>> After this fact the colliding block is itself very interesting,
>> aand it is also very likely that theis block will be stored and
>> archived just for this reason.
>
> Which will increase the chance of a failure ;-O
by how much? the fact that something *could* happen is often
meaningless. what is
lim{x->∞} 1+1/x?
the εδ argument made to proove the result always says that
if i control the input of a function this much i can control the
output that much. in the real world there are limits (ha!)
to how small or large something can get before it is practically
infinite or zero. this is because, .e.g., there is no such thing as
1e24 bytes of storage.
theoretically, i don't think a collision by itself would be all that
interesting. the number of possible bit patterns in, say, 8k blocks
would be
2^(8*8192).
while the number of possible bit patterns in a sha1 hashs
2^(8*20).
assuming an even distribution, there would be
ceil(2^(8*8192)/2^(8*20) - 1) =
2^(8*8172) - 1
collisions on average per hash value. (that's ~1.37e4095, btw.)
the only way a collision would be interesting is if it exposed
a weakness in sha1.
- erik
