From 1f862439170198372d09104ecd8b188dd2fd264c Mon Sep 17 00:00:00 2001 From: Alexander Rosenberg Date: Mon, 7 Oct 2024 01:29:11 -0700 Subject: [PATCH] Initial commit --- LICENSE | 674 ++++++++++++++++++++++++++++++++++++++++++++++ hill.lisp | 432 +++++++++++++++++++++++++++++ shift-affine.lisp | 142 ++++++++++ vigenere.lisp | 63 +++++ 4 files changed, 1311 insertions(+) create mode 100644 LICENSE create mode 100644 hill.lisp create mode 100644 shift-affine.lisp create mode 100644 vigenere.lisp diff --git a/LICENSE b/LICENSE new file mode 100644 index 0000000..f288702 --- /dev/null +++ b/LICENSE @@ -0,0 +1,674 @@ + GNU GENERAL PUBLIC LICENSE + Version 3, 29 June 2007 + + Copyright (C) 2007 Free Software Foundation, Inc. + Everyone is permitted to copy and distribute verbatim copies + of this license document, but changing it is not allowed. + + Preamble + + The GNU General Public License is a free, copyleft license for +software and other kinds of works. + + The licenses for most software and other practical works are designed +to take away your freedom to share and change the works. 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If your program is a subroutine library, you +may consider it more useful to permit linking proprietary applications with +the library. If this is what you want to do, use the GNU Lesser General +Public License instead of this License. But first, please read +. diff --git a/hill.lisp b/hill.lisp new file mode 100644 index 0000000..347b21a --- /dev/null +++ b/hill.lisp @@ -0,0 +1,432 @@ +;; hill.lisp - A very bad implementation of a hill cipher along with some +;; unnessesary, insecure PRNG key stuff. +;; +;; This file is free software under the terms of the GPL3 license. You can find +;; a copy of the license at: https://www.gnu.org/licenses/gpl-3.0.en.html + +;; PRNG subroutines +(defparameter *system-rng-stream* + (open "/dev/random" :element-type '(unsigned-byte 8))) + +(defun number-bits (num) + "Return the number of bits in NUM." + (ceiling (log num 2))) + +(defun get-random (&key (size 8) (rng-stream *system-rng-stream*)) + "Read SIZE random bytes from the systems high-quality random source." + (let ((read-buf (make-array size))) + (read-sequence read-buf rng-stream) + (loop with out-val = 0 + for part across read-buf do + (setq out-val (+ (ash out-val 8) part)) + finally (return out-val)))) + +(defun xorshift*64 (seed) + "Generate the next number in the xorshift* sequence starting or continuing +from SEED. That is, SEED should either be an initial, non-zero seed or the last +number generated in the sequence." + (setq seed (logxor seed (ash seed -12)) + seed (logxor seed (ash seed 25)) + seed (logxor seed (ash seed -27))) + ;; #x2545F4914F6CDD1DULL + (logand (* seed 2685821657736338717) + #xFFFFFFFFFFFFFFFF)) + +(defun rand-clamp (num &key (bits 64) (min 0 has-min-p) + (max (1- (expt 2 bits)) has-max-p)) + "Clamp the randomly generated number NUM to between [MIN, MAX]. The random +numbers is assumed to be between 0 and 2^BITS - 1." + (if (or has-min-p has-max-p) + (+ (* (- max min) (/ num (1- (expt 2 bits)))) min) + num)) + +(defclass xorshift*64-stream () + ((state :initarg :seed + :initform 1 + :documentation "The last generated number. If no number has yet been +generated, this is the seed.")) + (:documentation "A stream of random numbers generated and xorshift*64 +algorithm.")) + +(defun make-xorshift* (&key (seed (get-random)) (bits 64)) + (case bits + (64 (make-instance 'xorshift*64-stream :seed