Thread: Nuvi 58LMT "can't unlock maps".

Originally Posted by stevex

OK well I did offer to go away...

Yeah i know. But i hadn't finished torturing you had I? ...

Yes that makes sense. Storing the results of a computationally expensive result in NV will speed up boot times. I'd have expected it to check to see if the files had changed (if they exist, dates, sizes even hashes) but apparently not.

I happily admit i don't understand the detail behind the actual machinations and fancy footwork happening, i just know it does happen. Once the data is in NV there is no need for UNL/GMA files in visible memory to be inspected again by the device unless the existing map data changes or another map is added. Then, if the new map has a different FID, because the device can't find the appropriate associated codes in NV, it'll start searching the visible internal and external memory for them and even look inside the map IMG files for a relevant UNL code.

OK well here's a few things I'm curious about. If any of this is documented anywhere just point me in the right direction and I'll go off and study it.

Because of the fact that Garmin's OS is a closed shop i really don't know if you'll find anything more that what's talked about right here.

The .unl files seem like something relatively simple.

Yes they are really just a bog-standard TXT file renamed UNL. The file can contain just one single code string with matching map file name, or many individual codes unseparated and continuous. I think a file with multiple codes can only be named as "gmapsupp.unl" - i haven't had to think about this stuff for a while now.

Is their precise format documented anywhere?

Not much to tell. They're simply a string of 25 alpha numeric characters generated by a 'keygen' having the knowledge of a device's UID and a particular FID etc. for the relevant map data.

I was thinking of throwing together a quick parse/generation tool that wont trigger the stupid anti-virus warnings.

Are you getting warnings using JM? Many AV programs treat all such keygen tools as a threat. JM doesn't trigger anything in my Win 11 PC running Windows Security (aka Defender) but i may have excluded it years ago.

The .gma files? What's known about those?

Very little AFAIK. The initials stand for "Garmin Map Authentication" and they were introduced as an additional barrier to map piracy because the UNL codes were so easy to generate once JM circulated. They are certainly far more sophisticated than UNL files. Now we additionally have MSV and GSV (aka GVS) protection in modern devices' firmware which confirm the Map Signature Verification and GUPDATE Signature Verification, that means an unlocked map IMG file or modified GCD fw file is rejected. That's overcome by the universal firmware patcher janch referenced earlier.

They've got some ASCII stuff in what looks like a header and then nothing obvious. Might be binary data or some kind of cryptographic information to check authenticity.

I dunno what it is after "GARMIN GMA" in clear. All i know is what it does, which is check the authenticity of the map for use on the device. Far smarter ppl than me decided long again it was easier to strip the GMA protection from the map than try to generate GMA codes like was done for UNL codes.

What do the .sid files do?

Look here: [Only registered and activated users can see links. ], and [Only registered and activated users can see links. ]. Scroll down the alphabetic list to SID.

The one that I'm still puzzling over is how GN got the Full EU map. Creating data on an SD card (virtual or real) with just GarminDevice.xml is enough to keep GE happy and download maps details (presumably this is so it can recover if someone wipes the visible flash) along with .unl and .gma for the device but I've never been offered Full EU as an option when I change region.

It's not to do with selecting a different region on the device following a reset.

You can replicate what he did by putting your latest GarminDevice.xml file in a Garmin root folder on a USB or Virtual drive. This is simply exploiting what GarminExpress does, it only reads the XML to check entitlements for the specific Unit ID number on Garmin's servers. If it's got allowable updates, it'll offer them space permitting. If, like your UK-market travel edition, it has LM entitlement to maps other than UK/ROI you can get them too just like GN did:

Spoiler: Click for Image

The above page is reached from GE's Home Page>Map Details>Map Options>Change Map>Accept>Continue then the dropdown next to the current map detail will list all other maps available to the device.

Also actually, no i don't think the option in GE to restore existing maps or download new ones is to do with recovering an empty flash specifically.

PS: I see GN replied as i was still drafting this so i've probably repeated some of what he's just explained.