GSM Security And Encryption
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25-02-2009, 02:43 PM


GSM Security And Encryption

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Abstract:
The motivations for security in cellular telecommunications systems are to secure conversations and signaling data from interception as well as to prevent cellular telephone fraud. With the older analog-based cellular telephone systems such as the Advanced Mobile Phone System (AMPS) and the Total Access Communication System (TACS), it is a relatively simple matter for the radio hobbyist to intercept cellular telephone conversations with a police scanner. A well-publicized case involved a potentially embarrassing cellular telephone conversation with a member of the British royal family being recorded and released to the media.

Another security consideration with cellular telecommunications systems involves identification credentials such as the Electronic Serial Number (ESN), which are transmitted "in the clear" in analog systems. With more complicated equipment, it is possible to receive the ESN and use it to commit cellular telephone fraud by "cloning" another cellular phone and placing calls with it. Estimates for cellular fraud in the U.S. in 1993 are as high as $500 million. The procedure wherein the Mobile Station (MS) registers its location with the system is also vulnerable to interception and permits the subscriber's location to be monitored even when a call is not in progress, as evidenced by the recent highly-publicized police pursuit of a famous U.S. athlete.

The security and authentication mechanisms incorporated in GSM make it the most secure mobile communication standard currently available, particularly in comparison to the analog systems described above. Part of the enhanced security of GSM is due to the fact that it is a digital system utilizing a speech coding algorithm, Gaussian Minimum Shift Keying (GMSK) digital modulation, slow frequency hopping, and Time Division Multiple Access (TDMA) time slot architecture. To intercept and reconstruct this signal would require more highly specialized and expensive equipment than a police scanner to perform the reception, synchronization, and decoding of the signal.
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Security systems are used to secure conversations and signaling data from interception as well as to prevent cellular telephone fraud.Electronic Serial Number (ESN), is transmitted "in the clear" in analog systems. cellular telephone fraud can be done by "cloning" another cellular phone and placing calls with it after recieving this ESN . In Advanced Mobile Phone System (AMPS) and the Total Access Communication System (TACS)a radio hobbyist can simply intercept cellular telephone conversations with a police scanner.
GSM is the most secure mobile communication standard currently available. The reason for this is that it is a digital system utilizing a speech coding algorithm, slow frequency hopping,Gaussian Minimum Shift Keying (GMSK) digital modulation, and Time Division Multiple Access (TDMA) time slot architecture.It is very difficult to intercept and reconstruct this signal. The subscriberâ„¢s anonymity is ensured through the use of temporary identification numbers.

Cryptography
a.Symmetric Algorithms
They are algorithms in which the encryption and decryption use the same key. symmetric algorithms are functionally described as follows:

C=Ex(P)
P=Dx©
P=Dx(Ex(P))
where,
p:plaintext
c: ciphrtext,
Ex( ):encryption with key x
Dx( ):decryption with key x

A good example is the Data Encryption Standard (DES).Symmetric encryption algorithms may be further divided into block ciphers and stream ciphers.

GSM Security Features
It consists the following aspects:
1.subscriber identity authentication:
International Mobile Subscriber Identity (IMSI) along with individual subscriber authentication key (Ki)makes up the sensitive identification credentials.These are never transmitted over the radio channel, but a challenge-response mechanism is used to perform authentication.
2. subscriber identity confidentiality
This is done with the Temporary Mobile Subscriber Identity (TMSI). it is sent to the mobile station after the authentication and encryption procedures have taken place.To which the mobile station responds .
3. signaling data confidentiality
The user's SIM contains the ciphering key generating algorithm which is used to produce the 64-bit ciphering key (Kc). The ciphering key may be changed at regular intervals as required by network design and security considerations.
4. user data confidentiality.

The security mechanisms of GSM are implemented in three different system elements; the Subscriber Identity Module (SIM), the GSM handset or MS, and the GSM network.
SIM contains:
a) the IMSI
b)the individual subscriber authentication key (Ki)
c) the ciphering key generating algorithm (A8)
d) the authentication algorithm (A3)
e) Personal Identification Number (PIN)
GSM handset contains:
a) ciphering algorithm (A5)

