use UByteArray for SecretGenerator

src/commonMain/kotlin/com/infendro/otp/SecretGenerator.kt

@@ -2,13 +2,14 @@ package com.infendro.otp
 
 import com.infendro.hash.HashFunction
 import com.infendro.random.csprng.CSPRNG
+import kotlin.random.nextUBytes
 
 class SecretGenerator(
     val function: HashFunction,
 ) {
     private val random = CSPRNG.HMAC(function)
 
-    fun generate(): ByteArray {
-        return random.nextBytes(function.bytes)
+    fun generate(): UByteArray {
+        return random.nextUBytes(function.bytes)
     }
 }