ssh_hmac_start
if ((ctx = ssh_hmac_start(SSH_DIGEST_MD5)) == NULL)
struct ssh_hmac_ctx *ssh_hmac_start(int alg);
if ((ctx = ssh_hmac_start(SSH_DIGEST_SHA1)) == NULL ||
if ((mac->hmac_ctx = ssh_hmac_start(macalg->alg)) == NULL)