Hi folks, you remember that in a previous video, we talked about DES, single DES, and the problem we had with 64-bit block cryptography. Meaning, a train comes in, a 64-bit block comes in, I encrypt it and boom, out pops a 64-bit block output. And then the next one comes in, 64 in 64 out, 64 in 64 out. It allows for something called a covert channel. And you remember we talked about that previously, where you use a wall maybe to knock on, to communicate with somebody. It's a little crazy to think that that's how somebody would actually communicate, but cryptography clearly can include leaks of information as part of the plain text decipher, text transformation. It's crazy. So, we have to come up with a way to make the 64-bit block and 64-bit block output less prone to covert channel. Like if I just make the blocks all the same, then I get the same output each time and I can signal. So, back in the 1970s, there was this patent written by a group at IBM. And to this day, I think it's one of the fundamental patents in cyber security. It's something I'm actually going to have you read as part of the required reading. And here's what their idea was, they said let's chain the blocks. And if you guys hang out on tour or if you read about Bitcoin and block chaining, you know what a chaining of blocks is essentially putting things together in a way to make the cryptography more undecipherable. So here's the idea, here's how it works. Normally, I would say block A, F of block A is B and then F of block A prime is B prime and so on. That's block cryptography. But chained block cryptography works as follows. I take the output of one round and I make it part of the input domain to the next round. And then whatever pops out of that becomes input to the next round and on and on and on. So think of it this way. And there's one part that's kind of weird. You have to start somewhere. So you go, whenever you have something in chains you go, "Okay, how do you start?" And in block chain you have this concept called the Genesis block. So let's start with the Genesis which is me making up that I did one already and I make believe that's the output to some fictitious previous run. I say there's my Genesis block. So now, when I encrypt the block that comes in, I encrypt the block plus the Genesis from previous one to get a new output. Now, that new output gets encrypted with the next block to produce a second output and that second output gets encrypted with the third block to produce a fourth and on and on and on. Isn't that kind of a cool concept. Like this idea that you're chaining things makes it harder, not impossible but much harder, for some to create a covert channel. Now this really is the basis for block chain, for a lot of the really interesting concepts. But the thing you should keep in mind is that there's work involved in going right and left, right? So as I go from from where I am now out to subsequent block chain computations, I'm doing work to do that. If I came back and I fussed with something, I change something here I mess up the whole chain and I have to work real hard to catch up and might actually come up with some different outputs depending on what I'm actually doing. See, it gives you a little hint as to how powerful this chaining concept is going to be, but the main thing that the guys back in the 1970s at IBM were trying to do is to shut down a covert channel in single DES and obviously works also in triple DES. So I think it's a beautiful way of solving a problem in an elegant manner without having a real change to the base cryptography. Now, we have little quiz here to test your understanding here. And the answer is none of the above. None of them have anything to do with block chaining. Block chaining closes covert channels, okay? So threw that in for a little trick questions to see if you're listening. But I hope you enjoyed this. What we're going to do in subsequent videos is we're going to start moving in the direction of public key cryptography. We'll see in the next video in our series, we're going to talk about a basic limitation, a scaling limitation, in conventional crypto and then we'll dive right into the public key. So we'll see in the next one.
