We generate certificates that are intended to be used for server authentication in a couple of places:

- `bin/elasticsearch-certutil http` command  ( See `CertGenUtils.generateSignedCertificate` in `HttpCertificateCommand` )
- TLS auto-configuraion for node startup ( see [`AutoConfigureNode`](https://github.com/elastic/elasticsearch/blob/814f7f9f52cb8c05db6465b9bfc818ea5d76c575/x-pack/plugin/security/cli/src/main/java/org/elasticsearch/xpack/security/cli/AutoConfigureNode.java#L454) )

In these cases we know that the certificate is meant to be used for server authentication, so we can set Extended Key Usage extension to `1.3.6.1.5.5.7.3.1`
x-pack/plugin/security/cli/src/main/java/org/elasticsearch/xpack/security/cli/HttpCertificateCommand.java
if (approxDays < recommendedMinimumDays) {
                    terminal.println("The period '" + toString(parsed) + "' is less than the recommended period");
                    if (terminal.promptYesNo("Are you sure?", false) == false) {
                        return null;
                    }
                }
                return parsed;
            } catch (DateTimeParseException e) {
                terminal.println("Sorry, I do not understand '" + input + "' (" + e.getMessage() + ")");
                return null;
            }
        });
        return period;
    }
    private Integer readKeySize(Terminal terminal, int keySize) {
        return tryReadInput(terminal, "Key Size", keySize, input -> {
            try {
                final int size = Integer.parseInt(input);
                if (size < 1024) {
                    terminal.println("Keys must be at least 1024 bits");
                    return null;
                }
                if (size > 8192) {
                    terminal.println("Keys cannot be larger than 8192 bits");
                    return null;
                }
                if (size % 1024 != 0) {
                    terminal.println("The key size should be a multiple of 1024 bits");
                    return null;
                }