[BitTorrent] Merkle, URLs, etc

Joseph Ashwood ashwood at msn.com
Fri Mar 4 20:41:21 EST 2005

----- Original Message ----- 
From: "John Hoffman" <theshadow at shambala.net>
Subject: Re: [BitTorrent] Merkle, URLs, etc

> {SHA-1 is] not THAT broken, especially not for
> BitTorrent's purpose.  We may also see some new 20-byte hashes that
> don't have SHA's weakness.  "160 bits ought to be enough for everyone." 
> ;-)

It is unlikely that there will be any good new 160 bit hashes. The 2^80 
security offered by them has been considered just barely good enough for a 
decade, and an increasing number of cryptanalysts (including myself) are 
pushing that with the breaks seemingly coming faster and faster now we might 
as well just jump to the stronger hashes now, and prepare for a quick jump 
in the future. If you have your heart set on a 160 bit hash though take a 
look toward RIPEMD-160, it is the only other 160-bit hash that has been 
beaten on by cryptanalysts, then look to Tiger, and SHA-2 series. You could 
also simply take a larger hash and trim the output, so trimming SHA-1 to 138 
bits would balance the output with the most recent attack (although there is 
a chance it will open up new attacks, but that is highly unlikely). While 
everyone has their favorite method of doing this (mine if to drop the last 
bits) the absolute truth is that if the hash is strong, nothing you can do 
to the output will weaken it (except by the amount shortened) so feel free 
to use 1M time Whirlpool (using the previous output as an IV for the new 
hash), XOR all the hashes together, and then trim off bits, you won't affect 
the strength of the hash for good or bad.

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