Sweepprivkey proposal

sweepprivkey is a PROPOSED Bitcoin RPC command that, given a Bitcoin private key, allows sweeping of some or all of the funds to another address.

This function is useful for merchants who wish to accept typed or scanned Bitcoin private keys as payments, without imposing any specific methodology on how the payments must be processed. sweepprivkey has no direct effect on anything in the local wallet or transaction history -- the function simply generates a transaction in real-time to sweep the funds to another address, and broadcasts it. That destination address may or may not be in the local wallet.

The local wallet is only affected if the destination address is in the wallet, and only in an indirect sense: the wallet reacts to the sudden presence of a new transaction referencing it, just the same as if that transaction had been received from a peer.

This allows merchants to offer a "deposit by private key" method with trivial effort, specifically without having to create a separate deposit implementation for accepting the keys. The merchant can simply expose a way for users to submit private keys, which can then be passed to sweepprivkey with minimal validation along with the same deposit address that the merchant would normally have given to their customer in anticipation of payment.

Once the customer has entered a private key and allowed a sweep to take place, the merchant from its perspective will see what looks like a deposit with zero confirmations, and can treat it exactly the same as a normal deposit from outside (including requiring the customer to wait for confirmations as usual, where applicable).

Usage

The syntax of sweepprivkey is as follows:

sweepprivkey <privkey> <minconfirms> [<destinationtype> <destination>] [<fee>] [<amount>] [<changeaddress>]

• privkey is required. It can be in any private key format normally accepted for wallet imports with the importprivkey function.
• minconfirms determines how many confirmations the private key's prior transactions must have before they are considered eligible for being swept. Transactions having less than the minimum confirmations will be ignored. If this is 0, then sweepprivkey will also attempt to sweep unconfirmed transactions from the in-memory pool.
• destinationtype is either the word "address" or "account", and indicates what destination is. "address" means sweep funds to another Bitcoin address, and "account" will perform a getnewaddress operation to get an address on the local wallet and sweep the funds there. destinationtype and destination are optional, but must both be present together when used. If these are absent, the command is treated as a simple query which returns the total funds available to the private key.
• destination is either a Bitcoin address (if destinationtype is "address"), or is an account name (if destinationtype is "account"). Funds are sent here.
• fee is optional, but must be provided if amount is provided (so that the command knows which parameter is which). This determines how much of a transaction fee to pay. This can be either an amount, or the word "default", or the word "minimum". Default means to pay the transaction fee configured in the bitcoin client, and is the choice assumed if omitted. Minimum means to pay the minimum fee that the client believes will be required to ensure that the transaction is relayed by the network (which is based on the transaction's age, size, and other factors - see GetMinFee). In many cases, the fee may be zero. If you specify an amount but it is below the minimum, or the private key doesn't have a balance large enough to meet the minimum, the command reports failure. Fees are always paid out of the swept balance, not the wallet.
• amount is optional. When not present (or entered as "all"), the command will attempt to sweep all funds available via the private key to the extent those funds have enough confirmations to satisfy minconfirms. If an amount is present, the command will only sweep that amount, reporting a failure if amount (plus fee) is greater than the funds available. If the private key contains funds that satisfy minconfirms as well as funds that do not, only the sufficiently confirmed funds are swept. If a specific amount is requested and a transaction fee must be paid, it will be deducted out of the change rather than the requested transfer amount.
• changeaddress is optional and must only be provided in conjunction with amount and fee. This address specifies a destination for any leftover change that may be the result of sweeping a partial amount. If changeaddress is not provided, change is sent back to the original Bitcoin address corresponding to privkey. Not all partial-balance transactions will generate change, particularly if the private key's balance arrived in separate smaller transactions that can be used individually to satisfy the sweep amount. If there is no change, the change address is ignored.

Proposed flowchart

The sweepprivkey function depends on the ability to quickly find transactions in the block chain belonging to a given address. It relies on an in-memory hashtable that is built the first time it is needed, and is maintained in memory until bitcoind is shut down. As a result, the first time sweepprivkey is used, a delay of a few minutes can be expected. This delay can be avoided by starting bitcoind with the --(whatever) command line switch. (ideally this should be persistable to disk, as this delay only gets longer as the blockchain grows).

To conserve resources, the hash table is simple. It associates the first 32 bits of the Bitcoin address (in hash160 form) with a list of the blocks that reference addresses with that prefix. The hashtable is created upon first use, and updated each time a block is received.

sweepprivkey will also scan the in-memory transaction pool and incorporate unconfirmed transactions when the minconfirms parameter is zero.

When the sweepprivkey command is run, bitcoind will:

1. Ensure the address hash table is being maintained in memory. If not, it will be built before continuing.
2. Scan all of the blocks that might contain unspent transactions belonging to the address. If minconfirms=0, then scan the in-memory transaction pool as well, to create a list of such transactions.
3. Confirm that there is a non-zero balance available, and that (if amount is specified), that the balance is equal to or greater than it.
4. Do getnewaddress if a wallet account was specified.
5. If amount is specified, eliminate all transactions from the list from largest to smallest that will not be needed to satisfy the amount.
6. Construct a new transaction in memory that sends funds from the Bitcoin address represented by privkey to the destination address and change address, if necessary. Sign the transaction with privkey.
7. Treat the signed transaction the same as if it had come from a peer, including relaying it to all connected peers.
8. Return success with the txid and (if applicable) the new address that was gathered via getnewaddress.