Figure 5-11 GSM authentication in Visual C#.NET

Reader UPC-A in Visual C#.NET Figure 5-11 GSM authentication

5
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Figure 5-11 GSM authentication
Recognize Bar Code In C#
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Introduction to Cellular Networks
UPC Symbol Decoder In C#
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VLR Base Station RAND, A3, K RAND Phone K1,A3
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SRES
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SRES
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Do SRES calculations match
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SIM card and in the AuC, HLR, and VLR databases The SIM card is tamper-proof, and network operators restrict physical access to these databases to minimize the risk of compromising a specific subscriber s authentication key
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Protection is provided against brute force attacks A 128-bit RAND means that there are 2128 or 34 1038 possible combinations Even if a hacker knows the A3 algorithm, calculating all the possible SRES values for each given RAND is computationally complex This makes the probability of determining a valid RAND/SRES pair very remote
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GSM Confidentiality
After a successful authentication, the GSM network and handset complete a process to establish an encrypted digital link between the handset and the network Two simple steps must be completed First, an encryption key must be generated Second, that key must be used to encrypt the communication Figure 5-12 outlines the steps 1 A SIM card takes the previously received 128-bit RAND, combines it with the Ki, and uses an A8 algorithm to generate a 64-bit session key (Kc)
Figure 5-12
Part 2 Wireless Technologies and Applications
The GSM confidentiality process
K1 RAND
HANDSET
K1 RAND
NETWORK
2 The GSM network repeats the same calculation to verify the subscriber s identity because the network knows the RAND and has a copy of the Ki 3 The Kc is combined with the A5 algorithm to create encrypted voice and data communications between the handset and the GSM network Session keys may also be reused, as this helps improve network performance and reduces the potential for delays resulting from performing multiple cryptographic operations The last step involves subscriber identification It is essential for the operator to be able to provide real-time billing information Subscriber identification is conducted with two values: the Ki and the IMSI value Because the IMSI is a unique value and can be used for illegal cloning purposes, efforts must be made to minimize the broadcast of the IMSI over the airwaves Therefore, the IMSI is only transmitted during the first time a new handset is actually used For every subsequent connection, the networks rely on the Temporary Mobile Subscriber Identity (TMSI) The TMSI is sent to the mobile station after the authentication and encryption procedures have taken place The mobile station responds by confirming the reception of the TMSI The TMSI is valid in the location area in which it was issued For communications outside the location area, the Location Area Identification (LAI) is necessary in addition to the TMSI
5
Introduction to Cellular Networks
GSM Algorithm Analysis
The GSM cryptographic algorithms (A3, A5, and A8) were developed privately by the GSM Memorandum of Understanding (MoU) member nations No global cryptographic review of the protocols or design took place As mentioned previously, the public review of cryptographic algorithms is essential to the long-term viability of any security architecture Unfortunately, GSM members did not follow an open review process Instead, GSM members created a clandestine committee, the Security Algorithm Group of Experts (SAGE), to develop security algorithms privately Although SAGE was comprised of leading cryptographers and successfully developed the A3, A5, and A8 algorithms, the closed door review process meant that the GSM members did not benefit from a global analysis of the architecture Furthermore, efforts to keep the GSM algorithms secret proved impossible The Internet accelerated interest and debate in the A series of algorithms and during the mid-1990s, information about the algorithms slowly leaked into the public domain The sudden availability of information about these algorithms, combined with the high profile of the GSM architecture, made these algorithms a tempting target for amateur and professional cryptographers alike Not surprisingly, the greater scrutiny soon exposed some rather significant weaknesses in the algorithms The first published (and provable) attacks occurred in April 1998 when researchers at the University of California, Berkeley, uncovered weaknesses in the A3/A8 algorithms The specific weaknesses enabled the cloning of GSM phones Although the initial analysis required physical access to a GSM handset and a SIM card for a period of time to complete a successful cloning exercise, the researchers later noted that over-the-air cloning was increasingly possible as well5 The Berkeley researchers analysis uncovered several startling facts First, although A3 claimed to use 64-bit keys, the last 10 bits of the keys had been left blank, resulting in an equivalent key size of only 54 bits Since 56-bit keys had already proven susceptible to massive parallelcomputing-organized brute force attacks, this information now meant that 54-bit A3 keys could be successfully attacked with brute force in a reasonable period of time
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