Cryptography

This article explains the Cryptography present in GTA IV.

Hash Algorithms

GTA IV relies on many different hash algorithms work its operation, each for a different purpose. GTA IV's new usage of hashing has allowed it to explore a more binary focused way of hiding data in files, and strings away from plain site.

SHA1

The SHA1 hashing algorithm is used when comparing the files in versions 1.0 and 1.0.1. This check was later removed in 1.0.2.

CRC32

The Cyclic Redundancy Check 32 bit hashing algorithm is used in the GXT file to match text codes with their counterparts. A C++ implementation of GTA IV's CRC32 Hashing Algorithm can be displayed as follows:

unsigned int CRC32(char* text)
{
	size_t textLen = strlen(text);
	int i = 0;
	unsigned int retHash = 0;
	if(text[0] == '"')
		i = 1;
	for(i;i<textLen;i++)
	{
		char ctext = textLen[i];
		if(ctext == '"')
			break;
		if(ctext - 65 > 25)
		{
			if(ctext == '\\')
				ctext = '/';
		}
		else ctext += 32;
		retHash = (1025 * (retHash + ctext) >> 6) ^ 1025 * (retHash + ctext);
	}
	return 32769 * (9 * retHash ^ (9 * retHash >> 11));
}

As you can see it differs from most common hashing algorithms in the way it handles text (by not including " or \ characters).

One At A Time Hash

The One At A Time hashing function was originally created by Bob Jenkins. A C++ implementation can be found here and at Wikipedia.

unsigned int oneAtATimeHash(char* inpStr)
{
	unsigned int value = 0,temp = 0;
	for(size_t i=0;i<strlen(inpStr);i++)
	{
		char ctext = tolower(inpStr[i]);
		temp = hashchr;
		temp += value;
		value = temp << 10;
		temp += value;
		value = temp >> 6;
		value ^= temp;
	}
	temp = value << 3;
	temp += value;
	unsigned int temp2 = temp >> 11;
	temp = temp2 ^ temp;
	temp2 = temp << 15;
	value = temp2 + temp;
	if(value < 2) value += 2;
	return value;
}