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Date: Tue, 16 Dec 2014 13:46:16 +0100
From: Thomas Pornin <pornin@...et.org>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Re: Some KDF stumbling blocks, plus Common

On Tue, Dec 16, 2014 at 04:57:57AM +0000, Adam Back wrote:
> Recall g is a fixed base, y=g^e mod n is stored at setup (when p and q
> is known), and blinded work is r=m*g^b mod n , the computation is
> s=r^e mod n and unblinding is u=y^b mod n (fast as b is < n) and
> m^e=s/u mod n.

Ok, now I get what I had not understood previously. You do generate a
new blinding pair dynamically, with the b exponent. Actual blinding
pairs are not reused nor stored, so no issue there.

Let's see how it could be implemented efficiently. With a simple
square-and-multiply algorithm and a 2000-bit modulus, you need about
4000 squarings and 2000 other multiplications to compute x^b and y^b.
You can save on the multiplications by using a sliding window. Another
method which saves time is to precompute (and store) g, g^2, g^4, g^8...
and also y, y^2, y^4, y^8... and just multiply the values you need,
depending on the binary representation of b. At that point you are down
to an average of 2000 multiplications. But then, that's my blinding
method (instead of g, g^2, g^4... I use random base values, but I
don't think that really changes things here).

I also trim down the list to 300, because it saves space and time, and I
don't see "information theoretic security" as a worthwhile goal. It sure
looks elegant, but it makes sense only in a context where the rest of
the system is also information theoretic secure in some way. It really
depends on what you use the KDF for; the "brain wallet" instance is a
case where information theoretic security is useless since an adversary
with unbounded computational power can simply break the resulting ECDSA
key pair. Moreover, I am quite ready to assume that attackers cannot run
a 2^150 attack with non-negligible probability of success (even if they
"get lucky").

With 300 base pairs, cost is about 300 multiplications on average, i.e.
six times better than the "information theoretic" method, and 15 to 20
times faster than the g^b/y^b method with a sliding window
exponentiation. Depending on the available resources (both in CPU and
storage size), that kind of speed-up may or may not matter.


Another thought: if you use a shorter b, you no longer have the
"information theoretic security", but you get better performance, in the
same way as ElGamal or Diffie-Hellman with a smaller exponent. People
do that all the time; especially the small-exponent DH, which is at
work whenever there is a SSL connection with a DHE cipher suite, or for
about every SSH connection around the world. I don't think this is that
much a risk.

Anyway, all these methods can be applied to Makwa without changing the
function definition. If some notion of "information theoretic security"
is what you want for the delegation, then, by all means, go for it.


	--Thomas Pornin

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