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Date: Tue, 24 Nov 2015 13:01:26 -0800
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From: Chris Priest <cp368202@ohiou.edu>
To: Gavin Andresen <gavinandresen@gmail.com>
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Cc: Bitcoin Dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] OP_CHECKWILDCARDSIGVERIFY or "Wildcard Inputs" or
 "Coalescing Transactions"
Precedence: list

A coalescing transaction in my scheme is the same size as a normal
transaction. You only include one UTXO, the rest are implied based on
the presence of the OP_CHECKWILDCARDSIGVERIFY opcode.

The code that determines if a UTXO is spent or not will need to be
modified to include a check to see if any matching coalescing
transactions exist in any later block. Maybe there should be a
"coalescing pool" containing all coalescing transactions that make
such a check faster.

The part I'm not too sure about is the "wildcard signature". I'm not
too versed in cryptography to know how exactly to pull this off, but I
think it should be simple.
You'd just have to some way inject a flag into the signing process
that can be verified later.

I originally wanted the "wildcardness" of the transaction expressed by
the transaction version number.
Basically any input that exists within a "version 2 transaction" is
viewed as a wildcard input. Then I realized whats to stop someone from
modifying the transaction from version 1 to version 2 and stealing
someones funds. The "wildcardness" must be expressed in the signature
so you know that the private key holder intended all inputs to be
included. Hence the need for a new opcode.

btw, this scheme is definitely in the 10x or higher gain. You could
potentially spend an unlimited number of UTXOs this way.

On 11/24/15, Gavin Andresen <gavinandresen@gmail.com> wrote:
> On Tue, Nov 24, 2015 at 12:34 PM, Chris Priest via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> The technical reason for this is that you have to explicitly list each
>> UTXO individually when making bitcoin transactions. There is no way to
>> say "all the utxos". This post describes a way to achieve this. I'm
>> not yet a bitcoin master, so there are parts of this proposal that I
>> have not yet figured out entirely, but I'm sure other people who know
>> more could help out.
>>
>
> So every input has:
>  32-byte hash (transaction being spent)
>  4-byte output (output being spent)
>  4-byte sequence number
> ... plus the scriptSig. Which is as small as about 73 bytes if you're
> spending a raw OP_CHECKSIG (which you can't do as a bitcoin address, but
> could via the BIP70 payment protocol), and which is at least two serialized
> bytes.
>
> Best case for any scheme to coalesce scriptSigs would to somehow make
> all-but-the-first scriptSig zero-length, so the inputs would be 42 bytes
> instead of 40+73 bytes -- the coalesce transaction would be about one-third
> the size, so instead of paying (say) $1 in transaction fees you'd pay 37
> cents.
>
> That's in the gray are of the "worth doing" threshold-- if it was a 10x
> improvement (pay 10 cents instead of $1) it'd be in my personal "definitely
> worth the trouble of doing" category.
>
> RE: the scheme:  an OP_RINGSIGVERIFY is probably the right way to do this:
>   https://en.wikipedia.org/wiki/Ring_signature
>
> The funding transactions would be:  <public key> OP_RINGSIGVERIFY
> ... which might could be redeemed with <ring signature> for one input and
> then... uhh... maybe just <index_to_input_with_signature> for the other
> inputs that are part of the same ring signature group (OP_0 if the first
> input has the signature that is good for all the other public keys, which
> would be the common case).
>
> --
> --
> Gavin Andresen
>