seed)) + (t (error "Invalid prandom number size: ~d" bits)))) + +(defmethod next-int ((stream xorshift*64-stream) &key (max #xFFFFFFFFFFFFFFFF) + (min 0)) + "Get the next random integer between MIN and MAX from STREAM." + (with-slots (state) stream + (setf state (xorshift*64 state)) + (floor (rand-clamp state :bits 64 :max max :min min)))) + +(defparameter *default-rng* (make-xorshift*) + "The default RNG source to use when generating matrices.") + +;; Matrix subroutines +(defun mat-minor (transpose-mat i j) + "Find the minor of MAT for I and J." + (destructuring-bind (height width) + (array-dimensions transpose-mat) + (let ((minor (make-array (list (1- height) + (1- width))))) + (dotimes (row height minor) + (dotimes (col width) + (unless (or (= row i) + (= col j)) + (let ((out-row (if (> row i) + (1- row) + row)) + (out-col (if (> col j) + (1- col) + col))) + (setf (aref minor out-row out-col) (aref transpose-mat row col))))))))) + +(defun cofactor-sgn (i j) + "Return the sign of the cofactor at I and J." + (expt -1 (+ i j))) + +(defun cofactor (mat i j) + "Find the cofactor for I and J in MAT." + (* (cofactor-sgn i j) (det (mat-minor mat i j)))) + +(defun first-column-cofactors (mat) + "Find the cofactors for the first column of MAT." + (let* ((height (array-dimension mat 0)) + (out-arr (make-array height))) + (dotimes (i height out-arr) + (setf (aref out-arr i) (cofactor mat i 0))))) + +(defun det2x2 (mat) + "Find the determinate of a 2x2 matrix MAT." + (let ((a (aref mat 0 0)) + (b (aref mat 0 1)) + (c (aref mat 1 0)) + (d (aref mat 1 1))) + (- (* a d) (* b c)))) + +(defun det (mat &key first-column-cofactors) + "Find the determinant of MAT. If the cofactors for the first column have +already been calculated, they can be supplied in FIRST-COLUMN-COFACTORS." + (destructuring-bind (height width) + (array-dimensions mat) + (if (and (= height 2) + (= width 2)) + (det2x2 mat) + (loop for i below height + when first-column-cofactors + summing (* (aref mat i 0) (aref first-column-cofactors i)) + else + summing (* (aref mat i 0) (cofactor mat i 0)))))) + +(defun map-mat (function mat) + "Return a new matrix of the same dimensions as MAT, with each element being +the result of applying FUNCTION to each index of MAT. FUNCTION is passed two +values, (i j), where i and j are the row and column of the element." + (destructuring-bind (height width) + (array-dimensions mat) + (let ((out-mat (make-array (list height width)))) + (dotimes (row height out-mat) + (dotimes (col width) + (setf (aref out-mat row col) + (funcall function row col))))))) + +(defun extended-gcd (n1 n2) + "Return the greatest common denominator of N1 and N2. As a second value, +return a cons of (x . y) such char x * N1 + y * N2 = GCD." + (if (zerop n1) + (values n2 (cons 0 1)) + (multiple-value-bind (div rem) (floor n2 n1) + (destructuring-bind (gcd (x . y)) + (multiple-value-list (extended-gcd rem n1)) + (values gcd (cons (- y (* div x)) x)))))) + +(defun modular-multiplicative-inverse (n m) + "Find the modular multiplicative inverse of N with respect to M. That is, some +integer i such that (N * i) mod M = 1." + (destructuring-bind (gcd (x . y)) + (multiple-value-list (extended-gcd n m)) + (declare (ignorable y)) + (unless (= 1 gcd) + (error "N and M must be coprime. N: ~d, M: ~d" n m)) + (+ x m))) + +(defun modular-inverse-det (mat modulo &key first-column-cofactors) + "Calculate the inverse determinant of MAT under MODULO. If MODULO is nil, this +is just 1/det." + (let ((det (det