bool silc_client_add_private_message_key(SilcClient client,
                                             SilcClientConnection conn,
                                             SilcClientEntry client_entry,
                                             const char *cipher,
                                             const char *hmac,
                                             unsigned char *key,
                                             SilcUInt32 key_len,
                                             bool generate_key,
                                             bool responder);

    Adds private message key to the client library. The key will be used to
    encrypt all private message between the client and the remote client
    indicated by the `client_entry'. If the `key' is NULL and the boolean
    value `generate_key' is TRUE the library will generate random key.
    The `key' maybe for example pre-shared-key, passphrase or similar.
    The `cipher' and `hmac' MAY be provided but SHOULD be NULL to assure
    that the requirements of the SILC protocol are met. The API, however,
    allows to allocate any cipher and HMAC.

    If `responder' is TRUE then the sending and receiving keys will be
    set according the client being the receiver of the private key.  If
    FALSE the client is being the sender (or negotiator) of the private
    key.

    It is not necessary to set key for normal private message usage. If the
    key is not set then the private messages are encrypted using normal
    session keys. Setting the private key, however, increases the security.

    Returns FALSE if the key is already set for the `client_entry', TRUE
    otherwise.