What is Cryptography

Cryptography is not a new subject. In fact, we've been using cryptography for thousands of years. It's just that with all this global access and all this connectivity, the need for cryptography has grown tremendously. There's a whole industry behind this, and it's a very critical part of cyber security.

What the word means

What does this word cryptography mean? It's Greek, and it's made up of two components: crypto, which means conceal, hidden or secret, and graphy, which means drawing, writing, representing, recording, describing, or we can think of it as the art of. So you can think of cryptography as the art of concealing. When it comes to cyber security, we can think of it as the art of concealing information. The example that we're going to use throughout is that we are going to hide, or conceal, the word secret, so we're concealing it into some sort of code like this.

The basic terms

What does the term cryptographers mean? That's just the person who's going to be making or breaking these coded messages.

Plain text is just the message that you have before you actually convert it into a hidden message, whereas cipher text is just what we have after it's been converted. Encryption is what we call the process that happens when we take plain text and turn it into cipher text. Decryption is when we take the cipher text and convert it back to the plain text.

A cipher is just the process or algorithm that we use to either encrypt a message or decrypt a message, so it is the process that it goes through to do this. There are lots of different ciphers out there. It's a type of algorithm. Algorithm is just a generic term, but when it comes to cryptography algorithms, we call those ciphers. Depending on which cipher you choose, this output here, this encrypted format, the cipher text, is going to look quite a bit different.

What an algorithm is

If a cipher is a type of algorithm, what's an algorithm? It's some sort of process that happens. You have some sort of problem or some sort of computation that you need to solve, so there's some sort of output for this, and then you have some sort of input into it. There's an input, it goes through this process, and then there's an output, and that's an algorithm.

An example here is that our algorithm, or our cipher right here, has a set of instructions. This is the process that we go through. We start with the first letter. Our first letter is S, and we look it up on the chart. So we look up S on the chart, and then we substitute the letter with the new letter, which is V, so then we put the V right up here. Next we're going to repeat steps two and three for the next letter, so we're going to take E, which is going to be H, and so we're going to put H there, and so on and so forth. That's the cipher that it's going through to convert this plain text into some sort of cipher text.

This particular cipher is known as the Caesar cipher, because it was used by Julius Caesar around 58 BC. So this is a very old and very simplistic way that you could encrypt and decrypt messages.

Cryptoanalysis

Cryptoanalysis is the art of reverse engineering, or decrypting things without knowing all of the variables that go into it. As an example here, we have our cipher text right here, and being able to reverse engineer to find out what our plain text is without knowing all the details of how to decrypt this.

One thing that they look for is repeated letters. One of the most common letters in the English alphabet is the letter E, so if they see a repeated pattern here, they might try first of all saying, well, if H was an E, let's replace that with an E, and they could reverse engineer to figure out what the key is. In this particular one, the letters are shifted by three, so you'll notice that an A is now a D, a B is now an E, a C is now an F, and all those letters have just shifted by three places. So cryptoanalysis would be able to figure this out pretty quickly.

Keys

There are ways that we can make this a little more complex so that it'll be harder to break. For instance, maybe this time when I send you a message I send it to you with the alphabet shifted by three, but maybe next time it's going to be by six, and maybe the time after that it's going to be by 9. I'm going to shift it a different amount every time I send you a message. Somehow you're going to need to know how many letters you need to shift the alphabet in order to decrypt this message. That's where a key comes in.

A key is additional information that you need in order to encode or decode a secret message. In this case here the secret key is eight, so we shift our alphabet by eight characters, and now when I encode this message, or encrypt this message, an S becomes an A, an E becomes an M. And then when you reverse engineer this, or when you decrypt it, an M becomes an E and an A becomes an S, and you can decrypt that message.

Substitution, transposition and steganography

What we're doing here is a type of cryptography called substitution, where we're substituting in one letter for another letter. Although substitution is one of the most popular, it's not the only way that we can encrypt a message. Transposition is another way that we can encrypt a message, and we do that by shifting the letters: rather than substituting, we shift the letters around.

Steganography is another way that we can hide a message. We're concealing it in something else. An example is maybe I'm concealing this word secret inside a picture, or I've actually concealed the word secret into this little sentence right here. So it's different than cryptography. With cryptography we're actually converting, so secret gets converted into something that is supposedly undecipherable, or we need the key to be able to decrypt that message, versus steganography, which is hiding it into something else.

See if you can point out what the pattern is that spells out secret, that this word is inside this sentence right here, hidden inside there. If you take the first letter of every second word, then we can see that it spells out secret. But as mentioned, we could actually hide it into other things, like this picture could have an embedded message hidden inside of it.

As we move along through this module we're going to be covering a lot of cool information, and we're going to be using these terms as we go, so make sure you have a good understanding of what these terms are.