From 5031cf3498cdca14319b79229296aca188b47a37 Mon Sep 17 00:00:00 2001
From: magnasilvar <3359919+magnasilvar@users.noreply.github.com>
Date: Sat, 21 Mar 2020 19:46:30 +0100
Subject: [PATCH] Add vigenere cipher (decryption part)

Coded during the 2015 Parrot "hack mon annonce" event.
---
 README.md     | 27 +++++++++++++++-
 Vigenere.java | 87 +++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 113 insertions(+), 1 deletion(-)
 create mode 100644 Vigenere.java

diff --git a/README.md b/README.md
index 23c90d5..ecceaa6 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,27 @@
 # vigenere-cipher
-Java version of the Vigenère cipher (created in 1553), now breakable (since 1863)
+
+Java implementation of the **Vigenère cipher** (decryption part) as part of the "_hack mon annonce_" event organized by [Parrot](https://github.com/Parrot-Developers) in **2015**.
+
+## Setup
+
+* Java: `Oracle JDK 1.8`
+
+## How to
+
+At the root level of the project :
+* Compile Vigenere java class with the `javac` command.
+* Run the compiled class with the `java` command and the following arguments (respecting this order):
+  1. Decryption key (Hexadecimal format needed ! It was the key's format provided during the event),
+  2. Alphabet to use,
+  3. Encrypted words (with Vigenère cipher).
+
+> Powershell example:
+```powershell
+PS C:\GIT\vigenere-cipher> javac .\Vigenere.java
+PS C:\vigenere-cipher> java Vigenere 434c45 ABCDEFGHIJKLMNOPQRSTUVWXYZ JPPNZ
+Clé: CLE
+Alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZ
+Message clair: HELLO
+```
+
+> CAUTION - Use the same case for your 3 arguments !
diff --git a/Vigenere.java b/Vigenere.java
new file mode 100644
index 0000000..f3dac91
--- /dev/null
+++ b/Vigenere.java
@@ -0,0 +1,87 @@
+import javax.xml.bind.DatatypeConverter;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Set;
+
+/**
+ * Classe permettant de déchiffrer un message codé par le chiffrement de Vigenère
+ */
+public class Vigenere {
+
+    public static void main(final String[] args) {
+	    if (args != null && args.length == 3) {
+            final String key = hexaToString(args[0]);
+            final String alphabet = args[1];
+            final String cryptedMsg = args[2];
+            final String repeatedKey = generateRepeatedKey(key, cryptedMsg.length());
+            System.out.println("Cl\u00e9: " + key);
+            System.out.println("Alphabet: " + alphabet);
+            System.out.println("Message clair: " + vigenereDecrypt(repeatedKey, alphabet, cryptedMsg));
+        } else {
+            System.out.println("Bad use ! Syntax : \"key\" \"alphabet\" \"encrypted sequence\"");
+        }
+    }
+
+    /**
+     * Convertit une chaine hexadécimal en chaine de caractères
+     *
+     * @param hexaString Chaine hexadécimal
+     * @return Chaine de caractère
+     */
+    public static String hexaToString(final String hexaString) {
+        return new String(DatatypeConverter.parseHexBinary(hexaString));
+    }
+
+    /**
+     * Génère la clé répétée
+     *
+     * @param key Clé
+     * @param msgLength Taille du message codé
+     * @return La clé répétée
+     */
+    public static String generateRepeatedKey(final String key, final int msgLength) {
+        final StringBuilder repeatedKey = new StringBuilder();
+        while (repeatedKey.length() < msgLength) {
+            repeatedKey.append(key);
+        }
+        repeatedKey.setLength(msgLength);
+        return repeatedKey.toString();
+    }
+
+    /**
+     * Décrypte un message codé avec le chiffrement de Vigenère
+     *
+     * @param repeatedKey Clé répété
+     * @param alphabet Alphabet pour le décryptage
+     * @param cryptedMsg Message encrypté
+     * @return Message décrypté
+     */
+    public static String vigenereDecrypt(final String repeatedKey, final String alphabet, final String cryptedMsg) {
+        final StringBuilder result = new StringBuilder();
+        Map<Character, Map<Character, Character>> square = new HashMap<Character, Map<Character, Character>>();
+        final int nbAlphaEl = alphabet.length();
+        // Construction du carré de Vigenère
+        for (int i = 0; i < nbAlphaEl; i++) {
+            // On ne récupère que les colonnes qui nous intéressent, à savoir celles de la clé
+            if (repeatedKey.contains(String.valueOf(alphabet.charAt(i)))) {
+                final Map<Character, Character> column = new HashMap<>();
+                for (int j = 0; j < nbAlphaEl; j++) {
+                    column.put(alphabet.charAt(j), alphabet.charAt((j + i) % nbAlphaEl));
+                }
+                square.put(alphabet.charAt(i), column);
+            }
+        }
+        // Décryptage
+        for (int i = 0; i < cryptedMsg.length(); i++) {
+            Set<Character> lines = square.get(repeatedKey.charAt(i)).keySet();
+            for (final Character line : lines) {
+                if (square.get(repeatedKey.charAt(i)).get(line) == cryptedMsg.charAt(i)) {
+                    result.append(line);
+                    break;
+                }
+            }
+        }
+        return result.toString();
+    }
+
+}