Author:

Name: Don Yang
Location: US - United States of America (United States)

To build:

    make

To use:

The author suggested that after compilation one should look at the generated files. For instance:

    less generated1.c
    less generated2.c
    less generated3.c

Try:

    ./try.sh

Judges’ remarks:

Code should be readable in any direction! This tool will help you rotate your code so you don’t have to stare at it sideways or even upside down. Rotating this program left and recompiling will reveal other tools including a right rotate and a shift program.

Strange things happen when the world is upside down! It is entirely possible that this remark is completely misleading.

What exactly does the shift program do?

Like a great sliding puzzle (hint) this entry has six more programs that will reveal messages and one more tool that can be used to reveal the final message hidden in the original source. All of these can be created using combinations of ./left, ./right and ./shift and the additionally generated programs.

Author’s remarks:

Tools’ usage:

Nuko is a text rotator: given some text in stdin, Nuko will write the same text to stdout, but rotated 90 degrees counterclockwise.

    cc prog.c -o left
    ./left < input.txt > rotated_counterclockwise.txt

Due to static memory allocation, only the first 1K columns by 1K rows are rotated, the rest are silently ignored. But fear not, a separate tool with dynamic memory allocation is included:

    ./left < prog.c > r1.c
    cc r1.c -o right
    ./right < input.txt > rotated_clockwise.txt

For variety, this second tool rotates clockwise instead of counterclockwise. Also, it can handle files larger than 1024x1024, depending on how much memory you have.

Of course we wouldn’t stop with just two rotations, if we continue to rotate counterclockwise once more, we get another program. This program outputs a single message to stdout, which is the name of the series that Nuko came from.

    ./left < prog.c | ./left > r2.c
    cc r2.c -o msg0
    ./msg0

There is one final rotation, which produces a tool that removes leading whitespaces:

    ./left < prog.c | ./left | ./left > r3.c
    cc r3.c -o shift
    ./shift < input.txt > no_leading_space.txt

Where this might be useful, besides ruining the formatting of certain files, is that it completes the set of tools needed to solve the puzzle that is embedded in prog.c.

Puzzle box:

Notice how the edges of prog.c contain two notches. By rotating prog.c and removing leading space, the code would be shifted one space toward one of those notches (and create a new notch on the other side). This shifted code behaves slightly different from the original program. For example, here are two more messages that can be produced:

    ./shift < prog.c | ./right > msg1.c && gcc msg1.c -o msg1 && ./msg1
    ./shift < prog.c | ./left > msg2.c && gcc msg2.c -o msg2 && ./msg2

In total, there are 9 embedded strings that can be produced via a sequence of rotates and shifts, one of which can be used to extract the 10th final string from prog.c. The intent is to simulate those wooden puzzle boxes that can be opened by pushing and shifting various well-concealed seams. Thus, finding the correct sequence of rotates and shifts is left as an exercise to the reader (but if you are really lazy, just read the Makefile.

Source code for all the tools needed to solve this puzzle are embedded in prog.c, all you need is a C compiler.

Features

Code compiles when rotated 4 ways. This required a bit of patience to achieve. Code still compiles even with one column of text shifted. This required even more patience.
All rotated and shifted variants compile without warnings. This involves various tweaks to satisfy cases where the compiler is overly protective, including but not limited to the “1125” at line 4 as opposed to “1025”, to satisfy -Waggresive-loop-optimizations.
CRC32 of the code is embedded in the code itself.
Process for writing prog.c is available in obfuscation.html.

Compatibility:

Nuko and the rotated tools accept only ASCII files where each character maps to exactly one byte. Also, end-of-line sequence is assumed to be LF only, as opposed to CR-LF. All control characters are treated like normal printable characters, so files containing tabs will look weird after rotation, for example.

Nuko has been verified to work with these compiler / OS combinations:

gcc 4.8.4 on Linux
gcc 4.9.2 on Linux
gcc 6.1.0 on JS/Linux
gcc 6.3.0 on Linux
gcc 6.4.0 on Cygwin
clang 3.5.0 on Linux
clang 3.8.1 on Linux
clang 5.0.1 on Cygwin
tcc 0.9.25 on JS/Linux

Nuko compiles without warnings with all compilers above, even with -Wall -Wextra -pedantic for gcc and clang.

NOTICE to those who wish for a greater challenge:

If you want a greater challenge, don’t read any further: just try to understand the program without running the command below.

If you get stuck, come back and run the following command:

    ./msg9 < prog.c

But what combinations will generate ./msg3, ./msg4, ./msg5, ./msg6, ./msg7, ./msg8 and finally ./msg9?

For more hints try:

    make details

And for the last word, try:

    make the_last_word

Inventory for 2018/yang

Primary files

prog.c - entry source code
Makefile - entry Makefile
prog.orig.c - original source code
try.sh - script to try entry
obfuscation.html - JavaScript page with full recording of development

Secondary files

2018_yang.tar.bz2 - download entry tarball
README.md - markdown source for this web page
.entry.json - entry summary and manifest in JSON
.gitignore - list of files that should not be committed under git
.path - directory path from top level directory
index.html - this web page

The International Obfuscated C Code Contest

2018/yang - Most shifty