Bounded-Error Log

It’s December, so it’s time for a lot of things, including Advent of Code. I will not be able to be as competitive as I was last year, and already lost a lot of points to a really silly mistake on day 1, but I’ll be playing when I can and golfing the problems when I have time (so far: 7 + 14 bytes).

As one might expect, Day 1 is not too complex, but the second part can be analyzed to some mathematical depth and was discussed a bit on Reddit; plus, it occurred to me recently that I set up KaTeX on my blog but never used it, so I was looking for an excuse to write some equations anyway.

The problem statement for part 2, in brief: We are tasked with calculating the total mass of fuel required to launch a rocket module of a given mass. For something of mass \(m\), one can compute the directly required mass of fuel by dividing \(m\) by 3, rounding down, and subtracting 2; if the result is negative, it is taken to be 0 instead. However, the directly required fuel also requires fuel itself, calculated from its own mass by the same procedure, and that required fuel requires fuel based on its own mass, and so on until you reach fuel with 0 requirement.

Call me maybe? nc rev.chal.csaw.io 1001

A rev with a nasty binary. There are so many functions. I do not like this binary.

Static Analysis

After staring at the sea of functions in IDA for a little bit, I gave up and tried dumb things instead.

come get me

http://web.chal.csaw.io:1003

This was a web challenge with a few pages. The “User” page displayed some user information:

Name: Alice
Email: [email protected]
Group: CSAW2019
Intro: Alice is cool

Name: Bob
Email: [email protected]
Group: CSAW2019
Intro: Bob is cool too

The “About” page simply told us, “Flag is located at /flag.txt, come get it”. The most interesting page was “Upload”, where we could view an example users XML file:

Welcome to pwn.

nc pwn.chal.csaw.io 1005

Ahhh, CSAW CTF. Amidst all the other CTFs where we’re competing with security professionals who probably have decades of experience and who follow security developments for a living or whatever, there remains a competition where scrubs like me can apply our extremely basic CTF skills and still feel kinda smart by earning points. Now that I’ve graduated and am no longer eligible, our team was pretty small and I didn’t dedicate the full weekend to the CTF, but it means I got to do the really easy challenges in the categories that I was the worst at, by which I mean pwn.

baby_boi is pretty much the simplest possible modern ROP (the modern security protections NX and ASLR are not artificially disabled, but you get everything you need to work around them). We even get source code.

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv[]) {
  char buf[32];
  printf("Hello!\n");
  printf("Here I am: %p\n", printf);
  gets(buf);
}

So there’s nothing novel here for experienced pwners, but I feel like there is a shortage of tutorials that walk you through how to solve a textbook ROP the way you’d want to solve it in a CTF, so here is a writeup.

Imagine a word search.

Now imagine you aren’t told what words to look for.

Now imagine you aren’t told it’s a word search.

Now imagine it isn’t a word search.

This post aims to be a fairly comprehensive introduction to puzzlehunts and their puzzles, a single post where I can just point people. I erred towards comprehensiveness in this post because I am not aware of any similar resources, especially for puzzlers interested in trying harder puzzlehunts who might not know any more experienced puzzlers to solve with. It’s possible to start solving some puzzles after reading much shorter guides, e.g. Puzzled Pint’s “Puzzling Basics” (PDF), so feel free to skip around, stop reading midway through, or bookmark this to read only after you’ve spent more time solving.

What is a Puzzlehunt?

A puzzlehunt¹ is an event where people, usually in teams, solve a series of puzzles.

This is not a very useful definition. The more interesting question is, what is the kind of “puzzle”² that appears in a puzzlehunt? The concept of a puzzlehunt puzzle is fuzzy and difficult to define precisely. Just about any hard rule one might try to state will be broken by some puzzle, sometimes deliberately. Still, here are some common features of puzzlehunt puzzles:

When I was very young, I thought cooking was easy. I sliced plastic vegetables with a toy knife and then Velcroed them back together, ad infinitum. For at least some time, I wanted to be a chef when I grew up.

When I was slightly less young, I thought cooking was hard. My reference points were mostly (1) my parents, who seemed to know how to make a million different dishes in inscrutable ways without thinking, and (2) MasterChef contestants (who I assume were better at cooking than my parents because they were, well, on MasterChef) messing things up and getting kicked off the show.

Now, I think I probably elided some meaningful distinctions there in my youthful naïveté. Cooking food that will keep you from getting kicked off MasterChef is hard. Cooking edible food is easy.¹ Cooking storebought dumplings in particular is so stupidly easy it’s unfair. More generally, though, most recipes tolerate a lot of substitutions,² number fudging,³ and even straight-up skipping pesky instructions, like the ones in baking recipes where you mix two sets of ingredients separately in specific orders. There are reasons for those steps, but ignoring them and dumping everything into the same mixing bowl usually won’t make your results inedible. You can also just decide to omit ingredients you don’t like. Probably the least tolerant ingredient measurements in recipes are the measurements of baking soda or baking powder, which by the way are different things, in baking recipes. But otherwise you’d really be surprised how many corners you can get away with cutting — I’ve even completely winged one baking soda/powder measurement with decent results. I think this is especially important to know for people from technical backgrounds like me, who have an instinct to treat the numbers in recipes as precisely measured, painstakingly optimized choices to produce the best dish. They usually aren’t, and even if they are optimized for the recipe author’s palate, they probably won’t be optimized for yours.⁴ And they certainly aren’t optimized for any tradeoffs you might want to make between food quality versus the time and effort you’re putting into cooking. Make the tradeoffs you want. You’re not on MasterChef.

