Quote from: njcarlos on February 10, 2014, 03:05:09 PM

I think my point was that there is not real utility in allowing a TX ID to be modified. I'd be open to hearing what I might be overlooking but at face value it seems like a very poor decision in design.

It wasn't really a design decision. Malleability is naturally allowed unless they take steps to prevent it, which they presumably didn't think was necessary at the time. Which it isn't, really. Only one transaction makes it into the block chain, and that has a single, unambiguous hash. Even now, they have taken steps to tighten it up, but it's not a priority for devs compared to more fundamental issues (like scalability).

Quote from: dorobotsdream on February 10, 2014, 03:20:30 PM

I think this txid mutability doesn't cause double-spend by itself. But if the sender (i.e. Mt. Gox) thinks (erroneously) the coins didn't arrive because they didn't see the txid and somebody complained and they did the spend again, then it depends. If the sending address still holds enough coin, or if they use a different address then the sender does a double-spend.

That's not actually a Bitcoin double-spend, though. The second spend involves different bitcoins to the first. There are no double-spent coins in the block chain.

When you think you hold bitcoin in MtGox, you actually hold an IOU for the bitcoin, and you might be able to double-spend that IOU. That's a matter internal to MtGox. It's not a problem for the wider Bitcoin community, any more than MtGox getting hacked would be.