Re: ECC point compression trick

Pubkeybreaker wrote:
The square roots of 2 mod 7 are 3 and 4.

And -3 == 4

OMG WOW PONIES

Please tell me which one is the postive root and which one is the
negative? If you always want to (say) always select the one with
larger norm and declare it as 'positive', tell us how to do this over
an extension field and not just in the ground field.

I think it's obvious what I meant. The one which you're likely going
to have to negate to get to.

if you compute y' = sqrt(blah) and y' != y then -y' == y

That would be the negative. Aside from random root finding algorithms
the rest will always find the same root... e.g.

blah^(p+1)/4 mod p

will always return the same number for a fixed blah. that means you'll
ALWAYS have to negate the outcome if you want the OTHER root.

In short, GFY.

Tom

.

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