There’s some oft-cited psychology studies that suggest that once your salary goes above $75,000, additional money doesn’t make you happier. This sounds like a sage bit of life advice if it were true, the ultimate rebuff against excessive greed and materialism and sacrificing other things for a six-digit salary, but it overstates the case a bit. 80,000 Hours’ analysis of money and happiness is probably the analysis I’d trust the most here; I think it would be more accurate just to say that you get diminishing returns of happiness from salaries above $70,000.¹ Still, that was enough for me to decide fairly early on that I wasn’t interested in trying to get a high-paying job for its own sake, or in spending too much effort trying to invest my way to a fortune.² I wanted my job to be personally satisfying and good for the world, while paying enough for me and my family (current and future) to get by, but I planned to treat any additional money after that as little more than a bonus used for breaking ties.

I still mostly stand by that decision today, but over the intervening years I realized there were a whole host of reasons to want money that weren’t that selfish at all.

I can’t begin to count the number of times we were exhorted in high school to go to class. College is different, they said. Nobody is going to force you to go to class any more. This is what you came to college to do, what you paid so much time and money for. It’s on you to make sure you’re learning.

By and large I followed this advice, until I considered that I might have overcorrected given the exhortations. There are a lot of definitely bad reasons to skip class, chief among them being too lazy to get out of bed. There are also some non-obvious reasons to go to class, such as getting the professors to recognize you — this is a reason to go to office hours even if you’re not particularly struggling with the class, or if you know people who might be able to help you that you could ask more comfortably or more conveniently; professors who know you may eventually be able to give you career advice, research opportunities, or letters of recommendation. (Of course, you shouldn’t try to befriend professors purely for these selfish motives; they’re also good to know just as fellow humans.)

But there are also plenty of legitimate reasons to skip class. (It’s really unclear how many people out there need to hear this, but my past self did, and I want this draft out of the way-too-long queue of posts.) Here are some.

We interrupt the irregularly scheduled philosophical posts for some programming memes:

Over the last few days, the Internet has divided itself over what the value of the expression 8÷2(2+2) should be. Some say it should be evaluated as (8÷2)×(2+2) = 16. Some say it should be evaluated as 8÷(2×(2+2)) = 1.

At the risk of belaboring the obvious, the core dispute here is not really mathematical. There is not some sequence of mathematical operations that produces some number, where mathematicians disagree about what number it produces. Instead, this is a dispute about mathematical notation: what sequence of mathematical operations the expression corresponds to the way it’s written. Specifically, it is a dispute about whether multiplication written as juxtaposition (how “2” is written right next to “(2+2)”) has strictly higher precedence than division. It is closer to a linguistic or typographical dispute than a purely mathematical one, and the correct answer to the dispute is that whoever wrote the expression that way should learn to write math better.

This debate is not even new. The internet had fun arguing over 48÷2(9+3) and 6÷2(1+2), which are functionally identical ambiguous expressions, eight years ago. I don’t know why the debate is resurging now and why we still haven’t gotten tired of it.

But life is short, so since we’re here anyway, let’s make some additional memes.

Asking the computer

Some of my coworkers had the idea to ask some programming languages what the answer was. The results were underwhelming.

$ python3
Python 3.6.7 (default, Oct 22 2018, 11:32:17)
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 8/2(2+2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not callable

I hated timed essays in high school. It’s pretty clear if you skim that part of the blog archives:

My [SAT] essay got a 10 out of 12. It’s an essay I’d be ashamed of posting anywhere else; it’s disgustingly traditional and formulaic. […] This was simply because I knew that using my normal essay-writing mindset, I’d get maybe a 3, because I’d spend the first twenty minutes debating myself over which side I was on and rewrite the introduction ten times. Too bad. I wasn’t there to write a good essay; I was there to get a good score on the SAT.

The worst issue is that students do not need to give in-depth explanation of anything they learned. Due to its stringent time limit, the essay portion rewards quick reckless writing much more than deep thought. […] encouraging students to practice writing 25-minute essays in order to improve their college-bound skills is like encouraging people to play Grand Theft Auto in order to improve their driving skills.

I took the Grand Theft Auto analogy pretty far. I’m not proud enough of these posts to link them, but you can find them if you try. In short, past me thought timed essays rewarded writing too quickly, with a disregard not only for facts but for the opportunity to lay out your opinions and thoughts so you could clarify and revise them, which was actually the most valuable part of writing.¹ I conceded that the time limits made sense as a practical concession to allow you to test students’ writing skills fairly,² but I felt like there was still way too much emphasis on the raw speed.