android-aes-gcm-iv-aad-outputs.kt

const val AAD_LENGTH = 16
const val TAG_LENGTH = 16

class EncryptionOutput(val iv: ByteArray,
                       val aad: ByteArray,
                       val tag: ByteArray,
                       val ciphertext: ByteArray)

fun encrypt(key: SecretKey, message: ByteArray): EncryptionOutput {
    val cipher = Cipher.getInstance("AES/GCM/NoPadding")
    cipher.init(Cipher.ENCRYPT_MODE, key)
    val iv = cipher.iv.copyOf()
    val aad = SecureRandom().generateSeed(AAD_LENGTH)
    cipher.updateAAD(aad)
    val result = cipher.doFinal(message)
    val ciphertext = result.copyOfRange(0, result.size - TAG_LENGTH)
    val tag = result.copyOfRange(result.size - TAG_LENGTH, result.size)
    return EncryptionOutput(iv, aad, tag, ciphertext)
}

fun decrypt(key: SecretKey, iv: ByteArray, aad: ByteArray, tag: ByteArray, ciphertext: ByteArray): ByteArray {
    val cipher = Cipher.getInstance("AES/GCM/NoPadding")
    val spec = GCMParameterSpec(TAG_LENGTH * 8, iv)
    cipher.init(Cipher.DECRYPT_MODE, key, spec)
    cipher.updateAAD(aad)
    return cipher.doFinal(ciphertext + tag)
}

Why would you generate random aad just to store it? Aad is supposed to be meaningful data which is then authenticated upon decryption, it should be passed to encrypt as an argument. So the example is kinda misleading or am I missing something?

Also should not GCMParameterSpec appear in encrypt too?

@user7 You pass the aad to the cipher in line 14, which is then using it for authenticated encryption.