How to Make a Keyword Cipher: A Step-by-Step Guide

A keyword cipher is a simple yet effective encryption method that replaces letters in a message based on a keyword. It's a great way to learn the basics of cryptography or to create secret messages for fun. Here's how to make one.

What Is a Keyword Cipher?

A keyword cipher is a type of substitution cipher where a keyword is used to alter the alphabet. The remaining letters follow in order, excluding any duplicates from the keyword. This creates a unique cipher alphabet for encoding and decoding messages.

Step 1: Choose Your Keyword

Select a keyword that's easy to remember but not too common. For example, "SECRET" is a good choice. Avoid using repeated letters, as duplicates are skipped in the cipher alphabet.

Step 2: Remove Duplicate Letters

If your keyword has repeating letters, remove them. For "SECRET," the letters are already unique, so no changes are needed.

Step 3: Build the Cipher Alphabet

Write down your keyword first, then append the remaining letters of the alphabet in order, skipping any letters already in the keyword. For "SECRET," the cipher alphabet would start with S, E, C, R, T, followed by A, B, D, etc.

Step 4: Map the Original Alphabet

Write the standard alphabet (A-Z) above your cipher alphabet. Each letter in the original message will be replaced by the corresponding letter in the cipher alphabet.

Step 5: Encrypt Your Message

Replace each letter in your plaintext message with the corresponding letter from the cipher alphabet. For example, "HELLO" would become "TARRD" using the "SECRET" cipher.

Step 6: Decrypt the Message

To decrypt, reverse the process. Match each letter in the encrypted message to the cipher alphabet and replace it with the original alphabet letter.

Tips for a Stronger Cipher

For added security, use a longer keyword or combine the cipher with other encryption methods. Avoid common words or phrases that could be easily guessed.

Conclusion

Creating a keyword cipher is a fun and educational way to explore basic cryptography. With these steps, you can easily make and use your own cipher for secure or playful messaging.