Substitution Ciphers

A gentle introduction to substitution ciphers.

Throughout history, an ongoing battle to ensure the secrecy of communications has been raging. For every innovation in designing secret messages, there is an associated breakthrough in the effort to read them. This constant struggle over time produced advancements in subject areas ranging from business to computer science.

The field of cryptography (krip-tog-ruh-fee) focuses on the methods and techniques used for hiding the meaning in messages and making sense of them.

Cryptography — The art and science of reading and writing disguised messages.

There is a multitude of methods for disguising messages. These methods range from mathematically simple to complex and from easy to break to practically impenetrable. A secret code is a system where words or phrases in the original message are replaced by other words or symbols. A cipher (sy-fer) is a way of creating a secret message by replacing individual letters in a systematic way.

Cipher — A disguised or secret way of writing by systematically replacing letters.

There are many different types of ciphers used in everyday life. People use them, often unknowingly, when poking around on smart phones, accessing ATM machines, and shopping online. Classical substitution ciphers involve methods where letters are systematically replaced with other letters, numbers, or symbols. These types of ciphers come in many forms and have been around for a very long time.

Using a secret code or a cipher to disguise a message is called encryption (en-krip-shuhn).

Encode — To convert a message into a disguised form using some secret code or cipher.

Using some method to read a disguised message is called decryption (dee-krip-shuhn). The ability to read secret messages is of interest to the sender, receiver, and their adversaries.

Decode — To convert an encrypted message into a readable form.

Some ciphers are more secure than others. However, if your goal is to delay your adversary from knowing your short-term plans, you can choose a simple cipher. If your goal is to obscure your message for a longer period, choose a more complicated cipher. This series begins with simple ciphers and walks you through some techniques used to break them.

Now if you haven’t already decoded the secret message from the beginning of this introduction, here is what it says:

nggnp xsebz guryr sgjvg unepu ref
attac kfrom thele ftwit harch ers

attack from the left with archers

In each installment of this series, you will be tasked with completing several missions. Complete all 16 missions and then you will be ready begin your journey exploring the rich subject of cryptography.

Bonus Mission: After reading this series, determine the cipher used to code the secret message in this introduction.




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John Redden

John Redden

Professor and Author

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