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GSM SECURITY AND ENCRYPTION
INTRODUCTION:
The motivations for security in cellular telecommunications systems are to secur
conversations and signaling data from interception as well as to prevent cellular telephone fraud.
With the older analog-based cellular telephone systems such as the Advanced Mobile Phone
System (AMPS) and the Total Access Communication System (TACS), it is a relatively simple
matter for the radio hobbyist to intercept cellular telephone conversations with a police scanner Awell-publicized case involved a potentially embarrassing cellular telephone conversation with a
member of the British royal family being recorded and released to the media. Another security
consideration with cellular telecommunications systems involves identification credentials such asthe Electronic Serial Number (ESN), which are transmitted "in the clear" in analog systems. Withmore complicated equipment, it is possible to receive the ESN and use it to commit cellular
telephone fraud by "cloning" another cellular phone and placing calls with it. Estimates for cellularfraud in the U.S. in 1993 are as high as $500 million. The procedure wherein the Mobile Station(MS) registers its location with the system is also vulnerable to interception and permits thesubscriberâ„¢s location to be monitored even when a call is not in progress, as evidenced by the
recent highly-publicized police pursuit of a famous U.S. athlete.
The security and authentication mechanisms incorporated in GSM make it the most secure
mobile communication standard currently available, particularly in comparison to the analog
systems described above. Part of the enhanced security of GSM is due to the fact that it is a digitalsystem utilizing a speech coding algorithm, Gaussian Minimum Shift Keying (GMSK) digitalmodulation, slow frequency hopping, and Time Division Multiple Access (TDMA) time slotarchitecture. To intercept and reconstruct this signal would require more highly specialized andexpensive equipment than a police scanner to perform the reception, synchronization, and
decoding of the signal. In addition, the authentication and encryption capabilities discussed in thispaper ensure the security of GSM cellular telephone conversations and subscriber identificationcredentials against even the determined eavesdropper.
OVERVIEW OF GSM
GSM (group special mobile or general system for mobile communications) is the Pan-
European standard for digital cellular communications. The Group Special Mobile was establishedin 1982 within the European Conference of Post and Telecommunication Administrations (CEPT).
A Further important step in the history of GSM as a standard for a digital mobile cellular
communications was the signing of a GSM Memorandum of Understanding (MoU) in 1987 in
which 18 nations committed themselves to implement cellular networks based on the GSM
specifications. In 1991 the first GSM based networks commenced operations. GSM provides
enhanced features over older analog-based systems, which are summarized below:
Total Mobility: The subscriber has the advantage of a Pan-European system allowing him to
communicate from everywhere and to be called in any area served by a GSM cellular network
using the same assigned telephone number, even outside his home location. The calling party
does not need to be informed about the called person's location because the GSM networks are
responsible for the location tasks. With his personal chipcard he can use a telephone in a rental
car, for example, even outside his home location. This mobility feature is preferred by many
business people who constantly need to be in touch with their headquarters.
High Capacity and Optimal Spectrum Allocation: The former analog-based cellular
networks had to combat capacity problems, particularly in metropolitan areas. Through a more
efficient utilization of the assigned frequency bandwidth and smaller cell sizes, the GSM
System is capable of serving a greater number of subscribers. The optimal use of the available
spectrum is achieved through the application Frequency Division Multiple Access (FDMA),
Time Division Multiple Access (TDMA), efficient half-rate and full-rate speech coding, and
the Gaussian Minimum Shift Keying (GMSK) modulation scheme.
Security: The security methods standardized for the GSM System make it the most secure
cellular telecommunications standard currently available. Although the confidentiality of a call
and anonymity of the GSM subscriber is only guaranteed on the radio channel, this is a major
step in achieving end-to- end security. The subscriberâ„¢s anonymity is ensured through the use
of temporary identification numbers. The confidentiality of the communication itself on the
radio link is performed by the application of encryption algorithms and frequency hopping
which could only be realized using digital systems and signaling.
Services: The list of services available to GSM subscribers typically includes the following:
voice communication, facsimile, voice mail, short message transmission, data transmission and
supplemental services such as call forwarding.
GSM RADIO CHANNEL
The GSM standard specifies the frequency bands of 890 to 915 MHz for the uplink band,and 935 to 960 MHz for the downlink band, with each band divided up into 200 kHz channels.Other features of the radio channel interface include adaptive time alignment, GMSK modulation,discontinuous transmission and reception, and slow frequency hopping. Adaptive time alignmentenables the MS to correct its transmit timeslot for propagation delay. GMSK modulation providesthe spectral efficiency and low out-of-band interference required in the GSM system.Discontinuous transmission and reception refers to the MS powering down during idle periods and
serves the dual purpose of reducing co-channel interference and extending the portable unit's
battery life. Slow frequency hopping is an additional feature of the GSM radio channel interface
which helps to counter the effects of Rayleigh fading and co-channel interference.
TDMA Frame Structures, Channel Types, and Burst Types
The 200 kHz channels in each band are further subdivided into 577 ms timeslots, with 8timeslots comprising a TDMA frame of 4.6 ms. Either 26 or 51 TDMA frames are grouped intomultiframes (120 or 235 ms), depending on whether the channel is for traffic or control data.
Either 51 or 26 of the multiframes (again depending on the channel type) make up one superframe(6.12 s). A hyperframe is composed of 2048 superframes, for a total duration of 3 hours, 28minutes, 53 seconds, and 760 ms. The TDMA frame structure has an associated 22-bit sequencenumber which uniquely identifies a TDMA frame within a given hyperframe. Figure 1 illustratesthe various TDMA frame structures.
TDMA Frame Structures
The various logical channels which are mapped onto the TDMA frame structure may begrouped into traffic channels (TCHs) used to carry voice or user data, and control channels (CCHs)used to carry signaling and synchronization data. Control channels are further divided intobroadcast control channels, common control channels, and dedicated control channels.
Each timeslot within a TDMA frame contains modulated data referred to as a "burst".There are five burst types (normal, frequency correction, synchronization, dummy, and accessbursts), with the normal burst being discussed in detail here. The bit rate oftheradiochannel is270.833 kbit/sec, which corresponds to a timeslot duration of 156.25 bits. Thenormalburstiscomposed of a 3-bit start sequence, 116 bits of payload, a 26-bit training
sequence used to helpcounter the effects of multipath interference, a 3-bit stop sequence required by the channel coder,and a guard period (8.25 bit durations) which is a "cushion" to allow for different arrival times ofbursts in adjacent timeslots from geographically disperse MSs. Two bits from the 116-bit payloadare used by the Fast Associated Control Channel (FACCH) to signal Tthe structure of the normal
burst.
Normal Burst Structure
Speech Coding, Channel Coding, and Interleaving
The speech coding algorithm used in GSM is based on a rectangular pulse excited linearpredictive coder with long-term prediction (RPE-LTP). The speech coder produces samples at 20ms intervals at a 13 kbps bit rate, producing 260 bits per sample or frame. These 260 bitaredivided into 182 class 1 and 78 class 2 bits based on a subjective evaluation of their sensitivity tobit errors, with the class 1 bits being the most sensitive. Channel coding involves the addition ofparity check bits and half-rate convolutional coding of the 260-bit output of the speech coder. Theoutput of the channel coder is a 456-bit frame, which is divided into eight 57-bit components andinterleaved over eight consecutive 114-bit TDMA frames. Each TDMA frame correspondinglyconsists of two sets of 57 bits from two separate 456-bit channel coder frames. The result ofchannel coding and interleaving is to counter the effects of fading channel interference and othersources of bit errors.
Overview of Cryptography
This section provides a brief overview of cryptography, with an emphasis on the features that
appear in the GSM system.
Symmetric Algorithms
Symmetric algorithms are algorithms in which the encryption and decryption use the samekey. For example, if the plaintext is denoted by the variable P, the ciphertext by C, the encryption
