[tahoe-dev] UEB hash size

[Shawn Willden]

On Sunday 12 July 2009 07:37:12 pm Zooko Wilcox-O'Hearn wrote:
> I really don't like the idea of adding an insecure mode, if we can
> help it.  Could you explain more -- perhaps even quantify -- what
> impact the size of the cap has on your system?  Hopefully there's a
> solution which is both secure and efficient.

I'm not suggesting an "insecure" mode; I don't think reducing the UEB hash 
size from 32 bytes to 16 bytes (or even smaller) affects security at all.  In 
fact, as far as I can see the UEB hash is entirely redundant -- it's a sort 
of an integrity optimization that allows shares to be checked more quickly, 
but as far as ensuring the integrity of the file as a whole, it's redundant, 
because the read key is derived from the content hash, so you can use it to 
verify integrity.  And including the whole 32 bytes is further redundant, 
when a truncated version would do just as well.

As for the impact of the size, the smaller the URI data, the more read cap 
index entries I can pack into an index file in the grid-based burst trie 
(without making the files so big they're slow to upload and retrieve), and 
since those files have to get uploaded and downloaded frequently, tighter 
packing improves performance.

	Shawn.