, and sends it to the USIM.

The USIM then mixes RAND (the random string) the K key and OPc key and sends back the RES (Result).

Because we introduced a RAND value, every time the RAND is different the RES is different. This prevents against the replay attacks we were vulnerable to in our last example.

If the result the USIM calculated with the K key, OPc key and random data is the same as the USIM calculated with the same K key, OPc key and same random data, the user is authenticated.

While an attacker could reply with the same RES, the random data (RAND) will change each time the user authenticates, meaning that response will be invalid.

While an attacker could reply with the same RES, the random data (RAND) will change each time the user authenticates, meaning that response will be invalid.

The problem here is now the network has authenticated the USIM, the USIM hasn’t actually verified it’s talking to the real network.

This is why we don’t use this method.

GSM authentication worked like this, but in a GSM network you could setup your HLR (The GSM version of a HSS) to allow in every subscriber regardless of what the value of RES they sent back was, meaning it didn’t look at the keys at all, this meant attackers could setup fake base stations to capture users.

Option 4 – Mutual Authentication (Real World*)

So from the previous options we’ve learned:

  • Our network needs to authenticate our subscribers, in a way that can’t be spoofed / replayed so we know who to bill & where to route traffic.
  • Our subscribers need to authenticate the network so they know they can trust it to carry their traffic.

So our USIM needs to authenticate the network, in the same way the network authenticates the USIM.

To do this we introduce a new key for network authentication, called AUTN.

The AUTN key is generated by the HSS by mixing the secret keys and RAND values together, but in a different way to how we mix the keys to get RES. (Otherwise we’d get the same key).

This AUTN key is sent to the USIM along with the RAND value. The USIM runs the same mixing on it’s private keys and RAND the HSS did to generate the AUTN , except this is the USIM generated – An Expected AUTN key (XAUTN). The USIM compares XAUTN and AUTN to make sure they match. If they do, the USIM then knows the network knows their secret keys.

The USIM then does the same mixing it did in the previous option to generate the RES key and send it back.

The network has now authenticated the subscriber (HSS has authenticated the USIM via RES key) and the subscriber has authenticated the USIM (USIM authenticates HSS via AUTN key).

*This is a slightly simplified version of how EUTRAN / LTE authentication works between the HSS and the USIM – In reality there are a few extra values, such as SQN to take into consideration and the USIM talks to to the MME not the HSS directly.

I’ll do a follow up post covering the more nitty-gritty elements, AMF and SQN fields, OP vs OPc keys, SQN Resync, how this information is transfered in the Authentication Information Answer and how KASME keys are used / distributed.

Roll your own USIMs for Private LTE Networks

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.

LTE (4G) – USIM Basics

I’ve been working on private LTE recently, and one of the first barriers you’ll hit will be authentication.

LTE doesn’t allow you to just use any SIM to authenticate to the network, but instead relies on mutual authentication of the UE and the network, so the Network knows it’s talking to the right UE and the UE knows it’s talking to the right network.

So because of this, you have to have full control over the SIM and the network. So let’s take a bit of a dive into USIMs.

So it’s a SIM card right?

As a bit of background; the ever shrinking card we all know as a SIM is a “Universal integrated circuit card” – a microcontroller with it’s own OS that generally has the ability to run Java applets.

One of the Java applets on the card / microcontroller will be the software stack for a SIM, used in GSM networks to authenticate the subscriber.

For UMTS and LTE networks the card would have a USIM software stack allowing it to act as a USIM, the evolved version of the SIM.

Because it’s just software a single card can run both a USIM and SIM software stack, and most do.

As I’m building an LTE network we’ll just talk about the USIM side of things.

USIM’s role in Authentication

When you fire up your mobile handset the baseband module in it communicates with the USIM application on the card.

When it comes time to authenticate to the network, and authenticate the network itself, the baseband module sends the provided challenge information from the network to the USIM which does the crypto magic to generate responses to the authentication challenges issued by the network, and the USIM issues it’s own challenges to the network.

The Baseband module provides the ingredients, but the USIM uses it’s secret recipe / ingredients combo, known only to the USIM and HSS, to perform the authentication.

Because the card challenges the network it means we’ve got mutual authentication of the network.

This prevents anyone from setting up their own radio network from going all Lionel Ritche and saying “Hello, is it me you’re looking for” and having all the UEs attach to the malicious network. (Something that could be done on GSM).

It’s worth noting too that because the USIM handles all this the baseband module, and therefore the mobile handset itself, doesn’t know any of the secret sauce used to negotiate with the network. It just gets the challenge and forwards the ingredients down to the USIM which spits back the correct response to send, without sharing the magic recipe.

This also means operators can implement their own Crypto functions for f and g, so long as the HSS and the USIM know how to generate the RES and AUTN results, it’ll work.

What’s Inside?

Let’s take a look at the information that’s stored on your USIM:

All the GSM stuff for legacy SIM application

Generally USIMs also have the ability to operate as SIMs in a GSM network, after all it’s just a different software stack. We won’t touch on GSM SIMs here.


