A few years ago I wrote two posts on explaining the workings of some obfuscated programs. After these posts other stuff took priority and I forgot about the world of obfuscation for a while. Now though, I find myself returning to it. For reasons I’ll explain in a different post I find studying obfuscated code to be quite valuable. So, I thought to myself ‘Well, those last programs I examined were honestly pretty simple. What would it be like to research a truly obfuscated program?’

This post is the story of that journey. I will discuss the code, how I got such old and obscure code to run, as well as include snippets from my conversations with one of the original authors (who was very helpful in figuring some of this out). If all that wasn’t enough I managed to obtain the original PDP and VAX source code, it will be hosted here with permission. I want to give a huge thank you to Sjoerd Mullender and Don Libes for their assistance and permission in reproducing some of this material.


Because older obfuscated programs tend to be easier to dissect I decided to stay in the early years of the IOCCC (International Obfuscated C Code Contest). Among those early programs one sticks out as truly interesting - and that is the first IOCCC winner mullender.c, which I’ve also replicated below because it’s quite short.

  short main[] = {
	  277, 04735, -4129, 25, 0, 477, 1019, 0xbef, 0, 12800,
	  -113, 21119, 0x52d7, -1006, -7151, 0, 0x4bc, 020004,
	  14880, 10541, 2056, 04010, 4548, 3044, -6716, 0x9,
	  4407, 6, 5568, 1, -30460, 0, 0x9, 5570, 512, -30419,
	  0x7e82, 0760, 6, 0, 4, 02400, 15, 0, 4, 1280, 4, 0,
	  4, 0, 0, 0, 0x8, 0, 4, 0, ',', 0, 12, 0, 4, 0, '#',
	  0, 020, 0, 4, 0, 30, 0, 026, 0, 0x6176, 120, 25712,
	  'p', 072163, 'r', 29303, 29801, 'e'

Where do you start with a program like this? Well, the hint provided by the IOCCC is always a good first step. From this hint, we can learn a few things necessary to understanding how the program works:

  • Perhaps most importantly, this program will only run on a PDP-11 or VAX-11.
  • The C startup routine - provided by crt0.o - transfers control to a location named ‘main’ - usually intended to be a function, but in this case it’s an array of shorts
  • These shorts (represented in different formats to enhance obfuscation) form a meaningful set of PDP-11 and VAX-11 instructions
  • The first instruction is a branch to the PDP code, and due to the way VAX calls the main ‘function’ it skips the first word of the program (I’ll discuss this more later).
  • The program prints a string

Essentially, the program is a bunch of machine code instructions encoded in various formats. It runs because this code predates ANSI C by a half decade and old compilers seem to have had significantly less qualms about ‘main’ being a function or not (I tried to define ‘main’ as an array of shorts with a modern compiler and it did not like that one bit).

Running the thing on a PDP-11

I admit it, the PDP-11 was before my time. I grew up in the Windows 95/98/2000 era, and until recently I didn’t even know what a PDP-11 was. This ignorance left me kind of stuck, and a brief look on eBay confirmed that I was not going to get a working PDP-11 any time soon. Fortunately though, an emulator exists. Enter SimH, it’s more of a suite of emulators for a wide range of historical computers - of which the PDP-11 is one, and lucky for us the VAX-11 is another.

After a few false starts and misadventures with 2.11BSD, I realized I had no idea what I was doing so I decided I needed some help. The two original authors of this program are Robbert van Renesse and Sjoerd Mullender, why not try to contact them? So I did. Robbert didn’t answer my e-mail, but luckily Sjoerd got back to me and was happy to discuss the program. He told me that the PDP-11 OS they used was Unix v7, so I proceeded on that route.

I managed to find a guide on installing Unix v7 onto this emulator that worked; and now with my emulator running, and OS installed, I could slap it into a file, compile, and run the thing. Below is what I got.

Running the program on the PDP-11 using Unix v7

The animation is a little choppy, but even still you can hopefully see that it is printing “:-)” across the screen until it is forced to exit (there is no clean way to stop it).

