How to check the checksum of a RFID tag

Question

Following this steps I wrote my function to check that the value of the tags I scan is correct, but it always returns false. Here is my function:

int checksum(const char* string)
{
    int i;
    char hexPairs[6][2];
    long totXor;

    // load matrix
    for(i=0; i<6; i++)
    {
        hexPairs[i][0] = string[i*2];
        hexPairs[i][1] = string[i*2+1];
    }

    // perform xor
    totXor = strtol(hexPairs[0], NULL, 16);
    for(i=1; i<5; i++)
    {
        totXor = totXor^strtol(hexPairs[i], NULL, 16);
    }

    return (totXor == strtol(hexPairs[5], NULL, 16));
}

What am i doing wrong?

I've done the xor of the binary values manually to check that what I scan is a valid value.

EDITED

The code above didn't work because it didn't have '\0' at the end of each pair. Solved this, my code works in CodeBlocks but in Vinculum II IDE, that is where I need it to work, always returns false. Here's the list of versions I've tried:

int checksum(const char* string)
{
    int i;
    char hexPairs[6][3];
    long totXor = 0;

    // load matrix
    for(i=0; i<6; i++)
    {
        hexPairs[i][0] = string[i*2];
        hexPairs[i][1] = string[i*2+1];
        hexPairs[i][2] = '\0';
    }

    // perform xor
    totXor = strtol(hexPairs[0], NULL, 16);
    for(i=1; i<5; i++)
    {
        totXor = totXor^strtol(hexPairs[i], NULL, 16);
    }

    return (totXor == strtol(hexPairs[5], NULL, 16));
}


int checksum(const char* string)
{
    int i;
    char *hexPairs[6];
    long totXor = 0, chk = 0;

    // load matrix
    for(i=0; i<6; i++)
    {
        hexPairs[i] = malloc(3);
        memset(hexPairs[i], 0, 3);
        hexPairs[i][0] = string[i*2];
        hexPairs[i][1] = string[i*2+1];
    }

    // perform xor
    totXor = strtol(hexPairs[0], NULL, 16);
    for(i=1; i<5; i++)
    {
        totXor = totXor^strtol(hexPairs[i], NULL, 16);
    }

    chk = strtol(hexPairs[5], NULL, 16);

    return (totXor == chk);
}


int checksum(const char* string)
{
    char *hex0, *hex1, *hex2, *hex3, *hex4, *hexChk;
    long totXor = 0, chk = 0;

    hex0 = malloc(3);
    hex1 = malloc(3);
    hex2 = malloc(3);
    hex3 = malloc(3);
    hex4 = malloc(3);
    hexChk = malloc(3);

    memset(hex0, 0, 3);
    memset(hex1, 0, 3);
    memset(hex2, 0, 3);
    memset(hex3, 0, 3);
    memset(hex4, 0, 3);
    memset(hexChk, 0, 3);

    hex0[0] = string[0];
    hex0[1] = string[1];

    hex1[0] = string[2];
    hex1[1] = string[3];

    hex2[0] = string[4];
    hex2[1] = string[5];

    hex3[0] = string[6];
    hex3[1] = string[7];

    hex4[0] = string[8];
    hex4[1] = string[9];

    hexChk[0] = string[10];
    hexChk[1] = string[11];

    // perform xor
    totXor = strtol(hex0, NULL, 16);
    totXor = totXor^strtol(hex1, NULL, 16);
    totXor = totXor^strtol(hex2, NULL, 16);
    totXor = totXor^strtol(hex3, NULL, 16);
    totXor = totXor^strtol(hex4, NULL, 16);

    chk = strtol(hexChk, NULL, 16);

    return (totXor == chk);
}

int checksum(const char* string)
{
    char *hex0, *hex1, *hex2, *hex3, *hex4, *hexChk;
    long totXor = 0, chk = 0;

    hex0 = malloc(3);
    hex1 = malloc(3);
    hex2 = malloc(3);
    hex3 = malloc(3);
    hex4 = malloc(3);
    hexChk = malloc(3);

    memset(hex0, 0, 3);
    memset(hex1, 0, 3);
    memset(hex2, 0, 3);
    memset(hex3, 0, 3);
    memset(hex4, 0, 3);
    memset(hexChk, 0, 3);

    sprintf(hex0, "%c%c", string[0], string[1]);
    sprintf(hex1, "%c%c", string[2], string[3]);
    sprintf(hex2, "%c%c", string[4], string[5]);
    sprintf(hex3, "%c%c", string[6], string[7]);
    sprintf(hex4, "%c%c", string[8], string[9]);

    sprintf(hexChk, "%c%c", string[10], string[11]);

    // perform xor
    totXor = strtol(hex0, NULL, 16);
    totXor = totXor^strtol(hex1, NULL, 16);
    totXor = totXor^strtol(hex2, NULL, 16);
    totXor = totXor^strtol(hex3, NULL, 16);
    totXor = totXor^strtol(hex4, NULL, 16);

    chk = strtol(hexChk, NULL, 16);

    return (totXor == chk);
}

I call the function this way:

if(!checksum("6D003D302040"))
{
    return;
}

Did you verify what numbers you get from strtol(hexPairs[i], NULL, 16)? Since strtol expects a char*, it requires a terminating 0, which may not be after the two chars that make your digit. — corvairjo, Jul 15 '14 at 14:14
Yes, sorry for not updating. I'm working in Vinculum II IDE whose debugger doesn't let me see those values. I copied the code in CodeBlocks and could fix the terminating character issue. Now the problem I have is that the code works in CodeBlocks but not in Vinculum. I've tried declaring "char \*hexPairs[6]" and declaring 6 char\* variables but still not working. — adryledo, Jul 15 '14 at 15:21
Try to use a string array "char *hexPairs[]" and populate that in your first loop. — corvairjo, Jul 15 '14 at 15:25
Can you detail what does not work with you modified version? — corvairjo, Jul 15 '14 at 15:33

score 0 · Accepted Answer · answered Jul 16 '14 at 13:03

SOLVED

I could finally make it work changing the return. This is the final function:

int checksum(const char* string)
{
    int i;
    char *hexPairs[6];
    long totXor = 0;

    // load matrix
    for(i=0; i<6; i++)
    {
        hexPairs[i] = malloc(3);
        memset(hexPairs[i], 0, 3);
        hexPairs[i][0] = string[i*2];
        hexPairs[i][1] = string[i*2+1];
    }

    // perform xor
    totXor = strtol(hexPairs[0], NULL, 16);
    for(i=1; i<5; i++)
    {
        totXor = totXor^strtol(hexPairs[i], NULL, 16);
    }

    return ((totXor == strtol(hexPairs[5], NULL, 16)) ? 1 : 0);
}

It seems that Vinculum didn't like "return (totXor == strtol(hexPairs[5], NULL, 16));".

It didn't like the matrix "char hexPairs[6][3];" either.

How to check the checksum of a RFID tag

1 Answers1