Ernie Secure

Syscon 0xD0 four part key exchange

The 0F here appears to be a context id. Only 0F and 0B has been seen but only 0F seems to be supported by the update sm using the command 0xD0002 (with different sequence number for each part).

Part 1

1. Generate an empty buffer 30 00 00 0F 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 and send it to Syscon.
2. Syscon returns header 30 01 00 0F 00 00 00 00 + 8 byte challenge.

Part 2

1. F00D composes a data buffer that is 8 bytes of RNG value, 8 bytes copied from challenge, and 16 bytes of shared data. On 1.69, the shared data is C86B51FB019A207F32118E55462D5008
2. This data is encrypted using AES-128-CBC with all zero IV and a shared key. On 1.69, it is 50E4C3A77264167C409C72A9B57A8609
3. A header is prepended 30 02 00 0F 01 00 00 00 to the data and sent to Syscon

Part 3

1. Gets a 0x28 byte response from Syscon with a header 30 03 00 0F 00 00 00 00 and 0x20 buffer.
2. Decrypt with AES-128-CBC with IV from part 2 (it is the last ciphertext generated, or the last 16 bytes of the ciphertext sent from F00D) and a shared key. On 1.69 it is 9E34087C48985B4B351A63572D9B481B
3. Check that first 8 byte matches unknown and next 8 byte matches the F00D nonce generated in part 1.
4. Encrypt the same buffer back using a shared key for 16 bytes. On 1.69 it is EBE3460D84A41754AC441368CF0200D8 and the IV will be the last 16 bytes from the encrypted input buffer.
5. This is now the session key!

Part 4

1. Using the session key, encrypt a known value. On 1.69 it is 457798C6D9B8390DDCA49745746F65A000000000000000000000000000000000
2. Append the header 30 04 00 0F 00 00 00 00 and send to Syscon
3. Get a response back from Syscon (header 30 05 00 0F 00 00 00 00), decrypt the buffer with the session, and check that it matches the known value. Both the plaintext and ciphertext should match.
4. Keyslot 0x511 is programmed with the session key.
5. Keyslot 0x512 is programmed with a 32 bit random number from Bigmac.

Syscon 0xD2

Packets sent/received with syscon 0xD2 are encrypted with the session key and IV = 0. There is a 4 byte command field, 4 byte counter (from keyslot 0x512) that increments per send, 6 bytes of zeros, optional data, and a 2 byte checksum. The checksum is the sum of all previous bytes and then negated.