mat :first-column-cofactors first-column-cofactors))) + (if modulo + (modular-multiplicative-inverse det modulo) + (/ det)))) + +(defun invert-2x2-mat (mat &key modulo) + "Invert the 2x2 matrix MAT." + (let ((a (aref mat 0 0)) + (b (aref mat 0 1)) + (c (aref mat 1 0)) + (d (aref mat 1 1)) + (ood (modular-inverse-det mat modulo))) + (make-array '(2 2) + :initial-contents (list (list (* ood d) (* ood (- b))) + (list (* ood (- c)) (* ood a)))))) + +(defun invert-mat (mat &key modulo) + "Invert MAT. This naively assumes that MAT is not singular." + (destructuring-bind (height width) + (array-dimensions mat) + (if (and (= width 2) + (= height 2)) + (invert-2x2-mat mat :modulo modulo) + (let* ((first-column-cofactors (first-column-cofactors mat)) + (one-over-det (modular-inverse-det mat modulo + :first-column-cofactors + first-column-cofactors))) + (map-mat (lambda (i j) + ;; this calculates 1/det * adjugate[i][j] + (* one-over-det + (if (and first-column-cofactors (zerop i)) + (aref first-column-cofactors j) + (cofactor mat j i)))) + mat))))) + +(defun transpose-mat (mat) + "Transpose MAT." + (map-mat (lambda (i j) + (aref mat j i)) + mat)) + +(defun dot-row-col (mat1 mat2 row col) + "Take the dot (scalar) product of ROW of MAT1 and COL of MAT2." + (let ((sum 0)) + (dotimes (n (array-dimension mat2 0) sum) + (setq sum (+ sum (* (aref mat1 row n) + (aref mat2 n col))))))) + +(defun *mm (mat1 mat2 &key modulo) + "Multiply MAT1 by MAT2. This naively assumes that these matrices can be +multiplied without issue. The result will be taken under MODULO, if it is +non-nil." + (let* ((width (array-dimension mat1 0)) + (height (array-dimension mat2 1)) + (out-mat (make-array (list height width)))) + (dotimes (i height out-mat) + (dotimes (j width) + (let ((dot (dot-row-col mat1 mat2 i j))) + (setf (aref out-mat i j) + (if modulo + (mod dot modulo) + dot))))))) + +(defun *mv (mat vec &key modulo) + "Multiply MAT by VEC. This naively assumes that these can be multiplied +without issue. The result will be taken under MODULO, if it is non-nil." + (let* ((width (array-dimension mat 1)) + (height (array-dimension mat 0)) + (out-vec (make-array height))) + (dotimes (row height out-vec) + (setf (aref out-vec row) + (loop for col below width + summing (* (aref vec col) (aref mat row col)) into sum + finally (return (if modulo + (mod sum modulo) + sum))))))) + +(defun generate-matrix (width height &key (rng *default-rng*) max) + "Generate a random matrix of size WIDTH x HEIGHT. Use RNG to get the random +data for this matrix." + (map-mat (lambda (i j) + (declare (ignorable i j)) + (if max + (next-int rng :max max) + (next-int rng))) + (make-array (list height width)))) + +(defun generate-invertible-matrix (width height &key (rng *default-rng*) max) + "Generate a random invertible matrix of size WIDTH x HEIGHT. Use RNG to get +the random data for this matrix. This generates a strictly diagonally dominant +matrix." + (let ((rand-mat (generate-matrix width height :rng rng :max max))) + (dotimes (row height rand-mat) + (let ((sum 0)) + (dotimes (col width) + (setq sum (+ sum (abs (aref rand-mat row col))))) + (setf (aref rand-mat row row) sum))))) + +;; Base64 subroutines +(defparameter +base64-alphabet+ + "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/" + "Lookup table for base64 encode.") + +(defparameter +inverse-base64-alphabet+ + (loop with alph = (make-array 123 :initial-element 0) + for i below (length +base64-alphabet+) + for char = (aref +base64-alphabet+ i) + do (setf (aref alph (char-code char)) i) + finally (return alph)) + "Inverse lookup table for base64 decode.") + +(defun base64-encode-block (b1 &optional b2 b3) + "Base64 encode B1, B2, and B3, each of which should be a single byte. The +result of this function is a list of the 4 encoded characters." + ;; allow B2 and B3 to be nil, so we can't just check if they where passed, we + ;; need to know if they where nil when the function was called + (let ((has-b2-p b2) + (has-b3-p b3)) + (unless b2 + (setq b2 0)) + (unless b3 + (setq b3 0)) + (let ((e1 (ash (logand b1 #b11111100) -2)) + (e2 (logior (ash (logand b1 #b11) 4) + (ash (logand b2 #b11110000) -4))) + (e3 (logior (ash (logand b2 #b1111) 2) + (ash (logand b3 #b11000000) -6))) + (e4 (logand b3 #b111111))) + (cond + ((and has-b2-p has-b3-p) + (list (aref +base64-alphabet+ e1) + (aref +base64-alphabet+ e2) + (aref +base64-alphabet+ e3) + (aref +base64-alphabet+ e4))) + (has-b2-p + (list (aref +base64-alphabet+ e1) + (aref +base64-alphabet+ e2) + (aref +base64-alphabet+ e3) + #\=)) + (t + (list (aref +base64-alphabet+ e1) + (aref +base64-alphabet+ e2) + #\= #\=)))))) + +(defun base64-encode (data) + "Encode DATA, a vector of numbers, as base64." + (loop with output = () + for i upfrom 0 by 3 below (- (length data) 2) + do (setf output (nconc (nreverse (base64-encode-block + (aref data i) + (aref data (+ i 1)) + (aref data (+ i 2)))) + output)) + finally + (return + (coerce (if (< i (length data)) + (let ((b1 (aref data i)) + (b2 (when (< (1+ i) (length data)) + (aref data (1+ i))))) + (nreverse (nconc (nreverse + (base64-encode-block b1 b2)) + output))) + (nreverse output)) + 'string)))) + +(defun base64-decode-block (e1 e2 e3 e4) + "Base64 decode E1, E2, E3, and E4, each of which is a single character. The +result of this function is a list of 3 bytes." + (let ((i1 (aref +inverse-base64-alphabet+ (char-code e1))) + (i2 (aref +inverse-base64-alphabet+ (char-code e2))) + (i3 (aref +inverse-base64-alphabet+ (char-code e3))) + (i4 (aref +inverse-base64-alphabet+ (char-code e4)))) + (list (logior (ash i1 2) + (ash (logand i2 #b110000) -4)) + (unless (eq e3 #\=) + (logior (ash (logand i2 #b1111) 4) + (ash (logand i3 #b111100) -2))) + (unless (eq e4 #\=) + (logior (ash (logand i3 #b11) 6) + i4))))) + +(defun base64-decode (data) + "Decode DATA, a string, as base64." + (coerce (loop for i upfrom 0 by 4 below (length data) + append (delete-if 'null + (base64-decode-block (aref data i) + (aref data (+ i 1)) + (aref data (+ i 2)) + (aref data (+ i 3))))) + 'vector)) + +;; Hill subroutines +(defun nth-hill-block (plain n &key (size 4)) + "Get the Nth hill block in PLAIN. If there are not enough characters in PLAIN, +pad the block with zeros. Each block is SIZE characters." + (let* ((start (* size n)) + (end (+ start size))) + (when (< (length plain) end) + (setq end (+ start (- (length plain) start)))) + (cond + ((> start (length plain)) + (make-array size :initial-element 0)) + ((>= end (length plain)) + (loop with out-vec = (make-array size :initial-element 0 :fill-pointer 0) + for elem across (subseq plain start (length plain)) do + (vector-push elem out-vec) + finally + (setf (fill-pointer out-vec) size) + (return out-vec))) + (t + (subseq plain start end))))) + +(defun hill-block-count (plain &key (size 4)) + "Return the number of hill blocks in PLAIN." + (ceiling (length plain) size)) + +(defmacro do-hill-blocks ((var plain &optional retval) (&key (size 4)) + &body body) + "Evaluate