The International Mobile Subscriber Identity (IMSI) is a unique 15-digit number that identifies a mobile subscriber within a mobile network. This identifier is stored on the Subscriber Identity Module (SIM) card and is sent by the mobile device to the network operator for authentication and billing purposes.
The IMSI plays a crucial role in network authentication and is used to identify the subscriber’s home network and country of operation.
Understanding the IMSI Structure
The IMSI consists of three parts:
- Mobile Country Code (MCC): The first two or three digits of the IMSI represent the MCC, which identifies the subscriber’s home country. The MCC is always either two or three digits long.
- Mobile Network Code (MNC): Following the MCC, the next two or three digits form the MNC, which identifies the specific Mobile Network Operator (MNO) the subscriber is associated with within their home country.
- Mobile Subscription Identification Number (MSIN): The remaining nine or ten digits make up the MSIN, which is a unique number assigned by the MNO to identify the individual subscriber.
Together, these components form the complete 15-digit IMSI, which is stored on the SIM card and used by the mobile device to connect to the cellular network.
IMSI vs. IMEI
It is important to note the difference between the IMSI and the International Mobile Equipment Identity (IMEI). While the IMSI identifies the subscriber and their associated mobile network, the IMEI is a unique 15-digit number that identifies the mobile device itself.
The IMEI is used to identify the specific hardware, such as the make and model of the mobile phone, and is not directly related to the subscriber’s identity or mobile network.
The Role of IMSI in Mobile Connectivity
When a mobile device attempts to connect to a cellular network, it sends the IMSI to the network operator. The network operator uses the IMSI to determine the subscriber’s home network and whether the subscriber is authorised to access the requested services.
If the subscriber is roaming outside their home network, the IMSI is used to identify the subscriber’s home network and facilitate communication between the visited network and the home network.
IMSI Catchers and Security Concerns
IMSI catchers, also known as Stingrays, are devices that exploit vulnerabilities in the mobile network infrastructure to intercept mobile phone traffic and track the location of mobile devices. These devices work by mimicking legitimate cell towers and tricking nearby mobile devices into connecting to them. Once connected, IMSI catchers can intercept calls, text messages, and other data transmitted by the mobile devices.
To protect against IMSI catchers and other security threats, mobile users should take the following precautions:
- When not in use, we would recommend either putting your phone into aeroplane mode or switching it off completely.
- Update your phones’ software on a regular basis. This means you have the latest security which will help defend you against IMPS catchers.
- VPN (virtual private network) – a VPN with strong encryption will add an extra layer of security making it harder for criminals to intercept your data.
At present, the Police are known to use IMSI catchers (although they do not actively state this) for circumstances such as protests. This enables them to identify who was present in the vicinity of the protest. However, as cybercriminals become more tech savvy, they are now also using IMSI catchers.
How Advanced Sweeping Can Help
At Advanced Sweeping, we specialise in comprehensive technical surveillance countermeasures (TSCM) and mobile forensics services. Our team of experienced experts can help organisations and individuals identify and mitigate risks associated with mobile devices, including those related to IMSI vulnerabilities and IMSI catchers.
Contact Us for Professional TSCM Services
Don’t leave your privacy and security to chance. If you’re concerned about mobile surveillance or want to ensure your devices and premises are secure, contact Advanced Sweeping today.
Our expert team is ready to provide professional advice and tailor our TSCM and mobile forensics services to meet your specific security needs.
