[Cryptography] How does bitcoin find if a transaction output has already been used.

[Mike Lisanke]

On Tue, Jan 22, 2019 at 5:36 PM <jamesd at echeque.com> wrote:

> A new transaction is proposed.  How does the validator figure out if one
> of the transaction outputs has already been used?
>
> The blockchain is heading towards a terabyte.  So you cannot search the
> entire blockchain to make sure a particular transaction output has never
> been used.
>

Finally, my question exactly, the question nobody in cryptocurrency answers
(satisfactorily) ... how does verification work? it's too damn big
To be more complete... who the heck checks if the verifiers checked?
where's the procedure to assure it's correct? who verifies Contracts?
who verifies ACLs for blockchain DBs? I've asked a bunch of 'experts' who
seamed (very -  and I never use this) put off by the question.

If the Only thing you have is a chain of hashed transactions, and proof
Only that one version path a Proof of Work test But Not what verification
that 'node' did... then you don't have enough. Yes, everybody Might be
running the software common to a node And (assume it's open source)
I can check that it would do Something to verify Transaction double-spend
And smart-contract, etc/ But as other responses state, what if the
node that won the block mine Didn't do that work... we Wait until 'someone
else' finds the mistake? then what?

Any way, thanks for asking this question!

-- 
Best regards,  Mike
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