First, sorry for my English, I’m not a native English speaker, and I’m not very familiar with cryptography.

The idea of Delayed and Cancellable Transaction is add a special kind of address into Bitcoin protocol. And there are two private key associated with this kind of address, one is the usual private key to sign a transaction, another is a cancellation private key to cancel a transaction. Every transaction from this kind of address is marked as “delayed” while broadcast to the network, and does not considered as a spend, after it reach a time period specified by this address, for example 24 hours or one month, it becomes a normal transaction and processed by miners. During this delayed period, the transaction can be cancelled use the cancellation private key.

Delayed and Cancellable Transaction can guarantee that if you have the cancellation private key, no one could steal bitcoin from you. The only thing you need to do is watch on your address, cancel any transaction not made by you before delayed period end. This will greatly improve the security of Bitcoin. If you lost the cancellation private key, you could send it’s bitcoin to another wallet.

The only problem with Delayed and Cancellable Transaction is that if someone stole your usual private key and cancellation private key, and if he like, you can not spend any bitcoin in it. But the good thing is he can not spend it as well. If you are lucky, he give up or dead before you, you could get your bitcoin back. In the future maybe we can improve Delayed and Cancellable Transaction further, if you can prove the address is belongs to you and you provide a verification key to miners, and most miners willing to accept this verification key, your transaction would not be canceled. Hope this is feasible in cryptography.

Another usage of Delayed and Cancellable Transaction is to share private key with someone you are not completely trust. For example, your non-tech wife can not securely store the private key, but you have to share it, just in case you are dead in a car accident. You want to left a bitcoin bequest to your son, but you do not trust any third party even your son, you do not want him spend it before you die.

For online wallet like Coinbase, it’s very easy to implement it. Just create a saving account, delay withdraw from it, notify customer, customer could cancel the transaction before a specified time period. Then, even your account get hacked, you will not lose any bitcoin in your saving account.

I am sure Coinbase could implement this. But is this feasible for Bitcoin protocol?