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(defun square (x) (* x x))
(defun modexpt (x e m)
(cond ((= e 0) 1)
((= e 1) (mod x m))
((evenp e) (mod (square (modexpt x (/ e 2) m)) m))
(t (mod (* (square (modexpt x (floor e 2) m)) x) m))))
(defun modinv (x m &optional (b m) (a 0) (u 1))
(if (= x 0)
(when (= b 1) (mod a m))
(modinv (mod b x) m x u (- a (* u (floor b x))))))
(defun rsa (p q &optional e d m c)
(let ((n (* p q))
(l (* (1- p) (1- q))))
(cond ((not e) (rsa p q (modinv d l) d m c))
((not d) (rsa p q e (modinv e l) m c))
((not m) (modexpt c d n))
((not c) (modexpt m e n)))))
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