Cryptography

CTF cryptography challenge writeups by event and year

Cryptography writeups from Google CTF, DEF CON, picoCTF, and other global competitions. Organized by year and event.

---

2025 Challenges

Google CTF 2025

Google CTF 2025

WeakRSA

Easy

Category: Crypto · Points: 100 · Solves: 610

RSA cryptosystem configured with a small public exponent (e=3). Perform a cube root attack on the ciphertext to decrypt the message without factoring N.

RSASymmetric CryptoSmall ExponentCube Root

Writeup →

HTB Cyber Apocalypse 2025

HTB Cyber Apocalypse 2025

AES-Leak

Medium

Category: Crypto · Points: 250 · Solves: 198

AES-CTR mode encryption reusing the same nonce/IV across different messages. Recover the keystream and decrypt the flag.

AES-CTRNonce ReuseKeystream RecoveryKnown Plaintext

Writeup →