Length of passphrase beneficial?

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Feel free to skip to 'Actual Question:' below.

I am one to succumb to the assumed benefits of overkill.  I like
the fact that everyone's use of crypto can cause each
individual transmission to become less suspicious to prying
eyes.  I would love knowing that the govt. spent billions of CPU
cycles on one of my transmissions only to find my softball
schedule.  We could lure them by making our subject lines
'Fertilizer-Bomb Recipe' or 'CHILDPORN.GIF Attached', not
condoning either, of course.

I have a 2048-bit PgP key and pseudorandom a/n character
generator, from which I chose a large passphrase similar to:

f4VnI1G1mGcwTZ1vGoyPwN4NLojF8Ee9ff1aicOGn87x0nwwHhJUo6XSYKEawRne
(Yes, cut-n-paste, but my only in-house threat is my wife.)

Actual Question:
Does the length and randomness of a passphrase contribute at all
to the overall security of a cryptosystem?

Thanks in advance!

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vagab0nd@sd.cybernex.net
http://ww2.sd.cybernex.net/~vagab0nd/index.html
Visit web page for public key.

On Sun, 21 Jul 1996, Erle Greer wrote:
[snip]
> I have a 2048-bit PgP key and pseudorandom a/n character
> generator, from which I chose a large passphrase similar to:
>
> f4VnI1G1mGcwTZ1vGoyPwN4NLojF8Ee9ff1aicOGn87x0nwwHhJUo6XSYKEawRne
> (Yes, cut-n-paste, but my only in-house threat is my wife.)

Ugh.  Erle, you might want to check out the Diceware method for generating
passphrases.  It lets you generate a lengthy passphrase that is random and
that you might actually be able to remember :)

I don't have a URL handy, but if you go to Altavista and search for
"diceware" you should find it...  It might be indexed at Yahoo, too...

> Actual Question:
> Does the length and randomness of a passphrase contribute at all
> to the overall security of a cryptosystem?

Actual short answer: yes :)

Look for the passphrase FAQ, for a better explanation than I can give...

______________________________________________________________________
Rich Burroughs  richieb@teleport.com  http://www.teleport.com/~richieb
See my Blue Ribbon Page at http://www.teleport.com/~richieb/blueribbon
New EF zine "cause for alarm" - http://www.teleport.com/~richieb/cause

Rich Burroughs <richieb@teleport.com> wrote:

   > Actual Question:
   > Does the length and randomness of a passphrase contribute at all
   > to the overall security of a cryptosystem?

   Actual short answer: yes :)

Answer in his particular case, however: no

-- 
Paul Foley <mycroft@actrix.gen.nz>       ---         PGPmail preferred

	   PGP key ID 0x1CA3386D available from keyservers
    fingerprint = 4A 76 83 D8 99 BC ED 33  C5 02 81 C9 BF 7A 91 E8
----------------------------------------------------------------------
A bird in the bush usually has a friend in there with him.

Erle Greer writes:
> I have a 2048-bit PgP key and pseudorandom a/n character
> generator, from which I chose a large passphrase similar to:
> 
> f4VnI1G1mGcwTZ1vGoyPwN4NLojF8Ee9ff1aicOGn87x0nwwHhJUo6XSYKEawRne
> (Yes, cut-n-paste, but my only in-house threat is my wife.)
> 
> Actual Question:
> Does the length and randomness of a passphrase contribute at all
> to the overall security of a cryptosystem?

The passphrase only does one thing for you, which is protect your
keyring in case someone gets it. Since you keep the passphrase on
line, you are actually less secure than if you used a memorable
phrase.

BTW, since the passphrase is used to hash into an IDEA key, more than
128 bits of input entropy would be wasted.

Perry

Erle Greer wrote:

| f4VnI1G1mGcwTZ1vGoyPwN4NLojF8Ee9ff1aicOGn87x0nwwHhJUo6XSYKEawRne
| (Yes, cut-n-paste, but my only in-house threat is my wife.)
| 
| Actual Question:
| Does the length and randomness of a passphrase contribute at all
| to the overall security of a cryptosystem?

	Not directly.  The SECRECY of a passphrase does contribute.
If you do not provide it to your attacker (in the form of a file on
your computer which a Microsoft Worm macro carried in a message might
send out, that the search party might find, etc), then the length and
difficulty of guessing protect you.

	For random text (I'll assume you're rolling dice) like that,
figure you get about 5 bits of entropy per character.  Your PGP secret
key is IDEA encrypted with a 128 bit key, so you don't need any more
than 30 characters of random text to get a passphrase space (or
universe, to use Tim's metaphor) thats harder to search than the
keyspace.

	I think its a poor assumption that your home won't be searched
if you're doing something that makes you want a 2048 bit key.  A
thousand bits of keylength should be good enough for most things that
don't need to stay secret more than 5-10 years.

Adam

-- 
"It is seldom that liberty of any kind is lost all at once."
					               -Hume