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Task/RSA-code/Common-Lisp/rsa-code.lisp

@@ -0,0 +1,32 @@
+(defparameter *n* 9516311845790656153499716760847001433441357)
+(defparameter *e* 65537)
+(defparameter *d* 5617843187844953170308463622230283376298685)
+
+;; magic
+(defun encode-string (message)
+  (parse-integer (reduce #'(lambda (x y) (concatenate 'string x y))
+     (loop for c across message collect (format nil "~2,'0d" (- (char-code c) 32))))))
+
+;; sorcery
+(defun decode-string (message) (coerce (loop for (a b) on
+  (loop for char across (write-to-string message) collect char)
+    by #'cddr collect (code-char (+ (parse-integer (coerce (list a b) 'string)) 32))) 'string))
+
+;; ACTUAL RSA ALGORITHM STARTS HERE ;;
+
+;; fast modular exponentiation: runs in O(log exponent)
+;; acc is initially 1 and contains the result by the end
+(defun mod-exp (base exponent modulus acc)
+  (if (= exponent 0) acc
+    (mod-exp (mod (* base base) modulus) (ash exponent -1) modulus
+	     (if (= (mod exponent 2) 1) (mod (* acc base) modulus) acc))))
+
+;; to encode a message, we first convert it to its integer form.
+;; then, we raise it to the *e* power, modulo *n*
+(defun encode-rsa (message)
+  (mod-exp (encode-string message) *e* *n* 1))
+
+;; to decode a message, we raise it to *d* power, modulo *n*
+;; and then convert it back into a string
+(defun decode-rsa (message)
+  (decode-string (mod-exp message *d* *n* 1)))