SHA2017CTF Crypto100: Stack Overflow Writeup

題目提供了代碼跟一個使用該代碼加密的 pdf 檔:


CTFZone 2017 Forensics/Misc1000: Puzzle Writeup

Written by harrier, participated as 0x000505AD

No victory can dispense with mighty patron, hence the more powerful your supporter is the better. Yesterday you finally managed to get in contact with one secret society. It seems that they might back your candidate if you accomplish the task they sent you as a rite of passage. You may find it easy at first glance but I wouldn’t be so naive…

*flag format: ctfzone{flag_you_found}

For puzzle: The way to the magister’s mystery lies through the five degrees:

1. Find all pieces of the puzzle and solve it.
2. Realize the secret behind the names.
3. Find the ritual book.
4. Involve into ritual
5. Translate the flag


CTFZone 2017 Crypto409: e-Voting Writeup

這個挑戰是一個黑箱密碼系統,裡面有加密和解密的功能。經過一些測試後,發現是 ElGamal 密碼。測試過程如下:

加密 1,得到兩個數字 (是什麼並不重要);解密  1333333337 1,得到 1333333337

可以猜到它跟 Wikipedia 說的有點不同:它加密的 \((c_1, c_2)\) 跟維基的 \((c_1, c_2)\) 次序是相反的。


MeePwnCTF 2017 Crypto600: justpad Writeup

題目提供了一個 encryption oracle。第一層是 Proof of Work,目的是為了拖延時間,所以就不寫了。這個 encryption oracle 利用 RSA 加密,每次會產生不同的 public key:兩個質數 \(p, q\) 最多為 512 bits,而 \(e\) 是從 3, 5, 7, 11 跟 13 裡面隨機抽出來的。


MeePwnCTF 2017 Crypto100: |\/|/-\T|-| Writeup



MeePwnCTF 2017 Crypto100: Simpler than RSA? Writeup

題目提供了一個密碼系統還有一組 ciphertext,要求我們從中找出 flag。

Key generation:

  • 生成兩個 90 bits 的質數:\(p, q\), 並設 \(n = p^2 q\)
  • 生成一個亂數 \(g\) 使得 \(1 \leq g \leq n\)
  • 算出 \(h \equiv g^n (\text{mod }n)\)
  • 傳回 \((n, g, h)\)