The Truth About Gsm Cell Phone Security
Introduction
Every day millions of people use gsm cellular phones over radio links. With the increasing features, the mobile phone is gradually becoming a handheld computer. In the early 1980's, when most of the mobile telephone system was analog, the inefficiency in managing the growing demands in a cost-effective manner led to the opening of the door for digital technology (Huynh & Nguyen, 2003). According to Margrave (n.d), "With the older analog-based cellular telephone systems such as the Advanced Mobile Phone System (AMPS) and the Total Access Communication System (TACS)", cellular fraud is extensive. It's very simple for a radio hobbyist to tune in and hear cellular telephone conversations since without encryption, the voice and user data of the subscriber is sent to the network (Peng, 2000). Margrave (n.d) states that apart from this, cellular fraud can be committed by using complex equipment to receive the Electronic Serial Number so as to clone another mobile phone and place calls with that. To counteract the aforementioned cellular fraud and to make mobile phone traffic secure to a certain extent, GSM (Global System for Mobile communication or Group Special Mobile) is one of the many solutions now out there. According to GSM-tutorials, formed in 1982, GSM is a worldwide accepted standard for digital cellular communication. GSM operates in the 900MHz, 1800MHz, or 1900Mhz frequency bands by "digitizing and compressing data and then sending it down a channel with two other streams of user data, each in its own time slot." A GSM provides a secure and confidential method of communication.
Security provided by GSM
The limitation of security in cellular communication is a result of the fact that all cellular communication is sent over the air, which then gives rise to threats from eavesdroppers with suitable receivers. Keeping this in account, security controls were integrated into GSM to make the system as secure as public switched telephone networks. The security functions are:
1. Anonymity: It implies that it is not simple and easy to track the user of the system. According to Srinivas (2001), when a new GSM subscriber switches on his/her phone for the first time, its International Mobile Subscriber Identity (IMSI), i.e. real identity is used and a Temporary Mobile Subscriber Identity (TMSI) is issued to the subscriber, which from that time forward is always used. Use of this TMSI, prevents the recognition of a GSM user by the potential eavesdropper.
2. Authentication: It checks the identity of the holder of the smart card and then decides whether the mobile station is allowed on a particular network. The authentication by the network is done by a response and challenge method. A random 128-bit number (RAND) is generated by the network and sent to the mobile. The mobile uses this RAND as an input and through A3 algorithm using a secret key Ki (128 bits) assigned to that mobile, encrypts the RAND and sends the signed response (SRES-32 bits) back. Network performs the same SRES process and compares its value with the response it has received from the mobile so as to check whether the mobile really has the secret key (Margrave, n.d). Authentication becomes successful when the two values of SRES matches which enables the subscriber to join the network. Since every time a new random number is generated, eavesdroppers don't get any relevant information by listening to the channel. (Srinivas, 2001)
3. User Data and Signalling Protection: Srinivas (2001) states that to protect both user data and signalling, GSM uses a cipher key. After the authentication of the user, the A8 ciphering key generating algorithm (stored in the SIM card) is used. Taking the RAND and Ki as inputs, it results in the ciphering key Kc which is sent through. To encipher or decipher the data, this Kc (54 bits) is used with the A5 ciphering algorithm. This algorithm is contained within the hardware of the mobile phone so as to encrypt and decrypt the data while roaming. Algorithms used to make mobile traffic secure
Authentication Algorithm A3: One way function, A3 is an operator-dependent stream cipher. To compute the output SRES by using A3 is easy but it is very difficult to discover the input (RAND and Ki) from the output. To cover the issue of international roaming, it was mandatory that each operator may choose to use A3 independently. The basis of GSM's security is to keep Ki secret (Srinivas, 2001)
Ciphering Algorithm A5: In recent times, many series of A5 exists but the most common ones are A5/0(unencrypted), A5/1 and A5/2. Because of the export regulations of encryption technologies there is the existence of a series of A5 algorithms (Brookson, 1994).
A8 (Ciphering Key Generating Algorithm): Like A3, it is also operator-dependent. Most providers combine A3 and A8 algorithms into a single hash function known as COMP128. The COMP128 creates KC and SRES, in a single instance (Huynh & Nguyen, 2003).
