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!)

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