Information can be protected if it is transformed into a more secure format. In essence, cryptography is the science which allows ordinary information to be transformed into secure information. The process itself is called encryption, and it’s not something that’s come with the internet age – cryptography has been around for centuries, even when people used to send messages written by hand.
Of course, nowadays most cryptography techniques are used to protect data that is digital, not handwritten. Cryptography is a computer science which involves manipulating data into a format which can only be recognized by users who are authorized.
To illustrate how cryptography works, imagine a message where each letter is replaced by a different character – based on a regular map. So, to decode the content you would need the table that explains how letters are transposed. This grid is called a cipher. Grids can be simple: just mapping one letter to another, but for more security a more advanced algorithm can be used that makes the grid mapping much more complex.
If you’re just going to be handwriting a note you could probably work with a simple 1-1 grid mapping, but these types of codes are easy to decipher using a computer. Computers can analyze codes at the speed of billions of operations per second, so a code has to be very complex to withstand cracking attempts. In fact, cryptography used in modern secure applications must be complex enough to be difficult for even a super computer to crack.