GSM security flaws
Security by obscurity. According to (Li, Chen & Ma) some people asserts that since the GSM algorithms are not publicized so it is not a secure system. "Most security analysts believe any system that is not subject to the scrutiny of the world's best minds can't be as secure." For instance, A5 was never made public, only its description is divulged as part of the GSM specification.
Another limitation of GSM is that although all communication between the Mobile station and the Base transceiver station are encrypted, in the fixed network all the communication and signalling is not protected as it is transmitted in plain text most of the time (Li, Chen & Ma).
One more problem is that it is hard to upgrade the cryptographic mechanisms timely.
Flaws are present within the GSM algorithms. According to Quirke (2004) " A5/2 is a deliberately weakened version of A5/1, since A5/2
Oh, and speaking of Verizon, I got word today that we may be seeing another VZW handset or two thrown into the pot before Summer's over. So Happy Friday and Happy July 4th from your friends at PhoneDog and LG!
]]> PSP-Phone: Is Sony planning a Playstation phone and leaving Ericsson out? MarketingWeek has an article dated June 25 claiming that Sony is working on a Playstation-branded mobile phone. This isn't news, or isn't new news anyway, but beyond the strange use of the word "mooting" in the first sentence, the article is notable for claiming that the new device won't be a Sony Ericsson phone, but rather will bear only the Sony moniker.
The launch of the SE F305 gaming phone drew a few, "Hmm ... I thought a PSP phone was coming," comments from the media, but it seems that Sony may be looking to keep the PSP brand all to their selves. The Marketing Week article cites an industry source as saying, "the relationship between Sony and Sony Ericsson has become 'frosty' and he adds that Sony has learned from its 'strategic error' of giving its Walkman branding to Sony Ericsson."
SE's profits were off almost 50% in Q1 of this year, and they've fallen into fifth place in the global handset market. Whether or not Sony's looking to distance themselves from the Ericsson partnership remains to be seen, but a well-executed PSP phone could prove a big hit. Look for it in time for Christmas '09, if Marketing Week is to be believed.
Users need to use Safari on iPhone to get to Google Talk. When you navigate away from the page on your web browser, status is set to unavailable, and the session restarts when you return. So, it's not as integrated as a downloadable app, but it'll do for a quick fix.
You can select from a list of the people you talk to the most, search your contacts, and manage multiple conversations.
]]> Google maps for mobile with voice search for BlackBerry Pearl 8110, 8120, and 8130, US only). Using the same speech recognition engine as GOOG-411, the free directory assistance service, Voice Search for Google Maps is great in situations when you can text...like while driving in California as of this week.]]> Touch Diamond: Telus Becomes First to Launch Diamond in North America HTC Touch Diamond before too long, our friends to the north don't have to wait much longer. HTC just sent word that Telus has cracked open the seal on their fancy new Windows Mobile smartphone here in North America:
"Telus will launch a CDMA version of the HTC Touch Diamond later this summer for $149.99 with a 3 year contract. To make the HTC Touch Diamond even more attractive, they offer subscribers very affordable unlimited data plans like $15 per month for unlimited email and IM, or $30 for unlimited email, IM and web browsing. You and your readers can learn more about Telus and the HTC Touch Diamond at: http://www.telusmobility.com/on/pcs/data.shtml"
$149's a nice price, but a three year contract? Ouch.
MMS isn't a new technology by any means, but the first-gen iPhone doesn't have it. Conventional wisdom says that Apple would just as soon have you use their Mobile Me online service, and not MMS messaging, for your photo sharing needs since Apple charges $100/year for Mobile Me. But AT&T is laying out all kinds of cash to subsidize iPhone 3G's new, lower price, and they'd stand to recoup some of that money by way charging for iPhone MMS messages.
It's worth noting that the new iPhone plan does not include messaging of any sort (the "old" iPhone plan does). Current AT&T messaging plans start at $5/month for 200 messages. As with everything else iPhone 3G, all will become clear when the next-gen of Apple's mobile handset goes on sale July 11.
I am personally grateful for the free Samsung Mobile Charging Station service, which I coincidentally took advantage of yesterday at one of the 51 charging stations in Los Angeles at LAX.
