title: How to check CRC16 validity
body: <p>I'm working on receiving binary data from sensors for the first time.
  The data is base64-encoded, I should decode the data and validate it and then save it to the database. One step of the validation process is to check for the CRC-16 validity.</p>
  <p>Each payload I receive comes with a CRC code, I have the function that is supposed to calculate the CRC-16 code itself, all I want to know is if it's enough to check that by passing the decoded data to the CRC-16 calculation function and then comparing the result to zero? If it's non-zero, then the data has been corrupted.</p>
  <p>If everything went fine, I need unpack the binary data and loop over the result to get the sensors' data such as the <code>battery</code> and <code>air_temperature</code> according to specific offset info (according to the manufacturer's documentation). Then save the data as we normally do to the DB.</p>
  <p>The problem is: I get non-zero values when I apply the <code>crc16Calc</code> function to a valid dataset.</p>
  <p>Can that be because the CRC is added to the <em>beginning</em> of the data string, not to the <em>end</em>? I mean the structure of the payload is <code>&lt;CRC code&gt;&lt;Original code&gt;</code>, not the opposite!</p>
  <p>My code is:</p>
  <pre class="lang-php prettyprint-override"><code>public static $crc16_tbl = [
      0x0000, 0xC0C1, 0xC181, 0x0140, 0xC301, 0x03C0, 0x0280, 0xC241,
      0xC601, 0x06C0, 0x0780, 0xC741, 0x0500, 0xC5C1, 0xC481, 0x0440,
      0xCC01, 0x0CC0, 0x0D80, 0xCD41, 0x0F00, 0xCFC1, 0xCE81, 0x0E40,
      0x0A00, 0xCAC1, 0xCB81, 0x0B40, 0xC901, 0x09C0, 0x0880, 0xC841,
      0xD801, 0x18C0, 0x1980, 0xD941, 0x1B00, 0xDBC1, 0xDA81, 0x1A40,
      0x1E00, 0xDEC1, 0xDF81, 0x1F40, 0xDD01, 0x1DC0, 0x1C80, 0xDC41,
      0x1400, 0xD4C1, 0xD581, 0x1540, 0xD701, 0x17C0, 0x1680, 0xD641,
      0xD201, 0x12C0, 0x1380, 0xD341, 0x1100, 0xD1C1, 0xD081, 0x1040,
      0xF001, 0x30C0, 0x3180, 0xF141, 0x3300, 0xF3C1, 0xF281, 0x3240,
      0x3600, 0xF6C1, 0xF781, 0x3740, 0xF501, 0x35C0, 0x3480, 0xF441,
      0x3C00, 0xFCC1, 0xFD81, 0x3D40, 0xFF01, 0x3FC0, 0x3E80, 0xFE41,
      0xFA01, 0x3AC0, 0x3B80, 0xFB41, 0x3900, 0xF9C1, 0xF881, 0x3840,
      0x2800, 0xE8C1, 0xE981, 0x2940, 0xEB01, 0x2BC0, 0x2A80, 0xEA41,
      0xEE01, 0x2EC0, 0x2F80, 0xEF41, 0x2D00, 0xEDC1, 0xEC81, 0x2C40,
      0xE401, 0x24C0, 0x2580, 0xE541, 0x2700, 0xE7C1, 0xE681, 0x2640,
      0x2200, 0xE2C1, 0xE381, 0x2340, 0xE101, 0x21C0, 0x2080, 0xE041,
      0xA001, 0x60C0, 0x6180, 0xA141, 0x6300, 0xA3C1, 0xA281, 0x6240,
      0x6600, 0xA6C1, 0xA781, 0x6740, 0xA501, 0x65C0, 0x6480, 0xA441,
      0x6C00, 0xACC1, 0xAD81, 0x6D40, 0xAF01, 0x6FC0, 0x6E80, 0xAE41,
      0xAA01, 0x6AC0, 0x6B80, 0xAB41, 0x6900, 0xA9C1, 0xA881, 0x6840,
      0x7800, 0xB8C1, 0xB981, 0x7940, 0xBB01, 0x7BC0, 0x7A80, 0xBA41,
      0xBE01, 0x7EC0, 0x7F80, 0xBF41, 0x7D00, 0xBDC1, 0xBC81, 0x7C40,
      0xB401, 0x74C0, 0x7580, 0xB541, 0x7700, 0xB7C1, 0xB681, 0x7640,
      0x7200, 0xB2C1, 0xB381, 0x7340, 0xB101, 0x71C0, 0x7080, 0xB041,
      0x5000, 0x90C1, 0x9181, 0x5140, 0x9301, 0x53C0, 0x5280, 0x9241,
      0x9601, 0x56C0, 0x5780, 0x9741, 0x5500, 0x95C1, 0x9481, 0x5440,
      0x9C01, 0x5CC0, 0x5D80, 0x9D41, 0x5F00, 0x9FC1, 0x9E81, 0x5E40,
      0x5A00, 0x9AC1, 0x9B81, 0x5B40, 0x9901, 0x59C0, 0x5880, 0x9841,
      0x8801, 0x48C0, 0x4980, 0x8941, 0x4B00, 0x8BC1, 0x8A81, 0x4A40,
      0x4E00, 0x8EC1, 0x8F81, 0x4F40, 0x8D01, 0x4DC0, 0x4C80, 0x8C41,
      0x4400, 0x84C1, 0x8581, 0x4540, 0x8701, 0x47C0, 0x4680, 0x8641,
      0x8201, 0x42C0, 0x4380, 0x8341, 0x4100, 0x81C1, 0x8081, 0x4040
  ];
  
