How to verify TPM-generated signatures with OpenSSL?

Question

I'm working with a Trusted Platform Module (TPM) and need to generate a Signing Key (SK) that can be proven to reside within the same TPM as the Attestation Identity Key (AIK). My goal is to enable a server, which does not have a TPM, to verify that the SK originated from my TPM using OpenSSL. Here's what I have accomplished so far:

sudo tpm2_createek -c ek.ctx -G rsa -u ek.pub -P 123
sudo tpm2_createak -C ek.ctx -c ak.ctx -G rsa -g sha256 -s rsassa -u ak.pub -n ak.name -P 123
sudo tpm2_load -C ek.ctx -u ak.pub -r ak.priv -n ak.name -c ak.ctx
sudo tpm2_createprimary -C o -c primary.ctx -G rsa -P 123
sudo tpm2_create -C primary.ctx -G rsa -u sk.pub -r sk.priv -a "fixedtpm|fixedparent|sensitivedataorigin|userwithauth"
sudo tpm2_load -C primary.ctx -u sk.pub -r sk.priv -n sk.name -c sk.ctx
sudo tpm2_certify -C ak.ctx -c sk.ctx -g sha256 -o certify.signature -f plain

After generating the signature, I attempted to verify it using OpenSSL with the following command:

openssl dgst -sha256 -verify ak.pub -signature certify.signature sk.pub
Verification Failure

The certify.signature file is a binary file and I suspect it contains more than just the signature (likely it includes TPM structures). How can I extract just the raw signature from this binary file in a format that OpenSSL can verify?

Any help on how to correctly verify the TPM-generated signature using OpenSSL would be greatly appreciated.

Answer 1

I currently cannot reproduce your problem, but the following should help. Essentially tpm2_readpublic can give you a PEM-formatted key, which seems to be what you need, here.

From the tpm2-tools integration tests:

tpm2 createprimary -Q -C e -g sha256 -G rsa -c primary.ctx -p signedpass

tpm2 create -Q -g sha256 -G rsa:rsassa -u certify.pub -r certify.priv \
    -C primary.ctx -P signedpass -p certifypass

tpm2 load -Q -C primary.ctx -P signedpass -u certify.pub -r certify.priv \
    -n certify.name -c certify.ctx

tpm2 certify \
    -c primary.ctx -P signedpass \
    -C certify.ctx -p certifypass \
    -g sha256 -o attest.out -f plain -s sig.out

# Verify the signatures with openssl
tpm2 readpublic -Q -c certify.ctx -f pem -o certify.pem
openssl dgst -verify certify.pem -keyform pem -sha256 \
    -signature sig.out attest.out