Why can't we expire two year old transactions?

I understand any output in blockchain can be left un-redeemed forever. But If we expire all transactions that are more than two years old (specifically after some number of blocks), there are some benefits

Recycle lost/destroyed bitcoins by giving them back to miners
Miners will continue to get more transactions and thus more fees
Number of unspend transactions may become less speeding up verification
Size of blockchain to keep downloaded can be minimized
Broken cryptographic hahes/signatures can be made obsolute
New features can be easily introduced

Are there any alt coins that consider these? What are the advantages of keeping all transactions valid since genesis block?

balki

Posted 2015-03-06T15:23:25.387

Reputation: 265

Also see: Why doesn't Bitcoin return lost coins back into the block reward?

– Murch – 2015-03-07T09:16:07.043

Answers

One big reason against this sort of thing is that it creates a huge incentive for bad people to try to prevent you to do transactions (censor you).

If miners get the "lost" bitcoins they might simply refuse transactions with coins that are almost expired hoping to get a bigger reward later.
If miners don't get them but they are truly lost forever, then there is an incentive for big stakeholders to try to do the same thing: less coins means the value of everyone else's coins goes up.
It gives governments and other mafia huge power: don't want to comply? We'll just lock you up for 2 years and make your coins expire. Or more "humanely" use other censorship tricks (hacking/digital censoring) to prevent you from spending your coins before they expire.

Another reason is that people will en mass set up automated scripts to move all their coins regularly, which is a big security (and privacy) risk. It's much easier to keep private keys safe (on a piece of paper in a vault) than it is to keep an internet connected system powered up and safe 24/7.

Jannes

Posted 2015-03-06T15:23:25.387

Reputation: 5 823

When I mean give it back to miners. not directly. But extend the block reward. For e.g. current block reward is 25 BTC. Make it 25.1 BTC and next one as 12.505 BTC the number calculated according to number of bitcoins that are lost forever. I doubt it will be beneficial for miner to censor one person and loose the transaction fee as other miners will be willing to include the transaction – balki – 2015-03-09T16:57:17.923

Two years might be very small. May be it can be 5 years. Agree people will need automated scripts or have to just remember to reuse bitcoins atleast once in 5 years. – balki – 2015-03-09T16:59:16.267

I know you didn't mean directly but it's still a dangerous incentive. And there are other people that would love to be able to make someone else's bitcoins worthless even if they can't take them. 2 years or 5 years is only a parameter, i was discussing the concept. Either way it's a big unnecessary security and privacy reduction. – Jannes – 2015-03-09T23:43:21.760

Recycle lost/destroyed bitcoins by giving them back to miners

Miners will continue to get more transactions and thus more fees

If you already own bitcoin, this isn't a good thing as it increases the supply. That said, there is an altcoin called Freicoin that uses demurrage rather than an abrupt 2-year expiration that's got some of the property you're looking for.

Number of unspend transactions may become less speeding up verification

Size of blockchain to keep downloaded can be minimized

This may or may not be true as expiring coins gives people incentive to transfer them in order to not lose them. I would guess that given this fairly major economic incentive, the number of unspent transactions wouldn't actually go down that much.

Broken cryptographic hahes/signatures can be made obsolute

New features can be easily introduced

You seem to be of the opinion that what's keeping bitcoin from innovating is that there's all these obsolete transactions. That's simply not true. What's keeping bitcoin from innovating is the possibility of a hard fork. New features like lifting the 1MB block limit has nothing to do with old transactions being in the blockchain. It has everything to do with miners achieving concensus.

Jimmy Song

Posted 2015-03-06T15:23:25.387

Reputation: 7 067

Why would this increase the supply? Maximum limit of bit coins that can exist will still be the same. – balki – 2015-03-06T19:48:46.953

1Your question said you'd take coins that haven't been used in 2 years and give them to miners. These formerly dead/unused coins are now alive/being used again. So yes, that would increase the amount of bitcoins on offer, or the supply. – Jimmy Song – 2015-03-06T19:50:29.577

@balki Think of it like this - if Bitcoin changed so that there were an extra million Bitcoins that nobody could spend, would that change the amount of available Bitcoins significantly? – Nick ODell – 2015-03-06T21:28:20.260