Wrapping an HMAC key with sealed data under a parent for transport between a server and a client device

wrap_sealdata.py

#/usr/bin/env python3

from tpm2_pytss import *
from tpm2_pytss.utils import wrap

# This would come from the client, but we create it here for clarity. You
# could serialize and deserialize a TPM2Bw_PUBLIC of the key to use for object
# protections and just open the file here.
e = ESAPI()
primary_ctx, primary_public = e.create_primary(None)[0:2]

# Generate the new public and sensitive to wrap which produces the TPM2B_PRIVATE that can be sent to the client
# Wrap supports accepting or generating an inner wrapping key, but then you need to get this key to the remote, so
# most folks DONT use it, as one layer of wrapping with aes128cfb is enough. The output seed for import is encrypted
# with the parents public using an RSA OAEP or ECDH encryption scheme.
hmac_sens, hmac_pub = TPM2B_SENSITIVE.keyedhash_from_secret(b"Secret\n", objectAttributes=TPMA_OBJECT.USERWITHAUTH)
hmac_priv, hmac_seed = wrap(primary_public.publicArea, hmac_pub, hmac_sens)[1:3]

# Import on the client TPM.
hmac_priv_2 = e.import_(primary_ctx,
    TPM2B_DATA(), # No inner encryption used, wrap() supports it if desired.
    hmac_pub, hmac_priv,
    hmac_seed, # the seed is encrypted RSA OAEP with parent public or an ECDH scheme using ECC
    TPMT_SYM_DEF_OBJECT.parse("null") # Set this to aes128cfb if you want the inner encryption
    )

hmac_ctx = e.load(primary_ctx, hmac_priv_2, hmac_pub)
secret = e.unseal(hmac_ctx)
print(f"Secret: {bytes(secret)}")