code: final void randomizePrefix() {
          byte[] cb = new byte[preLen];
  
          // Use SecureRandom for prefix only
          srand.nextBytes(cb);
  
          for (int i = 0; i < preLen; i++) {
              pre[i] = digits[(cb[i] & 0xFF) % base];
          }
      }
docs: /*
  Generate a new prefix from random. This *can* drain entropy and will be
  called automatically when we exhaust the sequential range.