with key x by the function Ex( ), and the decryption with key x by Dx( ), then the symmetric
algorithms are functionally described as follows:
C=Ex(P)
P=Dx©
P=Dx(Ex(P))
For a good encryption algorithm, the security of the data rests with the security of the key,
which introduces the problem of key management for symmetric algorithms. The most widelyknownexample of a symmetric algorithm is the Data Encryption Standard (DES). Symmetricencryption algorithms may be further divided into block ciphers and stream ciphers.
Block Ciphers
As the name suggests, block ciphers encrypt or decrypt data in blocks or groups of bits.DES uses a 56-bit key and processes data in 64- bit blocks, producing 64-bits of encrypted data for64-bits of input, and vice-versa. Block algorithms are further characterized by their mode ofoperation, such as electronic code book (ECB), cipher block chaining (CBC) and cipher feedback(CFB). CBC and CFB are examples of modes of operation where the encryption of successiveblocks is dependent on the output of one or more previous encryptions. These modes are desirablebecause they break up the one-to-one correspondence between ciphertext blocks and plaintextblocks (as in ECB mode). Block ciphers may even be implemented as a component of a streamcipher.
Stream Ciphers
Stream ciphers operate on a bit-by-bit basis, producing a single encrypted bit for a singleplaintext bit. Stream ciphers are commonly implemented as the exclusive-or (XOR) of the datastream with the keystream. The security of a stream cipher is determined by the properties of thekeystream. A completely random keystream would effectively implement an unbreakable one-timepad encryption, and a deterministic keystream with a short period would provide very littlesecurityLinear Feedback Shift Registers (LFSRs) are a key component of many stream ciphers.LFSRs are implemented as a shift register where the vacant bit created by the shifting is a functionof the previous states. With the correct choice of feedback taps, LFSRs can function as pseudorandomnumber generators. The statistical properties of LFSRs, such as the autocorrelation
function and power spectral density, make them useful for other applications such as pseudo-noise(PN) sequence generators in direct sequence spread spectrum communications, and for distancemeasurement in systems such as the Global Positioning System (GPS). LFSRs have the additionaladvantage of being easily implemented in hardware.The maximal length sequence (or m-sequence) is equal to 2n-1 where n is the degree of the shiftregister. An example of a maximal length LFSR is shown below in Figure 3. This LFSR willgenerate the periodic m-sequence consisting of the following states (1111, 0111, 1011, 0101, 1010,
1101, 0110, 0011, 1001, 0100, 0010, 0001, 1000, 1100, 1110).
Four-Stage Linear Feedback Shift Register
In order to form an m-sequence, the feedback taps of an LFSR must correspond to aprimitive polynomial modulo 2 of degree n. A number of stream cipher designs consist of multipleLFSRs with various interconnections and clocking schemes. The GSM A5 algorithm, used toencrypt voice and signaling data in GSM is a stream cipher based on three clock-controlledLFSRs.
Public Key Algorithms
Public key algorithms are characterized by two keys, a public and private key, whichperformcomplementary functions. Public and private keys exist in pairs and ideally have theproperty that the private key may not be deduced from the public key, which allows the public keyto be openly distributed. Data encrypted with a given public key may only be decrypted with thecorresponding private key, and vice versa. This is functionally expressed as follows:
C=Epub(P), P=Dpriv©
C=Epriv(P), P=Dpub©
Public key cryptography simplifies the problem of key management in that two parties may
exchange encrypted data without having exchanged any sensitive key information. Digital
Signatures also make use of public key cryptography, and commonly consist of the output of a
one-way hash function for a message (discussed in Section 3.3) with a private key. This enables
security features such as authentication and non- repudiation. The most common example of a
public key algorithm is RSA, named after its inventors Rivest, Shamir, and Adleman. The securityfeatures of GSM, however, do not make use of any type of public key cryptography.
One-Way Hash Functions
Generally, one-way hash functions produce a fixed-length output given an arbitrary input.
Secure one-way hash functions are designed such that it is computationally unfeasible to determinethe input given the hash value, or to determine two unique inputs that hash to the same value.Examples of one-way hash functions include MD5 developed by Ron Rivest, which produces a128-bit hash value, and the Secure Hash Algorithm (SHA) developed by the National Institutes ofStandards and Technology (NIST), which produces a 160-bit output.A typical application of a one-way hash function is to compute a "message digest" whichenables the receiver to verify the authenticity of the data by duplicating the computation and Comparing the results. A hash function output encrypted with a public key algorithm forms the Basis for digital signatures, such as NIST's Digital Signature Algorithm (DSA).A key-dependentone-way hash function requires a key to compute and verify the hashValue. This is useful for authentication purposes, where a sender and receiver may use a key dependentHash function in a challenge-response scheme. A key-dependent one-way hash functionMay be implemented by simply appending the key to the message and computing the hash value.Another approach is to use a block cipher in cipher feedback (CFB) mode, with the output beingthe last encrypted block (recall that in CFB mode a given block's output is dependent on the outputofprevious blocks). The A3 and A8 algorithms of GSM are key- dependent one-way hash functions. The GSM A3 and A8 algorithms are similar in functionality and are commonly implemented as a single algorithm called COMP128.
DESCRIPTION OF GSM SECURITY FEATURES The security aspects of GSM are detailed in GSM Recommendations 02.09, "SecurityAspects," 02.17,SubscriberIdentityModules," 03.20, "Security Related Network Functions," and03.21, "Security Related Algorithms". Security in GSM consists of the following aspectsConfusedubscriber identity authentication, subscriber identity confidentiality, signaling data confidentiality,and user data confidentiality. The subscriber is uniquely identified by the International MobileSubscriber Identity (IMSI). This information, along with the individual subscriber authenticationkey (Ki), constitutes sensitive identification credentials analogous to the Electronic Serial Number(ESN) in analog systems such as AMPS and TACS. The design of the GSM authentication dencryption schemes is such that this sensitive information is never transmitted over the radio channel. Rather, a challenge-response mechanism is used to perform authentication. The actual conversations are encrypted using a temporary, randomly generated ciphering key (Kc). The MS
identifies itself by means of the Temporary Mobile Subscriber Identity (TMSI), which is issued bythe network and may be changed periodically (i.e. during hand-offs) for additional security.
The security mechanisms of GSM are implemented in three different system elements; theSubscriber Identity Module (SIM), the GSM handset or MS, and the GSM network. The SIM
contains the IMSI, the individual subscriber authentication key (Ki), the ciphering key generating
algorithm (A8), the authentication algorithm (A3), as well as a Personal Identification Number
(PIN). The GSM handset contains the ciphering algorithm (A5). The encryption algorithms (A3,
A5, A8) are present in the GSM network as well. The Authentication Center (AUC), part of the
Operation and Maintenance Subsystem (OMS) of the GSM network, consists of a database of
identification and authentication information for subscribers. This information consists of the
IMSI, the TMSI, the Location Area Identity (LAI), and the individual subscriber authentication
key (Ki) for each user. In order for the authentication and security mechanisms to function, all
three elements (SIM, handset, and GSM network) are required. This distribution of security
credentials and encryption algorithms provides an additional measure of security both in
ensuringthe privacy of cellular telephone conversations and in the prevention of cellular telephone fraud.Figure 4 demonstrates the distribution of security information among the three systemelements, the SIM, the MS, and the GSM network. Within the GSM network, the security
information is further distributed among the authentication center (AUC), the home location
register (HLR) and the visitor location register (VLR). The AUC is responsible for generating the
sets of RAND, SRES, and Kc which are stored in the HLR and VLR for subsequent use in the
authentication and encryption processes.
Distribution of Security Features in the GSM Network
Authentication
The GSM network authenticates the identity of the subscriber through the use of a
challenge-response mechanism. A 128-bit random number (RAND) is sent to the MS. The MS
computes the 32-bit signed response (SRES) based on the encryption of the random number
(RAND) with the authentication algorithm (A3) using the individual subscriber authentication key
