I made a truststore
keytool -import -alias ceph -file ceph.crt -keystore ceph.jks
and then passed the file and password:
ES_JAVA_OPTS=-Xms2048m -Xmx2048m \
-Djavax.net.ssl.trustStore=/usr/share/elasticsearch/config/ceph.jks \
-Djavax.net.ssl.keyStorePassword=secretpasswordyoudontknow"
And now it works !
Thanks !!