Issue with setting up a sealed object, protected via pcr policy, and accessed over an encrypted session

[terilenard]

Hello,

I have the following use-case. Create a sealed secret that is accessed only when a TPM PCR policy is met. Additionally, I want to have an authenticated and encrypted communication with the TPM. Please find below the TPM commands, software versions, setup, and issues.

Output of uname -a: Linux Ubuntu-beaver 5.4.0-144-generic Output of tpm2 -v: tool="tpm2" version="5.5-4-g90534beb" tctis="libtss2-tctildr" tcti-default=tcti-abrmd TPM: IBMTss virtual TPM with tpm2-abrmd

Issue: sealed secret is visible on the network communication when performing tpm2_unseal. I expected the sealed secret to be visible only at application level, not in the tpm to application communication.

Question: Is it possible to have the tpm2_startauthsession encrypt the communication between the application and tpm?

Commands:

tpm2_createprimary -C e -g sha256 -G ecc -c primary.ctx 

tpm2_pcrread -o pcr.dat "sha1:0,1,2,3" 

tpm2_startauthsession -S session.dat  #-G aes -g sha256

tpm2_policypcr -S session.dat -l "sha1:0,1,2,3" -f pcr.dat -L policy.dat 

tpm2_flushcontext session.dat 

tpm2_create -Q -u key.pub -r key.priv -C primary.ctx -L policy.dat -i- <<< "secret" 

tpm2_load -C primary.ctx -u key.pub -r key.priv -n unseal.key.name -c unseal.key.ctx 

tpm2_startauthsession --policy-session -S session.dat -c primary.ctx -G aes -g sha256  

tpm2_policypcr -S session.dat -l "sha1:0,1,2,3" -f pcr.dat -L policy.dat 

tpm2_unseal -psession:session.dat -c unseal.key.ctx 

tpm2_flushcontext session.dat 

At one point I also tried to run:

tpm2_create -Q -C primary.ctx -Gaes128 -u session.key.pub -r session.key.priv -L policy.dat

tpm2_load -C primary.ctx -u session.key.pub -r session.key.priv -c session.key.ctx

to create a aes key that could be used in the session. This was executed before:

tpm2_startauthsession --policy-session -S session.dat -c primary.ctx -G aes -g sha256 

Thank you! Teri

[terilenard]

Follow-up. There is a related known issue here 2569 on tpm2-tss. But this is on FAPI, not on ESAPI. Any toughts?

[JuergenReppSIT]

@terilenard with ESAPI you have to create a policy session with activated parameter encryption FAPI uses ESAPI for this purpose. At: https://github.com/JuergenReppSIT/tpm2-tss/blob/8e6aca9ce03c67cec7f3e6c01946d7398f816260/src/tss2-fapi/ifapi_policyutil_execute.c#L97 you can see how how FAPI does it. the SRK is here used as tpm key for the session. The example implements a state machine with the ESAPI async functions. You could create a simpler version (without context->state) with the ESAPI one call functions.

[terilenard]

@terilenard with ESAPI you have to create a policy session with activated parameter encryption FAPI uses ESAPI for this purpose. At: https://github.com/JuergenReppSIT/tpm2-tss/blob/8e6aca9ce03c67cec7f3e6c01946d7398f816260/src/tss2-fapi/ifapi_policyutil_execute.c#L97 you can see how how FAPI does it. the SRK is here used as tpm key for the session. The example implements a state machine with the ESAPI async functions. You could create a simpler version (without context->state) with the ESAPI one call functions.

Your help is much appreciated! Thank you for pointing me in the right direction.

[virgilhem]

Has the issue been resolved? Noticing that I2C bus sniffing attacks are becoming more straightforward to execute (for instance https://www.tomshardware.com/pc-components/cpus/youtuber-breaks-bitlocker-encryption-in-less-than-43-seconds-with-sub-dollar10-raspberry-pi-pico), I think it would be appreciated to add information to the wiki on the proper configuration of command parameter encryption.

[terilenard]

Has the issue been resolved? Noticing that I2C bus sniffing attacks are becoming more straightforward to execute (for instance https://www.tomshardware.com/pc-components/cpus/youtuber-breaks-bitlocker-encryption-in-less-than-43-seconds-with-sub-dollar10-raspberry-pi-pico), I think it would be appreciated to add information to the wiki on the proper configuration of command parameter encryption.

Hello, I think the issue was solved on the C library on #2569.