IMSI (International mobile subscriber identity)

Last updated on Apr 22, 2023

Introduction:

IMSI (International Mobile Subscriber Identity) is a unique identification number assigned to each subscriber of a mobile network operator. It is a 15-digit number that serves as a key to access a mobile network. The IMSI is stored in the SIM (Subscriber Identity Module) card, which is inserted into the mobile phone. IMSI is one of the essential elements of the GSM (Global System for Mobile Communications) architecture, which is the most widely used mobile phone technology in the world. IMSI plays a critical role in the secure communication between a mobile phone and a mobile network.

IMSI Structure:

The IMSI is composed of three parts: MCC, MNC, and MSIN.

MCC (Mobile Country Code) is a three-digit number that identifies the country where the mobile network operator is registered. MCC is assigned by the ITU (International Telecommunication Union). For example, the MCC for the United States is 310, and the MCC for the United Kingdom is 234.
MNC (Mobile Network Code) is a two or three-digit number that identifies the mobile network operator within a country. MNC is also assigned by the ITU. For example, the MNC for AT&T in the United States is 410, and the MNC for Vodafone in the United Kingdom is 15.
MSIN (Mobile Subscriber Identification Number) is a unique 10-digit number that identifies the subscriber within the mobile network operator's network. The first three digits of the MSIN are the combination of the MCC and MNC, and the remaining seven digits are randomly assigned by the mobile network operator.

IMSI Usage:

The IMSI is used for authentication and authorization purposes in a mobile network. When a mobile phone is powered on and tries to connect to a mobile network, it sends a request to the network with its IMSI. The mobile network checks the IMSI against its database to verify if the IMSI is valid and whether the subscriber is allowed to use the network. If the IMSI is valid and the subscriber is authorized, the mobile network sends a response to the mobile phone with a temporary identifier called TMSI (Temporary Mobile Subscriber Identity). The TMSI is used to protect the subscriber's privacy and security by changing the identifier frequently to avoid tracking and eavesdropping.

The IMSI is also used for location tracking in a mobile network. When a mobile phone is in use, the mobile network keeps track of the cell tower it is connected to and its location. This information is used for billing purposes and emergency services, such as E911 in the United States, which allows emergency services to locate a caller's position.

IMSI Security:

The IMSI is a critical security element in a mobile network because it is used to authenticate and authorize the subscriber. If an attacker can obtain a subscriber's IMSI, they can impersonate the subscriber and access the mobile network's resources. Therefore, the IMSI must be protected from unauthorized access and disclosure.

One way to protect the IMSI is to encrypt it when it is transmitted over the air interface between the mobile phone and the mobile network. The encryption algorithm used in GSM is A5/1 or A5/3, which provides confidentiality and integrity protection of the data transmitted over the air interface. However, the A5/1 encryption algorithm is vulnerable to attacks, and it has been cracked by hackers, making it easier for attackers to eavesdrop on the communication between the mobile phone and the mobile network.

Another way to protect the IMSI is to use a temporary identifier called TMSI instead of the IMSI when the mobile phone is in use. The TMSI is changed frequently, making it more difficult for attackers to track and eavesdrop on the communication between the mobile phone and the mobile network. The mobile network assigns a new TMSI to the mobile phone each time it connects to the network, and the TMSI is only valid for a limited time. The mobile network maintains a mapping between the TMSI and the IMSI in its database, allowing it to identify the subscriber without revealing the IMSI.

Another security measure used to protect the IMSI is SIM card authentication. When a mobile phone connects to a mobile network, the mobile network sends a challenge to the SIM card, which generates a response based on a secret key stored in the SIM card. The SIM card sends the response to the mobile network, which verifies it against its database. If the response is correct, the mobile network authenticates the SIM card and allows it to access the network. The SIM card authentication ensures that only authorized SIM cards can access the network, preventing attackers from using a fake or cloned SIM card to impersonate a subscriber.

IMSI Catcher:

An IMSI catcher, also known as a Stingray or a fake base station, is a device used to intercept and track mobile phone communication. An IMSI catcher works by pretending to be a legitimate mobile network base station and tricking nearby mobile phones to connect to it. Once a mobile phone connects to the IMSI catcher, it captures the IMSI and other identifying information, allowing the attacker to track the phone's location and intercept its communication.

IMSI catchers are often used by law enforcement agencies for surveillance purposes. However, IMSI catchers can also be used by criminals and hackers to eavesdrop on mobile phone communication and steal sensitive information. To protect against IMSI catchers, mobile phone users can use a secure messaging app, such as Signal or WhatsApp, that provides end-to-end encryption of messages. They can also use a virtual private network (VPN) to encrypt their internet traffic and protect their privacy.

Conclusion:

IMSI is a critical element of the mobile network architecture that enables secure communication between a mobile phone and a mobile network. It provides a unique identifier for each subscriber and is used for authentication, authorization, and location tracking. The IMSI must be protected from unauthorized access and disclosure to prevent attackers from impersonating subscribers and accessing the mobile network's resources. The use of temporary identifiers, SIM card authentication, and encryption can help protect the IMSI from attacks. However, IMSI catchers remain a threat to mobile phone security and privacy, and mobile phone users must take precautions to protect themselves.