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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