  
  // $crc is an integer between 0 and 0xFFFF
  // $dataByte is an integer between 0 and 0xFF
  // The result is an integer between 0 and 0xFFFF
  function addCRC($crc, $dataByte)
  {
      $index = ($crc &amp; 0xFF) ^ $dataByte;
      $crc16int = self::$crc16_tbl[$index];
      return ($crc &gt;&gt; 8) ^ $crc16int;
  }
  
  // $buffer is a string containing the binary data
  // The result is an integer between 0 and 0xFFFF
  function crc16Calc($buffer)
  {
      $crc16 = 0;
      $length = strlen($buffer);
      for ($i = 0; $i &lt; $length; $i++) {
          // Use ord() to go from a length-1 string to an integer between 0 and 0xFF
          $dataByte = ord($buffer[$i]);
          $crc16 = $this-&gt;addCRC($crc16, $dataByte);
      }
      return $crc16;
  }
  
  public function store(Request $request)
  {
      // 1. Decode the data from base64 string, and check for CRC validaity. 
      $content = file($request-&gt;file('data'));
      Storage::disk('local')-&gt;put('examples.bin', '');
      $file_handler = fopen('C:\laragon\www\medium-clone\storage\app\examples.bin', 'w+');
      foreach ($content as $line) {
          $decoded_data = base64_decode($line);
          // check for CRC validaty
          print ($this-&gt;crc16Calc($decoded_data)). '&lt;br /&gt;'; // this gives a different non-zero number eachtime
          if($this-&gt;crc16Calc($decoded_data) != 0)
              return &quot;Invalid Data&quot;;
          //else
          fwrite($file_handler, $decoded_data);
      }
      fclose($file_handler);
  }
  </code></pre>
  <p><strong>Edit</strong>
  This is the data below encoded with base64, contains 20 payloads &amp; the image below explains the structure of the payload,  all multi-byte binary fields are ordered little endian.</p>
  <pre><code>otykgAFuAGUAAEwBQAMfCqMI6g3zA+UDBQR8AXEBiQEyAiQCPQKh/nb+SwBKAAA=
  WVOWgAFuAGUAAEwBQAMOCgAA6g1nAVsBcAEuAi0CMgJLAUgBTgFK/kX+IgAiAAA=
  g5v5gAFvAGcAAPAAQAMRCs0IxiWrA54DsgMzAycDQQObAI0ApwCFAnYCFAATAA8=
  z/5qgAFvAGcAABkBQAPuCSMJLh+uAqgCtALoA+gD6APY/9j/2P+uAqgCAAAAAA8=
  XoVTgAFvAGcAAPgAQAMDCr8JZiq0Aa0BvAGhAkIC3gL+ANAARAGG/7n+GgAWAAA=
  SI5CgAFvAGcAAPgAQAPvCQAAWirJAMEA0AD8ALgATwHvAcEBFQKu+U/4NAAvAAU=
  RxA9gAFvAGcAAA8BQAMRCrgJUCVbAkwCcgLNAoQCCQPjALIAIAGBAOD/GQAUAAA=
  T+s1gAFvAGcAAPgAQAP0CQAATioEAfsADQHgAL4AIgEMAucBIgJe+bL4OAA0AAU=
  H+EqgAFvAGcAAPgAQAP8CQAAQip0AXIBdgH0AswB6AOjAND/jgG0/1P9EgAAAAU=
  CLUbgAFvAGcAAPgAQAMDCgAAJirIAa8B5AHoA+gD6APT/9L/0//IAa8BAAAAAA8=
  3nAQgAFvAGcAAPgAQAMFCq4IHCqtAKUAswAyACoAQgBgAlQCZwKx8gfyQQBAAAA=
  fDsKgAFvAGgAADEBQAMvCtYJOfgmAxsDNgM+AzADVwOKAHIAmAAZAgQCEQAPAA8=
  YD4pgAFvAGgAADEBQAP2CQAAOfiCAXABlgHbA84D6APf/9D/7f9wAV4BAAAAAA8=
  hCW9gAFvAGcAAOkAQAMgCjoAbh6xALEAswC9A7IDxQP7//L/BgB1AGUAAgACAA8=
  HRv7gAFvAGcAAL4BQAP5CQAASBPCBbgFzAXoA+gD6APw/+//8P/CBbgFAAAAAA8=
  lZPRgAFvAGcAANcAQAMqCnoJTiAoAhwCOALvAuICCAPGALEA0QCTAG4AFQAUAAA=
  9AfcgAFvAGcAAE4BQAMdCgAAAMBUCEcIYwi1Aa8BuwHJAr4C1QJQA0oDjgCMAAA=
  KHT7gAFuAGUAADwBQAMrCv0ItA9EADQAVADoA+gD6APK/8r/yv9DADQAAAAAAA8=
  fcjsgAFvAGcAAK0BQAMdCqMJtg1OA0EDWwOHA3QDpANCACUAVwC6AqcCCgAHAA8=
  LHArgAFvAGcAAJwBQAMLCsQJpBXhANAAAgHoA+gD6APO/83/zv/hANAAAAAAAA8=
  </code></pre>
  <p><a href="https://i.stack.imgur.com/yLqrY.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/yLqrY.png" alt="This is the payload structure,  all multi-byte binary fields are ordered little endian" /></a></p>
  <p>I also tried to move the first two bytes of the CRC to the end of the string, then calculate the result, it gave <code>0xB9AE</code> which is non-zero, The function is performing the calculation correctly because I compared the result with an online CRC-16 calculator.</p>
  <pre><code>  $new_string = mb_strcut($decoded_data,2,46).mb_strcut($decoded_data,0,2);
          print $new_string;
          print 'crc1: '.$this-&gt;crc16Calc($new_string).'  ';
  </code></pre>