drbig/cryptofloppy

The super crypto floppy that would make you secure 30 years ago

cryptoFloppy

x86 bare-metal password-protected notepad on a single floppy

Features

• All you need is an i386 personal computer with a floppy drive
• No Operating System required
• Encrypt and decrypt up to 9 512-character long messages
• All data is stored on that single floppy
• Perfect for dead-drops
• Advanced encryption will protect your secrets
• Simple interface, no manual required

And seriously

I wanted to try out writing something low-level in Assembly for x86. I thought writing a bootloader would be a nice exercise, but after I've written the first stage I had this idea for something more fun. Hence the cryptoFloppy.

TL;DR:

• This is a for-fun project
• It actually works - tested on a real computer (I'm that brave)
• The 'encryption' for now is a simple XOR
• I'd happily include a proper cipher, there is still plenty of space left
• Anyone up for the challenge of writing a modern cipher for x86 protected real mode? :) (yeah, I'm new here)
• UI can always use more love
• People actually knowing what they doing are welcome to improve the code in terms of performance, readability and general best practices
• I've tried to comment the code

Building and testing

For building on Unix-like systems you'll need NASM, dd and make. There is make target for testing for which you'll need QEMU - of course you can test on real hardware too.

Example build and test session:

$ make clean floppy.img emu

This will clean the bin files, assemble the sources, create a zero-filled file (space for saving the messages), bundle up the floppy image and run it via qemu-system-i386.

You should obviously always test your image in an emulator before even thinking about trying it on a real hardware, as at this level the code may wipe out your harddrives, produce seizure-inducing video and fax your mother a nasty letter. Though most probably bad code will just hang/reboot the machine.

For those brave enough and with an actual vintage working floppy drive writing the image to a real floppy diskette is just (adjust the device path if needed):

$ dd if=floppy.img of=/dev/fd0

Contributing

Fork the repo, hack away and remember to share back by making a Pull Request. All skill levels welcome. Feature ideas short-list:

• Better UI (and there is a lot under this one)
  • Proper new-line handling when writing the message
  • Cursor keys for editing
  • Character left counter
  • Password confirmation
  • Write confirmation
  • ...
• Work from any drive (not only A:)
• Read and write more than just 9 sectors
• Pad messages with random bytes to fill the whole sector

Feel free to add issues with ideas/concerns etc.

If you're a seasoned x86 assembly hacker you may go over the code and apply best practices and optimisations (with comments, so we can learn). Or if you're looking for a challenge - port some real crypto.

Learning corner

Links to readings I found useful and/or interesting:

• Bootloader introduction
• Full bootloader code
• BIOS interrupts
• Agner Fog's manuals
• Ciphers for protected mode

Licensing

Seriously? Standard two-clause BSD license, see LICENSE.txt for details.

Copyright (c) 2015 Piotr S. Staszewski