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If you created a certification authority above, you are creating a certificate (signing a certificate signing request). Otherwise, you send the myPLC.csr file for signing to the appropriate certification authority (e.g., the company's IT department).
The -days parameter specifies the certificate validity period in days.
openssl x509 -req -inmyPLCin myPLC.csr -CA caMQTT.crt -CAkey caMQTT.key -CAcreateserial -out myPLC.crt -days 1000
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