Digital keys is how information and communication on the Internet is protected. We all use them every day. Hence, generating those keys is crucial aspect of security. This is how it’s done on TKey.

Three factors for key generation

When TKey generates keys, three factors are needed:
– UDS, Unique Device Secret. This is unique for each TKey produced and part of the hardware design.
– USS, User Supplied Secret. This is a secret the user chose (know) for every application loaded.
– The TKey device app (that is loaded to the TKey via USB-C) is hashed at reception by the TKey. We write it like this:
hash(TKey device app).

All three factors are hashed again, i.e.;
hash(UDS, hash(TKey device app), USS).

The result of this hash is called CDI, Compound Device Id and is a unique identity. If you change any of the three factors, you will get a new CDI, i.e. new identity.

CDI is stored so it’s accessible for the TKey device app.

The hardware guarantees that the UDS is only read-once per power cycle. It lives for a very brief time and is then not available in the memory map anymore, not even for the firmware.

Read more about TKey memory here.

One important aspect, when it comes to security in devices, is how data stored in memory, is protected.

Program Storage

On the end user TKey, the entire FPGA design together with the firmware ROM is kept in the locked-down configuration memory (NVCM – Non Volatile Configuration Memory) within the FPGA chip. It’s not externally readable and only readable by the FPGA when configuring itself. This means the device secret (UDS) can’t be read, not even with the TKey programming board we provide.

Data Memory

There is no persistent data memory. Meaning that there is no way to store anything (e.g. passwords, keys) on the TKey between uses.

Since the TKey does not have persistent data memory, key material is generated at each use. This also means private keys can’t leak between TKey apps and that there are an unlimited number of key pairs, since they are generated each time.

Memory Modes

The TKey hardware offers two memory modes: firmware mode and app mode. The app mode has a constrained view of the memory map where some things are read-only and other things not even readable. The firmware switches to the constrained app mode just before jumping to the loaded device app.

Memory Protection

The entire firmware stack is kept in a special firmware stack that is 1) marked as non-executable, 2) only memory mapped while in firmware mode, and, 3) cleared just in case before switching to the constrained app mode.

We provide the memory execution protection to apps as well, so they for example can set up their stack to be non-executable.

Memory Scrambling

The TKey has a hardware-assisted RAM address and RAM content scramble function which the firmware sets up. This means addresses used are placed on random places in RAM and the content in the RAM is also scrambled. This is set up with new random keys from a true random number generator every power cycle and is transparent for the device apps – they see only linear RAM and the ordinary contents.