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Date: Tue, 01 Apr 2014 11:35:27 -0300
From: Marcos Simplicio <mjunior@...c.usp.br>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Questions about Lyra

Hi, there.

Thank you for pointing out unclear points! We try to provide some
answers inline and we will update the eprint/code to reflect them.

BTW, you may want to take a look at Lyra2, the newer version of the
algorithm available at http://lyra-kdf.net/ and that was actually
submitted to the PHC, which (we hope) already solves some of those issues.

BR,

Marcos.

On 31-Mar-14 18:39, Greg Zaverucha wrote:
> Hi
> After reading the Lyra ePrint and looking at the reference implementation I have a few questions about the spec (Algorithm 2, page 10).  I guess the designers would be the best to answer but others who've looked at the design might have input as well.
> 
> 1. For line 16: "col <- M[row][C-1] mod C", doesn't this require multiprecision modular arithmetic in the general case?  C will usually fit in a machine word, but elements of M will probably not.  In the reference implementation C is hard-coded to 64, so this is a non-issue.  Will C always be a power of two?   It's my preference that key derivation not depend on multiprecision arithmetic.

In the general case, it does require multiprecision arithmetic. The
point is that we did not want to unnecessarily constraint the underlying
design at the point it would prevent users from choosing "esoteric"
values of C.

However, we do agree that this liberty is likely to become an issue from
an implementation point of view (which was ignored in our
benchmark-oriented implementation). So we are going to do the same thing
we did with Lyra2: to use the truncation of M[row][col] to 1 word
whenever we need any modulo operation.

> 
> 2. On line 17, how is |R| defined?  I initially thought it was the number of bits required to represent R, but in the reference implementation |R| is fixed to 64.  Maybe it would be better to fix |R| = 64 and require R < 2^64. The implementation of line 17 is slightly more awkward when |R| varies.

Same issue as above, so one should go with the same solution.

> 
> 3. On lines 16 and 17 a byte array must be converted to an integer, but the endianness is not specified.  In the reference implementation the endianness of the processor is used; so a different result would be computed on a big and little endian machines.

Good point! And now I believe this was not made clear on Lyra2's
specification either... :(

We were thinking about x86 architectures, so the more usual is to go
with little-endian.

> 
> 4. Does the paper define what the initial state should be?  In the code it's the Blake2b IV, but for another choice of f how should it be chosen?

This is a tricky issue. Usually, sponges are initialized with zeros
(according to the specifications in http://sponge.noekeon.org/), so in
principle this also applies to any f used in Lyra/Lyra2.

With Blake2b, however, we added a disclaimer on the code (but not on the
specification... our bad...) saying that we are using Blake2b's IV to
initialize the sponge's state. The reason is that Blake2b's G function
has a fixed point at 0s. This should not be a huge issue for Lyra/Lyra2
unless both the password and the salt are strings of zeros long enough
to fill whole blocks (otherwise, the 10*1 padding would already avoid
the fixed point), but we went with the "better safe than sorry" approach
here.

> 
> A general comment: you might consider breaking up the current "Setup" phase (lines 1-8) into two pieces, Setup (lines 1-3) and Initialization (lines 4-8), because after line 3 the input password is no longer required and can be zeroized.  Or maybe make Setup be lines 1-3 and call lines 3-19 "Derive" or similar.
>
> Looking forward to your response, thanks
> Greg
> 

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