[BitTorrent] sha-0 and md5 breaks

Bram Cohen bram at bitconjurer.org
Tue Sep 7 10:32:48 EDT 2004

BitTorrent currently uses sha-1, a different algorithm from the recently
broken sha-0 and md5. While those attacks raise some concern, there aren't
critical right now, since -

* The attacks don't (yet) work on sha-1.

* The attacks are birthday attacks, not pre-image attacks, which are what
would be what would cause a security concern for BitTorrent.

That said, I'm going to hold off on deciding on what secure hash algorithm
to use for bt2 until the dust settles. My current best guess is it will be

In other bt2 news, I'm fairly certain at this point that the one-the-wire
piece size will be 16k, as a hard-coded magic number. The larger piece
size will probably also be a magic number, but I'm less sure of that
value, probably somewhere around a quarter meg. Yay magic numbers.

Also, for the merkle hash trees the very last piece will always be padded
out with zeros rather than be shorter than everything else, because I'm
sick of all the stupid edge cases that was causing. The padding will of
course not be actually sent down the wire.

Some of you may have noticed that work on ipv6 support has stalled. That's
because ipv6 addresses are bigger than ipv4 addresses, and the bandwidth
taken up by sending those around can hurt trackers, which are already the
bottleneck on the whole system. Since it's mostly something I want to
support just on principle with no immediate benefit, and it appears to be
a real problem, it's been put off until later.


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