Polished secure random number implementation (#191285)

## Summary

Polished secure random number implementation.

---------

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

src/plugins/interactive_setup/server/verification_code.ts

@@ -69,20 +69,24 @@ Your verification code is: ${highlightedCode}
 
   /**
    * Returns a cryptographically secure and random 6-digit code.
-   *
-   * Implementation notes: `secureRandomNumber` returns a random number like `0.05505769583xxxx`. To
-   * turn that into a 6 digit code we multiply it by `10^6` and result is `055057`.
    */
   private static generate(length: number) {
-    return Math.floor(secureRandomNumber() * Math.pow(10, length))
-      .toString()
-      .padStart(length, '0');
+    return secureRandomNumber(length).join('');
   }
 }
 
 /**
  * Cryptographically secure equivalent of `Math.random()`.
  */
-function secureRandomNumber() {
-  return crypto.randomBytes(4).readUInt32LE() / 0x100000000;
+function secureRandomNumber(length: number) {
+  const digits = [];
+  while (digits.length < length) {
+    const byte = crypto.randomBytes(1)[0];
+    if (byte >= 250) {
+      continue;
+    }
+    digits.push(byte % 10);
+  }
+
+  return digits;
 }