 Hey y'all, Mr. Gibson here and welcome to the first lesson in cryptography. In this video, we'll discuss what cryptography is, learn some key vocabulary, and preview different types of ciphers that will be covered in this video. Cryptography comes from the Greek words cryptos and graphy, meaning secret writing. In this course, we'll focus on using mathematics to make our writing secret while also learning how to harness the power of computing to chip away at the security that the mathematics can provide. Cryptography and secret codes have been popular for generations, making their way into popular cultures such as books, television shows, movies, and new technologies such as cryptography. It's no surprise that so many of you are interested in learning more about how cryptography works. At the end of this course, I hope that you'll be able to watch a movie and while doing so be able to determine what cryptography methods are real and which are just fantasy. Some key vocabulary to understand when discussing cryptography. Plain text, which is your readable message, this could just be any message of your choosing cipher text, which will be the unreadable message that you generate using algorithm and key. The cipher that you choose to use is the algorithm that will convert your plain text message into cipher text. We'll study a variety of these and the key key goes with the algorithm. It's just a piece of information. It could be a number or a phrase and the algorithm will use it to create that unique cipher. So if you use a particular algorithm with two different keys and the same plain text, you should get two different cipher. So for example, an example, plain text message could be North Carolina, which would turn into the cipher text Q, U, R, U, W, K, and so on. If we use the Caesar cipher with a key of three. A couple of things to point out here, you can note the formatting of these different elements. Plain text is written in all lowercase letters, cipher text in all uppercase letters. You also see that cipher text letters are written in groupings or blocks of five letters. This helps to ensure that you don't miss a letter when you're reading and writing these sometimes long messages. Keys are often written in italic font when you can. And then notice that your messages, they're written in what looks like maybe an old typewriter font. This is called a mono spaced font. The most common one of these is called Currier New. It's found on pretty much every computer, whether it's running Windows, Max, Linux, it doesn't matter. You can find that. In addition to converting between plain text and cipher text using algorithms and keys, we'll play the role of a crypto analyst in this course. Attending to determine what a plain text message is, even when we don't know exactly how the cipher text was created. This is the role that people often see portrayed in the movies when characters crack the code. This task requires a lot of probabilities, statistics, and other information to correctly obtain the plain text message. So for example, this message is part of the warm up activity in this course. You'll notice that the cipher text is not blocked into groups of five characters. And this probably allowed you to make some educated guesses about how that plain text message was created. Sorry, altered to create a second. Many of you correctly assumed that the message was a substitution cipher more on what that is in just a minute. Where each plain text letter was changed into a different cipher text letter, not that the substitution was consistent throughout the entire message. So for example, the letter T in the plain text message became an S in the cipher text message every time that the T appeared. Then using what you know about short words, one, two, three letter words, and where vowels appear in them, you probably made some good guesses about which plain text letters became which cipher text letters. And a little trial and error, and before you know it, you've got the entire plain text available to read. You didn't need to know the algorithm or the key used to generate the cipher text, but you are able to obtain it using single letter frequency analysis. Something we'll cover in more depth later on in the course. In this course, we'll cover two major types of ciphers for most of the content. And those are transposition ciphers and substitution ciphers. Transposition ciphers such as reverse cipher, rail fence and route. There are algorithms that just shuffle up the letters in a message without actually changing the characters. Whereas a substitution cipher will actually change those characters into new characters based on some sort of mapping function. That can be kind of real hands-on, really non-mathematical, or it can be very mathematical. So, simple, complex, we'll study a variety of those functions throughout the course. Here's an example of a transposition cipher. If we have a plain text message, just math, there are 24 possible cipher texts that could come using different transposition ciphers. So, for example, a two-row rail fence, which we'll cover in the next lesson, would yield the cipher text m, t, a, h, while the three-row rail fence would give m, h, a, t. And I'm sure there's many other transposition ciphers that we could use to get to each of those 24 different possible cipher texts. Here's a really well-known substitution cipher, the Caesar cipher. This type of cipher changes a plain text letter to another letter, found a certain number of positions further down the alphabet. So, in this example, we can see the shift was three as the plain text letter a becomes the cipher text letter d, b becomes e, and so on, changing the plain text word math to the cipher text word p, d, w, k. As we study ciphers in this course, we'll want to make sure that they meet certain criteria to ensure that they're valid. So, for example, the message should be easy to encipher and decipher. It should be easy to transmit the message so no kind of unusual characters or glyphs, standard text would be best. It should be possible to communicate and remember the key without having to write it down. So, nice short or easy to remember keys are preferred. The method or algorithm that's used shouldn't have to be secret to guarantee the security. The security should come from not having the key. So, for example, it's okay if somebody knows that I use the Caesar cipher as long as they don't have the key, it should be secure. It's not in practice, but we'll see certain ciphers that are. And then if the last bit, if somebody intercepts the message, it should be pretty hard for them to decipher without knowing the key. So, as we study different ciphers, we'll kind of come back to these criteria and see which, if any, that it meets. And hopefully with the end goal being finding a nice cipher that meets all these criteria so we can be reasonably sure that our messages are secure. And that's it. You've got a solid foundation to get started in your study of cryptography. Thanks y'all for watching and we'll catch you in the next one.