[BitTorrent] bt2 protocol features

Elliott Mitchell ehem at m5p.com
Sun Jun 6 17:54:30 EDT 2004

> From: Bram Cohen <bram at bitconjurer.org>
> Elliott Mitchell wrote:
> > Related thing, how about peers refering to pieces *strictly* by hash?
> It turns out that the size of those hashes can be fairly significant, and
> the program logic winds up doing a lookup to translate back from hash to
> index on the other side anyway.

That translation will need to be done at the final destination, but
helper clients need not do that, and can thus remain oblivious.

For bandwidth savings you can only use some of the hash. Though this
makes collisions a real danger, those are easily detectable (scan the
torrent ahead of time, when a cache is loading a new file it notes them)
and only those pieces will need to be referenced by the full hash value
(or merely a little bit more of the hash value). I'm also unsure the
bandwidth costs are that large an issue anyway; maybe 10 messages with 16
additional bytes of data for one 16K piece message?

Vitenka, I'm well aware of the Birthday Attack. For caches this is a
_concern_, not an actual issue yet. 2^80 pieces is one heck of a large
cache (even with disk space and bandwidth increasing fast). If you're a
*very* large government agency, this is in the realm of possibility for a
system that will last 20 years. But this is why SHA1's bigger brothers

(\___(\___(\______          --=> 8-) EHM <=--          ______/)___/)___/)
 \   (    |         EHeM at gremlin.m5p.com PGP 8881EF59         |    )   /
  \_  \   |  _____  -O #include <stddisclaimer.h> O-   _____  |   /  _/
    \___\_|_/82 04 A1 3C C7 B1 37 2A*E3 6E 84 DA 97 4C 40 E6\_|_/___/

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