64 lines
2.4 KiB
Common Lisp
64 lines
2.4 KiB
Common Lisp
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;; vigenere.lisp - Simple implementation of a Vigenere cipher.
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;;
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;; This file is free software under the terms of the GPL3 license. You can find
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;; a copy of the license at: https://www.gnu.org/licenses/gpl-3.0.en.html
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(defpackage vigenere
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(:use :cl)
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(:export
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:+vigenere-alphabet+ :+vigenere-inverse-alphabet+ :vigenere-letter-char
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:vigenere-char-letter :vigenere-add :vigenere-encrypt :vigenere-invert-char
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:vigenere-decrypt))
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(in-package vigenere)
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(defparameter +vigenere-alphabet+
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" abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789."
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"Alphabet for use in `vigenere-encrypt'.")
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(defparameter +vigenere-inverse-alphabet+
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(loop with alph = (make-array 123 :initial-element 0)
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for i below (length +vigenere-alphabet+)
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for char = (aref +vigenere-alphabet+ i)
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do (setf (aref alph (char-code char)) i)
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finally (return alph))
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"Inverse lookup table for `vigenere-encrypt'.")
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(defun vigenere-letter-char (letter)
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"Convert LETTER to a Lisp char object."
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(aref +vigenere-alphabet+ letter))
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(defun vigenere-char-letter (char)
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"Convert a Lisp char object CHAR to a letter."
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(aref +vigenere-inverse-alphabet+ (char-code char)))
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(defun vigenere-add (elem by)
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"Add ELEM and BY under the modulo (length `+vigenere-alphabet+')"
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(mod (+ elem by) (length +vigenere-alphabet+)))
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(defun vigenere-encrypt (plain key)
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"Encrypt PLAIN using a vigenere cipher. This works by shifting each element of
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PLAIN by each consecutive elements of KEY, wrapping around if there are no more
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elements left in KEY."
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(let ((done (make-array (length plain))))
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(dotimes (i (length plain) (map 'string 'vigenere-letter-char
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done))
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(setf (aref done i)
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(vigenere-add (vigenere-char-letter (aref plain i))
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(vigenere-char-letter
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(aref key (mod i (length key)))))))))
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(defun vigenere-invert-char (char)
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"Invert CHAR. That is, return the character that will invert the effect that
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CHAR had during encryption."
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(let ((letter (vigenere-char-letter char)))
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(vigenere-letter-char (- (length +vigenere-alphabet+) letter))))
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(defun vigenere-decrypt (cipher key)
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"Decrypt CIPHER using a vigenere cipher, with the key KEY."
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(vigenere-encrypt cipher (map 'string 'vigenere-invert-char key)))
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;; Local Variables:
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;; jinx-local-words: "vigenere"
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;; End:
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