(Ki). Upon receiving the signed response (SRES) from the subscriber, the GSM network repeats
the calculation to verify the identity of the subscriber. Note that the individual subscriber
authentication key (Ki) is never transmitted over the radio channel. It is present in the subscriber's
SIM, as well as the AUC, HLR, and VLR databases as previously described. If the received SRES
agrees with the calculated value, the MS has been successfully authenticated and may continue. If
the values do not match, the connection is terminated and an authentication failure indicated to the
MS. Figure 5 shown below illustrates the authentication mechanism.
GSM Authentication Mechanism
The calculation of the signed response is processed within the SIM. This provides
enhanced security, because the confidential subscriber information such as the IMSI or the
individual subscriber authentication key (Ki) is never released from the SIM during the
authentication process.
Signaling and Data Confidentiality
The SIM contains the ciphering key generating algorithm (A8) which is used to produce
the 64-bit ciphering key (Kc). The ciphering key is computed by applying the same random
number (RAND) used in the authentication process to the ciphering key generating algorithm (A8)with the individual subscriber authentication key (Ki). As will be shown in later sections, theciphering key (Kc) is used to encrypt and decrypt the data between the MS and BS. An additionallevel of security is provided by having the means to change the ciphering key, making the systemmore resistant to eavesdropping. The ciphering key may be changed at regular intervals as requiredby network design and security considerations. Figure 6 below shows the calculation of theciphering key (Kc).
Ciphering Key Generation Mechanism
In a similar manner to the authentication process, the computation of the ciphering key(Kc) takes place internally within the SIM. Therefore sensitive information such as the individualsubscriber authentication key (Ki) is never revealed by the SIM.Encrypted voice and data communications between the MS and the network isaccomplished through use of the ciphering algorithm A5. Encrypted communication is initiated bya ciphering mode request command from the GSM network. Upon receipt of this command, themobile station begins encryption and decryption of data using the ciphering algorithm (A5) and theciphering key (Kc). Figure 7 below demonstrates the encryption mechanism.
Ciphering Mode Initiation Mechanism
Subscriber Identity Confidentiality
To ensure subscriber identity confidentiality, the Temporary Mobile Subscriber Identity
(TMSI) is used. The TMSI is sent to the mobile station after the authentication and encryption
procedures have taken place. The mobile station responds by confirming 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. The
TMSI allocation/reallocation process is shown in Figure 8 below.
TMSK Reallocation Mechanism
DISCUSSION
This section evaluates and expands on the information presented in previous sections.
Additional considerations such as export controls on crypography are discussed as well.
GSM Encryption Algorithms
A partial source code implementation of the GSM A5 algorithm was leaked to the Internet in
June, 1994. More recently there have been rumors that this implementation was an early design
and bears little resemblance to the A5 algorithm currently deployed. Nevertheless, insight into the
underlying design theory can be gained by analyzing the available information. The details of this
implementation, as well as some documented facts about A5, are summarized below:
¢ A5 is a stream cipher consisting of three clock-controlled LFSRs of degree 19, 22, and 23.
¢ The clock control is a threshold function of the middle bits of each of the three shift
registers.
¢ The sum of the degrees of the three shift registers is 64. The 64-bit session key is used to
initialize the contents of the shift registers.
¢ The 22-bit TDMA frame number is fed into the shift registers.
¢ Two 114-bit keystreams are produced for each TDMA frame, which are XOR-ed with the
uplink and downlink traffic channels.
¢ It is rumored that the A5 algorithm has an "effective" key length of 40 bits.
Key Length
This section focuses on key length as a figure of merit of an encryption algorithm.
Assuming a brute-force search of every possible key is the most efficient method of cracking an
encrypted message (a big assumption), Table 1 shown below summarizes how long it would take
to decrypt a message with a given key length, assuming a cracking machine capable of one million
encryptions per second.
Table 1 Brute-force key search times for various key sizes
Key length in bits 32 40 56 64 128
Time required to test all possible
keys
1.19
hours
12.7
days
2,291
years
584,542
years
10.8 x 10^24
years
The time required for a 128-bit key is extremely large; as a basis for comparison the age of
the Universe is believed to be 1.6x10^10 years. An example of an algorithm with a 128-bit key is
the International Data Encryption Algorithm (IDEA). The key length may alternately be examined
by determining the number of hypothetical cracking machines required to decrypt a message in a
given period of time.
Table 2 Number of machines required to search a key space in a given time
Key length in bits 1 day 1 week 1 year
40 13 2 -
56 836,788 119,132 2,291
64 2.14x10^8 3.04x10^6 584,542
128 3.9x10^27 5.6x10^26 10.8x10^24
A machine capable of testing one million keys per second is possible by todayâ„¢s standards.
In considering the strength of an encryption algorithm, the value of the information being protected
should be taken into account. It is generally accepted that DES with its 56-bit key will have
reached the end of its useful lifetime by the turn of the century for protecting data such as banking
transactions. Assuming that the A5 algorithm has an effective key length of 40 bits (instead of 64),it currently provides adequate protection for information with a short lifetime. A common
observation is that the "tactical lifetime" of cellular telephone conversations is on the order of
weeks.
Export Restrictions on Encryption Technology
The goal of the GSM recommendations is to provide a pan- European standard for digitalcellular telecommunications. A consequence of this is that export restrictions and other legalrestrictions on encryption have come into play. This is a hotly debated, highly political issue whichinvolves the privacy rights of the individual, the ability of law enforcement agencies to conductsurveillance, and the business interests of corporations manufacturing cellular hardware for export.The technical details of the encryption algorithms used in GSM are closely held secrets.The algorithms were developed in Britain, and cellular telephone manufacturers desiring toimplement the encryption technology must agree to non-disclosure and obtain special licenses
from the British government. Law enforcement and Intelligence agencies from the U.S., Britain,
France, the Netherlands, and other nations are very concerned about the export of encryption
technology because of the potential for military application by hostile nations. An additionalconcern is that the widespread use of encryption technology for cellular telephone communicationswill interfere with the ability of law enforcement agencies to conduct surveillance on terrorists ororganized criminal activity.A disagreement between cellular telephone manufacturers and the British governmentcentering around export permits for the encryption technology in GSM was settled by acompromise in 1993. Western European nations and a few other specialized markets such as HongKong would be allowed to have the GSM encryption technology, in particular the A5/1 algorithm.A weaker version of the algorithm (A5/2) was approved for export to most other countries,including central and eastern European nations. Under the agreement, designated countries such asRussia would not be allowed to receive any functional encryption technology in their GSMsystems. Future developments will likely lead to some relaxation of the export restrictions,allowing countries who currently have no GSM cryptographic technology to receive the A5/2algorithm.
ACRONYMS
A3
Authentication Algorithm
A5
Ciphering Algorithm
A8
Ciphering Key Generating Algorithm
AMPS
Advanced Mobile Phone System
AUC
Authentication Center
BS
Base Station
CBC
Cipher Block Chaining
CEPT
European Conference of Post and Telecommunication Administrations
CFB
Cipher Feedback
CKSN
Ciphering Key Sequence Number
DES
Data Encryption Standard
DSA
Digital Signature Algorithm
ECB
Electronic Code Book
ETSI
European Telecommunications Standards Institute
GMSK
Gaussian Minimum Shift Keying
GSM
Group Special Mobile
HLR
Home Location Register
IMSI
International Mobile Subscriber Identity
Kc
Ciphering Key
Ki
Individual Subscriber Authentication Key
LAI
Location Area Identity
LFSR
Linear Feedback Shift Register
MoU
Memorandum of Understanding
MS
Mobile Station
MSC
Mobile Switching Center
NIST
National Institute of Standards and Technology1
OMS
Operation and Maintenance Subsystem
RAND
Random Number
RSA
Rivest, Shamir, Adleman
SHA
Secure Hash Algorithm
SRES
Signed Response
TACS
Total Access Communications System
TMSI
Temporary Mobile Subscriber Identity
VLR
Visitor Location Register
Applications Of GSM Modem

