Added an service for generating Backup codes and added some tests.

src/Services/TFA/BackupCodeGenerator.php

@@ -0,0 +1,60 @@
+<?php
+
+
+namespace App\Services\TFA;
+
+/**
+ * This class generates random backup codes for two factor authentication
+ * @package App\Services\TFA
+ */
+class BackupCodeGenerator
+{
+    protected $code_length;
+    protected $code_count;
+
+    /**
+     * BackupCodeGenerator constructor.
+     * @param  int  $code_length How many characters a single code should have.
+     * @param  int  $code_count How many codes are generated for a whole backup set.
+     */
+    public function __construct(int $code_length, int $code_count)
+    {
+        if ($code_length > 32) {
+            throw new \RuntimeException('Backup code can have maximum 32 digits!');
+        }
+        if ($code_length < 6) {
+            throw new \RuntimeException('Code must have at least 6 digits to ensure security!');
+        }
+
+        $this->code_count = $code_count;
+        $this->code_length = $code_length;
+    }
+
+    /**
+     * Generates a single backup code.
+     * It is a random hexadecimal value with the digit count configured in constructor
+     * @return string The generated backup code (e.g. 1f3870be2)
+     * @throws \Exception If no entropy source is available.
+     */
+    public function generateSingleCode() : string
+    {
+        $bytes = random_bytes(32);
+        return substr(md5($bytes), 0, $this->code_length);
+    }
+
+
+    /**
+     * Returns a full backup code set. The code count can be configured in the constructor
+     * @return string[] An array containing different backup codes.
+     * @throws \Exception If no entropy source is available
+     */
+    public function generateCodeSet() : array
+    {
+        $array = [];
+        for($n=0; $n<$this->code_count; $n++) {
+            $array[] = $this->generateSingleCode();
+        }
+
+        return $array;
+    }
+}