It's an excellent question to ask. We chose the CH552 in part due to the chip shortage at the time we locked in the design. According to the specification: It

It's an excellent question to ask. We chose the CH552 in part due to the chip shortage at the time we locked in the design. According to the specification: It is effectively a USB PHY with a small microcontroller attached. Firmware updates are done through the plug but can't be done from a USB host due to special electrical requirements that USB can't satisfy. One of our greatest concerns regarding the CH552 is any remote code execution vulnerabilities in the firmware that would allow an attacker from the host to gain persistence and use that to traverse an airgap. For what it's worth we have the same concerns for other dedicated USB chips as well. Many of them are in fact trivially upgradeable from the USB host. It should also be noted that while the lower layers of USB are easily designed in hardware logic, the upper layers would be quite expensive to do in logic. In other words, any USB IC that provides USB to UART or USB to SPI likely has a microcontroller in it. In our case I would love to find a USB IC with OTP memory. Our security model would even allow remote code execution vulns. A compromised host can upload anything they want to the TKey anyway by design. It is attacker persistence that would get annoying.