How to solve ADB device unauthorized in Android ADB host device?

Issue

When I'm using a rooted Android device as ADB host to send adb command "adb devices" to Samsung S4, I received device unauthorized error message. However when I tried adb to Samsung Galaxy Nexus, it is working fine. Can anyone advise how to solve my Samsung S4 problem?

=========================================
# adb devices
List of devices attached
4d00f9169907301b        unauthorized
=========================================

Thanks in advance.

Edit: Found that this problem only happened to Android 4.2.2 and above. The following link explained that Google has implemented some new security features for using adb.

http://nelenkov.blogspot.com/2013/02/secure-usb-debugging-in-android-422.html

Appreciate if anyone can help on this.

Solution

• Get the public key from the client phone (adb host)

  cat /data/.android/adbkey.pub

• copy the above public key to the target phone's /data/misc/adb/adb_keys location. (you may need to stop the adb daemon first with stop adbd)

  cat /data/misc/adb/adb_keys

verify both cat outputs match.

try restarting adb daemon on target start adbd or just reboot them.

If you are having problems reading or writing to ADB KEYS in above steps, try setting environment variable ADB_KEYS_PATH with a temporary path (eg: /data/local/tmp). Refer to that link it goes into more details

    "On the host, the user public/private key pair is automatically generated,
    if it does not exist, when the adb daemon starts and is stored in
    $HOME/.android/adb_key(.pub) or in $ANDROID_SDK_HOME on windows. If needed,
    the ADB_KEYS_PATH env variable may be set to a :-separated (; under
    Windows) list of private keys, e.g. company-wide or vendor keys.

    On the device, vendors public keys are installed at build time in
    /adb_keys. User-installed keys are stored in /data/misc/adb/adb_keys"

Answered By - ashoke