An Interesting Trick with XOR to Flip Bits in a Byte!

Nice bit flipping! Back to school again 🙂

Nice bitflip technique, the codebase at work actually uses if conditions for it lol

Using a single 'and' and a single 'xor', you can also do :

Set some bit
Reset other bits
Invert other bits,
Leave other bits unchanged.

Suppose you that you want to (counting from left to right) :

- invert bits 1 and 2
- leave bit 3 unchanged
- set bit 4 and 5
- invert bit 6
- leave bit 7 unchanged
- reset bit 8

How can you do that using just two operations ?
simply 'and' the byte with the following value
11100110
(this resets bits 4,5,8, leaving the others unchanged),
and xor the result with the following value:
11011100
This inverts bits 1,2
than it leaves unchanged bit 3
bit 4 and 5, that were cleared by the 'and', are now set
bit 6 is inverted by the xor
bit 7 is left unchanged
bit 8, that was cleared by the 'and', is left reset.

When is speedtestg returning

And another good 101 class on bit magic!

Why use XOR to flip bits when the NOT operator does the same thing?

Oh good. You mentioned RAID. You know, it took me a while to understand how a RAID5 array could retain all the data if any one drive was missing. But once I understood how XOR worked, I had it figured out.

XOR is just an ADD without the carry.

So General Motors used a McAfee or a Norton program (I forget which) that would expand a .ZIP or a .7z file that was being copied to a jump drive and scan its contents to determine if the ZIP file contained any sensitive data. This effectively limited the throughput of a copy routine to roughly 1GB per day. Kinda slow...

After quite a bit of poking around, I found the the program looked at just the first two bytes of the file to determine what kind of file it was. If it started out with "PK", then it was a ZIP file. If it started out with "7z", then it was a 7Zip file.

So I created a program that XOR'ed the first two characters of a file with my favorite number, 69, and stored them back to the file and saved it. The protection routine would not detect the nature of the file because the signature was changed and thus allowed the file to be copied to the removable media directly. Running the program a second time returned it to its original contents.

The exploit has been fixed (I wasn't trying to hide what I was doing, and I went as far as to share it with IT), so consider either XOR'ing the entire file, or do the sensible thing and avoid working for GM altogether.

In Python, swapping values has been made very easy:

a,b = b,a

Done.
But the underlying function probably uses the method Gary describes.

I'd rather flip burgers on a BBQ but it's winter and I'll settle for this.

very interesting, thanks Gary. Don't see code in repository.

Takes me back to my training at Arborfield Garrison 40 years ago. Subsequently, used for many years developing industrial robots for the pick & place industry.
Great video, many thanks. J

This is nice refresher for advanced programmer 😛