I just realize that if we have OP_CAT, OP_CHECKPRIVATEKEYVERIFY (aka OP_CHECKPRIVPUBPAIR) is not needed (and is probably better for privacy)

Bob has the prikey-x for pubkey-x. Alice and Bob will agree to a random secret nonce, k. They calculate r, in the same way as signing a transaction.

The script is:

SIZE <r-length + 1> ADD <0x30> SWAP CAT <0x02|r-length|r> CAT SWAP CAT <pubkey-x> CECHKSIGVERIFY <Bob pubkey hash> CHECKSIG

To redeem, Bob has to provide:

With k, s and sighash, Alice (and only Alice) can recover the prikey-x with the well-known k-reuse exploit

( https://en.wikipedia.org/wiki/Elliptic_Curve_Digital_Signature_Algorithm )

The script will be much cleaner if we remove the DER encoding in the next generation of CHECKSIG

The benefit is prikey-x remains a secret among Alice and Bob. If they don’t mind exposing the prikey-x, they could use r = x coordinate of pubkey-x, which means k = prikey-x (https://bitcointalk.org/index.php?topic=291092.0) This would reduce the witness size a little bit as a DUP may be used

From: bitcoin-dev-bounces@lists.linuxfoundation.org [mailto:bitcoin-dev-bounces@lists.linuxfoundation.org] On Behalf Of Tier Nolan via bitcoin-dev
Sent: Monday, 29 February, 2016 19:53
Cc: Bitcoin Dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] BIP CPRKV: Check private key verify

On Mon, Feb 29, 2016 at 10:58 AM, Mats Jerratsch <matsjj@gmail.com> wrote:

This is actually very useful for LN too, see relevant discussion here

http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-November/011827.html

Is there much demand for trying to code up a patch to the reference client? I did a basic one, but it would need tests etc. added.

I think that segregated witness is going to be using up any potential soft-fork slot for the time being anyway.