i don't think it's a trapboor function but rather a form of asymmetric cipher, it's not too distantly different from an elliptic curve.

https://wikimedia.org/api/rest_v1/media/math/render/svg/3dbe6cab1bc2c7f1c99757dc6e5d7a517cf9b4f8

y^2=x^3+a*x+b

your equation is:

p1^2 = (p1*p2)%x)

mod 1 is um... 1. i'm using the simple modulo notation % meaning "remainder", which is sorta like one step of a modulo, more or less the same operation, at least, one cycle of a clockwork arithmetic system. that mod value tells you the potential number of values that could be valid for y, relative to how p2 relates to it.

i'm used to working with these things in code not so much in math. the finite field is a different shape to the field of real numbers.