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Figuring out Samsung Accesory Protocol internals

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javispedro

Senior Member
Dec 6, 2008
55
46
Hello,

I want to figure out the Samsung Accesory Protocol in order to create a "open source" Gear Manager app replacement. This thread is to ask if anyone has been trying to do the same thing as well as try to gather as much information about this protocol as possible. Generic discussion is also accepted, in case anyone has better ideas.

Right now all I know is that this protocol is based on RFCOMM, albeit it can be transported over TCP too. It has a level 1 "framing" which consists basically on

Code:
packed struct Frame {
 uint16_be length_of_data;
 char data[length_of_data];
}

packed struct FrameWithCRC {
 uint16_be length_of_data;
 uint16_be crc_of_length;
 char data[length_of_data];
 uint16_be crc_of_data;
}

I also know that there are various types of packets. "Hello" packets are exchanged early during the connection and contain the product name, etc. Authentication packets are exchanged right after the initial "hello" and contain some varying hashes (crypto warning!). Then the normal data packets are "multiplexed", as in usbmuxd: they have 'session' IDs which described towards which watch program they are talking with. All Hello and authentication packets are sent without CRC, but normal data packets are. The CRC implementation used is crc16, same poly as in the linux kernel.

I suspect that whatever we uncover about this protocol might be useful to e.g. pair Gear with an iPhone, with a PC, things like that.

Note: most of this comes from viewing Bluetooth logs. However it's clear that reverse engineering will be required for the cryptographic parts. In this case I believe it's legally OK to do so in the EU because it's purely for interoperability reasons. I don't want to create a competitor to the Gear2, I just want to talk to it.


Motivation: I bought a Gear2 in order to replace a LiveView that was dying (buttons wearing out, broken wriststrap clips, etc.) . I used it both for notifications as well as map/navigation.
Since I have a Jolla, no programs are available to pair with most smartwatches, but I've been developing my own so far (MetaWatch, LiveView). Thus I decided on a replacement based purely on hardware characteristics and price. Also Tizen seems more open than Android, thus I figured out it would be easier for me to adapt to the watch.

However it seems that I understimated the complexity of the protocol that connects the Gear with the GearManager. So my options in order to make use of this watch are:

  1. Sell Gear2 back and buy something that's easier to hack (e.g. another LiveView :) ),
  2. Figure out the SAP protocol and write a replacement Gear Manager app (what this thread is about),
  3. Write replacement Tizen applications that don't use SAP. This involves writing new programs for Calls, Messages, Notifications, Alarms, Camera, watchOn, Pulse monitor, etc. i.e. a _lot_ of work if I want to exploit all features of the watch.
    But at least one can reuse the existing Tizen settings app, launcher, drivers, etc. (I started porting Qt to the Gear2 with this idea)
  4. Use a different Linux distro on the Gear 2. Such as Sailfish, Mer, etc. This involves all the work of option 3 + possibly driver work.
As of now I've not decided which option is easier for me so I'll keep trying to push them all.
 
Last edited:

noellenchris

Inactive Recognized Developer / Retired Moderator
May 11, 2007
2,599
886
Marlton, NJ
Hello,

I want to figure out the Samsung Accesory Protocol in order to create a "open source" Gear Manager app replacement. This thread is to ask if anyone has been trying to do the same thing as well as try to gather as much information about this protocol as possible. Generic discussion is also accepted, in case anyone has better ideas.

Right now all I know is that this protocol is based on RFCOMM, albeit it can be transported over TCP too. It has a level 1 "framing" which consists basically on

Code:
packed struct Frame {
 uint16_be length_of_data;
 char data[length_of_data];
}

packed struct FrameWithCRC {
 uint16_be length_of_data;
 uint16_be crc_of_length;
 char data[length_of_data];
 uint16_be crc_of_data;
}

I also know that there are various types of packets. "Hello" packets are exchanged early during the connection and contain the product name, etc. Authentication packets are exchanged right after the initial "hello" and contain some varying hashes (crypto warning!). Then the normal data packets are "multiplexed", as in usbmuxd: they have 'session' IDs which described towards which watch program they are talking with. All Hello and authentication packets are sent without CRC, but normal data packets are. The CRC implementation used is crc16, same poly as in the linux kernel.

I suspect that whatever we uncover about this protocol might be useful to e.g. pair Gear with an iPhone, with a PC, things like that.

