[BitTorrent] SHA-1 broken

Justin Cormack justin at street-vision.com
Wed Feb 16 09:06:36 EST 2005


> 
> 
>  
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> BitTorrent is a bit more secure about its overall use of SHA1 than
> just a basic hash since the hashed data has a known size. With that
> known size, the scope of data that can cause a colission is a LOT
> smaller.
> 
> The hash blocks are always fixed within a torrent (except for the
> final block) at a specified value within the .torrent metadata. When
> a hash algorithm finds collisions on two different-sized pieces of
> data, it is not as much of a concern for users using those algorithms
> for file hashing since a same-sized collision is required.
> 
> Invalidating hash algorithms for file verification operations is a
> lot harder than just invalidating them for general use because of the
> fixed-size restriction placed.
> 
> The amount of CPU time required to break SHA1's use in BT is far more
> than the amount of CPU time used to break SHA1's use in
> variable-sized messages.
> 
> SHA1 is still more secure in BT's use than general use because of the
> amount of information already known about the pieces.
> 
> The Edonkey implementation of MD4 is still more secure than the base
> MD4 implementation for similar reasons.
> 
> We don't need to worry at all unless a same-sized collision has been
> found.

The collisions found for md5 were same size. Because SHA1 has the size
hashed (at the end) it is quite likely they would be same size, though
not necessarily the case.

Some Merkle tree based BT versions might not have all messages of a known
size.

j

 


 
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