From 6d596b5951bacbe80eb9bd7aa0a0fe830b5ac406 Mon Sep 17 00:00:00 2001
From: Oleg Yukhnevich <whyoleg@gmail.com>
Date: Mon, 13 Mar 2023 19:48:26 +0300
Subject: [PATCH] add real test for randomness

---
 .../sources/common/CryptographyRandomTest.kt  | 111 ++++++++++++++----
 1 file changed, 85 insertions(+), 26 deletions(-)

diff --git a/cryptography-random/test/sources/common/CryptographyRandomTest.kt b/cryptography-random/test/sources/common/CryptographyRandomTest.kt
index 02a234bb..dcdce779 100644
--- a/cryptography-random/test/sources/common/CryptographyRandomTest.kt
+++ b/cryptography-random/test/sources/common/CryptographyRandomTest.kt
@@ -1,36 +1,19 @@
 package dev.whyoleg.cryptography.random
 
-import kotlin.math.*
 import kotlin.test.*
 
-class CryptographyRandomTest {
-    @Test
-    fun testInt() {
-        assertNotEquals(CryptographyRandom.nextInt(), CryptographyRandom.nextInt())
-    }
-
-    @Test
-    fun test65536() {
-        //WebCrypto specific test - improve it somehow?
-        assertTrue(CryptographyRandom.nextBytes(65535).any { it != 0.toByte() })
-        assertTrue(CryptographyRandom.nextBytes(65536).any { it != 0.toByte() })
-        assertTrue(CryptographyRandom.nextBytes(65537).any { it != 0.toByte() })
-    }
+class DefaultRandomTest : CryptographyRandomTest(CryptographyRandom.Default)
 
+abstract class CryptographyRandomTest(private val random: CryptographyRandom) {
     @Test
-    fun testArray() {
-        repeat(8) { n ->
-            val size = 10.0.pow(n).toInt()
-            val array = CryptographyRandom.nextBytes(size)
-            assertTrue(array.any { it != 0.toByte() })
-            assertEquals(size, array.size)
-        }
+    fun testInt() {
+        assertNotEquals(random.nextInt(), random.nextInt())
     }
 
     @Test
     fun testEmpty() {
-        assertTrue(CryptographyRandom.nextBytes(0).isEmpty())
-        assertTrue(CryptographyRandom.nextBytes(ByteArray(0)).isEmpty())
+        assertTrue(random.nextBytes(0).isEmpty())
+        assertTrue(random.nextBytes(ByteArray(0)).isEmpty())
     }
 
     @Test
@@ -38,17 +21,93 @@ class CryptographyRandomTest {
         val bytes = ByteArray(10) { it.toByte() }
         bytes.copyOf().also { copy ->
             assertContentEquals(bytes, copy)
-            assertFalse(bytes.contentEquals(CryptographyRandom.nextBytes(copy)))
+            val inPlace = random.nextBytes(copy)
+            assertContentEquals(inPlace, copy)
+            assertFalse(bytes.contentEquals(inPlace))
             assertFalse(bytes.contentEquals(copy))
         }
         bytes.copyOf().also { copy ->
-            assertContentEquals(bytes, CryptographyRandom.nextBytes(copy, 5, 5))
+            val inPlace = random.nextBytes(copy, 5, 5)
+            assertContentEquals(inPlace, copy)
+            assertContentEquals(bytes, inPlace)
             assertContentEquals(bytes, copy)
         }
         bytes.copyOf().also { copy ->
-            assertFalse(bytes.contentEquals(CryptographyRandom.nextBytes(copy, 0, 5)))
+            val inPlace = random.nextBytes(copy, 0, 5)
+            assertContentEquals(inPlace, copy)
+            assertFalse(bytes.contentEquals(inPlace))
             assertFalse(bytes.contentEquals(copy))
         }
     }
 
+    @Test
+    fun testComposite() {
+        fun targetValues(value: Int): List<Int> = buildList {
+            fun addLocal(value: Int) {
+                add(value)
+                add(value + 1)
+                add(value - 1)
+            }
+
+            addLocal(value)
+            (2..8).forEach { split ->
+                val div = value / split
+                val multi = value * split
+                addLocal(value)
+                addLocal(div)
+                addLocal(multi)
+                addLocal(multi - div)
+            }
+        }
+
+        val sizes = listOf(
+            32, //min jdk native prng chunk
+            256, //linux chunk
+            512,
+            4096,
+            65536, //webcrypto chunk | max jdk native prng chunk
+            1000000, // 1 MB
+            1048576, // 1 MiB (via targetValues will test values up to 8 MiB)
+        ).flatMap(::targetValues).distinct().sorted()
+
+        sizes.forEach { size ->
+            val bytes = random.nextBytes(size)
+            val refillBytes = random.nextBytes(bytes.copyOf())
+            assertEquals(bytes.size, size)
+            assertEquals(refillBytes.size, size)
+
+            var nonEmptyCount = 0
+            var changedCount = 0
+
+            repeat(size) {
+                val original = bytes[it]
+                val changed = refillBytes[it]
+                if (original != 0.toByte()) ++nonEmptyCount
+                if (original != changed) ++changedCount
+            }
+
+            val nonEmptyProb = 1.0 - nonEmptyCount.toDouble() / size
+            val changedProb = 1.0 - changedCount.toDouble() / size
+
+            // empirical values
+            val tolerance = when {
+                size < 4      -> 0.7
+                size < 6      -> 0.5
+                size < 20     -> 0.3
+                size < 50     -> 0.2
+                size < 100    -> 0.1
+                size < 200    -> 0.05
+                size < 500    -> 0.03
+                size < 1000   -> 0.025
+                size < 5000   -> 0.02
+                size < 10000  -> 0.01
+                size < 100000 -> 0.0075
+                else          -> 0.005
+            }
+
+            assertEquals(0.0, nonEmptyProb, tolerance, "size=$size EMPTY diff = $nonEmptyProb")
+            assertEquals(0.0, changedProb, tolerance, "size=$size CHANGED diff = $changedProb")
+        }
+    }
+
 }