In addition to MSP, power thirsty and travelers desperate for a little battery boost can take advantage of Samsung's generosity at the following airports: JFK, LAX, LaGuardia, Orlando International and of course, Dallas/Fort Worth.
iPhone 3G will be available for $199 for the 8GB model and $299 for the 16GB model. These prices require two-year contracts and are available to the following customers:
iPhone customers who purchased before July 11
Customers activating a new line with AT&T
Current AT&T customers who are eligible, at the time of purchase, for an upgrade discount
Existing AT&T customers who are not currently eligible for an upgrade discount can purchase iPhone 3G for $399 for the 8GB model or $499 for the 16GB model. Both options require a new two-year service agreement. In the future, AT&T will offer a no-contract-required option for $599 (8GB) or $699 (16GB).
Current customers can also wait until they become eligible for the upgrade discount. Current AT&T customers who are upgrading to iPhone 3G will pay an $18 upgrade fee and new AT&T customers will pay the standard $36 activation fee.
Voice, Data and Text Messaging Plans
AT&T NationSM Unlimited: Includes unlimited Anytime Minutes for $129.99 a month.
AT&T Nation 1350: Includes 1350 Anytime Minutes and unlimited Night & Weekend Minutes for $109.99 a month.
AT&T Nation 900: Includes 900 Anytime Minutes and unlimited Night & Weekend Minutes for $89.99 a month.
AT&T Nation 450: Includes 450 Anytime Minutes and 5,000 Night & Weekend Minutes for $69.99 a month.
iPhone 3G goes on sale at AT&T retail stores at 8 a.m. local time on Friday, July 11.
Looks like that July ship date is now looking more like August - even mid-August according to Boy Genius Report. RIM showed Bold off at a press event in New York City Wednesday night (which I couldn't attend because I'm headed out to Montana for a wedding and mini-vacation, thankyouverymuch), and a RIM representative made it clear that the Bold was planned for an August launch all along.
"We didn't say anything about 'July' or 'August.' It will become available in August. So we're still on track for [availability] this summer," said Erik Van Drunen, RIM's PR manager, according to BetaNews.com. Van Drunen also denied knowledge of purported overheating problems with the device. "I don't know where that's coming from at all, but it's probably from somewhere in the blogosphere," he said.
Set up to help web developers create more function-rich, user friendly and relevant applications, the initiative is designed to drive the use of data services on mobile.
But now a photo of Paris in a new color scheme - black and orange - has surfaced over on Just Another Mobile Phone Blog. So what gives? It's possible that this is a photo of a prototype built before SE decided to scrap the project, and it's possible that SE's going forward with Paris and just wasn't ready to announce the handset last week. Many a blog post, comment, and forum entry decried Paris' rumored specs as not being robust enough for a modern smartphone, so it could well be that SE brass sent the device back to its engineering labs for a processor transplant before deeming it ready for the marketing folk.
]]> Unlimited: Virgin Mobile to offer $80/mo unlimited calling Virgin Mobile. The "Totally Unlimited" plan will work nationwide with no contract necessary. You'll be able to pay via cash or credit, and you're not going to get hit with roaming charges, which is a big advantage over similarly priced regional offerings.
Another $10 each month will get you unlimited SMS/MMS/Email/IM messaging on top of all of those minutes. Looks like a pretty good deal for anyone who makes it a personal practice to avoid contractual agreements like the plague.
]]> Wi-Fi on the go: Alltel announces $70 3G/Wi-Fi plan with Boingo Alltel, announced a partnership with hotspot provider Boingo to add some WiFi to their customers' 3G data diet. $69.98 each month will now get you unlimited handset data via EV-DO along with unlimited access to Boingo Hotspots around the globe.
Alltel's calling the plan the Internet Anywhere Bundle, and it rings up at $10/mo. more than their 3G only Internet plan. I guess it's a decent buy if you use 3G on your handset a lot, but also rely on the faster speeds of WiFi for moving larger chunks of data around with your laptop.
We'll have to see how this all shakes after that Verizon purchase of Alltel goes through.
I no longer have a Sprint Touch and so can't personally vouch for this file, but from the comments on the XDA Developers forum and over at Engadget Mobile, it looks like this is the real deal and "is a breeze to install." Proceed at your own risk, kids.
