[bittorrent] Why are blocks smaller than pieces?

Matt Pearson 404emailnotfound at gmail.com
Tue Sep 11 12:08:39 EDT 2007


However, these time complexities are for birthday collision attacks,
where the person posting the torrent could find two different pieces
that have the same hash.  This is different from a preimage attack of
finding something that hashes to a specific hash.  The complexity of
that would be 2^160 if SHA-1 was perfect, and even if it has been
reduced it's still probably quite high.  So as long as you trust the
uploader (which you have to anyway...), there's no problem here.  The
birthday attacks tend to be used more for cryptographic signing.

On 9/10/07, Thad Ward <coderjoe at grnet.com> wrote:
> It appears that, as of August 17, 2005, we're down to a
> collision-finding complexity of 2^63. Yes, this is still rather high for
> the time being, but it will continue to go down with time.



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