Re-implementation of the "iLnk"/"iLnkP2P"/"PPPP" protocol used on some cheap (\<$5) IP cameras (sometimes branded as 'X5' or 'A9').

* Bought [this X5](https://www.aliexpress.com/item/1005006287788979.html) and [this A9](https://www.aliexpress.com/item/1005006117593880.html).
* App is [YsxLite](https://play.google.com/store/apps/details?id=com.ysxlite.cam&hl=en&gl=US)


Per pictures of the [X5](https://github.com/DavidVentura/cam-reverse/blob/master/pics/pcb.jpg?raw=true), [A9](https://github.com/DavidVentura/cam-reverse/blob/master/pics/pcb_a9.jpg?raw=true) the main chip is TXW817 ([chinese](https://www.taixin-semi.com/Product/ProductDetail?productId=306), [eng, google translate](https://www-taixin--semi-com.translate.goog/Product/ProductDetail?productId=306&_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp))


## Running
To execute the HTTP server, run `make run`; you can access the JPEG stream at http://localhost:1234/ and a file `audio.pcm` will be created.

The roundtrip delay when using MJPEG is [~350ms](pics/delay.jpg?raw=true).

There's a basic UI which can display multiple cameras:

![](pics/web-ui.jpg?raw=true)

The server will send a broadcast packet every few seconds to discover all the cameras available; this means that it *must* run in the same broadcast domain (VLAN) as your cameras.

## Pairing a new camera

Connect to the device's access point and run `SSID=<your ssid> PSK=<your password> make pair`.

## Protocol

The protocol is weirdly complex, though very little communication is necessary to use the device

The base structure of a packet is:

![](diagrams/packet.svg)

The payload is command-dependent; most commands have only a literal payload, but the `Drw` (`0xf1d0`) command has a framing scheme:

By using the second byte in the payload as a discriminant, we can split the payload into two types of subcommands:

**Control packets**:

![](diagrams/control_packet.svg)

The payload on control packets is "encrypted" when the length is > 5.

**Data packets**:

![](diagrams/data_packet.svg)

Data packets further discriminate based on the first 4 bytes into: Audio Data (0x55aa15a8), Video data.

### Session

To establish a session, a few _control packets_ are sent.
```mermaid
---
title: Establish session
---

sequenceDiagram
	autonumber
    App->>+Cam: [C] LanSearch
    Cam->>-App: [C] PunchPkt (SerialNo)
    App->>+Cam: [C] P2PRdy
    Cam->>-App: [C] P2PRdy
    App->>+Cam: [C] ConnectUser
    Cam->>-App: [C] ConnectUserAck (Ticket)
   
   loop Every 400-500ms
        Cam-->>+App: [C] P2PAlive
        App-->>-Cam: [C] P2PAliveAck
    end
```

To start a stream, a single _control packet_ is sent.

The received stream is broken up into 1028 byte payloads, along with a sequence number.

Stitching the payloads together yields JPEG frames for video, and 8KHz A-law PCM for audio.

```mermaid
---
title: Stream audio/video
---

sequenceDiagram
    App->>Cam: [C] StreamStart (with Ticket)
   
   loop
        Cam-->>+App: [D] Audio/Video Payload
        App-->>-Cam: [C] DrwAck
    end
```

### Serial

The A9 cameras have a TX/RX test points - connecting with UART at 921600 8N1 gives _read only_ access to some debug logs.

### Discrepancies between cameras

1. Wifi Strength
	- A9 reports '100%' strength
	- X5 reports different strength values

I bricked two cameras by patching out part of the WiFi setup - unclear yet which commands.

After bricking itself, it reports very broken configuration via serial:

```
network interface: ƀ (Default)
MTU: 51050
MAC: 06 18 40 06 3e 51 b4 e2 c6 80 06 3f 77 30 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 74 00 00 00 01 00 00 00 9c ea 01 20 00 00 00 00 00 00 00 00 00 28 60 00 00 00 00 00 00 00 00 00 06 4e 00 20 2a 00 2a 00 80 00 00 00 00 00 ff ff ff ff   ff ff 3e 51 b4 e2 c6 80 08 06 00 01 08 00 06 04 00 01 3e 51 b4 e2 c6 80 01 01 01 01 00 00 00 00 00 00 01 01 01 01 00 28 74 00 00 00 00 00 00 00 00 00 58 4e 00 20 48 00 48 00 80 00 00 00 00 00 ff ff ff ff ff ff 3e 51 b4 e2 c6 80 08 06 45 00 00   48 00 51 00 00 ff 11 c8 be 01 01 01 01 23 9c cc f7 7d 6c
FLAGS: DOWN LINK_DOWN IGMP
ip address: 1.1.1.1
gw address: 1.1.1.1
net mask  : 1.1.1.1

network i
nterface: ^@^@
MTU: 0
MAC: 
FLAGS: DOWN LINK_DOWN  
ip address: 127.0.0.1
gw address: 127.0.0.1
net mask  : 255.0.0.0   
```

## Spyware

When connecting the camera to a network, it tries to send a HELLO (?) to 4 IP addresses:
```
139.155.68.77 - Shenzhen Tencent Computer Systems Company Limited 
119.45.114.92 - Shenzhen Tencent Computer Systems Company Limited
162.62.63.154 - Tencent Building, Kejizhongyi Avenue
3.132.215.40 - ec2-3-132-215-40.us-east-2.compute.amazonaws.com
```

With the payload
```
0000   f1 10 00 28 42 41 54 43 00 00 00 00 00 09 4d 2c   ...(BATC......M,
0010   48 56 44 43 53 00 00 00 08 00 02 01 00 00 6c 7d   HVDCS.........l}
0020   65 28 a8 c0 00 00 00 00 00 00 00 00               e(..........
```

which is `DevLogin`


These addresses are decoded (script at `scripts/dec_svr.py`) from the string `SWPNPDPFLVAOLNSXPHSQPIEOPAIDENLXHXEHIFLKPGLRHUARSTLQEEEPSUIHPDLSPEAOICLOSQEMLPPALNIBIAERHZLKHXEJHYHUEIEHELEEEKEG`.

Every 8-10s

There are some other strings in the APK ending in `-$$` which decode to other ips/hostnames.


## Reversing

The interesting implementation is in `libvdp.so`, part of the apk bundle.

Protocol reversing was done with a combination of static analysis of the shared object with [Ghidra](https://ghidra-sre.org/) and dynamic analysis with [Frida](https://frida.re/docs/javascript-api/).

The headers reversed with Ghidra are at `types/all.h`. They are almost not used by this minimal implementation though.

The hooks used with frida are at `frida-hooks.js`, but it's mostly a playground - some useful functions got deleted once I understood the protocol.

There's also a partial Wireshark dissector at `dissector.lua`. You can install it with `make install-wireshark-dissector`.

### Take APK from emulator/sacrificial device
```
adb shell pm list packages | grep ysx
adb shell pm path com.ysxlite.cam
adb shell pm path com.ysxlite.cam | while read -r line ; do adb pull $(echo $line | cut -d: -f2-) ;  done
```
### Push to sacrificial device
```
adb install-multiple *apk
```

### Frida install Android

[docs](https://frida.re/docs/android/)

### Start frida server

```
adb shell 'su -c nohup /data/local/tmp/frida-server-16.1.11-android-arm64 &'
```