Because a USIM is just an application running on a Universal Integrated Circuit Card, it’s got a ICCID or Universal Integrated Circuit Card ID. Generally this is the long barcode / string of numbers printed on the card itself.

The network generally doesn’t care about this value, but operators may use it for logistics like shipping out cards.


PINs and PUKs are codes to unlock the card. If you get the PIN wrong too many times you need the longer PUK to unlock it.

These fields can be written to (when authenticated to the card) but not read directly, only challenged. (You can try a PIN, but you can’t see what it’s set too).

As we mentioned before the terminal will ask the card if that’s correct, but the terminal doesn’t know the PIN either.


Each subscriber has an IMSI, an International Mobile Subscriber Identity.

IMSIs are hierarchical, starting with 3 digit Mobile Country Code MCC, then the Mobile Network Code (MNC) (2/3 digits) and finally a Mobile Subscription Identification Number (MSIN), a unique number allocated by the operator to the subscribers in their network.

This means although two subscribers could theoretically have the same MSIN they wouldn’t share the same MNC and MCC so the ISMI would still be unique.

The IMSI never changes, unless the subscriber changes operators when they’ll be issued a new USIM card by the new operator, with a different IMSI (differing MNC).

The MSIN isn’t the same as the phone number / MSISDN Number, but an IMSI generally has a MSISDN associated with it by the network. This allows you to port / change MSISDN numbers without changing the USIM/SIM.

K – Subscriber Key

Subscriber’s secret key known only to the Subscriber and the Authentication Center (AuC/ HSS).

All the authentication rests on the principle that this one single secret key (K) known only to the USIM and the AuC/HHS.

OP – Operator Code

Operator Code – same for all SIMs from a single operator.

Used in combination with K as an input for some authentication / authorisation crypto generation.

Because the Operator Code is common to all subscribers in the network, if this key were to be recovered it could lead to security issues, so instead OPc is generally used.

OPc – Operator Code (Derived)

Instead of giving each USIM the Operator Code a derived operator code can be precomputed when the USIM is written with the K key.

This means the OP is not stored on the USIM.


PLMN (Public Land Mobile Network)

The PLMN is the combination of MCC & MNC that identifies the operator’s radio access network (RAN) from other operators.

While there isn’t a specific PLMN field in most USIMs it’s worth understanding as several fields require a PLMN.

HPLMNwAcT (HPLMN selector with Access Technology)

Contains in order of priority, the Home-PLMN codes with the access technology specified.

This allows the USIM to work out which PLMN to attach to and which access technology (RAN), for example if the operator’s PLMN was 50599 we could have:

  • 50599 E-UTRAN
  • 50599 UTRAN

To try 4G and if that fails use 3G.

In situations where operators might partner to share networks in different areas, this could be set to the PLMN of the operator first, then it’s partnered operator second.

OPLMNwACT (Operator controlled PLMN selector with Access

This is a list of PLMNs the operator has a roaming agreement with in order of priority and with the access technology.

An operator may roam to Carrier X but only permit UTRAN access, not E-TRAN.

FEHPLMN (Equivalent HPLMN)

Used to define equivalent HPMNs, for example if two carriers merge and still have two PLMNs.

FPLMN (Forbidden PLMN list)

A list of PLMNs the subscriber is not permitted to roam to.

HPPLMN (Higher Priority PLMN search period)

How long in seconds to spend between each PLMN/Access Technology in HPLMNwAcT list.

ACC (Access Control Class)

The ACC allows values from 0-15, and determines the access control class of the subscriber.

In the UK the ACC values is used to restrict civilian access to cell phone networks during emergencies.

Ordinary subscribers have ACC numbers in the range 0 – 9. Higher priority users are allocated numbers 12-14.

During an emergency, some or all access classes in the range 0 – 9 are disabled.

This means service would be could be cut off to the public who have ACC value of 0-9, but those like first responders and emergency services would have a higher ACC value and the network would allow them to attach.

AD (Administrative Data)

Like the ACC field the AD field allows operators to drive test networks without valid paying subscribers attaching to the network.

The defined levels are:

  • ’00’ normal operation.
  • ’80’ type approval operations.
  • ’01’ normal operation + specific facilities.
  • ’81’ type approval operations + specific facilities.
  • ’02’ maintenance (off line).
  • ’04’ cell test operation.

GID 1 / 2 – Group Identifier

Two group identifier fields that allow the operator to identify a group of USIMs for a particular application.

SPN (Service Provider Name)

The SPN is an optional field containing the human-readable name of the network.

The SPN allows MVNOs to provide their own USIMs with their name as the operator on the handset.

ECC (Emergency Call Codes)

Codes up to 6 digits long the subscriber is allowed to dial from home screen / in emergency / while not authenticated etc.


Mobile Station International Subscriber Directory Number. The E.164 formatted phone number of the subscriber.

This is optional, as porting may overwrite this, so it doesn’t always match up.