Okay. Now that we've talked about random and pseudorandom numbers, we're going to talk about one of their uses namely cryptography. The random and pseudorandom numbers are used typically as cryptographic keys. So what do you do once you've got them? This video five, of lesson eight, is going to talk a little bit about cryptographic basics. The basics here simply to orient yourself. We assume that you know how to use cryptography in general, at least in the theoretical sense, and we're going to talk about how to use it in practice. The biggy (phonetic) for cryptography and surprisingly is not the cryptographic algorithms, it's the policy. When do you want to use it? What do you want to use it for? You'll see examples of both of these later on. But the gist of this is, in many cases for example, if you want to protect the file by encrypting it, well, you can use whole disk encryption and that gives you the encryption that you want right there. So there's no need to do the additional step of encrypting it. The second question is, when you say you use cryptography, why are you using it? Are using it to ensure that data is secret? Or that the data will only be changed by authorized people? Or are you simply trying to associate that data with someone else provenance if you like, or some combination. Each of these requires a slightly different use of cryptography. Now, just as a very quick review, there are two types of cryptosystems that we'll be dealing with. The first is called secret key or symmetric. This is the cipher that was used in classical times. The sender and the recipient share a key, or they have a key that, recipient's key can be easily derived from the sender's and vice versa. The Caesar cipher or shift cipher is a classic example of this. Where A becomes D, B becomes E, C becomes F, and so forth. We're going to call the key for this sort of thing secret, and these ciphers hold either secret key, or symmetric secret key. Because you have to keep the key secret from everybody but the sender and the recipient, and symmetric because both of them have the same key, in essence. The second form which was first discovered in the classified community in 1969, but later rediscovered publicly by Diffie and Hellman in 1976, and by the way they did not know about the classified work, the classified work only became public in the mid 1990s in a book Dorothy Denning wrote called, Information Warfare. In public key cryptography, as it's normally called, the sender and the recipient have two different keys. The keys cannot be easily derived from one another. They can be generated easily, but not, given one, it's computationally infeasible to get the other. Maybe as one of the keys will be used for enciphering and the other for deciphering. The enciphering key is public. Everybody knows that it's put out there, it's typically protected by a certificate or something like that. I won't get into a discussion of public key infrastructure because that's very messy and not particularly germane to this, except for one point that I'll bring out later. So anyone can get the public key, but no one can get the private key. At least in theory, only the owner can get the private key. The private key is typically stored protected on the local, on the recipient, or on the owner's system