Envelope cipher

pro.aduki.crypto.cipher.Envelope is AES-256-GCM authenticated encryption.

object Envelope {
    fun encrypt(plain: ByteArray, key: SecretKey): ByteArray
    fun decrypt(cipherText: ByteArray, key: SecretKey): ByteArray
}

Format

encrypt returns IV (12 bytes) || ciphertext || tag (16 bytes). The IV is the one the cipher generates (a fresh random 12 bytes on every call; the Android Keystore refuses a caller-supplied IV, so the cipher is initialised without one and Cipher.getIV() is stored). The layout is unchanged, so blobs written earlier still decrypt. The transformation is AES/GCM/NoPadding with a 128-bit tag.

decrypt requires at least 28 bytes (IllegalArgumentException otherwise) and throws javax.crypto.AEADBadTagException when the data or tag was modified or the key is wrong.

val key = Provider().get()
val sealed = Envelope.encrypt("secret".toByteArray(), key)
val plain = Envelope.decrypt(sealed, key)