exporting partial wallets (non-network transfers)

[chaord]

I'm not sure if this has already been discussed, but I was thinking that the following would be a really nice feature.  Let's say that I have a mybitcoin account, or a standard client running on my computer.  The account wallet.dat has 1000 BTC in it.  I would really like the following:

• be able to export a portion (say 400) BTC to a new (partial?) wallet.dat file

This would allow for a number of things:

• Allows users of service sites (mt gox, mybitcoin, etc) to back up portions of their account.
• Allows p2p transfers (among trusted individuals) without immediately needing to pass through the network.

Since with bitcoin, every user pretty much runs their own bank, this is the equivalent of being able to write a cheque to yourself (eg to another account that you have at a different branch), and never cashing the cheque (submitting it to the blockchain) unless the original node experiences problems. 

I hope what I'm saying here makes sense?  Having a legitimate transaction on "standby" (not in any block chain yet) for the majority of your "balance" at a site like mtgox/mybitcoin would be really neat.

So essentially what I'm proposing syntactically would be something like "bitcoind generatewallet 50" would generate a valid wallet.dat file with a balance of 50 BTC (if it is ever "merged" into the block chain).  Is this possible?

[caveden]

From what I understand, it would hardly be possible, at least with the flexibility you ask.

The only way to transfer bitcoins without using the block chain is by moving around the private keys that own the coins. But then, I don't see how you could specify arbitrary amounts like that. You would only be able to create a new wallet with 50 BTC if you could separate a group of your private keys that together sum 50 BTC...

To transfer coins from one key to another, you need the block chain...

Sites like mtgox or mybitcoin could do that internally, when transferring from/to users of their system... they don't need to really do the transfer, just update the balance of each user. Of course that in such case, if I a user checks the balance of his addresses with the block explorer, he might see a different value than the actual one.

I don't see why would such a feature be useful by the way, specially now that transfers are mostly free. Or you're already predicting the scenario where transactions are almost never free?

[theymos]

You could create a new address, send some coins to that address, and then export just that address.

Since you trust yourself not to double-spend, you can create a transaction and save it to disk without broadcasting it. However, spending any of the specific coins used by this transaction would make the transaction invalid, so you'd be essentially freezing those funds, which is unlike your example of writing a check.

[ribuck]

I don't see why would such a feature be useful

You could give a gift from one phone to another, using nothing more than Bluetooth file transfer.

[chaord]

I don't see why would such a feature be useful

You could give a gift from one phone to another, using nothing more than Bluetooth file transfer.

Yeah, I thought this feature would be useful for a number of reasons.  One would be in the case where network transactions aren't free, like you mentioned.  The other would be for backing up funds (but not freezing them).  And lastly (and possible most important) would be a way to migrate coins out of nodes that have been compromised.  If such a feature was standard, then everyone could still access their bitcoins even if a site like mtgox went down.