Re: [Cfrg] Curve manipulation, revisited
Michael Hamburg <mike@shiftleft.org> Wed, 07 January 2015 18:34 UTC
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From: Michael Hamburg <mike@shiftleft.org>
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Date: Wed, 07 Jan 2015 10:33:58 -0800
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To: Peter Dettman <peter.dettman@bouncycastle.org>
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Cc: Adam Langley <agl@imperialviolet.org>, cfrg@irtf.org
Subject: Re: [Cfrg] Curve manipulation, revisited
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> On Jan 1, 2015, at 2:16 AM, Peter Dettman <peter.dettman@bouncycastle.org> wrote: > > On 29/12/2014 8:31 pm, Adam Langley wrote: >> (An implementation that wants to use a windowed method with a Montgomery-X *input* will need to perform a square-root first, but that's the same cost as a design where compressed Edwards points are sent. Note: smarter people than I might be able to eliminate the square-root but I don't see it) > > I believe I can give an example of how to eliminate the square-root for a random-base scalar-mult. I'll use short Weierstrass to describe it though, as I'm less familiar with Edwards and Montgomery. I assume we can convert easily enough between forms, and that twist security carries across. > > - Given input X0, use curve equation to calculate {Y0}^2, let K = {Y0}^2. > - Initial point P0 is (X0,{Y0}) where {Y0} is unevaluated. > - Change of variables to an isomorphic curve: x' = u^2.x, y' = u^3.y, a' = u^4.a, with u = {Y0} (b' = u^6.b, but not typically needed). > - Can now write P0' as (K.X0, K^2) without unknowns. > - Proceed with scalar multiplication of P0' to result point Pn'. > - Recover output Xn from Pn' as Xn'/(K.Zn^2). > > Note that the doubling formula is affected by the changed a' curve parameter (unless a==0); assume modified-Jacobian coordinates are used. > > In the case where the original curve has a==-3, the extra cost of the windowing using the isomorphism in modified-Jacobian coordinates is +2S(quares) per addition (doubling cost is unchanged). The total extra cost using width 5 windows is therefore < 40% of the best-case cost of a sqrt. > > However, there are actually good performance reasons to already be using an isomorphism: to allow mixed addition with precomputed points without needing an inversion in the precomputation. In that case, there is very little additional overhead for the scheme above. > > Regards, > Pete Dettman This is neat. Have you tested it? Does it work, and do you know if it’s better than a Montgomery ladder? From your high-level description it looks like it should be competitive with the short Weierstrass XZ Montgomery ladder (8M + 7S + 5m IIUC). Cheers, — Mike
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