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

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 addictive 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:

Look at this table for Occurrences of the alphabet

• 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.

• Example Output 2(Plain-text) – EXECUTE

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.

Conclusion

Monoalphabetic cipher is a better cipher technique than Caesar cipher. Since the monoalphabetic cipher is easy to guess using the English letter, a countermeasure is to provide multiple substitutes known as homophones for a single letter.

Frequently Asked Questions on Monoalphabetic Cipher – FAQs

Advantages Of Monoalphabetic Cipher over Caesar Cipher

In Caesar Cipher, the difference between the characters mapped is uniform due to which it becomes easy for unknowns to covert the cipher-text back to plain-text , whereas in Monoalphabetic cipher the difference is not uniform and thus it makes difficult for unknowns to crack the text.

Why Polyalphabetic Cipher is better than Monoalphabetic Cipher?

Polyalphabetic Ciphers are more secure than monoalphabetic cipher as they use multiple substitution alphabets to encrypt a message.

Why does the Monoalphabetic Cipher come under the substitution technique and not under the transposition technique?

A substitution cipher technique replaces letter with other letters whereas a transposition technique keeps the letters same but rearranges the order. So, monoalphabetic cipher comes under substitution technique.