Monoalphabetic Cipher is a part of the substitution technique in which a single cipher alphabet is used per message (mapping is done from plain alphabet to cipher alphabet). Monoalphabetic cipher converts plain text into cipher text and re-convert a cipher text to plain text. Monoalphabetic Cipher eliminates the brute-force techniques for cryptanalysis. Moreover, the cipher line can be a permutation of the 26 alphabetic characters.
In Cryptography, various encryption techniques are used to provide data security. The classical Encryption technique is categorized into two divisions:
1. Substitution Cipher Technique
- Caesar Cipher
- Monoalphabetic Cipher
- Polyalphabetic Cipher
- Playfair Cipher
- Hill Cipher
- One-time Pad
2. Transposition Cipher Technique
The techniques which come under this division are as follows -
- Rail Fence.
- Row Column Transposition.
Types of Monoalphabetic Substitution Ciphers
Additive Cipher
It is also Known as Shift Cipher which shifts plain text to form Cipher-text.
- Mathematical Expression:
- For Encryption: C=( P + K ) mod 26 where 'P' is the character in plain text, 'K' is the key, and 'C' is the Cipher.
- For Decryption: P=( C-K ) mod 26.
- Example : Input : P= GEEKS ,Key= 4 . Output : C=KIIOW
Caesar Cipher
A type of Addictive Cipher but the value of key is always '3' here.
- Mathematical Expression:
- For Encryption: C=( P + K ) mod 26 where 'P' is the character in plain text, 'K' is the key, and 'C' is the Cipher.
- For Decryption: P=( C-K ) mod 26
- Example: Input: P=GEEKS , Output : C=JHHNV
Multiplicative Cipher
Letters are changed here using a multiplication key.
- Mathematical Expression:
- For Encryption: C=( P * K ) mod 26 where, 'P' is the character in plain text, 'K' is the key, and 'C' is the Cipher.
- Example: Input : P=VMH , Key= 3 . Output : C=HEL
Affine Cipher
A mathematical function is used to convert plain text into cipher text.
- Mathematical Expression:
- For Encryption : C=( P * K1+K2 ) mod 26 where, 'P' is the character in plain text, 'K1' is the multiplicative key, 'K2' is the additive key and 'C' is Cipher.
- For Decryption : P=(( C-K2 )/ K1) mod 26.
- Example : Input : P=ARM , Key1=3,Key2=5 . Output : C=HEL
How Does Monoalphabetic Cipher Work?
Let us understand how this cipher technique works with an example, let us suppose the mapping of plaintext using the table below.
0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
A | B | C | D | E | F | G | H | I | J | K | L | M |
13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
N | O | P | Q | R | S | T | U | V | W | X | Y | Z |
Example Input 1(Plain-text)- GFG
- Explanation: In Monoalphabetic cipher, the mapping is done randomly and the difference between the letters is not uniform. Here, the word is mapped to S (G->S), F is mapped to R(F->R) and G was already mapped to S so we cannot change it (G->S).
- Example Output 1(Cipher-text)- SRS
Example Input 2 (Cipher-text) - GZGEWVGRNCP
- Explanation:
- Using the table, use the alphabet which has occurred most times. So we see, G is used most so we replace G with E (G->E) and then V->T, R->P, N->L, C->A, Z->X, E->C, and W->U. Thus, we found the plain text.
Advantages of Monoalphabetic Cipher
- Better Security than Caesar Cipher.
- Provides Encryption and Decryption to data.
- Monoalphabetic Cipher maintains a frequency of letters.
Disadvantages of Monoalphabetic Cipher
- Monoalphabetic ciphers are easy to break because they reflect the frequency data of the original alphabet.
- Prone to guessing attack using the English letters frequency of occurrence of letters.
- The English Language is used so the nature of plain text is known.
- Less secure than a polyalphabetic cipher.