Using TPM key handle for device CA key and device Identity Keys - identity

Question

Does anyone tried using the TPM key for the device CA and identity certificates in Edge Device?
Currently the device CA and the identity keys are generated in PEM files and set the path in the config.yaml as URI link.
I have generated a TPM key and generate device CA and Identity certificate with a root CA. How do I use the TPM key instead of the PEM key file by referencing to the handle example 0x81000002?

Answer 1

Objective is to secure the certificates used by edge device using TPM for upstream(device identity) and downstream (device CA)operations.
Currently both keys above are using PEM key files which is unsafe.
Operation example:
Step1: User Create TPM keys for device CA and identity keys with peristent handle under a SRK primary key using tpm2-tools
example : device identity at 0x81020000 and device CA at 0x81000002
echo ">>>>>>>> Create SRK primary"
tpm2_createprimary -C o -g sha256 -G ecc -c SRK_primary.ctx
tpm2_evictcontrol -C o -c SRK_primary.ctx 0x81000001
echo "create persistent IDevID Key"
tpm2_create -C 0x81000001 -g sha256 -G ecc -r ID_Priv.key -u ID_Pub.key
tpm2_load -C 0x81000001 -u ID_Pub.key -r ID_Priv.key -n ID_key_name_structure.data -c ID_keycontext.ctx
tpm2_evictcontrol -C o -c ID_keycontext.ctx 0x81020000
echo "create persistent devCA Key"
tpm2_create -C 0x81000001 -g sha256 -G rsa -r DevCA_Priv.key -u DevCA_Pub.key
tpm2_load -C 0x81000001 -u DevCA_Pub.key -r DevCA_Priv.key -n DevCA_key_name_structure.data -c DevCA_keycontext.ctx
tpm2_evictcontrol -C o -c DevCA_keycontext.ctx 0x81000002
Step2: Create CSR and certificates using the above key handles
openssl req -new -engine tpm2tss -key 0x81020000 -passin pass:"" -keyform engine -subj /CN=DeviceIdentity -out dev_iden.csr
Step3: need modification in the security daemon to make this work
modify config.yaml to use the above handles for the keys of device ca and idendtity and specify certs as URI path

Answer 2

Great question!
The IoT Edge runtime needs to access the TPM to automatically provision your device. See how to do it here: https://learn.microsoft.com/en-us/azure/iot-edge/how-to-auto-provision-simulated-device-linux#give-iot-edge-access-to-the-tpm
The way the attestation process works is like this:
When a device with a TPM first connects to the Device Provisioning Service, the service first checks the provided EK_pub against the EK_pub stored in the enrollment list. If the EK_pubs do not match, the device is not allowed to provision. If the EK_pubs do match, the service then requires the device to prove ownership of the private portion of the EK via a nonce challenge, which is a secure challenge used to prove identity. The Device Provisioning Service generates a nonce and then encrypts it with the SRK and then the EK_pub, both of which are provided by the device during the initial registration call. The TPM always keeps the private portion of the EK secure. This prevents counterfeiting and ensures SAS tokens are securely provisioned to authorized devices.
Ref: https://learn.microsoft.com/en-us/azure/iot-dps/concepts-tpm-attestation
I believe your case is different though (you want to add your CA certificate to your TPM and then retrieve it from there?). I saw your feedback request, sharing here for others to vote:
Using TPM keys for Device CA and Identity - https://feedback.azure.com/forums/907045-azure-iot-edge/suggestions/40920013-using-tpm-keys-for-device-ca-and-identity
If your idea is identical to the one already added in IoT Edge Feedback forum, please merge it and vote:
Store Private key for X.509 based DPS securely on HSM - https://feedback.azure.com/forums/907045-azure-iot-edge/suggestions/39457678-store-private-key-for-x-509-based-dps-securely-on