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when we say that HSM stores keys securely we mean that non-authoriszed requester cannot access the keys.

but the key is stored physically anyway in the hardware, so access is still possible

if the keys are encrypted, that means that there is another key in the hardware that encrypted those keys. this key is not encrypted then !!

I'm confused

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The key is stored physically anyway in the hardware, so access is still possible

The assumption is that it is practically impossible to read the keys that reside (permanently or temporarily) inside the HSM. They will be present in usable form, and used, only inside the HSM, and it is the HSM's job to protect these keys (including enforcing restrictions about what can be done with them).

Preventing access to information stored and processed in the HSM (and deliberate interference with that processing) is accomplished to some useful degree by physical barriers that prevent connecting probes to the insides of the HSM. There are sensors intended to detect if these barriers are pierced or operating conditions otherwise not normal, checks to detect incorrect processing... In such case, the HSM typically attempts to destroy the keys and data being processed, so that there is no longer anything useful that can be extracted.

Additionally, it is often insured that the keys are just not there (thus can not leak) unless there is good assurance that the HSM is operating under rightful conditions (for example, it might be necessary to reunite several partial keys, some in Smart Cards under control of security officers and protected by a PIN code with counter, in order to obtain and use the actual keys inside the HSM).

If the keys are encrypted, that means that there is another key in the hardware that encrypted those keys

Yes. That's typical.

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but the key is stored physically anyway in the hardware, so access is still possible

Yes, in the end the value of the key is required to perform the calculations. And storing digital data still requires a carrier.

However, various measures can be taken to make sure that only the right users can use the keys, and that nobody is able to retrieve the key values.

This means that often HSM's are protected against power fluctuations and movement. Side channel attacks are often protected against. Sensors will detect when you are trying to open the outer shell. Disallowign accesss to the inner workings of an HSM is the property of being tamper-proof.

Not all HSM's are created equal. Some low end HSM's are just Smart Card-type chips with a USB connection, e.g. Yubikey HSM's are those kind of crypto tokens. Of course, that also means that the storage capacity and speed are rather different from network or PCI based cards. It could also mean that some operations such as key pair generation are not as secure as their larger siblings.

Note that there are also software / cloud based HSM's nowadays that rely on homomorphic operations accross heterogenous environments. In that case the location of the key is distributed over various devices.

if the keys are encrypted, that means that there is another key in the hardware that encrypted those keys. this key is not encrypted then !!

It is not said that keys in a HSM are encrypted, but generally they probabably will be, if just because destroying them would be that much easier.

In the end the key needs to exist. You cannot have "turtles all the way". However, the final key or keys may be stored into specially designed silicon, making it near impossible to retrieve the final key value.

And as ciphers are considered pretty strong, many security devices even store their keys outside the device, loading the keys into the device only when they are required. That way the key storage is unlimited and the key value and operations using the key are still protected against leakage. This is the general way that a TPM works, for instance.

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The keys stored in HSM's are stored in secure memory.

Hardware tamper events are detectable events that imply intrusion into the appliance interior.One such event is removal of the lid (top cover). The lid is secured by anti-tamper screws, so any event that lifts that lid is likely to be a serious intrusion.

Another event that is considered tampering is opening of the bay containing the ventilation fans.

You can use the thumbscrew to access the mesh air filter in front of the fans, without disturbing the system. However, if you open the fan-retaining panel behind that, then the system registers a tamper.

Therefore, cleaning of the filter is encouraged, especially if you work in a dusty environment, but fan module removal and replacement are discouraged unless you have good reason to suspect that a fan module is faulty (see Fan Maintenance).

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  • $\begingroup$ "You can use the thumbscrew to access the mesh air filter in front of the fans" is specific to a particular model of HSM. $\endgroup$
    – fgrieu
    Dec 5, 2019 at 15:45
  • $\begingroup$ @fgrieu it is specific to Safenet HSM. $\endgroup$
    – krishna T
    Dec 6, 2019 at 6:44

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