Arduino RFID checksum calculation and key visualization

Question

I\'m using this RFID module for Arduino Ethernet R3 and I need to retrieve from the Software Serial the card (TAG) ID that is written outside the tag. The module\'s datasheet sa

Answer 1

Here's a walkthrough of how to calculate the checksum.

Take your card number (this is just directly quoted from your text)

I received: 2
I received: 51
I received: 67
I received: 48
I received: 48
I received: 67
I received: 69
I received: 55
I received: 53
I received: 52
I received: 67
I received: 67
I received: 66
I received: 3

This would give you a number that is equivalent to the following:

2 51 67 48 48 67 69 55 53 52 67 67 66 3

The first numer (2) indicates that this is the beginning of a request.

The last number (3) indicates that this is the end of a request.

2 51 67 48 48 67 69 55 53 52 67 67 66 3

For the purposes of calculating the checksum, we are going to remove these two numbers. So your new number is now:

51 67 48 48 67 69 55 53 52 67 67 66

The last two numbers that you have are your checksum. The remaining numbers are your card number. So:

Your card number is:

51 67 48 48 67 69 55 53 52 67

And your checksum is:

67 66

Next you need to convert your Card Number and your Checksum to ASCII values:

Your card number is:

3 C 0 0 C E 7 5 4 C

And your checksum is:

C B

Next, grab each number into pairs:

Your card number is:

3C 00 CE 75 4C

And your checksum is:

CB

Then you need to treat each pair as a HEXIDECIMAL value and do an XOR against them. So basically you need to prove the following:

3C ^ 00 ^ CE ^ 75 ^ 4C == CB

(3C ^ 00) = 3C

3C ^ CE ^ 75 ^ 4C == CB

(3C ^ CE) = F2

F2 ^ 75 ^ 4C == CB

(3C ^ CE) = 87

87 ^ 4C == CB

(87 ^ 4C) = CB

CB == CB

Because CB == CB, this is a valid transaction.

No doubt someone else can come up with a better approach than this, but there should be enough pseudo code here for you to write it yourself.