#!/usr/bin/env tclsh

gets stdin cpub
gets stdin dpub

# From rosettacode
proc tcl::mathfunc::modexp {a b n} {
    if {$b == 0} {return 1}
    set c [expr {modexp($a, $b / 2, $n)**2 % $n}]
    if {$b & 1} {
	set c [expr {($c * $a) % $n}]
    }
    return $c
}

# Call the C program to find the private key (loop size) for one of the
# inputs.  Then use modexp to calculate the encryption key using the
# other input.

set cpriv [exec ./25c $cpub]
puts [expr {modexp($dpub,$cpriv,20201227)}]