[darcs-users] Why Bitkeeper still wins

Daan Leijen daan at cs.uu.nl
Tue Mar 22 15:00:20 UTC 2005


Hi Jamie,

Jamie Webb wrote:

>On Tue, Mar 22, 2005 at 02:34:26PM +0100, Daan Leijen wrote:
>  
>
>>>That depends on your definition of useful. The break in SHA1 is
>>>largely theoretical at this point. 
>>>
>>ell, Bruce Schneier seems to say that it is definitely broken (as of 
>>February 15, 2005):
>>    
>>
>
>Yes. It's broken, as I said. It now takes 2^69 ops to find a
>collision, rather than the 2^80 required for a brute force attack.
>
>Which in a few years time might be a reasonable number of ops for a
>supercomputer to perform. But, you must understand what they mean by
>'find a collision'. They can find two random strings of garbage with
>the same hash. This has a lot of theoretical significance, and may
>pave the way to more severe attacks, but it does not allow them to
>take one meaningful document written by someone else, modify it to
>introduce a malicious meaning, and then add a few bytes to give it the
>same hash. That is /much/ harder. I.e. doing it by brute force
>requires typically 2^159 ops, not 2^80.
>
>There may be social engineering workarounds that allow a
>two-chosen-texts attack (AFAIK it's not been suggested that the
>current break provides this, but it at least lays the groundwork), but
>due to Monotone's tree-versioning approach I couldn't think of a
>practical one. Also, no such attack would be possible against existing
>tree versions (because this attack depends on 'planting' a patch and
>at present there's not enough computing power to calculate a patch to
>plant) and so could only be mounted against the trees that are
>'current' in a few years time (when 2^69 ops is feasible), by which
>time Monotone will presumably have changed hash.
>
>In summary: feel free to object to Monotone. I don't use it either.
>But the fact that it currently uses SHA1 is not a sound reason.
>
>-- Jamie Webb
>
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>  
>

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