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Certificates and keys must be in PEM format.
PEM-format files can have a variety of extensions (.pem, .key, .cer, .csr, and so on). Be aware that the listed extensions can also be binary, which is not supported.
To validate the certificates of the TLS clients and servers you communicate with, their CA signing certificates must be in the certificate store.
The certificate store already contains the certificates for many commercial CAs, and you can import additional certificates into the store if you need.
If you do not want to purchase a digital certificate from a third-party CA, or if you want to use digitally signing immediately, you can create your own self-signed TLS certificate.