typedef void (*SilcAskPassphrase)(const unsigned char *passphrase,
                                        SilcUInt32 passphrase_len,
                                        void *context);

    Ask passphrase callback. This is called by the application when the
    library calls `ask_passphrase' client operation.  The callback delivers
    the passphrase to the library.  The passphrases in SILC protocol
    MUST be in UTF-8 encoding, therefore the `passphrase' SHOULD be UTF-8
    encoded, and if it is not then library will attempt to encode it.