YCEP 2025 - Sum Checker

Challenge Description

Description

30 files, each locked behind a SHA-256 checksum, and the task is clear: figure out which one has the real flag. It’s like being handed a stack of cryptic treasure maps and told, “Good luck, no pressure!”

Checksum: a91561aa9ce79c721f66a7d846128df4f81d11937fd723860ad92547ff2e814e

• Author: Jun Wei
• Category: forensics
• Difficulty: easy
• Discord: syn3pz

Hints

• How can we find the SHA256 checksum of files? (100 points)

Files

• check.zip

Sum Checker - Solution

We are provided with a zip file, containing 30 files named flag(n).txt.

After unzipping the zip file, we must find the file with a SHA256 checksum that matches the one given in the challenge: Checksum of correct file: a91561aa9ce79c721f66a7d846128df4f81d11937fd723860ad92547ff2e814e

To display the checksum of all 30 files in the check folder, we can run the following command:

sha256sum check/*

Out of the 30 files, 29 of them contain fake flags. To find the correct file, we can do:

sha256sum check/* | grep a91561aa9ce79c721f66a7d846128df4f81d11937fd723860ad92547ff2e814e

We now know that the file that contains the correct flag is flag28.txt.

We can cat the flag28.txt to obtain the flag.

Flag: YCEP25{UNPR3D1CT4BL3_0UTPUT}