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In a simple substitution cipher, you can assigning letter values to other letters. For example, A may equal C and and F might equal J. Make sure you don’t choose easy combinations like A = B or A = Z. They’ll be easily broken.  Make decoder rings. Those decoder rings you got in a box of cereal years ago may come in handy, but you can make your own too if you want. Write your letters, A through Z, in a large circle. Then, write the substituted letters inside the large circle to form their own smaller circle. Now you’ll have a key for your cipher. Randomize the cipher. Once you’ve got your keys and your friends have theirs too, you can cut out both rings from the paper. Turn the smaller ring inside the larger outer ring to create new substitutions. When you pass your note, write what A equals, and the reader will know where to set their rings. To make a slightly more complicated substitution cipher, you can make your own simple symbols to stand in for letters. The more each symbol looks different than the letter it represents, the harder the code will be to crack. Make sure each symbol is also simple and easy to draw quickly.  Create a key for your new language. Make a master key that shows which letter matches up with each symbol. As you practice writing in your new script, you’ll become more familiar with the symbols, but you’ll need a key at first to remember which is which. Share the key with friends. Your language won’t be of much use if only you can read the notes you pass! Share the key with your friends, advising them to keep it safe so that no one else finds and learns the symbols. Write your messages in the new language. Practice the symbols until you can read and write them as quickly as your native language. It’ll take a while, but you’ll get to know the symbols well over time. For example, swap A with (α) Alpha, B with (β) Beta, C with (Χ) Chi, etc. from the Greek alphabet..  Use similar characters for missing letters. Some foreign alphabets may not contain all the letters in your language. Greek doesn't have Y, but there's a visually similar Upsilon (Y in capitals), so you might use that instead. Make sure your reader knows that you’re going to be substituting some letters for others not in the language. Test some examples.  For example: "See you tonight" becomes "ΣΕΕ ΤΗΟΥ ΤΟΝΙΓΗΤ,” with “you” subbed out for “thou” because Greek doesn’t have a character for “Y”. Numbers tend to stick out in messages with a lot of text, so try to find a better way to to hide them in your cipher. For example, replace the "8" in "See you tonight, 8 o'clock" with "√(128/2)". The 8 is replaced by a simple math: the square root of (128/2=64)=8. It’s relatively easy to crack a substitution code, because you’re still using your native language and only swapping out the letters for other letters or symbols. Words like “and”, “you” and “the”, which will show up frequently in your messages, and can be found out and used to piece together parts of a key. Common letters like E, T and A also help code-breakers to decipher your message.
Substitute letters for other letters. Substitute your own symbols for letters. Substitute letters from another language. Substitute equations for numbers. Be aware that substitution ciphers can be broken.