We will call the lib_BN library for big numbers:

{require lib_BN}

In this library {BN.compare a b} returns 1 if a > b, 0 if a = b and -1 if a < b.
For a better readability we define three small functions

{def BN.= {lambda {:a :b} {= {BN.compare :a :b} 0}}}
-> BN.=
{def BN.even? {lambda {:n} {= {BN.compare {BN.% :n 2} 0} 0}}}
-> BN.even?
{def BN.square {lambda {:n} {BN.* :n :n}}}
-> BN.square

{def mod-exp
 {lambda {:a :n :mod}
  {if {BN.= :n 0}
   then 1
   else {if {BN.even? :n}
   then {BN.% {BN.square {mod-exp :a {BN./ :n 2} :mod}} :mod}
   else {BN.% {BN.* :a   {mod-exp :a {BN.- :n 1} :mod}} :mod}}}}}
-> mod-exp

{mod-exp
  2988348162058574136915891421498819466320163312926952423791023078876139
  2351399303373464486466122544523690094744975233415544072992656881240319
  {BN.pow 10 40}}
-> 1527229998585248450016808958343740453059   // 3300ms