patrickfav · February 27, 2024 11:32 · Feb 25, 2023 · Apr 19, 2020 · Apr 19, 2020 · Apr 19, 2020
diff --git a/AesGcmTest.java b/AesGcmTest.java
@@ -1,3 +1,5 @@
+package at.favre.lib.bytes.otherPackage;
+
 import org.junit.Test;
 
 import javax.crypto.Cipher;
@@ -11,12 +13,16 @@
 
 import static org.junit.Assert.assertEquals;
 
+/**
+ * A simple showcase for encryption and decryption with AES + GCM in Java
+ */
 public class AesGcmTest {
     private final SecureRandom secureRandom = new SecureRandom();
     private final static int GCM_IV_LENGTH = 12;
 
     @Test
     public void testEncryption() throws Exception {
+        //create new random key
         byte[] key = new byte[16];
         secureRandom.nextBytes(key);
         SecretKey secretKey = new SecretKeySpec(key, "AES");
@@ -30,6 +36,15 @@ public void testEncryption() throws Exception {
         assertEquals(message, decrypted);
     }
 
+    /**
+     * Encrypt a plaintext with given key.
+     *
+     * @param plaintext      to encrypt (utf-8 encoding will be used)
+     * @param secretKey      to encrypt, must be AES type, see {@link SecretKeySpec}
+     * @param associatedData optional, additional (public) data to verify on decryption with GCM auth tag
+     * @return encrypted message
+     * @throws Exception if anything goes wrong
+     */
     private byte[] encrypt(String plaintext, SecretKey secretKey, byte[] associatedData) throws Exception {
 
         byte[] iv = new byte[GCM_IV_LENGTH]; //NEVER REUSE THIS IV WITH SAME KEY
@@ -50,6 +65,15 @@ private byte[] encrypt(String plaintext, SecretKey secretKey, byte[] associatedD
         return byteBuffer.array();
     }
 
+    /**
+     * Decrypts encrypted message (see {@link #encrypt(String, SecretKey, byte[])}).
+     *
+     * @param cipherMessage  iv with ciphertext
+     * @param secretKey      used to decrypt
+     * @param associatedData optional, additional (public) data to verify on decryption with GCM auth tag
+     * @return original plaintext
+     * @throws Exception if anything goes wrong
+     */
     private String decrypt(byte[] cipherMessage, SecretKey secretKey, byte[] associatedData) throws Exception {
         final Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
         //use first 12 bytes for iv

diff --git a/AesGcmTest.java b/AesGcmTest.java
@@ -1,5 +1,3 @@
-package at.favre.lib.bytes.otherPackage;
-
 import org.junit.Test;
 
 import javax.crypto.Cipher;
@@ -9,6 +7,7 @@
 import java.nio.ByteBuffer;
 import java.nio.charset.StandardCharsets;
 import java.security.SecureRandom;
+import java.security.spec.AlgorithmParameterSpec;
 
 import static org.junit.Assert.assertEquals;
 
@@ -53,12 +52,14 @@ private byte[] encrypt(String plaintext, SecretKey secretKey, byte[] associatedD
 
     private String decrypt(byte[] cipherMessage, SecretKey secretKey, byte[] associatedData) throws Exception {
         final Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
-        cipher.init(Cipher.DECRYPT_MODE, secretKey, new GCMParameterSpec(128, cipherMessage, 0, GCM_IV_LENGTH));
+        //use first 12 bytes for iv
+        AlgorithmParameterSpec gcmIv = new GCMParameterSpec(128, cipherMessage, 0, GCM_IV_LENGTH);
+        cipher.init(Cipher.DECRYPT_MODE, secretKey, gcmIv);
 
         if (associatedData != null) {
             cipher.updateAAD(associatedData);
         }
-
+        //use everything from 12 bytes on as ciphertext
         byte[] plainText = cipher.doFinal(cipherMessage, GCM_IV_LENGTH, cipherMessage.length - GCM_IV_LENGTH);
 
         return new String(plainText, StandardCharsets.UTF_8);

diff --git a/AesGcmTest.java b/AesGcmTest.java
@@ -0,0 +1,66 @@
+package at.favre.lib.bytes.otherPackage;
+
+import org.junit.Test;
+
+import javax.crypto.Cipher;
+import javax.crypto.SecretKey;
+import javax.crypto.spec.GCMParameterSpec;
+import javax.crypto.spec.SecretKeySpec;
+import java.nio.ByteBuffer;
+import java.nio.charset.StandardCharsets;
+import java.security.SecureRandom;
+
+import static org.junit.Assert.assertEquals;
+
+public class AesGcmTest {
+    private final SecureRandom secureRandom = new SecureRandom();
+    private final static int GCM_IV_LENGTH = 12;
+
+    @Test
+    public void testEncryption() throws Exception {
+        byte[] key = new byte[16];
+        secureRandom.nextBytes(key);
+        SecretKey secretKey = new SecretKeySpec(key, "AES");
+        byte[] associatedData = "ProtocolVersion1".getBytes(StandardCharsets.UTF_8); //meta data you want to verify with the secret message
+
+        String message = "the secret message";
+
+        byte[] cipherText = encrypt(message, secretKey, associatedData);
+        String decrypted = decrypt(cipherText, secretKey, associatedData);
+
+        assertEquals(message, decrypted);
+    }
+
+    private byte[] encrypt(String plaintext, SecretKey secretKey, byte[] associatedData) throws Exception {
+
+        byte[] iv = new byte[GCM_IV_LENGTH]; //NEVER REUSE THIS IV WITH SAME KEY
+        secureRandom.nextBytes(iv);
+        final Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
+        GCMParameterSpec parameterSpec = new GCMParameterSpec(128, iv); //128 bit auth tag length
+        cipher.init(Cipher.ENCRYPT_MODE, secretKey, parameterSpec);
+
+        if (associatedData != null) {
+            cipher.updateAAD(associatedData);
+        }
+
+        byte[] cipherText = cipher.doFinal(plaintext.getBytes(StandardCharsets.UTF_8));
+
+        ByteBuffer byteBuffer = ByteBuffer.allocate(iv.length + cipherText.length);
+        byteBuffer.put(iv);
+        byteBuffer.put(cipherText);
+        return byteBuffer.array();
+    }
+
+    private String decrypt(byte[] cipherMessage, SecretKey secretKey, byte[] associatedData) throws Exception {
+        final Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
+        cipher.init(Cipher.DECRYPT_MODE, secretKey, new GCMParameterSpec(128, cipherMessage, 0, GCM_IV_LENGTH));
+
+        if (associatedData != null) {
+            cipher.updateAAD(associatedData);
+        }
+
+        byte[] plainText = cipher.doFinal(cipherMessage, GCM_IV_LENGTH, cipherMessage.length - GCM_IV_LENGTH);
+
+        return new String(plainText, StandardCharsets.UTF_8);
+    }
+}