Cancelling transactions I know it is currently not possible to cancel transactions in the Bitcoin. However, say someone wants to implement such mechanism in an altcoin. Is it theoretically possible without damaging the open nature of cryptocurrencies ? Do we have the required primitives for it ?
One of the fundamental principals of cryptocurrency is to be able to operate without the need of a trusted central entity.
I can't see how a transaction can be cancelled or reversed without involving a trusted central entity to arbitrate the transaction.

However we also have cryptocurrency like Ripple. In my opinion Ripple is not a cryptocurrency because of the centralized nature, but well what do I know.

Maybe in the future some genius will discover a method to implement this without a central entity :)