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PFX : PFX defines a file format commonly used to store private with accompanying public key certificates, protected with a password-based symmetric key (standard-PKCS12).
PEM : Openssl usages PEM (Privacy Enhanced Mail Certificate) to store the private key.
If you have the openssl then go to command promt and run the following commands (If not, download it from openssl, you can either download binary or source and then compile).
If you want to extract private key from a pfx file and write it to PEM file
>>openssl.exe pkcs12 -in publicAndprivate.pfx -nocerts -out privateKey.pem
If you want to extract the certificate file (the signed public key) from the pfx file
>>openssl.exe pkcs12 -in publicAndprivate.pfx -clcerts -nokeys -out publicCert.pem
To remove the password from the private key file.
>> openssl.exe rsa -in privateKey.pem -out private.pem
This is required as, at the time of exporting privateKey, you have added a password to the private key to secure it. If you left the password with it, it will keep asking the password as any application tries to access it.