;;;; util.lisp -- functions that come in handy in crypto applications (in-package :crypto) (defun byte-array-to-hex-string (vector &key (start 0) end (element-type 'character))   "Return a string containing the hexadecimal representation of the subsequence of VECTOR between START and END.  ELEMENT-TYPE controls the element-type of the returned string."   (let* ((end (or end (length vector)))          (length (- end start))          (hexdigits "0123456789abcdef"))     (loop with string = (make-string (* length 2)                                      :element-type element-type)           for i from start below end           for j from 0 below (* length 2) by 2           do (let ((byte (aref vector i)))                (setf (aref string j)                      (aref hexdigits (ldb (byte 4 4) byte))                      (aref string (1+ j))                      (aref hexdigits (ldb (byte 4 0) byte))))           finally (return string)))) (defun ascii-string-to-byte-array (string &key (start 0) end)   "Convert STRING to a (VECTOR (UNSIGNED-BYTE 8)).  It is an error if STRING contains any character whose CHAR-CODE is greater than 255."   (let ((vec (make-array (length string) :element-type '(unsigned-byte 8)))         (end (or end (length string))))     (loop for i from start below end do           (let ((byte (char-code (char string i))))             (unless (< byte 256)               (error "~A is not an ASCII character" (char string i)))             (setf (aref vec i) byte))           finally (return vec))))