What is the current behavior / cost that this proposal is trying to avoid?
Are ancient utxos required to be kept in memory always in a fully
validating node, or can ancient utxos get pushed out of memory like a
normal LRU caching db?

Thanks,
-Danny
On Dec 12, 2015 1:55 PM, "jl2012--- via bitcoin-dev" <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> It is a common practice in commercial banks that a dormant account might
> be confiscated. Confiscating or deleting dormant UTXOs might be too
> controversial, but allowing the UTXOs set growing without any limit might
> not be a sustainable option. People lose their private keys. People do
> stupid things like sending bitcoin to 1BitcoinEater. We shouldn’t be
> obliged to store everything permanently. This is my proposal:
>
> Dormant UTXOs are those UTXOs with 420000 confirmations. In every block X
> after 420000, it will commit to a hash for all UTXOs generated in block
> X-420000. The UTXOs are first serialized into the form:
> txid|index|value|scriptPubKey, then a sorted Merkle hash is calculated.
> After some confirmations, nodes may safely delete the UTXO records of block
> X permanently.
>
> If a user is trying to redeem a dormant UTXO, in addition the signature,
> they have to provide the scriptPubKey, height (X), and UTXO value as part
> of the witness. They also need to provide the Merkle path to the dormant
> UTXO commitment.
>
> To confirm this tx, the miner will calculate a new Merkle hash for the
> block X, with the hash of the spent UTXO replaced by 1, and commit the hash
> to the current block. All full nodes will keep an index of latest dormant
> UTXO commitments so double spending is not possible. (a "meta-UTXO set")
>
> If all dormant UTXOs under a Merkle branch are spent, hash of the branch
> will become 1. If all dormant UTXOs in a block are spent, the record for
> this block could be forgotten. Full nodes do not need to remember which
> particular UTXO is spent or not, since any person trying to redeem a
> dormant UTXO has to provide such information.
>
> It becomes the responsibility of dormant coin holders to scan the
> blockchain for the current status of the UTXO commitment for their coin.
> They may also need to pay extra fee for the increased tx size.
>
> This is a softfork if there is no hash collision but this is a fundamental
> assumption in Bitcoin anyway. The proposal also works without segregated
> witness, just by replacing "witness" with "scriptSig"
>
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