Hi,

From the previous messages, I learned how salted sessions exchange keys and are encrypted.

However, I have yet to get an idea to prevent MITM attacks.

I was able to get an salted session in the following way.

Sealing:
> tpm2_startauthsession -S session.ctx
> tpm2_policypcr -Q -S session.ctx -l sha256:0,2,4 -L pcrs.sha256.policy
> tpm2_flushcontext session.ctx
> tpm2_createprimary -C o -c tpm-primary.ctx
> tpm2_startauthsession --hmac-session -c tpm-primary.ctx -S session.ctx
> tpm2_create -g sha256 -u seal.pub -r seal.priv -i INPUT_KEY -C tpm-primary.ctx -S session.ctx -L pcrs.sha256.policy
> tpm2_load -C tpm-primary.ctx -u seal.pub -r seal.priv -n seal.name -c tpm-seal.ctx
> tpm2_evictcontrol -C o -c tpm-seal.ctx 0x81000002
> tpm2_flushcontext session.ctx

Unsealing:
> tpm2_startauthsession --policy-session -S session.ctx
> tpm2_policypcr -S session.ctx -l sha256:0,2,4
> tpm2_unseal -p session:session.ctx -c 0x81000002 -o OUTPUT_KEY
> tpm2_flushcontext session.ctx

However, in my opinion, from the tpm2_startauthsession part of the unsealing process, an MITM attack is performed to establish a session between the attacker-PC and the TPM-attacker session is established so that the attacker will be able to obtain plaintext data for subsequent unsealing.


Thanks & Regards,
Joseph.

------ Previous Message ------

"Steven Clark" <davolfman@gmail.com> wrote on 08/02/2021 01:26:56 PM:

> I think it may be an optional standard but my TPM has some certs
> permanently stored in nv-indices in the 0x1c0000x range that can be
> checked against the manufacturer cert.  I haven't learned how to
> leverage those into trusted parameter encryption keys yet but they
> should be able to verify there's a real TPM at the other end at the
> very least (and more if you learn to use them correctly).


The EK certificates in NV are in theory optional, but every TPM

I have encountered has them.

Checking the certificate against the manufacturer's CA is
a standard crypto library function.

Once you have an authentic EK, create a salted session using
the EK.

Once you have the salted session, set the encrypt and/or decrypt bit
when running the command.

Underneath, there's some complicated crypto, but it's all
hidden from the application.

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