In my discussions with Sjoerd, it was revealed that the string it prints is not exactly “:-)” but instead " :-)\b\b\b\b” (NOTE: there should be 2 space characters at the start but my templating engine is removing one for some reason). It is exactly 9 bytes long. The first 2 characters are whitespace, followed by the :-), finally followed by four ‘\b’ characters. The ‘\b’ characters indicate a backspace. What does the backspace accomplish? Well, if you count 4 characters back from when they begin you’ll see we’re left with a single space - it is at this point that the program loops and prints the string again. This happens quickly enough that to our eyes it looks like the “:-)” is scrolling across the screen - a clever effect.

Examining the PDP-11 code

As it turns out, the C code is not very useful to examine directly. As it also turns out, the assembly isn’t much better. Below is a small sample of what the assembly looks like (from the PDP-11’s perspective):

    .globl  _main
    .word   0425 
    .word   04735
    .word   -010041
    .word   031
    .word   0
    .word   0735
    .word   01773
    .word   05757

So, what to do? Well, you can actually set up a cross compiler for the PDP-11 and if you go through that effort you can use the objdump program targeted at PDP-11 binaries.

Below is the relevant portion of the objdump - the whole thing is here, but not all of it is useful. Sjoerd confirmed that the PDP code is from _main+0x2c to _main+0x4a (inclusive). Of course, the first word is also a PDP-11 instruction. Everything I’ve ommitted is related to the VAX portion of the program which we will get back to. (It’s also not proper VAX code since the disassembler thinks everything is PDP-11).

   0:   0115            br  0x2c
  2c:   11c4            mov pc, r4
  2e:   0be4            tst -(r4)
  30:   e5c4 0009       sub $11, r4
  34:   1137 0006       mov r4, $0x3e
  38:   15c0 0001       mov $1, r0
  3c:   8904            sys 4
  3e:   0000            halt
  40:   0009            .word   11
  42:   15c2 0200       mov $1000, r2
  46:   892d            sys 55
  48:   7e82            sob r2, 0x46
  4a:   01f0            br  0x2c

Before going further there are 2 important things to know about PDP-11 assembly code:

  • The PDP-11 works primarily with octal. All the addresses and instructions are encoded in octal.
  • In an instruction like mov pc, r4, you are moving pc INTO r4, not r4 into pc. Personally, I got used to this pretty quickly.

The first number in the original program is 277, put that into a hex converter and you’ll get 115 like we see in the objdump. More interesting though is what you get translating it into octal - 425. How does 0425 get you br 0x2c though? Well for that you need to grab some manuals. I got my hands on a lot of old manuals for these computers but the most useful one was the PDP-11/45 Handbook from 1973. It would helpfully describe the instruction format, and exactly what it did. Below is an example of the page for the unconditional branch instruction.

PDP-11/45 Handbook Describing the Branch instruction

As you can see, it sets the program counter to pc + 2x the offset - which is 25 (octal) in our case. Since the program counter at the start of our program is 2 (because it points to the next word to execute), we get 2 + (25 x 2), or 2 + 52 = 54, which translating back into hex gives us 2c. Cool!

The meat of the code

Now that we know how the branch works we can examine the main stuff - I won’t go into huge depth with the manuals for each little instruction but doing so is fun if you’re bored and enjoy that sort of thing.

The next relevant portion of code is this:

  mov pc, r4
  tst -(r4)
  sub $11, r4
  mov r4, $0x3e
  mov $1, r0
  sys 4
  .word   11

We move the program counter into the r4 register and then use the ‘test’ (tst) instruction. This has the effect of clearing the some flags in the PSW (processor status word), and because the register is using ‘Autodecrement mode’ it will subtract 2 from the value stored at r4. When using autoincrement / decrement, it will go by 1 in byte instructions or 2 in word instructions. After this we subtract 11 from r4, but 11 is actually 9 (used to octal yet?). Then we store r4 into…0x3e? Well, if you look back at the objdump you’ll see that 0x3e is ‘halt’ or 0000. This mov instruction overwrites that value with the contents of r4. We move 1 into r0 (1 indicating STDOUT), and call ‘sys 4’. The next line will be the value of r4 as just mentioned, and .word 11 simply inserts octal 11 directly into that spot. If you’re wondering what exactly is in r4 and why we’re doing all this stuff, be patient - we’re getting there.

