[tahoe-lafs-trac-stream] [tahoe-lafs] #1813: Choice of tree-hash

tahoe-lafs trac at tahoe-lafs.org
Sat Sep 22 10:41:41 UTC 2012


#1813: Choice of tree-hash
------------------------------+------------------------
     Reporter:  CodesInChaos  |      Owner:  davidsarah
         Type:  enhancement   |     Status:  new
     Priority:  minor         |  Milestone:  undecided
    Component:  unknown       |    Version:  n/a
   Resolution:                |   Keywords:  newcaps
Launchpad Bug:                |
------------------------------+------------------------

Comment (by CodesInChaos):

 The CPU overhead of tree-hashing depends a lot on the particular hash
 function, and how you use it. In particular the interaction of the
 blocksize, hashsize, padding and finalization. But I estimate that
 it's only about 15% compared to a flat hash, for a leaf size of 1 KiB.
 So the increase in software complexity using a higher branching factor
 doesn't seem worthwhile. If those 15% worry you, I'd rather double the
 leaf size.

 The official Skein treehash skips finalization for anything but the
 root, since finalization is only required for full pseudo-randomness,
 and not for pre-image/collision resistance. This halves the overhead
 from 15% to 8%.

-- 
Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1813#comment:2>
tahoe-lafs <https://tahoe-lafs.org>
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