CS 166 tips and tricks: CS 166

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+---
+title: "Tips and tricks: CS 166 Information Security taught by Mark Stamp"
+date: 2025-03-04T12:14:54-08:00
+---
+
+# Chapter 5
+## Problem 4 - hash collision
+The expected answer is the square root approximation that’s much glossed over in the textbook, and seen here:
+https://en.m.wikipedia.org/wiki/Birthday_problem#Square_approximation
+
+But, for the sake for information, I did end up in quite a rabbit hole trying to find the exact solution. These maybe 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
+
+## 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.
+
+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.
+
+## 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")`