test: throw upon reaching too high nonces

Thanks to @breynders-cb and @tvandriessel-cb for spotting the risk.

src/test/kotlin/CipherTest.kt

@@ -0,0 +1,37 @@
+package nl.sanderdijkhuis.noise
+
+import nl.sanderdijkhuis.noise.cryptography.AssociatedData
+import nl.sanderdijkhuis.noise.cryptography.CipherKey
+import nl.sanderdijkhuis.noise.cryptography.Nonce
+import nl.sanderdijkhuis.noise.cryptography.Plaintext
+import nl.sanderdijkhuis.noise.data.Data
+import org.junit.jupiter.api.assertThrows
+import kotlin.test.Test
+
+@OptIn(ExperimentalStdlibApi::class)
+class CipherTest {
+    private val data = AssociatedData(Data.empty)
+    private val plaintext = Plaintext(Data.empty)
+
+    @Test
+    fun `throws upon reaching nonce maximum while encrypting`() {
+        val nonceTooHighToToUse = Nonce(ULong.MAX_VALUE - 1uL) // 2^64-2
+
+        assertThrows<IllegalStateException> { cipher(nonceTooHighToToUse).encrypt(data, plaintext) }
+    }
+
+    @Test
+    fun `throws upon reaching nonce maximum while decrypting`() {
+        val nonceTooHighToEncrypt = Nonce(ULong.MAX_VALUE - 2uL) // 2^64-3
+        val (cipher, ciphertext) = cipher(nonceTooHighToEncrypt).encrypt(data, plaintext)
+
+        assertThrows<IllegalStateException> { cipher.decrypt(data, ciphertext) }
+    }
+
+    private fun cipher(nonce: Nonce) =
+        Cipher(
+            JavaCryptography,
+            CipherKey(Data("76fef1ab184aa7539e3b62a43019ecafc621248b3ac2f5297dd5814e3bd560d3".hexToByteArray())),
+            nonce
+        )
+}