Classical Ciphers
Index of classical cipher pages for CTF crypto - Caesar, Vigenère, substitution, transposition, and frequency analysis.
Classical Ciphers
Classical ciphers are the most beginner-friendly crypto challenges. They're solvable with statistical analysis and freely available online tools.
Pages in This Subsection
| Page | Difficulty | Key Tool |
|---|---|---|
| Caesar / ROT | Beginner | Brute force 25 shifts |
| Vigenère | Beginner | dcode.fr, Kasiski test |
| Substitution | Beginner | quipqiup.com |
| Transposition | Beginner | dcode.fr |
| Frequency Analysis | Beginner | Manual + automated |
Identification Flowchart
The Three Questions
Answer these three questions in order - they narrow down the cipher type in under a minute.
For any classical cipher challenge, ask:
- Are the original letters present? (yes → transposition; no → substitution)
- What is the index of coincidence? (: monoalphabetic; : polyalphabetic)
- Is there a key hint in the challenge title or description?
Calculate Index of Coincidence
from collections import Counter
def ic(text):
t = [c for c in text.upper() if c.isalpha()]
n = len(t)
freq = Counter(t)
return sum(f*(f-1) for f in freq.values()) / (n*(n-1))Interpret: ~0.067 → monoalphabetic or transposition; ~0.045 → Vigenère.
Check letter frequencies
Are the most frequent letters roughly E, T, A, O, I? If yes: transposition. If shifted uniformly: Caesar. If scrambled but same distribution: monoalphabetic substitution.
Check challenge metadata
Look at the challenge title, description, and hints. Words like "shift", "rot", "fence", "key", or "columnar" directly indicate the cipher type.
Apply the appropriate attack
Route to the correct page and apply the standard attack for that cipher type.
Online Tools
| Tool / URL | Description |
|---|---|
| dcode.fr/cipher-identifier | Paste text → get list of likely ciphers |
| dcode.fr/vigenere-cipher | Solve Vigenère with key or key-length |
| dcode.fr/rail-fence-cipher | Rail fence decoder |
| dcode.fr/columnar-transposition-cipher | Columnar transposition decoder |
| dcode.fr/frequency-analysis | Visual letter frequency chart |
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