The service itself is distributed as Docker container. Check the [`http-adapter`](https://github.com/mainflux/mainflux/blob/master/docker/docker-compose.yml#L245-L262) service section in
Setting `MF_HTTP_ADAPTER_CA_CERTS` expects a file in PEM format of trusted CAs. This will enable TLS against the Things gRPC endpoint trusting only those CAs that are provided.