BODY once for each hill cipher block in PLAIN, with VAR bound to +that block." + (let ((i-var (gensym)) + (plain-var (gensym))) + `(let ((,plain-var ,plain)) + (dotimes (,i-var (hill-block-count ,plain-var :size ,size) ,retval) + (let ((,var (nth-hill-block ,plain-var ,i-var :size ,size))) + ,@body))))) + +(defun make-hill-matrix (key &key (size 4)) + "Get the hill cipher matrix for KEY and block size SIZE." + (let ((rng (make-xorshift* :seed key))) + (generate-invertible-matrix size size :rng rng :max 128))) + +(defun hill-process (data mat size dict-size) + "Perform hill encryption or decryption on DATA. Each block is SIZE bytes. MAT +is taken to be the key matrix (or its inverse). DICT-SIZE is the modulus." + (let ((processed (make-array 0))) + (do-hill-blocks (bl data (map 'vector (lambda (elem) + (mod elem dict-size)) + processed)) + (:size size) + (setq processed (concatenate 'vector processed + (*mv mat bl)))))) + +(defun hill-encrypt (plain key &key (size 4) (dict-size 251)) + "Encrypt PLAIN using a hill cipher, using KEY as the secret and SIZE as the +block size. DICT-SIZE is the largest character code in PLAIN." + (hill-process (map 'vector 'char-code plain) + (make-hill-matrix key :size size) + size dict-size)) + +(defun make-inverse-hill-matrix (key &key (size 4) (dict-size 251)) + "Create an inverse hill matrix for KEY. SIZE and DICT-SIZE are as they are to +`hill-decrypt'." + (invert-mat (make-hill-matrix key :size size) :modulo dict-size)) + +(defun hill-remove-padding (plain-vec) + "Remove trailing null bytes from PLAIN-VEC." + (loop for i downfrom (1- (length plain-vec)) + while (and (>= i 0) (eq (aref plain-vec i) #\Nul)) + finally (return (subseq plain-vec 0 (1+ i))))) + +(defun hill-decrypt (cipher key &key (size 4) (dict-size 251)) + "Perform the inverse operation of `hill-encrypt', restoring the plaintext +of CIPHER that was encrypted using KEY with a block size of SIZE. DICT-SIZE must +be the same value originally used to encrypt the text." + (coerce (hill-remove-padding + (map 'vector 'code-char + (hill-process cipher + (make-inverse-hill-matrix key :size size + :dict-size dict-size) + size dict-size))) + 'string)) + +;; Local Variables: +;; jinx-local-words: "adjugate cofactor cofactors det plaintext prandom unnessesary xorshift" +;; End: diff --git a/shift-affine.lisp b/shift-affine.lisp new file mode 100644 index 0000000..13549e8 --- /dev/null +++ b/shift-affine.lisp @@ -0,0 +1,142 @@ +;; shift-affine.lisp - Some simple subroutines for working with affine and shift +;; ciphers. +;; +;; This file is free software under the terms of the GPL3 license. You can find +;; a copy of the license at: https://www.gnu.org/licenses/gpl-3.0.en.html + +(defpackage shift-affine + (:use :cl) + (:export + :char-letter :letter-char :affine-encrypt :extended-gcd + :modular-multiplicative-inverse :affine-encrypt :affine-decrypt + :shift-encrypt :shift-decrypt :*english-letter-frequencies* + :relative-frequencies :chi-squared :count-letters :shift-guess-key + :shift-try-break)) + +(in-package shift-affine) + +(defun char-letter (char) + "Return the index of CHAR in the alphabet. The returned number is 0 for A, 1 +for B, 2 for C, etc. Note that this will return the save value for upper and +lower case letters." + (- (char-code char) + (if (upper-case-p char) + (char-code #\A) + (char-code #\a)))) + +(defun letter-char (letter) + "Return the Lisp character object for LETTER. Letter should be the index of a +letter in the alphabet. For