Get a private key from http_ca.crt

Hi

How i can get a private key from /etc/elasticsearch/certs/http_ca.crt file ?
I need that key to create cert for logstash


./bin/elasticsearch-certutil cert --name logstash --ca-cert /etc/elasticsearch/certs/http_ca.crt --ca-key XXXXXX --dns elk.example.com --ip 1.2.3.4 --pem