Android unlocked

Is your Android phone's bootloader unlocked? Is it SIM unlocked? What's the difference?

There's been a bit of confusion in the blogs the past few days over unlocking phones. Maybe you're wondering about an unlocked bootloader. Or maybe you need something that's SIM unlocked. Or maybe you want to unlock your phone's bootloader, but you can't, because it's encrypted.

It's confusing, we know. Even bloggers have a hard time keeping it all straight. But you've come to the right place. We don't have that problem here. 

So let's have a little refresher course on what we mean when we talk about unlocking things, shall we?

Unlocking bootloaders

Galaxy Nexus unlock

Here in the United States, this is the one you're more likely to be concerned with. 

Every Android smartphone has what's called a "bootloader." Think of it as a set of instructions that runs before the Android operating system kicks in. No bootloader, no phone. 

Now, 99.99 percent of Android smartphones ship with a "locked" bootloader. (The other .01 percent are phones you probably don't want to know about anyway.) It's "locked" insofar as you can't overwrite it with files that won't work -- or overwrite it with any files for that matter -- either accidentally, or on purpose if you're the tinkering type.

It's also a security measure. You can't unlock a bootloader without erasing all the data that's already on the phone. That means someone can't steal your phone, unlock the bootloader and circumvent your log-in password or anything to get to your data -- it'll be erased in the process! So when you start seeing headlines that say "Such-and-such phone ships with a locked bootloader!", you know not to freak out, because nearly every single Android phone ships with a locked bootloader. And you want it to.

The question is whether you're actually able to unlock the bootloader.

But let's back up for a second. Why on god's green Earth would you want to mess around with a bootloader?

HTC Dev unlockThe simple answer is so you can tinker. So you can flash new kernels. Or ROMs. Or even just a custom recovery. The bootloader is the gatekeeper for all this stuff. Again, it's locked out of the box so that you can't accidentally break something, or so that someone (or something) can't purposefully break things.

The act of encrypting a bootloader means that it can't be unlocked without a key. No key, no unlocked bootloader, and no hackery. That's what folks are upset about. They want to tinker, and whomever decided an encrypted bootloader is the way to go -- and it really doesn't matter if it was the carrier or the manufacturer -- isn't making it easy. There are arguments on both sides. Locking down the phone so it can't be tinkered with means fewer customer-service calls, which cost companies money. On the other hand, you bought the phone, you should damn well be able to do whatever you want with it. We get that, too.

Not every phone throws the wall of encryption at you. Google's Nexus line of devices can be unlocked after about 30 seconds and a three little words in a command line: fastboot oem unlock. Many new HTC phones are easily unlocked with a tool provided by HTC. Sony has its own method. It requires a little more time and brain power, but it's not all that hard to do. 

So that's bootloader unlocking -- opening up a way to more easily tinker with your phone.

SIM unlocking

SIM unlock

The other kind of "unlocking" is SIM unlocking. If you've been living in the GSM world (ie not on Sprint or Verizon here in the States), chances are you've heard this before.

A brief primer: To use a phone on a GSM network anywhere, it needs a SIM card. That's the little card that lets the network recognize your phone, so that you can make phone calls and get data. (You also can store a little bit of information on it, most often contacts.) GSM phones use SIM cards. In the U.S., that means AT&T and T-Mobile. Now, to use an LTE network -- like what Verizon's had for more than a year now, and Sprint's just getting going -- your phone will need a SIM card as well. That's what's relatively new for Verizon and Sprint, and likely where some of the confusion is creeping in. 

Think of it like this: LTE is a GSM standard, therefore it uses a SIM card. Easy enough. Recently, Verizon announced that a couple of its phones -- including the Droid RAZR/MAXX and the Galaxy S III -- would receive software updates so that you could use them outside the United States, on GSM networks. In other words, you update the phone, add international data and calling to your plan, and things should just work.

SIM cardSo back to SIM unlocking. There are two ways to purchase GSM-capable phones -- SIM locked, or SIM unlocked. The difference is simple. With a SIM-unlocked phone, you can take a SIM card from any carrier -- AT&T, T-Mobile, Three, Vodafone, O2, etc. -- pop it in the phone, and it should work after a few changes in your settings. There's nothing else keeping it from working.

