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Thu, 3 Sep 2020 03:51:02 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=riseup.net; s=squak; t=1599130262; bh=eFiRp1IUEkIL32UniFEGMpXu2nwXpvan+zP1h5peJm8=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=eTpO9ctDN918xKrs2wAvlTww60dV3qJuMCL8GYQe5VNd9csaOnZO10vEGG6SdRmnr O1Vl5qcRbS4t2ltn2f2uSpi0Dg8zdNBojwcz6h2yWomvs25wHgpM5AbuUEWWGlsSfY EAQcaVxXnRwppjqZ7l3ojezDmoUyYOv+lFahqU9w= X-Riseup-User-ID: 65C532AC5EAB847D083D5DD08DD3FD01E71BE858A300ED6C027E2CA37EE0C098 Received: from [127.0.0.1] (localhost [127.0.0.1]) by capuchin.riseup.net (Postfix) with ESMTPSA id 4BhyK140SXz8tqB; Thu, 3 Sep 2020 03:51:01 -0700 (PDT) To: ZmnSCPxj References: <813e51a1-4252-08c0-d42d-5cef32f684bc@riseup.net> <0ac3fecb-012b-e210-55bb-36809682634a@riseup.net> From: Chris Belcher Autocrypt: addr=belcher@riseup.net; prefer-encrypt=mutual; keydata= xsFNBFPk74oBEACzBLjd+Z5z7eimqPuObFTaJCTXP7fgZjgVwt+q94VQ2wM0ctk/Ft9w2A92 f14T7PiHaVDjHxrcW+6sw2VI2f60T8Tjf+b4701hIybluWL8DntG9BW19bZLmjAj7zkgektl YNDUrlYcQq2OEHm/MGk6Ajt2RA56aRKqoz22e+4ZA89gDgamxUAadul7AETSsgqOEUDI0FKR FODzoH65w1ien/DLkG1f76jd0XA6AxrESJVO0JzvkTnJGElBcA37rYaMmDi4DhG2MY4u63VE 8h6DyUXcRhmTZIAj+r+Ht+KMDiuiyQcKywCzzF/7Ui7YxqeAgjm5aPDU2E8X9Qd7cqHQzFM7 ZCqc9P6ENAk5a0JjHw0d0knApboSvkIJUB0j1xDIS0HaRlfHM4TPdOoDgnaXb7BvDfE+0zSz WkvAns9oJV6uWdnz5kllVCjgB/FXO4plyFCHhXikXjm1XuQyL8xV88OqgDFXwVhKrDL9Pknu sTchYm3BS2b5Xq1HQqToT3I2gRGTtDzZVZV0izCefJaDp1mf49k2cokDEfw9MroEj4A0Wfht 0J64pzlBYn/9zor5cZp/EAblLRDK6HKhSZArIiDR1RC7a6s7oTzmfn0suhKDdTzkbTAnDsPi Dokl58xoxz+JdYKjzVh98lpcvMPlbZ+LwIsgbdH4KZj7mVOsJwARAQABzR9DaHJpcyBCZWxj aGVyIDxmYWxzZUBlbWFpbC5jb20+wsF+BBMBAgAoBQJT5O+KAhsDBQkSzAMABgsJCAcDAgYV CAIJCgsEFgIDAQIeAQIXgAAKCRDvc06md/MRKS8jD/9P9fSYSIVjltL9brAMfIu7wJn0H0lX TbcuCM2uQitJ3BNxI3c7aq5dEby27u5Ud54otncDJuRPQVDKs6H7t1rInitgJ1MTQ9/aQGFA btKcgtVIMFbeClzTTfWr4W7fE45NI7E9EANgk5JfmWh3U+KINYLF5RtqynYocrsP6zOV+G9A HCpBemd9TN60CoMLMyMzTHEW1oQffaVAXY8DgthEYO/odWYIod7VTmEm0zU1aSysPqMwPWNm 8XIl0f8SfKQyZlAU8e1eCFVCenkE44FKC5qQNYc2UxexEYtfCWChTGc4oHKxIyYmTCCefsQF LvgwtvlNHRXHSDKSPSNcRcpl8DFpNEKrmMlkJ8Mx+YR05CydlTQ0bI3FBohJC+UHrjD5I3hA wJUC1o+yVSOEd+zN3cG1EECIwkEQSmBgG5t/le2RdzfXOdpf9ku2/zoBpq00R54JxUKlfRM7 OPTv7X+1AKHkxOySdCZwGgvdh2Whuqs4kTvtco00gCFM9fBd5oi1RJuHtxHsj8+/XU15UItb jeo96CIlM5YUeoRLPT5mxZYWgYAARFeSFReNq/Tuwq9d8EokUrtAyrPayznliy53UJfWDVzl 925c0Cz0HWaP2fWj+uFcj/8K0bhptuWJQy0Poht1z3aJC1UjEgr1Xz8I7jeSJmIlA9plcJw2 k4dhWc7BTQRT5O+KARAAyFxAM28EQwLctr0CrQhYWZfMKzAhCw+EyrUJ+/e4uiAQ4OyXifRr ZV6kLRul3WbTB1kpA6wgCShO0N3vw8fFG2Cs6QphVagEH8yfQUroaVxgADYOTLHMOb7INS8r KI/uRNmE6bXTX27oaqCEXLMycqYlufad7hr42S/T8zNh5m2vl6T/1Poj2/ormViKwAxM+8qf xd8FNI4UKmq2zZE9mZ5PiSIX0qRgM0yCvxV39ex/nhxzouTBvv4Lb1ntplR/bMLrHxsCzhyM KDgcX7ApGm+y6YEsOvzw9rRCRuJpE4lth8ShgjTtNTHfklBD6Ztymc7q7bdPWpKOEvO5lDQ6 