Note: most of this comes from viewing Bluetooth logs. However it's clear that reverse engineering will be required for the cryptographic parts. In this case I believe it's legally OK to do so in the EU because it's purely for interoperability reasons. I don't want to create a competitor to the Gear2, I just want to talk to it.


Motivation: I bought a Gear2 in order to replace a LiveView that was dying (buttons wearing out, broken wriststrap clips, etc.) . I used it both for notifications as well as map/navigation.
Since I have a Jolla, no programs are available to pair with most smartwatches, but I've been developing my own so far (MetaWatch, LiveView). Thus I decided on a replacement based purely on hardware characteristics and price. Also Tizen seems more open than Android, thus I figured out it would be easier for me to adapt to the watch.

However it seems that I understimated the complexity of the protocol that connects the Gear with the GearManager. So my options in order to make use of this watch are:

  1. Sell Gear2 back and buy something that's easier to hack (e.g. another LiveView :) ),
  2. Figure out the SAP protocol and write a replacement Gear Manager app (what this thread is about),
  3. Write replacement Tizen applications that don't use SAP. This involves writing new programs for Calls, Messages, Notifications, Alarms, Camera, watchOn, Pulse monitor, etc. i.e. a _lot_ of work if I want to exploit all features of the watch.
    But at least one can reuse the existing Tizen settings app, launcher, drivers, etc. (I started porting Qt to the Gear2 with this idea)
  4. Use a different Linux distro on the Gear 2. Such as Sailfish, Mer, etc. This involves all the work of option 3 + possibly driver work.
As of now I've not decided which option is easier for me so I'll keep trying to push them all.

I think your thread should probably go in the Dev section for Tizen. Have you made any development? If your want it moved, report your own post with the button in top right labeled report. You can then suggest your thread be moved to the new Tizen Development section. Ok, I wish you all the luck, you seem to be very talented programmer/dev. Thanks for your contributions.

Chris
 

javispedro

Senior Member
Dec 6, 2008
55
46
I think your thread should probably go in the Dev section for Tizen.
Well, some mod already moved this thread from Development, where I originally posted it, into Q&A. This is not exactly "Tizen" development (SAP is used in may Samsung devices seemingly).

Have you made any development?

Yes, lots of progress. I have been able to write a program that connects to the Gear2 from my PC, succesfully "completes" the setup program and synchronizes the date&time. Things like changing the background color etc. are now trivial. I will soon port it to my Jolla.

I am now looking into how to send notifications to the watch. I've not been able to get Gear Manager to actually send any notifications (to use as "reference"), because goproviders crashes when I try to simulate notifications on my android_x86 VM :(
If anyone can send me an HCI / Bluetooth packet capture of their Android device while it is sending notifications to the Gear2 I would really appreciate it.


Unfortunately, the main problem here is that Samsung uses some cryptographic authentication as a form of "DRM". I am not exactly sure why.
There was no way for me to discover how the crypto worked so I took the unclean approach and dissasembled their crypto code (libwms.so). That means there's no way I would be able to distribute the code now without risking a lawsuit from Samsung.

Sadly this means that while I can distribute the protocol specifications I obtained, legally distributing "Gear Manager replacements" is probably impossible.
 

Antartica

Member
Sep 13, 2011
23
4
Unfortunately, the main problem here is that Samsung uses some cryptographic authentication as a form of "DRM". I am not exactly sure why.
There was no way for me to discover how the crypto worked so I took the unclean approach and dissasembled their crypto code (libwms.so). That means there's no way I would be able to distribute the code now without risking a lawsuit from Samsung.

Sadly this means that while I can distribute the protocol specifications I obtained, legally distributing "Gear Manager replacements" is probably impossible.

I would gladly write a MIT-licensed C library implementing your protocol specifications. That would be correctly following the chinese-wall approach to reverse-engineering, right?

Anyway, AFAIK, being in Europe decompiling for interoperability purposes is allowed -- I know that wikipedia is not to be taken at face value, but: en.wikipedia.org/wiki/Reverse_engineering#European_Union
 

javispedro

Senior Member
Dec 6, 2008
55
46
I would gladly write a MIT-licensed C library implementing your protocol specifications. That would be correctly following the chinese-wall approach to reverse-engineering, right?