If you’re familiar with system calls, you probably know what’s going on. If not, all of this is to setup the write system call. I actually made a post on the retro-computing stackexchange asking about the workings of this call internally - I recommend checking it out if you want further clarity on how this works.

After this we have the final 3 lines of PDP code which are:

  mov $1000, r2
  sys 55
  sob r2, 0x46

We put 1000 into r2, and call system call 55. After this, we use the ‘sob’ instruction - which is ‘subtract one and branch’ and not an invitation to cry - to see if r2 is 0 yet. If it isn’t, we branch to the previous line and call system call 55 again. loop until r2 is 0. Syscall 55 eh? Which one is that? If you look up system call numbers you might be tempted to believe it’s the reboot system call - but alas, it is not. This is an undefined system call, and Sjoerd states that these 3 lines were only included to introduce a slight delay to the program (presumably for more legible printing on the screen).

Finally, after the delay has delayed things, we branch back up to 0x2c and the fun begins all over again.

Where is the string?

So that’s all well and good. We setup a system call to print the string, we delay a bit, then we repeat. Where is the string though? If you guessed that r4 is a pointer to the string you’d be correct - but the string is not in the PDP-11 code. So you might think it’s in the VAX code, but it is not there either. It is at this point that it’s useful to understand the ‘segments’ of this program - i.e, how it is laid out. It is very deliberate, and part of the design.

I hope you find my ASCII art explanation useful, it doesn’t look good on mobile though so if you’re reading on mobile here is the same diagram as a picture

           PDP-11 Entry
                 |       VAX Entry
                 |           +
                 v           |
         +-------+------+    |
   +-----|Initial Branch|------------> Address  _main+0x0
   |     +--------------+    |
   |     +--------------+    |
+--------|   VAX Code   |<---+-------> Address  _main+0x2 - _main+0x22
|  |     +--------------+
|  |     +--------------+
|  |     | String Bytes |------------> Address  _main+0x23 - _main+0x2B
|  |     +--------------+
|  |     +--------------+
|  +---->|   PDP Code   |------------> Address  _main+0x2C - _main+0x4A
|        +--------------+
|        +--------------+
+------->|   VAX Code   |------------> Address  _main+0x4C - _main+0xA0

As you can see above, the printed string is not in either portions ‘code’ - rather it exists between the two sections. For the PDP, it is right before it. This is why we move the program counter (pc) into r4 and then decrement it by 2 and subtract 9 - the string is 9 bytes long. This leaves us with the address of the beginning of the string in r4.

This also demonstrates why the PDP-11 objdump file is only partly useful, it interprets these bytes as instructions when they are not. This initially misled me into believing that they have some purpose in terms of the codes execution but nothing that complex is going on. That is, the bytes do not do ‘double duty’ as code and data - it is simply data carefully placed between two code sections.

We can verify this in adb (‘Advanced Debugger’ - not Android Debug Bridge, also; if this is advanced I’d hate to see the regular ‘db’ program). Below is a picture of an adb session showing the subtraction, a dump of the registers, as well as the printed string and the octal representation of the bytes.

ADB Session showing how to locate the string

You’ll probably notice the numbers are all octal (even the bytes of the string). They also do not match up with the objdump numbers - this is because of the C startup code.

Looking at the VAX code

I know what you’re thinking - all this old stuff is neato but it sure would be better if we added docker to the mix. Well, I couldn’t agree more. I didn’t want to go through the hassle of installing another OS on one of these old machines and luckily for me someone has distributed a docker image with VAX-11 running 4.3BSD on SimH.

