Signature Verification.srt

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in this video I'm gonna show you how to

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sign and verify messages to assign a

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message you first create a message to

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sign and then you hash the message and

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then you sign the hash of the message to

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sign the hash you're gonna need your

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private key of your eternium account and

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you want to keep your private key or

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secret so you will do the actual signing

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without interacting with us my kata and

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to verify a signature you will first

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recreate the hash from the original

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message and then you would recover the

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signer from the signature and hash and

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finally you would compare the recovered

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signer to the claimed signer let's

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imagine that we're signing a check from

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a bank account so we need three

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informations the person that were

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signing the check to the amount that

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this person can withdraw from our

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account and what they don't know about

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what this check is for and lastly we're

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gonna need a unique identifier called

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announce so that we can invalidate the

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check wants the person withdrawals from

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our account will declare this function

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as public peer and it's gonna be tender

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32 bytes hash of the inputs and inside

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our get message hash function will

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return the catch at 256 Bob our inputs

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so this function here covers steps one

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and two of signing now when we sign the

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message - it's actually not the message

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hash itself that is being signed what's

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actually being signed is this you prefix

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the message that you're going to sign

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with ether um sign message followed by

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the length of the message and then

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finally the message itself you take all

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of this and hash it using catch up to 56

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and that is the message that is actually

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being signed so when we sign the message

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hash the actual message that will be

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signed is the prefix

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eternium sign message since the message

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has is 32 bytes here the length will be

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32 and then

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followed by the message ash and then you

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take the cat chapter 56 of the whole

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thing so you'll write a function to

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recreate that the input of this function

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will be the message hash from the

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function of up and since it's going to

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create another hash it's going to return

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by it's 32 inside the function it's

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gonna return the catch up to 56 of the

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prefix eternam sighing message followed

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by the length of the message which will

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be 32 and the actual message which is

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going to be the message hash that is

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passed from the input so that completes

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the code for the function that recreates

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the hash that is actually being signed

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now to actually sign the message I'll

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show you later

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after we deploy this contract moving on

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I'm gonna show you how to verify a

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signature given a message hash first I'm

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gonna write the function that paints the

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big picture and then we'll work our way

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down into the details to verify a

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signature we need several inputs and the

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address that claims to be the signer of

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the message the parameters that was used

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to create the message hash and the

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signature itself and the function will

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return true if the signer is indeed the

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owner of the signature you'll first

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recreate the message hash from the

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inputs and then compute the hash that

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was actually signed the function recover

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signer which we all write out later will

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take in the hash that was signed and the

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signature and it will be turned the

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address of the signer

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so we are compared with the signer that

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we think should have signed the message

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and then we turn the comparison the

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function recover Steiner is gonna fake

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each side message hash and the signature

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as input and it's gonna output an

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address based on the message hash and

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the signature to recover the signer we

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first need to split the signature into

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three parameters or s and B now you

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don't need to understand what these

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parameters

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all you need to understand here is that

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these parameters I needed to call the

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built-in function EC recover to call the

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function GC recover you first need a

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pass in the hash that was signed and

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then the parameter is B R in s that we

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split from the signature and this

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function will try to compute the address

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that signed the hash and return a 0

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address if the signature is invalid so

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that completes the call to recover

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designer let's now work on the function

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split signature which we have not

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defined yet the function split signature

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is going to take the signature as input

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and it's going to return the T

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parameters are as can be needed to call

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EC recover will first require that the

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length of the signature is equal to 65

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since 32 first output 32 for the second

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output and 1 for the last output is

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equal to 65 now to actually split the

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bytes the only way to do it at this

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moment is to use assembly and don't

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worry if you don't understand this part

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since you won't be using assembly much

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now to get the first output from the

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signature this is how you do it at X Y

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will return X plus y so what does it

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mean to that 32 to the signature well

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you're not going to be adding 32 to the

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actual signature signature is a dynamic

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data type so what's being stored inside

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the variable C is a pointer to where the

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actual signature is stored in other

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words the starting position of the

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signature being stored in memory and

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when you add 32 you move the starting

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position by 32 effectively skipping the

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first 32 bytes

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why do you skip the first 32 bytes and

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this is because dynamic array store the

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length of the array in the first 32

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bytes so the signature itself starts

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after skipping the first 32 bytes

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when you call the function endo with an

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input of P it builds the next 32 bytes

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starting at the memory address P so

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putting it all together

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this line of code will skip the first 32

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bytes and then load the next 32 bytes

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and then assign it to the variable R and

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to get the value for the s we will do

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something similar to what we did to get

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the R value here we need to skip 64

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bytes since the first 32 bytes stores

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the length of the signature and the next

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study 2 bytes stores the value for R so

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the value for s starts after skipping

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the first 64 bytes and to get the value

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B we need to skip the first 96 bytes and

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then get the first bite after so this is

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how you get our SMB value from the

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signature and that covers the code for

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how to verify a signature let's now

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compile and deploy the contract and I'll

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show you example of how to sign and

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verify a message and for this example

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you're going to need meta mask which is

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a eternium wallet that you can install

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inside your browser so here I deployed

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the contract I will create the message

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hash to sign and then later verify the

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signature and let's now go through these

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steps first we'll create the message

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hash to sign so you'll call the get

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message hash function and for the inputs

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I'll pass in the second account for the

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number I'll pass in 1 2 3 4 the message

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all passing in coffee and donuts and

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then for the nouns or pass in one and

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then call the function and that's the

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hash that we're going to be signing and

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to sign the hash we need to allow remix

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to connect to metal mask and you can do

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that by hitting f12 and opening the

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JavaScript console and then typing the

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30 end of enable this will open up the

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metal mask so go ahead and type in your

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password to unlock it and then hit

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connect so now you'll have access to

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meta mask inside the browser console and

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we can sign the hash inside the browser

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now first of all assign the hash to a

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variable named hash and to sign the hash

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you'll need to call what the personal

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dot sign pass in the hash followed by

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the account that we're going to use to

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sign - in this case we will use the

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default account that is available to

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meta mask and you can access that by

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calling web 3.8 the default hotel and

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then finally we will console the result

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so you'll see a pop-up like this and hit

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sign and that is our signature and the

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signature will be different for

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different accounts now to verify the

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signature we're gonna need the address

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of the signer and we can get it by

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calling web 3 the the default account

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again so that was the address that was

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used to sign the message and to verify

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the signature I'm gonna paste the signer

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in here and then passing the parameters

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that was used to sign it and then the

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signature itself

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then head verified and you can see that

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the function verified

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returned to now if you change the

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parameter even by a little for example

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instead of sending one two three will

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add extra zero and then hit verify and

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you can see that the function returns

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false likewise if we keep the message

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the same but changed the signature so

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for example changing the signature by

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one number and then hit verify it would

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again return false so in summary if you

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changed the original message or the

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signature even by a little bit then the

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signature is no longer valid that's all

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I got to say about signature

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verification thanks for watching and

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have a nice day