Substitution Cipher

A substitution Cipher is a method of encryption by which units of plain text are replaced with cipher text.The receiver deciphers the text by performing an inverse substitution.For example, the letter "a" is encrypt to "z", while "b" change to "y" and so on.

How To Break Substitution Cipher
Most people break substitution cipher is using the frequency of English letter/alphabet.

Use the frequency of letter
Some research found that the most frequently used alphabet/letter in a text is "e" which is 12.7 %, the second letter is "t" is 9.7% ,third letter is  "a" 8.1%.

Then we use the frequency of pair letter(digrams) like "he","an","in","th" .Following is looking for trigrams.

This kind of attack is call the cipher only attack.






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