[Cryptography] Entropy is forever ...

Bill Cox waywardgeek at gmail.com
Fri Apr 24 19:12:38 EDT 2015

On Fri, Apr 24, 2015 at 3:59 PM, Bill Cox <waywardgeek at gmail.com> wrote:

> On Tue, Apr 21, 2015 at 9:54 PM, <dj at deadhat.com> wrote:
>> I've never seen the term surprisal used in any crypto math paper. I don't
>> remember seeing it all before this email chain, but I have far from
>> perfect memory.
> I use surprise as described in this document:
>     http://www.av8n.com/turbid/paper/turbid.htm
Here's a good Turbid quote:

"Let’s be clear: The surprisal is a property of the symbol, while the
entropy is a property of the source. In particular, the entropy is the
appropriately-weighted average surprisal.

For a long string, we expect the surprisal of the string to converge to the
entropy of the source times the length of the string.

Beware: In the literature it is common to see the word “entropy” used in
places where surprisal would make more sense. In particular, the code and
documentation for the Linux /dev/random talks about depleting the “entropy”
in its buffer."

We can't fix how everyone else uses the word entropy.  Linux has an entropy
pool... end of story.  However, we can be more clear in our discussions of
TRNGs, randomness, and entropy.  In common usage, it is simpler to say
entropy, because no one knows what you're talking about if you say
surprisal :-)

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