What is an OP and OPc in SIM card?

Haseeb Awan
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June 15, 2023
Modified On
June 15, 2023

Introduction

A SIM card, short for Subscriber Identity Module, is a small electronic chip that is inserted into mobile devices such as smartphones, tablets, and some other connected devices. It is used to identify and authenticate the subscriber on a mobile network. SIM cards store important information like the subscriber's phone number, network authorization data, contacts, and text messages. They also allow users to switch between devices while retaining their mobile identity and personal data. SIM cards play a crucial role in enabling voice calls, text messaging, mobile data access, and other services provided by mobile network operators.

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What Is A SIM Card Made Up of?

A SIM card is a compact electronic device that encompasses several key components. At its core is a microprocessor, which serves as the brain of the card, executing various operations and managing data.. The microprocessor relies on the support of different types of memory. The ROM, or Read-only Memory, the RAM, or Random Access Memory and the EEPROM, or Electrically Erasable Programmable Read-only Memory. 

Furthermore, a SIM card features an operating system, often based on a Java environment. This operating system provides a platform for executing applications and services on the SIM card. It enables compatibility with various mobile devices and facilitates the smooth functioning of SIM-related operations.

Authentication Algorithm

What is the authentication algorithm, and what are the associated values? 

Under the realm of the operating system lies the authentication algorithm. The Authentication Algorithm in a SIM card refers to the specific cryptographic algorithm used to authenticate the SIM card and verify its identity to the mobile network. there are various versions and variations of authentication algorithms used in SIM cards, depending on the specific generation and standards employed. These algorithms are designed to provide secure authentication, protect against unauthorized access, and ensure the privacy of user communications within the mobile network.

In SIM card configuration related to the authentication algorithm, there are two key elements: XOR and Mileanage. XOR is primarily utilized for testing purposes, while Mileanage is predominantly used in production environments. Several parameters associated with authentication are stored within the SIM card, including:

  1. Subscriber Authentication Key (Ki): This is a 128-bit key specific to the subscriber. It plays a critical role in authentication processes to ensure the security and integrity of subscriber information.
  2. Operator Code (OP): The OP is an identifier assigned to a particular mobile network operator. It remains the same for all SIM cards belonging to that operator. The OP helps differentiate between different operators and is used in various cryptographic operations.
  3. Derived Operator Code (OPc): The OPc is derived from the OP value but is unique for each individual SIM card. It is generated using specific algorithms, typically involving encryption techniques. The OPc serves as a secure and distinct code for each SIM, ensuring that compromising one OPc does not jeopardize the security of other SIM cards.

What Is OP And OPc In Sim Card?

To get deeper into what OP and OPc are, Operator Code (OP) is an identifier assigned to a mobile network operator, which is used in the generation of cryptographic keys for 3G and 4G networks. Unlike user-specific information, OP is not directly inputted into key generation algorithms. It is because this code remains constant for all subscribers and SIM cards associated with a particular operator.

To maintain security, the OP value is not exposed and is kept fixed for all subscribers. However, due to its fixed nature, the OP poses a risk as unauthorized access could potentially compromise all SIM cards using the same OP value. 

To address this concern, operators employ a solution by provisioning the OPc (Operator Variant Algorithm Configuration Field) instead of the OP in the Authentication Center (AuC) or Home Location Register/Home Subscriber Server (HLR/HSS). The OPc is derived from the OP and a secret key using the "RijndaelEncrypt" encryption algorithm, and it is unique to each SIM card. 

Essentially, OPc serves as the secure key derived from OP and the secret key, providing unique values for each SIM card. Its usage ensures that compromising one OPc would only affect a single SIM card, rather than compromising all SIM cards associated with an operator.

By provisioning OPc instead of OP, each SIM card receives a distinct value for vector generation in functions such as f1, f2, f3, f4, and f5. This prevents reverse engineering of the OP from OPc and mitigates the risk of unauthorized access to multiple SIM cards.

These parameters, including the Subscriber Authentication Key (Ki), Operator Code (OP), and Derived Operator Code (OPc), are essential in maintaining the security and functionality of SIM cards. They contribute to the authentication process and help establish secure connections between the SIM card, the subscriber, and the mobile network operator.

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Haseeb Awan
CEO, Efani Secure Mobile

I founded Efani after being Sim Swapped 4 times. I am an experienced CEO with a demonstrated history of working in the crypto and cybersecurity industry. I provide Secure Mobile Service for influential people to protect them against SIM Swaps, eavesdropping, location tracking, and other mobile security threats. I've been covered in New York Times, The Wall Street Journal, Mashable, Hulu, Nasdaq, Netflix, Techcrunch, Coindesk, etc. Contact me at 855-55-EFANI or haseebawan@efani.com for a confidential assessment to see if we're the right fit!

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