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GSM (Global System for Mobile Communications) is worlds most famous Mobile platform. Mobile phones with SIM cards use GSM technology to help you communicate with your family, friends and business associates.
GSM systems have following advantages over basic land line telephony systems:
1.Mobility
2.Easyavailability
3.Highuptime

GSM technology is being mostly used for talking to family, friends and business colleagues. we use communication feature of Telephone landlines for internet, e-mail, data connectivity, remote monitoring, computer to computer communication, security systems. In the same way we can use GSM technology and benefit from its advantages.
Applications of GSM GPRS Modems GSM Guwahati, GSM Gurgaon, GSM Haridwar, GSM Hyderabad, GSM Indore, GSM Jaisalmer, GSM Jodhpur, GSM Kanyakumari, GSM Kashmir, GSM Khajuraho, GSM Kodaikanal, GSM Kolkata, GSM Lonavala, GSM Lucknow, GSM Ludhiana, GSM Madurai, GSM Manali, GSM Mumbai, GSM Mussoorie, GSM Munnar, GSM Mysore, GSM Nainital, GSM Ooty, GSM Patna, GSM Pune, GSM Pushkar, GSM Rajkot, GSM Shimla, GSM Udaipur, GSM Varanasi,GPRS and GPS Tracking & Security System for Car, Truck, Fleet & Asset management.

