java 45 lines · 7 steps

Authenticated encryption with AES-GCM in Java

A small helper that encrypts and decrypts strings using AES-GCM, packing the random IV alongside the ciphertext.

Explained by highlit
Intermediate cryptography aes-gcm authenticated-encryption base64 byte-buffer
1public final class AesGcmCipher {
2 
3 private static final String TRANSFORMATION = "AES/GCM/NoPadding";
4 private static final int GCM_TAG_BITS = 128;
5 private static final int IV_LENGTH = 12;
6 
7 private final SecretKey key;
8 private final SecureRandom random = new SecureRandom();
9 
10 public AesGcmCipher(SecretKey key) {
11 this.key = key;
12 }
13 
14 public String encrypt(String plaintext) throws GeneralSecurityException {
15 byte[] iv = new byte[IV_LENGTH];
16 random.nextBytes(iv);
17 
18 Cipher cipher = Cipher.getInstance(TRANSFORMATION);
19 cipher.init(Cipher.ENCRYPT_MODE, key, new GCMParameterSpec(GCM_TAG_BITS, iv));
20 byte[] ciphertext = cipher.doFinal(plaintext.getBytes(StandardCharsets.UTF_8));
21 
22 byte[] combined = ByteBuffer.allocate(iv.length + ciphertext.length)
23 .put(iv)
24 .put(ciphertext)
25 .array();
26 return Base64.getEncoder().encodeToString(combined);
27 }
28 
29 public String decrypt(String encoded) throws GeneralSecurityException {
30 byte[] combined = Base64.getDecoder().decode(encoded);
31 if (combined.length <= IV_LENGTH) {
32 throw new IllegalArgumentException("Malformed ciphertext");
33 }
34 
35 ByteBuffer buffer = ByteBuffer.wrap(combined);
36 byte[] iv = new byte[IV_LENGTH];
37 buffer.get(iv);
38 byte[] ciphertext = new byte[buffer.remaining()];
39 buffer.get(ciphertext);
40 
41 Cipher cipher = Cipher.getInstance(TRANSFORMATION);
42 cipher.init(Cipher.DECRYPT_MODE, key, new GCMParameterSpec(GCM_TAG_BITS, iv));
43 return new String(cipher.doFinal(ciphertext), StandardCharsets.UTF_8);
44 }
45}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1GCM needs a fresh random IV for every encryption, so a shared key stays secure across many messages.
  2. 2Prepending the IV to the ciphertext lets you decrypt later with no separate metadata to track.
  3. 3The 128-bit GCM tag provides integrity, so tampered ciphertext fails to decrypt instead of returning garbage.

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