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 E0B56B5D for ; Thu, 7 Jan 2016 19:02:07 +0000 (UTC) X-Greylist: whitelisted by SQLgrey-1.7.6 Received: from mail-lb0-f182.google.com (mail-lb0-f182.google.com [209.85.217.182]) by smtp1.linuxfoundation.org (Postfix) with ESMTPS id D0934CC for ; Thu, 7 Jan 2016 19:02:06 +0000 (UTC) Received: by mail-lb0-f182.google.com with SMTP id sv6so205461981lbb.0 for ; Thu, 07 Jan 2016 11:02:06 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:date:message-id:subject:from:to:content-type; bh=KoSTMEczF4hxan5oEdQ9U7lhEPxOlpwIrXmMlUoGHUQ=; b=kh8UTSdRYpBLvEU3cVBcd7KtPvbr2yclHm3Ygbi3zHeB6ef+MHDdh6TRehk1GpfZad MLRyPZd0AZ5M6VcQXKTfnakQAqNaS/+UvIWgOVs0KGjuVAsqTUZTp8taR6g4LMQG9R98 R0AG4/mJmnmKiyLhdqpcTn84csCBskMAgxO+p9lgY4BN29m79UYxdBWyC57gigaY02nu ES8U2NDMt4GlMy6hjzv+THVwOirJUajtp26ywLCekIzLoNxzFDrEILxfq9hD6yA13fS+ 7xppeYAe2I5zHyXXUfbXazCJTBaBnWQGzUU8gm7BBXB3lypYqQF9hfao52jvvJhqHNbV oV5w== MIME-Version: 1.0 X-Received: by 10.112.219.101 with SMTP id pn5mr10112863lbc.123.1452193325211; Thu, 07 Jan 2016 11:02:05 -0800 (PST) Received: by 10.25.25.78 with HTTP; Thu, 7 Jan 2016 11:02:05 -0800 (PST) Date: Thu, 7 Jan 2016 14:02:05 -0500 Message-ID: From: Gavin Andresen To: Bitcoin Dev Content-Type: multipart/alternative; boundary=001a11c3c26c4b6c4c0528c31b10 X-Spam-Status: No, score=-2.7 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FROM,HTML_MESSAGE,RCVD_IN_DNSWL_LOW 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: Thu, 07 Jan 2016 19:05:19 +0000 Subject: [bitcoin-dev] Time to worry about 80-bit collision attacks or not? X-BeenThere: bitcoin-dev@lists.linuxfoundation.org X-Mailman-Version: 2.1.12 Precedence: list List-Id: Bitcoin Development Discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 07 Jan 2016 19:02:08 -0000 --001a11c3c26c4b6c4c0528c31b10 Content-Type: text/plain; charset=UTF-8 I'm hoisting this from some private feedback I sent on the segregated witness BIP: I said: "I'd also use RIPEMD160(SHA256()) as the hash function and save the 12 bytes-- a successful preimage attack against that ain't gonna happen before we're all dead. I'm probably being dense, but I just don't see how a collision attack is relevant here." Pieter responded: "The problem case is where someone in a contract setup shows you a script, which you accept as being a payment to yourself. An attacker could use a collision attack to construct scripts with identical hashes, only one of which does have the property you want, and steal coins. So you really want collision security, and I don't think 80 bits is something we should encourage for that. Normal pubkey hashes don't have that problem, as they can't be constructed to pay to you." ... but I'm unconvinced: "But it is trivial for contract wallets to protect against collision attacks-- if you give me a script that is "gavin_pubkey CHECKSIG arbitrary_data OP_DROP" with "I promise I'm not trying to rip you off, just ignore that arbitrary data" a wallet can just refuse. Even more likely, a contract wallet won't even recognize that as a pay-to-gavin transaction. I suppose it could be looking for some form of "gavin_pubkey somebody_else_pubkey CHECKMULTISIG ... with the attacker using somebody_else_pubkey to force the collision, but, again, trivial contract protocol tweaks ("send along a proof you have the private key corresponding to the public key" or "everybody pre-commits pubkeys they'll use at protocol start") would protect against that. Adding an extra 12 bytes to every segwit to prevent an attack that takes 2^80 computation and 2^80 storage, is unlikely to be a problem in practice, and is trivial to protect against is the wrong tradeoff to make." 20 bytes instead of 32 bytes is a savings of almost 40%, which is significant. The general question I'd like to raise on this list is: Should we be worried, today, about collision attacks against RIPEMD160 (our 160-bit hash)? Mounting a successful brute-force collision attack would require at least O(2^80) CPU, which is kinda-sorta feasible (Pieter pointed out that Bitcoin POW has computed more SHA256 hashes than that). But it also requires O(2^80) storage, which is utterly infeasible (there is something on the order of 2^35 bytes of storage in the entire world). Even assuming doubling every single year (faster than Moore's Law), we're four decades away from an attacker with THE ENTIRE WORLD's storage capacity being able to mount a collision attack. References: https://en.wikipedia.org/wiki/Collision_attack https://vsatglobalseriesblog.wordpress.com/2013/06/21/in-2013-the-amount-of-data-generated-worldwide-will-reach-four-zettabytes/ -- -- Gavin Andresen --001a11c3c26c4b6c4c0528c31b10 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
I'm hoisting this from some private feedback I sent on= the segregated witness BIP:

I said:

"I'd als= o use RIPEMD160(SHA256()) as the hash function and save the 12 bytes-- a su= ccessful preimage attack against that ain't gonna happen before we'= re all dead. I'm probably being dense, but I just don't see how a c= ollision attack is relevant here."

Pieter responded:<= /font>

"The problem case is where someone in a contract setup shows you a = script, which you accept as being a payment to yourself. An attacker could = use a collision attack to construct scripts with identical hashes, only one= of which does have the property you want, and steal coins.

So you really want collisi= on security, and I don't think 80 bits is something we should encourage= for that. Normal pubkey hashes don't have that problem, as they can= 9;t be constructed to pay to you."

... but I'm unconvinced:

=
"But it is trivi= al for contract wallets to protect against collision attacks-- if you give = me a script that is "gavin_pubkey CHECKSIG arbitrary_data OP_DROP"= ; with "I promise I'm not trying to rip you off, just ignore that = arbitrary data" a wallet can just refuse. Even more likely, a contract= wallet won't even recognize that as a pay-to-gavin transaction.=

I= suppose it could be looking for some form of "gavin_pubkey somebody_e= lse_pubkey CHECKMULTISIG ... with the attacker using somebody_else_pubkey t= o force the collision, but, again, trivial contract protocol tweaks ("= send along a proof you have the private key corresponding to the public key= " or "everybody pre-commits pubkeys they'll use at protocol s= tart") would protect against that.

Adding an = extra 12 bytes to every segwit to prevent an attack that takes 2^80 computa= tion and 2^80 storage, is unlikely to be a problem in practice, and is triv= ial to protect against is the wrong tradeoff to make."

<= /div>
20 bytes instead of 32 bytes is a savings of almost 40%, which is= significant.

The general question I'd like to= raise on this list is:

Should we be worried, toda= y, about collision attacks against RIPEMD160 (our 160-bit hash)?
=
Mounting a successful brute-force collision attack would req= uire at least O(2^80) CPU, which is kinda-sorta feasible (Pieter pointed ou= t that Bitcoin POW has computed more SHA256 hashes than that). But it also = requires O(2^80) storage, which is utterly infeasible (there is something o= n the order of 2^35 bytes of storage in the entire world).=C2=A0 Even assum= ing doubling every single year (faster than Moore's Law), we're fou= r decades away from an attacker with THE ENTIRE WORLD's storage capacit= y being able to mount a collision attack.

References:=C2=A0




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Gavin Andresen

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