A SIM-locked phone blocks the use of other carriers' SIM cards. Take, say, a Vodafone SIM card and put it into an AT&T Galaxy S III, and the phone's going to ask your for an unlock code. If you don't have the code, you're not going to be using another carrier's SIM card. (Getting the code is easy enough. If your account is in good standing, chances are the carrier will give it to you. Or there are services online that can help, for a fee.)

Why use another carrier's SIM card? Simple: It can be much cheaper than adding international roaming features onto your current plan. Because your carrier loses money when you do that, they don't make it all that easy. (But neither is it all that hard.)

This is not rooting

It's worth noting that nowhere have we mentioned the words "root" or "rooting." Unlocking bootloaders and having a SIM-unlocked phone has nothing to do with having root access on your phone. Having an unlocked bootloader can make gaining root access easier, but a locked bootloader isn't necessarily a deal-breaker.

It's something to keep in mind.

The tl;dr version

Wrapping it all up in two handy bullet points:

  • Bootloader unlock: Needed for some serious hacking.
  • SIM unlock: Needed to use your phone on other GSM networks.

They're not the same thing. At all. One doesn't have anything to do with the other.


Reader comments

Ask AC: What is 'unlocked'?


I love these Android 101's. You put technical terms into ordinary language. I pick up something new now and then also. But this helps me explain to some friends who are android noobs or iPhone converts. Thanks Phil!

Well said! People often ask me questions about these things when I discuss how I unlocked and rooted my phone; now I have a post I can direct them to for a great explanation! Thanks Phil!

The galaxy lineup from Samsung is also easy to get access to things as well. Bootloader access is not encrypted. SIM unlocking can be done via software (with root access) that's easily obtained in the play store.

People love to bash Sammy but at least they leave the device mostly open (unless your a GS3 VZW user).

Just a quick factoid: the Galaxy Nexus on Sprint does not have a SIM card for LTE. I'm not sure about the other LTE phones in their lineup, but there isn't one in my Nexus.

A common misconception is that LTE needs a SIM. The LTE standard does not require a SIM card as it can use other methods to identify the subscriber (though 3GPP recommends using SIM cards). The spec list for the Sprint Evo 4G LTE list a non-removable SIM card (insultingly listed as if its a "feature") so it sounds like Sprint is using the SIM card standard but sealing them in the phones.

Is there a way to relock your bootloader after you put a custom ROM on your device? That way you could have your cake and eat it too.

Depends on the phone. But usually even if re-lockable, the bootloader will still record the fact that it has been unlocked in the past.

Very true. Pretty much all devices have a "tamper" flag in the bootloader that will be set if you unlock the bootloader and is (for all intents and purposes) impossible to turn off. Think of it like the digital "tamper tape" you'll find on a lot of electronic devices covering screws and the like.

The more important question, here, is "what happens if I have to take my phone to the store and the tamper flag is set?" Honestly, probably nothing. It all depends on what's wrong with the phone, of course, but most of the carriers don't really care, so long as you're not trying to get a new phone because of something *you* did as a result of hacking at it. I'm on Sprint, and they seem especially indifferent about the whole thing.

Basically, this flag is just a way for the warranty guys to be able to determine if a fault was actual device failure, or likely the result of somebody "tinkering" with the device. The carriers aren't really as concerned about you unlocking your bootloader as they are about not having to buy you a new phone if you screw something up.

The article should clarify one thing. A locked bootloader does not mean you cant flash *anything*. It means you can only flash ROMs that have the proper digital signature from the phone manufacturer. Unlocked bootloader will flash anything you ask it to.

Verizon is the most notorious for making manufacturers lock their bootloaders. GS3 is a good example of this, which is the only version of a GS3 with a bootloader that cannot be unlocked. On one hand they have a legitimate argument. Customers who dont know what they are doing end up bricking their device and costing VZW in customer support. Even worse customers who do know what they are doing purposefully brick their device just to get a shiny new one. Technically it is not verizon's obligation to replace these devices as it is really a manufacturer warranty issue but what customer wouldnt raise hell if they didnt walk out of the verizon store with a working phone?

Verizon struck a compromise with the GS3. The version they sell in the store is locked but you can buy the "developer edition" direct from Samsung with an unlockable bootloader. This way they can say you didnt buy the phone from them so they are not responsible for your warranty issues. I can live with that. Sure, the phone might not be subsidized with a contract but you could buy a subsidized phone from VZW and sell it to help offset that. Samsung probably still says parts of the warranty are void if you unlock the bootloader. This is ok as long as they still honor warranty claims that have nothing to do with the bootloader.

