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November 19, 2016

exFat patent avoidance

I've been looking at the patents that Microsoft holds on the exFat filesystem, to see whether they might be avoided, so that support could be included in Linux distributions. One, relating to hashes, https://www.google.com/patents/US8321439 seems quite straight forward, because it gives a specific way of calculating the hashes using rotations. I've created some code, included below, that shows that the same values can be calculated in a way that doesn't involve rotations, nor any operation that can be construed as a rotation.

Another patent https://www.google.com/patents/US8583708 deals with the structure of the file system. It's even more of junk patent than the first one, but be that as it may. An odd requirement in the claims is  "based on a determination that the critical primary directory entry is not recognized, preventing the volume from being mounted". It's difficult to see why that's in a claim at all, unless it was intended to disguise how little is being invented. Anyway, it seems to offer a way out - mount the volume anyway. Indeed it appears to me that the exfat-nofuse package actually does that.

There may be other patents that I'm not aware of.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main(int argc, char *argv[]) {
        if(argc < 2) {
                fprintf(stderr, "%s string\n", argv[0]);

        /* Microsoft patent algorithm */
                unsigned char *cp = (unsigned char *) argv[1];

                unsigned x = 0;
                while(*cp) {
                        /* Do rotate. */
                        x = ((x & 1) << 31) + (x >> 1);

                        /* Add. */
                        x += *cp++;
                printf("MS  value: %u\n", x);

        /* Alternative algorigthm. */
                unsigned rx;
                int i = 0;
                long long xl = 0;
                unsigned char *cp = (unsigned char *) argv[1];
                while(*cp) {
                        long long xc = ((long long) *cp++) << i;
                        long long av = ((xl & (1 << (i - 1))) << 32);
                        xl += av + xc;

                        if(i == 32) {
                                xl >>= 32;
                                i = 0;

                rx = (unsigned) (xl >> (i - 1));

                printf("ALT value: %u\n", rx);

        return 0;


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