This content explains how public-private key cryptography and digital signatures are used to secure transactions on a blockchain, ensuring that only the owner of the private key can authorize the movement of funds.
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Welcome back
Last time we looked at a blockchain, and how it works
particularly in the financial context
and we have these transactions that we were creating that move money from one person to another but there's a
big problem with this and that is what's to stop somebody from just adding a
Transaction that spends all of someone else's money to them
There's seems to be no
Protection here for that so what we're gonna do is we're gonna look at ways to add
transactions to a blockchain
that
Keep it. So not just anyone can create these transactions in order to do this. We need to look at
another cryptographic primitive and that is
Public private key pairs, and then we'll use that for signatures, so let's take a look at that right now
so here's a
public/private key pair
The private key here
It's just you know
It's a really really long number and any number is a private key right you could
Make this be one one is a private key
It's not a very good one lots of people have thought of the number one before
but you could you know you could pick some really really long number and it's sort of a random number and
We can use that as a private key. Now. You can see every time. I'm adding digits here its
Recomputing the public key that relates to that private key, and that's why the stuff down
Here starts changing so as the name would imply this
Private key is to be kept private only you have this private key, and you never tell it to anyone else
Okay, and just as the name implies
Public key the public key here, you just tell everyone this is something that you want everybody to know
This is something that there's no harm in letting everyone know
There is not a way to derive from this public key
What the private key is okay? So it's just a kind of a public
Version of this private key that does not reveal what the private key is
Okay, so I'm gonna hit my little ray number generator, so I get a nice really long private key
And it's not something you know where the numbers are close together where I typed
On the keyboard it's something a little bit more random looking and then the system has derived a public key
From this so this is going to be my private key, and I'm gonna use this to do signatures
So let's do that right now, so
Here's a message signature
So here is a message. I'm gonna type. You know hello on ders. That's me
and I have
My private key here six five six whatever it is is my private key only
I have that and here's the message that only I have
And I can hit sign and come up with a message signature here now this message
Signature I can pass to someone else
I'm gonna hit my little verify button I'm gonna pass this message signature to someone else to anybody else okay
now of course they don't have my private key because I keep that private nobody else can see that but I
Publicize my public key, and they know that everybody knows that this is my public key
So given this message and everybody knows my public key and given the signature that I just made
You should be able to verify this now of course if I hit verify sure enough the screen goes green
This is a valid message
And I have verified that whoever signed this message and came up with a signal
Signature had access to the private key behind this public key
Okay, and if I have kept that private key secret that must be me okay
So that's a message signature
And how you can sign something and verify it now instead of just using this freeform text box
Let's put some structure around this. I'm gonna make a transaction here instead all right. This is similar to what we saw before
the message I'm gonna say is I'm gonna send twenty dollars from this happens to be my
public key to
Somebody else's public key whoever whoever this is that I'm sending money to okay? I
Of course because I'm Simon smashes. I have my private key
I never tell anybody the private key, but I can use this private key to sign this message
Which consists of these three things up here?
And if I hit sign I get a message signature great all right now
I send this whole thing out I send out my message and my signature to to
somebody else and
They know that I'm trying to send twenty dollars from my public key to this somebody else's public key
You'll notice this little blue box around the from public key that
suggests that
You can check this signature against this public key to see whether or not the private key behind, this public key
actually sign this message
so let's hit verify sure enough it verifies so I know that the person in possession with the
Private key behind this public key must only beyonders is sending twenty dollars
To some other public key, okay, now, let's use this in the blockchain
Let's go back to the blockchain case where we were that we're looking at before now
You'll notice a couple of different things here all right first of all
There's not names here anymore right there
There are just public keys in the from and the two and you'll also notice that that I added a signature
section here, okay, so
In this case this is this
Public key sending two dollars to this public key
And here's a message signature that says that it is well
What happens if I change this to you know 25 dollars okay?
Of course it broke the block
But it also broke the signature the signature is not verified and that's why the signature is turning red so Wow?
we couldn't press the little mine button you know a miner could take this altered block and
Remind this block
They're gonna end up with something where the block is signed
which should eventually happen and
there we go and
the signature though is still
invalid because the the minor has no they don't have my
Private key, they only have my public key, so they can't come up with the right signature. Okay, so that's the way
We can make sure that
The message this transaction here was posed
By the person that had the money and only that person not just anyone else on on the Internet
So that's how public private key pair
Message signing is used to protect
transactions and make sure that they are
from the people that they
proposed that they're from
Now if you think about it it actually works really well because in order to create a new
address a new public key the only thing you have to do is go back and
Come up with a new private key a new random number. You didn't have to go to a centralized Authority to
Public/private key pair, you know you just make up a private key
And you use it you derive the public key from it and you put that out there
And and that's how people can pay you so
That's a blockchain and again. It's it's a financial context all successful production block chains that are
Distributed use a token of some sort so they did they're all they'll have a financial
context so that's
You know how a blockchain?
Will will work I mean I have glossed over a couple of
the details
But for the most part the the overall idea here that you're looking at is is very similar to the way that
Bitcoin works
And many other cryptocurrencies as well, so that's a blockchain. I hope it's helpful to you
please leave me some notes down below and let me know what you think of this and
I hope to see you next time in the next video
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