Name: End to End Encryption (E2EE) - Computerphile
Uploaded: 2020-04-13T16:55:32.000Z
Duration: 8 min 12 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

Past simple

So, recently the Home Secretary of the United Kingdom Government was on

breakfast news or something along those lines

Present continuous

and was talking about how criminals are using

end-to-end encryption to essentially evade detection

Um, now, in some sense that's very much true: it is unacceptable.

Um, but... What they're suggesting is that we

or we find a way of only allowing certain parties, like trusted government parties,

So, before we declare that as insane, let's look at what that means

Let's imagine that I am using Whatsapp or

Facebook Messenger or some other end-to-end encrypted messenger with you, right?

So, you have a phone here, right? It could be a phone, it could be a computer

it's not really important - some device, right, with a screen

- this is why I'm not employed to design these things.

This is you, but I'm gonna call you Alice this time because we always do that

(Mike) >> It does. So we've got Alice and Bob here having a communication between two phones

There's gonna be some communication mechanism between these two devices, right?

It could be SMS or, you know, GSM phone signal

or it could be something like Wi-Fi over the internet.

In all of these cases there's usually gonna be an intermediary server

This phones aren't capable of connecting to each other on their own

apart for things like NFC, where you come really close.

So, there's going to be some server in here, which I'm just gonna label "S"

which in the case of Whatsapp will be a Whatsapp server

and obviously it's gotta be a server for whatever product you're using

Now, anytime that Bob sends Alice a message it's going to go via the server, by definition

because that's the thing that relays the message to Alice.

It knows how to communicate with Alice, you know, it knows what her phone number is,

it has a list of you contacts and things, you know, this is how it works.

This could be a phone provider, and there's gonna be, you know, phone antennas and things in this mix, but

So, this message here is gonna come in this way from Bob and it's gonna go to Alice like this.

The issue is if we want to encrypt this channel, right, we want certain people not to be able to read it,

if I'm sitting on a router somewhere on the internet, here

we do want me to go "Oh, that's a nice message with your credit card details" and I'll have that, right?

So that's what we're trying to avoid here

(Brady) >> Because that's how email works, right? (Mike) >> Yeah (Brady) >> You could sit there and-

(Mike) >> Absolutely. And people do. Encryption of channels is nothing new, right?

We've seen it for a long long time, right? These techniques, things like public key cryptography

and some of these cyphers has been around for many years.

So how do we do this? Well, there's really two options:

the first is that Alice could negotiate some shared secret key with the server.

So that key there could be used by Alice to talk to the server

and she could send a message encrypted by KAS to the server and say

"Please, can you forward this message to Bob?"

Bob would have another key with the server, KBS,

Obviously here Alice does not know what KBS is, and Bob doesn't know what KAS is.

The server decrypts a message using KAS, that it knows

and it re-encrpyts it with KBS and forwards it to Bob.

because obviously it's been decrypted halfway through.

In some sense that's a good thing, right?

If I'm a terrorist, or a criminal, and I send a message,

this server could perform some kind of rudimentary checks to make sure I wasn't doing anything untoward.

But for obvious reasons a lot of people don't like this idea.

What end-to-end encryption does is replace these two keys

The idea being that this server's quite happy to relay the packages back and forth

but it doesn't have any idea of what's in them.

And this works you very well for this server as well, because when someone says "Can you give us this data?"

they can reasonably say "No. Not because we don't want to, but because we actually can't."

The process we use for this is something called a "key exchange".

The obvious problem here is that at some point Alice and the server have got to share a key,

without an encrypted channel. When she first ever connects

And so, how do we get the key? There's a sort of chicken and egg problem.

The solution was proposed by Diffie and Hellman

which is the Diffie-Hellman key exchange, right?

We're not gonna go into the details of mathematics of Diffie-Hellman in this video

but I'll simply say that Alice and Bob both have public and private components of this key.

They share the public ones, and then they use the private ones in secret

to create a shared key that no one else can know.

That's essentially how it works. So it is a way

of even via the server, producing a shared key "KAB" that no one else knows.

So now they have this shared communication channel.

So when you first connect, you will send some identifiers to the server,

you will establish a public and private key pair, and then from then on

anytime you want to connect to anyone new you'll generate one of these keys.

It's called "ephemeral" which means that basically generate one almost every message,

if not every message for some of these apps.

The important thing is that the server, although they relay these messages,

is not involved in this key exchange process and can't

inject itself in the middle, which means that it doesn't know what KAB is

and it can't decrypt the message physically.

When the Minister or someone in the media says

"What we really want to do is allow some kind of entry for government into the system"

you can quite reasonably say "That is impossible."

because you'd have to inject something in the middle of this key exchange which would completely undo it.

So let's think about the different ways we could do it and discuss whether they're practical.

Okay, so the first one is we could go back to this system here.

So, we could have Alice talking to the server in a secure way, using a key exchange.

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End to End Encryption (E2EE) - Computerphile

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