Using TPM2_LoadExternal() for loading an HMAC key into the TPM?

Hi!

I was wondering, if anyone has an idea how precisely to set up a pair
of TPM2B_PUBLIC and TPM2_SENSITIVE structures for loading an
HMAC-SHA256 key into the TPM via TPM_LoadExternal()?

I am currently setting things up more or less like this:

        TPM2B_PUBLIC auth_hmac_public = {
                .size = sizeof(TPMT_PUBLIC),
                .publicArea = {
                        .type = TPM2_ALG_KEYEDHASH,
                        .nameAlg = TPM2_ALG_SHA256,
                        .objectAttributes = TPMA_OBJECT_DECRYPT | TPMA_OBJECT_SIGN_ENCRYPT /* | TPMA_OBJECT_USERWITHAUTH */,
                        .parameters.keyedHashDetail.scheme.scheme = TPM2_ALG_SHA256,
                        .unique.keyedHash.size = buffer.size,
                },
        };

        TPM2B_SENSITIVE auth_hmac_private = {
                .size = sizeof(TPMT_SENSITIVE),
                .sensitiveArea = {
                        .sensitiveType = TPM2_ALG_KEYEDHASH,
                        .sensitive.sym.size = buffer.size,
                },
        };

        memcpy(auth_hmac_private.sensitiveArea.sensitive.sym.buffer, buffer.buffer, buffer.size);

And then use TPM2_LoadExternal() with this, for the NULL hierarchy.

tpm2-tss responds with these errors:

         ERROR:esys:src/tss2-esys/api/Esys_LoadExternal.c:184:Esys_LoadExternal_Async() SAPI Prepare returned error. ErrorCode (0x0009000b)
         ERROR:esys:src/tss2-esys/api/Esys_LoadExternal.c:85:Esys_LoadExternal() Error in async function ErrorCode (0x0009000b)

But, uh, what am I supposed to make of this?

I figure it's not even the TPM that refuses this, but it's tpm2-tss
already?

Anyone has an idea?

(Background: I am trying to protect an nvindex that I want to use with
TPM2_AuthorizeNV, with an TPM2_PolicySigned access policy. I want to
use an HMAC key for the signature scheme. If you want to know even
more, see → https://github.com/systemd/systemd/pull/31790. The above
is more or less a copy of the topmost commit of that)

Any help appreciated!

Lennart