[curves] Torsion-safe representatives (was: Ed25519 "clamping" and its effect on hierarchical key derivation)

Oleg Andreev oleganza at gmail.com
Mon Mar 27 13:00:49 PDT 2017

>> I have a lame question, though. You mention that `a*B = a'*B` holds
>> for the base point. But is it also true for any point in the B's
>> subgroup? The reason I ask is that I need to have not just regular
>> EdDSA signatures, but also DLEQs (discrete log equality proofs) with
>> random generator points.
> Yes.  Proof: If P is a point in B's subgroup, then P = p*B for some
> scalar p.  Thus
>   a*P = a*p*B = p*a*B = p*a'*B = a'*p*B = a'*P,
> since multiplication of scalars is associative with multiplication of
> curve points, and multiplication of scalars is commutative.

Oh, thanks for pointing this out to me! That was a lame question indeed :)

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