Anyway, AFAIK, being in Europe decompiling for interoperability purposes is allowed -- I know that wikipedia is not to be taken at face value, but: en.wikipedia.org/wiki/Reverse_engineering#European_Union
Well, the problem is not the protocol specifications per se, which I'm actually quite confident I'd be able to redistribute (I'm in EU). The problem is the cryptography part, which is basically ripped off from the Samsung lib "libwsm.so" . Unless we can find out what cryptographic method that lib uses, distributing alternate implementations Is a no-go.
 

Antartica

Member
Sep 13, 2011
23
4
Well, the problem is not the protocol specifications per se, which I'm actually quite confident I'd be able to redistribute (I'm in EU). The problem is the cryptography part, which is basically ripped off from the Samsung lib "libwsm.so" . Unless we can find out what cryptographic method that lib uses, distributing alternate implementations Is a no-go.

If you have the time, I don't mind researching the possible crypto used (although I've only studied DES/3DES, AES and Serpent, hope that whatever scheme used is not very different from them).

Some ideas to start from somewhere:
1. As you have used its functions, it is a block cipher? I will assume that it is.
2. What is the key size and the block size?
3. Are there signs that it is using a stack of ciphers? (that is, applying one cipher, then another to the first result and so on)
 

javispedro

Senior Member
Dec 6, 2008
55
46
If you have the time, I don't mind researching the possible crypto used (although I've only studied DES/3DES, AES and Serpent, hope that whatever scheme used is not very different from them).

Some ideas to start from somewhere:
1. As you have used its functions, it is a block cipher? I will assume that it is.
2. What is the key size and the block size?
3. Are there signs that it is using a stack of ciphers? (that is, applying one cipher, then another to the first result and so on)

Hello, I've not forgotten about this, just somewhat busy and been using the MetaWatch lately :)

1. Yes it is clearly a block cipher, and the block size Is 16bytes.
2. I don't know about the key size, it is obfuscated.
3. Doesn't seem like a stack of ciphers. It looks like some overcomplicated AES. But to be honest AES is the only encryption I know of :)

By the way I think I will upload my current test "manager" source code to somewhere after removing the crypto specific files . Since the protocol itself has been obtained cleanly. Note I've used Qt (not the GUI parts) so it's useless for creating a library; the code will probably need to be rewritten to do so, but it may be useful as "protocol specs".
 
Last edited:

Antartica

Member
Sep 13, 2011
23
4
Hello, I've not forgotten about this, just somewhat busy and been using the MetaWatch lately :)

No problem. Curiously, I've transitioned from the metawatch to the Gear1 fully (null rom, not pairing with bluetooth to the phone but gear used as a standalone device).
[off-topic]I'm not using my metawatch anymore. I was modifying Nils' oswald firmware to make it prettier and to have some features I wanted (calendar, stopwatch), but it was very inaccurate, supposedly because of missing timer interrupts (the existing LCD drawing routines were too slow). I rewrote the graphics subsystem just to stumble into a known mspgcc bug, and trying to use the new redhat's mspgcc resulted in more problems (memory model, interrupt conventions). In the end I couldn't commit enough time to fix that and my metawatch is now in a drawer[/off-topic]

Returning to the topic:
1. Yes it is clearly a block cipher, and the block size Is 16bytes.
Good. We can at least say it isn't DES/3DES nor blowfish (64 bits block size). Regrettably there are a lot of ciphers using 128-bits block size; that I know: AES, Twofish and serpent.

Perusing the wikipedia there are some more of that size in use: Camellia, sometimes RC5 and SEED.

2. I don't know about the key size, it is obfuscated.
3. Doesn't seem like a stack of ciphers. It looks like some overcomplicated AES. But to be honest AES is the only encryption I know of :)

I understand that to mean that you cannot use that library passing your own key, right?

What a pity! One way to test for these ciphers would have been to just cipher a known string (i.e. all zeroes) with a known key (i.e. also all zeroes) and compare the result with each of the normal ciphers :-/.

By the way I think I will upload my current test "manager" source code to somewhere after removing the crypto specific files . Since the protocol itself has been obtained cleanly. Note I've used Qt (not the GUI parts) so it's useless for creating a library; the code will probably need to be rewritten to do so, but it may be useful as "protocol specs".

