Pass Phrases

Maybe I'm just being a little dense about this.

If I am the only person who knows what pair of texts I'm using and what
permutation algorithm, and what the random number I'm going to salt the
pass phrase with, and where I'm going to put the random digits, how is it
insecure?

Say I use the following two key phrases

The Quick Brown Fox Jumps Over The Lazy Dog
Oh Be A Fine Girl Kiss Me

I decide my method is going to be first letter of each word of the first
phrase, last letter of each word of the second phrase

I get

THQEBAFLJSOMTHLEDA

I then throw in 1701 as follows

1701THQ1EBA7FLJ0SOM1THL1EDA7

Without knowing the phrases, method, or number, what makes this insecure?

I'm not deliberately trying to be dense, I'd like to know why I shouldn't use
this sort of mnemonic method to remember the pass phrase.

jpb@gate.net

> 
> Say I use the following two key phrases
> 
> The Quick Brown Fox Jumps Over The Lazy Dog
> Oh Be A Fine Girl Kiss Me
> 
> I decide my method is going to be first letter of each word of the first
> phrase, last letter of each word of the second phrase
> 
> I get
> 
> THQEBAFLJSOMTHLEDA

Really? How about

  THQEBAFEJLOSTELHDE

> 
> I'm not deliberately trying to be dense, I'd like to know why I shouldn't use
> this sort of mnemonic method to remember the pass phrase.

Maybe it's not really an effective mnemonic after all? Somebody's .sig file
says "one man's mnemonic is another man's crypto." Can you really type this
from memory using the key phrases without writing them down?

(Based on your first attempt, some would say "apparently not".)  ;-)

=D.C. Williams

> 
> jpb@gate.net
>

On Mon, 4 Jul 1994, D.C. Williams wrote:
> > The Quick Brown Fox Jumps Over The Lazy Dog
> > Oh Be A Fine Girl Kiss Me
Lets say you pick these from a set of books.

All this does is give you a larger dictionary, with say 10^6 vs 10^3 entries.
It could be even longer if you use fragments of sentences.

This means you will need half the number of sentences you needed words for.

Some one might be clued in by the fact that your books show considerable 
use at certain pages.

The mixing up stuff adds bits, but not that many, perhaps 10 if you 
really do a good job.

so I would say you have 10^5(4 digit number) *(10^6)^2(two sentences) 
*10^3(choosing the nth letter, or stagering) or about 10^20.

Seems ok to me, about 60 bits.

If I bust you and look at your books, though, you could be screwed.
This is not much of a concern in a reasonably free country, but....
Also, if you don't have your books, you can't get into your computer.

Roger.

> 
> On Mon, 4 Jul 1994, D.C. Williams wrote:
> > > The Quick Brown Fox Jumps Over The Lazy Dog
> > > Oh Be A Fine Girl Kiss Me

Not exactly. Note the extra >>. Mine was a reply to the original
post, where the original proposal/question was offered.

I disavow any credit for the pass phrase mnemonic proposed by Joseph
Block. I only pointed how difficult it apparently is to use.

=D.C. Williams

Joseph Block writes:

> Maybe I'm just being a little dense about this.
> 
> If I am the only person who knows what pair of texts I'm using and what
> permutation algorithm, and what the random number I'm going to salt the
> pass phrase with, and where I'm going to put the random digits, how is it
> insecure?
...

> I then throw in 1701 as follows
> 
> 1701THQ1EBA7FLJ0SOM1THL1EDA7
> 
> Without knowing the phrases, method, or number, what makes this insecure?
                                                                  ^^^^^^^^^

It's not that this password is "insecure" on the face of it, it's that
the password has much less entropy than its 25 or 30 characters would
otherwise suggest. Dividing passwords into "secure" and "insecure"
is not very useful...intstead, one talks about entropy, a measure of
randomness or unpredictability.

The "structure of password space" is rich and crufty, filled with
nooks and crannies of easily-guessed (relatively) n-bit passwords in a
sea of nearly unguessable passwords. The trick is not let human
psychology lead you into picking a relatively easy to guess
passphrase. 

It may seem "really hard to guess" a password that takes the opening
lines of "Atlas Shrugged" and twiddles and salts them a bit, but
"opening line" attacks may be programmed to run in a few seconds on
the Crays that do these sorts of things. Entropy that just isn't there
can't be conjured up.

(As usual, I'm not saying this is a pressing concern. I still use an
11-character nonsense word as my password. This partly reflects my
judgement on where the attacks on my PGP use are likely to be.)


--Tim May


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