# Re: constructing a specified hash function

*From*: "bobic" <fbloveu@xxxxxxxxxxx>*Date*: 31 Oct 2006 23:58:19 -0800

David Wagner wrote:

bobic wrote:

Hi, all. Recently, I want to construct a specified hash function, which

satisfies

y=h(x), x can be any string, and y must belong to [g^a mod p,g^{a+1}

mod p,...,g^b mod p], and no body knows the discrete logrithm of y

based on g. a,b are known.

How about h(x) = G * g^H(x), where G = g^a mod p and

H(x) = SHA256(x) mod (b-a)/ Does that do it? Note that you will

have to publish the values G and b-a, but you nobody needs to know

the discrete log of G to the base g, nor the discrete log of y to

the base x.

Out of curiousity, why do you want this hash function? Do you

have any particular security goals for it?

No! :( It cannot catch the goal.

In your way, though nobody knows the discrete logrithm, everybody knows

that h(x) is the product of a constant and a number with a known

discrete logrithm.

If the constant changes to a variant, it is ok!

Or if the number changes to be with a unknown discrete logirhtm, it is

also ok!

Thanks for your reply!

.

**Follow-Ups**:**Re: constructing a specified hash function***From:*David Wagner

**Re: constructing a specified hash function***From:*Peter van Liesdonk

**References**:**constructing a specified hash function***From:*bobic

**Re: constructing a specified hash function***From:*David Wagner

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