Perfect. I don't need anything more :).
 

javispedro

Senior Member
Dec 6, 2008
55
46
Ok, so I've uploaded my SAP protocol implementation: https://git.javispedro.com/cgit/sapd.git/ . It's "phone" side only, ie it can be used to initiate a connection to the watch but not to simulate one. In addition, it's missing two important files: wmscrypt.cc and wmspeer.cc which implement the closed crypto required to "pair" the watch. The most important file is sapprotocol.cc which implements the packing/unpacking of the most important packet types. The license of those files is GPLv3 albeit I'm very happy if you use the information contained on them to build your "Gear Manager" program under whichever license you'd prefer.

For anyone who hasn't been following the above discussion: I've figured out a large part (useful for at least establish contact with the watch and syncing time/date) of the SAP protocol used between the Gear watch and the Gear manager program on the phone. This has been done mostly by studying traces and afterwards talking to the watch using my test implementation above to figure out the remaining and some error codes. The debug messages left by the watch's SAP daemon were also immensely helpful. As long as I understand this is perfectly safe to do, publish and use as I'm in the EU and is basically the same method Samba uses.

Unfortunately, the protocol contains some crypto parts required for the initial sync (subsequent connections require authentication). However, the communication itself is not encrypted in any way, which helped a lot with the process. Because it's impossible for me to figure out whatever authentication method is used, I had to disassemble the library implementing this stuff (libwms.so). This is still OK according to EU law, but I'm no longer to release that information to the public. I'm looking for alternatives or ideas on how to handle this fact.

In the meanwhile, let's talk about the protocol. It's basically a reimplementation of the TCP(/IP) ideas on top of a Bluetooth RFCOMM socket. This means that it's connection oriented and that it can multiplex several active connections (called "sessions") over a single RFCOMM link. Either side of the connection can request opening a connection based on the identifier of the listening endpoint (called a "service"). Strings are used to identify services instead of numeric ports as in TCP. For example, "/system/hostmanager" is a service that listens on the watch side. Once you open a session towards this service (i.e. once you connect to it) you can send the time/date sync commands. In addition to be the above the protocol also seems to implement QoS and reliability (automatic retransmission, ordering, etc.). It's not clear to me why they reimplemented all of this since RFCOMM is a STREAM protocol, and thus reliability is already guaranteed!! So I've not focused much on these (seemingly useless) QoS+reliability parts of the protocol.

Let's start with the link level. There are two important RFCOMM services exposed by the watch: {a49eb41e-cb06-495c-9f4f-aa80a90cdf4a} and {a49eb41e-cb06-495c-9f4f-bb80a90cdf00}. I am going to respectively call those two services "data" and "nudge" from now on. These names, as many of the following ones, are mostly made up by me :) .

The communication starts with Gear manager trying to open a RFCOMM socket towards the "nudge" service in the watch. This causes the watch to immediately reply back by trying to open a connection to the "data" service _on the phone_ side. So obviously this means that your phone needs to expose the "data" RFCOMM service at least. In addition, the watch will try to open a HFP-AG connection (aka it will try to simulate being a headset) to your phone. Most phones have no problem doing this so no work is required. Of course, if your phone is a PC (as in my case :) ) then you'll need to fake the HFP profile. I give some examples in my code above (see scripts/test-hfp-ag and hfpag.cc).

Once the RFCOMM socket from the watch to the phone "data" service is opened, the watch will immediately send what I call a "peer description" frame. This includes stuff such as the model of the watch as well as some QoS parameters which I still don't understand. The phone is supposed to reply back to this message with a peer description of its own. See sapprotocol.cc for the packet format.

After the description exchange is done, the watch will send a "authentication request" packet. This is a 65 byte bigint plus a 2 byte "challenge". The response from the phone should contain a similar 65 byte bigint, the 2 byte response, and an additional 32 byte bigint. If correct, the watch will reply with some packet I don't care about. Otherwise the connection will be dropped. It obviously looks like some key exchange. But this is the crypto part that's implemented in libwms.so....

After these two exchanges link is now set up. The first connection that needs to be opened is towards a service that is always guaranteed to be present, called "/System/Reserved/ServiceCapabilityDiscovery". It is used by both sides of the connection to know the list of available services present on the other side. Despite this, you cannot query for all services; instead, you must always know the name of the remote service you're looking for. There's some 16-byte checksum there which I don't know how to calculate, but fortunately the watch seems to ignore it!! I suspect that you're expected to actually persist the database of available services in order to shave a roundtrip when connection is being established. But this is not necessary for normal function. This service is implemented in capabilityagent.cc, capabilitypeer.cc . This part was actually one of the most complex ones because of the many concepts. I suggest reading the SDK documentation to understand all the terms ("service", "profile", "role", etc.).

