Re: Make a unique filesystem path, without creating the file

From	Chris Angelico <rosuav@gmail.com>
Newsgroups	comp.lang.python
Subject	Re: Make a unique filesystem path, without creating the file
Date	2016-02-23 11:56 +1100
Message-ID	<mailman.59.1456188988.20994.python-list@python.org> (permalink)
References	(10 earlier) <slrnncn8u3.16b.jon+usenet@wintry.unequivocal.co.uk> <mailman.55.1456186740.20994.python-list@python.org> <slrnncn9uc.16b.jon+usenet@wintry.unequivocal.co.uk> <mailman.57.1456187639.20994.python-list@python.org> <slrnncnb1a.16b.jon+usenet@wintry.unequivocal.co.uk>

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On Tue, Feb 23, 2016 at 11:44 AM, Jon Ribbens
<jon+usenet@unequivocal.co.uk> wrote:
> On 2016-02-23, Chris Angelico <rosuav@gmail.com> wrote:
>> On Tue, Feb 23, 2016 at 11:26 AM, Jon Ribbens
>><jon+usenet@unequivocal.co.uk> wrote:
>>> On 2016-02-23, Chris Angelico <rosuav@gmail.com> wrote:
>>>> On Tue, Feb 23, 2016 at 11:08 AM, Jon Ribbens
>>>><jon+usenet@unequivocal.co.uk> wrote:
>>>>>> If you generate 2**128 + 1 such numbers, you are *guaranteed* to
>>>>>
>>>>> ... have expired due to the heat death of the universe.
>>>>
>>>> Maybe... but by the time you get to 2**64 of them, you have a 50%
>>>> chance of a collision. (That's either utterly intuitive or completely
>>>> counter-intuitive, depending on who you are.)
>>>
>>> Um, did you mean to say 2**127? Are you thinking of the
>>> birthday paradox or something, which doesn't apply here?
>>
>> By the time you generate 2**64 of them, you have a 50% chance that
>> some pair of them collides. Yes, the birthday paradox does apply here.
>
> Oh, I see, you're thinking of it differently. I was thinking of it as
> Alice is choosing a filename and Mallet is trying to guess it, in which
> case the birthday paradox doesn't apply. You're thinking of it as Alice
> is generating many random filenames and, even though she could avoid
> collisions with 100% certainty by remembering what she's already had,
> isn't doing so, and must avoid colliding with herself. I don't think
> your version makes has much relevance as an attack model.

Ah. Steven was talking about collisions; once you have 2**128+1 of
them, you're guaranteed a collision (pigeonhole principle). What
you're talking about gives certainty slightly sooner - specifically,
once you've tried 2**128 of them, you're guaranteed to have hit it :)

