README.md
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# i2c-stub toy
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Recently I had a chance to play with ``i2c-stub``. The goal was to send and receive encrypted data to/from I2C connected device. I didn't want to play with real I2C device, so I needed to emulate it somehow, which is possible with ``i2c-stub`` on linux. Here below is description how it was done.
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# Requirements
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The solution needs to be implemented in C and have following functionalities
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* Possibility to connect to I2C slave
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* Send encrypted data
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* Receive and decrypt data
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* Possibility to check connection status
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# The code
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The code itself is here. To compilie with ``gcc`` simply download and ``make``.
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## Initialization
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In order to use the code (read/write data to I2C) I'm using ``i2c-stub`` linux
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module and i2c-tools package (ArchLinux). ``i2c-stub`` creates a fake I2C adapter(Controller/Master) and emulates i2C hardware (using array to store data). We also will need ``i2c-dev`` module as a frontend.
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Following command will load the module, initialize slave device with an address 0x03, and read it's initial state:
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```
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> modprobe i2c-dev
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> modprobe i2c-stub chip_addr=0x03
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> i2cdetect -l
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i2c-1 i2c i915 gmbus dpc I2C adapter
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i2c-2 i2c i915 gmbus dpd I2C adapter
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...
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i2c-8 smbus SMBus stub driver SMBus adapter <-- this one
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...
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> i2cdump -y 8 0x03
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No size specified (using byte-data access)
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0 1 2 3 4 5 6 7 8 9 a b c d e f 0123456789abcdef
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00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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30: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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60: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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```
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We can see that module was loaded. ``i2cdetect`` as detected is as a character device ``/dev/i2c-8`` and ``i2c-dump`` shows memory state of slave device with an ID ``0x03``.
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## Sending data
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Test program has ``-s`` option that needs to be used in order to send data. As an argument, device ID needs to be provided.
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```
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> ./bin/main -s 8 && echo $?
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0
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```
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On a success program return 0. We can now verify if data has been stored in the i2c device with ``i2cdump``.
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```
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0 1 2 3 4 5 6 7 8 9 a b c d e f 0123456789abcdef
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00: 1f f8 47 8e 7f 24 1d 2b 47 ca 64 be ce 0a 3f bd ??G??$?+G?d?????
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10: 08 1c 05 87 b0 31 6c 85 46 94 6f c8 9e 49 dd b2 ?????1l?F?o??I??
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20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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30: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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60: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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```
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Before sending program uses Poly1305-ChaCha20 to encrypt and authenticate data.
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## Receiving data
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Test program has ``-r`` option to indicate that user want's to receive data from I2C device. On exit program prints received data.
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```
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[root@cryptoden final]# ./bin/main -r 8
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RECEIVED DATA:
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HELLO WORLD!!!
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```
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As data is authenticated any change to data stored in the I2C will result
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in decryption error. In order to see this behaviour one can dump the I2C
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memory, change it, load to I2C and try to read again. Let's see this:
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```
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> i2cdump -y 8 0x03 b > dump
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> cat dump
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[root@cryptoden ~]# cat dump
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0 1 2 3 4 5 6 7 8 9 a b c d e f 0123456789abcdef
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00: 1f f8 47 8e 7f 24 1d 2b 47 ca 64 be ce 0a 3f bd ??G??$?+G?d?????
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10: 08 1c 05 87 b0 31 6c 85 46 94 6f c8 9e 49 dd b3 ?????1l?F?o??I??
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20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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30: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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60: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
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## Here I modify 32-nd byte from b3 to b2 and load data to i2c-stub
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> i2c-stub-from-dump 0x03 dump
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256 byte values written to 8-0003
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## Trying to read
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> ./bin/main -r 8
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[i2c_recv() src/i2c.c:165] Error occured when decrypting
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[i2c_recv() src/i2c.c:172] ERROR: can't receive encrypted data
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[test_receive() src/main.c:88] Error occured when receiving data
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```
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## Testing
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Program has a ``-t`` option which can be used to test program and see
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that connection stays persistent after connecting to the device.
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```
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> ./bin/main -t 8 && echo $?
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0
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```
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