In this video, you will learn to describe important cryptography terms such as plain text, cipher text, symmetric key, public key, substitution cipher, Data Encryption Standard. So before starting, one of the things we should do is set our lexicon or dictionary about cryptography. So there's a couple of key points to be made here. Looking at this diagram, we see Alice communicating to Bob again. Alice and Bob are the sender and receiver respectively. Alice has a plain text message that she wishes to send to Bob. So the plain text message is human readable. This is a clear text. It can be an email, a Microsoft Word document, a web-page link, anything that Alice may wish to send to Bob. Like I said, it can be clear text and in this simple form, readable by any. So then there's Alice runs through an encryption algorithm to create the cipher-text. So the cipher-text is the encrypted message. We talk about a message but could be a Word document. Once again, any number of types of content. You'll notice that Alice has an encryption key. So this encryption key is designated by the letter K and you will notice there's a small subscript right here that indicates that is Alice's key. So K_A is Alice's key. Once again, this creates the cipher-texts which is put on the communication channel and sent to Bob who's the recipient. Bob decrypts the cipher-text to recover the plain text using his key which is designated by K is for key lowercase b subscript b for Bob. So K_A Alice's key, K_B Bob's key. Truly, in the center here is the interceptor, the eavesdropper. Now well there's a basic architectural difference between two types of cryptography architecture. One is a symmetric key. This is where the receiver key, Bob's key and Alice's keys are identical. So in this case, K_A equals K_B. Public-key cryptography uses a difference of a key. There is a secret key that Bob has so K_A does not equal K_B. So let's move on and let's take a look at some principles of symmetric cryptography. Let's take some time and investigate the principles behind symmetric key cryptography. There is a couple of architectures, a couple of styles of symmetric key cryptography that we will look at. So one of the first ones is the substitution cipher. So this is the equivalent of a magic decoder warning that we have a simple substitution of one letter. It's a mono alphabetic cipher which means that we substitute one letter to another and that substitution does not change for the entire message. We'll take a look here at a plain text or are we just simply run a through z and the cipher-text is m through cube. So m is the 13th letter of the alphabet. So this is k equals 13, meaning that we'll shift the cipher-text 13 characters to the right in the mono alphabetic presentation of a through z and our plain text as in the example from Alice to Bob says, "Bob, I love you Alice," and the cipher-text as you can see is nkn and you can certainly read the rest. So one of the questions before us is how difficult is it to break this simple cipher. Well, the answer is it's not very hard at all because there's a very uneven distribution of the use of letters in the English language. So we know for example, that the letter e occurs most frequently. So a simple histogram of the occurrence of letters in the cipher text will reveal what e is. In this case, e will be c. So this frequency histogram will quickly yield the cipher-text. So in fact, this is not a very secure method to use it. Now from a graphic at what a symmetric key cryptography architecture looks like. So once again, Alice, who is sending a plain text message and we will designate the message as M, encrypts the plain text with the key especially designed between Alice and Bob. That's what the subscripts of A-B indicates. Lets go through the encryption algorithm to create the cipher-text. Now look the designation right here. So we have the cipher-text is identified as the key, Alice to Bob, parenthetically message. So this is the distribution that we saw earlier with a one-letter shift. For example, Bob here receives the cipher-text, applies the decryption key which is identical to the encryption key. So this is K_A-B and recovers the plain texts. So mathematically, the message here is found by applying the decryption key to the encrypted message or the cipher-text that Bob has received from Alice. So this element right here. This is the message. This is the decryption key and that will result in the extraction or recovery of the plain text message. So Bob and Alice, for this to work, we have to share the distribution key K_A-B. Now the question is, how does Bob and Alice agree on the key value? That is actually the weakness for symmetric key cryptography. The actual encryption on that and we'll take a look at some other methods that are just not Manuel alphabetic. Well, no strong or no worse than asymmetric or public-key cryptography. But the issue is, this is about key distribution, how does Bob get the key from Alice? So she could e-mail it, but could truly intercept that key and then use that for decryption of that message, and the answer was obviously yes. So the problem, and we'll talk about this in more detail, for the foundational problem for symmetric key cryptography is actually in key distribution. So let's take a look there are another symmetric key method. We'll take a look at some of the technology behind [inaudible] by six. Let's talk about DES. So this is a IBM historical cryptography approach. So this was actually built to a standard, that NIST published. It's a 56-bit symmetric key. So that means that the key from Alice to Bob is 56-bits long. When you see a 64-bit plaintext input, that just simply means that the algorithm, the DES encryption algorithm, ingests digests text in 64 bit chunks. So if you had a 640-bit plaintext message, you'd have 10, 64-bit groups that are going to be encrypted. So one of the questions, of course, is how secure is DES, the data encryption standard? Well, 56-bit keys, it like I said is the encrypted key life. It is a brute force approach which was undertaken about several years ago, and said this could be broken in about four-months. So how to defeat defeat that? Well, change the key every three months, and then they have to simply start it over. So there's no known back door. This has been gone through peer review within the cryptography community and those people will report on the slightest vulnerability in an encryption standards. So this has never been published, so we have a sense that there's some strength right here. So we'll take a look at how to make this a little more secure, we could simply use three keys on each of these data blocks, that's the 64-kbit that we see coming through. There's an architecture called cipher-block chaining. Let's take a look at that briefly on the next slide. Here on Slide 7, is an architectural element for DES. So there's actually you can take a look at just do a little finger walk right here, that you can see that there's a left and a right part of the 64-bit and those are reversed. Then we apply 48 bits out of the 56 bits against this element, and there are swapped left and right and there's a permutation elements. The point being is that there are 16 rounds of this segmentation and encryption per encryptions cycle on each of the 64 bits. So once I said earlier we had a 640-bit input, this would happen 10 times and we would concatenate those tendons send that as the encrypted message. Following DES in November of 2001, NIST published a new standard. So what we did was is that NIST moved the ingest block by a factor of two. So went from 64 bits to 128 bits. The key length move from 56 bits to these larger numbers that we've seen here 128, 192, or 256 bits. Why are there three? Well, this is a user selected key length. Now keep in mind the longer the key, the more computationally intensive the algorithm will be. So that if we got information that is at one level of sensitivity and information, it's at a higher level of sensitivity. There's an argument for using the longer bits, the a 128-bit verses the 64-bit makes for a more efficient algorithm. So if you remember on the previous slide, we mentioned that we had a brute force approach that with the high-end computers that are available today we would take just a second to find the DES key as you can see, moves to a 149 trillion years for AES. So you've seen the attraction that brute force is essentially off the table when it comes to the advanced encryption standard. So the salient point for this training module is to know that the first commercially available, electronic encryption algorithms, DES, and then the second follow one was AES which effectively removed brute force.
