Published on December 10th, 2018 | by Sunit Nandi


What is an IMSI catcher?

The abbreviation IMSI stands for “International Mobile Subscriber Identity”, which could be translated as “international mobile subscriber identity”. Behind it hides a number that is stored on the SIM card of a mobile phone. And this IMSI number is unique in the world. Each SIM card has its own number. The number has 15 digits and reveals, for example, from which country and from which network operator a card comes. The IMSI is not visible to the mobile phone user and has nothing to do with the phone number. Also unique is the IMEI number of a mobile phone. This is given for the device as such, not for the SIM card that is in it.

The IMSI Catcher is a device that can read the IMSI and also the IMEI number of a mobile phone. The catcher is used for military investigations to monitor suspicious persons. The device pretends to be a base station and thus builds up a small mobile radio cell. The data of all connected mobile phones within this cell are collected by the catcher.


Conversations, which are led with mobile phones, are basically inaudible. This may seem surprising at first because the network operators and device manufacturers have long been promoting encryption for this new communication technology. In this context, however, it was not mentioned that network operators can turn off the encryption by a command. This feature is necessary because in some European countries only unencrypted communication is possible. Whether encrypted or unencrypted is transmitted, is not displayed on the phones so far.

As part of the deliberations on a law accompanying the Telecommunications Act was discussed, intelligence services and law enforcement agencies to allow the use of devices (so-called IMSI catcher), which can selectively disable the encryption of individual phones and thus allow the monitoring and recording of conversations. In addition, these devices should be used to determine the network internal numbers of mobile phones, the so-called IMSI (International Mobile Subscriber Identity – network-internal subscriber identifier that belongs to a specific mobile network) to be able to accurately access individual phones.

Construction of the IMSI catcher

The basic device is no bigger than an average PC. The control is done by a standard laptop. The IMSI Catcher can work in two modes of operation (catch, eavesdrop). Devices for catching and listening are identical; to listen in addition, only a software supplement and a downstream cell phone is needed. IMSI catchers can be used in various radio networks. The operation can be done out of a car. This makes a quick change of location unproblematic.

IMSI catcher in catch mode

n order to be able to listen carefully, the knowledge of the telephone number is usually required. The listening devices simulate a base station for this purpose by creating an additional own radio cell, which behaves exactly like an original cell. Because the listening devices operate at a slightly higher power level, all devices log in to this new cell and not to the actual base station. About this station then run all connection requests of mobile phones. The users notice nothing of this “catching”. Of all mobile phones in its range, all mobile phones within a radius of about 100 meters. In addition to the IMSI, the listening device can also call up the IMEI (International Mobile Station Equipment Identity). For technical reasons, during this procedure [ Note: a few minutes ] no one can make or receive calls with the affected mobile phone. Even emergency calls to the police, fire brigade or medical emergency service are not possible by any of the mobile phones registered in the new radio cell.

IMSI catcher in listening mode

In listening mode, IMSI catchers use the ability to turn off encryption. So if the calls of a mobile phone are to be intercepted, the encryption is turned off when establishing a connection, so that the conversation content is still available in digital form, but now available unencrypted and with appropriate software and can be recorded. As long as the listening device works in this mode, no connection can be established with any cell phone caught in the area of ​​influence of the listening device. Only outgoing calls of the cell phones are possible.

The data protection officers of the federal and state governments have rejected the use of the IMSI catcher in particular because the right to unobserved communication of uninvolved third parties is impaired when determining the number and listening to those affected with an unprecedented intensity.

Even if these listening devices are initially not intended to be used by intelligence services and law enforcement authorities, however, the described risks for the users of mobile phones remain. On the one hand, it can not be ruled out that these devices are produced, for example, for export. On the other hand, experience has shown that it does not take long for building instructions to be published for individual components or for the entire device. It would be surprising if what was available was not used by anyone.

It becomes clear that only a switched off mobile phone guarantees a really safe protection against misuse. But this is just the purpose of mobile phone use, which lies in the constant accessibility, subverted. However, mobile phone users need to be aware of the risks outlined here, so they can consciously decide for themselves whether and how long to turn on their phones on which occasions. It would be nice if mobile devices were so developed and the network operation would be designed so that abuse possibilities are excluded from the outset as far as possible. Device manufacturers and network operators are called upon to take appropriate measures to ensure that their customers can communicate with each other confidentially, within the limits of applicable law.

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I'm the leader of Techno FAQ. Also an engineering college student with immense interest in science and technology. Other interests include literature, coin collecting, gardening and photography. Always wish to live life like there's no tomorrow.

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