If everything's gone well, now the watch will try to open a connection to a service in your phone called "/system/hostmanager". Once you get to this message things start to get fun, because the protocol used for this service is JSON! It's implementation resides in hostmanageragent.cc, hostmanagerconn.cc . For example, Gear Manager sends the following JSON message once you accept the EULA: {"btMac":"XX:XX:XX:XX:XX:XX", "msgId":"mgr_setupwizard_eula_finished_req", "isOld":1}. At this point, the watch hides the setup screen and goes straight to the menu.

Well, this concludes my high-level overview of the SAP protocol. Hope it is useful for at least someone!

Things to do:
  • Personally I'm looking for some traces of the notification service. Ie the one that forwards Android notifications towards the watch. For some reason it doesn't work on my phone, so I can't get traces. I suspect it's going to be a simple protocol so a few traces will be OK. It's the only stuff I'm missing in order to be able to actually use the Gear as a proper smartwatch with my Jolla.
  • We still need to tackle the problem of the cryptographic parts. Several options: either "wrap" the stock libwms.so file, try to RE it the "proper way", .... I'm not sure of the feasibility of any of these.
  • Many other services.
 
Last edited:

Antartica

Member
Sep 13, 2011
23
4
After the description exchange is done, the watch will send a "authentication request" packet. This is a 65 byte bigint plus a 2 byte "challenge". The response from the phone should contain a similar 65 byte bigint, the 2 byte response, and an additional 32 byte bigint. If correct, the watch will reply with some packet I don't care about. Otherwise the connection will be dropped. It obviously looks like some key exchange. But this is the crypto part that's implemented in libwms.so....

About that 65-byte bigint... that is a 520-bit key. The usual length of ECDSA keys is exactly 520-bits, so we may have something there: it is possible that they are using ECDSA signing (just like in bitcoin, so there are a lot of implementations of that code).
 

Antartica

Member
Sep 13, 2011
23
4
Not forgotten about this!

Just an status update:
I'm still in the process of defining the API of the C library using javispedro's sources as template.
It's tougher than I originally supposed because the C++ code has a lot of forward-declarations of classes, which is very difficult to map into C. To counter that I have to move elements between structures and I'm not so comfortable with the codebase yet.
And then there is still the hard work of translating the Qt signals/slots to plain' old callbacks... and implementing the bluetooth part using bluez API... and... well, I hope that is all.
Anyway, patience :).
 

javispedro

Senior Member
Dec 6, 2008
55
46
I've now had access to a Samsung S2 and thus I have been able to obtain more traces. The latest Git now contains code to connect to the notification manager service, thus allowing to send notifications from the phone to the watch.

That was the last missing part to be able to use the Gear 2 as a 'daily' smartwatch with my Jolla, so I've now also ported the code to run under Sailfish. In fact I'm using this setup at the moment. My first comment is "wow the vibrator IS weak".

You can find a log of sapd's (ie my code) startup qDebug() messages; they may be useful (if you can't yet get your code to run)

I suspect that there may still be some important battery issues because the watch keeps printing error messages about SAP services it can't find on the phone (and instead of sleeping, it starts busy polling for them.... :/ ). It does not seem to happen while the watch is out of the charging cradle, so it may not be important, but not sure yet.

As for the encryption, I'm not sure how to proceed. I could describe the code to you, but that would be risky, because I don't understand what it does. Thus the only way (for me) to describe it would be to pass on the mathematical formulas/pseudocode ... Apart from that, we also have the problem of the keys...

The usual length of ECDSA keys is exactly 520-bits, so we may have something there: it is possible that they are using ECDSA signing
They do use ECDH indeed, and they link with OpenSSL and import the ECDH functions. However it's not clear if they use ECDSA; while the crypto algorithm DOES resemble DSA, I cannot fully identify it.
 

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Antartica

Member
Sep 13, 2011
23
4
Congratulations for managing to make it work with the Jolla :).

I have finally found a suitable "flattened" class hierarchy as to be able to map your code into C; see the attachs. Basically, I have to move the functionality of SAPConnectionRequest, SAPSocket, CapabilityPeer and SAPConnection into SAPPeer, and then it is suitable for my needs.

As for the encryption, I'm not sure how to proceed. I could describe the code to you, but that would be risky, because I don't understand what it does. Thus the only way (for me) to describe it would be to pass on the mathematical formulas/pseudocode ... Apart from that, we also have the problem of the keys...

