(define (pre-process-add-length padded-data original-length)
  (define str (~a #:width 64 #:pad-string "0" #:align 'right
                  (number->string original-length 2)))

  (foldl
   (λ (b data)
     (bytes-append data b))
   padded-data
   (for/list ([start (in-range 0 64 8)])
     (define end (+ start 8))
     (bytes (string->number (substring str start end) 2)))))