phc-discussions - Re: [PHC] Re: Some KDF stumbling blocks, plus Common

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Date: Sat, 13 Dec 2014 21:34:43 +0100
From: Thomas Pornin <pornin@...et.org>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Re: Some KDF stumbling blocks, plus Common

On Sat, Dec 13, 2014 at 04:28:49PM +0000, Adam Back wrote:
> I think you are saying this would be slower, take more storage than the
> method I describe.  Do you see some advantage?

In my method, the user needs not know the factorization of n at any
point. This means that n can be shared by all users; the 300 blinding
pairs can be shared as well. Thus, they can be hardcoded in the
application, without requiring per-user storage of such data.

> I think your method loses information theoretic security because the 
> way you generate the blinding factor has number of values 
> c(300,150)<|n|.  With my method b is uniform random from [0,n).

Ah, I see: the _delegation_ is information-theoretic secure. However,
if the attacker can get a copy of the resulting hash value (the
classic situation with a server authenticating user against a
database of hashed passwords), then we are back to "normal" security
(if you know the factorization of n, you can compute square roots
modulo n).

	--Thomas Pornin