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From Kev Jackson <kevin.jack...@it.fts-vn.com>
Subject Re: cvs commit: ant/docs/manual/CoreTasks checksum.html
Date Thu, 17 Feb 2005 04:10:19 GMT

>>
>> But can the forged file with identical MD5 masquerade as the original
>> file, i.e. still be a Zip file, or tar'd gzipped or bzipped file?
>>
>> Sure, what you describe sounds bad, but I'm trying to figure out
>> (without too much research of my own ;-) if it's a real problem in
>> practice. --DD
>>
In practice the algorithm described "only" takes 2**69 to produce the 
same hash instead of (brute-force)2**80.  It's still a *long* time for a 
desktop PC.  The problem I see is that if you are using Ant to generate 
a checksum as part of your ultra-secure-NSA-application, then you really 
need to consider switcing to a different hash algorithm *now*.  Indeed 
as SHA-1 was so trusted, pretty much everyone uses it, so a lot of 
things will need to be upgraded (not just for hashes to verify the file 
contents).

>>  
>>
> Would it be feasible to publish instead of just the SHA-1 all, the 
> SHA-1, MD5 and the size of the file.
>
> Is Modifying a file while fulfilling all of the following conditions:
> - the file format valid
> - the size the same
> - the SHA-1 the same
> - the MD5 the same
> - the working of ant not obviously broken
> practically possible?
>
> And would it be worth the wile to spend that much effort on forging it 
> on the ant distribution (in the release timeframe?)

I don't think that this is the major problem.  It's very very very 
unlikely that anyone would want to tamper with Ant (why bother, a user 
can always get teh source and build themselves?).  The problem is that 
when using Ant to build new code (and to generate a checksum for that 
distribution), now you as the developer of new-shiny-applictaion have to 
decide whether anyone is going to take the time to create a fake version 
of your app.  If so, use a different checksum technique and don't rely 
on Ant's checksum task.

At least that's how I see it.  www.schneier.com for more details

Kev

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