GCD
no efficient method is known for computing them in general for the well-chosen {g,p,a,b}
GCD Algorithms
Greatest common divisor
In mathematics, the greatest common divisor (GCD) of two or more integers, which are not all zero, is the largest positive integer that divides each of the integers. For two integers x, y, the greatest common divisor of x and y is denoted
gcd
(
x
,
y
)
{\displaystyle \gcd(x,y)}
. For example, the GCD of 8 and 12 is 4, that is,
gcd
(
8
,
12
)
=
4
{\displaystyle \gcd(8,12)=4}
.[1][2]
https://en.wikipedia.org/wiki/Greatest_common_divisor

Seonglae Cho