Monday, November 25, 2013

Code 60 : Monoalphabetic Substitution

Monoalphabetic Substitution


The monoalphabetic substitution takes a letter of an alphabet and substitutes it with another letter, this way a ciphertext is generated. The way of converting is fixed. A character of the plaintext will be replaced by the same ciphertext character, during the entire ciphertext. There is no additional key. The only way of security is to keep the substitution-table secret. 
A popular example for the monoalphabetic substitution is the caesar cipher.


The first step is to write down the plaintext alphabet. It includes all characters needed for the message. In this example only upper case letters will be used. 

The second step is to build an additional random alphabet:

An becomes  /B is replaced by QC replaced by W, etc. 
The word „EXAMPLE“ would be encoded by: „RV/GSHR“.


The Monoalphabetic substitution can easily be cracked with a frequency-analysis.



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