example, 0 for A, 1 for B, 2 for C, etc." + (code-char (+ letter (char-code #\A)))) + +(defun affine-encrypt (plain coef offset) + "Encrypt PLAIN using an affine encryption algorithm. COEF cannot be coprime +with 26. That is, it cannot be a multiple of 2 or 13. The algorithm used is: + f(p) = (COEF * p + OFFSET) mod 26 +where this function is used to transform every letter of PLAIN." + (when (or (zerop (mod coef 2)) + (zerop (mod coef 13))) + (error + "COEF must be coprime with 26 (not divisible by 2 or 13), but it was ~d" + coef)) + (map 'string (lambda (char) + (if (alpha-char-p char) + (let ((letter (char-letter char))) + (letter-char + (mod (+ (* coef letter) offset) 26))) + char)) + plain)) + +(defun extended-gcd (n1 n2) + "Return the greatest common denominator of N1 and N2. As a second value, +return a cons of (x . y) such char x * N1 + y * N2 = GCD." + (if (zerop n1) + (values n2 (cons 0 1)) + (multiple-value-bind (div rem) (floor n2 n1) + (destructuring-bind (gcd (x . y)) + (multiple-value-list (extended-gcd rem n1)) + (values gcd (cons (- y (* div x)) x)))))) + +(defun modular-multiplicative-inverse (n m) + "Find the modular multiplicative inverse of N with respect to M. That is, some +integer i such that (N * i) mod M = 1." + (destructuring-bind (gcd (x . y)) + (multiple-value-list (extended-gcd n m)) + (declare (ignorable y)) + (unless (= 1 gcd) + (error "N and M must be coprime. N: ~d, M: ~d" n m)) + (+ x m))) + +(defun affine-decrypt (cipher coef offset) + "Decrypt CIPHER, which should be a string encrypted by `affine-encrypt'. COEF +and OFFSET should be the same as the values passed when encrypting CIPHER." + (let ((ic (modular-multiplicative-inverse coef 26))) + (affine-encrypt cipher ic (- (* offset ic))))) + +(defun shift-encrypt (plain offset) + "Encrypt PLAIN by shifting each letter by OFFSET." + (affine-encrypt plain 1 offset)) + +(defun shift-decrypt (cipher offset) + "Decrypt CIPHER by shifting each letter by -1 * OFFSET." + (affine-encrypt cipher 1 (- offset))) + +(defparameter *english-letter-frequencies* + #(8.2 1.5 2.8 4.3 12.7 2.2 2.0 6.1 7.0 0.15 0.77 4.0 2.4 6.7 7.5 1.9 0.095 6.0 + 6.3 9.1 2.8 0.98 2.4 0.15 2.0 0.074) + "Percent frequency of each letter of the English alphabet in standard writing. +Source: https://en.wikipedia.org/wiki/Letter_frequency") + +(defun relative-frequencies (seq) + "Find the relative percent frequency of the sample data in SEQ." + (let ((sample-count (reduce '+ seq))) + (map 'vector (lambda (elem) + (/ elem sample-count)) + seq))) + +(defun chi-squared (data ref-data &key (offset 0)) + "Return the sum of the square of the error between each index of DATA and +REF-DATA. Before performing the calculation, shift each element in DATA by +OFFSET indexes, wrapping. That is, with OFFSET 1, index 0 becomes index 1, and +the last index becomes index 0." + (loop for ref across ref-data + for i upfrom offset + for sample = (aref data (mod i (length data))) + summing (/ (expt (- sample ref) 2) ref))) + +(defun count-letters (str) + "Count the number of times each letter appears in STR. Return a vector with +each index being the number of times that letter appeared." + (loop with freq-data = (make-array '(26) :initial-element 0) + for char across str + for letter = (char-letter char) + when (alpha-char-p char) + do (incf (aref freq-data letter)) + finally (return freq-data))) + +(defun shift-guess-key (cipher &key (reference *english-letter-frequencies*)) + "Try to guess the shift offset for CIPHER. This