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
next prev parent 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 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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=5C2809F9-286D-49E4-89DB-7109B73F6076@gmx.com \
--to=peter_r@gmx$(echo .)com \
--cc=bitcoin-dev@lists$(echo .)linuxfoundation.org \
--cc=greg@xiph$(echo .)org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox