implement HMAC PRNG

src/commonMain/kotlin/com/infendro/random/HMAC.kt

@@ -0,0 +1,46 @@
+package com.infendro.random
+
+import com.infendro.bytearray.toInt
+import com.infendro.hash.SHA256
+import com.infendro.mac.HMAC
+import kotlin.random.Random
+
+class HMAC internal constructor() : Random() {
+    private val random = Default
+    private val hmac = HMAC(SHA256)
+    private var K: ByteArray
+    private var V: ByteArray
+
+    init {
+        K = ByteArray(32).also {
+            random.nextBytes(it)
+        }
+        V = ByteArray(32).also {
+            random.nextBytes(it)
+        }
+    }
+
+    fun reseed(
+        entropy: ByteArray,
+    ) {
+        K = hmac.hash(K, V + byteArrayOf(0x00) + entropy)
+        V = hmac.hash(K, V)
+        K = hmac.hash(K, V + byteArrayOf(0x01) + entropy)
+        V = hmac.hash(K, V)
+    }
+
+    override fun nextBits(
+        count: Int,
+    ): Int {
+        if (count !in 0..32) throw Exception()
+        if (count == 0) return 0
+
+        val bytes = hmac.hash(K, V)
+        val bits = bytes.take(4).toInt() ushr (32 - count)
+
+        K = hmac.hash(K, V + byteArrayOf(0x00))
+        V = hmac.hash(K, V)
+
+        return bits
+    }
+}