Inspired by a recent CTF, but I made the binary much easier for you to solve.
Hint: The decompiled binary seems to look similar to this...
Hint 2: There is no FFI to C, what remains seems to be a flag checker with a simple cipher.
Well, just a simplified copy of SekaiCTF. Only rot3 is involved. Just get the const array and shift 3 characters :p
Source code:
(import (chicken io))
(import (chicken string))
(import string-utils)
(print "Enter the flag: ")
(define input (read-line))
(define input (string->list input))
(define arr '(96 115 96 113 99 120 50 96 101 48 106 48 92 95 49 95 118 91 91 122))
(define good 1)
(define (nth n l)
(if (or (> n (length l)) (< n 0))
(error "Wrong.")
(if (eq? n 0)
(car l)
(nth (- n 1) (cdr l)))))
(if (or (> (length input) 20) (< (length input) 20))
(set! good 0)
(do ((i 0 (+ i 1))) ((> i 19))
(if (not (= (nth i arr) (- (char->integer (nth i input)) 3)))
(set! good 0)
)
)
)
(if (= good 1)
(print "Flag is correct!")
(print "Flag is wrong..."))