Switch to even-y tiebreaker for pubkeys

[sipa]

Addresses #191.

TODO: adjust reference code, but we'll probably only want to do that after addressing #190 as well.

[jonasnick]

looks good to me otherwise

[jonasnick]

nit: perhaps slightly better returns ''P = lift_x(int(x))'' if ''y(P) & 1 = 0'' and ''-P'' otherwise

[real-or-random]

Mathematically this is correct but algorithmically defining point via lift_x somehow abuses lift_x: the latter makes effort to get the squared-y point, but this effort is then discarded by the former anyway.

Previously point and lift_x were the same except for the parsing of an integer via int. I suggest to get rid of one of them and instead have point_even_y and point_squared_y (or lift_x_even_y and lift_x_squared_y). The resulting functions should not perform the integer parsing, because everywhere else in the pseudocode we have an explicit call to int.

[real-or-random]

by the way, I think this discussion confirms the obversation of #191 (comment)

[ajtowns]

"where x is a 32-byte array, calculates P = lift_x(int(x)) and returns P if is_even(P) or -P otherwise" might read better?

Alternatively, "point(x) returns the point P where x(P)=x and is_even(P). This may be achieved by calculating P=lift_x(x) and returning P if is_even(P) and -P otherwise" or similar might work?

[real-or-random]

Concept ACK, looks good except nits. :)

• Can you define &? We define a lot of notation in this bip, and then for consistency it makes sense to define that one too.
• Should we call this evenness instead of oddness?

[ajtowns]

Can you define &?

Saying ** The function ''is_even(P)'', where ''P'' is a point, returns ''y(P) = 0 mod 2''. should work, we've already got blah ≠ 0 mod p elsewhere.

Should we call this evenness instead of oddness?

We only use "oddness" once when talking about choosing which R point, so doesn't seem worth worrying about to me.

[sipa]

Changed things a bit.

[real-or-random]

ACK :)

[jonasnick]

ACK

[ajtowns]

ACK

[sipa]

Fixed a few more remaining mentions of "point".