TKey is a RISC-V computer in a USB-C case, that can run security applications

• knox-hsm

  1 21 5.3 Verilog

  Circuits and hardware security modules formally verified with Knox 🔐

This is really neat!

We've been working on some research to formally verify the hardware/software of such devices [1, 2]. Neat how there are so many shared ideas: we also use a PicoRV32, run on an iCE40 FPGA, use UART for communication to/from the PicoRV32 to keep the security-critical part of the hardware simple, and use a separate MCU to convert between USB and UART.

Interesting decision to make the device stateless. Given that the application keys are generated by combining the UDS, USS, and the hash of the application [3], it seems this rules out software updates? Was this an intentional tradeoff, to have a sort of "forward security"?

In an earlier project I worked on [4], we had run into a similar issue (no space for this in the write-up though); there, we ended up using the following approach: applications are _signed_ by the developer (who can use any keypair they generate), the signature is checked at application load time, and the application-specific key is derived using the hash of the developer's public key instead of the hash of the application. This does have the downside that if the developer is compromised, an adversary can use this to sign a malicious application that can leak the key.

[1]: https://github.com/anishathalye/knox-hsm

• tkey-verification

  1 39 0.0 Go

  Vendor signing and user verification of TKey genuineness

  

Yes. I've described some of the flexibility characteristics elsewhere in this thread, but here's how you could do it:

1. The ROM stage measures the first mutable application stage A1, and makes the Compound Device Identifier (CDI) available to A1. CDI=Hash(UDS, Hash(A1), USS).

2. A1 creates a key pair for itself using CDI.

3. A1 measures A2 and creates a key pair for A2.

4. A1 signs A2's public key. This is the attestation.

5. A1 scrubs CDI and its private key from memory while ensuring that the attestation as well as A2's key pair is available to A2. Note that the TKey only supports two-stage boot chains at the moment.

A1's key pair could also be signed by the vendor before shipping. We do it like this: https://github.com/tillitis/tkey-verification

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