You can use GSM technology for following applications:
Access control devices: Now access control devices can communicate with servers and security staff through SMS messaging. Complete log of transaction is available at the head-office Server instantly without any wiring involved and device can instantly alert security personnel on their mobile phone in case of any problem. RaviRaj Technologies is introducing this technology in all Fingerprint Access control and time attendance products. You can achive high security any reliability.

Transaction terminals: EDC machines, POS terminals can use SMS messaging to confirm transactions from central servers. The main benefit is that central server can be anywhere in the world. Today you need local servers in every city with multiple telephone lines. You save huge infrastructure costs as well as per transaction cost.

Supply Chain Management: Today SCM require huge IT infrastructure with leased lines, networking devices, data centre, workstations and still you have large downtimes and high costs. You can do all this at a fraction of the cost with GSM M2M technology. A central server in your head office with GSM capability is the answer, you can receive instant transaction data from all your branch offices, warehouses and businessassociateswithnildowntimeandlowcost.
If your application needs one or more of the following features, GSM will be more cost-effective then other communication systems.

Short Data Size: You data size per transaction should be small like 1-3 lines. e.g. banking transaction data, sales/purchase data, consignment tracking data, updates. These small but important transaction data can be sent through SMS messaging which cost even less then a local telephone call or sometimes free of cost worldwide. Hence with negligible cost you are able to send critical information to your head office located anywhere in the world from multiple points. You can also transfer faxes, large data
through GSM but this will be as or more costly compared to landline networks.

Multiple remote data collectio: n points If you have multiple data collections points situated all over your city, state, country or worldwide you will benefit the most. The data can be sent from multiple points like your branch offices, business associates, warehouses, agents with devices like GSM modems connected to PCs, GSM electronic terminals and Mobile phones. Many a times some places like warehouses may be situated at remote location may not have landline or internet but you will have GSM network still availableeasily.

High uptime: If your business require high uptime and availability GSM is best suitable for you as GSM mobile networks have high uptime compared to landline, internet and other communication mediums. Also in situations where you expect that someone may sabotage your communication systems by cutting wires or taping landlines, you can dependonGSMwirelesscommunication.

Large transaction volumes: GSM SMS messaging can handle large number of transaction in a very short time. You can receive large number SMS messages on your server like e-mails without internet connectivity. E-mails normally get delayed a lot but SMS messages are almost instantaneous for instant transactions. consider situation like shop owners doing credit card transaction with GSM technology instead of conventional landlines. many a time you find local transaction servers busy as these servers use multiple telephone lines to take care of multiple transactions, whereas one GSM connection is enough to handle hundreds oftransactionperminute.

