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Protect Code Distribution For Flash-Based Microcontrollers

By Richard F. Zarr

March 03, 2003

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Full article begins on Page 2

Because it's fairly ubiquitous and accessible, the Internet is a great way to provide code up-dates to customers. But it's an open medium, so it's subject to attacks or theft. By encrypting machine code and placing a wrapper around it to define its function, a simple program can be used to load firmware to a target device.

Wrapper information might consist of the intended target device, code revision, size, date, and other useful user information. This information could warn operators that they might be downgrading a piece of equipment or attempting to load an unsupported target device.

Protecting a company's intellectual property (IP) is a high priority in today's market. Years of development may go into a design that contains trade secrets and proprietary algorithms. Thus, companies are moving away from traditional one-time-programmable microprocessors and to flash-based versions.

Flash allows just-in-time programming at the end of the production line and code changes after production. But when moving this feature to the field via the Internet, designers risk exposing IP to competitors. This article examines ways to provide field updates without exposing a product's IP, plus other features to control update licenses.

HIGHLIGHTS:
Code Protection There are many ways to protect software from unauthorized copying, but vulnerability exists with the Internet. For firmware, protecting the IP is most critical.
Simple Ciphers A rolling-code generator consists of serially connected flip-flops, with several of their outputs implemented as a rolling key.
Transport Methods A firmware loader determines whether the current load is valid. It uses data organized into packets that are delivered to the microcontroller and then to the decryption engine.
Example Design The example COP8 firmware loader with decryption uses a modified version of the Intel hex format. A table gives record types for normal Intel hex format.

 

Full article begins on Page 2

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