Can't connect Nexus 4 to adb: unauthorized

Question

I had similar situation. Here is what I did:

Try to check and uncheck the USB Debugging option in the device. (if not working, try to unplug/plug the USB)

At some point, the device should show up a messagebox to ask you if you authorize the computer. After you click yes, the device is then authorized and the connection is hooked.

I solved this problem without deleting any keys. Here's how:

1. My device was connected to my computer via USB
2. I made sure Eclipse and android device monitor weren't running.
3. Typed at the commandline prompt: adb kill-server
4. Typed at the commandline prompt: adb start-server
5. After adb was restarted, I got a prompt on my device to authorize which I allowed.

Сhange USB connection mode from MTP to Camera (for Nexus 7) or, possibly, to Mass Storage or something else (for other devices). This option is usually under Settings -> Storage. Then connect the device again, you'll get the authorization dialog.

MTP has been known to interfere with USB debugging -- these two did not work together at all on majority of older devices. Nexus 7 and many newer devices do allow both to work alongside, but this particular issue suggests it's not all that smooth yet.

Bonus -- checklist for when adb is not behaving well:

• adb kill-server followed by adb start-server
• (on device) Settings -> Developer Options -> USB Debugging -- switch off and on
• [Windows] Make sure you have the proper driver (your best bet is Universal Adb Driver by Koushik Dutta -- will handle any device)
• [Windows] Try turning off all fancy on-the-fly anti-malware scanners/firewalls
• [Linux] Make sure you have the proper UDEV rule in /etc/udev/rules.d/51-android.rules (again, universal solution: https://github.com/snowdream/51-android)
• [Linux] Make sure everything under ~/.android is owned by you, not root (and upvote this answer)
• restart device (yes, surprisingly, this is a valid measure, too)
• (Obviously) replug cable, try different cable, different port, remove any extender cables

I think it has an error when the device tries to display the screen asking for permission, so it does not appear.

This works for me (commands are given in the adb shell):

1. rm /data/misc/adb/adb_keys;
2. I sent the public key (adbkey.pub in ~/.android/) from my computer to my device via email;
3. Invoke stop adbd;
4. cat adbkey.pub >> /data/misc/adb/adb_keys (authorize myself);
5. start adbd (restart adb with new keys).

After you ensure you have enabled USB debugging unlock your phone and plug it into your machine via USB. You will be then asked to authorize communication with the computer you have connected to. It will also show computers RSA key fingerprint.

Accept it and you are good to go!