Compression of random binary data

[Chris Angelico]

On Sun, Oct 29, 2017 at 1:18 PM, Gregory Ewing
<greg.ewing at canterbury.ac.nz> wrote:
> You're missing something fundamental about what
> entropy is in information theory.
>
> It's meaningless to talk about the entropy of a single
> message. Entropy is a function of the probability
> distribution of *all* the messages you might want to
> send.

Which is where a lot of "password strength" confusion comes from. How
much entropy is there in the password "U3ZINVp3PT0="? Strong password
or weak? What about "dark-next-sure-secret"?
"with-about-want-really-going"? They were generated by, respectively:
double-MIME-encoding four bytes from /dev/random (32 bits of entropy),
picking four words from the top 1024 (40 bits), and picking 5 words
from the top 64 (30 bits). But just by looking at the passwords
themselves, you can't tell that.

ChrisA