ChrisA

Thread

Re: Make a unique filesystem path, without creating the file Ethan Furman <ethan@stoneleaf.us> - 2016-02-22 10:11 -0800
  Re: Make a unique filesystem path, without creating the file Jon Ribbens <jon+usenet@unequivocal.co.uk> - 2016-02-22 18:17 +0000
    Re: Make a unique filesystem path, without creating the file Chris Angelico <rosuav@gmail.com> - 2016-02-23 05:25 +1100
      Re: Make a unique filesystem path, without creating the file Jon Ribbens <jon+usenet@unequivocal.co.uk> - 2016-02-22 18:39 +0000
        Re: Make a unique filesystem path, without creating the file Marko Rauhamaa <marko@pacujo.net> - 2016-02-22 20:48 +0200
          Re: Make a unique filesystem path, without creating the file Steven D'Aprano <steve@pearwood.info> - 2016-02-23 10:37 +1100
            Re: Make a unique filesystem path, without creating the file Jon Ribbens <jon+usenet@unequivocal.co.uk> - 2016-02-23 00:08 +0000
              Re: Make a unique filesystem path, without creating the file Chris Angelico <rosuav@gmail.com> - 2016-02-23 11:18 +1100
                Re: Make a unique filesystem path, without creating the file Jon Ribbens <jon+usenet@unequivocal.co.uk> - 2016-02-23 00:26 +0000
                Re: Make a unique filesystem path, without creating the file Chris Angelico <rosuav@gmail.com> - 2016-02-23 11:33 +1100
                Re: Make a unique filesystem path, without creating the file Jon Ribbens <jon+usenet@unequivocal.co.uk> - 2016-02-23 00:44 +0000
                Re: Make a unique filesystem path, without creating the file Chris Angelico <rosuav@gmail.com> - 2016-02-23 11:56 +1100
        Re: Make a unique filesystem path, without creating the file Chris Angelico <rosuav@gmail.com> - 2016-02-23 06:04 +1100
          Re: Make a unique filesystem path, without creating the file Paul Rubin <no.email@nospam.invalid> - 2016-02-22 11:22 -0800
            Re: Make a unique filesystem path, without creating the file Steven D'Aprano <steve@pearwood.info> - 2016-02-23 10:45 +1100
          Re: Make a unique filesystem path, without creating the file Jon Ribbens <jon+usenet@unequivocal.co.uk> - 2016-02-22 19:22 +0000
            Re: Make a unique filesystem path, without creating the file Marko Rauhamaa <marko@pacujo.net> - 2016-02-22 21:32 +0200
              Re: Make a unique filesystem path, without creating the file Random832 <random832@fastmail.com> - 2016-02-22 14:41 -0500
                Re: Make a unique filesystem path, without creating the file Marko Rauhamaa <marko@pacujo.net> - 2016-02-22 22:41 +0200
                Re: Make a unique filesystem path, without creating the file Paul Rubin <no.email@nospam.invalid> - 2016-02-22 13:05 -0800
                Re: Make a unique filesystem path, without creating the file Marko Rauhamaa <marko@pacujo.net> - 2016-02-22 23:22 +0200
                Re: Make a unique filesystem path, without creating the file Paul Rubin <no.email@nospam.invalid> - 2016-02-22 15:26 -0800
              Re: Make a unique filesystem path, without creating the file Steven D'Aprano <steve@pearwood.info> - 2016-02-23 11:33 +1100
                Re: Make a unique filesystem path, without creating the file Marko Rauhamaa <marko@pacujo.net> - 2016-02-23 08:54 +0200
                Re: Make a unique filesystem path, without creating the file Paul Rubin <no.email@nospam.invalid> - 2016-02-22 23:18 -0800
                Re: Make a unique filesystem path, without creating the file Marko Rauhamaa <marko@pacujo.net> - 2016-02-23 21:04 +0200
                Re: Make a unique filesystem path, without creating the file Steven D'Aprano <steve@pearwood.info> - 2016-02-24 12:40 +1100
                Re: Make a unique filesystem path, without creating the file Marko Rauhamaa <marko@pacujo.net> - 2016-02-24 09:20 +0200
                Re: Make a unique filesystem path, without creating the file Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-02-25 16:38 +1100
                Re: Make a unique filesystem path, without creating the file Marko Rauhamaa <marko@pacujo.net> - 2016-02-25 08:54 +0200
                Re: Make a unique filesystem path, without creating the file Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-02-25 19:21 +1100
                Re: Make a unique filesystem path, without creating the file Jon Ribbens <jon+usenet@unequivocal.co.uk> - 2016-02-25 10:05 +0000
            Re: Make a unique filesystem path, without creating the file Chris Angelico <rosuav@gmail.com> - 2016-02-23 06:37 +1100
            Re: Make a unique filesystem path, without creating the file Steven D'Aprano <steve@pearwood.info> - 2016-02-23 11:03 +1100
              Re: Make a unique filesystem path, without creating the file Jon Ribbens <jon+usenet@unequivocal.co.uk> - 2016-02-23 00:11 +0000
              Re: Make a unique filesystem path, without creating the file Paul Rubin <no.email@nospam.invalid> - 2016-02-22 18:27 -0800
                Re: Make a unique filesystem path, without creating the file Chris Angelico <rosuav@gmail.com> - 2016-02-23 13:53 +1100
                Re: Make a unique filesystem path, without creating the file Paul Rubin <no.email@nospam.invalid> - 2016-02-22 19:26 -0800
                Re: Make a unique filesystem path, without creating the file Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-02-23 08:09 +0000
                Re: Make a unique filesystem path, without creating the file Paul Rubin <no.email@nospam.invalid> - 2016-02-23 00:22 -0800
                Re: Make a unique filesystem path, without creating the file Peter Otten <__peter__@web.de> - 2016-02-23 09:40 +0100
                Re: Make a unique filesystem path, without creating the file Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-02-23 09:00 +0000
                Re: Make a unique filesystem path, without creating the file Grant Edwards <invalid@invalid.invalid> - 2016-02-23 15:14 +0000
                Re: Make a unique filesystem path, without creating the file Steven D'Aprano <steve@pearwood.info> - 2016-02-25 11:41 +1100
                Re: Make a unique filesystem path, without creating the file Random832 <random832@fastmail.com> - 2016-02-25 10:03 -0500
    Re: Make a unique filesystem path, without creating the file Steven D'Aprano <steve@pearwood.info> - 2016-02-23 11:18 +1100

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