Ok, you are right. I will use the user password as passphrase. Thanks for the suggest.
Now, to import the certificate in your iPhone, you could:
– export the certificate in .pem format (ex. fulvio.pem)
– by using openssl convert it in pkcs12 with passphrase:
openssl pkcs12 -export -in fulvio.pem -out fulvio.pfx
Of course you will be asked for the passphrase.