To know who did what when, use a log file with auditors. A blockchain does not make things magically immutable - it does so through the use of consensus algorithms among a set of trusted parties (PBFT, private blockchains), or through economic incentives in public (Proof of Work, public network). The latter only works when there is a subsidy to incentivize convergence, something that isn't available for general purpose logs. – Pieter Wuille – 2017-01-20T18:41:29.560

you are perfectly right for a specific context or environment, eg. a organisation, a company, etc. Then of course auditable logs are the best fit for this and a blockchain doesn't make sense at all. I was thinking of a environment of several parties, probably even in competition, which do not have processes or rules to set up a "auditable log" since there is no admin, no processes, no agreed-upon rules. Then a common "infrastructur component" which answers the question permissionless and decentrally is what you need. – Ice09 – 2017-01-21T21:11:32.360

If you want permissionless, you need economic incentives instead of trusted parties. Logging can't provide those, a currency can. – Pieter Wuille – 2017-01-21T21:54:54.913

For sure, that's why logging is completely different use case here. Permissionless must go with economic incentives and since there are no trusted parties, the blockchain with incentives is the only possibility to enable "who did what when" in a public permissionless environment. In a permissioned, (private/consortium) chain with trusted parties you could argue that auditable logging or a central audit database might also fit, but I would challenge this since "trust" is not identical to having an common infrastructure and processes you need to set up a central solution. – Ice09 – 2017-01-23T09:55:47.920

In short: having a "currency" (ie. economic incentives) in a blockchain is a fundamental part of permissionless public chains to work, but that's all it is, a basis for other things to build upon and not the key feature of a blockchain. The key feature is the answer to "who did what when" on protocol level. One "implemetation" of this key feature is currency. Another one is voting, "proofing", auditing, timestamping, "who did what when", permission- and trustless. – Ice09 – 2017-01-23T10:22:56.990

You can't pay a miner with timestamps. – Pieter Wuille – 2017-01-23T15:46:06.217

yes, that's why you pay for timestamping, not with. – Ice09 – 2017-01-23T20:29:52.613

I'm not talking about the users of the system, but the incentives of the system itself. Bitcoin is only secure because the subsidy pays miners more than what a fraudster could reasonably pay them to revert transactions. Paying a miner to censor votes, or include fraudulent proofs is trivial otherwise. – Pieter Wuille – 2017-01-23T20:37:39.403

Timestamping can be done off-chain, with only non-invasive commitments inside an otherwise functioning monetary chain, but won't protect you from having commitments to conflicting data (which is not an issue is all you want to know whether some piece of data existed at a certain point in time). Voting is much harder, as you're essentially letting the highest-paying voters determine what gets included (if miners are behaving rationally). If your chain has no monetary function at all, you can't pay for things, and you can't prevent reorganizations (as those would become cheap). – Pieter Wuille – 2017-01-23T20:56:01.637

I don't object all this, the only thing which I think is important: the blockchain answers "who did what when". It does this by using economic incentives. Timestamping, voting, proofing, etc. is not difficult at all because miners are not interested in the payload, but in the value of the transaction or what they can earn by including it, which is measured in a cryptocurrency. So that's the infrastructure part of the blockchain and it works pretty well. – Ice09 – 2017-01-24T21:08:58.410

Ah, you want all of this in a blockchain that does have a currency associated. Yes, I agree that it can work, but only as long as these non-financial uses don't interfere with the operation of the currency (and not for example what happened to Namecoin: overcrowded by data storage making it useless as a currency, and thus indirectly killing its security). – Pieter Wuille – 2017-01-24T21:41:23.417

All of that works fine for pure timestamping (which only needs a hash). I'm not convinced it can work for auditing (needs linearly growing data storage), voting (those who pay most win? seems a bad property), or in general anything that needs identities (as miners can always choose to censor who they don't like - or be paid to do so). – Pieter Wuille – 2017-01-24T21:44:33.293