A substitution code is a very basic cipher in which every letter of the plain text alphabet is randomly substituted for another letter creating a cipher text. First you would write out the plain text alphabet (a-z).

ABCDEFGHIJKLMNOPQRSTUVWXYZ

Next, you would randomly substitute letters for another to create the cipher alphabet.

ABCDEFGHIJKLMNOPQRSTUVWXYZ

GNOAQSBCRDNEXFYZKHIJMPLTVW

Now that you have your cipher alphabet you simply rewrite the plain text using the cipher alphabet. Thus,

“Life is like a box of chocolates

Forrest Gump”

Becomes,

“ERSQ RF ERUQ G NYM YS OCYOYEGJQI

Read more in Mathematical Theory

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SYHHQIJ BKXZ”

In this cipher all the letters of the normal alphabet are simply and randomly substituted for another creating the cipher text. This kind of cipher is created, usually with the intentions of delivering a message without anyone being privy to what it says with the exception of the intended recipient. The recipient will usually have a copy of the cipher alphabet, allowing him to crossreference the cipher to the key. This enables him to convert the cipher back to plain text, thus revealing the hidden message.

To crack a substitution code you would simply use a cipher only attack using frequency analysis. For more on frequency analysis please read my other article entitled “Frequency Analysis”.