Sets the private key for a server certificate or client certificate.
A secure connection using this SecurityContext will use this key with
the server or client certificate to sign and decrypt messages.
file is the path to a PEM or PKCS12 file containing an encrypted
private key, encrypted with password. Assuming it is well-formatted, all
other contents of file are ignored. An unencrypted file can be used,
but this is not usual.
NB: This function calls ReadFileAsBytesSync, and will block on file IO.
Prefer using usePrivateKeyBytes.
Source
void usePrivateKey(String file, {String password});