If a system were ever implemented that allowed exchanges/businesses to refuse to accept coins because at some point in the past they were tainted I would never again accept btc for payment. I would sell all that I have that are not tainted and never use Bitcoin again. I seriously doubt I'm alone in that regard. Could you imagine the Dollar surviving if you could have your money confiscated because some kid bought a bag of pot five years ago with the money in your pocket!
how about this one?
i intentionally send 1 Satoshi's worth of tainted coin to EVERY single address in existence on the blockchain.
Most people don't understand 'taint'. Or 'tarnish' which was probably invented to help understand, but made things worse. It is perfectly possible to compute the percentage of a transaction which had a 'bad' history. So, your idea to ruin everything would only have the effect of ruining everything by a tiny fraction. And you your $1 might be worth $.95 due to the 5 year old pot purchase. Better than $0 though, right? And we solve a lot of crime after all.
And also it would be difficult to make an algorithm which was able to characterize 'bad'. For this we need a trusted body consisting at least partially of humans (until great strides are made in AI) to decide how bad is bad. These people will need compensation as will the infrastructure they operate to perform the calculations and communicate with the rest of the network.
Don't feel bad. Someone in the audience insisted on making an ass out of himself by not getting it (twice!) in the QA session.
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Speaking of the Q&A, another interesting thing about that session was the suggestion about sending private keys around. When the round-table figured out what he was saying, they pretty much all turned a few shades more pale.
This tells me that the analysis which is being performed is probably not yet sophisticated enough to overcome that extra degree of freedom.
It is said to be a 'bad idea'. It actually works fine. I've done it. It seems like a bad idea for the recipient unless he knows what he's doing. I don't see any real dis-advantages for the person giving up the funds. I mean, they are assumed to be a total loss by the nature of the transaction.