(defun generate-k (m)
  (mod (+ (parse-integer (sha256 m) :radix 16)
          (get-internal-real-time))
       *n*))

;;; CL-USER> *n*
;;; 115792089237316195423570985008687907852837564279074904382605163141518161494337
;;; CL-USER> (sha256 "hi")
;;; "8f434346648f6b96df89dda901c5176b10a6d83961dd3c1ac88b59b2dc327aa4"
;;; CL-USER> (mod (+ (parse-integer (sha256 "hi") :radix 16)
;;;                  (get-internal-real-time))
;;;               *n*)
;;; 64799580415538975799983076416639475941656422820260801829320304467620837323285
;;; CL-USER> (generate-k "hi")
;;; 64799580415538975799983076416639475941656422820260801829320304467620881419869
;;; CL-USER> (let ((k 0))
;;;            (setq k (generate-k "hi"))
;;;            k)
;;; ; Evaluation aborted on #<TYPE-ERROR expected-type: VECTOR datum: 0>.
;;; CL-USER>