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From: Peter R <peter_r@gmx.com>
To: Gregory Maxwell <greg@xiph.org>,
	Bitcoin Development Discussion
	<bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Compact Block Relay BIP
Date: Mon, 9 May 2016 16:37:00 -0700	[thread overview]
Message-ID: <5C2809F9-286D-49E4-89DB-7109B73F6076@gmx.com> (raw)
In-Reply-To: <CAAS2fgRL1=YSrAZVES0WBySyL1brZcvQsvZdsqUEY2-8UOFFiA@mail.gmail.com>

Greg Maxwell wrote:

> What are you talking about? You seem profoundly confused here...
> 
> I obtain some txouts. I write a transaction spending them in malleable
> form (e.g. sighash single and an op_return output).. then grind the
> extra output to produce different hashes.  After doing this 2^32 times
> I am likely to find two which share the same initial 8 bytes of txid.

[9 May 16 @ 4:30 PDT]

I’m trying to understand the collision attack that you're explaining to Tom Zander.  

Mathematica is telling me that if I generated 2^32 random transactions, that the chances that the initial 64-bits on one of the pairs of transactions is about 40%.  So I am following you up to this point.  Indeed, there is a good chance that a pair of transactions from a set of 2^32 will have a collision in the first 64 bits.  

But how do you actually find that pair from within your large set?  The only way I can think of is to check if the first 64-bits is equal for every possible pair until I find it.  How many possible pairs are there?  

It is a standard result that there are 

    m! / [n! (m-n)!] 

ways of picking n numbers from a set of m numbers, so there are

    (2^32)! / [2! (2^32 - 2)!] ~ 2^63

possible pairs in a set of 2^32 transactions.  So wouldn’t you have to perform approximately 2^63 comparisons in order to identify which pair of transactions are the two that collide?

Perhaps I made an error or there is a faster way to scan your set to find the collision.  Happy to be corrected…

Best regards,
Peter



  reply	other threads:[~2016-05-09 23:37 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-05-02 22:13 [bitcoin-dev] Compact Block Relay BIP Matt Corallo
2016-05-03  5:02 ` Gregory Maxwell
2016-05-06  3:09   ` Matt Corallo
2016-05-08  0:40 ` Johnathan Corgan
2016-05-08  3:24   ` Matt Corallo
2016-05-09  9:35     ` Tom Zander
2016-05-09 10:43       ` Gregory Maxwell
2016-05-09 11:32         ` Tom
     [not found]           ` <CAAS2fgR01=SfpAdHhFd_DFa9VNiL=e1g4FiguVRywVVSqFe9rA@mail.gmail.com>
2016-05-09 12:12             ` [bitcoin-dev] Fwd: " Gregory Maxwell
2016-05-09 23:37               ` Peter R [this message]
2016-05-10  1:42                 ` [bitcoin-dev] " Peter R
2016-05-10  2:12                 ` Gregory Maxwell
2016-05-09 13:40           ` Peter Todd
2016-05-09 13:57             ` Tom
2016-05-09 14:04               ` Bryan Bishop
2016-05-09 17:06 ` Pieter Wuille
2016-05-09 18:34   ` Peter R
2016-05-10  5:28   ` Rusty Russell
2016-05-10 10:07     ` Gregory Maxwell
2016-05-10 21:23       ` Rusty Russell
2016-05-11  1:12         ` Matt Corallo
2016-05-18  1:49   ` Matt Corallo
2016-05-08 10:25 Nicolas Dorier

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