From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from sog-mx-4.v43.ch3.sourceforge.com ([172.29.43.194] helo=mx.sourceforge.net) by sfs-ml-1.v29.ch3.sourceforge.com with esmtp (Exim 4.76) (envelope-from ) id 1WrgfL-0003Ig-7O for bitcoin-development@lists.sourceforge.net; Tue, 03 Jun 2014 04:49:39 +0000 X-ACL-Warn: Received: from smtprelay02.ispgateway.de ([80.67.29.24]) by sog-mx-4.v43.ch3.sourceforge.com with esmtps (TLSv1:AES256-SHA:256) (Exim 4.76) id 1WrgfJ-0003nN-7z for bitcoin-development@lists.sourceforge.net; Tue, 03 Jun 2014 04:49:39 +0000 Received: from [89.13.84.41] (helo=anonymous) by smtprelay02.ispgateway.de with esmtpsa (TLSv1:AES256-SHA:256) (Exim 4.68) (envelope-from ) id 1WrgMT-0005SM-4V for bitcoin-development@lists.sourceforge.net; Tue, 03 Jun 2014 06:30:09 +0200 From: xor To: bitcoin-development@lists.sourceforge.net Date: Tue, 03 Jun 2014 06:29:55 +0200 Message-ID: <2341954.NpNStk60qp@1337h4x0r> Organization: Freenet Project User-Agent: KMail/4.11.5 (Linux/3.11.0-22-generic; KDE/4.11.5; x86_64; ; ) MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" X-Df-Sender: eG9yQGZyZWVtYWlsLmJvZ2VydC5kZQ== X-Spam-Score: 1.0 (+) X-Spam-Report: Spam Filtering performed by mx.sourceforge.net. See http://spamassassin.org/tag/ for more details. -0.0 RCVD_IN_DNSWL_NONE RBL: Sender listed at http://www.dnswl.org/, no trust [80.67.29.24 listed in list.dnswl.org] -0.0 SPF_HELO_PASS SPF: HELO matches SPF record 1.0 SPF_SOFTFAIL SPF: sender does not match SPF record (softfail) X-Headers-End: 1WrgfJ-0003nN-7z Subject: [Bitcoin-development] Lets discuss what to do if SHA256d is actually broken X-BeenThere: bitcoin-development@lists.sourceforge.net X-Mailman-Version: 2.1.9 Precedence: list Reply-To: xor@freenetproject.org List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 03 Jun 2014 04:49:39 -0000 -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Hi, I thought a lot about the worst case scenario of SHA256d being broken in a way which could be abused to A) reduce the work of mining a block by some significant amount B) reduce the work of mining a block to zero, i.e. allow instant mining. Bitcoin needs to be prepared for this as any hash function has a limited lifetime. Usually crypto stuff is not completely broken instantly by new attacks but gradually. For example first the attack difficulty is reduced from 2^128 to 2^100, then 2^64, etc. This would make scenario A more likely. Now while B sounds more dangerous, I think in fact A is: Consider how A would happen in real life: Someone publishes a paper of a theoretical reduction of SHA256d attacks to 2^96 bit. Mathematicians will consider this as a serious attack and create a lot of riot. If no plan is made early enough, as in now, the Bitcoin Core team might then probably want to just do the easiest approach of replacing the hash function after a certain block number, i.e. a hard fork. But what about the Bitcoin miners, those who need to actually accept a change of mining algorithm which renders their hardware which cost MILLIONS completely worthless? Over the years they have gotten used to exponential growth of the Bitcoin networks hashrate, and therefore exponential devaluation of their mining hardware. Even if the attack on SHA256d causes a significant growth of difficulty, the miners will not *believe* that it is an actual attack on SHA256d - - maybe it is just some new large mining operation? They are used to this happening! Why should they believe this and switch to a new hash function which requires completely new hardware and therefore costs them millions? They will just keep mining SHA256d. Thats why this is more dangerous, because changing the hash funciton won't be accepted by the miners even though it is broken. Something smarter needs to be thought of. Now I must admit that I am not good at cryptography at all, but I had the following idea: Use the altcoin concept of having multiple hash functions in a chain. If SHA256d is broken, it is chained with a new hash function. Thereby, people who want to mine the new replacement hash function still will need ASICs which can solve the old SHA proof of work. So existing ASIC owners can amend their code to do SHA256d using the ASIC, and then the second hash function using a general purpose CPU. This would also allow a smooth migration of difficulty - I don't even know how difficulty would react with the naive approach of just replacing SHA with something else: It would probably be an unsolvable problem to define new rules to make it decrease enough so new blocks can actually be mined by the now several orders of magnitude slower CPU-only mining community but still be high enough to be able to deal with the fact that millions of people will try their luck with mining at the release date. While this sounds simple in theory, it might be a lot of work to implement, so you guys might want to take precautions for it soon :) Greetings, xor - A Freenet project developer -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.14 (GNU/Linux) iQIcBAEBCAAGBQJTjU9DAAoJEMtmZ+8tjWt5pNEP/2460eHu7ujrUSxinJXY7+wF E759/NcpNuakqu4NsS3ndi8lSiVIeixiOWZxPwLYkzC0pgPd5JrK5hdrYewsgreL Ltkh6LKB4YZLYrV3jm62ZPMTzCopYQ1l872xbN3PJQJoXhEp4fKu99++LDzVg9Gk n7rvrk6Iy/nSsZ1IMANpKghbU8/Gtn6ppCJv9rxRE//CZdTso1tTyOXXkEEMTHcV y/iv6CHXtTXPvOgEgciU0oCPq0NOUKdIAOD//ybcKzncOoHSmwr1rZdreCTH6/Ek 9uwq/HaQnseHPrq9qrIkIKrZDlnjKu7Tqw1BlbyBeCrWdJPCeDJg2kyCXgTvIzFD oXwZ6r16tb2QPR4ByyO1lZy9G2Pp26thk12BnadnPYTf1rMvsY15BjfUrCU9ppt/ YpFAZSFlXUGOuOBKUznUeO8U1bXJylcTTnyER/cudOpcKR8Jt9l5tfm5LTHCB6Q2 Tjmvsmd0BzwafLEhHD5FHoTZFNVdXWvEUO/w4I/2UWTS7CacbE1qk0rVpsF/4L1K /oFVnZIUKqsm5mMMb6WTQq+MjP2TF/eAAwm2UtFYmj0FVML9HBNwyiAc5UKwnD4Y Yq3Pl5QfRobwu6pgT3zO7vK+saOl8sePWbU8Skj41OTEDrJM4QIQGAqs1U8xke8+ YnUYiyzreJ8ofHhNBs4/ =dkuk -----END PGP SIGNATURE-----