classical-cipher-analysis

Name: classical-cipher-analysis
Author: Prohao42

91stars8forks

How to install classical-cipher-analysis?

prohao42/aimy-skill/classical-cipher-analysis

$npx -y skills add Prohao42/aimy-skill --skill classical-cipher-analysis

Installs into the current project.

Summary

Classical cipher analysis playbook. Use when encountering substitution ciphers, Vigenere, transposition, XOR, or encoded text in CTF challenges that requires frequency analysis, Kasiski examination, or known-plaintext cryptanalysis.

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SKILL.md

1	# SKILL: Classical Cipher Analysis — Expert Cryptanalysis Playbook
2
3	> AI LOAD INSTRUCTION: Expert classical cipher identification and breaking techniques for CTF. Covers cipher identification methodology (frequency analysis, IC, Kasiski), monoalphabetic substitution, Caesar/ROT, Vigenere, Enigma, affine, Hill, transposition ciphers, Bacon/Polybius/Playfair, and XOR ciphers. Base models often skip the identification step and jump to the wrong cipher type, or fail to recognize encoded (base64/hex) ciphertext that needs decoding before analysis.
4
5	## 0. RELATED ROUTING
6
7	- [symmetric-cipher-attacks](../symmetric-cipher-attacks/SKILL.md) when dealing with modern symmetric ciphers (AES/DES) rather than classical
8	- [hash-attack-techniques](../hash-attack-techniques/SKILL.md) when the challenge involves hash-based constructions
9	- [lattice-crypto-attacks](../lattice-crypto-attacks/SKILL.md) when knapsack-based ciphers are encountered
10
11	### Quick identification guide
12
13	\| Observation \| Likely Cipher \| First Action \|
14	\|---\|---\|---\|
15	\| All uppercase letters, uneven frequency \| Monoalphabetic substitution \| Frequency analysis \|
16	\| All uppercase, flat frequency distribution \| Polyalphabetic (Vigenere) \| IC + Kasiski \|
17	\| Only A-Z shifted uniformly \| Caesar/ROT \| Brute force 25 shifts \|
18	\| Base64 alphabet (A-Za-z0-9+/=) \| Base64 encoded (decode first) \| Base64 decode \|
19	\| Hex string (0-9a-f) \| Hex encoded (decode first) \| Hex decode \|
20	\| Binary (0s and 1s) \| Binary encoded \| Convert to ASCII \|
21	\| Dots and dashes \| Morse code \| Morse decode \|
22	\| Raised/normal text pattern \| Bacon cipher \| Map to A/B, decode \|
23	\| 2-digit number pairs (11-55) \| Polybius square \| Grid lookup \|
24	\| Text appears scrambled (right letters, wrong order) \| Transposition \| Anagram analysis \|
25	\| Non-printable bytes XOR-like \| XOR cipher \| Single/repeating key XOR analysis \|
26
27	---
28
29	## 1. CIPHER IDENTIFICATION METHODOLOGY
30
31	### 1.1 Step 1: Character Set Analysis
32
33	```python
34	def analyze_charset(ciphertext):
