Re: Hash functions' strength

From: Bill Unruh (unruh_at_string.physics.ubc.ca)
Date: 06/05/04 

Date: Sat, 5 Jun 2004 18:35:48 +0000 (UTC)

"Roger Schlafly" <rogersc1@mindspring.com> writes:

]"Ivan Voras" <ivoras@__geri.cc.fer.hr> wrote
]> lacking :) ). Since hash functions are one-way, could, for example, the
]> CRC function, implemented in 160 bits be as safe as SHA-1? (I'm not
]> proposing to do that, I'm just interested :) )

Two different definitions of one-way. when people say that a hash is one
way, they mean that there is no unique input for any output. In the reverse
direction it is not a function.
 
When one talks about one way functions on the other hand, one can also mean
a function which is easily calculated by the inverse, which does exist is
very difficult to calculate.

All Hashes are one way in the first sense. crytographic functions are one
way in the second (as well as the first if they are hashes, in which case
one means find any one of the inverse values, not all of them)
CRC is one way in the first sense but not in the second. It is easy to find
some input which produces the given output.

]It takes a lot of cleverness to put in the right one-way properties.
]Typical CRC functions are not one-way. That is, it is easy to
]find an input to match a given output.

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