Correction: You are referring to *encrypted* bootloaders. Not "locked". All carrier's phone have locked bootloaders. Even the Nexus series.


"ROM" stands for "Read Only Memory". This term differentiates it from RAM (Random Access Memory), which is the memory your electronic devices uses to load and run applications. It is also different because RAM memory is erased when the device is turned off.

The term "Image" refers to a collection of all the bits and pieces of data in an exact placement and order. You can have an image of the OS of a device, a CD/DVD, etc. This term gets misused a lot in the mobile environment, as a typical ROM that you download from XDA is not an "Image" in the strictest sense of the term.

Both these things are really the same thing. Most of the time, though, you'll see flash-able zip files that replace the OS on your phone referred to as a "ROM".

This subject I've never needed to study---till I read Phil's clear explanations here, and now I want to understand it.

" . . . an unlock code. If you don't have the code, you're not going to be using another carrier's SIM card." Does this unlock code refer to the phone or to the phone's initial SIM card or to a SIM card from another carrier that the user wants to insert into the phone? (Please excuse my ignorance.)

The unlock code refers to the phone.  As soon as you insert a SIM card from a different carrier it will ask for this code.  It's device specific, not SIM specific.

These are almost always either review or old info for me, but they are really great for using as a method to point my less tech-savvy friends in the right direction. Very well written, great job!

Have a quick question about bootloader unlock. So when I unlock the bootloader, say on a Nexus, but do not flash any new ROM or kernel, will I continue to get the updates from Google automatically. Or do I have to do something to be updated every time some kind of update releases.

You can still get OTAs if you're unlocked. However, usually before an OTA runs it checks that various files are what it expects, eg. system files, radio, etc haven't been altered. If you've changed any of these, the OTA will probably fail. The process of unlocking won't change these, nor will flashing factory images from Google (because this will always result in a legitimate set of files), but having an unlocked bootloader means that you *can* change these. For example, you might flash a factory image and then flash a different radio or kernel. An OTA will probably fail in this case.

Yeah i really enjoy these articles. see this is why i read all of mobile nations. whether its bb, android or wp. i learn something new everyday and then i can educate the average joe.

btw someone explain to me why Sprint is going the way of 'embedded LTE' ??

I don't know the answer for certain, but I would imagine that it's because their existing CDMA network doesn't use SIM cards to identify the phone, giving users the ability to "swap" out LTE SIM cards would result in a clerical nightmare.

Most GSM-based LTE phones (such as AT&T) use a single SIM card to identify the phone on both networks. Swap it to a new device, and the entire account follows you. It makes sense for Sprint to not want you using the voice connection on one device and swapping *just* the LTE over to another.

Phil, please read the following comment. Partway through the article, I was like, this must be Jerry talking, so I scrolled back up to the top. Bit no, it's Phil, writing on something technical! Not only that, but you made it pretty simple to understand. Well done, sir.

Thanks for a great explanation. I had picked up most of it from reading AC but this filled in a few missing pieces.

Question on Sim unlocking. I've read about having certain radio bands or phones only being able to get lower speeds on other networks. Where does this fit in with using other carriers. Is there more to it than just an unlocked SIM?

Each phone's radio antenna governs the frequencies on which it can receive and transmit. As a "global" phone, Verizon's Motorola Droid 3 ordinarily conducts voice communication on Verizon's CDMA network but has a full-size GSM SIM card for use outside the US. The Droid 3 is incompatible with either AT&T or T-Mobile's 700 MHz spectrum in the US, since the Droid 3's GSM antennae are tuned to the 900-1800-2100 MHz spectra used elsewhere.

Thanks for sharing the knowledge... and not keeping it for yourself ! This is one of the main reasons I like Androidcentral so much !

Thanks know I'm new with all this wanting to unlock the bootloader and wanting to root my phone so I can be a "super User," and the truth is, like a lot of people out here- they/me don't really know what it is and what you gain. Just watching a video on Youtube makes it look easy enough, but terrifying I/we might screw something up with the feeling of 'no return' to the original settings... Anyway your article really made it simple for me to understand...the simplicity was perfect!!!

I have a note 2 that was flashed to page plus.... how do I get it fixed so that it can be activated on us cellular again?