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---
title: "Tips and tricks: CS 166 Information Security taught by Mark Stamp"
date: 2025-03-04T12:14:54-08:00
tags: ["SJSU"]
---
A collection of troubleshooting notes, general tips and tricks, or personal thoughts on the CS 166 Information Security class taught by Mark Stamp at SJSU.
Part of this, dealing with specific homework problems, is written with the intention of being a last-resort rescue manual. I only include information you need to get out of potential deep water. No solutions to hard problems. No hand holding, especially no "here is how you solve this problem".
The other part, the tips and thoughts, is just that. I will not stop myself blabbering on for forever.
---
# Recommended Tools
## Hex editor
I'm only listing two of my favorites here.
HxD <https://mh-nexus.de/en/hxd/>
A very light, Windows-only hex editor. I use it on my laptop for quick things. It definitely will suffice for this class.
ImHex <https://imhex.werwolv.net/>
Much more powerful, but heavier. It has builtin support for pattern matching, processing, disassembly, etc. It also just looks _really really nice_, 200% eye candy factor.
## C compiler
Some assignments supply C source code that you'll have to compile. In general, they are not compatible with MSVC (Visual Studio), so special care needs taken on Windows.
- macOS: install Xcode Command Line Tools, which contains an Apple-flavored clang. Alternatively, install either clang or gcc from [Homebrew](https://brew.sh/).
- Liunx: your distro's gcc will do. Or clang if you like that. Honestly if you use Linux why are you even reading this section, go away. :P
- Windows: install some flavor of gcc
- I highly recommend <https://nuwen.net/mingw.html>
It’s tiny, just a zip file. Unzip it, you get a `open_distro_window.bat`, which when opened gives you a terminal with everything setup. There is _zero_ room for `PATH` to go wrong.
- Otherwise, https://www.msys2.org/
{{< details summary="What's MinGW and what does it have to do with msys2 and cygwin" >}}
In short, gcc is to MinGW as Linux is to distros.
gcc is a whole bunch of code that can turn C source code, among other things, into an executable. It's designed to run on various \*nix platforms.
MinGW is a bunch of extra code _on top of gcc_ to make it (1) run on Windows, and (2) produce Windows ("PECOFF") executable.
Both projects are just code, they don't provide downloads ("builds"). For gcc, various Linux distros compile them and ship it as a package.
For MinGW, these projects do the same job:
- https://winlibs.com/
- https://github.com/niXman/mingw-builds-binaries
- https://cygwin.com/ \
This one tries to emulate the *nix environment on Windows. Comes with quite a few extra programs like bash. _See google._
- https://www.msys2.org/ \
This one builds a whole bunch of softwares in addition to MinGW: bash, make, etc. It's quite complicated, due to those software requring compatibility layers like cygwin to function. I won't explain here. _See google._
- etc.
Regardless of which one you download, you get a copy of gcc (and MinGW). The difference is the default configs, and the extra software they ship with gcc.
In fact, you'll hear people call these things "MinGW distros".
The MinGW project has a [list of notable distros](https://www.mingw-w64.org/downloads/).
I would have linked something instead of writing this myself, but I literally can't find anything of good quality on the internet...
{{< /details >}}
# Chapter 5
## Problem 4 - hash collision
Hint: the expected answer _is not_ the exact solution. That's way too complicated
I did end up in quite a rabbit hole trying to find the exact solution. These are not relevant to the textbook at all, but they're still interesting reads:
- https://math.stackexchange.com/questions/407307/second-pair-of-matching-birthdays
- https://math.stackexchange.com/questions/2313215/birthday-paradox-2-pairs (same thing, just less general and less verbose)
- https://math.stackexchange.com/questions/1539271/probability-of-exactly-two-pairs-share-a-birthday-and-each-pair-shares-differen
{{< details summary="More hint" >}}
It's the square root approximation that’s implied, but much glossed over in the textbook. Fuller explaination here:
https://en.m.wikipedia.org/wiki/Birthday_problem#Square_approximation
{{< /details >}}
## Problem 24 - MD5 collision
### I'm getting different hashes
The messages are supposed to be binary files, but the textbook gave them in hex codes.
You probably need something like `xxd -r -p`, or [powershell](https://stackoverflow.com/a/64927815), or your hex editor of choice, to turn it into a binary message.
{{< details >}}
Something like these will work:
```sh
$ cut -d ' ' -f 2- <<'EOF' | xxd -r -p > msg1.bin
00000000 d1 31 dd 02 c5 e6 ee c4 69 3d 9a 06 98 af f9 5c
00000010 2f ca b5 87 12 46 7e ab 40 04 58 3e b8 fb 7f 89
00000020 55 ad 34 06 09 f4 b3 02 83 e4 88 83 25 71 41 5a
00000030 08 51 25 e8 f7 cd c9 9f d9 1d bd f2 80 37 3c 5b
00000040 96 0b 1d d1 dc 41 7b 9c e4 d8 97 f4 5a 65 55 d5
00000050 35 73 9a c7 f0 eb fd 0c 30 29 f1 66 d1 09 b1 8f
00000060 75 27 7f 79 30 d5 5c eb 22 e8 ad ba 79 cc 15 5c
00000070 ed 74 cb dd 5f c5 d3 6d b1 9b 0a d8 35 cc a7 e3
EOF
$ cut -d ' ' -f 2- <<'EOF' | xxd -r -p > msg2.bin
00000000 d1 31 dd 02 c5 e6 ee c4 69 3d 9a 06 98 af f9 5c
00000010 2f ca b5 07 12 46 7e ab 40 04 58 3e b8 fb 7f 89
00000020 55 ad 34 06 09 f4 b3 02 83 e4 88 83 25 f1 41 5a
00000030 08 51 25 e8 f7 cd c9 9f d9 1d bd 72 80 37 3c 5b
00000040 96 0b 1d d1 dc 41 7b 9c e4 d8 97 f4 5a 65 55 d5
00000050 35 73 9a 47 f0 eb fd 0c 30 29 f1 66 d1 09 b1 8f
00000060 75 27 7f 79 30 d5 5c eb 22 e8 ad ba 79 4c 15 5c
00000070 ed 74 cb dd 5f c5 d3 6d b1 9b 0a 58 35 cc a7 e3
EOF
$ md5sum msg1.bin msg2.bin
```
FYI, `cut` is used to strip the address column from the string.
{{< /details >}}
## Problem 39 - stenography
### Getting a blank PDF on windows
The given `stegoRead.c` and `stego.c` is using `fopen(2)` in text mode, and CRT on Windows _may_ expand byte sequence 0A (\n) to 0D 0A (\r\n). I'm honestly not sure when it decides to do that.
Add `b` to the mode of all instances of `fopen(...)`. For example, change `fopen(outfname, "w")` into `fopen(outfname, "wb")`
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