]]> Samsung Instinct-M800 in-depth review - At Glance What's Good: Supports every feature Sprint offers; EV-DO Rev. A data speeds; Good touchscreen response; Bright, clear display; Excellent GPS/navigation and voice command features; Good visual voicemail feature; 3.5mm headphone jack and stereo Bluetooth; Second battery included in packaging; Excellent value at $129 retail price
What's Not Good: "Full HTML" Web browser is disappointing; No IM support; No calendar synching; No support for user-created ringtones or background images on menu screens; No phone as modem support on mandatory Simply Everything plan
Bottom Line: Sprint's new flagship media phone is a very good device, even if I wish they'd skipped the whole "Instinct vs iPhone" marketing campaign. The Web browser needs work and mobile IM addicts will be left in the cold by Instinct, but otherwise Instinct offers solid support across a huge variety of features and excellent implementation of GPS-based navigation and voice activated commands in particular. Though plans start at a hefty $70/mo, if you use the unlimited data, navigation, and Sprint TV it's actually a good value. And you can't beat the $129 price tag.
Specs: Make/Model: Samsung Instinct (M800) Network: CDMA 850 / 1900 Data: 1x EV-DO Rev. A (3G) Carrier: Sprint Size: 116 x 55 x 12.4 mm Weight: 128 g Form Factor: Candybar with Full Touchscreen Display: 3.2" Color LCD, 240 x 432 resolution, 262,000 Colors Memory: 32 MB built-in, microSD card slot, 2GB memory card included Notable Features: GPS with navigation; Support for Sprint Music Store, Sprint TV, Streaming music services; Voice control system; First Sprint EV-DO Rev. A device for consumers
]]> Shadow II: T-Mobile Smartphone May Support HotSpot@Home TmoNews has a little more dirt on the upcoming Shadow II, and whlie its nothing groundbreaking it's at least promising. The next-gen Shadow looks much like the current version, with a few more rounded corners and some other cosmetic updates. The big deal here is that it looks like Shadow II will support TMo's Hotspot@Home UMA calling service, making it the carrier's first H@H-compatible WindowsMobile device.
While it'd be nice to see a high-end T-Mobile device with 3G support, I really liked Shadow when I reviewed it awhile back, and HotSpot@Home keeps getting better and better. TMo's really playing up the value angle as of late (well, as of always), and while they're often last to the party when it comes to new phone tech, they also often get it right when they finally do it. With many of the kinks worked out of the whole UMA calling thing - using the performance of the recent BlackBerry Curve and Pearl as an indicator - here's hoping that T-Mo will soon unleash some uber-phones combining "Value Tech" like myFaves and H@H with some sex appeal like high-speed data.
]]> Rock On: T-Mobile Launches MOTO ROKR E8 I was pretty impressed by the E8 when I played with it at CES in January. The "ModeShift" morphing keypad is a pretty clever way of offering context-sensitive controls and menus on a non-touchscreen phone, and the music player looked pretty nice. Think of the E8 as a higher-end Nokia 5310.
You can head on over to TMo's site for some product shots and specs, but there's no pricing or availability info up yet (as of 10 PM Pacific Time Wed night, anyway). If you're in the market for a relatively thin candybar phone with a 2MP camera, 3.5mm headphone jack and Stereo Bluetooth support, and a unique look, the E8 might just be your ticket. You'll be limited to EDGE data speeds, but if you're on TMo you're used to that already.
We've covered Dare quite a bit already in these pages so I won't go into too much more detail here, except to say that I wish they'd gone with a true Xenon flash instead of an LED flash on the camera. But hey, LED's better than nothing. I guess I can be bothered to cut and paste the full spec sheet from the press release, too. More just as soon as I get my paws on a review sample.
From the Press Release:
The LG Dare's touch-screen offers the optimal HTML Web browsing experience and runs on the nation's most reliable wireless network to ensure a smooth and easy navigation experience. The touch-screen also supports handwriting recognition and tactile feedback so that customers can more easily select their options on the phone. The phone's camera has a Schneider-Kreuznach certified lens for outstanding image quality, and the camera offers advanced features, such as face detection and an LED flash so customers can take enhanced-quality photos from their mobile phones. Additionally, customers can use the photo editing tool to personalize their digital photos with features that include borders and landscapes, and they can draw on their pictures directly on the screen using their fingertips.
