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Unlocking the bootloader

This is a preliminary guide in unlocking your device's bootloader.
Unlocking your device's bootloader is the first step to be done in order to install a custom firmware. This is vital as your device's bootloader normally disallows installing third-party firmware, unless you unlock it.
You can skip this portion if you have already unlocked your bootloader.
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What is a 'bootloader' anyway?

A bootloader is a vendor-proprietary image responsible for bringing up the kernel on a device. It guards the device state and is responsible for initializing the Trusted Execution Environment and binding its root of trust. The bootloader also verifies the integrity of the device itself before moving execution to the kernel, and displays boot state warnings.
In short terms, a bootloader checks if the software installed on your device is valid, genuine, and secure before the device attempts to boot. As the first step in an Android device's boot process, it checks for the signatures and disallows booting if it detects a corrupted or unsigned software.
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Unlocking a Samsung bootloader

Unlocking your bootloader does not void your warranty immediately. Your warranty becomes void once you install custom software on your device.
If you are uncomfortable with this, immediately stop proceeding with this guide.

What you will need for this step

  • A PC, preferably running Microsoft Windows(TM)
  • A USB Type-C to Type-C cable, make sure the cable does not lose connection immediately when plugged in
  • Your device, preferably charged to at least 60%.

Steps in unlocking your device's bootloader

Proceeding with this step will completely wipe your data and remove all files. Make sure you have backed up all important data before proceeding.
  1. 1.
    Enable developer options if you haven't enabled it yet. Open the Settings app, tap About phone then Software information, and tap the Build number until you see "Developer options has been turned on."
  2. 2.
    Open the Settings app.
  3. 3.
    If not yet enabled, turn on the OEM unlocking option. This will allow you to fully unlock the booloader.
  4. 4.
    Completely turn off the device by long-pressing the power button and tapping Power off. Tap the icon again to confirm. You can now plug in the USB cable to your PC while your device is shutting down.
  5. 5.
    Once your device has turned off completely, press both volume up and volume down buttons, and plug in the USB cable to your phone.
  6. 6.
    Your device will turn on to a teal screen. This is your device's download mode.
  7. 7.
    Long-press the volume up button until you see a screen titled "Unlock bootloader".
  8. 8.
    Press the volume up button to confirm the bootloader unlock.
  9. 9.
    Your device will reboot twice while it removes all data completely. It will take a while before it turns on to a setup screen.
The Download Mode feature of your device is useful in installing custom software, as well as the manufacturer's official software. More on this later.

Setting up the device after unlocking the bootloader

This is a very important step to do after unlocking your bootloader or re-installing the manufacturer's official software. Samsung disallows devices from installing custom software for at least 7 days if the steps below are not done.
  1. 1.
    Set up your device as usual. Ensure you connect your device to the internet, this is very important in ensuring Samsung immediately 'unlocks' your device.
  2. 2.
    If needed, enter your existing Google Account's information. You can choose to restore data at this point, if you're not installing Fresh immediately.
  3. 3.
    After setting up, go to the Settings app.
  4. 4.
    Find and tap Developer options.
  5. 5.
    Check if the OEM unlocking option is permanently enabled (grayed out), with a caption saying "Device bootloader is unlocked".
  6. 6.
    You're ready to install custom software on your device.
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Re-locking the device's bootloader

Here are the steps in the case you decide to re-lock your device's bootloader.
  1. 1.
    Ensure you are running the manufacturer's official software for your device. See the steps here for instructions.
  2. 2.
    Completely turn off the device by long-pressing the power button and tapping Power off. Tap the icon again to confirm. You can now plug in the USB cable to your PC while your device is shutting down.
  3. 3.
    Once your device has turned off completely, press both volume up and volume down buttons, and plug in the USB cable to your phone.
  4. 4.
    Your device will turn on to a teal screen. This is your device's download mode.
  5. 5.
    Long-press the volume up button until you see a screen titled "Lock bootloader".
  6. 6.
    Press the volume up button to confirm the bootloader lock.
  7. 7.
    Your device will reboot twice while it removes all data completely. It will take a while before it turns on to a setup screen.
  8. 8.
    You have re-locked your device's bootloader.
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For RMM/KG Prenormal-locked devices

Your device is considered to be locked by Samsung's KnoxGuard if any of the following criteria matches:
  • Your device refuses to install custom software from Download Mode and would only install firmware officially released by the manufacturer; or
  • The OEM unlocking option in the device's Developer options menu is missing.
To resolve this issue, you may try doing any of the following:
  • Keep your device connected to the internet (either Wi-Fi or mobile data)
  • Regularly check for software updates
  • Refer to this guide by XDA Portal for an alternative way