Crypto Analysis

---
name: crypto-analysis
description: 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.
allowed-tools: Bash, Read, Write, Grep, Glob
---

# 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!)

```bash
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](reference/rsa-attacks.md) |
| 16/32 byte key, IV | AES | [reference/aes-attacks.md](reference/aes-attacks.md) |
| XOR operations | XOR/Stream | [reference/classical.md](reference/classical.md) |
| Polynomial mod | Lattice | [reference/lattice.md](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](reference/rsa-attacks.md)

## Quick Commands

```bash
# 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](reference/rsa-attacks.md)**: Small e, Wiener, Hastad, Common Modulus, Fermat, FactorDB
- **[AES Attacks](reference/aes-attacks.md)**: ECB detection, CBC flip, Padding Oracle
- **[Classical/XOR](reference/classical.md)**: Ciphey, xortool, frequency analysis, Vigenère
- **[Lattice](reference/lattice.md)**: Coppersmith, LLL, HNP