is done by analyzing the +frequencies of letters in CIPHER and finding the shift offset that most closely +matches those listed in REFERENCE, which defaults to `*english-letter-frequencies*'." + ;; We also try CIPHER as is, just in case + (let ((best-chi-squared most-positive-fixnum) + (best-offset 0) + (freq-data (relative-frequencies (count-letters cipher)))) + (dotimes (i (length reference) best-offset) + (let ((chi-squared (chi-squared freq-data reference + :offset i))) + (when (< chi-squared best-chi-squared) + (setq best-chi-squared chi-squared + best-offset i)))))) + +(defun shift-try-break (cipher &key (reference *english-letter-frequencies*)) + "Attempt to decrypt CIPHER by guessing the shift offset. This uses +`shift-guess-key' (which see) internally. REFERENCE is passed verbatim to +`shift-guess-key'. As a second value, return the key used for decryption." + (let ((key (shift-guess-key cipher :reference reference))) + (values (shift-decrypt cipher key) + key))) + +;; Local Variables: +;; jinx-local-words: "coprime" +;; End: diff --git a/vigenere.lisp b/vigenere.lisp new file mode 100644 index 0000000..3d049ea --- /dev/null +++ b/vigenere.lisp @@ -0,0 +1,63 @@ +;; vigenere.lisp - Simple implementation of a Vigenere cipher. +;; +;; This file is free software under the terms of the GPL3 license. You can find +;; a copy of the license at: https://www.gnu.org/licenses/gpl-3.0.en.html + +(defpackage vigenere + (:use :cl) + (:export + :+vigenere-alphabet+ :+vigenere-inverse-alphabet+ :vigenere-letter-char + :vigenere-char-letter :vigenere-add :vigenere-encrypt :vigenere-invert-char + :vigenere-decrypt)) + +(in-package vigenere) + +(defparameter +vigenere-alphabet+ + " abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789." + "Alphabet for use in `vigenere-encrypt'.") + +(defparameter +vigenere-inverse-alphabet+ + (loop with alph = (make-array 123 :initial-element 0) + for i below (length +vigenere-alphabet+) + for char = (aref +vigenere-alphabet+ i) + do (setf (aref alph (char-code char)) i) + finally (return alph)) + "Inverse lookup table for `vigenere-encrypt'.") + +(defun vigenere-letter-char (letter) + "Convert LETTER to a Lisp char object." + (aref +vigenere-alphabet+ letter)) + +(defun vigenere-char-letter (char) + "Convert a Lisp char object CHAR to a letter." + (aref +vigenere-inverse-alphabet+ (char-code char))) + +(defun vigenere-add (elem by) + "Add ELEM and BY under the modulo (length `+vigenere-alphabet+')" + (mod (+ elem by) (length +vigenere-alphabet+))) + +(defun vigenere-encrypt (plain key) + "Encrypt PLAIN using a vigenere cipher. This works by shifting each element of +PLAIN by each consecutive elements of KEY, wrapping around if there are no more +elements left in KEY." + (let ((done (make-array (length plain)))) + (dotimes (i (length plain) (map 'string 'vigenere-letter-char + done)) + (setf (aref done i) + (vigenere-add (vigenere-char-letter (aref plain i)) + (vigenere-char-letter + (aref key (mod i (length key))))))))) + +(defun vigenere-invert-char (char) + "Invert CHAR. That is, return the character that will invert the effect that +CHAR had during encryption." + (let ((letter (vigenere-char-letter char))) + (vigenere-letter-char (- (length +vigenere-alphabet+) letter)))) + +(defun vigenere-decrypt (cipher key) + "Decrypt CIPHER using a vigenere cipher, with the key KEY." + (vigenere-encrypt cipher (map 'string 'vigenere-invert-char key))) + +;; Local Variables: +;; jinx-local-words: "vigenere" +;; End: