Connection Filebeat to Logstash SSL: certificate specifies an incompatible key usage

[peterol]

Hey everyone,

I want to establish a connection with SSL between Filebeat and Logstash according to this. Filebeat runs natively whereas Logstash is running in a Docker container. I converted my keys to the PKCS8 format, using the command:

 openssl pkcs8 -topk8 -inform PEM -outform PEM -in infile.key -out outfile.pkcs8.key -nocrypt 

But in Filebeat I get the error:

Failed to connect to backoff(async(tcp://logstash.domain.de:5045)): x509: certificate specifies an incompatible key usage

and in Logstash as soon as Filebeat tries to connect:

Caused by: javax.net.ssl.SSLHandshakeException: Received fatal alert: bad_certificate

I also tried a traditional PEM key in Filebeat with the same result.

This is my configuration:
input.conf:

input {
  beats {
    port => 5045
    ssl => true
    ssl_certificate_authorities => ["/usr/share/logstash/config/certs/cacert.pem"]
    ssl_certificate => "/usr/share/logstash/config/certs/cert.pem"
    ssl_key => "/usr/share/logstash/config/certs/key.pkcs8.key"
    # deactivated for now
    ssl_verify_mode => "none"
    client_inactivity_timeout => 120
  }
}

filebeat.yml:

output.logstash:
  # The Logstash hosts
  hosts: ["logstash.domain.de:5045"]

  # Optional SSL. By default is off.
  # List of root certificates for HTTPS server verifications
  ssl.certificate_authorities: ["/etc/certs/cacert.pem"]
  
  # Certificate for SSL client authentication
  ssl.certificate: "/etc/certs/cert.pem"
  
  # Client Certificate Key
  ssl.key: "/etc/certs/key.pkcs8.key"

I use the same certificates and keys to communicate with my Elasticsearch cluster with no problems, so they should be okay. The only difference is the PKCS8 format used here.

Thanks a lot for any help!