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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def ssh_init_sec_context( self, target, desired_mech=None, username=None, recv_token=None ):
 """ Initialize a GSS-API context. :param str username: The name of the user who attempts to login :param str target: The hostname of the target to connect to :param str desired_mech: The negotiated GSS-API mechanism ("pseudo negotiated" mechanism, because we support just the krb5 mechanism :-)) :param str recv_token: The GSS-API token received from the Server :raises: `.SSHException` -- Is raised if the desired mechanism of the client is not supported :return: A ``String`` if the GSS-API has returned a token or ``None`` if no token was returned """

        from pyasn1.codec.der import decoder

        self._username = username
        self._gss_host = target
        targ_name = gssapi.Name(
            "host@" + self._gss_host, gssapi.C_NT_HOSTBASED_SERVICE
        )
        ctx = gssapi.Context()
        ctx.flags = self._gss_flags
        if desired_mech is None:
            krb5_mech = gssapi.OID.mech_from_string(self._krb5_mech)
        else:
            mech, __ = decoder.decode(desired_mech)
            if mech.__str__() != self._krb5_mech:
                raise SSHException("Unsupported mechanism OID.")
            else:
                krb5_mech = gssapi.OID.mech_from_string(self._krb5_mech)
        token = None
        try:
            if recv_token is None:
                self._gss_ctxt = gssapi.InitContext(
                    peer_name=targ_name,
                    mech_type=krb5_mech,
                    req_flags=ctx.flags,
                )
                token = self._gss_ctxt.step(token)
            else:
                token = self._gss_ctxt.step(recv_token)
        except gssapi.GSSException:
            message = "{} Target: {}".format(sys.exc_info()[1], self._gss_host)
            raise gssapi.GSSException(message)
        self._gss_ctxt_status = self._gss_ctxt.established
        return token