phc-discussions - Re: [PHC] Re: Mechanical tests

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Date: Sat, 05 Apr 2014 19:04:16 +0000
From: "Poul-Henning Kamp" <phk@....freebsd.dk>
To: discussions@...sword-hashing.net, Peter Maxwell <peter@...icient.co.uk>
Subject: Re: [PHC] Re: Mechanical tests

In message <CA+aY-u64u8ULtYRwMd6qZaiL0-qUC0OS8umKUSYksyJue0CzeQ@...l.gmail.com>
, Peter Maxwell writes:

>Ok, to avoid this being an intractably long evening, please define
>"entropy".

My "private" definition of entropy, in the password scrambling context,
is the numer of bits which an attacker has to attack brute force.

How the bits are distributed, encoded or represented is not material.

If the user choses a password of "password", there isn't much we
can do to protect it from attack, because it contains only a couple
of bits of entropy. 

But if the user does "strings -16 < /dev/random" and uses a password
of "0Ju>a,6`%/NCxEX@$C", then the scrambler must ensure that it takes
on average (96^16 / 2) attempts before there is a hit.

This is not a/the strict scientific definition, but neither does
it conflict with that definition, and I have found it very effective
when I have had to explain the role of a password scrambler to
people.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@...eBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.