They do use ECDH indeed, and they link with OpenSSL and import the ECDH functions. However it's not clear if they use ECDSA; while the crypto algorithm DOES resemble DSA, I cannot fully identify it.

If you manage to describe it using mathematical formulas as in
http://en.wikipedia.org/wiki/Ellipt...ture_Algorithm#Signature_generation_algorithm
it would be perfect, but I reckon that to be able write that you need intimate knowledge of the code and don't know if you have time for that :angel:
And identifying the hash function used would be a problem in itself...
One idea: how about a ltrace so we have the calls to the openssl library? That may uncover new hints.

Anyway, I have a lot of work before me until I need that, so don't fret over it.
 

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javispedro

Senior Member
Dec 6, 2008
55
46
Congratulations for managing to make it work with the Jolla :).

I have finally found a suitable "flattened" class hierarchy as to be able to map your code into C; see the attachs. Basically, I have to move the functionality of SAPConnectionRequest, SAPSocket, CapabilityPeer and SAPConnection into SAPPeer, and then it is suitable for my needs.
You may want to look at the official Samsung SDK docs to match their class hierarchy. I tried to match my hierarchy to theirs, but this happened very late in the development process, so there is some weirdness.

One idea: how about a ltrace so we have the calls to the openssl library? That may uncover new hints.
I more or less know what it is doing with OpenSSL, but that's because I looked at the dissassembly. They use OpenSSL for key derivation (ECDH), but the actual cryptographic algorithm is their own. This 'block cipher' is the part they have tried to obfuscate. Not much, but still enough to require more time than what I have available :) It is basically a set of arithmetical operations with some tables hardcoded in the libwsm.so binary, so no external calls to any library. The hardcoded tables are probably derivated from their private key, which is most definitely not on the binary. In fact I suspect this is basically AES with some changes to make it hard to extract the actual key used, so that's where I've centered my efforts.

Technically it should not even be copyrightable, so maybe I could just redistribute my C reimplementation of the algorithm, but as with any other DRM who knows these days... and that still leaves the problem of the tables/"private key".

agreed. Needs iPhone support please.
Well you are welcome to implement one such iPhone program yourself. Will be happy to resolve all the protocol questions you have.
(But please stop with the nagging).
 

Digiguest

Senior Member
Dec 26, 2010
413
29
Ontario
Wasn't nagging at all. Just agreeing with him. I am no programmer so I have to rely on others for answers. Sorry if you thought otherwise.

Looking for to see more work on it though. Keep it up.
 

Duobix

Member
Jun 29, 2010
31
5
Hi there! Nice work on getting Gear2 to work with Jolla.
I'd love to get Gear1 to work with WP8.1. Do you have the code for Jolla
on github/bitbucket so I could give it a peek? :D Thanks in advance.
 

Antartica

Member
Sep 13, 2011
23
4
Hi there! Nice work on getting Gear2 to work with Jolla.
I'd love to get Gear1 to work with WP8.1. Do you have the code for Jolla
on github/bitbucket so I could give it a peek? :D Thanks in advance.

javispedro had the sources in gitorius, but they are not there anymore (surely related to gitlab buying gitorius).

I attach a tarball with javispedro sources as of 19 October 2014.

Note that it lacks the files implementing the crypto, so just porting it is not enough to be able to communicate to the gear. OTOH, I know that there are some differences in the protocol between the Android Gear1 and the Tizen Gear2 (if the gear1 has been updated to Tizen, it uses the same protocol as gear2). Specifically, to be able to communicate with both watches, the gear manager package has both gear manager 1.7.x and gear manager 2.x. javispedro's code implements the gear 2 protocol.

Personally, I have my port on hold (I have problems with bluetooth in my phone, so there is no point in porting sapd right now as I would not be able to use it).
 

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    Hello,

    I want to figure out the Samsung Accesory Protocol in order to create a "open source" Gear Manager app replacement. This thread is to ask if anyone has been trying to do the same thing as well as try to gather as much information about this protocol as possible. Generic discussion is also accepted, in case anyone has better ideas.

