(let ((hex-string "d41d8cd98f00b204e9800998ecf8427e"))
  (mapconcat
   (lambda (x)
     (nth (string-to-number x 16)
          '("0000" "0001" "0010" "0011"
            "0100" "0101" "0110" "0111"
            "1000" "1001" "1010" "1011"
            "1100" "1101" "1110" "1111")))
   (split-string hex-string "" t) ""))