[Cryptography] In the latest unexpected ransomware twist ...

[Adam P. Goucher]

> > My understanding of Bitcoin is somewhat limited, but I think Bitcoin
> > tokens aren't fungible.
>
> They are sort-of fungible - the bitcoins themselves in the wallets are
> fungible, the wallets aren't. If you mix up bitcoins from several
> sources in a single wallet then you can't tell which bitcoins are which
> - in fact bitcoins do not have an individual identity as such, just
> "there are 4.2 bitcoins in wallet x".

That's almost but not quite true; specifically, it becomes true if you
replace every instance of 'wallet' with 'UTXO' (unspent transaction output).
See here for a helpful analogy:

https://en.bitcoin.it/wiki/Coin_analogy

But yes, one of the important details is that if you have a transaction
with three different inputs of 5 BTC and three different outputs of 5 BTC,
there is no way to define which input 'corresponds' to which output; doing
so is meaningless. This is the basis of the CoinJoin privacy-enhancing
technique, which in practice is implemented trustlessly using secure
multi-party computation.


Best wishes,


Adam P. Goucher