Ernie Secure

Shared Secrets

Class	Shared Key A	Shared Key B	Shared Key C	Shared Data A	Shared Data B
0xB	BB644721CB4C55072E83177BEB3BBEE9	DC6B6EE0F457DF0E7BAD1C5EA338027F	00000000000000000000000000000000	CF2E93E9F94E28CCA48026134C7C77CE
0xF	50E4C3A77264167C409C72A9B57A8609	9E34087C48985B4B351A63572D9B481B	EBE3460D84A41754AC441368CF0200D8	C86B51FB019A207F32118E55462D5008	457798C6D9B8390DDCA49745746F65A000000000000000000000000000000000

Syscon four part key exchange

The 0F here appears to be a class 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: Ernie challenges Kermit

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: Kermit responds and challenges Ernie

1. F00D composes a data buffer that is 8 bytes of RNG value, 8 bytes copied from challenge in part 1.2, and 16 bytes of Shared Data A.
2. This data is encrypted using AES-128-CBC with all zero IV and Shared Key A as the key.
3. A header is prepended 30 02 00 0F 01 00 00 00 to the data and sent to Syscon

Part 3: Session Key Establishment

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 all zero IV and the key Shared Key B.
3. Check that first 8 byte matches the challenge from part 1.2 and next 8 byte matches the F00D nonce generated in part 2.1. The remaining 16 bytes are zero.
4. Encrypt the same buffer back using Shared Key C for 16 bytes. The IV will be the last 16 bytes from the encrypted input buffer.
5. This is now the session key!

Part 4: Verification

1. Using the session key, encrypt a known value, Shared Data B.
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.

This part is only seen used with command 0xD0 and not 0xA0.

Channel Protocol

Packets sent/received with command 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.

Syscon 0xA0

This command uses class 0xB and is only seen in secure_loader. No data transfer is seen. The session key derived here is actually not used. The hypothesis is that this handshake serves as a checkpoint for the Syscon to know that F00D has successfully decrypted IDPS and OpenPSID since in between the handshake each decryption happens. The decryption does not use any data from this handshake.

Syscon 0xD0

Establishes a secure channel for class 0xF used by updater 0xD0002 as well as second_loader.

After handshake:

Keyslot 0x511 is programmed with the session key.

Keyslot 0x512 is programmed with a 32 bit random number from Bigmac.

Syscon 0xD2

Data transfer for class 0xF.

Seen at boot

First transaction

F00D => Syscon: 00 00 20 00 47 D3 65 F4 00 00 00 00 00 00 6C FD

F00D <= Syscon: 00 00 20 00 47 D3 65 F4 00 00 00 00 00 00 01 DD C7 AB 57 AD 28 9E 00 93 51 F8 D3 D2 29 2E 78 B6 8C 63 5A 26 7A A9 6B C8 E3 AC F3 3D 61 77 F0 EC

Second transaction

F00D => Syscon: 20 00 20 00 48 D3 65 F4 00 00 00 00 00 00 4B FD

F00D <= Syscon: 20 00 20 00 48 D3 65 F4 00 00 00 00 00 00 95 AD 79 D1 FE 5E 96 4B 3F 66 7D 47 04 28 05 E9 EB D1 26 86 E2 C1 9B 7B 53 B6 D3 11 76 8F 2D 3F 80 ED