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Smart Cards Lab COMPGA12 University College London

14 GSM SIM Card Exploration

We mean a classical (2G) SIM card, what we present here also works on

many (but not all as it seems) 3G cards that are (not always?) backwards

compatible with 2G phones.

Quiz SIM stands for:

Standard ICC for Mobile (phones) Subscriber Identity Module

Bibliography: The exact speciﬁcations of how a mobile phone commu-

nicates with a SIM card can be found in the ETSI GSM 11-11 document.

14.1 GSM SIM Card During a Phone Call

We will perform the following sequence of operations:

1. Select the ﬁle DF

GSM

, which is one of the two main directories on

each SIM card, the second being DF

T ELEC OM

2. Verify the PIN (not needed if PIN inactive).

3. Run the GSM algorithm to obtain cryptographic keys for the authen-

tication and encryption during the current phone call.

This is the strict minimum that needs to be executed for each phone call

(the phone does NOT know the secret key of the card and therefore cannot

replace the SIM card in these 3 operations, it can however cache the PIN so

that the user does not have to enter it each time).

14.2 Accessing DF

GSM

First we execute the following:

GSM SELECT FILE (by FID)

CLA INS P1 P2 Le FID

A0 A4 00 00 02 7F 20

Where the FID of DF

GSM

is ’7F 20’.

The answer should be:

SW1 SW2

Where is the length of the response data

that we will now recover with GET RESPONSE command:

Nicolas T. Courtois 2009-10

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Classic Electronics Smart 4F Especificaciones Pagina 32