decode.rkt

#lang racket/base
(require [only-in srfi/13 string-index])

(define alphabet "ABCDEFGHIJKLMNOPQRSTUVWXYZ")

(define table
  (for/list ([i (in-range 26)])
    (string-append
     (substring alphabet i)
     (substring alphabet 0 i))))

(define key "SOLVECRYPTO")
(define code "UFJKXQZQUNB")

(list->string
 (for/list ([k (in-string key)]
            [c (in-string code)])
   (string-ref alphabet
               (string-index (list-ref table (string-index alphabet k))
                             c))))