[mbed-tls] psa_raw_key_agreement does not like PSA_ALG_ECDSA(PSA_ALG_SHA_256)

I am not sure I completely understand the PSA API for key exchange.

I have been creating a private key, exporting the public key, passing that successfully to the peer, obtaining the peer's public key, and I want to use psa_raw_key_agreement to obtain the shared secret. But it fails in:

if (!PSA_ALG_IS_KEY_AGREEMENT(alg)) { status = PSA_ERROR_INVALID_ARGUMENT; goto exit; }

The value of "alg" is 0x06000609, computed as PSA_ALG_ECDSA(PSA_ALG_SHA_256). That's probably wrong, but I do not know why...

-- Christian Huitema