Hello Everyone,
I want to calculate CRC-16/X-25 for my APDU tranmission. I have a C code that calculates CRC-16/CCITT-FALSE, But I don't know what exactly is the difference between these two algorithms/protocols and how to calculate CRC-16/X-25?
I have used the following online calculator (https://crccalc.com/), and the values are fine, but I want to write a C program for that.
And one more thing is that what is RefIn and RefOut parameters given in the above mentioned link, because these two are the only things that are different in these two algorithms.
The C code I am using for CRC-16/CCITT-FALSE is given below:
#include
#define CRC_POLYNOMIAL 0x1021
#define USHORT_TOPBIT 0x8000
short crc_ret;
// function initialisation
unsigned short crc16(const char *buf, unsigned int buf_len)
{
unsigned short ret = 0xFFFF; // Initial Value
int bit = 8;
do {
if (!buf) {
break;
}
unsigned int i = 0;
for (i=0; i> 8));
for (bit = 8; bit > 0; --bit)
{
if (ret & USHORT_TOPBIT)
ret = (unsigned short)((ret >> 1) ^ CRC_POLYNOMIAL);
else
ret = (unsigned short)(ret >> 1);
}
}
} while (0);
return ret;
}
Can someone please guide me, what changes do I need to make so that it can also work with CRC-16/X-25?
Thank you
Raza Javed
The difference between the two algorithms (CRC-16/CCITT-FALSE and CRC-16/X-25) lies in the polynomial value and the initial value used in the calculation. The polynomial value and initial value used in the calculation of the CRC-16/X-25 are different from those of the CRC-16/CCITT-FALSE.
To calculate the CRC-16/X-25, you need to change the polynomial value in the code to the appropriate value (0x1021) and change the initial value to 0xFFFF. Additionally, the RefIn and RefOut parameters specify the bit order (MSB or LSB first) for the calculation of the checksum. In the code you posted, RefIn is set to 1 (MSB first), which is the standard for the CCITT-FALSE. If you want to change it to LSB first, you would set RefIn to 0.
Here's the updated code that implements the CRC-16/X-25 algorithm:
#include
#define CRC_POLYNOMIAL 0x1021
#define USHORT_TOPBIT 0x8000
short crc_ret;
// function initialisation
unsigned short crc16_x25(const char *buf, unsigned int buf_len)
{
unsigned short ret = 0xFFFF; // Initial Value
int bit = 8;
do {
if (!buf) {
break;
}
unsigned int i = 0;
for (i=0; i 0; --bit)
{
if (ret & 1)
ret = (unsigned short)((ret >> 1) ^ CRC_POLYNOMIAL);
else
ret = (unsigned short)(ret >> 1);
}
}
} while (0);
return ret;
}