GreyCTF 2025 Crypto writeup - Uwusignatures..

🧠 Uwusignatures - Crypto Challenge Walkthrough | GreyCTF 🔐

In this video, we’re diving into Uwusignatures, a cryptography challenge from GreyCTF. The vulnerability? The server leaks and reuses its secret value k, making the digital signature scheme completely breakable.

📌 What you’ll see:

A clear breakdown of the vulnerable code in uwusignatures.py

A step-by-step plan to recover the private key

A full demo of the exploit script in action

Getting the flag by forging a valid signature

👨‍💻 What you'll learn:

How leaking and reusing k in ElGamal-style digital signatures completely compromises security

How to derive the private key x when k, r, s, and the message hash are known

How to forge a signature for any message once the private key is recovered

🛠️ Tools & Concepts used:

Modular arithmetic & exponentiation

Hashing and signature structure

Python scripting for automation

📽️ Video Flow:

Scene 1: Intro to the challenge and highlighting the bug (reused k)

Scene 2: High-level attack plan — what we need and how to use it

Scene 3: Running the exploit and walking through the output step-by-step

Scene 4: Wrapping up with the takeaway — never leak or reuse k!

🚩 Flag captured successfully at the end! ⚠️ Difficulty: Intermediate–Advanced 🧩 Challenge: Uwusignatures
🎯 Category: Cryptography
🏁 CTF: GreyCTF 2025
👨‍💻 Presented by: Team Bitsec

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