Thread: *.gma - What's inside?!

Originally Posted by Swall

For me, it seems the GMA mechanism is designed to facilitate many usage scenarios and it's up to map vendor which one to choose.

GMA was designed to counter forged unlock codes obtained using JetMouse's keygen.

Originally Posted by Swall

While in another scenario, the .gma file just supersedes the unlocking code file ( .unl), not "validates" it.

It's not like that.
GMA file of course contains unlock codes, the validity of which is confirmed by a digital signature in the file. The main thing in this file is a valid signature.

Here's the official Overview by Garmin:

Code:

Please Login or Register to see the links

Originally Posted by Cnfhbr

It's not like that.
GMA file of course contains unlock codes, the validity of which is confirmed by a digital signature in the file. The main thing in this file is a valid signature.

Shure it's some kind of signature. Anyway that's my experience. Some time ago my vendor said that starting from the next map version current "license" is no more valid (as the map's FID value has increased) so each customer have to "renew" it individually according to the procedure. So I'd sent them ID of my unit and the vendor did responded with the .gma file generated. Actually, in order to get the map working, all I have to do is to place that .gma file next to gmapsupp.img in the ext. storage media. There are no other related files like .unl, etc. The .gma file missing results in auth error so the map is indeed protected. And that .gma file is the sole mean to unlock the map. I've no idea if that's right or wrong but it in my case it works in that manner.

Thanks for the quote from the official Overview. Could you provide a direct link to the related Garmin page?

Originally Posted by Swall

Some time ago my vendor said that starting from the next map version current "license" is no more valid (as the map's FID value has increased) so each customer have to "renew" it individually according to the procedure. So I'd sent them ID of my unit and the vendor did responded with the .gma file generated. Actually, in order to get the map working, all I have to do is to place that .gma file next to gmapsupp.img in the ext. storage media. There are no other related files like .unl, etc. The .gma file missing results in auth error so the map is indeed protected. And that .gma file is the sole mean to unlock the map. I've no idea if that's right or wrong but it in my case it works in that manner.

No wonder.
An additional GMA-check was added to the firmware of modern devices for maps with FID > 2200.
As I mentioned before, the GMA file contains unlock codes that are digitally signed, so it was enough in your case.
However, many legacy devices don't have additional GMA-check and don't support GMA files at all, only UNL.

Originally Posted by Swall

Thanks for the quote from the official Overview. Could you provide a direct link to the related Garmin page?

Garmin does not publish such overviews on its pages available to the public.

Originally Posted by Giomen

As another option there are unlock code in img file. It may be distributed by vendor with ready for use file ...

Most likely that's the case, since the customer has updated the map on an individual basis.

Originally Posted by Giomen

Garmin device read it and content of the file be saved in non-volume memory. So until you will reset device there is no longer needed in this file. Did you use a unlock file with new FID sometime?

Of course the device can save the code in NVM because it's feasible. On the same ground one can claim that if your computer is networked then all your sensitive data you ever placed on the computer are (or will be soon) stolen. Honestly, I don't think it saves an unlocking codes. Simply because it's not necessary. Moreover it will weaken the map protection strength.

Originally Posted by Giomen

As another option there are unlock code in img file. It may be distributed by vendor with ready for use file or with mapset for mapsource.
We do not know anything about your map vendor...

Well the whole truth is that the map on the vendor's web site is available as an executable file. To get the map, customer have to download and run that file, which rises a dialog in the process asking for the unlocking code. The customer have to type the code and the program creates the map image file for use on the device. So it seems the unlocking code is buried in the map image. It's no more necessary to put the unlocking code file (.unl) to the SD card. For my device anyway

Originally Posted by Giomen

The both of things are fact!

Another fact is the device performs the map enabling procedure on each cold start. So it does not relies on the stored history data

Originally Posted by Swall

Another fact is the device performs the map enabling procedure on each cold start. So it does not relies on the stored history data

For most devices, the info on the successful authentication of the map is stored in non-volatile memory after the first success and kept there right down to the next hard/master reset or NonVol clearing.
For example, some tricks for temporary map authentication are based on this property.

Originally Posted by Swall

So it seems the unlocking code is buried in the map image.

It's easy to verify via ImgTool or GMapTool.
The 25-digit unlock code is written to the MPS subfile of the map as a U-record.