Quote from: johoe on May 18, 2015, 07:07:59 PM

Quote from: sangaman on May 16, 2015, 05:41:13 PM

Quote from: johoe on May 13, 2015, 01:25:48 PM

similar to malleability it changes the transaction ids of your valid inputs
it may invalidate your partially signed transaction that you got from a third party on a common multisig address
it may make your bitcoins lower priority, since their age is diminished

How could the first point be practically exploited? I'm failing to see any issue with inputs being changed. If you're expecting a payment with a certain txid, that txid will still exist even if it is later combined with other utxos right?

You buy a coffee at your local shop. At the same time you send the transaction, someone combines your UTXOs with another transaction that uses the same input. Only one of the two transactions wins. So you may accidentally double spend on your coffee and do not even know it.

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I'm not familiar enough with how the second point might pose an issue, do you have an example?

GreenAddress would be an example. They have a service to keep your bitcoins safe against someone stealing your private key by requiring that every transaction is additionally signed by them after you logged in with a second factor. They also give you a pre-signed time-locked transaction that gives you access to your bitcoins after a few weeks/months without their intervention in case they go out of service or lose their private keys. However if someone in the mean time compresses your UTXOs this transaction would no longer be valid.

Other scenarios are payment channels where you also get a time-locked transaction to reclaim your coins if the server crashes.

Thanks that makes a lot of sense now. Not sure how to work around those problems. I do hope we come up with some new ways to reduce the UTXO set in the near future, though.