How do I know a Longitudinal redundancy check is correctly calculated and how is it used to ensure that data before is not corrupt?

Question

How do I know a Longitudinal redundancy check is correctly calculated and how is it used to ensure that data before is not corrupt?

This is how I'm calculating it right now in Java but how do I know I am calculating it correctly given the data?

byte[] testMessage1 = {0x02, // STX
        0x10,0x2,0xA,0x10,0x10,0x7,0x8, // Data 02, A, 10, 7, 8
        0x03, // ETX
        0x2^0xA^0x10^0x7^0x8^0x03}; // LRC calculated from the data (with the DLE removed) plus the ETX 


public static byte calculateLRC2(byte[] bytes) {
    byte checksum = 0;
    for (int i = 1; i < bytes.length - 1; i++) {
        checksum ^= (bytes[i] & 0xFF);          
    }
    return checksum;
}

Answer 1

You are looking for answer of a wrong question in my opinion because LRC check is to find out weather you are reading the data correctly or not not the other way around. But if you just want to validate your implementation create unit tests from real examples. Please remember that LRC is to validate if the data read from the source was correct (and not tempered or faulty) so LRC must be available within the data your code is just to make sure that LRC you are calculating and LRC available in the source are matching. If in any case you don't have access to sample data then you can try online tools e.g. asecuritysite.com. Coming back to code following code is working for me in java for validation of credit card magnetic strip read. see it running online here.

String str = ";4564456445644564=1012101123456789?";
byte lrc = 0x00;
byte [] binaryValue = str.getBytes();

for(byte b : binaryValue) {

    lrc ^= b;
}

// lrc msut be between 48 to 95
lrc %= 48; 
lrc += 48;

System.out.println("LRC: " + (char) lrc);