Newbie needs help please

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Re: Newbie needs help please

Postby RaydPanda » Mon Nov 09, 2009 11:49 am

I came to the same conclusion just before I saw your post... and what wonder: Now the decryption works as well. :D
I was thinking too strictly in terms of the already existing functions so I didn't notice the algebraic error, silly me. Well, the nice thing is that I now have a fully functional program. This is the complete project:

Code: Select all
; Encryption-Project by Cara Petrovitsch

; 1) Program that takes in a word(given as a string) plus a number and encrypts the word using Caesar's Cipher in the modulo given as the number:

; Extended alphabet as parameter plus one white space at index 0:

(defparameter *alphabet* " aAbBcCdDeEfFgGhHiIjJkKlLmMnNoOpPqQrRsStTuUvVwWxXyYzZ,0.1?2!3;4:5@6-7_8'9")

; Finds the index of char in  *alphabet*:

(defun alphabet-index (char)
  (position char *alphabet*))
 
; Finds the char shifted n places to the right in the alphabet:

(defun rotate (char n)
  (elt *alphabet* (mod (+ (alphabet-index char) n)(length *alphabet*))))

; Makes the rotate function easier to use with map:

(defun e-helper (n)
  #'(lambda (char)
      (rotate char n)))

; Returns the encrypted string (when no modulo is specified explicitly, modulo 7 is used):
 
(defun c-encrypt (str &optional (n 7))
  (map 'string (e-helper n) str))
 

; 2) Program that decrypts an already encrypted  word given as a string (first argument) using Caesar's cipher in the modulo given (second argument)

; Rotates the encrypted chars back to their original meaning:

(defun rotate-back (char n)
  (elt *alphabet* (mod (- (alphabet-index char) n)(length *alphabet*))))

; Makes the rotate-back function easier to use with map:

(defun d-helper (n)
  #'(lambda (char)
      (rotate-back char n)))

; Returns the decrypted string (when no modulo is specified explicitly, modulo 7 is used):

(defun c-decrypt (str &optional (n 7))
  (map 'string (d-helper n) str))


; 3) Program that takes a plaintext and a keyword (given in stringformat) and encrypts the plaintext using Vigenere cipher

 
; Function that rotates the characters of the plaintext modulo the key's index:

(defun v-rotate (P K)
  (rotate P (alphabet-index K))))

; Returns the encrypted string:

(defun v-encrypt (string key)
  (coerce (loop for i from 0 to (1- (length string))
                collect (v-rotate (char string i) (char key (mod i (length key)))))
          'string))

; 4) Program that takes a ciphertext and a keyword (given in stringformat) and decrypts the ciphertext using Vigenere cipher
   

; Function that rotates the Ciphertexts char back to its Plaintext position:

(defun v-rotate-back (C K)
  (rotate-back C (alphabet-index K)))

; returns the decrypted string:

(defun v-decrypt (string key)
  (coerce (loop for i from 0 to (1- (length string))
                collect (v-rotate-back (char string i) (char key (mod i (length key)))))
          'string))


I am so happy with it, my first working program! :P

Thank you once again! (I start sounding like a scratched plate... don't I?)
RaydPanda
 
Posts: 8
Joined: Thu Oct 22, 2009 11:13 am
Location: Bergen, Norway

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