    Right now all I know is that this protocol is based on RFCOMM, albeit it can be transported over TCP too. It has a level 1 "framing" which consists basically on

    Code:
    packed struct Frame {
     uint16_be length_of_data;
     char data[length_of_data];
    }
    
    packed struct FrameWithCRC {
     uint16_be length_of_data;
     uint16_be crc_of_length;
     char data[length_of_data];
     uint16_be crc_of_data;
    }

    I also know that there are various types of packets. "Hello" packets are exchanged early during the connection and contain the product name, etc. Authentication packets are exchanged right after the initial "hello" and contain some varying hashes (crypto warning!). Then the normal data packets are "multiplexed", as in usbmuxd: they have 'session' IDs which described towards which watch program they are talking with. All Hello and authentication packets are sent without CRC, but normal data packets are. The CRC implementation used is crc16, same poly as in the linux kernel.

    I suspect that whatever we uncover about this protocol might be useful to e.g. pair Gear with an iPhone, with a PC, things like that.

    Note: most of this comes from viewing Bluetooth logs. However it's clear that reverse engineering will be required for the cryptographic parts. In this case I believe it's legally OK to do so in the EU because it's purely for interoperability reasons. I don't want to create a competitor to the Gear2, I just want to talk to it.


    Motivation: I bought a Gear2 in order to replace a LiveView that was dying (buttons wearing out, broken wriststrap clips, etc.) . I used it both for notifications as well as map/navigation.
    Since I have a Jolla, no programs are available to pair with most smartwatches, but I've been developing my own so far (MetaWatch, LiveView). Thus I decided on a replacement based purely on hardware characteristics and price. Also Tizen seems more open than Android, thus I figured out it would be easier for me to adapt to the watch.

    However it seems that I understimated the complexity of the protocol that connects the Gear with the GearManager. So my options in order to make use of this watch are:

    1. Sell Gear2 back and buy something that's easier to hack (e.g. another LiveView :) ),
    2. Figure out the SAP protocol and write a replacement Gear Manager app (what this thread is about),
    3. Write replacement Tizen applications that don't use SAP. This involves writing new programs for Calls, Messages, Notifications, Alarms, Camera, watchOn, Pulse monitor, etc. i.e. a _lot_ of work if I want to exploit all features of the watch.
      But at least one can reuse the existing Tizen settings app, launcher, drivers, etc. (I started porting Qt to the Gear2 with this idea)
    4. Use a different Linux distro on the Gear 2. Such as Sailfish, Mer, etc. This involves all the work of option 3 + possibly driver work.
    As of now I've not decided which option is easier for me so I'll keep trying to push them all.
    2
    Not forgotten about this!

    Just an status update:
    I'm still in the process of defining the API of the C library using javispedro's sources as template.
    It's tougher than I originally supposed because the C++ code has a lot of forward-declarations of classes, which is very difficult to map into C. To counter that I have to move elements between structures and I'm not so comfortable with the codebase yet.
    And then there is still the hard work of translating the Qt signals/slots to plain' old callbacks... and implementing the bluetooth part using bluez API... and... well, I hope that is all.
    Anyway, patience :).
    1
    Hi there! Nice work on getting Gear2 to work with Jolla.
    I'd love to get Gear1 to work with WP8.1. Do you have the code for Jolla
    on github/bitbucket so I could give it a peek? :D Thanks in advance.

    javispedro had the sources in gitorius, but they are not there anymore (surely related to gitlab buying gitorius).

    I attach a tarball with javispedro sources as of 19 October 2014.

    Note that it lacks the files implementing the crypto, so just porting it is not enough to be able to communicate to the gear. OTOH, I know that there are some differences in the protocol between the Android Gear1 and the Tizen Gear2 (if the gear1 has been updated to Tizen, it uses the same protocol as gear2). Specifically, to be able to communicate with both watches, the gear manager package has both gear manager 1.7.x and gear manager 2.x. javispedro's code implements the gear 2 protocol.

    Personally, I have my port on hold (I have problems with bluetooth in my phone, so there is no point in porting sapd right now as I would not be able to use it).
    1
    I'm still alive too :)
    sapd has been moved to https://git.javispedro.com/cgit/sapd.git/ .

    I actually started cleaning it a bit , but so far i only moved the sailfish specific parts into an external library.
    I'm planning to get a Gear S via a promotion which means that the project might get a bit more steam from my part.

    The crypto/DRM "problem" is still unsolved (meaning: the only way to actually use this publicly is by using samsung's binary). I've tried putting some pressure on Samsung from the Tizen side to actually open up SAP, but silence has been the only answer so far...