Secure AES-GCM encryption with random IV and explicit tag split

secure-aes-gcm-encryption.kt

override fun encryptAesGcm(plaintext: ByteArray, aad: ByteArray?): CryptoHelper.AesGcmResult {
    val key = currentKey()
    val iv = ByteArray(12)
    random.nextBytes(iv)
    val cipher = Cipher.getInstance("AES/GCM/NoPadding")
    val spec = GCMParameterSpec(128, iv)
    cipher.init(Cipher.ENCRYPT_MODE, key, spec)
    if (aad != null) cipher.updateAAD(aad)
    val out = cipher.doFinal(plaintext)
    // Split cipherText and tag (last 16 bytes) for teaching clarity
    val tagLen = 16
    val ct = out.copyOfRange(0, out.size - tagLen)
    val tag = out.copyOfRange(out.size - tagLen, out.size)
    return CryptoHelper.AesGcmResult(iv, ct, tag)
}