Re: Algorithms used in PGP 5.x; Relative strength of Blowfish

[David Honig <honig@otc.net>]

At 11:36 PM 2/9/98 -0500, Alan Tu wrote:
>anything besides 3DES and IDEA, what are their relative strengths to 3DES
>or IDEA? 

Before you can measure something's strength, you have to define
the stress.  Are you twisting it or bending it?  Compressing or pulling?

Are you trying an exhaustive keyspace search?
Are you trying to find a weak key?  Or some information leaking through?
Can you pick the plaintext to put through the system, and twiddle bits?
Or does your adversary only have access to some plain + cipher pairs?

None of the algorithms has been shot out of the water.  All of them annoy
statists, since
they can use more bits than is convenient to search through.  That's why
40-bit
anything is laughable.  

>Also, what's the relative strength of Blowfish to 3DES or IDEA?  Its key
>length is variable, as I recall.  Could someone tell me the range?

3DES uses 3 x 56 bits of user key, 56 bits being the DES keylength, defined
by the Feds
by neutering IBM's Lucifer system.  DES uses some fixed tables with unknown
(but upon investigation, apparently good) properties.

Blowfish uses over 4Kbytes of internal key, derived from a user-key
of up to 448 bits by mixing the user-key with random digits in a
computationally
expensive way that slows keysearch.  

Both Blowfish and DES are based on Feistel networks, ie, iterations
of rounds containing permutations, substitutions, addition and xors,
including data-dependant operations (ie, one fraction of the input data
(as well as the user's key, or derivations thereof) modifies the other
fraction in each round).  The 'fractions' are usually halves, but
unbalanced feistel networks have been studied too (e.g., McGuffin
algorithm).  BF was
designed with modern 32-bit cpus (and their cache sizes) in mind.
3-DES is a temporary kludge.

IDEA doesn't use tables, but uses the shift operator which the others
don't.  This is 
newer (less historical scrutiny) than DES or BF's structure.  IDEA is
patented, the
others aren't.

You should find the papers on these, there's descriptions and code
on line.




------------------------------------------------------------
      David Honig                   Orbit Technology
     honig@otc.net                  Intaanetto Jigyoubu

		Lewinsky for President '2012