Cryptography: The Science of Secret Writing

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A code uses words or phrases to represent other words or phrases. For instance, a tank might be called a turtle, or “the egg is in the nest” could mean that a delivery has been made. To decode a message, you must have a codebook that defines each word or phrase—almost like a language dictionary.

A cipher is a set of rules—an algorithm—that is applied to each character or letter of a message. The original message is called the plaintext, and the encrypted message that you can’t read is called the ciphertext. There are two main types of ciphers: substitution ciphers and transposition ciphers. In a substitution cipher, each letter is replaced with a different character—a substitute. In a transposition cipher, the letters are not replaced, but all the letters of the message are rearranged. Think about it as a change of position. When you use more than one cipher on the same message, it is called a product cipher.

Cryptography (meaning secret writing) is the science of using codes or ciphers to keep messages safe. When you think of codes and ciphers, you probably think about spies or treasure hunters. But did you know that you use cryptography all the time? Any time you use a computer, cell phone, or copy machine, the information must be encrypted (i.e., a cipher applied to it) to be sent electronically. This is very important for keeping your personal information private and safe.

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Because a cipher is a set of rules to follow, it always works the same way every time you use it. You can follow the steps in reverse to decrypt it—to undo the cipher so that you can read the plaintext again. Cryptography depends on consistent rules and predictable patterns. In other words, we can only use ciphers because God created an orderly universe that is governed by unchanging laws of logic.

There are predictable patterns in the world because God, who created (Genesis 1) and upholds the universe (Hebrews 1:3), does not change (Malachi 3:6). In an evolutionary world—one that results from chance and random processes—we cannot expect order and consistency. It is only because of God’s unchanging nature that we can expect math, science, logic, and even ciphers to work in a consistent and predictable way.

Many ciphers also have a key. If you locked your house, what do you need to get back in? You need a key. But not just any key; you need the right key. Your friend’s key won’t open the door. You need the key that matches the lock. Ciphers work the same way. If you secure the message with a key, that same key is needed to unlock the message. The Bible is a little bit like this. Paul explained that God’s Word cannot be understood without the help of the Holy Spirit (1 Corinthians 2:6–16). The Spirit is the key to interpreting God’s Word.

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Sending secret messages can be a lot of fun, but it can also have some very important uses. Most cryptography today is done with computers, but there are many classic ciphers that you can learn to do by hand! Check out these instructions to write your own secret messages using a Caesar Shift Cipher or a Columnar Cipher.