Breaking Down SHA-1 and SHA-256 Authentication in Battery Fuel Gauges

Original Equipment Manufacturers (OEMs) commonly utilize SHA-1 and SHA-256-based authenticators, such as the DS2784, DS2776, and MAX17312, to bolster the security of their 1-Wire battery fuel gauge devices. These devices employ challenge-response protocols, where a host presents a random challenge, and the device responds with a calculated value derived from its secret key. This mechanism is designed to thwart unauthorized access and safeguard sensitive data stored within the battery fuel gauge, such as battery health information, cycle counts, and remaining capacity estimates.

The Foundation of SHA-Based Authentication

At the core of SHA-based authentication in battery fuel gauges lies a framework of shared secrets and cryptographic safeguards. Each device possesses unique identification data (ID Data) ingrained during manufacturing. This ID Data, in conjunction with a securely held Master Authentication Secret on the host system, is leveraged to generate a Unique Secret specific to each device.

This Unique Secret serves as a shared key between the host and the device, facilitating secure communication. When the host seeks to access protected data or execute sensitive operations, it transmits a command accompanied by a Message Authentication Code (MAC). This MAC is computed using the Unique Secret, ensuring that only the authorized host can interact with the device.

The utilization of cryptographic hash functions like SHA-1 or SHA-256 adds another layer of defense. These hash functions are designed to be one-way, making it computationally impractical to reverse-engineer the Unique Secret even if an attacker intercepts the communication. This robust mechanism protects against unauthorized access and manipulation, guaranteeing the integrity and confidentiality of data exchanged between the host and the 1-Wire device.

Real-World Applications and Consumer Impact

The deployment of SHA-based authenticators in battery fuel gauges can significantly impact consumers, often limiting their choices and inflating costs:

  • Proprietary Batteries: OEMs often use authentication in battery fuel gauges to enforce the use of their own proprietary batteries. This practice prevents consumers from using more affordable third-party batteries or refurbishing existing battery packs, leading to increased costs and potential e-waste.

  • Limited Repair Options: Authentication mechanisms can hinder independent repair shops and individuals from servicing or replacing battery packs, forcing consumers to rely on expensive OEM services or prematurely discard devices with failing batteries.

The Inner Workings of the DS2784

Let's take a closer look at the DS2784 as a prime example of how SHA-based authentication is implemented in battery fuel gauges. This 1-Cell Stand-Alone Fuel Gauge IC incorporates SHA-1 authentication alongside its core functions of precise voltage, temperature, and current measurement, capacity estimation, and Li+ protection circuitry.

The DS2784 employs a 32-bit wide SHA-1 engine with a 64-bit secret and challenge words, providing a robust defense against brute-force and other attacks. The authentication process involves a challenge-response protocol where the host sends a random challenge, and the DS2784 responds with a calculated MAC based on its secret key. The host then compares this MAC with its own calculated MAC to verify the authenticity of the device.

Jumtee's Expertise

At Jumtee Security, we specialize in 1-Wire hardware security and possess a profound understanding of the vulnerabilities inherent in these systems. We have cultivated an array of capabilities that can overcome the obstacles OEMs erect, including:

  • Recovering Master Keys and Root Signing Keys: The bedrock of SHA-based authentication lies in the secrecy of these keys. We have devised techniques to extract these keys, circumventing the entire authentication process.

  • Bypassing Protection Modes: OEMs implement protection modes like Write Protection and Read Protection to shield critical data within the fuel gauge. Our expertise enables us to navigate around these protections, granting unrestricted access to the device's memory and configuration.

  • 1-Wire Emulators: We can engineer emulators that replicate the behavior of genuine 1-Wire devices, empowering us to interact with systems just like OEM devices.

  • Data Analysis and Reverse Engineering: Through sophisticated data analysis and die-level reverse engineering, we can unearth concealed vulnerabilities and exploit weaknesses in the implementation of SHA-1 and SHA-256 authentication in battery fuel gauges.

Conclusion

SHA-based authenticators furnish a valuable security layer for 1-Wire devices like the DS2784, DS2776, and MAX17312 across diverse applications. Nevertheless, they are not impervious to circumvention. Jumtee Security's proficiency in 1-Wire hardware security allows us to identify and surmount the barriers OEMs put in place, promoting consumer choice and enabling cost-effective battery replacements and repairs.