Part of cloak.c from UnrealIRCd

cloak.c

static char *hidehost_ipv4(char *host)
{
unsigned int a, b, c, d;
static char buf[512], res[512], res2[512], result[128];
unsigned long n;
unsigned int alpha, beta, gamma;

	/* 
	 * Output: ALPHA.BETA.GAMMA.IP
	 * ALPHA is unique for a.b.c.d
	 * BETA  is unique for a.b.c.*
	 * GAMMA is unique for a.b.*
	 * We cloak like this:
	 * ALPHA = downsample(md5(md5("KEY2:A.B.C.D:KEY3")+"KEY1"));
	 * BETA  = downsample(md5(md5("KEY3:A.B.C:KEY1")+"KEY2"));
	 * GAMMA = downsample(md5(md5("KEY1:A.B:KEY2")+"KEY3"));
	 */
	sscanf(host, "%u.%u.%u.%u", &a, &b, &c, &d);

	/* ALPHA... */
	ircsprintf(buf, "%s:%s:%s", KEY2, host, KEY3);
	DoMD5(res, buf, strlen(buf));
	strcpy(res+16, KEY1); /* first 16 bytes are filled, append our key.. */
	n = strlen(res+16) + 16;
	DoMD5(res2, res, n);
	alpha = downsample(res2);

	/* BETA... */
	ircsprintf(buf, "%s:%d.%d.%d:%s", KEY3, a, b, c, KEY1);
	DoMD5(res, buf, strlen(buf));
	strcpy(res+16, KEY2); /* first 16 bytes are filled, append our key.. */
	n = strlen(res+16) + 16;
	DoMD5(res2, res, n);
	beta = downsample(res2);

	/* GAMMA... */
	ircsprintf(buf, "%s:%d.%d:%s", KEY1, a, b, KEY2);
	DoMD5(res, buf, strlen(buf));
	strcpy(res+16, KEY3); /* first 16 bytes are filled, append our key.. */
	n = strlen(res+16) + 16;
	DoMD5(res2, res, n);
	gamma = downsample(res2);

	ircsprintf(result, "%X.%X.%X.IP", alpha, beta, gamma);
	return result;
}