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MathGroup Archive 2004

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Re: Reverse Hash Function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg52909] Re: Reverse Hash Function
  • From: David Bailey <dave at Remove_Thisdbailey.co.uk>
  • Date: Thu, 16 Dec 2004 03:41:17 -0500 (EST)
  • References: <cpp1u5$b01$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Linda Pan wrote:
> Dear experts,
> 
> I met a problem, and I don't have experience to solve it. Thus, please give 
> me a help.
> 
> 
> 
> My question is:
> 
> 
> 
> I have a hash function: h().
> 
> I have a plain text: A
> 
> 
> 
> So I can get a hash value B = h(A)
> 
> 
> 
> Suppose that I have a reverse hash function: h'()
> 
> 
> 
> Then I can get a fade plain text A'= h'(B), probably A' is not equal to A, 
> but it doesn't matter!
> 
> 
> 
> I want to ask: Is it possible to build a reverse hash function h'() so that 
> h(h'(B)) is equal to B?
> 
> 
> 
> I do appreciate if you could answer me or send me a hint.
> 
> 
> 
> Merry Christmas!
> 
> 
> 
> P. L.
> 
> 
> 
Hi,

Think of the number of bits in your original text and in the resultant 
hash value. Normally a lot of information is lost in a hash function - 
so you can't reverse the process. Algorithms that use hash functions 
always make provision for the possibility that several things hash to 
the same number.

David Bailey
dbaileyconsultancy.co.uk


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