Re: [gentoo-dev] (crazy?) proposal to reduce load and disk on mirrors

[Fred Van Andel]

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On July 24, 2003 12:30 am, you wrote:

>
> I'd be VERY careful with this.
> http://www.rsasecurity.com/rsalabs/faq/3-6-6.html

I am familiar with this.  But in practice its not an issue.

Each chunk of the file would be protected by a MD5 hash and then the
file as a whole is protected by a different MD5, any change to the data
that passes the MD5 protecting one chunk would fail the MD5 protecting
the file as a whole.

I suspect the referenced machine would work only looking at hashes that
belong in a reduced domain, ie the upper X bits being zero. In our
application that is not an option. Our problem is not a birthday attack
but rather a search for a small set of possible values.

As well anyone who has the resources to spend on creating a machine such
as the one described would have much more profitable uses for it.

I am thinking of protecting each chunk with a smaller hash (say 64 bits)
and use it just to detect transmissions errors. A full MD5 or SHA1
would protect the file as a whole.  This would reduce the bandwidth
requirements of the central tracker.  In the event that a bad chunk
passes the reduced hash test then the full file hash would cause the
entire download to be rejected rather than just one chunk.  So in the
worst case a malicious user could cause a lot of failed downloads but
no corrupted files.
- --
Fred Van Andel
fava@g.o
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