Today's Dumb Question

[Matt Thomlinson]

David Merriman <merriman at metronet.com> writes:

What Happens If, instead of using prime numbers or logarithms for the
basis for a public-key crypto system, we instead generated out public key
thus:

1> pick an arbitrary bit stream (large [pseudo?]random number, binary
representation of selected chunk of text or data file, etc).  1024 bits or
more (in 256 bit chunks?)
2> enter a passphrase
3> XOR the bit stream with the binary representation of the passphrase,
cycling the passphrase as necessary.  This makes the 'large' component of
our public key.
4> hash the passphrase to 128 or more (in blocks of 64?) bits.  This makes
the 'small' component of the public key.

5> We then use these components as in normal public-key algorithms.

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(matt says:)

Okay, you're forgetting one thing. In public key systems, the two 
numbers you have are related -- the algorithm you use needs a pair of 
numbers that create a function and inverse function pair.

The pair you suggest above [((random #) xor (passphrase)), hash(passphrase)] 
have nothing in common. Good, as you point out, since you can't create 
one from the other, but also bad for a public key system, since you can't 
use one to decrypt what the other encrypted (they're not inverses)!

mt

Matt Thomlinson                               
University of Washington, Seattle, Washington.      phone: (206) 548-9804
Check my home page -- ftp://ftp.u.washington.edu/public/phantom/home.html 
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