Cryptography, An Introduction – CompTIA Security+ Lesson 16

Cryptography is the science of altering informations with a key to protect it. It’s becoming an area of concern for goverments, businesses and individuals.

The study of cryptographic algorithms is called cryptography. Cryptolanalysis on the other hand is the study on how to break them.

Both combined are called cryptology.

However, many textbooks (and specially the one I’m reading) use the terms cryptology and cryptography interchangeable.

This topic is so broad and important, that I will make a multi-post-series of it.

This post will be an introduction to cryptography. I will cover “old” / historical cryptography. It’s less heavy on math and easier to get in.

The following posts will be about modern cryptography: symmetric, asymmetric, hashing, quantom cryptography, etc…

cryptography learning

Historical Cryptography

In the olden days, it was much simpler and less math heavy (I miss those days). The two primary types were substitution and transpositions. Both easily hackable.


Important substitution methods for the exam are caesar and vigenère ciphers.

Simply put, substitution methods shift the alphabet to the right in a certain number (key). A caesar cipher would look like this:

Key: 3

I like trains –> L olnh wudlqv

The vigenère cipher would do the same, but shift each letter with an individual key.


Usually, with this method, a message is broken into blocks and each block is scrambled.

cryptography cipher 1

Another classic example is the Rail Fence Cipher. You write the message letters out diagonally over a number of rows and then read off the cipher row by row:

cryptography ciper 2


This is the process of hiding a message in a medium such as an image, audio file or anything else.

The most commonly used methode is called the least significant bit (lsb). In an image for example, every pixel is 24 bits (on a windows machine, according to my book). If you chnge the last bit (least significant one), no noticeable change would appear in the image.

You can try it yourself with QuickStego.

Other important systems