Mobility, Quick installation: GSM technology allow mobility, GSM terminals, modems can be just picked and installed at other location unlike telephone lines. Also you can be mobile with GSM terminals and can also communicate with server using your mobile phone. You can just purchase the GSM hardware like modems, terminals and mobile handsets, insert SIM cards, configure software and your are ready for GSM communication. GSM solutions can be implemented within few weeks whereas it may take many months to implement the infrastructure for other technologies.
Indian cities GPRS Agra, GPRS Ahmedabad, GPRS Ajmer, GPRS Allahabad, GPRS Amritsar, GPRS Aurangabad, GPRS Bangalore, GPRS Bharatpur, GPRS Bhopal, GPRS Bikaner, GPRS Calicut, GPRS Chandigarh, GPRS Chennai Madras, GPRS Cochin-Kochi, GPRS Coimbatore, GPRS Darjeeling, GPRS Dehradun, GPRS Gangtok, GPRS Guwahati, GPRS Gurgaon, GPRS Haridwar, GPRS Hyderabad, GPRS Indore, GPRS Jaisalmer, GPRS Jodhpur, GPRS Kanyakumari, GPRS Kashmir, GPRS Khajuraho, GPRS Kodaikanal, GPRS Kolkata, GPRS Lonavala, GPRS Lucknow, GPRS Ludhiana, GPRS Madurai, GPRS Manali, GPRS Mumbai, GPRS Mussoorie, GPRS Munnar, GPRS Mysore, GPRS Nainital, GPRS Ooty, GPRS Patna, GPRS Pune, GPRS Pushkar, GPRS Rajkot, GPRS Shimla, GPRS Udaipur, GPRS Varanasi,GSM and USA states GSM Alabama, GSM Alaska, GSM Arizona, GSM Arkansas, GSM California, GSM Colorado, GSM Connecticut, GSM Delaware, GSM Columbia, GSM Florida, GSM Georgia, GSM Hawaii, GSM Idaho, GSM Illinois, GSM Indiana, GSM Iowa, GSM Kansas, GSM Kentucky, GSM Louisiana, GSM Maine, GSM Maryland, GSM Massachusetts, GSM Michigan, GSM Minnesota, GSM Mississippi, GSM Missouri, GSM Montana, GSM Nebraska, GSM Nevada, GSM New Hampshire, GSM New Jersey, GSM New Mexico, GSM New York, GSM North Carolina, GSM North Dakota, GSM Ohio, GSM Oklahoma, GSM Oregon.
GSM Pennsylvania, GSM Rhode Island, GSM South Carolina, GSM South Dakota, GSM Tennessee, GSM Texas, GSM Utah, GSM Vermont, GSM Virginia, GSM Washington, GSM West Virginia, GSM Wisconsin, GSM Wyoming,GPRS and USA states GPRS Alabama, GPRS Alaska, GPRS Arizona, GPRS Arkansas, GPRS California, GPRS Colorado, GPRS Connecticut, GPRS Delaware, GPRS Columbia, GPRS Florida, GPRS Georgia, GPRS Hawaii, GPRS Idaho, GPRS Illinois, GPRS Indiana, GPRS Iowa, GPRS Kansas, GPRS Kentucky, GPRS Louisiana, GPRS Maine, GPRS Maryland, GPRS Massachusetts, GPRS Michigan, GPRS Minnesota, GPRS Mississippi, GPRS Missouri, GPRS Montana, GPRS Nebraska, GPRS Nevada, GPRS New Hampshire, GPRS New Jersey, GPRS New Mexico, GPRS New York, GPRS North Carolina, GPRS North Dakota, GPRS Ohio, GPRS Oklahoma, GPRS Oregon, GPRS Pennsylvania, GPRS Rhode Island, GPRS South Carolina, GPRS South Dakota, GPRS Tennessee, GPRS Texas, GPRS Utah, GPRS Vermont, GPRS Virginia, GPRS Washington, GPRS West Virginia, GPRS Wisconsin, GPRS Wyoming,GSM and USA cities GSM Amarillo, GSM Baton Rouge, GSM Bartow, GSM Billings, GSM Charleston, GSM Clovis, GSM Columbia, GSM Des Moines, GSM Dodge, GSM Greeley, GSM Jackson , GSM Jefferson, GSM Kearney, GSM Lexington, GSM Little Rock, GSM Louisville, GSM Madison, GSM Montgomery, GSM Moses Lake, GSM Nashville, GSM New Holland, GSM Oklahoma, GSM Phoenix, GSM Portland, GSM Raleigh, GSM Richmond, GSM St. Joseph, GSM San Angelo, GSM Salt Lake, GSM Sioux Falls, GSM Springfield, GSM South St. Paul, GSM West Fargo, GSM Thomasville, GSM Torrington, GSM Washington,GPRS and USA cities GPRS Amarillo, GPRS Baton Rouge, GPRS Bartow, GPRS Billings, GPRS Charleston, GPRS Clovis, GPRS.



CONCLUSION
The security mechanisms specified in the GSM standard make it the most
secure cellular telecommunications system available. The use of authentication,
encryption, and temporary identification numbers ensures the privacy and anonymity
of the system's users, as well as safeguarding the system against fraudulent use.
Even GSM systems with the A5/2 encryption algorithm, or even with no encryption
are inherently more secure than analog systems due to their use of speech coding,
digital modulation, and TDMA channel access.
REFERENCES
1. Van der Arend, P. J. C., "Security Aspects and the Implementation in the GSM System,"
Proceedings of the Digital Cellular Radio Conference, Hagen, Westphalia, Germany,
October, 1988.
2. Biala, J., "Mobilfunk und Intelligente Netze," Friedr., Vieweg & Sohn Verlagsgesellschaft,
1994.
3. Cooke, J.C.; Brewster, R.L., "Cyptographic Security Techniques for Digital Mobile
Telephones," Proceedings of the IEEE International Conference on Selected Topics in
Wireless Communications, Vancouver, B.C., Canada, 1992.
4. European Telecommunications Standards Institute, Recommendation GSM 02.09,
"Security Aspects".
5. European Telecommunications Standards Institute, Recommendation GSM 02.17,
"Subscriber Identity Module".
6. European Telecommunications Standards Institute, Recommendation GSM 03.20,
"Security Related Network Functions".
7. Hodges, M.R.L., "The GSM Radio Interface," British Telecom Technology Journal, Vol. 8,
No. 1, January 1990, pp. 31-43.
8. Hudson, R.L., "Snooping versus Secrecy," Wall Street Journal, February 11, 1994, p. R14
9. Schneier, B., "Applied Cryptography," J. Wiley & Sons, 1994.
10. Williamson, J., "GSM Bids for Global Recognition in a Crowded Cellular World,"
Telephony, vol. 333, no. 14, April 1992, pp. 36-40.
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19-10-2010, 12:50 PM


