Crypto Corner
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  • Monoalphabetic Substitution Ciphers
    • Atbash Cipher
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    • Affine Cipher
    • Mixed Alphabet Cipher
    • Other Examples
    • Frequency Analysis: Breaking the Code
    • Homophonic Substitution
  • Simple Transposition Ciphers
    • Rail Fence Cipher
    • Route Cipher
    • Columnar Transposition Cipher
    • Myszkowski Transposition Cipher
    • Permutation Cipher
    • Anagramming: Jumbling words
    • Combining Monoalphabetic and Simple Transposition Ciphers
  • Polyalphabetic Substitution Ciphers
    • Vigenère Cipher
    • Kasiski Analysis: Breaking the Code
    • Autokey Cipher
    • Other Examples
  • Fractionating Ciphers
    • Polybius Square
    • Straddling Checkerboard
    • Transposing Fractionated Text
    • Other ways to Alter Fractionated Text
  • Digraph Substitution Ciphers
    • Playfair Cipher
    • Two-Square Cipher
    • Four-Square Cipher
    • Hill Cipher

Monoalphabetic Substitution Ciphers

Substitution ciphers are probably the most common form of cipher. They work by replacing each letter of the plaintext (and sometimes puntuation marks and spaces) with another letter (or possibly even a random symbol).

A monoalphabetic substitution cipher, also known as a simple substitution cipher, relies on a fixed replacement structure. That is, the substitution is fixed for each letter of the alphabet. Thus, if "a" is encrypted to "R", then every time we see the letter "a" in the plaintext, we replace it with the letter "R" in the ciphertext.
A simple example is where each letter is encrypted as the next letter in the alphabet: "a simple message" becomes "B TJNQMF NFTTBHF". In general, when performing a simple substitution manually, it is easiest to generate the ciphertext alphabet first, and encrypt by comparing this to the plaintext alphabet. The table below shows how one might choose to, and we will, lay them out for this example.
Picture
The ciphertext alphabet for the cipher where you replace each letter by the next letter in the alphabet
There are many different monoalphabetic substitution ciphers, in fact infinitely many, as each letter can be encrypted to any symbol, not just another letter.
The history of simple substitution ciphers can be traced back to the very earliest civisilisations, and for a long time they were more than adequate for the purposes for which they were needed. By today's standards they are very weak, and incredibly easy to break, but they were a very important step in developing cryptography.

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Next Page: Atbash Cipher
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  • Home
    • Crypto Corner Challenges
    • Glossary
    • Help with Activities
    • Educational Uses
    • Downloadable Resources
  • Introduction to Cryptography
    • Steganography
    • Codes and Ciphers
    • Conventions in Cryptography
  • Monoalphabetic Substitution Ciphers
    • Atbash Cipher
    • Pigpen Cipher
    • Caesar Shift Cipher
    • Affine Cipher
    • Mixed Alphabet Cipher
    • Other Examples
    • Frequency Analysis: Breaking the Code
    • Homophonic Substitution
  • Simple Transposition Ciphers
    • Rail Fence Cipher
    • Route Cipher
    • Columnar Transposition Cipher
    • Myszkowski Transposition Cipher
    • Permutation Cipher
    • Anagramming: Jumbling words
    • Combining Monoalphabetic and Simple Transposition Ciphers
  • Polyalphabetic Substitution Ciphers
    • Vigenère Cipher
    • Kasiski Analysis: Breaking the Code
    • Autokey Cipher
    • Other Examples
  • Fractionating Ciphers
    • Polybius Square
    • Straddling Checkerboard
    • Transposing Fractionated Text
    • Other ways to Alter Fractionated Text
  • Digraph Substitution Ciphers
    • Playfair Cipher
    • Two-Square Cipher
    • Four-Square Cipher
    • Hill Cipher