When TLS is enabled, this optional parameter specifies the name of the file containing the private key of the certificate configured with TLS_CertificateFile. It is sometimes in the same file as the TLS_CertificateFile. Special formatting characters are supported.

The private key file must be in PEM or ASN1 format, depending on the setting of the TLS_CertificateType parameter.

Usually the private key is encrypted, use TLS_PrivateKeyPassword is the key to decrypt it. For more information, see Section 3.11.4. TLS_CertificateFile and Section 3.11.8. TLS_PrivateKeyPassword.