1 Introduction

There are many methods to authenticate the slave. Some methods are listed below.

One simple method is to add one resistor in the slave. The host detects its resistance. And if the resistance is in the expected range, then the host will consider the slave is qualified. This method is simple and low cost, but easy for cheap copy.

Another method is to add one EEPROM device (like TI BQ2022A) in the slave. EEPROM stores some specific data. The host read the data from EEPROM, and compare with the data stored in the host. If they are matched, the host will consider the slave is qualified. But the data is transmitted transparently via communication line. So, it is easy to capture the data, and also easy for cheap copy.

The more complicated method is to add authentication device with SHA-1/SHA-256 algorithm in the slave. The host will generate a serial of random data (challenge), and send to the slave. Both host and slave will use the same challenge, key and function to calculate the result (digest). Then the host will read the digest from the slave, and compare with the digest that host itself calculates. If they are matched, the host will consider the slave is qualified. With this method, it is not easy for cheap copy.

The following section describes the BQ26100 and SHA-1 in detail.