Secure private key with passphrase when exporting

[dolphinsd]

Once private key is exported is not secure.

Proposal to add ability to encrypt the private key using a symmetric encryption key derived from a passphrase and then decrypted again.

[ektrah]

Yes, that would be a good enhancement. What specification should NSec implement to provide that? I was thinking of RFC 5958, Section 3 (EncryptedPrivateKeyInfo), but that covers only asymmetric keys.

[dolphinsd]

@ektrah it could be an option to Export call?

[ektrah]

An experimental implementation has been added in PasswordBasedKeyExporter.cs. I'm not sure how useful it is (and it's probably coming way too late too; sorry for that). Feedback is welcome.