q8+KfENv862cOLlWLk7YR2+mHZ1PXGhWC7ggwEkfGJoXo0x8X+zgUKe2+9Jj4yEhfL0IbFYC z2J5d+cWVIBktI3xqkwLUZWuAbE3vgYA4h8ztR6l18NTPkiAvpNQEaL4ZRnAx22WdsQ8GlEW dyKZBWbLUdNcMmPfGi5FCw2nNvCyN6ktv5mTZE12EqgvpzYcuUGQPIMV9KTlSPum3NLDq8QI 6grbG8iNNpEBxmCQOKz2/BuYApU2hwt2E44fL8e6CRK3ridcRdqpueg75my6KkOqm8nSiMEc /pVIHwdJ9/quiuRaeC/tZWlYPIwDWgb8ZE/g66z35WAguMQ+EwfvgAUAEQEAAcLBZQQYAQIA DwUCU+TvigIbDAUJEswDAAAKCRDvc06md/MRKaZwD/9OI3o3gVmst/mGx6hVQry++ht8dFWN IiASPBvD3E5EWbqWi6mmqSIOS6CxjU0PncxTBPCXtzxo/WzuHGQg/xtNeQ0T8b2lBScZAw93 qm1IcHXLUe5w/Tap6YaDmSYCIZAdtbHzYfPW4JK7cmvcjvF8jhTFOBEOFVQkTi19G7caVot0 +wL1e2DRHDXAe5CinEpaLBlwHeEu/5j6wc3erohUZlK9IbAclj4iZTQbaq3EyqUXl59dBOON xmL5edJxzVishIYQGIyA9WP1SylXt+kO82NEqZG2OxdXAlzjuJ8C2pAG+nbLtDo4hcsiN/MA aX9/JB7MXclT5ioerF4yNgKEdfq7LmynsTUd8w/Ilyp7AD+BWoujyO94i8h9eKvjf9PvSwxQ uAjRpxne7ZJD8vCsMNXBHSbeEK2LiwStHL/w473viXpDD53J6OLxX6a5RummR+rixbMH7dgK MJQ7FlyDphm3or6CSkGEir1KA0y1vqQNFtHhguFapAWMDKaJjQQNgvZUmOo6hbZqmvUF1OWc d6GA6j3WOUe3fDJXfbq6P9Jmxq64op887dYKsg7xjQq/7KM7wyRcqXXcbBdgvNtVDP+EnzBN HyYY/3ms4YIHE5JHxQ9LV4yPcWkYTvb1XpNIFVbrSXAeyGHVNT+SO6olFovbWIC3Az9yesaM 1aSoTg== Message-ID: <8f387b40-8212-9807-70cc-b527902609c2@riseup.net> Date: Thu, 3 Sep 2020 11:50:57 +0100 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Cc: Bitcoin Protocol Discussion Subject: Re: [bitcoin-dev] Detailed protocol design for routed multi-transaction CoinSwap X-BeenThere: bitcoin-dev@lists.linuxfoundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Bitcoin Protocol Discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 03 Sep 2020 10:51:03 -0000 Hello ZmnSCPxj, On 03/09/2020 10:45, ZmnSCPxj wrote: > Good morning Chris, > >> A big downside is that it really ruins the property of allowing coins to >> remain unspent indefinitely. That has privacy implications: if a coin >> remains unspent for longer than 2 weeks (or another short locktime) then >> for sure the transaction was not a CoinSwap, and so the anonymity set of >> the CoinSwap system would be far smaller For this reason I'm pretty >> desperate to solve the vulnerability without losing the coins remaining >> unspent indefinitely feature. > > Ah, right.... accept no small privacy leaks! I'd argue its not even a small leak. A huge amount of coins remain unspent for weeks, months and years, and it would be great to add them to our CoinSwap anonymity set. And also have them benefit from CoinSwap's anonymity set even if they didn't use CoinSwap. > This seems a great solution! > > Since B is the one offering HTLCs, the taker of a CoinSwap sequence can be B as well. > This means, the taker has to have *some* collateral input, of at least value K, that it cannot swap (because if it tried to swap that amount, it would be unable to provide a collateral as well). > > How much does C need to know about the B collateralized contract transaction? > At the minimum, it