[BitTorrent] Back to Merkle Hash Trees...

Elliott Mitchell ehem at m5p.com
Fri Feb 11 22:45:16 EST 2005


>From: Justin Cormack <justin at street-vision.com>
> > As to collisions. You've claimed knowledge of Cryptography, so you should
> > know the numbers just as well as I do. 2^80th blocks before you hit a 50%
> > chance of a collision with SHA1. We're discussing block sizes of 16KB or
> > 32KB. So the combined torrents will have to be 2^96 bytes for a 50%
> > chance. More than 65,536 yottabytes (the listing of prefixes I found
> > topped out at 10^24/2^80/yotta).
> > 
> > Such a large collection is a vague concern for a super-huge cache in
> > maybe 30 years time. By that point we'd be worrying about having to go to
> > something larger than 64-bit data types in the protocol to handle so much
> > data. I'll admit this is something to be aware of and a concern for the
> > future, but the chance of two leaves on two distinct torrents on the
> > whole of the planet Earth having the same hash value within the next ten
> > years is pretty well zero. You're okay to mention it as a concern for the
> > future, but your Cryptography reputation with me it fading for suggesting
> > it is a severe concern now.
> 
> I am talking about the case where the leaf data is the same hence the hash
> the same for 2 different torrents.

Two things. First, thought possible it is quite unlikely. Large files
tend to be compressed data which is unlikely to share with other files.
Second, if it happens this is irrelevant. A good client will notice the
identical data and only transfer the piece once (or in the worst case
transfer it twice, and as it knows what requests are out, what harm is
done?).


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