Check if EK cert from NV Index is an valid certificate

[THS-on]

We have seen some TPMs where the EK certificate for RSA2048 are padded with ones up to the length of 1600 bytes.

This causes the agent to fail to register, as we don't expect that large certificates and the full NVR is also not a valid certificate.
To work around this, one option is to parse the data to check the actual length of the DER data structure.

[THS-on]

@ansasaki should we just implement the check by parsing the DER structure? I have that code mostly ready anyway for that

[ansasaki]

@ansasaki should we just implement the check by parsing the DER structure? I have that code mostly ready anyway for that

Yes, I think that could be sufficient to avoid this issue