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There is no secure element but there is a hwrng built in to the SoC.

So it isn't an HSM.

Calling a device an HSM comes with an expectation that it will resist tampering or intensive analysis. A standard microcontroller on an exposed PCB cannot fulfill that expectation.

It is an HSM notwithstanding that it does not contain a separate component marketed as a secure element. The SoC is not a "standard microcontroller" (well, it is, but not in the sense that you mean it here). The STM32F405RG has security features built in designed to secure proprietary firmware against industrial espionage. The same hardware features can be used to secure keys. I'd be happy to provide you with additional details, but I'd suggest starting by reading the documentation of the SC4-HSM and the the SoC.

I'm quite familiar with the characteristics of the STM32.

The STM32F405 is a standard microcontroller. Its security features are generally comparable to those which are present on other STM32 microcontrollers, as well as most microcontrollers in general. The PCROP (proprietary code readout protection) features which you appear to be referencing here don't help, as your application code is already open-source, and the key data that you're trying to protect must be readable to be used. Moreover, PCROP functionality is absent on the STM32F405.

STM32 readout protection is not particularly robust. There are a number of known attacks which can be used to bypass it on certain families without decapping the chip. I'm not sure if there are any attacks currently known against the F4 series, but I wouldn't doubt that they exist.

Most STM32 parts actually have a tamper detection feature which can be used to clear the contents of a battery-backed RAM when triggered. This sort of configuration still doesn't make a HSM, but it would be a step in the right direction.

Now this has degenerated into a quibble about semantics, and the difference between "it's not an HSM" and "it is an HSM, but it might have a vulnerability due to an unpublished zero-day against the hardware." Unpublished attacks against your silicon are always a concern, even (perhaps especially) if your silicon is marketed as a "secure element".

The SC4-HSM costs $75. It is intended to compete against the likes of the Trezor, the Ledger, and the Yubikey, not the SafeNet Luna. Its primary threat model is a compromised host, not loss of physical possession. Is it the most secure HSM you can buy? No. Is it the best choice for use in a data center? No. Is it secure against loss of physical possession to a state actor? Probably not (though you can always choose to encrypt the keys, which has a pretty good shot of holding up even against state actors). Is it fair to say flat-out, "It's not an HSM" implying that it provides no security at all? No.

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