# To create a shared secret shared among M people with N+1 needed to reconstruct
./grothe -secret 1-/dev/urandom 2-/dev/urandom ... N-/dev/urandom \
1+shared1 2+shared2 ... M+sharedM

# To reconstruct the original (an arbitrary combination of N+1 or
# more shared files with their proper numbers)
./grothe I-sharedI J-sharedJ K-sharedK ... +reconstructed

Try:

./grothe -grothe.c 1+grothe.c.1 2+grothe.c.2

./grothe -cookie_recipe.txt 1-/dev/urandom 2-/dev/urandom \
1+cookie_piece1.dat 2+cookie_piece2.dat 3+cookie_piece3.dat 4+cookie_piece4.dat

./grothe 1-cookie_piece1.dat 2-cookie_piece2.dat 4-cookie_piece4.dat
+cookie_recipe_restore.txt

Selected Judges Remarks:

Also known as Best abuse of the judging process.

The IOCCC 2012 submission page didn't enforce the 2048 meaningful character limit. This entry, as submitted, weighed in at 2222 chars but was, luckily for it, extremely easy to bring down below the limit. There were a number of strings that when concatenated allowed the program to slide under the limit.

It was clear to the judges that the extra size was an entry beautification step. Had it not, this entry would have been rejected in round 0.

The judges also took some liberty obfuscating this entry a litte more.

How does this entry work? The only place multiplication is used is to perform the atoi conversion.

Author's comments:

Description

This program implements a version of Shamir's Secret Sharing. Shamir's Secret Sharing allows you to encrypt a file into N parts where it will take M parts to recreate the original file. E.g. you can split the secret recipe for Crystal Pepsi into 3 parts and require two of them be available to extract the original recipe.

Actually surprisingly useful for an IOCCC entry :-)

Usage

To share a secret

E.g.

file to be shared - cookie_recipe.txt number of pieces - 2 + file to be shared total number of pieces created by program 4 pieces needed to reassemble recipe 3 or greater

example usage

# to share secret into 4 pieces of which at least 3 will be needed to get back the recipe

./grothe \
    -cookie_recipe.txt 
    1-/dev/urandom 2-/dev/urandom 
    1+cookie_piece1.dat 2+cookie_piece2.dat 3+cookie_piece3.dat 4+cookie_piece4.dat

# put recipe back together using 1, 2 and 4th piece

./grothe \
    1-cookie_piece1.dat 2-cookie_piece2.dat 4-cookie_piece4.dat
    +cookie_recipe_restore.txt

Limitations

program is limited to 256 input and output files and will slow down dramatically when you get past 20-50 inputs and outputs

Obfuscations

nothing is done consistently throughout the program E.g i+=1, i=i+1, i++ and ++i are all used in the program. The mark of true code buggery. - "A foolish consistency is the hobgoblin of little minds" - Ralph Waldo Emerson
uses the most powerful obfuscation of all "Math". "Any sufficiently advanced cryptography is indistinguasble from magic" - Apologies to Arthur C. Clarke
shadows variables by using brackets in main program to create local copies of variables that are the same as global scope variables. -Wshadow should probably be a part of -Wall in gcc
uses array_index[array_name] instead of array_name[array_index] in several places. Still don't know why C continues to allow this ( error strings are encoded with offsets that are also used to select them in the case statement. All strings are located in one function for easy localization
liberal use of octal numbers, hexadecimal numbers and regular decimal numbers throughout program
variables and function names are semi-random and upper and lower case characters are used
use of #defines from stdio.h, stdlib.h for values _STDIO_H and EXIT_SUCCESS used to represent 1 and 0 respectively in the program
arrays only need to be 256x256 most are set to greater values than they need to be so trying to figure it out can be a bit tough

References

Shamir's Secret Sharing - http://en.wikipedia.org/wiki/Shamir%27s_Secret_Sharing
David Madore's implementation of Shamir's Secret Sharing - http://www.madore.org/~david/programs/programs-1.36.html
Steve's Recipe Database - source for the $25k Cookie Recipe - http://recipes.stevex.net/