CaesarBruteForce

src/main/java/com/ciphers/CaesarBruteForce.java

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+package com.ciphers;
+
+public class CaesarBruteForce {
+
+    /**
+     * Recursively Brute forces a parsed encrypted text, trying out all shifting keys from 1-26, printing out all decryption attempts
+     * @param message (String) The encrypted text.
+     * @param Key (int) The key used to decrypt the encrypted text and is increment upon a recursive call.
+     * @return (String) Concatenated string of all decryption attempts (For unit testing purposes).
+     */
+    public String decrypt(String message, int Key) {
+        final String LETTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+        if (Key > 26){ System.out.println(); return null; }
+
+        StringBuilder plainText = new StringBuilder();
+        for (char character : message.toUpperCase().toCharArray()) {
+            int index = LETTERS.indexOf(character);
+
+            if (index != -1) {
+                index -= Key;
+                //Wrap around index value range[1-26]
+                if (index < 0) { index += LETTERS.length(); }
+                plainText.append(LETTERS.toCharArray()[index]);
+            } else { plainText.append(character); }
+        }
+        System.out.println(String.format("Current Decryption Key %d : %s", Key, plainText));
+        return plainText.append(decrypt(message, Key+1)).toString();
+    }
+}

src/test/java/com/ciphers/CaesarBruteForceTest.java

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+package com.ciphers;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+class CaesarBruteForceTest {
+    @Test
+    void testCaesarBruteForce() {
+        CaesarBruteForce cipher = new CaesarBruteForce();
+
+        Assertions.assertSame(true, cipher.decrypt("TKFK", 1).contains("JAVA"));
+        Assertions.assertSame(true, cipher.decrypt("QHCH", 1).contains("JAVA"));
+        Assertions.assertSame(true, cipher.decrypt("LUHREIU", 1).contains("VERBOSE"));
+        Assertions.assertSame(true, cipher.decrypt("Mkockb", 1).contains("CAESAR"));
+        Assertions.assertSame(true, cipher.decrypt("olssv", 1).contains("HELLO"));
+        Assertions.assertSame(true, cipher.decrypt("Zvksxdohd", 1).contains("PLAINTEXT"));
+        Assertions.assertSame(true, cipher.decrypt("XGVKRIM", 1).contains("ENCRYPT"));
+        Assertions.assertSame(true, cipher.decrypt("OZRRVNQC123", 1).contains("PASSWORD123"));
+        Assertions.assertSame(true, cipher.decrypt("GEQDZMYQ", 1).contains("USERNAME"));
+        Assertions.assertSame(true, cipher.decrypt("IKHMXVMXW", 1).contains("PROTECTED"));
+        Assertions.assertSame(true, cipher.decrypt("ZPSRC-DMPACB", 1).contains("BRUTE-FORCED"));
+        Assertions.assertSame(true, cipher.decrypt("VCTKJ!", 1).contains("PWNED!"));
+        Assertions.assertSame(true, cipher.decrypt("JKLMNOP", 1).contains("ABCDEFG"));
+        Assertions.assertSame(true, cipher.decrypt("QFMDHCUFODVWQ", 1).contains("CRYPTOGRAPHIC"));
+        Assertions.assertSame(true, cipher.decrypt("Dmbncdc", 1).contains("ENCODED"));
+    }
+}