Hi there - neat project! I'm always interested in open hardware for cryptography, so thanks for contributing to the space. :) That said, this quote concerns me:

>The SC4-HSM is designed to defend against a compromised client machine, i.e. an attacker who pwns your laptop or desktop machine. If you think about it, this is the only threat model that makes sense for dedicated secure hardware. If you can trust that your client machine is secure, you don't need an HSM.

From my prospective, that's the bare minimum threat model for an open secure hardware device. I also include unsupervised physical access to the device. My ideal HSM would also provide robust protections against a myriad of complex hardware-level attacks - JTAG debugging, power & RF analysis, glitching, de-encapsulation just to name a few.

Of course, a cheaper HSM which lacks advanced hardware level protections can still offer a lot of protection against common threat vectors, and physical access threats can be mitigated to some extent by keeping the device in a secure location or on your person.

this is really cool! thanks for sharing it here, I'll definitely dig in deeper on it. I've been following Tomu development for over a year now (two?) I've got one on desk right now, the ideas that mithro cares about, the open hardware, are very important to me and I'm glad to see as much development in the space as possible.