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From: "David A. Harding" <dave@dtrt.org>
To: Jeremy <jlrubin@mit.edu>,
	Bitcoin Protocol Discussion
	<bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] A Replacement for RBF and CPFP: Non-Destructive TXID Dependencies for Fee Sponsoring
Date: Sat, 19 Sep 2020 09:37:16 -0400	[thread overview]
Message-ID: <20200919133716.d5ofags2tprlvpqm@ganymede> (raw)
In-Reply-To: <CAD5xwhi6+Q-UX2xVnD4TE9uEbe-omQ748tpJJpYdrMNnG6D5vA@mail.gmail.com>

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On Fri, Sep 18, 2020 at 05:51:39PM -0700, Jeremy via bitcoin-dev wrote:
> I'd like to share with you a draft proposal for a mechanism to replace
> CPFP and RBF for increasing fees on transactions in the mempool that
> should be more robust against attacks.

Interesting idea!  This is going to take a while to think about, but I
have one immediate question:

> To prevent garbage sponsors, we also require that:
> 
> 1. The Sponsor's feerate must be greater than the Sponsored's ancestor fee rate
> 
> We allow one Sponsor to replace another subject to normal replacement
> policies, they are treated as conflicts.

Is this in the reference implementation?  I don't see it and I'm
confused by this text.  I think it could mean either:

1. Sponsor Tx A can be replaced by Sponsor Tx B if A and B have at least
   one input in common (which is part of the "normal replacement policies")

2. A can be replaced by B even if they don't have any inputs in common
   as long as they do have a Sponsor Vector in common (while otherwise
   using the "normal replacement policies").

In the first case, I think Mallory can prevent Bob from
sponsor-fee-bumping (sponsor-bumping?) his transaction by submitting a
sponsor before he does; since Bob has no control over Mallory's inputs,
he can't replace Mallory's sponsor tx.

In the second case, I think Mallory can use an existing pinning
technique to make it expensive for Bob to fee bump.  The normal
replacement policies require a replacement to pay an absolute higher fee
than the original transaction, so Mallory can create a 100,000 vbyte
transaction with a single-vector sponsor at the end pointing to Bob's
transaction.  This sponsor transaction pays the same feerate as Bob's
transaction---let's say 50 nBTC/vbyte, so 5 mBTC total fee.  In order
for Bob to replace Mallory's sponsor transaction with his own sponsor
transaction, Bob needs to pay the incremental relay feerate (10
nBTC/vbyte) more, so 6 mBTC total ($66 at $11k/BTC).

Thanks,

-Dave

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  parent reply	other threads:[~2020-09-19 13:38 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-19  0:51 [bitcoin-dev] A Replacement for RBF and CPFP: Non-Destructive TXID Dependencies for Fee Sponsoring Jeremy
2020-09-19  1:39 ` Cory Fields
2020-09-19 16:16   ` Jeremy
2020-09-19 13:37 ` David A. Harding [this message]
2020-09-19 15:01   ` nopara73
2020-09-19 16:30   ` Jeremy
2020-09-19 17:24     ` David A. Harding
2020-09-19 18:39 ` Antoine Riard
2020-09-19 19:13   ` Antoine Riard
2020-09-19 19:46     ` Jeremy
2020-09-20 23:10       ` Antoine Riard
2020-09-21 14:52         ` David A. Harding
2020-09-21 16:27           ` Jeremy
2020-09-21 23:40             ` Antoine Riard
2020-09-22 18:05             ` Suhas Daftuar
2020-09-23 22:10               ` Jeremy
2020-09-24  4:22                 ` Dmitry Petukhov
2020-09-22  6:24 ArmchairCryptologist
2020-09-22 13:52 ` Antoine Riard

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