I booted that all up and compiled the program and it worked, as expected. No gif of this one running - it looks exactly the same as last time.

The assembly code for the VAX program is unhelpful, much like the PDP assembly code. Below is a sample:

		.long   0x9dd0115
		.long   0x19efdf
		.long   0x1dd0000
		.long   0xbef03fb
		.long   0x32000000
		.long   0x527fff8f
		.long   0xfc1252d7


To complicate matters, there is no objdump program (that I could find) available for the VAX architecture. I suspect this was largely because it was used in proprietary spaces and never really had a market outside of large businesses / universities. So basically, the only way to look at this is with the dbx debugger - a debugger originally shipped with BSD.

So, let’s see what dbx says…

00000408  bleq  40b
0000040a  pushl $9
0000040c  pushal        42b
00000412  pushl $1
00000414  calls $3,426
0000041b  cvtwl $7fff,r2
00000420  decl  r2
00000422  bneq  420
00000424  brb   40a
00000426  halt
00000427  halt
00000428  chmk  $4
0000042a  ret
0000042b  addp4 $20,$3a,$2d,$29
00000430  cvtps $8,$8,$8,-7151(r4)
00000437  crc   2533(r4),$0,$37,$11
0000043e  ldpctx
0000043f  halt
00000440  addl2 $15,$1
00000443  halt
00000444  ret
00000445  bisb3 $0,$0,$9
00000449  halt
0000044a  subl2 $15,$0
0000044d  rei
0000044e  cmpc5 (r9)+,(r2)+,-(sp),*1537(r0),$0
00000458  ret
00000459  halt
0000045a  halt
0000045b  rsb
0000045c  remque        $0,$0

Double Yikes

Yes, it is significantly more complicated than the PDP stuff. Trying to figure out what is even relevant is difficult, and perhaps that is because this code was never written by a person. Here is what Sjoerd said:

I have never known a lot about the VAX assembly, so we used the C compiler to create the VAX code. We didn’t write it ourselves from scratch as we did with the PDP code. This is the reason why the VAX code is more complex, including the extra data after the PDP code.

I tried to get up to speed on some VAX assembly but most of it is horribly complex - especially compared to the PDP. The amount of times the manual told me that the behaviour of something was ‘UNPREDICTABLE’ (it always used all caps) was…a lot.

So to save my sanity I gave up on understanding all of it (if you’re a VAX expert though I would be curious to hear your thoughts - get in touch). That being said some things do deserve explanation.

  • The ‘calls’ instruction. This invokes a procedure with a number of arguments on the stack. This instruction also uses the first word at the destination as a mask of registers to be saved, this is how we’re able to skip over the first branch to PDP code when running on VAX. (In that case, it is the C startup routine invoking ‘calls’)
  • ‘chmk $4’. This instruction switches to kernel mode, and I can only assume $4 is the write system call. I did not find great explanations about how this really works

Procedures being invoked with calls explains a bit of the above code. Specifically this bit:

0000040a  pushl $9
0000040c  pushal        42b
00000412  pushl $1
00000414  calls $3,426

This is putting on the stack the length of the string, the pointer to the string, and the file descriptor we’re writing to - STDOUT.

That’s all to really say about the VAX stuff. I wish I could explain what the bit after the PDP code does but it seems like no one has a good idea.

How the program happened in Sjoerd’s own words

To close this out, here is the story of the program. I was curious how it came about, and how Sjoerd and Robbert heard of the IOCCC - afterall this was the first year it existed.

Robbert and I were students at the VU (Free University in Amsterdam) at the time (mathematics with CS as our major since there was no CS curriculum when we started). We had an assignment to create a pair of programs for the computer networks course. The programs were supposed to send data reliably from one program to the other over an unreliable channel. This channel was simulated with a pair of pipes.

