The Simple Substitution Cipher is one of the most straightforward and commonly known encryption techniques. In this method, each letter in the plaintext is replaced with a letter from a fixed substitution alphabet. The cipher was used throughout history for various forms of communication, particularly in the early modern period, when simpler forms of cryptography were favored for their ease of use.
The Simple Substitution Cipher does not change the letter's position but rather swaps it with another letter. The key can be a shuffled alphabet that dictates which letter corresponds to which. While this method provides a basic level of security, it is vulnerable to frequency analysis, where an attacker can study the frequency of letters in the ciphertext to break the code.
Example:
Consider the plaintext message "HELLO". Using the substitution key where:
- A = D
- B = F
- C = H
- D = J
- E = K
- F = L
- G = N
- H = P
- I = R
- J = T
- K = V
- L = W
- M = X
- N = Y
- O = Z
- P = A
- Q = B
- R = C
- S = E
- T = G
- U = I
- V = M
- W = O
- X = Q
- Y = S
- Z = U
- Encrypt each letter:
- H → P
- E → K
- L → W
- L → W
- O → Z
- Resulting Ciphertext:
- The plaintext "HELLO" can be encrypted to "PKWWZ".
Mapping Table:
Here’s how the Simple Substitution Cipher encryption works for the letters involved:
| Plaintext | Ciphertext |
|---|---|
| A | D |
| B | F |
| C | H |
| D | J |
| E | K |
| F | L |
| G | N |
| H | P |
| I | R |
| J | T |
| K | V |
| L | W |
| M | X |
| N | Y |
| O | Z |
| P | A |
| Q | B |
| R | C |
| S | E |
| T | G |
| U | I |
| V | M |
| W | O |
| X | Q |
| Y | S |
| Z | U |
This table illustrates how each letter in the plaintext is substituted with a corresponding letter from the cipher alphabet. The simplicity of the Simple Substitution Cipher makes it easy to understand and implement, but its security is limited due to its vulnerability to various forms of cryptanalysis.