title: RFID tag block no longer accessible after writing data to it
body: <p>I just got a RFID-RC522 module for Arduino and a RFID tag. This is all new to me. In this tag the memory block 7 (sector 1) data is:</p>
  
  <pre><code>00 00 00 00 00 00
  FF 07 80 69
  FF FF FF FF FF FF
  </code></pre>
  
  <p>Originally, the first 8 bytes oof the block (key A) were set to <code>0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF</code>.</p>
  
  <p>Playing with it I wrote this data to block 7:</p>
  
  <pre><code>00 00 00 00 00 00
  00 00 00 00
  FF FF FF FF FF FF
  </code></pre>
  
  <p>And now I can no longer access it. Reading the tag with NXP TagInfo (Android app) says the default key is <code>0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF</code>. However, using this value, or a key consisting of all-zero bytes, I can't authenticate to the block.</p>
  
  <p>How can I find out the new authentication key? How exactly did I change it?</p>