We decided for fun to create an obfuscated set of programs, only for the PDP, to do this, but cirucumventing the channel. (I.e. cheating, hence the needed obfuscation.) Our programs worked and we handed them in.

Of course, the teacher had a good laugh and then rejected our submission. (We knew him well, so we could get away with this.)

Then the IOCCC came along. I don’t remember how we heard about it, but at the time there was a world-wide messaging network Usenet where we read a bunch of newsgroups. I’m sure it was announced there and we saw it.

Since we had just recently created these obfuscated programs we decided we could use the same technique for an obfuscated C program. We upped the ante a bit by making it “portable”.

To add to the obfuscation, we used different formats for the integers in the array, some in decimal, some in octal, some in hexadecimal, and when the value would fit, some as an ASCII character.

The rest is history.

Since this was the first contest, we hadn’t seen any old entries, nor had any of the other contestants. Of course we knew about #define and tricks you could do with that, but we didn’t need that for this program. In fact, we made it as “standard” as possible. At the time there was this program called “cb” for C beautifier which would reindent your program to make the layout look better. Our program is idempotent under cb.

The Original Source

Throughout the course of this research Sjoerd mentioned to me a book from the early 90s called “Obfuscated C and Other Mysteries” by Don Libes. It contained its own analysis of mullender.c and he thought it was akin to what I was doing. Naturally, I had to have it. So I found a used copy for a reasonable price and patiently waited for it to arrive (Note: if you want this book keep checking eBay - not amazon!!).

It did arrive, and it’s quite wonderful. Below is the cover, table of contents, and the 5 and 1/4 floppy disk it came with (the only floppy of that size I’ve ever owned).

Obfuscated C and Other Mysteries

The interesting thing is not that I have this book though. No, this book is interesting because it contains original source code for the PDP and VAX programs as well as the C program that generated the mullender.c submission. Without these there is no avenue to explore this program except through debugging tools. Graciously, Sjoerd Mullender and Don Libes are allowing me to Host the excerpt regarding mullender.c here for all to read (PDF Warning).

All of the code credit goes to Sjoerd Mullender and Robbert van Renesse. Copyright (1993) for the book belongs to Don Libes and has been reprinted with permission.

Extra bits

This is just a collection of random things that happened that couldn’t really find an appropriate spot in the main post.

  • My backspace key didn’t work on unix v7. This is a known issue, and I tried to patch the kernel, but no luck.
  • Because I’m working in a modern shell, sometimes certain bits of old code would display with font ligatures which I found quite funny
  • I had to learn the ‘ed’ editor to input the program on unix v7. 4.3BSD had vi but I wouldn’t say it worked well
  • Originally I thought I could recreate the programs behaviour on a bare-metal PDP-11, talk about ambitious
  • VAX is by far the most complicated old computer I’ve read about. Find a manual and check it out
  • SimH documentation is unfortunately pretty poor. Their PDP-11 documents only describe the peripherals available and what various bits mean. It does not explain much about working on the PDP-11 command line (i.e, what you get before booting unix). I suspect this stuff is largely used by people who know what they’re doing.
  • 4.3BSD had this weird bug where the current date and time would just print on the screen no matter what I was doing
  • dbx has a ‘gripe’ command which lets you complain to the programs maintainer - unfortunately it didn’t work.
    dbx gripe command

Documents and Websites I used

  • Many PDP-11 handbooks, mostly the PDP-11/45 Handbook
  • VAX-11 Architecture Reference Manual revision 6.1
  • UNIX Assembler Reference Manual
  • KDJ11-A CPU Module User’s Guide (PDP-11 CPU)
  • Operating Systems, or how Unix V6 really works
  • Unix Programmer’s Manual 7th Edition, Vol 2a
  • PDP-11 Architecture
  • Common VAX Instructions
  • Hex, Dec, Oct, Bin Converter (MVP!!)
  • ADB Documentation
  • dbx overview - Even though this is old it is not old enough to cover the version of dbx I used, it still helped a bit though.