Crypto Analysis Skill

Quick Workflow

Progress:

• Try Ciphey auto-decrypt first
• Identify crypto type (RSA/AES/XOR/classical)
• Check for known weaknesses
• Implement attack
• Decrypt flag

Step 1: Auto-Decrypt (Try First!)

ciphey -t "ENCODED_TEXT" # Auto-detects and decrypts ciphey -f encrypted.txt # From file

Step 2: Identify Crypto Type

Pattern Crypto Type Reference

n, e, c variables RSA reference/rsa-attacks.md

16/32 byte key, IV AES reference/aes-attacks.md

XOR operations XOR/Stream reference/classical.md

Polynomial mod Lattice reference/lattice.md

RSA Attack Decision Tree

├── e small (≤5)? → Direct eth root ├── e very large? → Wiener's Attack ├── Multiple n,e,c? → Hastad's Broadcast ├── Same n, diff e? → Common Modulus ├── GCD(n1,n2) > 1? → Common Factor ├── p ≈ q? → Fermat Factorization ├── dp/dq leaked? → Partial Key Recovery └── Default → FactorDB / yafu

Full implementations: reference/rsa-attacks.md

Quick Commands

Auto-decrypt

ciphey -t "text"

XOR analysis

xortool encrypted.bin xortool -c 20 encrypted.bin # Expect spaces

Factor large n

yafu "factor(<n>)"

RSA tool

python3 RsaCtfTool.py -n <n> -e <e> --uncipher <c>

Lattice (SageMath)

sage solve.sage

Reference Files

• RSA Attacks: Small e, Wiener, Hastad, Common Modulus, Fermat, FactorDB

• AES Attacks: ECB detection, CBC flip, Padding Oracle

• Classical/XOR: Ciphey, xortool, frequency analysis, Vigenère

• Lattice: Coppersmith, LLL, HNP