The concept of protecting information so it cannot be read while it is being transported over an insecure medium is thousands of years old. The leaders in Roman armies sent coded messages to each other using a code called the “Caesar Cipher”. The simplest version of this approach is to take each of the characters of the actual message (we call this “plain text”) and shift each character a fixed distance down the alphabet to produce the scrambled message or “ciphertext”.
Then we send the ciphertext via the courier or other insecure transport to the other person. The courier cannot read the message because it appears to be random characters unless you know the technique used to encode the message.
As long as the person receiving the message knew the number used to shift the message, they could unshift the characters in the encoded message to reproduce the original message.
Here is a simple example of plain text and ciphertext using a shift of one:
Plain text: Go to the river
Cipher text: Hp up uif sjwfs
We use the word “encrypt” to describe transforming the plain text to the ciphertext and “decrypt” to describe the reverse process.
The Caesar Cipher is very simple to defeat, but it was used to protect important messages until about 150 years ago. Modern encryption techniques are far more sophisticated than a simple character shift, but all encryption systems depend on some kind of a secret key that both parties are aware of so they can decrypt