Crypto Analysis Skill
crypto-analysis
Breaks cryptographic systems and decrypts ciphertext. Use when working with RSA, AES, XOR, classical ciphers, hash functions, or when challenge involves encryption, decryption, keys, or mathematical crypto attacks.
SKILL.md
Full skill instructions
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