The 0xC days of Christmas

08 January 1997

The first piece of obfuscated code I ever encountered was this famous (in some circles1) one liner, original author unknown. I don't think I was ever quite the same afterward.


If it isn't immediately obvious, this program prints out its own source code. It's a member of the class of self-reproducing programs, and when I was new to computer science, I found it a startling fact in its own right that this class even has members. This program has one of the primary virtues of great obfuscated code: When executed, it does something impressive.

The true greats in the field, though, do much more. A really fine obfuscated program will be artistic and inscrutable in source form, execute in a highly circuitous fashion, and produce output that (often only in hindsight) follows obviously from the source. Two of my favorites have their source shaped into an ASCII square root sign and pi symbol. Their outputs are left as rather trivial exercises for the reader.

Obfuscated code has a special place in the heart of many hackers. The popularity of the annual "International Obfuscated C Contest" (IOCCC) is the best evidence; some of the programs from years past obviously required herculean efforts by the entrants. I was lucky enough to attend the unveiling of the winners at USENIX one year, and I sat transfixed, mouth unswayingly agape, as the ever more unbelievable winners were trotted across the stage.

Writing an obfuscated program is such an interesting exercise because it's hard to break every rule of understandable programming, simultaneously. Unintentionally, clarity slips through the cracks, comprehension becomes possible, and the flawed grokkable code must be mercilessly hunted down and shaken apart. An interesting walkthrough on writing an obfuscated "Hello World" is here.

Now, obfuscation is itself a worthy subject, but I am here to talk about my experience with the deobfuscation2 of a program that turned out to be an IOCCC winner from 1988, though I did not know that during its unravelling. Someone had hacked the code to remove all the whitespace, introduced a small bug in the output, and posted it to Usenet sometime last year.

The challenge was to fix the bug in this code. This version prints out the words to 'The Twelve Days of Christmas', but the partridge is in a 'per tree' instead of its expected 'pear tree.' I encountered it years ago, marvelled at it along with some friends, and left it in an archive, gone but not forgotten. When I realized this column would appear on Christmas day3, its subject matter suddenly became obvious.

What to do? The first step was to introduce some sensible whitespace. I have limited experience with emacs, but having hung out with various wizards in my life, I knew it must hold some magic. So I approached our resident emacs fanatic and solicited his wisdom. He too was intrigued by the task, and we ended up hovering over his machine for the next several hours.

We tried both the Unix program "indent" and some emacs modes, but neither was terribly fruitful, so we ended up adding most of the whitespace by hand. This lent an appearance of readability without adding much in the way of insight, so we went on to rename the variables based on their positions in the parameter list: arg1, arg2, arg3.

Interesting, but still a mess. Pull out the two large string literals and #define them to variable names. Some help. Group the various expressions with parentheses, because there are few alive who can eyeball an expression like


and immediately know the order of evaluation. A fair amount of help. Take the glut of ternary operators (C's much loved "inline if-then-else") and change them to actual if-then-else constructs. This was without a doubt the biggest step forward; a clear sort of state machine now emerged, with various key control flows altered by the values of the parameters to main. Through a huge series of recursive calls with different parameters, the machine passed through all the states necessary to output the song.

But where was the song?

Obviously the string literals held the answer. At first we thought perhaps the large string literal had all the information bit shifted or XORed with something else, possibly involving the smaller literal. I looked at the large literal in a hex editor to see if any obvious patterns emerged, but no dice. Then we noticed that the end of the smaller string literal was composed of normal ascii letters, the sort that would go into typical English text. Aha! It must be a translation table of some sort. We counted 31 characters of weirdness and 31 characters of normalcy in the 62 character string, lending further support to the theory.

The final confirmation lay in this nifty oneliner.

./xmas | perl -ne 'foreach(split(//,$_)) { print "$_\n"; };' | sort -u | wc -l

The xmas binary was the compiled obfuscated C source. This pipeline counts the number of unique characters in the entire song, and -- wonder of wonders -- there were exactly 31.

What exactly was in the large string, then, such that this bizarre code was able to unravel it to produce the song? We wrote this C program to find out. It loops through the main string and uses the translation table in the second to determine output. If I'd been at the keyboard instead of the magic maker, this would have been written in Perl, but what can you do.

This is the translated string. The '/' characters act as sequence points; the code operates by counting past various numbers of slashes before beginning to print. Now we can see how it prints each verse, and of course it is quite clear how to fix the bug; simply introduce the missing (untranslated) character into the main string literal.

A call to main is identified as a "day starting" call if arg1 == 2. Then arg2 holds the value of the day to print (plus one.) From this point, "On the" is printed, then the ordinal number of the day, then "day of Christmas my true love gave to me," all by calling main with different trigger parameters. Then, by stacking recursive calls to main, each telling it to count further back in the string before printing, the right number of phrases are printed.

Most of the numbers used to control the flow of execution are arbitrary. One rather clever trick is that the third argument to main is almost always ignored initially, then stealthily switched to a pointer to the large string literal containing the obfuscated song on a future call. This allows the passing of red herring arguments like "%s %d %d" that actually do nothing. Each character is output in a spectacularly wasteful manner; the untranslated character is compared to each one in the small string literal until a match is found, with each comparison involving a recursive call! Once it's identified, the character 31 later in the string is actually output.

One last mystery remained. How was the initial call to main distinguished from the later calls? There was no obvious means of distinguishing them, but execution obviously started with the first day. Let's see, what are the values of the arguments to main on the initial call? The value of arg1 (argc) will be 1 unless we supply arguments, and the argv and env pointers will point off at some arrays of strings.

Aha. The first major if/then/else hinges on whether arg1 is greater than 1, and a block in the else clause checks whether it is greater than 0. The only way it can satisfy this is if it's exactly 1, and that code calls this:

return main(2, 2, "%s");

The first 2 indicates the start of a day, and the second means to output the first day (it's always day + one.) Here, finally, is the fully commented and largely understandable version.

For the sheer thrill of it, and because there's supposed to be Java code with all the Deep Magic articles, I translated the same version we deobfuscated into Java (leaving the bug intact.) This was an interesting task for a number of reasons. Java is not fond of the cavalier way the original source interchanges integers and pointers. Java has a real boolean type that it insists you use for such constructs as conditionals. And worst of all, Java has no comma operator. We worked around all this, and added a bit of extra obfuscation in the process. Have a look.

This was a neat exercise. True, it has no practical value, but neither do any number of other geek activities. That's no reason not to do it!

May all the shrouds of obfuscation fall before your penetrating gaze. *

Source code referenced in this article: phillipps.c (the actual IOCCC entry), christmas-obfus.c, xlate.c, christmas.c,

-- Paul Phillips <> visualizes whirled peas.