Project Sekai - ✅-forensics-crconfusion

Project Sekai

🔒 BYUCTF 2023 / ✅-forensics-crconfusion

CRConfusion - 500 points

Category: Forensics Description: I told my friend that I want to learn how CRC works, so he sent me these three files. Apparently, each line of the text file uses a modified CRC to calculate a checksum. He said that if I can find the hidden message, I'll truly understand CRC... Files:

Tags: Hard

Sutx pinned a message to this channel. 05/19/2023 10:01 AM

@Violin wants to collaborate

@Legoclones wants to collaborate

@Utaha wants to collaborate

79 lines, does that mean 10 bytes of data?

18:27

I thought it's crc, but with diff polynomial, but it seems like there's no solution

10 bytes seems proper length chal is split to 3 files, each file maybe has a different formula?

theoretically, the crc is essentially a matrix mult

18:31

so we just have to find the 32 * 8 matrix

18:31

but I wasn't able to find it somehow

18:34

hmm wait

18:34

if I can find the 32*8 matrix, then there's no place to hide message

18:35

maybe it's RHS = crc(LHS) ^ message

18:35

or sth similar

@Legoclones this is crypto or some forensics (like no hardcore maths)

or maybe each line is one of the crc...?

18:43

~~it's looking non-misc now~~

sahuang

10 bytes seems proper length chal is split to 3 files, each file maybe has a different formula?

I can't distinguish if it's 79 bytes (which is the same for all 3 files), or 10 bytes each

18:44

what's the flag format btw

byuctf

it's forensics

18:49

not crypto

yeah i just dunno how mathy it is, is it just inspection or we need at least calculate sth (in this "forensics"

(i'm bad at math)

Legoclones

(i'm bad at math)

you need maths for crc algo

well basic maths lol (edited)

too guessy for me lol

19:01

didn't find a way to solve this

Utaha

I can't distinguish if it's 79 bytes (which is the same for all 3 files), or 10 bytes each

i have no idea why there are 79 lines instead of 80 if 10 characters

19:11

should be 80

missing the first 0 bit (?

19:14

idk

@Legoclones is flag in format? weird that there's 1 less line to form character bits

sahuang

@Legoclones is flag in format? weird that there's 1 less line to form character bits

yes, flag is in format byuctf{.*}

Utaha

missing the first 0 bit (?

i think this is correct, they sent a 10-byte string in each file, and then bytes->binary, without zfill, so its 79 bits

@kanon wants to collaborate

@Sheep wants to collaborate

@4n0nym4u5 wants to collaborate

I just had an idea and I'll try it (maybe wrong)

As a base idea, I was thinking of a distinction between two types of CRC8, where the left bit sequence is the message and the right bit sequence is the inspection code.

@rubiya wants to collaborate

@TheBadGod wants to collaborate

TheBadGod

used /ctf submit

✅ Well done, challenge solved!

byuctf{cyclic_redundancy_checks_are_used_to_detect_errors_in_data_transmission}

04:19

generator polynomial was the flag characters, so first line in all three files was crc with polynomial of b

rikkawow

2

04:19

2.45 KB

04:20

crc code from https://gist.github.com/Lauszus/6c787a3bc26fea6e842dfb8296ebd630

Generic CRC-8, CRC-16 and CRC-32 calculations in Python

Generic CRC-8, CRC-16 and CRC-32 calculations in Python - crc.py

TheBadGod

generator polynomial was the flag characters, so first line in all three files was crc with polynomial of b

bruh wtf

06:27

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