A Caesar cipher encrypts a string by replacing each letter with the letter k later in the alphabet, wrapping around at the end.

(define (caesar str k)
  (let* ((n (string-length str))
         (new (make-string n)))
    (dotimes (i n new)
      (string-set! new i
        (caesar-char (string-ref str i) k) ) ) ))

(define (caesar-char char k)
  (if (char-alphabetic? char)
      (let ((base (if (char-upper-case? char)
                      (char->integer #\A)
                      (char->integer #\a))))
        (integer->char
          (+ base
             (modulo (+ k (- (char->integer char)
                             base))
                          26))) )
      char))

; decryption
(define (decaesar str k) (caesar str (- 26 k)))

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