QSslKey reading / writing DER with a passphrase is bogus

QSslKey provides a constructor which is supposed to be able to read DER data, optionally encrypted with a passphrase. Conversely it provides a toDer method which outputs DER data, optionally encrypted with a passphrase.

The problem is that "encrypted DER" on its own is unusable : we lack information about the crypto algorithm and the initialization vector (IV) which were used.

Note that this problem only affects DER input / output, the PEM format is fine as in addition to the encrypted data it has RFC822-style headers which convey the algorithm / IV information.