From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org [172.17.192.35]) by mail.linuxfoundation.org (Postfix) with ESMTPS id 963FBB88; Thu, 29 Nov 2018 19:37:57 +0000 (UTC) X-Greylist: from auto-whitelisted by SQLgrey-1.7.6 X-Greylist: from auto-whitelisted by SQLgrey-1.7.6 Received: from mail.bluematt.me (mail.bluematt.me [192.241.179.72]) by smtp1.linuxfoundation.org (Postfix) with ESMTPS id D0AF0840; Thu, 29 Nov 2018 19:37:56 +0000 (UTC) Received: from [10.233.42.100] (gw.vpn.bluematt.me [144.217.106.88]) by mail.bluematt.me (Postfix) with ESMTPSA id 7CDA5120243; Thu, 29 Nov 2018 19:37:55 +0000 (UTC) To: bitcoin-dev@lists.linuxfoundation.org From: Matt Corallo Message-ID: Date: Thu, 29 Nov 2018 19:37:54 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.0 MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on smtp1.linux-foundation.org X-Mailman-Approved-At: Fri, 30 Nov 2018 14:47:14 +0000 Cc: lightning-dev@lists.linuxfoundation.org Subject: [bitcoin-dev] CPFP Carve-Out for Fee-Prediction Issues in Contracting Applications (eg Lightning) X-BeenThere: bitcoin-dev@lists.linuxfoundation.org X-Mailman-Version: 2.1.12 Precedence: list List-Id: Bitcoin Protocol Discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 29 Nov 2018 19:37:57 -0000 (cross-posted to both lists to make lightning-dev folks aware, please take lightning-dev off CC when responding). As I'm sure everyone is aware, Lightning (and other similar systems) work by exchanging pre-signed transactions for future broadcast. Of course in many cases this requires either (a) predicting what the feerate required for timely confirmation will be at some (or, really, any) point in the future, or (b) utilizing CPFP and dependent transaction relay to allow parties to broadcast low-feerate transactions with children created at broadcast-time to increase the effective feerate. Ideally transactions could be constructed to allow for after-the-fact addition of inputs to increase fee without CPFP but it is not always possible to do so. Option (a) is rather obviously intractible, and implementation complexity has led to channel failures in lightning in practice (as both sides must agree on a reasonable-in-the-future feerate). Option (b) is a much more natural choice (assuming some form of as-yet-unimplemented package relay on the P2P network) but is made difficult due to complexity around RBF/CPFP anti-DoS rules. For example, if we take a simplified lightning design with pre-signed commitment transaction A with one 0-value anyone-can-spend output available for use as a CPFP output, a counterparty can prevent confirmation of/significantly increase the fee cost of confirming A by chaining a large-but-only-moderate-feerate transaction off of this anyone-can-spend output. This transaction, B, will have a large absolute fee while making the package (A, B) have a low-ish feerate, placing it solidly at the bottom of the mempool but without significant risk of it getting evicted during memory limiting. This large absolute fee forces a counterparty which wishes to have the commitment transaction confirm to increase on this absolute fee in order to meet RBF rules. For this reason (and many other similar attacks utilizing the package size limits), in discussing the security model around CPFP, we've generally considered it too-difficulty-to-prevent third parties which are able to spend an output of a transaction from delaying its confirmation, at least until/unless the prevailing feerates decline and some of the mempool backlog gets confirmed. You'll note, however, that this attack doesn't have to be permanent to work - Lightning's (and other contracting/payment channel systems') security model assumes the ability to get such commitment transactions confirmed in a timely manner, as otherwise HTLCs may time out and counterparties can claim the timeout-refund before we can claim the HTLC using the hash-preimage. To partially-address the CPFP security model considerations, a next step might involve tweaking Lightning's commitment transaction to have two small-value outputs which are immediately spendable, one by each channel participant, allowing them to chain children off without allowng unrelated third-parties to chain children. Obviously this does not address the specific attack so we need a small tweak to the anti-DoS CPFP rules in Bitcoin Core/BIP 125: The last transaction which is added to a package of dependent transactions in the mempool must: * Have no more than one unconfirmed parent, * Be of size no greater than 1K in virtual size. (for implementation sanity, this would effectively reduce all mempool package size limits by 1 1K-virtual-size transaction, and the last would be "allowed to violate the limits" as long as it meets the above criteria). For contracting applications like lightning, this means that as long as the transaction we wish to confirm (in this case the commitment transaction) * Has only two immediately-spendable (ie non-CSV) outputs, * where each immediately-spendable output is only spendable by one counterparty, * and is no larger than MAX_PACKAGE_VIRTUAL_SIZE - 1001 Vsize, each counterparty will always be able to independantly CPFP the transaction in question. ie because if the "malicious" (ie transaction-delaying) party bradcasts A with a child, it can never meet the "last transaction" carve-out as its transaction cannot both meet the package limit and have only one unconfirmed ancestor. Thus, the non-delaying counterparty can always independently add its own CPFP transaction, increasing the (A, Tx2) package feerate and confirming A without having to concern themselves with the (A, Tx1) package. As an alternative proposal, at various points there have been discussions around solving the "RBF-pinning" problem by allowing transactors to mark their transactions as "likely-to-be-RBF'ed", which could enable a relay policy where children of such transactions would be rejected unless the resulting package would be "near the top of the mempool". This would theoretically imply such attacks are not possible to pull off consistently, as any "transaction-delaying" channel participant will have to place the package containing A at an effective feerate which makes confirmation to occur soon with some likelihood. It is, however, possible to pull off this attack with low probability in case of feerate spikes right after broadcast. Note that this clearly relies on some form of package relay, which comes with its own challenges, but I'll start a separate thread on that. See-also: lightning-dev thread about the changes to lightning spec required to incorporate this: https://lists.linuxfoundation.org/pipermail/lightning-dev/2018-November/001643.html Matt