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From Alex Besogonov <alex.besogo...@gmail.com>
Subject Re: Why MD5 is used for hashes, also about non-deterministic IDs.
Date Tue, 15 Nov 2011 05:45:47 GMT
On Tue, Nov 15, 2011 at 12:19 AM, Dustin Sallings <dustin@spy.net> wrote:
> On Nov 14, 2011, at 8:41 PM, Alex Besogonov wrote:
>> Now, it might not sound too threatening, but this attack breaks the
>> main invariant of
>> CouchDB - database replicas won't ever be eventually consistent!
>> Also, I'd like to use stronger hash just on general principles.
>        I'd prefer to get rid of this functionality altogether.  It's wrong even
in cases where people aren't being malicious.
That also, though it's a judgment call which approach is better.

>        Example:
>        I have a document that represents how many things I've got.
>        On node A, I increment the number of things.  I go from 5 things to 6 things.
>        On node B, I increment the number of things.  I go from 5 things to 6 things.
>        Replication catches up, sees the same digest, and now I have six things --
but this is incorrect.  I have seven things (or at least a conflict).
>        This does will happen with any hash, but no UUID.
Hash allows to trace ancestry unforgeably, though. Git uses this approach.

But IMHO it's better to either have secure hash-based implementation
with cryptographically strong hashes authenticating canonic
representations of documents or throw it out altogether and use
UUID-based implementation.

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