35	"""Identify encoding/cipher by character set."""
36	chars = set(ciphertext.strip())
37
38	if chars <= set('01 \n'):
39	return "Binary encoding"
40	if chars <= set('.-/ \n'):
41	return "Morse code"
42	if chars <= set('0123456789abcdef \n'):
43	return "Hex encoding"
44	if chars <= set('ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=\n'):
45	if '=' in ciphertext or len(ciphertext) % 4 == 0:
46	return "Base64 encoding"
47	if chars <= set('ABCDEFGHIJKLMNOPQRSTUVWXYZ \n'):
48	return "Uppercase only — classical cipher"
49	if all(c in '12345' for c in ciphertext.replace(' ', '').replace('\n', '')):
50	return "Polybius square (digits 1-5)"
51
52	return "Mixed charset — needs further analysis"
53	```
54
55	### 1.2 Step 2: Frequency Analysis
56
57	```python
58	from collections import Counter
59
60	def frequency_analysis(text):
61	"""Compute letter frequency distribution."""
62	text = text.upper()
63	letters = [c for c in text if c.isalpha()]
64	total = len(letters)
65	freq = Counter(letters)
66
67	print("Letter frequencies:")
68	for letter, count in freq.most_common():
69	pct = count / total * 100
70	bar = '#' * int(pct)
71	print(f" {letter}: {pct:5.1f}% {bar}")
72
73	return freq
74
75	# English letter frequency (for comparison):
76	# E T A O I N S H R D L C U M W F G Y P B V K J X Q Z
77	# 12.7 9.1 8.2 7.5 7.0 6.7 6.3 6.1 6.0 4.3 4.0 2.8 ...
78	```
79
80	### 1.3 Step 3: Index of Coincidence (IC)
81
82	```python
83	def index_of_coincidence(text):
84	"""
85	IC ≈ 0.065 → English / monoalphabetic substitution
86	IC ≈ 0.038 → random / polyalphabetic cipher
87	"""
88	text = [c for c in text.upper() if c.isalpha()]
89	N = len(text)
90	freq = Counter(text)
91
92	ic = sum(f * (f - 1) for f in freq.values()) / (N * (N - 1))
93	return ic
94
95	# Interpretation:
96	# IC > 0.060 → monoalphabetic (Caesar, simple substitution, Playfair)
97	# IC ≈ 0.045-0.055 → polyalphabetic with short key (Vigenere key < 10)
98	# IC ≈ 0.038-0.042 → polyalphabetic with long key or random
99	```
100
101	### 1.4 Step 4: Kasiski Examination (for Polyalphabetic)
102
103	```python
104	from math import gcd
105	from functools import reduce
106
107	def kasiski(ciphertext, min_len=3):
108	"""Find repeated sequences and their distances → key length."""
109	text = ''.join(c for c in ciphertext.upper() if c.isalpha())
110	distances = []
111
112	for length in range(min_len, min(20, len(text) // 3)):
113	for i in range(len(text) - length):
114	seq = text[i:i+length]
115	j = text.find(seq, i + 1)
116	while j != -1:
117	distances.append(j - i)
118	j = text.find(seq, j + 1)
119
120	if not distances:
121	return None
122
123	# Key length is likely GCD of common distances
124	common_gcds =

