ecto / otter

:see_no_evil: one time pad lisp toy

intro

I decided I wanted to try my hand at lisp. I couldn't think of a good first project, but I remembered something from my childhood. When I was a kid, I "helped" my dad write a chapter in a C++ book, meaning I mostly sat on his lap and asked dumb questions. Our chapter was on crypto, and he wrote a one-time pad program for one of the examples. I figured it would be fun to do it on my own! So.. that's why this exists. You probably shouldn't use it for anything important.

install

Aparently there are a million different lisp implementations (I guess polish notation is easy to write interpreters for?). I use OSX and I chose clisp because it was the first in a brew search lisp. So on OSX:

brew install clisp

Download my code:

git clone https://github.com/ecto/otter.git
cd otter

and run it:

./otter

Usage: otter [command]

Commands:
  help                           display this message
  gen [bytes=4096]               generate [bytes] of random data
  enc [input] [pad] [offset=0]   encrypt [input] file with [pad] file starting at [offset] bytes of [pad] file
  dec [input] [pad] [offset=0]   decrypt [input] file with [pad] file starting at [offset] bytes of [pad] file

If you want, you can symlink it:

ln -s $(echo `pwd`/otter) /usr/local/bin/otter

use

Create a file you'd like to encrypt:

echo "hello, world" > in
cat in
# hello, world

Create a pad:

otter gen > pad

Send the pad to your friend on a secure medium (not the Internet)

Encrypt the file:

otter enc in pad > out
cat out
# L\}hd2><`
# yours will be different, because the pad is different

Send the file to your friend however you want!

Decrypt the file:

otter dec out pad > message
cat message
# hello, world

considerations

The security of one time pads relies on the key material not being used more than once, and that it remains secret (hopefully this is obvious). A common OTP practice is to generate a very large pad, and share it ahead of time.

If you'd like to generate a 10 megabyte pad, you can do so by adding the pad size to the gen command:

otter gen 10000000 > pad

When you encrypt or decrypt something, keep a running count of the bytes you've traversed, and pass them to the enc/dec commands.

For example, we can find the size of the message we encrypted earlier:

ls -l in
-rw-r--r--  1 ecto  staff  13 Apr  4 18:51 in

So we've used 13 bytes of the pad. The next time we want to encrypt something, we use 13 for the offset:

echo "hope all is well" > second-in
otter enc second-in pad 13 > second-out

Now when our friend decrypts the message, they will also need to specify the pad output:

otter dec second-out pad 13 > second-message

license

        DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
                    Version 2, December 2004

 Copyright (C) 2004 Sam Hocevar <[e-mail]>

 Everyone is permitted to copy and distribute verbatim or modified
 copies of this license document, and changing it is allowed as long
 as the name is changed.

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

  0. You just DO WHAT THE FUCK YOU WANT TO.