#15 Threshold pool feature

[Trek Star]

This patch add the threshold feature (and configuration
option) as described in "From a Trickle to a Flood:
Active Attacks on Several Mix Types"
<http://freehaven.net/doc/batching-taxonomy/taxonomy.ps>
to improve the minimum anonymity.

Now RATE operates on all messages minus the POOLSIZE
and not on all messages present in the pool. Also a
sanity check in RATE option is maked and the default
value is changed to 90 (from 95). It should be smaller
(like 70-40).

NOTE: I don't have all the current CVS tree, then i
can't test it.

[Trek Star]

patch for the current CVS tree (20021002)

[Trek Star]

second patch for the CVS tree (20021002)

[Trek Star]

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I'm not confident with my previus patch, because i don't
understand all of the code; here attached a new smaller
version that only add threshold and don't make any sanity
check; THRESHOLD is not exactly the threshold to send pool,
but is threshold-1 (because we make a <= check and not a <
in pool:432).

Now what I don't understand in pool.c:445:

max = size * RATE / 100; // here RATE is calculated on
all messages
if (max < 0) // this check is useful ONLY if RATE is
negative.. true?
max = 1;

well if we make a sanity check on RATE, we can remove the
check (max < 0); then I must understand if RATE should be
applied on all messages (size) or only to size-POOLSIZE
messages.

Finally some sanity check could be done with a special
read_conf_i() that takes minimal and maximal values too.

[Peter Palfrader]

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Roger did not really convnce me that the additional
threshold parameter increases anonymity as an operator could
always just increase the poolsize anyway.

Therefore I do not think changing and complicating our
mixing strategy will have any bonus and reject the patch.