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classical-cipher-analysis

91stars8forks

View on GitHub

How to install classical-cipher-analysis?

prohao42/aimy-skill/classical-cipher-analysis

$npx -y skills add Prohao42/aimy-skill --skill classical-cipher-analysis

Installs into the current project.

Summary

Files · 1

View on GitHub

SKILL.md

1	# SKILL: Classical Cipher Analysis — Expert Cryptanalysis Playbook
2
3	> AI LOAD INSTRUCTION: Expert classical cipher identification and breaking techniques for CTF. Covers cipher identification methodology (frequency analysis, IC, Kasiski), monoalphabetic substitution, Caesar/ROT, Vigenere, Enigma, affine, Hill, transposition ciphers, Bacon/Polybius/Playfair, and XOR ciphers. Base models often skip the identification step and jump to the wrong cipher type, or fail to recognize encoded (base64/hex) ciphertext that needs decoding before analysis.
4
5	## 0. RELATED ROUTING
6
7	- [symmetric-cipher-attacks](../symmetric-cipher-attacks/SKILL.md) when dealing with modern symmetric ciphers (AES/DES) rather than classical
8	- [hash-attack-techniques](../hash-attack-techniques/SKILL.md) when the challenge involves hash-based constructions
9	- [lattice-crypto-attacks](../lattice-crypto-attacks/SKILL.md) when knapsack-based ciphers are encountered
10
11	### Quick identification guide
12
13	\| Observation \| Likely Cipher \| First Action \|
14	\|---\|---\|---\|
15	\| All uppercase letters, uneven frequency \| Monoalphabetic substitution \| Frequency analysis \|
16	\| All uppercase, flat frequency distribution \| Polyalphabetic (Vigenere) \| IC + Kasiski \|
17	\| Only A-Z shifted uniformly \| Caesar/ROT \| Brute force 25 shifts \|
18	\| Base64 alphabet (A-Za-z0-9+/=) \| Base64 encoded (decode first) \| Base64 decode \|
19	\| Hex string (0-9a-f) \| Hex encoded (decode first) \| Hex decode \|
20	\| Binary (0s and 1s) \| Binary encoded \| Convert to ASCII \|
21	\| Dots and dashes \| Morse code \| Morse decode \|
22	\| Raised/normal text pattern \| Bacon cipher \| Map to A/B, decode \|
23	\| 2-digit number pairs (11-55) \| Polybius square \| Grid lookup \|
24	\| Text appears scrambled (right letters, wrong order) \| Transposition \| Anagram analysis \|
25	\| Non-printable bytes XOR-like \| XOR cipher \| Single/repeating key XOR analysis \|
26
27	---
28
29	## 1. CIPHER IDENTIFICATION METHODOLOGY
30
31	### 1.1 Step 1: Character Set Analysis
32
33	```python
34	def analyze_charset(ciphertext):
35	"""Identify encoding/cipher by character set."""
36	chars = set(ciphertext.strip())
37
38	if chars <= set('01 \n'):
39	return "Binary encoding"
40	if chars <= set('.-/ \n'):
41	return "Morse code"
42	if chars <= set('0123456789abcdef \n'):
43	return "Hex encoding"
44	if chars <= set('ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=\n'):
45	if '=' in ciphertext or len(ciphertext) % 4 == 0:
46	return "Base64 encoding"
47	if chars <= set('ABCDEFGHIJKLMNOPQRSTUVWXYZ \n'):
48	return "Uppercase only — classical cipher"
49	if all(c in '12345' for c in ciphertext.replace(' ', '').replace('\n', '')):
50	return "Polybius square (digits 1-5)"
51
52	return "Mixed charset — needs further analysis"
53	```
54
55	### 1.2 Step 2: Frequency Analysis
56
57	```python
58	from collections import Counter
59
60	def frequency_analysis(text):
61	"""Compute letter frequency distribution."""
62	text = text.upper()
63	letters = [c for c in text if c.isalpha()]
64	total = len(letters)
65	freq = Counter(letters)
66
67	print("Letter frequencies:")
68	for letter, count in freq.most_common():
69	pct = count / total * 100
70	bar = '#' * int(pct)
71	print(f" {letter}: {pct:5.1f}% {bar}")
72
73	return freq
74
75	# English letter frequency (for comparison):
76	# E T A O I N S H R D L C U M W F G Y P B V K J X Q Z
77	# 12.7 9.1 8.2 7.5 7.0 6.7 6.3 6.1 6.0 4.3 4.0 2.8 ...
78	```
79
80	### 1.3 Step 3: Index of Coincidence (IC)
81
82	```python
83	def index_of_coincidence(text):
84	"""
85	IC ≈ 0.065 → English / monoalphabetic substitution
86	IC ≈ 0.038 → random / polyalphabetic cipher
87	"""
88	text = [c for c in text.upper() if c.isalpha()]
89	N = len(text)
90	freq = Counter(text)
91
92	ic = sum(f * (f - 1) for f in freq.values()) / (N * (N - 1))
93	return ic
94
95	# Interpretation:
96	# IC > 0.060 → monoalphabetic (Caesar, simple substitution, Playfair)
97	# IC ≈ 0.045-0.055 → polyalphabetic with short key (Vigenere key < 10)
98	# IC ≈ 0.038-0.042 → polyalphabetic with long key or random
99	```
100
101	### 1.4 Step 4: Kasiski Examination (for Polyalphabetic)
102
103	```python
104	from math import gcd
105	from functools import reduce
106
107	def kasiski(ciphertext, min_len=3):
108	"""Find repeated sequences and their distances → key length."""
109	text = ''.join(c for c in ciphertext.upper() if c.isalpha())
110	distances = []
111
112	for length in range(min_len, min(20, len(text) // 3)):
113	for i in range(len(text) - length):
114	seq = text[i:i+length]
115	j = text.find(seq, i + 1)
116	while j != -1:
117	distances.append(j - i)
118	j = text.find(seq, j + 1)
119
120	if not distances:
121	return None
122
123	# Key length is likely GCD of common distances
124	common_gcds =

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