Re: [Monotone-devel] Re: RFC: Fake IDs

[Nathaniel Smith]

On Wed, Jul 19, 2006 at 12:27:42AM -0700, Graydon Hoare wrote:
> Ok. I've argued with myself about my knee-jerk perception of risk here 
> and convinced myself that I've been irrationally mis-perceiving relative 
> probabilities. You're right: a fixed plaintext collision -- even all 
> zero -- is actually *less* likely than the birthday-collision of "any 
> two files". It's unnerving, but I have to accept the reasoning. Score 
> one for unreasonable psychology.

Ah-hah.  Probability is Weird.

> I propose an Explicit and Sound Mechanism:
> 
> Make a class fake_rid_source that wraps a u64 counter and sequentially 
> assigns "%40.40llX" rids. Construct one and have it hand out fake rids 
> within the scope that you're using them. That makes the scope 
> relationships of your fakes somewhat explicit (which is half of what I 
> wanted with "checking the database") and still lets you generate a few 
> within a function and throw them out without dragging a db handle into it.

Hmm.  I guess as compared to the single global counter of Zack's
patch (IIUC), this has an advantage:
  -- you must explicitly mark the scope of these rids in your code
and a disadvantage:
  -- there's nothing to enforce you did this right, except careful
     code auditing[1]; in particular you might accidentally let two
     fake rids from two different sources bump into each other
     accidentally
My knee-jerk reaction is that the advantage is only really an
advantage if there's enforcement, but I dunno.


[1] You know, it would be pretty awesome if you had a type system
that let you say "this dynamic value right here must not escape this
static scope", a sort of very-refined tainting scheme.  I guess you
run into some problem with derived values; it's useless to have the
value in the first place if no other values that result from
computations involving it can escape either, but then code that does a
non-trivial identity function would let it escape... err, anyway.  I
guess some E auditors do stuff along these lines.

-- Nathaniel

-- 
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