has to know the output pays out to the correct contract, so it seems to me it has to know the entire B collateralized contract transaction, meaning it learns another input of B ("collateral(B)") that is not otherwise involved in the CoinSwap. > This is important, again, if B is a taker, as it means an unrelated input of B is now learned by C as having the same ownership as B. Yes, in fact that's why in my example I talk about a CoinSwap between two makers Bob and Charlie. Makers can be reasonably expected to own multiple UTXOs, but takers cannot. As you say because collateral payments breaks the ability of takers to sweep their entire wallet through CoinSwap. Happily, I think takers themselves don't need to use collateral payments. Here's an argument to why: Riskless theft attempts by the taker who no longer controls the coins actually isnt riskless or costless: Because it reduces the privacy of the previously-owned coins. If a taker genuinely wanted good privacy (Which, after all, they're paying for via miner fees and CoinSwap fees) then they would benefit if the coins they no longer own remain unspent for a long time, because it increases their anonymity set by making them hide among a greater crowd of coins which also don't get spent for a long time. Assuming that all peers, especially makers, deploy multiple redundant watchtowers then we can assume the success rate of such a theft attempt is very low. Because of the very low payoff, and privacy benefit of leaving coins unspent, then it can be argued that taker software which attempts such theft will never get popular. Of course this privacy argument only applies to takers, and if the CoinSwap contract is between two makers as part of a multi-transaction CoinSwap then it doesn't apply. So a maker-to-maker CoinSwap must use collateral payments. == Leak of first hop == Collateral inputs only applying to maker-maker CoinSwaps adds an additional information leak, which is that makers can now tell whether their previous peer was a taker or maker, based on whether they used a collateral input or not. This should be okay because the first maker doesn't know the final destination of the coins. This is similar to Tor, where this information is already leaked, for example when the user connects to a Tor bridge. The operator of the Tor bridge knows that everyone connecting to it is not a Tor relay node but an actual user. The operator of the tor bridge still has no idea where the user's internet traffic goes. Our situation is actually better than Tor, because in Tor the final relay always knows that they are an exit node, while the final maker in a CoinSwap might not know that. Also, if the taker does happen to own an extra UTXO, they may choose to use a collateral input anyway, just to pretend that they're a maker. Regards CB