Hi @matthewc
Asymetric authentication does the verification process on the message hash. Mbed TLS API suggests the hash to be the input. ( You can sign the full message, however sometimes it may take long time, if the message is long).
In mbedtls_ecdsa_sign_det(), the input is expected to be hashed and the md_alg is The hash algorithm used to hash the original data.. It is used to initialize the hmac_drbg() that is used as part of the deterministic ecdsa.
Regards,
Ron