Additional features and capabilities of LG Dare include:
V CAST Music-capable access more than 3 million full-length songs from well-known and independent artists that can be purchased and downloaded over-the-air directly to the phone
Music Player for .mp3, .wma, unprotected .aac, and unprotected .aac+ files
microSD? card memory port with up to 8 GB support with an optional memory card
USB Mass Storage transfer files between microSD card and PC
3.2 Megapixel Camera and Camcorder
Schneider-Kreuznach certified lens for optimal picture quality
Camera Resolutions: 2048 x 1536, 1600 x 1200, 1280 x 960, 640 x 480, 320 x 240 pixels
SmartPic technology and manual ISO adjustment for improved image quality
Face Detection detects a face when taking a picture
Panorama, split shot and frame camera settings
Scene Mode automatically makes adjustments to camera settings for improved quality in different environments
Zoom: up to 2x
Image Editor zoom, rotate, crop, add frames and icons and write over images
Video Resolutions: 640 x 480, 320 x 240, 176 x 144 pixels
Video Recording Speed: up to 120 frames per second for slow video playback
Video Recording Time: 30 seconds (for sending); up to 1 hour (for saving) depending on memory
Video Player for WMV, MP4, 3GP, 3G2 formats
Customizable brightness, white balance, shutter sound, color effects, night and preview mode, photometry, self-timer
Bluetooth® Version 2.1 + EDR (Enhanced Data Rate)
Listen to music with optional stereo Bluetooth headset
Send all contacts and calendar events via Bluetooth
Print and send customer-generated pictures (JPEG) via Bluetooth
Save up to 20 Bluetooth pairings
Supported Profiles: headset, hands-free car kits, dial-up networking, stereo Bluetooth, phone book access, basic printing, object push for vCard and vCalendar, file transfer, basic imaging, human interface device
One-touch speakerphone
Speaker-independent voice commands
Voice Recording: 1 minute or 1 hour (standby)
Music ringer support (clips from hit songs)
HTML Web browsing with touch navigation and favorites
Text, picture, video messaging-capable
Mobile Email access MSN® Hotmail®, America Online® and Yahoo!® accounts to exchange e-mail
Mobile IM capabilities
VZ NavigatorSM-capable get visual and audible directions to thousands of destinations, locate businesses and other points of interest, get maps of a location, and share directions with others
Phone Book with 1,000 contacts: each contact entry stores five numbers, two e-mail addresses and a Picture ID
]]> The Sound of Money: Sprint's Selling Loads of Instincts Samsung Instinct is Sprint's flashiest, most hyped device ever, has received mainly good reviews since it launched a week ago, and was priced at a bargain basement $129 on contract, this shouldn't come as much of a surprise. Sprint sent out word this morning that Instinct's broken all sorts of sales records in its first week on the shelves, and word around the InterWeb is that the handset is sold out all over the place.
"We had high expectations going into the launch so our initial order to Samsung was the largest for any Sprint EVDO handset to date," said John Garcia, President of Sprint's Wireless Division. "The strong early response tells us that wireless customers recognize Instinct as a highly-innovative and convenient touch-screen device combined with the fast speeds available on the largest national mobile broadband network. In the first few days of availability, many Instinct devices were purchased by existing customers upgrading their wireless device we thank our customers for their endorsement of this device, our Simply Everything pricing plan and our company."
If you're interested, I've posted about three thousand minutes worth of video detailing Instinct's workings (okay, slight exagguration ... but only slight). A full written review is forthcoming, but in a nutshell Instinct is a very good device with excellent GPS and voice command features and solid media capabilities. The Web browser and messaging are lacking a bit, but if you're on Sprint and can live without Mobile IM, that $129 price tag must be lookin' awful nice right about now.
]]>
can be cracked on the order of about 216".
Security breaches
Time to time, people have tried to decode GSM algorithms. For instance, according to Issac press release (1998) in April 1998, the SDA (Smartcard Developer Association) along with two U.C Berkeley researchers alleged that they have cracked the COMP128 algorithm, which is stored on the SIM. They claimed that within several hours they were able to deduce the Ki by sending immense numbers of challenges to the authorization module. They also said that out of 64 bits, Kc uses only 54 bits with zeros padding out the other 10, which makes the cipher key purposefully weaker. They felt government interference might be the reason behind this, as this would allow them to monitor conversations. However, they were unable to confirm their assertion since it is illegal to use equipment to carry out such an attack in the US. In reply to this assertion, the GSM alliance stated that since the GSM network allows only one call from any phone number at any one time it is of no relevant use even if a SIM could be cloned. GSM has the ability to detect and shut down duplicate SIM codes found on multiple phones (Business press release, 1998).
