Manually Testing USIM Authentication¶

This page describes how you can use Osmocom tools to manually test USIM / ISIM authentication against a SIM card.

Prerequisites¶

• A USIM card of which you know the secret key K + OP/OPc
• A smart card interface device ("reader") supported by pysim (such as any pcsc-lite / libccid compatible reader)
• osmo-auc-gen program (part of libosmocore)
• pySim-shell program (part of pysim)

Step-by-step guide¶

Generate a 16-byte random challenge¶

Let's use /dev/random to generate 16 bytes of randomness; conver it to hexadecimal

$ dd if=/dev/random bs=16 count=1 2>/dev/null | xxd -p -l 100
8188388ad5cdd481b02298ff29827791

Generating the actual quintuple using osmo-auc-gen¶

This process mimics what is happening inside the Authentication Centre part of the HLR/HSS of your 2G/3G/4G network: Deriving RES from K, OP/OPc and SEQ/SQN.

We use the card-specific K + OPc values we received from the card manufacturer, as well as the random value we generated in the previous step

$ osmo-auc-gen --3g --algorithm MILENAGE --key 77291F1E17132ADD86DC23A3AF601C89 --opc 831AFD01EF48692EC6FD18AEAB6CF381 --rand 8188388ad5cdd481b02298ff29827791
osmo-auc-gen (C) 2011-2012 by Harald Welte
This is FREE SOFTWARE with ABSOLUTELY NO WARRANTY

RAND:   8188388ad5cdd481b02298ff29827791
AUTN:   7f62c464f6d60000b77b88e0f6b9449c
IK:     43c7bc1e8e193ed2e0e7e164126bbed5
CK:     a0188b5f7724878b86b9a336d8f2e327
RES:    b4013d66d107a2b6
IMS nonce:      gYg4itXN1IGwIpj/KYJ3kX9ixGT21gAAt3uI4Pa5RJw=
IMS res:        tAE9ZtEHorY=
SRES:   65069fd0
Kc:     8581751333a4e4ab
SQN:    32
IND:    0

This shows us the following information

• input data (RAND, IND, SQN)
• the challenge the network would send via radio interface to the UE/phone/modem (RAND, AUTN)
• the data the network keeps on the network side and does not send over radio:
  • the A3 authentication result value (RES)
  • the A8 generated keys for integrity protection and ciphering (IK, CK)
  • the derived GSM values in case of fall-back to 2G (Kc, SRES)

Performing authentication with the card, using pySim-shell¶

We now perform what the phone/modem does with the SIM card when it receives the RAND + AUTN values from the cellular network via a AUTHENTICATION REQUEST.

For this, we use pysim, specifically the authentication command in it. It takes two parameters: rand and autn

As we want to do UMTS-AKA against the USIM application on the card, we must first select ADF.USIM.

$ ./pySim-shell.py -p0
Using PC/SC reader interface
Waiting for card...
Autodetected card type: sysmoISIM-SJA2
Info: Card is of type: UICC-SIM
AIDs on card:
 USIM: a0000000871002ffffffff8907090000 (EF.DIR)
 ISIM: a0000000871004ffffffff8907090000 (EF.DIR)
 ADF.ISD: a000000003000000
 ARA-M: a00000015141434c00
Detected CardModel: SysmocomSJA2
Welcome to pySim-shell!
pySIM-shell (MF)> select ADF.USIM
pySIM-shell (MF/ADF.USIM)> authenticate 8188388ad5cdd481b02298ff29827791 7f62c464f6d60000b77b88e0f6b9449c
{
    "successful_3g_authentication": {
        "res": "b4013d66d107a2b6",
        "ck": "a0188b5f7724878b86b9a336d8f2e327",
        "ik": "43c7bc1e8e193ed2e0e7e164126bbed5",
        "kc": "8581751333a4e4ab" 
    }
}

In this successful case, we see the card has accepted the AUTN nonce and generated the following output parameters:

• authentication result (RES); would be sent back to the network, where the network compares it with the expected RES value it has computed earlier. If it matches, authentication is OK.
• integrity protection (IK) and ciphering (CK) keys
  • used for air interface protection on 3G/UMTS after a @CIPHERING MODE COMMAND
  • used in derived forms for air interface protection and NAS protection in 4G/LTE
• GSM ciphering key (Kc); used for encryption on 2G/GSM/GPRS after a CIPHERING MODE COMMAND