[openssl-users] X25519 - why openssl shows server temp key as 253 bits?
mcr at sandelman.ca
Tue Sep 4 16:10:04 UTC 2018
Robert Moskowitz <rgm at htt-consult.com> wrote:
> A curve point needs an x and a y. But do you need the y for the
> computation. Do you only need its sign? I don't know. I am not a
My understanding is that you need x and y to do the computation.
(And I observe this in code)
However, since x and y have to be on the curve, if you know x, then
that constrained y to be one or two values... so you need to know the *sign*
of y, which is transmitted as a single bit. Then you can calculate y.
The fundamental reason behind this is because sqrt(4) = 2, and sqrt(4) = -2...
Since some bits of the x are required to be 0, it's possible to encode the
sign of Y into the encoded X bit-stream...
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