Public Key Cryptography - Computerphile

The thing I wanted to talk about today, um. . .

One of the things is a. . the coolness that is public-private key cryptography The basic idea of how it works I think is uh.

. . is just cool and such clever idea that I want to explain it So everyone is familiar with the basic idea of crypto, where you umm You have a secret key You used that to encode some information which converts it from a readable format to garr garbage effectively ah.

. it looks like a random noise You then transfer it to somebody else and they then able to decrypt it and get the message out and anyone who is listening here on your communication um. .

. isn't able to find out anything about the information that you are communicating the way you intuitively think about, especially if you doing these simple codes, something like caesar cypher that you might do when you are a kid there's one there's one secret that you both have, right, this. .

. this is a symmetric system so you have got your message saying HELLO and then you do some process to it to convert it using a key that some secret piece of information which then converts this into nonsense and then you send this to the other person and they decrypt it with the process that kind of same process in reverse using the same key and then again HELLO backout at the other end and that's nice and simple it works um. .

. and it was once sort of only way people did this but it has a problem which is you both need to know what this key is so if it is you and me. And we want to communicate with one another privately we have to agree on a key that's nobody else gonna guess and we have to share it with one anther um so we might meet in the park in secret and exchange envelopes or whatever this is the kind of things spy used to do, right and the problem with that is first its very inconvinient second, you sometimes can't do it, might be physically separated or we want to do something over internet and we never met the problem is that how do I send this key to you without just sending the key in the clear, as it were unencrypted inorder for us to share the key safely we need a secure encrypted connection but we can't establish a secure encrypted connection without a key there's a way of solving this problem which is asymmetric encryption um.

. . where what you do is generate two keys key a and key b let's just call then and i m drawing a line asymmetric system you have two keys and then basically it's same as before you have got your message says hello you encrypt it with key a to say whatever it was that I decided it was and then back out you get HELLO again and decryption in this case uses key b you can't guess one key from other but they are linked in such a way that anything you encrypt with key a can only be decrypted with key b and anything you encrypt with key b can only be decrypted with key a so there's two and what you do is you generate a pair of these keys, which is called key pair and you just pick one of them and say this is my public key right and your public key is public you publish it everywhere you put it you put it at the end of emails your forum posts you.

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