According to Srinivas (2001), one of the other claims was made by the ISAAC security research group. They asserted that a fake base station could be built for around $10,000, which would allow a "man-in-the-middle" attack. As a result of this, the real base station can get deluged which would compel a mobile station to connect to the fake station. Consequently, the base station could eavesdrop on the conversation by informing the phone to use A5/0, which is without encryption.
One of the other possible scenarios is of insider attack. In the GSM system, communication is encrypted only between the Mobile station and the Base Transceiver station but within the provider's network, all signals are transmitted in plain text, which could give a chance for a hacker to step inside (Li, Chen & Ma).
Measures taken to tackle these flaws
According to Quirke (2004), since the emergence of these, attacks, GSM have been revising its standard to add newer technologies to patch up the possible security holes, e.g. GSM1800, HSCSD, GPRS and EDGE. In the last year, two significant patches have been implemented. Firstly, patches for COMP 128-2 and COMP128-3 hash function have been developed to address the security hole with COMP 128 function. COMP128-3 fixes the issue where the remaining 10 bits of the Session Key (Kc) were replaced by zeroes. Secondly, it has been decided that a new A5/3 algorithm, which is created as part of the 3rd Generation Partnership Project (3GPP) will replace the old and weak A5/2. But this replacement would result in releasing new versions of the software and hardware in order to implement this new algorithm and it requires the co-operation of the hardware and software manufacturers.
GSM is coming out of their "security by obscurity" ideology, which is actually a flaw by making their 3GPP algorithms available to security researchers and scientists (Srinivas, 2001).
Conclusion
To provide security for mobile phone traffic is one the goals described in GSM 02.09 specification, GSM has failed in achieving it in past (Quirke, 2004). Until a certain point GSM did provide strong subscriber authentication and over-the-air transmission encryption but different parts of an operator's network became vulnerable to attacks (Li, Chen, Ma). The reason behind this was the secrecy of designing algorithms and use of weakened algorithms like A5/2 and COMP 128. One of other vulnerability is that of inside attack. In order to achieve its stated goals, GSM is revising its standards and it is bringing in new technologies so as to counteract these security holes. While no human-made technology is perfect, GSM is the most secure, globally accepted, wireless, public standard to date and it can be made more secure by taking appropriate security measures in certain areas.
Bibliography
Business Wire Press release (1998). GSM Alliance Clarifies False & Misleading Reports of Digital Phone Cloning. Retrieved October 26th, 2004 Web site: http://jya.com/gsm042098.txt
Brookson (1994). Gsmdoc Retrieved October 24th, 2004 from gsm Web site: http://www.brookson.com/gsm/gsmdoc.pdf
Chengyuan Peng (2000). GSM and GPRS security. Retrieved October 24th, 2004 from Telecommunications Software and Multimedia Laboratory Helsinki University of Technology Web site: http://www.tml.hut.fi/Opinnot/Tik-110.501/2000/papers/peng.pdf Epoker Retrieved October 27th, 2004 from Department of Mathematics Boise State University, Mathematics 124,Fall 2004 Web site:http://math.boisestate.edu/~marion/teaching/m124f04/epoker.htm Huynh & Nguyen (2003). Overview of GSM and GSM security. Retrieved October 25th, 2004 from Oregon State university, project Web site: http://islab.oregonstate.edu/koc/ece478/project/2003RP/huynh_nguyen_gsm.doc
Li, Chen & Ma (n.d). Security in gsm. Retrieved October 24th, 2004 from gsm-security Web site: http://www.gsm-security.net/papers/securityingsm.pdf
Quirke (2004). Security in the GSM system. Retrieved October 25th, 2004 from Security Website:http://www.ausmobile.com/downloads/technical/Security in the GSM system 01052004.pdf
Margrave (n.d). GSM system and Encryption. Retrieved October 25th, 2004 from gsm-secur Web site: http://www.hackcanada.com/blackcrawl/cell/gsm/gsm-secur/gsm-secur.html
Press release (1998). Smartcard Developer Association Clones Digital GSM 1998). Retrieved October 26th, 2004 from is sac Web site: http://www.isaac.cs.berkeley.edu/isaac/gsm.html
Srinivas (2001). The GSM Standard (An overview of its security) Retrieved October 25th, 2004 from papers Web site:http://www.sans.org/rr/papers/index.php?id=317
Stallings (2003). Cryptography and Network Security: Principles and practices. USA: Prentice Hall.
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