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If RingCT hides amounts (so dust would no longer be a problem, as I've heard) and addresses, is there any downside to using a mixin of 0?

Edit: I am aware that RingCT won't be available for the public network until the next hard fork in January. I was just playing around with Testnet and was wondering for the future.

Quentin Skousen
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Yes.

The intent of mixin is to make it ambiguous which output in a ring is being spent. This still applies, whether the amount of the output is visible or not.

As an example of a case where this matters:

Alice sends Bob output P in a transaction. Alice knows the size of that output (she created it). Alice can then track on the blockchain to see when that output gets included in a transaction. With mixin 0, Alice knows Bob respent that output (though does not know where to). With mixin > 0, Alice does not know, since it could be either Bob, or someone else using that output as a fake out for their own ring signature.

user36303
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  • Thanks, that makes sense. So, if I am not mistaken, the reason RingCT can use dust outputs is because of the fake transactions? – Quentin Skousen Oct 10 '16 at 23:34
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    Because of itself actually. The reason dust outputs are not used in current mixing is because it's hard to find another output of the same amount to mix with. If you hide the amounts with ringCT, you can mix any output with any output, unlike now where we have to mix the same denominations (and that's why all transactions are broken up into these denominations - to ensure theres plenty outputs to mix with). – JollyMort Oct 10 '16 at 23:42
  • @JollyMort, thanks. That answers all my questions :) – Quentin Skousen Oct 10 '16 at 23:47