Re: Using two one-way-functions => no one-way-function anymore

Stephan Wehner wrote:
I think one of the basics of one-way-functions in use, is that pairing
them is not a good idea.

In other words, if f1 and f2 are two one-way-functions which one deems
secure, then the function
x -> ( f1(x), f2(x) ) might be much less secure.

Is this "true"?

Yes. For instance, suppose that f1(x) maps 512-bit inputs to 512-bit
outputs as follows: f1(x) = (SHA256(x), lefthalf(x)). Suppose that f2(x)
= (SHA256(x), righthalf(x)). Then both f1 and f2 are one-way functions
(under suitable assumptions), yet f(x) = (f1(x), f2(x)) is not one-way.

Is there a reference/name for this phenomenon?
Is there a real-life example of exposure caused by this?

I don't know.
.