Add mitigations to threat model

Describe under each release what kind of threat mitigations we have
added.

doc/threat_model/threat_model.md

@@ -16,7 +16,6 @@ cases, the threat model tries to capture and describe the threats that
 needs to be mitigated in order for the device app to work in a secure
 and trustworthy manner.
 
-
 ## Assumptions
 
 * There are no backdoors or vulnerabilities in Lattice iCE40 UltraPlus
@@ -176,18 +175,30 @@ are out of scope and what mitigations are in place.
 
 ### TKey Unlocked
 
-Note that the threat model as described for the TK1-23.03.2-Bellatrix
-release (see below) applies to TKey Unlocked devices as long as the
-TKey has been provisioned with:
+Note that the threat model and the mitigations per release (see below)
+applies to TKey Unlocked devices too as long as they have been
+provisioned with:
 
-- the Tillitis TKey TK1-23.03.2-Bellatrix FPGA design and Tillitis
-  TKey firmware.
+- the bitstream from the release,
 - A unique, random UDS
 - A unique UDI
 
 The configuration must have been written into the NVCM and
 locked by blowing the fuses.
 
+### TK1-24.03-Bellatrix
+
+#### Mitigations
+
+- USB port attacks - boot protocol:
+
+  - Instead of exiting to an eternal loop on errors, firmware now
+    forces a CPU trap state that requires a reboot.
+
+- Software attacks:
+
+    Access outside of physical RAM forces the CPU into a trap state
+    that requires a reboot.
 
 ### TK1-23.03.2-Bellatrix
 This release contains a BOM update to the Tkey hardware for the touch
@@ -216,6 +227,37 @@ The FPGA design as well as the firmware has been audited, and
 hardening of these has been performed to some degree. For more
 information, see the [Release Notes](/doc/release_notes.md)
 
+#### Mitigations
+
+- To protect the UDS the hardware design allows only one read per word
+  of the UDS per power-cycle.
+
+- USB port attacks - boot protocol:
+
+  - The firmware has a more strict protocol state machine and exits out
+    to an eternal loop on any errors.
+
+  - Firmware stack is protected by hardware for execution.
+
+- Software attacks:
+
+  - Firmware uses its own FW_RAM for sensitive computations which is
+    not available in app mode.
+
+  - Device apps can protect arbitrarly parts of RAM, typically heap +
+    stack, with hardware support.
+
+- Hardware attacks:
+
+  - The reading and handling of the UDS is randomized so it doesn't
+    always occur on the same cycle.
+
+  - Firmware turns on hardware assisted RAM address and data
+    scrambling mechanisms. It makes it harder for an outside attacker
+    to find assets generated by and stored in the RAM by applications.
+    Note that this mitigates an attack from outside the CPU, not from
+    an exploit towards applications running on it.
+
 #### Known possible weakneses
 
 The CH552 MCU providing USB host communication contains firmware that