Passphrase entropy (was Re: Symmetric encryption)

[Chris De Young]

> i have several passphrases that are 20+ characters. 22 characters of 
> uppercase, lowercase, numbers and spaces is *stronger* than 128 bit.
> 
> that's not counting the full range of characters that can be used... 
> assuming that 95 characters are generally suitable (read: safe) for use in 
> passphrases, it only takes a theoretical 19.5 characters to equal 128 bits 
> and 39 characters to hit 256 bits.

How does that work?  (Ok, this is probably a dumb question, but...) 

I had thought that English has only somewhere around 1.5 bits worth of
entropy per character.  A passphrase certainly could have more than
that because it's not necessarily real English, uses a wider character
set, and so on... is that difference really enough?  19.5 8-bit
characters is 156 bits; that seems (intuitively, which granted can be
misleading) to be getting closer to real randomness than a passphrase
would allow.  At least, any passphrase that someone could
remember. :-)  It's only 3.5 characters longer than 128 bits, after
all. 

Cheers,
Chris
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