Received: from sog-mx-2.v43.ch3.sourceforge.com ([172.29.43.192] helo=mx.sourceforge.net) by sfs-ml-1.v29.ch3.sourceforge.com with esmtp (Exim 4.76) (envelope-from ) id 1WTuNd-00073Q-Sm for bitcoin-development@lists.sourceforge.net; Sat, 29 Mar 2014 14:37:05 +0000 Received-SPF: pass (sog-mx-2.v43.ch3.sourceforge.com: domain of gmail.com designates 209.85.215.48 as permitted sender) client-ip=209.85.215.48; envelope-from=gmaxwell@gmail.com; helo=mail-la0-f48.google.com; Received: from mail-la0-f48.google.com ([209.85.215.48]) by sog-mx-2.v43.ch3.sourceforge.com with esmtps (TLSv1:RC4-SHA:128) (Exim 4.76) id 1WTuNd-00018H-0P for bitcoin-development@lists.sourceforge.net; Sat, 29 Mar 2014 14:37:05 +0000 Received: by mail-la0-f48.google.com with SMTP id gf5so4508848lab.21 for ; Sat, 29 Mar 2014 07:36:58 -0700 (PDT) MIME-Version: 1.0 X-Received: by 10.112.85.6 with SMTP id d6mr9827727lbz.8.1396103818374; Sat, 29 Mar 2014 07:36:58 -0700 (PDT) Received: by 10.112.89.68 with HTTP; Sat, 29 Mar 2014 07:36:58 -0700 (PDT) In-Reply-To: References: <1878927.J1e3zZmtIP@crushinator> <1894130.91FUH3Vu6n@crushinator> Date: Sat, 29 Mar 2014 07:36:58 -0700 Message-ID: From: Gregory Maxwell To: Watson Ladd Content-Type: text/plain; charset=UTF-8 X-Spam-Score: -1.6 (-) X-Spam-Report: Spam Filtering performed by mx.sourceforge.net. See http://spamassassin.org/tag/ for more details. -1.5 SPF_CHECK_PASS SPF reports sender host as permitted sender for sender-domain 0.0 FREEMAIL_FROM Sender email is commonly abused enduser mail provider (gmaxwell[at]gmail.com) -0.0 SPF_PASS SPF: sender matches SPF record -0.1 DKIM_VALID_AU Message has a valid DKIM or DK signature from author's domain 0.1 DKIM_SIGNED Message has a DKIM or DK signature, not necessarily valid -0.1 DKIM_VALID Message has at least one valid DKIM or DK signature X-Headers-End: 1WTuNd-00018H-0P Cc: Bitcoin Dev Subject: Re: [Bitcoin-development] Presenting a BIP for Shamir's Secret Sharing of Bitcoin private keys X-BeenThere: bitcoin-development@lists.sourceforge.net X-Mailman-Version: 2.1.9 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 29 Mar 2014 14:37:06 -0000 On Sat, Mar 29, 2014 at 7:28 AM, Watson Ladd wrote: > This is not the case: one can use MPC techniques to compute a > signature from shares without reconstructing the private key. There is > a paper on this for bitcoin, but I don't know where it is. Practically speaking you cannot unless the technique used is one carefully selected to make it possible. This proposal isn't such a scheme I beleieve, however, and I think I'd strongly prefer that we BIP standardize a formulation which also has this property. The paper you want is http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.67.9913 There will soon be a paper coming out from some princeton folks about refining that and applying it to Bitcoin. You can use the secret sharing from threshold ecdsa in the not-super-useful way where you just recombine the private key and sign... but you can also use it to compute a secret shared signature and then interpolate back the signature... avoiding the need for any trusted device in holding the signature.