Info : Ubuntu 18.04
ELK : 7.4.0
OpenSSL : 1.1.1
I just followed the tutorial Configuring SSL, TLS, and HTTPS to secure Elasticsearch, Kibana, Beats, and Logstash | Elastic Blog
Connected logstash + kibana + elastic nodes kibana in stack monitoring works fine see logstash etc.
input {
beats {
port => 5044
ssl => true
ssl_key => '/etc/logstash/config/certs/logstash.pkcs8.key'
ssl_certificate => '/etc/logstash/config/certs/logstash.crt'
}
}
output {
elasticsearch {
hosts => ["https://node1.elastic.ucti:9200","https://node2.elastic.ucti:9200"]
cacert => '/etc/logstash/config/certs/ca.crt'
user => 'logstash_writer'
password => password
}
}
cat just displays the key in dirs.
add the instance information to yml file
instances:
- name: 'node1'
dns: [ 'node1.elastic.ucti' ] - name: "node2"
dns: [ 'node2.elastic.ucti' ] - name: "node3"
dns: [ 'node3.elastic.ucti' ] - name: 'my-kibana'
dns: [ 'kibana.ucti' ] - name: 'logstash'
dns: [ 'logstash.ucti' ] 
IF i do :
curl -v --cacert ca.crt https://logstash.ucti:5044
- Rebuilt URL to: https://logstash.ucti:5044/
 - Trying 172.21.1.254...
 - TCP_NODELAY set
 - Connected to logstash.ucti (172.21.1.254) port 5044 (#0)
 - ALPN, offering h2
 - ALPN, offering http/1.1
 - successfully set certificate verify locations:
 - CAfile: ca.crt
CApath: /etc/ssl/certs - TLSv1.3 (OUT), TLS handshake, Client hello (1):
 - OpenSSL SSL_connect: SSL_ERROR_SYSCALL in connection to logstash.ucti:5044
 - stopped the pause stream!
 - Closing connection 0
curl: (35) OpenSSL SSL_connect: SSL_ERROR_SYSCALL in connection to logstash.ucti:5044 
But this ca.crt works fine with curl elastic.node1.ucti and kibana.ucti for example
curl -v --cacert ca.crt https://kibana.ucti:5601
- Rebuilt URL to: https://kibana.ucti:5601/
 - Trying 172.21.1.253...
 - TCP_NODELAY set
 - Connected to kibana.ucti (172.21.1.253) port 5601 (#0)
 - ALPN, offering h2
 - ALPN, offering http/1.1
 - successfully set certificate verify locations:
 - CAfile: ca.crt
CApath: /etc/ssl/certs - TLSv1.3 (OUT), TLS handshake, Client hello (1):
 - TLSv1.3 (IN), TLS handshake, Server hello (2):
 - TLSv1.2 (IN), TLS handshake, Certificate (11):
 - TLSv1.2 (IN), TLS handshake, Server key exchange (12):
 - TLSv1.2 (IN), TLS handshake, Server finished (14):
 - TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
 - TLSv1.2 (OUT), TLS change cipher, Client hello (1):
 - TLSv1.2 (OUT), TLS handshake, Finished (20):
 - TLSv1.2 (IN), TLS handshake, Finished (20):
 - SSL connection using TLSv1.2 / ECDHE-RSA-AES128-GCM-SHA256
 - ALPN, server accepted to use http/1.1
 - Server certificate:
 - subject: CN=my-kibana
 - start date: Oct 16 16:33:51 2019 GMT
 - expire date: Oct 15 16:33:51 2022 GMT
 - subjectAltName: host "kibana.ucti" matched cert's "kibana.ucti"
 - issuer: CN=Elastic Certificate Tool Autogenerated CA
 - SSL certificate verify ok.
 
GET / HTTP/1.1
Host: kibana.ucti:5601
User-Agent: curl/7.58.0
Accept: /
< HTTP/1.1 302 Found
< location: /login?next=%2F
< kbn-name: kibana
< kbn-xpack-sig: f41b69eb682d8c0c5b1087f7ba1c86bf
< cache-control: no-cache
< content-length: 0
< Date: Tue, 22 Oct 2019 08:34:37 GMT
< Connection: keep-alive
<
- Connection #0 to host kibana.ucti left intact
 
Errors :
[2019-10-22T10:41:51,085][ERROR][logstash.inputs.beats    ][main] Looks like you either have a bad certificate, an invalid key or your private key was not in PKCS8 format.
[2019-10-22T10:41:51,086][WARN ][io.netty.channel.ChannelInitializer][main] Failed to initialize a channel. Closing: [id: 0xb5cacbf2, L:/172.21.1.254:5044 - R:/172.21.1.250:38896]
java.lang.IllegalArgumentException: File does not contain valid private key: /etc/logstash/config/certs/logstash.pkcs8.key
Caused by: java.security.KeyException: could not find key file: /etc/logstash/config/certs/logstash.pkcs8.key
at io.netty.handler.ssl.PemReader.readPrivateKey(PemReader.java:114) ~[netty-all-4.1.30.Final.jar:4.1.30.Final]
at io.netty.handler.ssl.SslContext.toPrivateKey(SslContext.java:1015) ~[netty-all-4.1.30.Final.jar:4.1.30.Final]
at io.netty.handler.ssl.SslContextBuilder.keyManager(SslContextBuilder.java:268) ~[netty-all-4.1.30.Final.jar:4.1.30.Final]
... 20 more
[2019-10-22T10:42:00,193][WARN ][org.logstash.beats.Server][main] Exception caught in channel initializer
java.lang.IllegalArgumentException: File does not contain valid private key: /etc/logstash/config/certs/logstash.pkcs8.key