🔐 The Substitution Cipher
A substitution cipher replaces each letter with another letter
according to a fixed rule. Unlike Caesar cipher, the replacement
doesn't follow a simple shift pattern.
How It Works
Create a scrambled alphabet to use as the key:
Plain: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Cipher: Q W
E R T Y U I O P A S D F G H J K L Z X C V B N M
Each letter in the plaintext is replaced with its corresponding cipher
letter.
Example
Plaintext: HELLO
H → I E → T L → S L → S O → G
Ciphertext: ITSSG
Frequency Analysis
The key to cracking substitution ciphers is knowing how often letters
appear in English:
| Letter |
E |
T |
A |
O |
I |
N |
S |
H |
R |
| Frequency |
12.7% |
9.1% |
8.2% |
7.5% |
7.0% |
6.7% |
6.3% |
6.1% |
6.0% |
How to Crack It
- Count letter frequencies in the ciphertext
-
Match to English frequencies: The most common
letter is probably E
-
Look for patterns:
- Single-letter words → A or I
- Most common 2-letter words → OF, TO, IN, IT, IS
- Most common 3-letter words → THE, AND, FOR, ARE
- Double letters → LL, SS, EE, OO, TT
-
Make educated guesses and check if words make sense
-
Use context clues from partially decoded words
Common Patterns
- Q is almost always followed by U
-
TH is the most common digraph (two-letter combo)
- -ING, -TION, -ED are common word endings
- THE is the most common word in English
Tips
- Start with the most frequent letters and common words
- Write out your substitution alphabet as you discover letters
- Be prepared to backtrack if your guesses don't work out
-
Longer texts are easier to crack due to more data for frequency
analysis