GSM Mobile Communications

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Computer Security and their Data

Section A

  • What is GSM?

Global System for Mobile communications (GSM) is an open, digital cellular technology used for transmitting mobile voice and data services using digital technology and time division multiple access transmission method. GSM is a circuit-switched system that divides each 200 kHz channel into eight 25 kHz time-slots. GSM operates in the 900 MHz and 1.8 GHz bands in Europe and the 1.9GHz and 850MHz bands in the US. The 850 MHz band is also used for GSM and 3GSM in Australia, Canada and many South American countries. GSM also provides Short Message Service (SMS) at a data transfer rate of 9.6 Kbps. Users can also benefit of the roaming capability which gives them access to the same services when travelling in any of the 210 countries that use GSM. GSM satellite roaming is also used when no terrestrial coverage is available.[1] “Another advantage is that the standard includes one worldwide Emergency telephone number, 112” which enables the user to dial the emergency number without knowing the emergency number of the country he's in.[2]

  • Introduction to GSM Security:

“GSM was designed with a moderate level of security. The system was designed to authenticate the subscriber using a pre-shared key and challenge-response. Communications between the subscriber and the base station can be encrypted. GSM only authenticated the user to the network (and not vice versa). The security model therefore offers confidentiality and authentication, but limited authorization capabilities, and no non-repudiation.

GSM uses several cryptographic algorithms for security. The A5/1 and A5/2 stream ciphers are used for ensuring over-the-air voice privacy. Serious weaknesses have been found in both algorithms: it is possible to break A5/2 in real-time with a cipher text-only attack, and in February 2008, Pico Consulting, Inc revealed its ability and plans to commercialize FPGAs that allow A5/1 to be broken with a rainbow table attack. The system supports multiple algorithms so operators may replace that cipher with a stronger one.”[3]

  • GSM Security:

There are three purposes in GSM security: Authentication, Encryption and Key Generation. [4]

GSM uses the A3 algorithm for authentication. It is implemented in the Subscriber Identity Module (SIM) card. “A3's task is to generate the 32-bit Signed Response (SRES) utilizing the 128-bit random challenge (RAND) generated by the Home Location Register (HLR) and the 128-bit Individual Subscriber Authentication Key (Ki) from the Mobile Station's Subscriber Identity Module (SIM) or the Home Location Register (HLR). A3 actually generates 128 bits of output. The first 32 bits of those 128 bits form the Signed Response.”[5]

For encryption, GSM uses the A5 algorithm which is implemented in the Mobile Station (MS). “The stream cipher is initialized with the Session Key (Kc) and the number of each frame. The same Kc is used throughout the call, but the 22-bit frame number changes during the call, thus generating a unique key stream for every frame. The same Session Key (Kc) is used as long as the Mobile Services Switching Center (MSC) does not authenticate the Mobile Station again. In practice, the same Session Key (Kc) may be in use for days. Authentication is an optional procedure in the beginning of a call, but it is usually not performed.”[6]

The A8 algorithm is used for key generation by the GSM and is implemented in the SIM card. “A8's task is to generate the 64-bit Session Key (Kc), from the 128-bit random challenge (RAND) received from the Mobile Services Switching Center (MSC) and from the 128-bit Individual Subscriber Authentication Key (Ki) from the Mobile Station's Subscriber Identity Module (SIM) or the Home Location Register (HLR). One Session Key (Kc) is used until the MSC decides to authenticate the MS again. This might take days. A8 actually generates 128 bits of output. The last 54 bits of those 128 bits form the Session Key (Kc). Ten zero-bits are appended to this key before it is given as input to the A5 algorithm.”[7]

Now how does the security in GSM work?

“Encryption in the GSM network utilizes a Challenge/Response mechanism.

  • The Mobile Station (MS) signs into the network.
  • The Mobile Services Switching Center (MSC) requests 5 triples from the Home Location Register (HLR).
  • The Home Location Register creates five triples utilizing the A8 algorithm. These five triples each contain:
  • A 128-bit random challenge (RAND)
  • A 32-bit matching Signed Response (SRES)
  • A 64-bit ciphering key used as a Session Key (Kc).
  • The Home Location Register sends the Mobile Services Switching Center the five triples.
  • The Mobile Services Switching Center sends the random challenge from the first triple to the Base Transceiver Station (BTS).
  • The Base Transceiver Station sends the random challenge from the first triple to the Mobile Station.
  • The Mobile Station receives the random challenge from the Base Transceiver Station and encrypts it with the Individual Subscriber Authentication Key (Ki) assigned to the Mobile Station utilizing the A3 algorithm.
  • The Mobile Station sends the Signed Response to the Base Transceiver Station.
  • The Base Transceiver Station sends the Signed Response to the Mobile Services Switching Center.
  • The Mobile Services Switching Center verifies the Signed Response.
  • The Mobile Station generates a Session Key (Kc) utilizing the A8 algorithm, the Individual Subscriber Authentication Key (Ki) assigned to the Mobile Station, and the random challenge received from the Base Transceiver Station.
  • The Mobile Station sends the Session Key (Kc) to the Base Transceiver Station.
  • The Mobile Services Switching Center sends the Session Key (Kc) to the Base Transceiver Station.
  • The Base Transceiver Station receives the Session Key (Kc) from the Mobile Services Switching Center.
  • The Base Transceiver Station receives the Session Key (Kc) from the Mobile Station.
  • The Base Transceiver Station verifies the Session Keys from the Mobile Station and the Mobile Services switching Center.
  • The A5 algorithm is initialized with the Session Key (Kc) and the number of the frame to be encrypted.
  • Over-the-air communication channel between the Mobile Station and Base Transceiver Station can now be encrypted utilizing the A5 algorithm.

This process authenticates the GSM Mobile Station (MS) to the GSM network. One known security limitation of GSM networks is that the GSM network is never authenticated by the GSM Mobile Station (MS).

This one-way authentication makes it possible for an attacker to pretend to be a GSM network provider.”[8]

  • Conclusion:

Security is about feeling safe. GSM technology somehow ensures that the data and voice communications we are sending will reach destination safely. However the problem remains in whether we are dealing with a real network provider or with an attacker pretending to be a network provider. The question is: “do we trust our network providers?”

  • References:
  • http://www.gsmworld.com/technology/what.shtml
  • http://en.wikipedia.org/wiki/GSM
  • http://en.wikipedia.org/wiki/GSM#GSM_security
  • http://www.gsm-security.net/faq/gsm-encryption.shtml
  • http://www.gsm-security.net/faq/gsm-authentication-algorithm-a3-comp128.shtml
  • http://www.gsm-security.net/faq/gsm-encryption-algorithm-a5-cipher.shtml
  • http://www.gsm-security.net/faq/gsm-key-generation-algorithm-a8-comp128.shtml
  • http://www.gsm-security.net/faq/gsm-authentication-key-generation.shtml

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