Since RouterOS 6.1 it is possible to interface router using RouterOS API over a secure connection using api-ssl service.
Service is available in '/ip services' menu
API-ssl service is capable to work in two modes - with and without a certificate. In the case no certificate is used in '/ip service' settings then anonymous Diffie-Hellman cipher have to be used to establish connection. If certificate is in use TLS session can be established