title: Sideloading Error 21 Signature Verification Failed
body: <p>I've read that it's because the bootloader is locked so I won't be able to flash the whole image. But if I unlock the bootloader, won't that wipe all my data? I would like to to see if sideloading can fix my issues (freezing, bootlooping) before going nuclear.</p>
  
  <p>After the package is verified it returns:</p>
  
  <pre><code>E: footer is wrong
  Update package verification took 0.1s (result 1).
  E: Signature verification failed
  E: error 21
  
  Install from ADB completed with status 2
  Installation aborted
  </code></pre>