.docx   GSM Modems.docx (Size: 81 KB / Downloads: 37)
GSM Modems


GSM Modems

A GSM modem can be an external modem device, such as the Siemens MC35 or Wavecom FASTRACK external modems. Insert a GSM SIM card into this modem, and connect the modem to an available serial port on your computer.
A GSM modem can be a PC Card installed in a notebook computer, such as the Sierra Wireless Aircard 750.
A GSM modem could also be a standard GSM mobile phone with the appropriate cable and software driver to connect to a serial port or USB port on your computer. Any phone that supports the "extended AT command set" for sending/receiving SMS messages, as defined in the ETSI GSM 07.05 Specification can be supported by the Now SMS/MMS Gateway.
A dedicated GSM modem (external or PC Card) is usually preferable to a GSM mobile phone. This is because of some compatibility issues that can exist with mobile phones. For example, if you wish to be able to receive inbound MMS messages with your gateway, most GSM phones will only allow you to send MMS messgaes. This is because the mobile phone automatically processes received MMS message notifications these messages, without forwarding them via the modem interface. Similarly some mobile phones will not allow you to correctly receive SMS text messages longer than 160 bytes (known as "concatenated SMS" or "long SMS"). This is because these long messages are actually sent as separate SMS messages, and the phone attempts to reassemble the message before forwarding via the modem interface. (We've observed this latter problem utilizing the Ericsson R380, while it does not appear to be a problem with many other Ericsson models.)
When you install your GSM modem, or connect your GSM mobile phone to the computer, be sure to install the appropriate Windows modem driver from the device manufacturer. To simplify configuration, the Now SMS/MMS Gateway will communicate with the device via this driver. If a Windows driver is not available for your modem, you can use either the "Standard" or "Generic" 19200 bps modem driver that is built into windows. A benefit of utilizing a Windows modem driver is that you can use Windows diagnostics to ensure that the modem is communicating properly with the computer.
The Now SMS/MMS gateway can simultaneously support multiple modems, provided that your computer hardware has the available communications port resources.
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What is GSM ?
Global System for Mobile (GSM) is second generation cellular standard developed to cater voice services and data delivery using Digital modulation.
GSM in India
Tele-Services

 Telecommunication services that enable voice communication via mobile phones.
 Offered services
- Mobile telephony
 Bearer Services
 Include various data services for information transfer between GSM and other networks like PSTN, ISDN etc at the rate from 300 to 9600 bps.
 Short message services
- up to 160 character alphanumeric data transmission to/from the mobile terminal.
 Unified Messaging Services (UMS).
Supplementary Services
 Call related services-
 Call Waiting- Notification of an incoming call while on the handset.
 Call Hold- Put a caller on hold to take another call.
 Call Barring- All calls, outgoing calls, incoming calls.
 Call forwarding- Call can be send to various numbers defined by user.
 Multi Party Call Conferencing- Link multiple calls together.
 CLIP- Caller line identification presentation.
 CLIR- Caller line identification restriction.
GSM System Architecture-1
Mobile Station (MS)

Mobile Equipment (ME)
Subscriber Identity Module (SIM)
Base Station Subsystem (BSS)
Base Transceiver Station (BTS)
Base Station Controller (BSC)
Network Switching Subsystem (NSS)
Mobile switching Center (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Authentication Center (AUC)
Equipment Identity Register (EIR)
System Architecture Mobile Station
 The Mobile Station is made up of two entities:
1. Mobile Equipment (ME)
2. Subscriber Identity Module (SIM)
Therefore, ME+SIM=MS
Mobile Station
Mobile Equipment (ME)

 Uniquely identified by an IMEI (International Mobile Equipment Identity)
 Voice & data transmission
 Monitoring power & signal quality of surrounding cells for optimum handover
 Power level: 0.8W-20W
Mobile Station
Subscriber Identity Module (SIM)

 Smart card contains the International Mobile Subscriber Identity (IMSI).
 Allows user to send & receive calls and receive other subscribed services.
 Protected by a password or PIN.
System Architecture Base Station Subsystem (BSS)
 BSS is composed of two parts.
1.Base Transceiver Station (BTS)
2.Base Station Controller (BSC)
Base Station Subsystem (BSS)
Base Transceiver Station (BTS)

 Consist of Transceivers (TRX) units.
 Encodes, multiplexes, modulates & feed the RF signal to the antenna.
 Communicates with Mobile Station & BSC.
Base Station Controller (BSC)
 Manages resources for BTS.
 Handles call set up.
 Radio power control.
 It communicates with MSC & BTS.
System Architecture Network Switching System (NSS)
Mobile Switching center (MSC)

 Heart of the network.
 Manages communication between GSM & other networks.
 Call setup function & basic switching.
 Call routing.
 Billing information & collection.
Network Switching System (NSS)
Home Location Register (HLR)
-It is a Static database, when a user applied for mobile services, all data about this subscriber will be store in HLR.
-Database contains IMSI ,MSISDN, prepaid/postpaid,roaming restrictions, supplementary services.
Visitor Location register (VLR)
-VLR is Dynamic database used by MSC for information index, it stores all related information of mobile subscriber that enter into the coverage area.
-Reduces the no. of queries to HLR.
-VLR can built together with MSC or set separately.
Network Switching System (NSS)
Authentication Center (AUC)

-Element to prevent illegal subscriber from accessing
GSM network.
-It can generate the parameters to confirm subscriber’s identity.
-Generally associated with HLR.
Equipment Identity Register (EIR)
-Database that is used to track handsets using IMEI.
-Made up of three sub class; White list, Black list &
Gary list.
Operation & Maintenance Subsystem (OSS)
 Used for network planning to enhance the overall working efficiency and service quality of the system.
Parts of OSS
 It contains two parts;
1.OMC-S: Responsible for the operation &
maintenance related to MSS.
2.OMC-R: Responsible for the operation &
maintenance related to BSS.
Call Routing Outgoing Call
1. MS sends dialled number to BSS
2. BSS sends dialled number to MSC
3,4 MSC checks VLR if MS is allowed the requested service.If so MSC asks BSS to allocate resources for call.
5 MSC routes the call to GMSC
6 GMSC routes the call to local exchange of called user
7, 8,
9,10 Answer back(ring back) tone is routed from called user to MS via GMSC,MSC,BSS
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i want to know more information about the topic "GSM Security And Encryption"
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