README.md

Decryption Example {#decryption-example}

This example assumes the inputKey derived from the pairing process is 7bd32320 3a26f003 05d76353 6ef3bfd0 01edc6fe 602a0c81 526f2a11 4880ac12 (32 bytes) and the current inputNonce is 00000000 0b000000 00000000 (12 bytes).

The nonce is 0000000 + counter (little endian order) which must be incremented each time we decrypt something. A different counter is used for encryption.

Original encrypted data received over the tcp connection:

SERVER -> CLIENT

19bbc5d6 67362344 a13ebbf0 c7b4e2aa 03029710 0f0f3b33 fbe3

It's prefixed with its length as a protobuf base 128 varint, 0x19 so 25 bytes. So the encrypted payload to decrypt is:

bbc5d667 362344a1 3ebbf0c7 b4e2aa03 0297100f 0f3b33fb e3

Using the python cryptography ChaCha20Poly1305 module we can now decrypt it:

> from cryptography.hazmat.primitives.ciphers.aead import ChaCha20Poly1305
> data = filter(str.isalnum, "bbc5d667 362344a1 3ebbf0c7 b4e2aa03 0297100f 0f3b33fb e3").decode('hex')
> inputKey = filter(str.isalnum, "7bd32320 3a26f003 05d76353 6ef3bfd0 01edc6fe 602a0c81 526f2a11 4880ac12").decode('hex')
> inputNonce = filter(str.isalnum, "00000000 0b000000 00000000").decode('hex')
> chacha = ChaCha20Poly1305(inputKey)
> decryptedData = chacha.decrypt(inputNonce, data, None)
> decryptedData.encode('hex')
'08112000b201020800'

Then from the shell:

echo -n "08112000b201020800" | xxd -r -p | protoc --decode_raw
1: 17
4: 0
22 {
  1: 0
}