public inbox for bitcoindev@googlegroups.com
 help / color / mirror / Atom feed
From: Gregory Maxwell <greg@xiph.org>
To: Peter R <peter_r@gmx.com>
Cc: Bitcoin Development Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Compact Block Relay BIP
Date: Tue, 10 May 2016 02:12:03 +0000	[thread overview]
Message-ID: <CAAS2fgR8j5QJkVb2rEfpi27OvN4gVw2ROaehLRvsojQd7yrpXg@mail.gmail.com> (raw)
In-Reply-To: <5C2809F9-286D-49E4-89DB-7109B73F6076@gmx.com>

On Mon, May 9, 2016 at 11:37 PM, Peter R <peter_r@gmx.com> wrote:
> 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…

$ echo -n Perhaps. 00000000f2736d91 |sha256sum
359dfa6d4c2eb2ac81535392d68af4b5e1cb6d9c6321e8f111d3244329b6a4d8
$ echo -n Perhaps. 0000000011ac0388 |sha256sum
359dfa6d4c2eb2ac44d54d0ceeb2212500cb34617b9360695432f6c0fde9b006

Try search term "collision", or there may be an undergrad Data
structures and algorithms coarse online-- you want something covering
"cycle finding".

(Though even ignoring efficient cycle finding, your factorial argument
doesn't hold... you can simply sort the data... Search term
"quicksort" for a relevant algorithm).


  parent reply	other threads:[~2016-05-10  2:12 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               ` [bitcoin-dev] " Peter R
2016-05-10  1:42                 ` Peter R
2016-05-10  2:12                 ` Gregory Maxwell [this message]
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=CAAS2fgR8j5QJkVb2rEfpi27OvN4gVw2ROaehLRvsojQd7yrpXg@mail.gmail.com \
    --to=greg@xiph.org \
    --cc=bitcoin-dev@lists.linuxfoundation.org \
    --cc=peter_r@gmx.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
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