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Since both Randpool and HMAC_RNG fed the input into a MAC anyway, this
works nicely. (It would be nicer to use tr1::function but, argh, don't
want to fully depend on TR1 quite yet. C++0x cannot come soon enough).
This avoids requiring to do run length encoding, it just dumps everything
as-is into the MAC. This ensures the buffer is not a potential narrow pipe
for the entropy (for instance, one might imagine an entropy source which
outputs one random byte every 16 bytes, and the rest some repeating pattern -
using a 16 byte buffer, you would only get 8 bits of entropy total, no matter
how many times you sampled).
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Combine the fast and slow polls, into a single poll() operation.
Instead of being given a buffer to write output into, the EntropySource is
passed an Entropy_Accumulator. This handles the RLE encoding that xor_into_buf
used to do. It also contains a cached I/O buffer so entropy sources do not
individually need to allocate memory for that with each poll. When data
is added to the accumulator, the source specifies an estimate of the number
of bits of entropy per byte, as a double. This is tracked in the accumulator.
Once the estimated entropy hits a target (set by the constructor), the
accumulator's member function predicate polling_goal_achieved flips to true.
This signals to the PRNG that it can stop performing polling on sources,
also polls that take a long time periodically check this flag and return
immediately.
The Win32 and BeOS entropy sources have been updated, but blindly; testing
is needed.
The test_es example program has been modified: now it polls twice and outputs
the XOR of the two collected results. That helps show if the output is consistent
across polls (not a good thing). I have noticed on the Unix entropy source,
occasionally there are many 0x00 bytes in the output, which is not optimal.
This also needs to be investigated.
The RLE is not actually RLE anymore. It works well for non-random inputs
(ASCII text, etc), but I noticed that when /dev/random output was fed into
it, the output buffer would end up being RR01RR01RR01 where RR is a random
byte and 00 is the byte count.
The buffer sizing also needs to be examined carefully. It might be useful
to choose a prime number for the size to XOR stuff into, to help ensure an
even distribution of entropy across the entire buffer space. Or: feed it
all into a hash function?
This change should (perhaps with further modifications) help WRT the
concerns Zack W raised about the RNG on the monotone-dev list.
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